Mechanical grapple for grabbing and holding sacks and bags

Description

FIELD OF THE INVENTION

This invention relates to material handling devices. More specifically, this invention is a grapple that can be used with robotic or material handling devices for grabbing and holding compliant objects with undefined shapes such as sacks and bags. The grapple of this invention can grab and hold filled sacks from any point on the sack and regardless of the sack orientation and position.

BACKGROUND OF THE INVENTION

Delivery and postal services across the world currently use sacks to hold letters, magazines and small boxes. These sacks are handled manually by mail handlers in the postal service distribution centers. Most sacks used by the US Postal Service (USPS) do not have eyelets, handles or any form of operator interface for lifting and carrying. The shape, size and the weight of a sack depends on the items in the sack and how it is rested on the floor. During several visits of USPS distribution centers, the inventors have observed sacks that were fully filled with magazine bundles and weighed up to seventy pounds. The heavy weight of these sacks, the lack of handles, eyelets or other operator interface on the sacks, and the unpredictable shape and size of the sacks create awkward and uncomfortable handling situations for U.S. mail handlers at all USPS distribution centers. This awkward sack handling, in particular during repeated maneuvers, increases the risk of wrist, finger and back injuries among mail handlers. To minimize the risk of injuries to workers, the inventors carried out an engineering research effort, on a solid scientific foundation, to design a grapple for grasping and holding sacks. The grapple of this invention grabs and holds a sack regardless of the shape and size of the sack from any point on the sack (i.e. it is not necessary to gather and flatten the edge of the sack or orient the sack prior to grasp.) This invention has been tested and proven to be effective in grabbing and holding sacks.

Inventors have created several devices to hold sacks and bags. For example, Achelpohl, U.S. Pat. No. 4,181,345, describes a manually operated clamping device for lifting filled sacks. The device includes a frame supporting a clamping mechanism. The clamping mechanism includes two spaced parallel bars, mounted at their end to two parallel plates and disposed to be rotated in unison about a common parallel axis by a hand-crank. One of the bars is individually rotatable about its own axis and displaceable towards and away from the other bar. To use the device of Achelpohl U.S. Pat. No. 4,181,345, the gathered top of a filled sack is manually placed between the two bars. The hand crank is manually operated to wrap the top of the sack around the two bars and clamp it between them. Once secured in the clamping mechanism, the combined clamping device and sack may then be lifted by the device's frame. Although useful for its purpose, the Achelpohl U.S. Pat. No. 4,181,345 device has the disadvantage of requiring considerable manual manipulation of both the sack and the device to accomplish its utility.

To try to overcome some of the disadvantages of the Achelpohl U.S. Pat. No. 4,181,345 device, Achelpohl, U.S. Pat. No. 4,226,458, discloses a device similar to the U.S. Pat. No. 4,181,345 device. In the U.S. Pat. No. 4,226,458 device, the clamping bars are mechanically separable and are mounted only at one end to a frame, such that they form parallel cantilever arms. This allows the gathered ends of a sack to be either inserted or slide between the clamping bars. The end of the sack is then engaged by mechanically operating the clamping mechanism. This device improves the manual operation required of the U.S. Pat. No. 4,181,345 device wherein it is necessary to manually introduce the end of the sack between the clamping bars. Although the Achelpohl U.S. Pat. No. 4,226,458 device has advantages over the earlier Achelpohl U.S. Pat. No. 4,181,345 device, it still requires the gathered end of a sack to be manually placed between the clamping bars, or that the sack be placed on its bottom and the top of the sack gathered and flattened and presented in a proper configuration before the U.S. Pat. No. 4,226,458 device can slideably engage the top of the sack and secure it for lifting.

In another example, Minenko et al., U.S. Pat. No. 4,549,760, discloses a device for gripping and hoisting packed sacks that utilizes a similar mechanism to grab sacks as the Achelpohl U.S. Pat. No. 4,226,458 device and has similar advantages and disadvantages. However in this case one of the bars moves relative to the other one with the help of a hydraulic linear actuator. The Minenko et al. device still requires the gathered end of a sack to be manually placed between the clamping bars. In this case, the sack is placed on its bottom next to the unpowered bar. The second bar, powered by an actuator moves around the first bar causing the throat of the sack to be secured in between the bars.

A later Minenko disclosure, U.S. Pat. No. 4,852,927, describes using two clamping bars to grip and hold the neck of a sack for lifting and transport. The Minenko U.S. Pat. No. 4,852,927 device includes a locking mechanism to secure the clamping bars in a closed position while gripping a sack. Although useful for its intended purpose, the U.S. Pat. No. 4,852,927 device requires that the neck of the sack be manually inserted between the clamping bars and that the device then be further manually operated to close and lock the neck of the sack in the device. Additionally, the sack cannot be released from the device without first removing the load from the locking mechanism.

All of the devices described in the above patents have the following common characteristics:

The gathered and flattened edge of the sack must be carefully placed between two adjacent bars by an operator prior to grasping.

One of the bars rotates around the other one or both bars rotate along a common axis. Then the gathered and flattened edge of the sack wraps around the bars. The weight of the sack itself pushes the rods against each other therefore locks the edge of the sack in between the bars.

It is necessary that the sack be placed on its bottom so it can be grasped by the device's bars. If a sack cannot be placed on its bottom in a stable form, then the devices described above cannot be used.

In general, a great deal of operator intervention is required for proper and safe operation of the devices described above. This usually results in slow lifting operation and therefore these devices have not been employed in USPS and many other distribution centers where large flow the sacks need to be lifted repeatedly from a shoot or from a conveyor belt.

SUMMARY OF THE INVENTION

The basic idea in design of the grapple of this invention is to create a device that allows at least two rollers, powered by at least one actuator, to turn in opposite directions along their own axes while they are pushed toward each other by force of at least one spring. The inward spinning of the rollers and the friction between the rollers and the sack material causes the sack material to be dragged in between the rollers. When the rollers are prevented from spinning, the sack material is kept secure in between the rollers due to the force of spring and friction between the rollers and the sack material. When rollers rotate outwardly, the sack material will come out of the rollers and the sack will be released. This application describes the hardware architecture, the control method and the design issues associated with the grapple.

OBJECTS AND ADVANTAGES

In view of the above prior art, the object of the present invention is to design a grapple that can grab any point of a sack without any operator intervention and regardless of how the sack is laid on the floor, on a table, or on a conveyor belt. To achieve this objective, an entirely different and effective concept for grasping sacks was developed and is described here. When any of the grapples described in this invention comes in contact with a sack, the sack material will be grabbed and pulled quickly into the grapple without any intervention from the operator. Unlike the devices of the above prior arts, the grapple described here

grabs a sack from any point on the sack.

does not require the edge of the sack to be gathered and flattened prior to grasp.

does not require the sack to be placed on its bottom prior to grasp (i.e. the sack can be laid on the floor or on a conveyor belt from any side.)

does not require operator intervention for grasp.

does not use the weight of the sack to lock and secure the sack in the grapple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B

illustrate the basic underlying principle of operation of the grapple.

FIGS. 2A and 2B

illustrate two perspective views of the grapple that has the basic functional characteristics depicted by

FIGS. 1A and 1B

.

FIG. 3

illustrates the schematic concept of the mechanism of the grapple of

FIGS. 4A and 4B

where a sack has been grabbed and held by the grapple.

FIGS. 4A

,

4

B and

4

C illustrate three perspective views of the grapple that has the basic functional characteristics depicted by FIG.

3

.

FIG. 5

illustrates the operational phases of the grapple as a function of the states of three control signals.

FIG. 6A

illustrates how the relays are wired with switches and detectors to create the behavior described in Table of FIG.

5

.

FIG. 6B

illustrates how the contacts of three relays are connected to the electric actuators and the brakes.

FIGS. 7A and 7B

show the first embodiment of the grapple where a proximity switch is installed on the grapple to indicate the nearness of the sack to the grapple.

FIGS. 8A and 8B

show the second embodiment of the grapple where a proximity switch is installed on the grapple to indicate the nearness of the sack to the grapple.

FIGS. 9A and 9B

show two perspective views of the second embodiment of the grapple with a switch that issues a signal when enough sack material has been collected in between the grapple rollers.

FIG. 10A

shows the mechanism in the second embodiment of the grapple where sufficient sack material is not collected in between the rollers.

FIG. 10B

shows the mechanism in the second embodiment of the grapple where sufficient sack material is collected in between the rollers and a switch has been activated.

FIG. 11

shows the grapple installed on a manual material handling system where the operator is able to use her left hand to push a button and release the sack.

FIGS. 12A and 12B

show two perspective views of the second embodiment of the grapple used in the material handling device of FIG.

11

.

FIGS. 13A and 13B

show two perspective views of the first embodiment grapple as it could be installed on the material handling device of FIG.

11

.

FIG. 14

shows the second embodiment of the grapple installed on a robot for grasping, lifting and maneuvering sacks.

FIG. 15

shows the contact forces and friction forces between the rollers and sack when the sack is held in between the rollers.

