Open loop control with velocity threshold for pneumatic hoist

Information

  • Patent Grant
  • 6547220
  • Patent Number
    6,547,220
  • Date Filed
    Wednesday, January 31, 2001
    23 years ago
  • Date Issued
    Tuesday, April 15, 2003
    21 years ago
Abstract
A control system for a pneumatic hoist limits the velocity or acceleration of a load attached to the hoist. The control system includes a controller and a valve. The controller provides a variable gain between an actuator, which controls the speed of the hoist, and the valve, which controls fluid flow into the hoist, thereby controlling the velocity or acceleration of the load. As the velocity or the acceleration of the load reaches a predetermined maximum value, the control system proportionally reduces the gain between the actuator and the valve, thereby closing the valve and reducing the velocity or acceleration of the hoist.
Description




BACKGROUND OF THE INVENTION




Pneumatic balancers are in widespread use in the material handling industry. The balancers are formed of a large air cylinder that rotates about a fixed axis, reeling a wire rope in or out as the cylinder is pressurized with air. There are several advantages to this type of pneumatic lifting device. One advantage is that once a heavy load has been lifted, the load “floats” or is “balanced” by the air that is pressurized inside the cylinder. This lets the user maneuver the heavy load up or down a short distance, while the load is positioned and secured onto an assembly. The combination of heavy lifting capacity and load suspension or “balancing” makes pneumatic balancers useful.




One of the drawbacks with pneumatic balancers involves controlling the speed of pressurization of the cylinder. If the cylinder is pressurized at a relatively slow rate, the load is lifted at a slow rate which slows down the operator. If the cylinder is pressurized at a relatively high rate, a smaller or lighter load can be violently “launched” across the room.




A traditional, all mechanical, solution to this problem is to use thumb screw adjustable needle valves to set the rate at which the cylinder pressurizes, and to employ a mechanical (centrifugal) brake which grabs and stops the cylinder if the velocity of the wire rope is too high. This solution, however, has drawbacks. The needle valves must be set based on the weight of the load being lifted. If the needle valves are set to lift a very heavy load by a first user, and at a later point a second user operates the balancer to lift a very light load without adjusting the needle valves, operation of the balancer causes the wire rope to accelerate much too quickly. At some later point, the mechanical brake “grabs” the cylinder, stopping it instantly. This violent series of events can still “launch” the light load, and in any case, is unsettling to the unexpecting user.




Another solution to this problem involves the use of an electronic control system to control the rate at which the load is lifted. The control system requires a device to measure the speed of the wire rope, a valve which controls the speed of air entering the cylinder, and a device which allows the user to set the rate at which the load is being lifted. A drawback to the use of the electronic control system involves requiring the control system to exhibit a desirable response to a user's speed request inputs over a full range of loads.




Classical control system theory assumes the use of a setpoint variable, which is, for example, a user's joystick setting that represents the desired velocity of the wire rope, and a process variable, which is the actual velocity of the wire rope. The difference between the set point variable and the process variable is the process error. The process error is then differentiated and integrated with respect to time. These three representations of the error are then scaled with three empirically determined constants, and the resulting three products are summed, generating an output which is used as the driving function for the process. These three parts of the driving function are referred to as Proportional, Integral and Differential (PID). This entire function, which occurs in real time, feeds back process variable information into the function such that the process error is driven to zero. This type of control is referred to as a closed loop control.




One issue in using a PID control with a balancer is that the “empirically determined constants” (or PID settings) are nominally set using a nominal weight on the balancer. This concept does not insure desirable performance over the full range of expected loads. Another complication is that the pressurized air, which is lifting the load, acts like a large spring (i.e. the air is compressible) where the spring constant changes depending on the weight being lifted (pressure in the cylinder).




SUMMARY OF THE INVENTION




A preferred embodiment of the present invention relates to a hoist whereby the lifting or lowering velocity of the hoist is independent of the load attached to the hoist.




More specifically, a velocity and acceleration limiter has been implemented within a hoist to decouple velocity and the effect of the load.




The user's force sensitive inputs can control the pneumatic valve directly, i.e. with a fixed gain) as long as the wire rope's velocity and acceleration are below pre-determined maximum values. If either the velocity or acceleration approaches the maximum value, the gain between the user's inputs and the pneumatic valve is automatically reduced using a gain control or gain reduction algorithm, thus opening the pneumatic valve less and reducing the velocity or acceleration of the hoist.