FIG. 16

shows roller in its initial engagement with the sack.

FIG. 17

shows the pressure profile on a roller.

FIG. 18

shows a grapple where only one roller is powered by an actuator.

FIG. 19

shows a grapple where the rollers move relative to each other on a linear path and two actuators power the rollers.

FIG. 20

shows a grapple where the rollers move relative to each other on a linear path and one actuator powers the rollers.

FIGS. 21A and 21B

show a grapple where three rollers are used for grasping and holding sacks.

FIGS. 22A

,

22

B and

22

C illustrate the schematic concept of the mechanism of the grapple that uses non-circular rollers.

FIG. 23

illustrates the schematic concept of the mechanism of the grapple where the rollers have mating lobes.

FIG. 24

illustrates how the second embodiment of the grapple is used to grab a box.

DETAILED DESCRIPTIONS OF DRAWINGS

Referring now to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Like elements in the drawings may be represented by like numbers.

FIG.

1

A and

FIG. 1B

schematically illustrate the basic principle behind this invention. The grapple

10

comprises two rollers

11

and

12

, which are able to rotate along their own axes

13

and

14

respectively. Both rollers

11

and

12

are powered and can rotate in both clockwise and counterclockwise directions. There are many ways to transmit torque to rollers

11

and

12

, however, FIG.

1

A and

FIG. 1B

are drawn without any source of power for the sake of clear illustration and understanding of the basic principle of this invention. Other figures in this document show the source of power and the method of transmitting power to the rollers. Two holding brackets

18

and

19

pivot relative to each other at one end around a pivot or hinge

20

, and hold rollers

11

and

12

at their other ends. A bias spring

15

is connected to holding brackets

18

and

19

and biases holding brackets

18

and

19

toward each other. The surfaces

16

of rollers

11

and

12

are gripping surfaces. A gripping surface may be accomplished in the present invention by any of a number of means known to one of ordinary skill in the art. Examples include the surface

16

being covered by frictional material such as soft rubber, or being knurled, grooved, stippled or the like. As shown in

FIG. 1A

, when roller

11

is turned clockwise and roller

12

is turned counterclockwise, and the rollers come in contact with a sack, sack material

17

will be grabbed and dragged into grapple

10

due to the interaction (e.g., friction forces) between the rollers and sack material

17

. As rollers

11

and

12

continue to turn, more sack material

17

will be dragged in between the rollers as shown in FIG.

1

B. When sufficient sack material

17

has been grabbed, rotation of rollers

11

and

12

is stopped. This can be facilitated by a detector switch (described in later paragraphs) disposed in grapple

10

which issues a signal to stop rotation and lock rollers

11

and

12

when sufficient sack material

17

has been dragged into the inter-roller region between rollers

11

and

12

. The friction between the rollers and sack material

17

will not allow the sack to slide out of grapple

10

. Depending on sack material

17

, an appropriate roller surface

16

can be selected to provide sufficient friction between rollers

11

and

12

and sack material

17

to hold the sack. As long as rollers

11

and

12

are locked and prevented from rotating and bias spring

15

pushes rollers

11

and

12

tightly together, and as long as the coefficient of friction between the sack material

17

and the rollers is sufficiently large, the sack will not slide out of grapple

10

. While secured in this manner, the sack can be maneuvered by manipulating grapple

10

with a material handling device such as a robot arm, a hoist or similar means. When rollers

11

and

12

are rotated in the opposite directions, (i.e., roller

11

turns in counterclockwise direction and roller

12

turns in clockwise direction) sack material

17

, which had been grabbed by rollers

11

and

12

will pass out of grapple

10

and the sack will be released. Another method of releasing the sack is to separate rollers

11

and

12

from each other. Bias spring

15

is disposed on holding brackets

18

and

19

to pull rollers

11

and

12

toward each other. However, any biasing mechanism that causes holding brackets

18

and

19

to push rollers

11

and

12

toward each other can be practiced in present grapple

10

. Examples of such biasing means that can be used instead of or in combination with bias spring

15

include electric solenoids, electrostatic actuators, electric motors, pneumatic and hydraulic actuators.

FIG.

2

A and

FIG. 2B

show two different views of a preferred embodiment of the grapple

26

of this invention. A first roller

27

is able to rotate along its own axis while being received between two holding brackets

29

and

30

. Similarly, a second roller

28

is able to rotate along its own axis while being received between two other holding brackets

31

and

32

. Standard ball bearings, roller bearings or bushings can be installed in holding brackets

29

,

30

,

31

and

32

at the roller receiving points to allow smooth rotation of rollers

27

and

28

.

A shaft

33

is held tightly between two horizontal shaft brackets

34

and

35

. Fasteners

36

are used to hold shaft brackets

34

and

35

tightly together, thereby fixing shaft

33

between them. Shaft brackets

34

and

35

provide a mounting point at which grapple

26

may be attached to a robot or a material handling system. Attachment points

37

(e.g., threaded holes) may be provided on shaft brackets

34

and

35

to facilitate attaching grapple

26

to a material handling device. The overall function of two shaft brackets

34

and

35

is to hold shaft

33

and connect grapple

26

to a robot or to a material handling system.

Two holding brackets

29

and

30

holding the first roller

27

are free to rotate on shaft

33

. Ball bearings, roller bearings, or bushings can be installed between shaft

33

and holding brackets

29

and

30

to allow smooth rotation of holding brackets

29

and

30

. Holding brackets

31

and

32

for the second roller

28

are tightly fixed to shaft

33

and therefore do not turn or pivot on shaft

33

. Fixing the second roller's holding brackets

31

and

32

to shaft

33

is accomplished in

FIG. 2A

by means of tightening screws

38

and

39

, however, other fixing means are known in the art. This arrangement allows the first roller

27

to move relative to the second roller

28

. Two biasing springs

40

and

41

pull holding brackets

29

and

30

of the first roller

27

toward holding brackets

31

and

32

of the second roller

28

.

Rollers

27

and

28

are powered by respective actuators

43

and

44

via flexible shafts

45

and

46

. Both actuators

43

and

44

are secured to a main bracket

52

via standard fasteners

53

and

54

. Actuator

43

, which turns the first roller

27

, consists of an electric motor

48

coupled to a speed reducer transmission

50

. Similarly actuator

44

, which turns the second roller

28

, consists of an electric motor

47

coupled to a speed reducer transmission

49

. By properly powering actuators

43

and

44

, rollers

27

and

28

are able to turn in both clockwise and counterclockwise directions.

Electric motors

47

and

48

employed in this embodiment were single phase 0.2 HP motors powered by a 24 VDC power supply via two multi-stranded power cables

42

and

51

. Both speed reducer transmissions

49

and

50

have a speed ratio of 36. The output torque of the transmission speed reducer at 180 RPM is 65 lbf-inch. Two brakes

55

and

56

used in this embodiment were powered by a 24 VDC power supply through two multi-stranded power cables

42

and

51

. The brakes are normally engaged when not powered electrically, and prevent the motor shafts from turning. When brakes

55

and

56

are electrically powered, they are disengaged, and the motor shafts were free to turn. Brakes

55

and

56

used in this embodiment were manufactured by Inertia Dynamics and produce 3 lbf-inch braking torque. Other actuator components are known in the art and are practicable in the present invention by the ordinary skilled artisan.

Two motors

48

and

47

are wired such that their respective rollers

27

and

28

turn in opposite directions when motors

48

and

47

are operated. When both motors

48

and

47

are operated such that rollers

27

and

28

turn inwardly, sack material

17

contacted by rollers

27

and

28

, is grabbed and drawn in between the rollers. When sufficient sack material is grabbed in between rollers

27

and

28

, the grapple controller (described in later paragraphs) stops motors

48

and

47

, causing brakes

56

and

55

to engage and prevent rollers

27

and

28

from rotating. With the motors prevented from turning and rollers

27

and

28

locked (zero angular speed is generated for the rollers), sack material

17

will be secured between the rollers and the sack can be maneuvered by manipulating shaft brackets

34

and

35

of grapple

26

. As long as rollers

27

and

28

pushed toward each other sufficiently by springs

40

and

41

, and the coefficient of friction between sack materials

17

and rollers is sufficiently large, the sack will not slide out of grapple

26

. When rollers

27

and

28

rotate outwardly, sack material

17

grabbed by rollers

27

and

28

will come out of grapple

26

, and the sack will be released. Of course, an alternative means to release the sack material from grapple

26

is to separate rollers

27

and

28

from one another.

Although two biasing springs

40

and

41

were used in the illustrated embodiment, there are other methods of pushing the rollers together. For example, an active force generating component could be used to force the rollers against each other, such as an electric solenoid, a motor, or a pneumatic or hydraulic translational actuator (cylinder).

FIG. 3

is a schematic representation of a second embodiment of the grapple

57

of this invention. Two holding brackets

60

and

61

are pivotally connected at one end and pivot relative to each other about a pivot or hinge

68

. The other end of holding brackets

60

and

61

receive and hold the ends of two rollers

70

and

71

. A biasing spring

69

is connected to holding brackets

60

and

61

and pulls them toward each other. Two motors

58

and

59

rotate two rollers

70

and

71

of grapple

57

. Motors

58

and

59

are installed on holding brackets

60

and

61

. Motor

58

is connected to a drive sprocket

63

which in turn is coupled via a drive chain

66

to a driven sprocket

64

. The driven sprocket

64

is in operative communication with the end of roller

70

to rotate roller

70

in response to operation of motor

58

. Similarly motor

59

is connected to a drive sprocket

62

which in turn is coupled via a drive chain

67

to a driven sprocket

65

. The driven sprocket

65

is in operative communication with the end of roller

71

to rotate roller

71

in response to operation of motor

59

. A brake may be installed on at least one of the motors to prevent the rotation of rollers

70

and

71

when the sack needs to be held securely. The surfaces

16

of the rollers

70

and

71

are grabbing surfaces as described above.

As the first roller

70

is turned counterclockwise by its motor

58

and the second roller

71

is turned clockwise by its motor

59

, sack material

17

will be grabbed and dragged into the grapple

57

due to friction forces between the rollers and sack material

17

. When sufficient sack material

17

has been grabbed, rollers

70

and

71

will stop turning, and the friction between the rollers and sack material

17

will retain the sack in grapple

57

. This can be accomplished by installing a switch in grapple

57

as described in later sections.

FIGS. 4A

,

4

B, and

4

C show three different views of the actual mechanism which is designed based on the schematic concept of FIG.

3

.

FIGS. 4A

,

4

B, and

4

C show three different views of the actual grapple

76

of the second embodiment of this invention. A first roller

77

is able to rotate along its own axis while being held by two holding brackets

79

and

80

. A second roller

78

is able to rotate along its own axis while held by two other holding brackets

81

and

82

. As in other embodiments, standard ball bearings, roller bearings and bushings can be installed in ends of holding brackets

79

,

80

,

81

and

82

to receive the ends of rollers

77

and

78

and allow their smooth rotation.

Shaft

83

is held tightly between two horizontal brackets

84

and

85

. Fasteners

86

hold two horizontal brackets

84

and

85

tightly together, thereby holding shaft

83

stationary in between horizontal brackets

84

and

85

. Attachment points

87

(threaded holes) are included on horizontal brackets

84

and

85

to facilitate connecting grapple

76

to a material handling device. An object of horizontal brackets

84

and

85

in addition to holding shaft

83

is to enable connecting grapple

76

to a material handling system.

Two holding brackets

81

and

82

holding the second roller

78

, are free to pivot or rotate on shaft

83

. Bearings or bushings can be installed between shaft

83

and these holding brackets

81

and

82

to allow them to rotate smoothly. However, holding brackets

79

and

80

holding the first roller

77

are secured tightly to shaft

83

with the help of dowel pins

88

and

89

, and therefore do not pivot or rotate on shaft

83

. This arrangement allows the second roller

78

and its holding brackets

81

and

82

to rotate and move relative to the first roller

77

and its holding brackets

79

and

80

. Biasing springs

90

and

91

bias rollers

77

and

78

toward each other.

Respective rollers

77

and

78

are powered by actuators

93

and

94

via two chains

95

and

96

. As shown in

FIG. 4A

, actuator

93

is connected to a drive sprocket

72

, which in turn is coupled via a drive chain

95

to a driven sprocket

73

. The driven sprocket

73

is in operative communication with the end of roller

77

to rotate roller

77

in response to operation of actuator

93

. Similarly actuator

94

(shown in

FIG. 4C

) is connected to a drive sprocket

74

, which in turn is coupled via a drive chain

96

to a driven sprocket

75

. The driven sprocket

75

is in operative communication with the end of roller

78

to rotate roller

78

in response to operation of actuator

94

. Both actuators

93

and

94

are secured to holding bracket

79

and

80

respectively using standard fasteners

97

and

98

. Actuator

93

comprises an electric motor

101

coupled to a speed reducer transmission

102

. Similarly, actuator

94

comprises an electric motor

99

coupled to a speed reducer transmission

100

. By properly powering actuators

93

and

94

, rollers

77

and

78

are able to turn in both clockwise and counterclockwise directions.

Electric motors

99

and

101

employed in this embodiment are single phase motors which are powered by a 24 VDC power supply via two power cables

103

and

104

. Two speed reducer transmissions

100

and

102

both have a speed ratio of 36. The output torque of the transmission speed reducer at 180 RPM is 65 lbf-inch. Brakes

105

and

106

are installed on motors

99

and

101

respectively. Brakes

105

and

106

employed in this embodiment are powered by a 24 VDC and are connected to the electric motor shafts. When the brakes are not powered electrically, they are engaged preventing the motor shafts from turning. When the brakes are electrically powered, they are not engaged and the motor shafts are free to turn. Brakes

105

and

106

used in the first embodiment are manufactured by Inertia Dynamics and produce 3 lbf-inch of braking torque. As we will discuss later, motors

99

and

101

will be powered such that both rollers turn in opposite directions. When both motors are powered such that both rollers turn inwardly, the sack material will be drawn in between the rollers.

When sufficient sack material is grabbed in between rollers

77

and

78

, the grapple controller (described below) will stop the rollers and keep them stationary. Now if the rollers are locked or the motors are prevented from turning so zero angular speed is generated for the rollers, the sack will be secured in between the rollers and one can maneuver the sack by holding onto horizontal bracket

84

of the grapple. As long as the rollers are pushed tightly together by two springs

90

and

91

and the coefficient of friction between the sack material and rollers is sufficiently large, the sack will not slide out. When rollers

77

and

78

turn outwardly, the sack material which was grabbed by the rollers will come out and the sack will be released. Of course an alternative approach to release the sack is to separate rollers

77

and

78

from each other.

The control of the present invention can include a system of detectors or switches installed on the grapple to control its operation. The grapple of this invention (either

26

or

76

) has three primary operational phases:

(1) Grabbing: rotating the rollers inward;

(2) Holding: preventing the rollers from rotating in any direction; and

(3) Releasing: rotating the rollers outwardly.

Depending on the application and sequence of operation, the grapple (either

26

or

76

) can be forced into one of the three phases. The logic of how the grapple can be forced into a particular phase depends on how and where the grapple is being used. Users may need to consider many operational issues and safety concerns when developing a control system operating grapple

26

or

76

. The following is an exemplary scenario of how a control system for grapple

26

or

76

may be accomplished.

A logic signal, S

G

, is used to indicate the proximity of the grapple to a sack or an object to be grasped. A proximity detector is installed on the grapple and generates a signal (S

G

becomes 1) when the grapple is in close proximity of a sack or other object to be grasped. Rather than using a proximity detector to recognize the nearness of the sack to the grapple, one could use a electromechanical switch installed on the bottom of the grapple to issue a logic signal when the switch contacts the sack. Depending on the application, there are many ways of accomplishing a proximity detector that are known to and practicable in the present invention by one of ordinary skilled in the art. For example, the proximity detector can be an optical system or a vision system, for which control software can be used to identify the closeness of the sack to the grapple. In general, a logic signal (called S

G

) is necessary to declare the closeness of the grapple to a sack. In its simplest form (e.g. when this grapple is used with a manual material handling device) the S

G

signal can be issued by the operator at the push of a switch.

Another logic signal, S

H

, should be issued when sufficient sack material has been dragged in between the rollers. An electromechanical switch was installed in grapple

26

or

76

to send a signal (S

H

becomes 1) when sufficient sack material has been dragged in between the rollers.

Finally a third logic signal, S

R

, is needed to flag that the sack must be released. Note that in many applications one may not want to release the sack until the sack is completely put on the floor while in other situations one may desire to release the sack upon a command from a computer or from an operator. In the first embodiment of this invention, a momentary switch was used to issue a release signal by the operator. A momentary switch is a switch that shorts (or opens) a circuit as long as it is held pressed.

The table of

FIG. 5

illustrates the operational phases of the grapple for all possible combinations of the states of three signals S

G

, S

H

and S

R

. Note that there is only one combination of signals S

G

, S

H

and S

R

which forces the grapple into the “Grab” phase. This combination is shown in row 5 of the table where S

G

is “1” (the grapple is close to the sack); S

H

is “0” (the sack is not completely grabbed) and S

R

is “0” (no command is issued to release the sack). Also note that there are three possibilities (rows 1, 3 and 7) that force the grapple into the “Hold” phase. Row 1 indicates the operation of the grapple when it is being maneuvered without any sack in the grapple while the grapple is not close to any sack and no signal is issued for release of the sack. Row 3 and row 7 indicate that sufficient sack material has been grabbed, therefore the grapple should hold the sack regardless of the state of S

G

. The remaining combinations (rows 2, 4, 6, and 8) indicate the situation where the grapple is forced into the “Release” phase. The grapple is forced into the “Release” phase when S

R

is “1” regardless of the states of S

G

and S

H

.