In order to cause the control system to exhibit a desired response to a user's speed request, the velocity and/or acceleration of the wire rope of a hoist is limited, rather than controlled, by the gain control algorithm. The user controls the velocity of the hoist in an open loop fashion. One benefit of the velocity limiting function is that as long as the user maintains the velocity below the maximum allowed, the output of the speed setting joystick is essentially connected directly to the input of the spool valve, with minimal effect of the gain control algorithm between the joystick and the hoist. This provides a direct feel for the user when moving a load at a slow rate of speed. A second benefit is that a velocity limiting function is not a closed loop function. There is no “setpoint variable” and there is no “process error” that is driven to zero, which are fundamentally part of a closed loop system.




In one embodiment, the hoist includes a housing having a first end wall and a second end wall, the housing, first end wall and second end wall forming a chamber. An inlet mechanism is attached to the housing the inlet mechanism and allows the passage of a fluid, such as air, into the chamber. The hoist includes a piston mounted within the chamber and a valve connected to the inlet mechanism. The valve controls the amount of fluid entering the chamber. The hoist also includes a pressure sensor attached to the chamber and a position measuring device connected to the housing. The pressure sensor is in fluid communication with a fluid within the chamber and the position measuring device is in positional communication with the piston. Also included is an actuator in electrical communication with the valve, the actuator controlling the positioning of the valve, and a control system in electrical communication with the valve, the pressure sensor, the position measuring device and the actuator. The control system has a variable gain between the actuator and the valve, where the gain is reduced as the velocity or acceleration of a load attached to the piston approaches a preset maximum value.




The variable gain of the control system includes a gain reduction algorithm. This gain reduction algorithm preferably includes a square root function.




The actuator has a plurality actuator control inputs. Engagement of an actuator control input pulses the valve in a first direction. The actuator also includes a deadband value. Engagement of the deadband value after engagement of an actuator control input pulses the valve in a second direction, opposite to the first direction, which stops the motion of the piston. Preferably, the control system includes a pulse magnitude algorithm that scales a moving average of a valve magnitude upon engagement of the deadband value, based upon first direction of the valve.




The hoist can also include a load selector that adjusts a maximum allowed velocity value and a maximum allowed acceleration value of the hoist with respect to a load attached to the hoist. The control system of the hoist can also include a closed loop control system. The closed loop control system can include a position control or a pressure control.




An embodiment of the present invention also relates to a method for adjusting the velocity of a load attached to a pneumatic hoist. Another embodiment of the invention also relates to a method for stopping the motion of a load attached to a hoist











BRIEF DESCRIPTION OF THE DRAWINGS




The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.





FIG. 1

shows a control system for a pneumatic hoist.





FIGS. 2A and 2B

show gain results of the control system based on a non-square root and a square root function, respectively.





FIG. 3

illustrates a graph of a square root look-up table.





FIG. 4

shows a gain calculation table for the gain of the control system.





FIG. 5

illustrates a load selection mechanism for the control system.





FIG. 6

illustrates actuator controls for the actuator.











DETAILED DESCRIPTION OF THE INVENTION





FIG. 1

illustrates a control system for a pneumatic hoist, given generally as


10


. The control system


10


includes a hoist


15


having a housing


12


. The housing


12


includes a first end wall


14


and a second end wall


16


and forms a hollow chamber


18


. The housing


12


also includes a lead screw


20


and a piston


22


. The lead screw


20


is mounted along a longitudinal axis


24


of the housing. The piston


22


is slidably mounted to the lead screw


20


. The housing


12


includes an inlet mechanism


26


which allows the flow of a fluid into the chamber


18


. Preferably, the fluid is a pressurized gas such as air, for example. The entrance of fluid into the chamber


18


causes rotational movement of the piston


22


on the lead screw


20


about the longitudinal axis


24


. The faster the fluid from the inlet


26


enters the chamber


18


, the greater the rotational velocity or acceleration of the piston


22


.




The piston


22


includes a cable


28


attached to and wrapped around an outer circumference of the piston


22


. The cable


28


can be made from a wire material, such as a wire rope. The cable


28


connects the piston


22


to a load


30


and allows for raising and lowering of the load


30


by the hoist


15


. An actuator


38


is attached to the cable


28


and controls the rotational velocity and acceleration of the piston


22


and controls the movement and positioning of the cable


28


. The actuator


38


includes actuator controls that are force sensitive such that the greater the amount of force a user exerts as the actuator controls


50


, the faster the load


30


moves in either an upward or downward direction. The force sensitive controls can be load cells, for example and the actuator can include ajoystick.