It is straightforward to generate the “Grab” and “Release” phases of the grapple: “Grab” indicates inward rotation of rollers

77

and

78

(to draw sack material into the grapple) while “Release” indicates outward rotation of rollers

77

and

78

(to eject material from the grapple). However there are many methods of forcing the grapple into “Hold” phase (preventing the rollers from turning in either directions.) One method is to install brakes on the motor shafts, on the transmission shafts, or on the rollers themselves or on any rotating component connected to the rollers. Brakes

105

and

106

(in grapple

76

of

FIGS. 4A

,

4

B and

4

C), and brakes

55

and

56

(in grapple

26

of

FIGS. 2A and 2B

) were electrically powered, and engaged the motor rotating shaft and stopped the shaft when they were not electrically powered. These brakes employ a spring to push its brake pad onto the rotating shaft when the brake coil is not electrically powered. When voltage is imposed on the brake coil, then the brake will disengage allowing the shaft to rotate. When the grapple is in the “Hold” phase, the power will be disconnected both from the actuators and the brakes. This causes the brakes to engage and no power is given to the actuators. It is also recommended that the terminals of the electric motors to be shorted when the grapple is forced into “Hold” phase. This option adds more braking torque (regenerative brake) to the rollers. An alternative to regenerative brake is to develop a closed loop position controller for the motors that drive the rollers. When the system is forced into “Hold” phase, a position controller controls the angular position of the rollers at their current positions and prevents the rollers from rotating in either directions. This approach, although more effective than shortening the terminals of the motors, might be relatively costly since it requires installation of the position sensors and feedback circuitry. Another attractive method for preventing the rollers from turning in either direction is to use a ratchet mechanism to stop the rollers. The ratchet, when engaged, would allow for inward rotation of rollers

77

and

78

(

FIGS. 4A

,

4

B and

4

C) during the grab mode, however it would prevent outward rotation. To release the sack, the ratchet would need to be disengaged, using a solenoid or manually, from the rollers for outward rotation of the rollers. Other means of locking or preventing the rollers from rotating are known to and practicable in the present invention by the ordinary skilled artisan.

FIG.

6

A and

FIG. 6B

illustrate an exemplary control system practicable in the present invention to accomplish the operational phases described above and in the truth table of FIG.

5

. Depending on the application, the three logic signals S

G

, S

H

, and S

R

can be generated by a variety of devices individually or in combination. In the present exemplary embodiments, the S

G

signal is generated by an electronic proximity detector and the S

H

signal and S

R

signal are generated by electromechanical switches (see later figures). Three relays A, B, and C are used to achieve the operational phases described above and shown in the truth table of FIG.

5

.

FIG. 6A

is a schematic of how three signal sources (detectors/switches S

G

, S

H

and S

R

) were wired to power three relays A, B, and C for accomplishing the events and operational phases shown the table of FIG.

5

.

FIG. 6B

is a schematic of how relays A, B and C (all with two contacts) are interconnected to form a control system or operation of the present invention. In the present example, it is assumed that all of the electrical components can utilize the same power source (24 Volt DC in this case). The normally open contacts of relays A and B (A

1

, A

2

, B

1

, and B

2

shown in

FIG. 6B

) disconnect each motor's terminals from the voltage source. Relay C has two contacts: a normally closed contact C

1

shorts the electric motors terminals, and a normally open contact C

2

disconnects power from the brakes. When relay A and relay C are energized and relay B is de-energized, the terminals of the electric motors are connected to the power source via contacts A

1

and A

2

, the brakes are connected to the power source via contact C

2

(disengaging the friction pads) and the electric motors turn the rollers inward, and the “Grab” phase is accomplished. When the relays B and C are energized and relay A is de-energized, the terminals of the electric motors are connected to the power source via contacts B

1

and B

2

, and the brakes are connected to the power source via contact C

2

(engaging the friction pads) and the electric motors turn the rollers outwardly, and the “Release” phase is accomplished. When all relays A, B and C are de-energized, the terminals of both motors are shorted by contact C

1

, creating regenerative braking, and the brakes being disconnected from power source by contact C

2

causes the friction pads to engage, and the rollers are held stationary, and the “Hold” phase is accomplished.

The schematic of

FIG. 6A

exemplifies how the signal sources S

G

, S

H

and S

R

, may be interconnected to power three relays A, B and C. The relay circuit can be powered by any appropriate power source as are known to and selectable by the ordinary skilled artisan, although here a 24 VDC is illustrated. In fact it is recommended that both circuits of FIG.

6

A and

FIG. 6B

be powered by the same power. Signal source S

G

comprises a double pole proximity detector switch wired to have two normally-open contacts. Signal source S

H

is a double pole momentary switch and is wired to have two normally-closed contacts. Signal source S

R

is a triple pole momentary switch and wired to have two normally-open contacts and one normally-closed contact.

By inspection of FIG.

6

A and

FIG. 6B

, one can see that the grapple of this invention can be forced into any of the operational phases shown in the table of

FIG. 5

depending on the combined signal conditions of detectors and switches S

G

, S

H

and S

R

. For example, if detector/signal source S

R

is activated, regardless of signaling states of detectors S

G

and S

H

, both relays B and C are powered and A is unpowered, and the “Release” phase is accomplished (see Row 2, 4, 6 and 8 of the truth table). If signal source S

G

is activated, while signal sources S

R

and S

H

are not activated, then both relays A and C are powered while relay B is unpowered, and the “Grab” phase is accomplished (see Row

5

). Finally, if signal source S

H

is activated, regardless of the signaling state of detector S

G

but as long as signaling source S

R

is not activated, then all relays, A, B and C, are unpowered, and the “Hold” phase is accomplished (see rows 1, 3, and 7).

FIG.

7

A and

FIG. 7B

illustrate one possible means for accomplishing a proximity detector to generate a S

G

signal. An electronic proximity detector

107

was used to detect the nearness of grapple

26

to an object to be grabbed or gripped. The proximity detector is disposed on the grapple to issue a S

G

signal when the object is detected within a predetermined distance. In

FIGS. 7A and 7B

, proximity detector

107

is installed on a detector mounting plate

108

via fasteners

109

, and detector plate

108

is rigidly connected to mounting bracket

35

via fasteners

110

. Detector plate

108

is configured to have an appropriate angle, so that the detector beam aims at the object or sack. The proximity detector issues a S

G

signal (S

G

becomes 1) when the grapple is close to the object. The proximity detector used in this embodiment was manufactured by Banner and sent a 5 VDC signal when an object is within 12 inches of the grapple. Alternatively, an electromechanical switch can be used as the proximity detector to generate the S

G

signal. Such electromechanical switch as a proximity detector can be activated either by the operator or by its contact with the object.

A similar arrangement can be used for installation of a switch on grapple

76

of

FIGS. 4A and 4B

(second embodiment of the invention) to issue a S

G

Signal. As shown in

FIGS. 8A and 8B

, an electronic proximity detector

111

was used to detect the distance from grapple

76

to an object to be grabbed. The proximity detector

111

is installed on a detector mounting plate

112

using fasteners

113

and detector mounting plate

112

is rigidly connected to mounting bracket

85

via fasteners

114

. The detector mounting plate

112

is configured at an appropriate angle so that the detector beam aims at the object. Similar to grapple

26

shown in

FIGS. 7A and 7B

, proximity detector

111

issues a S

G

signal (S

G

becomes 1) when grapple

76

is close to a sack (e.g. 12 inch). There are other methods of generating a grab signal. One, for example, can install an electro-mechanical switch on the grapple to generate a grab signal. The eletcro-mechanical switch issues a signal (S

G

becomes 1) when it contacts the sack.

FIG.

9

A and

FIG. 9B

show one possible configuration for installation of a switch to issue the S

H

signal for the second embodiment of this invention. These figures show perspective views of grapple

76

where holding brackets

79

and

81

are removed for clearer illustration of a switch that issues a S

H

signal. The detector assembly comprises a momentary wedge detector switch

116

, installed on a first angle bracket

117

, is disposed so it is activated when a sufficient amount of sack material has passed between rollers

77

and

78

. The first angle bracket

117

is secured to a swivel plate

118

via fasteners

119

. Swivel plate

118

is rigidly connected to a swivel shaft

120

via fasteners

121

. Swivel shaft

120

is rotatably supported by holding brackets

79

(not shown) and

80

, and is free to rotate around its own axis (i.e. in the direction of the arrow

122

). A second angle bracket

123

is secured to a third angle bracket

124

via a fastener

125

. A slot

127

on second angle bracket

123

allows for adjustment of the position of second angle bracket

123

relative to third angle bracket

124

. The adjustment is necessary to assure that an appropriate degree of rotation of the swivel plate

118

along arrow

122

corresponds to a sufficient wedge of sack material, and causes wedge detector switch

116

to be pressed against second angle bracket

123

. Third angle bracket

124

is secured to holding bracket

80

via fasteners

128

. This arrangement allows second angle bracket

123

to be rigidly connected to holding bracket

80

.

FIG. 10A

shows grapple

76

where swivel plate

118

is in its neutral position and wedge detector switch

116

is not activated.

FIG. 10B

shows grapple

76

when swivel plate

118

has turned in a clockwise direction due to the force from wedge

115

of sack material

17

, and wedge detector switch

116

is pressed against second angle bracket

123

. The more second angle bracket

123

is moved toward the right hand side along slot

127

, the more swivel plate

118

must turn (in a clockwise direction) to press wedge detector switch

116

against second angle bracket

123

. The S

H

signal generated by wedge detector switch

116

is transmitted via signal cable

126

to the rest of the control system. The method described above uses wedge

115

of sack material

17

to activate wedge detector switch

116

and cause a S

H

signal to issue. Alternatively, the wedge detector can be a displacement switch installed on grapple

76

that can be activated by displacement of rollers

77

and

78

apart. In other words, once rollers

77

and

78

are separated a predetermined distance due to a sufficient thickness of sack material

17

drawn between them, then the displacement switch (not shown) is activated and a S

H

signal issued. To issue a S

H

signal in grapple

26

, a similar arrangement can be practiced.

S

R

signal must be issued to release the sack. Depending on the application, there are many ways of creating this logic signal upon a command from a computer or from an operator. The grapples described in this invention can be used with a variety of material handling devices.

FIG. 11

shows grapple

76

when used in conjunction with a hoist

136

. Such hoist devices are used often on auto assembly lines, in warehouses and similar situations to manipulate loads. Grapple

76

is connected to a line

135

. The grapple is equipped with a handle which is gripped by the human operator's hand

139

and contains a sensor. Using the measurement from the sensor, the hoists lift grapple

76

. U.S. Pat. Nos. 5,915,673 and 5,865,426 describe some manual material handling devices and hoists that can be used with the present invention. As shown in

FIG. 11

, a cord

137

brings electric power to operate the grapple's electrical components. FIG.

12

A and

FIG. 12B

are detailed views of grapple

76

when equipped to be used with a hoist

136

. An eyelet

134

is used to hang grapple

76

from a line or cable

135

. Handles

138

and

141

are installed on grapple

76

to be held by the operator's hands and include operator input devices that cause the vertical movement of grapple

76

. A momentary switch

140

is installed on handle

141

. When momentary switch

140

is pressed by the operator's left hand

142

, a S

R

signal will be issued and the sack will be released.

FIGS. 13A and 13B

show similar arrangement when grapple

26

of

FIGS. 2A and 2B

is equipped to be used with a hoist

136

. A momentary switch

140

is installed on handle

141

, which when pressed by the operator, a S

R

signal is issued and the sack is released. An enclosure

143

houses the electronics and control system of the present invention.

FIG. 14

shows grapple

76

used with a robot

144

. The L shaped connecting bracket

145

is used to connect grapple

76

to robot

144

. As can be seen there is no manual switch in this case to issue the S

R

signal. The robot operating control system issues a S

R

signal when the sack reaches its target location.

A detailed description of the some of the important design issues associated with the grapple is given below. One important design issue is the calculation of the required torque during the “Hold” phase (i.e. when the sack material is dragged in between the rollers and the rollers have stopped turning).

FIG. 15

shows that when sack material

17

is held between rollers

27

and

28

and lifted, the total upward friction forces imposed on sack

17

by rollers

27

and

28

is calculated by equation (1):

Upward Friction Forces=2

μN

H

(1)

Where N

H

is the normal force imposed by rollers

27

and

28

onto sack material

17

during the “Hold” phase, and μ is the coefficient of friction between the rollers and sack material

17

. To prevent the sack from sliding out of the grapple, the upward friction forces (described in equation 1) must be larger than the total of the maximum weight and the inertial force due to the maximum upward acceleration of the grapple as shown by inequality (2):

\begin{matrix} 2 μ N_{H} \geq W_{\max} (1 + \frac{α}{g}) & (2) \end{matrix}

where g is the gravitational acceleration, W

max

is the weight of the heaviest sack to be lifted, and α is the maximum upward acceleration of the grapple induced by the robot or by the material handling device. If inequality (2) is not satisfied, sack material

17

will slide out of rollers

27

and

28

. Therefore one must design the grapple with a large N

H

and large p to guarantee that the heaviest sack that must be lifted by a grapple cannot slide out. Inspection of

FIG. 15

shows that the required torque to keep roller

27

stationary during the “Hold” phase is

T

H

27

=μN

H

R

27

(3)

where R

27

is the radius of roller

27

and T

H

27

is the holding torque that should be imposed on roller

27

during the “Hold” phase. Comparing inequality (2) with equation (3) results in inequality (4) for the required holding torque on roller

27

during the “Hold” phase.

\begin{matrix} T_{H}^{27} \geq (1 + \frac{α}{g}) R_{27} \frac{W_{\max}}{2} & (4) \end{matrix}

Similarly the holding torque on roller

28

can be calculated from inequality (5):

\begin{matrix} T_{H}^{28} \geq (1 + \frac{α}{g}) R_{28} \frac{W_{\max}}{2} & (5) \end{matrix}

If the heaviest sack to be lifted by a particular grapple is 70 pounds, and the maximum maneuvering acceleration is 0.3 g, then if the rollers radius is 0.7″, according to inequalities (4) and (5), one must impose at least 31.85 lbf-inch torque on each roller during the “Hold” phase.

Two electric brakes

55

and

56

are used in grapple

26

of

FIGS. 2A and 2B

to perform the “Hold” process. One must guarantee that brakes

55

and

56

generate sufficient holding torque on rollers

27

and

28

during the “Hold” phase. If the ratio of the angular speed of input shaft to the angular speed of output shaft of speed reducer transmission

49

is N, then the required braking torque for brake

55

can be calculated from inequality (6).

\begin{matrix} T_{B}^{55} = \frac{T_{H}^{27}}{N} \geq (1 + \frac{α}{g}) R_{27} \frac{W_{\max}}{2 N} & (6) \end{matrix}

where T

B

55

is the minimum required torque for brake

55

. A similar inequality can also be calculated for the minimum required torque for brake

56

. Inequality (6) can also be used to size brakes

105

and

106

of grapple

76

in

FIGS. 4A

,

4

B and

4

C. It is preferred to choose a brake with excess braking torque capacity to compensate for inefficiencies and uncertainties in various components of the grapple.

In embodiments exemplified in the figures, a normally engaged electric brake manufactured by Inertia Dynamics was used. Normally engaged brake means that the brake does not allow rotation of the motor shaft when the brake is not electrically powered. Note that the holding torque of a brake is a function of the stiffness of the spring that is installed in the brake. The stiffer the brake spring, the more holding torque can be generated. Although more holding torque during the “Hold” phase assures that heavier sacks can be lifted, one must consider a trade-off: a brake with a stiff brake spring, and consequently large holding torque, requires a large amount of electrical energy to disengage. Designers must make sure that there is sufficient energy available in the electric power source that supplies the brakes. Usually the required electric current to disengage a brake at a given voltage is given by brake manufacturers and here it is referred to as I

B

. The holding torque for the brake used in the first embodiment (grapple

26

in FIGS.

2

A and

2

B), when the brake is not energized electrically, is 3 lbf-inch. Since the transmission ratio is 36, the holding torque on each roller will be 108 lbf-inch. This is about 2.8 times larger than the required holding torque calculated by inequality (4) or (5). The required electric current to disengage the brake is 0.19 Amp at 24 VDC.

As discussed earlier, rather than using a brake, one can use other mechanisms (e.g. ratchet) to lock the rollers during the “Hold” phase. In design of any locking systems such as locking ratchets, one must guarantee that the required torque on each roller

27

and

28

during the “Hold” phase, given by inequalities (4) and (5), can be generated by the locking system.

Note that speed reducer transmissions

49

and

50

(in grapple

26

of

FIGS. 2A and 2B

) will not be back-drivable if they have large speed reduction ratios. This helps the grapple device during the “Hold” phase since the rollers will not spin outwardly by the force of sack weight and therefore sack material will not be released. In general the use of speed reducers that are non-back drivable (such as worm gears) may eliminate the need for brakes in the grapple device. Although worm gears and other non-back drivable speed reducers may be used in the grapples of the invention here, the use of brakes and latches is recommend to insure that the rollers are not spawn by sack weight and therefore sack is held securely between the rollers and is not released during the “Hold” phase.

The calculation of the torque required during the “Grab” phase needs more understanding of the “Grab” process. Three methods for calculating the “Grab” torque for three different scenarios are described below:

Method 1

Usually during the “Grab” phase, the sack is rested on a floor or other surface.

FIG. 16

shows a roller

27

in its initial engagement with sack material

17

. The normal vertical force between roller

27

and sack material

17

is N

G

. N

G

is the function of the normal vertical force being imposed on the grapple and the weight of the grapple. For example, if the grapple is connected to a manual material handling device (as shown in FIG.

11

), N

G

is the function of the operator force on the manual material handing device. The more the operator pushes on the grapple, the greater the normal vertical force, N

G

, will be generated. The friction forces onto the sack from each roller, μN

G

, should be larger than the tension force, T

S

, of sack material

17

.