The control system


10


also includes a position measuring device


36


, a pressure sensor


34


and a valve


32


attached to the inlet mechanism


26


. The position measuring device


36


is attached to the housing


12


and can be in communication with either the cable


28


or the piston


22


. The position measuring device


36


measures the position of the piston


22


or cable


28


and determines the distance that either the cable


28


or the piston


22


has traveled from some starting position. The pressure sensor


34


is also attached to the housing


12


and is in fluid communications with the fluid located within the chamber


18


of the housing


12


. The pressure sensor


34


measures the pressure of the fluid inside the chamber


18


. The valve


32


attached to the inlet


26


controls the amount of fluid entering the chamber


18


through the inlet


26


. Regulation of the amount of fluid entering the chamber


18


, in turn, controls the rotational velocity and acceleration of the piston


22


. The valve


32


can be a pneumatic valve, for example.




The control system


10


also includes a controller


40


. The controller


40


receives data from the actuator


38


, position measuring device


36


and pressure sensor


34


by an actuator data line


42


, a position device data line


44


and a pressure sensor data line


46


, respectively. Based upon this data, the controller


40


can control the closure of the valve


32


by way of a valve data line


48


. Certain parameters of data can cause the valve


32


to be fully opened by the controller


40


or fully closed. The control system


40


can include a computer, for example.




The controller


40


allows the velocity of the piston


22


and the cable


28


to be independent from the mass or weight of the load


30


. The controller


40


stores a predetermined maximum value for the velocity and acceleration of the cable


28


. The controller


40


also stores a predetermined value for gain which is implemented between the actuator


38


and the valve


32


.




When an operator uses the hoist


15


, he engages the actuator


38


to move the load


30


in either an upward or downward direction. As the user moves the load


30


, the position measuring device


36


measures the distance the cable


28


has traveled over some period of time. From this measurement, the velocity or acceleration of both the cable


28


and the load


30


can be calculated. The controller


40


compares the calculated velocity or acceleration of the cable


28


to the predetermined maximum allowed value of velocity or acceleration of the hoist


15


. If the calculated velocity or acceleration of the cable


28


is less than the predetermined maximum value, the controller


40


makes minimal changes to the gain of the system, thereby allowing the direct control of the valve


32


by the actuator


38


. By directly controlling the valve


32


, the actuator


38


controls the velocity and acceleration of the piston


22


, the cable


28


and the load


30


. The user's direct control of the velocity or the acceleration of the piston


22


, cable


28


and load


30


is an example of an open loop control system.




As the calculated velocity or acceleration of the cable


28


approaches the predetermined maximum value, the control system


40


proportionally reduces the gain between the actuator


38


and the valve


32


. Reducing the gain reduces the valve


32


opening on the inlet mechanism


26


, thereby preventing fluid from entering the chamber


18


. This, in turn, reduces the velocity or acceleration of the piston


22


, the cable


28


and the load


30


. As long as the velocity and/or acceleration of the cable


28


is below the maximum allowed value, the user has direct control of valve, with minimal effect of the gain control algorithm. As the velocity and/or acceleration approaches the maximum allowed value, the gain is proportionally reduced, thus reducing the valve


32


opening and thus reducing the velocity of the load


30


and the cable


28


. This reduction in velocity occurs even though the user has the joystick or actuator


38


engaged at a maximum level of operation.




There is a complication in implementing the gain control. Consider that the user moves the actuator until the velocity begins to approach a preset maximum value. The controller


40


then reduces the actuator gain, reducing the opening of the valve


32


orifice, and the velocity of the piston


22


is reduced. At this point the velocity is no longer near a preset maximum value, so the controller


40


restores the original gain, thereby opening the valve


32


to increase the velocity, thereby increasing the velocity of the piston to a level near the preset maximum value. Notice that this oscillation takes place with the user's actuator


38


at a constant setting. One solution to this problem is to use continuously updated peak velocity and peak acceleration measurements, rather than the actual velocity or acceleration values, when calculating the gain reduction. The addition of the peak function serves to give the measurements a static attribute that breaks the oscillatory operation described above. The length of time that the peak is held before being reset to zero is empirically determined, and allows new peaks to be captured and updated. Use of the updated peak velocity and acceleration are described as follows.




A user operates an actuator


38


or joystick to control the velocity or acceleration of the hoist


15


. The joystick produces signed integers or joystick integer values representing the user's velocity request. The maximum “up” velocity request is +512 and the maximum down velocity request is −511. A value of 0 indicates the joystick


38


is at a center position that requests no movement.