μ

N

G

≧T

S

(7)

The rollers of the grapple might not be able to properly engage with the sack material if the grapple is not pushed downward with sufficient force and if the coefficient of friction between the sack and the roller is small. To initiate the “Grab” phase successfully, therefore, both μ and N

G

should be sufficiently large to satisfy inequality (7). The torque needed to be imposed on roller

27

during the “Grab” phase is:

T

G

27

=N

G

μR

27

(8)

Considering inequality (7), the torque needed to be imposed on each roller

27

and

28

during the “Grab” phase are:

T

G

27

≧T

S

R

27

(9)

T

G

28

≧T

S

R

28

(10)

If both inequalities (9) and (10) are satisfied during the “Grab” phase, then the grabbing process will start successfully and sufficient sack material will be drawn between rollers

27

and

28

. Over-stuffed sacks can result in a large tensile force and therefore it can be difficult to start the “Grab” process.

Method 2

As shown in

FIG. 17

, after enough sack material

17

is collected between the rollers, the pressure built up in between rollers

27

and

28

pushes them apart from one another as sack material

17

is squeezed between them. Suppose the pressure between sack material

17

and the roller per unit length of the roller's perimeter (circumference) is P, and that both rollers

27

and

28

have equal diameters, then equation (11) represents the force balance for roller

27

along the horizontal direction.

\begin{matrix} R_{27} \int_{0}^{\frac{π}{2}} (p Sin (θ) + p μCos (θ)) ⅆ θ = N_{H}^{27} & (11) \end{matrix}

where N

H

27

the horizontal force on roller

27

due to the force of bias springs (

40

and

41

in FIGS.

2

A and

2

B). It is rather difficult to know the exact shape of the pressure profile on rollers

27

and

28

, but since sack material

17

is compliant, sack material

17

will move between rollers

27

and

28

so an almost uniform pressure is created on the rollers. Substituting a constant value for P into equation (11) results in equation (12) for force N

H

27

:

\begin{matrix} R_{27} P_{o} \int_{0}^{\frac{π}{2}} (Sin (θ) + μCos (θ)) ⅆ θ = N_{H}^{27} & (12) \end{matrix}

Or:

p

o

R

27

(1+μ)=

N

H

27

(13)

where P

o

is the constant pressure on the rollers. The torque that turns the rollers should be sufficiently large to overcome the friction forces due to the pressure on the rollers.

FIG. 17

shows that the torque on roller

27

during the “Grab” phase, T

G

27

, should be larger than the torque imposed on roller

27

by the friction forces:

\begin{matrix} T_{G}^{27} \geq \int_{0}^{\frac{π}{2}} p R_{27}^{2} μ ⅆ θ & (14) \end{matrix}

Substituting the constant value of P

o

for P in inequality (14) results in inequality (15) for the torque on roller

27

during the “Grab” phase.

\begin{matrix} T_{G}^{27} \geq p_{o} R_{27}^{2} μπ \frac{1}{2} & (15) \end{matrix}

Substituting for P

∘

from equation (13) into inequality (15) results in a relationship between the force N

H

27

and the “Grab” torque T

G

27

.

\begin{matrix} T_{G}^{27} \geq \frac{μπ}{2 (1 + μ)} N_{H}^{27} R_{27} & (16) \end{matrix}

Inequality (16) shows that the grab torque, T

G

27

, on roller

27

is proportional to the normal force generated by springs

40

and

41

. Inequality (16) also shows that the larger the force between the rollers due to bias springs, the larger torque is needed from each motor and transmission. If electric motor

48

and the transmission

50

cannot provide the torque represented by inequality (16), then roller

27

will be stalled.

Method 3

During high-speed operations, it is possible for the grapple to be moved upwardly by the robot before the “Grab” phase is completed. In other words, before the grapple is in the “Hold” phase, the grapple is moved upwardly by a robot or by a material handling device. In situations of this nature, to prevent the sack from falling, electric motors

47

and

48

and speed reducers transmissions

49

and

50

(

FIGS. 2A and 2B

) should generate enough torque on rollers

27

and

28

to assure that the rollers turn inwardly and draw enough sack material

17

between the rollers so the grapple goes into the “Hold” phase. This means that the required torque to be imposed on roller

27

during the “Grab” phase should be equal to or larger than the “Hold” torque from inequality (4):

\begin{matrix} T_{G}^{27} \geq W_{\max} (1 + \frac{α}{g}) \frac{R_{27}}{2} & (17) \end{matrix}

Similarly the required torque to impose on the other roller

28

during the “Grab” phase can be calculated:

\begin{matrix} T_{G}^{28} \geq W_{\max} (1 + \frac{α}{g}) \frac{R_{28}}{2} & (18) \end{matrix}

Of course inequality (2) must also be satisfied. Three inequalities (9), (16) and (17) offer three values for the grab torque on roller

27

. An actuator and transmission must be selected such that the steady state output torque is larger than the largest torque value generated by inequalities (9), (16) and (17). The largest value for T

S

, the tension force in sack material

17

, occurs when the sack is lifted. As T

S

gets larger, inequality (9) approaches inequality (17). In other words, inequality (17) yields a larger value for grab torque than inequality (9). Also note that inequality (16) usually results in a smaller value for the grab torque than inequality (17). It is preferred to choose an actuator and transmission such that their torque capability is more than what inequalities (17) and (18) prescribe. In other words, the torque capability during the “Grab” phase should be the same as the torque capability during the “Hold” phase. Although inequalities (17) and (18) prescribe a conservative value for the grab torque, it demonstrates more assurance for a successful grasp during the “Grab” phase. In applications where the sacks are not totally filled and are not lifted before the “Grab” phase is completed, one might use a smaller actuator to guarantee inequalities (9) and (10) only. In the first embodiment of this invention, an actuator and a transmission speed reducer that has 65 lbf-in steady state output torque were used to drive each roller. It is preferred that users choose a DC motor with excess torque capacity to compensate for inefficiencies and uncertainties in various components of the grapple. Of course the actuator and the transmission must be able to provide more torque, for a short time, to accommodate for the transient inertial torque due to acceleration of rotating elements of the grapple and friction in moving components of the grapple.

Through many experiments, it was observed that rollers with radii 0.7″ should turn with the speed of about three revolution/second for optimal operation. Small angular speeds for the rollers yield a slow grabbing process, while high speed rotation for the rollers may not allow the rollers to engage and grab the sack material. If the angular speed of roller

27

is ω revolution/seconds, the required power during the “Grab” is:

\begin{matrix} Power = \frac{T_{G}^{27} ω}{1050} HP & (19) \\ Power = \frac{T_{G}^{27} ω}{1.4} Watt & (20) \end{matrix}

where the unit of T

G

27

is lbf-inch and ω is in revolution/second. Substituting for T

G

27

(i.e. 31.85 lbf-inch) and ω (i.e. 3 rev/sec) into equations (19) and (20) results in 0.091 HP or 68.25 Watts for the required power for the electric motor at 3 revolution/second (180 RPM). The above analysis also yields a value for the required current if an electric DC motor is used to impose torque on roller

27

. If a DC power supply, with the voltage V, is used to power motor

48

, then the current required by motor

48

is calculated by inequality (21).

\begin{matrix} I_{M} \geq \frac{T_{G}^{27} ω}{1.4 V} Amp & (21) \end{matrix}

If a 24 VDC power supply is used to power the motor, then the current drawn by motor

48

is calculated by inequality (22).

\begin{matrix} I_{M} \geq \frac{31.85 \times 3}{1.4 \times 24} = 2.84 Amp & (22) \end{matrix}

In the first embodiment, both the actuators and the brakes are powered with the same power supply to reduce the overall system cost. In that case inequality (23):

\begin{matrix} I_{T} \geq \frac{T_{G}^{27} ω}{1.4 V} + I_{B} Amp & (23) \end{matrix}

represents the total required current from the power supply to power both motor

48

and brake

56

, where I

B

is the electric current required to disengage brake

56

. Similar calculations can be developed to determine the current required to power motor

47

and brake

55

.

The design issue associated with the friction between the rollers and the sack material is described below. A large coefficient of friction between rollers and the sack material can be achieved in many ways. One can knurl the rollers. Knurling is a process of putting a grip or roughened surface on a roller. The die which is engraved with a female impression of the design is pressed against the roller and sufficient pressure is applied to produce desired depth of impression. Knurl rollers are attractive, but they might damage the sacks. Another method of creating friction is to wrap the rollers with a rubber or rubber-like material that has a large coefficient of friction. However rubber with a large coefficient of friction usually wears off soon because it is soft. Inspection of inequality (2) shows that large values for μ and for N

H

allow the grapple to lift heavy sacks. However there is a trade-off and one cannot arbitrarily design a grapple with a large normal force, N

H

, and a large and a large coefficient of friction, μ. As can be seen from inequality (16), large values for N

H

and μ require high torque actuators. In other words, one should not arbitrarily choose a stiff spring to generate a large N

H

; if large N

H

and μ are chosen to guarantee inequality (2), then a large actuator should also be chosen to overcome friction forces between the rollers as prescribed by inequality (16). Stiff springs create large normal force N

H

between the rollers and the sack material. A soft rubber surface on rollers

27

and

28

creates a large coefficient of friction between the rollers and the sack material. Practitioners must arrive at a value for the spring stiffness and rubber coefficient of friction so inequality (2) is satisfied with a reasonable margin. Over designed systems (i.e., very a large μ and N

H

) will lead to an unnecessary large actuator and power supply. On the other hand, if the bias springs (

40

and

41

in

FIGS. 2A and 2B

) are not stiff enough to generate a sufficiently large N

H

to satisfy inequality (2), the rollers will not be pushed against or oppose each other sufficiently, and the sack will slide down. Also note that rubber material with a large coefficient of friction wears off quickly and has a short life. We suggest that the grapple be designed with replaceable rollers. Once an optimal material (good coefficient of friction while the rubber has a long life) is chosen for the rubber on the rollers, one must choose a spring with proper stiffness for the grapple to yield an appropriate normal force to satisfy inequality (2). In general, a large coefficient of friction for rubber requires softer springs, and a small coefficient of friction requires stiffer springs. Low durometer Neoprene with μ=1 was used for the first embodiment. The pre-load of the spring is adjusted to yield 50 lbf between the rollers to satisfy inequality (2). If the heaviest sack to be lifted by a particular grapple is 70 pounds and the maximum maneuvering acceleration is 0.3 g then inequality (2) will be satisfied as shown below:

\begin{matrix} 2 \times 1 \times 50 \geq 70 (1 + \frac{0.3 g}{g}) & (24) \end{matrix}

or:

100≧91 (25)

Alternative Embodiments

1. Grapple with Two Rollers Where Only One Roller is Powered.

In this embodiment, an actuator powers only one roller. The actuator can be connected either directly to one of the rollers or indirectly via chains, belts or gears. Although rather simple and low cost, this configuration may lead to some slipping between the rollers, and between the rollers and the sack. Also note that this configuration can work with rollers having unequal diameters.

FIG. 18

shows a grapple

149

where roller

150

is powered by an actuator

151

. Two sprockets

152

and

153

and a chain

154

are used to transfer power to roller

150

. As can be seen, the other roller

155

is not powered, but it turns in direction opposite to the direction of roller

150

since it is forced against roller

150

by use of a spring

15

. Similar to the device depicted in

FIGS. 2A and 2B

, one can install brakes on grapple

149

to prevent the rotation of rollers

150

and

155

when the sack needs to be held securely. Alternatively, a ratchet mechanism can be employed to stop the rollers rotation when the sack needs to be held securely between the rollers. To release the sack one can either separate the rollers from each other or power roller

150

counterclockwise.

2. Grapple with Two Rollers Where the Rollers Move Linearly Relative to One Another

In all above described embodiments, the rollers separate from each other or get close to each other on a circular path. However it is possible to design a grapple in which the rollers move relative to each other on a linear path.

FIG. 19

is a schematic of a grapple

160

wherein rollers

161

and

162

are powered by actuators

163

and

164

. Ball bearings or roller bearings

165

,

166

,

167

and

168

support rollers

161

and

162

while allowing for their rotation. Two parallel linear bearings

169

and

170

allow for linear motion of the housing of bearings

165

,

166

,

167

and

168

. In other words both rollers can spin while they can move linearly relative to each other on two parallel linear bearings

169

and

170

. Arrow

171

shows the direction of the movement of the rollers relative to each other. Tensile springs

172

and

173

, connecting bearing

166

to bearing

165

and bearing

168

to bearing

167

, pull rollers

161

and

162

toward each other. Again in this configuration, the rollers can be either powered directly or indirectly via gears, chains and belts by electric, pneumatic or hydraulic motors. Similar to the device shown in

FIG. 18

, one can power one roller only.

FIG. 20

shows how grapple

175

is powered using flexible shafts. In this embodiment of grapple

175

, one actuator

176

powers both rollers

161

and

162

. The rotary motion from actuator

176

is transmitted to flexible shafts

177

and

178

via two mating gears

179

and

180

, causing flexible shafts

177

and

178

to rotate in opposite directions.

3. Multi Roller Grapple

FIGS. 21A and 21B

are schematic representations of another embodiment of the present invention, wherein three rollers

191

,

192

and

193

are employed to grab sack material and hold sacks. The rollers of this grapple

190

are not parallel to each other and each roller axis is at a 60 degree angle with the others.

FIGS. 21A and 21B

show the case where the rollers are powered independently by three actuators

194

,

195

and

196

. The design principle for this architecture is similar to the system with two rollers in the sense that three rollers can separate from each other and allow the sack material to be grabbed in between the rollers.

FIG. 21B

shows the situation where roller

192

has been separated from the other two rollers to allow the sack materials to be dragged in between the rollers. Depending on the sack material and how the sack is left on the floor, it is possible that all rollers separate to hold the sack material in between them. Similar to the previous cases, to release the sack, one can either turn the rollers outwardly or separate them from one another.

4. Use of Non-Circular Rollers

Circular rollers were utilized throughout our experimental designs. It is also possible to use rollers having an oval shaped cross-section.

FIGS. 22A

,

22

B and

22

C are schematic representations of a grapple

199

in which two oval-shape rollers

197

and

198

are employed to grab and hold sack material

17

.

FIG. 22A

shows the initial configuration of the grapple where rollers

197

and

198

are initiating the grab process.

FIG. 22B

shows an intermediate position where the oval-shape rollers are forced toward each other on their smaller profiles creating a large normal force between the rollers.

FIG. 22C

shows the final configuration where rollers

197

and

198

have rotated and sufficient sack material

17

has been collected in between them. As shown in

FIG. 22C

, sack material

17

is locked in between the oval-shape rollers

197

and

198

. Similar to grapples described previously, a spring

15

, connected to holding brackets

18

and

19

, is employed to bias oval-shape rollers

197

and

198

toward each other. One could use either one or two actuators to power rollers

197

and

198

of grapple

199

.

One might use rollers with non-smooth surfaces.

FIG. 23

shows a configuration of a grapple

182

where two rollers

183

and

184

have mating lobes. The increased surface contact between the lobes and sack material

17

will lead to larger friction forces for grasping and holding sacks. A spring

15

, connected to holding brackets

18

and

19

, is employed to force rollers

183

and

184

toward each other. Furthermore, one could use either one or two actuators to power rollers

183

and

184

of grapple

182

.

Although particular embodiments of the invention are illustrated in the accompanying drawings and described in the foregoing detailed description, it is understood that the invention is not limited to the embodiments disclosed, but is intended to embrace any alternatives, equivalents, modifications and/or arrangements of elements falling within the scope of the invention as defined by the following claims. For example, while many of the embodiments described above are for lifting a sack, these embodiments can also be used for lifting other types of loads that have components that can be squeezed in between the rollers. For example one can use grapple

76

(or grapple

26

) to grab letter bins

146

as shown in FIG.

24

. In general boxes without any top cover can be grabbed by this grapple easily where the grapple rollers can grab vertical edge of the box. Also note that one can use unequal diameters for rollers. However, the rollers' speed should be controlled such that the linear speeds of the rollers at the point of contact between the rollers remain equal. In all embodiments described here brakes were used on the motors to prevent the rotation of the rollers when the sack needs to be held securely. Alternatively, a ratchet mechanism can be used to stop the rollers' rotation in order to secure the sack. Additionally, to prevent the rotation of the rollers when the sack needs to be held securely, one might want to use non-back drivable actuators to power the rollers. To release the sack one can either separate the rollers from each other or power the rollers to turn outwardly. The following claims are intended to cover all such modifications and alternatives.

While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof. Many other variation are possible, which would be obvious to one skilled in the art. Accordingly, the scope of the invention should be determined by the scope of the appended claims and their equivalents, and not just by the embodiments.

Claims

1. A device mountable on a transport mechanism for contacting, grasping, holding and releasing an object having a deformable surface, comprising:a mounting bracket mountable to said transport mechanism; a first and a second roller, each roller having a roller axis, and each being rotatably supported on said mounting bracket and rotatable about its axis; further each of said rollers having a roller surface being a dragging surface for engaging and drawing said deformable surface of said object; a drive means for rotating at least one of the said rollers; a holding means selectively operable to releaseably engage at least one of the said rollers, and prevent rotation of the roller when engaged; wherein when said holding means is not engaged, said drive means turns said first roller along a first direction, and said roller surface of said first roller is placed into contact with said deformable surface of said object, said first roller surface draws said deformable surface of said object into an inter-roller region between said first and second rollers, thereby grasping said object; and wherein when said holding means is engaged and said deformable surface of said object is located in said inter-roller region, the contact between said deformable surface of said object and said first and second rollers causes said object to be held by said device.
2. The device of claim 1 wherein the holding means for preventing the rotation of said rollers is a friction brake.
3. The device of claim 1 wherein the holding means for preventing the rotation of said rollers is a ratchet mechanism.
4. The device of claim 1, further comprising a releasing means operatively connected to at least one of the said rollers and selectively operable to allow said deformable surface of said object to pass out of said inter-roller region causing said object to be released.
5. The device of claim 4, wherein said releasing means comprises said drive means being selectively operable to rotate said first roller in a second direction to draw said deformable surface of the said object out of said inter-roller region, causing said object to be released.
6. The device of claim 4, wherein said releasing means comprises a roller separator operatively connected to at least one of the said rollers and selectively operable to move said rollers sufficiently apart to allow said deformable surface of said object to pass out of said inter-roller region, causing said object to be released.
7. The device of claim 1 where said axes of said first roller and said second roller are substantially parallel to each other.
8. The device of claim 1 further including a biasing means to bias said rollers toward each other with sufficient force to facilitate holding the deformable surface of said object in place when said deformable surface is between said rollers.
9. The device of claim 1 wherein said rollers are forced toward each other by force of a spring.
10. The device of claim 1 wherein said drive means is an electrically powered actuator.
11. The device of claim 1 wherein said drive means is an air-powered actuator.
12. The device of claim 1 wherein said first and second rollers are of cylindrical shape.
13. The device of claim 1 wherein said rollers are covered by rubber.
14. The device of claim 1 wherein said rollers are knurled.
15. The device of claim 1 wherein said object with said deformable surface is a sack that contains an interior object.
16. The device of claim 1, further comprising a controller which controls said device three operational phases: grab, hold, and release.
17. The device of claim 1, further comprising a proximity signal generating element that generates a proximity signal when the device is in close proximity of an object.
18. The device of claim 17, wherein said proximity signal generating element is a proximity switch, which issues said proximity signal when the device is close to the object.
19. The device of claim 17, wherein said proximity signal generating element is an electromechanical switch, which issues said proximity signal when said electromechanical switch contacts the object.
20. The device of claim 17, further comprising:a holding signal generating element that generates a holding signal when sufficient portion of said deformable surface of said object is collected between said rollers; and a releasing signal generating element that generates a releasing signal to release said object, wherein said drive means turns said rollers inwardly in response to said proximity signal when said holding signal generating element has not generated any holding signal and said releasing signal generating element has not enerated any releasing signal.
21. The device of claim 1, further comprising a holding signal generating element that generates a holding signal when sufficient portion of said deformable surface of the object is collected between said rollers.
22. The device of claim 21, wherein said holding signal generating element is an electromechanical switch.
23. The device of claim 22, where said deformable surface of the object collected between said rollers causes said electromechanical switch to issue said holding signal.
24. The device of claim 1, further comprising a releasing signal generating element that generates a releasing signal to release said object.
25. The device of claim 24, wherein said releasing signal generating element is a momentary switch pressed by an operator.
26. The device of claim 24, wherein said releasing signal generating element generates a releasing signal to release said object based on a command from a computer.
27. The device of claim 24, wherein a releasing means causes said deformable surface of said object to pass out of said inter-roller region causing said object to be released in response to said releasing signal.
28. The device of claim 24 further comprising a holding signal generating element that generates a holding signal when sufficient portion of said deformable surface of said object is collected between said rollers, wherein if said holding signal generating element generates a holding signal, as long as said releasing signal generating element has not generated any releasing signal, said holding means is engaged.
29. A device mountable on a transport mechanism for contacting, grasping and holding an object having a deformable surface, comprising:a mounting bracket mountable to said transport mechanism; a first and a second holding brackets, movable relative to each other and held by said mounting bracket wherein said first holding bracket is fixedly attached to said mounting bracket and said second holding bracket is pivotably attached to said mounting bracket and moves relative to said first holding bracket; a first roller, rotatable about a first roller axis and rotatably supported on said first holding bracket, said first roller further including a dragging surface for engaging and drawing said deformable surface when contacting said deformable surface of said object; a second roller, rotatable about a second roller axis and rotatably supported on said second holding bracket, said second roller further including a dragging surface for engaging and drawing said deformable surface when contacting said deformable surface of said object; and a drive means for rotating at least one of the said rollers; wherein when said first roller turns along a first direction, and said roller surface of said first roller is placed into contact with said deformable surface of said object, said first roller surface draws said deformable surface of said object into an inter-roller region between said first and second rollers, thereby grasping said object.
30. The device of claim 29 further including a biasing means to bias said holding brackets toward each other with sufficient force to facilitate holding the deformable surface of said object in place when said deformable surface is between said rollers.
31. The device of claim 29 wherein said holding brackets are forced toward each other by force of at least one spring.
32. The device of claim 29, further comprising a holding means selectively operable to releaseably engage at least one of the said rollers, and prevent rotation of the roller when engaged, so that when said deformable surface of said object is located in said inter-roller region and said holding means is engaged, the contact between said deformable surface of said object and said first and second rollers causes said object to be held by said device.
33. The device of claim 32 wherein the holding means for preventing the rotation of said rollers is a friction brake.
34. The device of claim 32 wherein the holding means for preventing the rotation of said rollers is a ratchet mechanism.
35. The device of claim 29, further comprising a releasing means operatively connected to at least one of the said rollers and selectively operable to allow said deformable surface of said object to pass out of said inter-roller region causing said object to be released.
36. The device of claim 35, wherein said releasing means comprises said drive means being selectively operable to rotate said first roller in a second direction to draw said deformable surface of the said object out of said inter-roller region, causing said object to be released.
37. The device of claim 35, wherein said releasing means comprises a roller separator operatively connected to at least one of the said rollers and selectively operable to move said rollers sufficiently apart to allow said deformable surface of said object to pass out of said inter-roller region, causing said object to be released.
38. The device of claim 29 where said axes of said first roller and said second roller are substantially parallel to each other.
39. The device of claim 29 further including a biasing means to bias said rollers toward each other with sufficient force to facilitate holding the deformable surface of said object in place when said deformable surface is between said rollers.
40. The device of claim 29 wherein said rollers are forced toward each other by force of a spring.
41. The device of claim 29 wherein said drive means is an electrically powered actuator.
42. The device of claim 29 wherein said drive means is an air-powered actuator.
43. The device of claim 29 wherein said first and second rollers are of cylindrical shape.
44. The device of claim 29 wherein said rollers are covered by rubber.
45. The device of claim 29 wherein said rollers are knurled.
46. The device of claim 29 wherein said object with said deformable surface is a sack that contains an interior object.
47. A method of manipulating an object;mounting a grapple device to a transport mechanism, wherein said grapple device comprises (i) a supporting bracket assembly (ii) a first roller, having a roller axis, said roller rotatably mounted on said supporting bracket assembly, and said first roller being rotatable about its axis, wherein said first roller further comprises a gripping surface for engaging the graspable portion of the object, (iii) a second roller, having a roller axis, and said second roller being rotatable about its axis, wherein said second roller further comprises a gripping surface for engaging the graspable portion of said object, (iv) a biasing means to bias said rollers toward each other with sufficient force to facilitate holding said graspable portion of said object in place between said rollers, (v) a driving means for rotating said rollers, and (vi) a holding means selectively operable to releaseably engage at least one of said rollers, and prevent rotation of said roller when engaged; positioning said grapple device close to said object; activating a grabbing operation of said grapple device, wherein said driving means rotates said first roller in a first direction, such that said first roller and said second roller rotate in opposite directions inwardly, and when said roller surface of said first roller contacts said object, a graspable portion of said object is drawn between said first and the second rollers, thereby grasping said object; stopping the rotation of said rollers by said holding means when a sufficient portion of said object is grabbed between said rollers, thereby holding said object; maneuvering said grapple device and object, via said transport mechanism, to a desired location; and releasing said object from said grapple wherein said driving means rotates said first roller in a second direction, said first roller and said second roller turn outwardly in opposing rotations.

CROSS REFERENCE TO RELATED APPLICATION

This application is entitled to the benefit of Provisional Patent Application Serial No. 60/251,931 filed on Dec. 7, 2000.

US Referenced Citations (9)

Number	Name	Date	Kind
3197250	Trotta et al.	Jul 1965	A
3239263	Farmer et al.	Mar 1966	A
3653708	Merola	Apr 1972	A
4181345	Achelpohl	Jan 1980	A
4226458	Achelpohl et al.	Oct 1980	A
4549760	Minenko et al.	Oct 1985	A
4578013	Barillec et al.	Mar 1986	A
4852927	Minenko et al.	Aug 1989	A
5303910	McGill et al.	Apr 1994	A

Foreign Referenced Citations (3)

Number	Date	Country
2749-908	May 1979	DE
1-231732	Sep 1989	JP
1611-822	Dec 1990	SU

Provisional Applications (1)

	Number	Date	Country
	60/251931	Dec 2000	US

Mechanical grapple for grabbing and holding sacks and bags

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

CPC

US Classifications

Field of Search

US

International Classifications