As the actuator


38


is manipulated, valve data is sent to the valve


32


where the valve data comprises signed integers representing the magnitude and direction of spool valve excitation. The maximum valve opening in the “up” direction (pressurizing) is +128 and the maximum valve opening in the down direction (relieving pressure) is −127. A value of 0 indicates the valve is in center position has no opening in either direction.




Velocity and acceleration are calculated based on the position measuring device


36


or encoder which keeps track of the cable


28


position. The hoist


15


or balancer and encoder


36


combination provides approximately 6 feet of cable


28


extension with a resulting position count (from fully retracted to fully extended) of approximately 15,000 counts. The actual position count is always preset at power up to 30,000 counts in that the position of the cable


28


is always a positive integer. Actual position of the cable


28


or piston


22


is updated every 6.25 msec. Velocity of the cable


28


is a signed integer and is calculated every fourth position update (25 msec rate) using the formula: velocity=current position—position four updates ago. Acceleration of the cable


28


is a signed integer and is also calculated every fourth position update (25 msec rate) using the formula: acceleration current velocity−last velocity calculated.




The peak velocity and peak acceleration of the cable are also calculated. The held or stored peak velocity and peak acceleration values are reset at regular intervals. Preferably, the values are reset at an interval within the range of 75 ms and 150 ms.




Gain reduction for the purpose of velocity and acceleration limiting occurs when either the peak velocity or peak acceleration begins to approach maximum allowed settings. Ordinals 1-9 are used to represent the maximum allowed settings for the velocity and acceleration as follows:




maximum allowed velocity settings:




1=velocity of 50 counts is allowed




2=velocity of 75 counts is allowed




3=velocity of 100 counts is allowed




4=velocity of 125 counts is allowed




5=velocity of 150 counts is allowed




6=velocity of 175 counts is allowed




7=velocity of 200 counts is allowed




8=velocity of 225 counts is allowed




9=velocity of 250 counts is allowed




maximum allowed acceleration settings:




1=acceleration of 4 counts is allowed




2=acceleration of 8 counts is allowed




3=acceleration of 12 counts is allowed




4=acceleration of 16 counts is allowed




5=acceleration of 20 counts is allowed




6=acceleration of 24 counts is allowed




7=acceleration of 28 counts is allowed




8=acceleration of 32 counts is allowed




9=acceleration of 36 counts is allowed




The full implementation of the gain control or gain reduction algorithm is preferably performed using the following steps:




First, a gain number is calculated based on maximum allowed velocity, maximum allowed acceleration, peak velocity, and peak acceleration of the cable


28


.

FIG. 4

illustrates a table used to calculate the gain based upon these variables. To calculate a gain number, a first result is calculated by subtracting the absolute value of the peak velocity from the maximum allowed velocity. If the first result is a negative number, the first result is set equal to zero. Similarly a second result is calculated by subtracting the absolute value of the peak acceleration from the maximum allowed acceleration. If the second result is a negative number, the second result is set equal to zero.




A third result is then determined from the table shown in FIG.


4


. The maximum allowed velocity and maximum allowed acceleration settings are used to determine or lookup the value of the third result.




The first, second and third results are multiplied together to form a product. The product is equal to approximately 65,535 if both the peak velocity and peak acceleration values are equal to zero. The product is equal to approximately zero if either the peak velocity or peak acceleration is at the maximum allowed value. This product is a number between 0 and 65,535.




Next, the product described above is divided by the value


128


. The result of this division is used as a pointer for the x-axis value in a square root look up table.

FIG. 3

illustrates a graph of the square root look up table. The division result is used as the xvalue pointer in the table where the corresponding y-value is determined based on the division result. The table


84


includes both x-values


80


and corresponding y-values


82


. The x-values range between 1 and 512. The corresponding y-values range between


96


and 62,736. The y-values are based upon the square root function:








y=


sqrt((1024)(1024)(8)(


x


))−2800






X-values between 1 and 512 are placed in the formula and yield corresponding y-values between 96 and 62,736.




The reasoning behind the use of a square root function is shown in

FIGS. 2A and 2B

. The square root function prevents the variable gain from having a value that is small relative to mid-scale velocity and acceleration values. In

FIGS. 2A and 2B

the velocity and acceleration numbers are normalized such that the value “1” represents the highest actual velocity, the highest acceleration value, the maximum allowed velocity value and the maximum allowed acceleration.





FIG. 2A

shows a table wherein the values within the table are given by the equation:






(1−actual velocity)*(1-actual acceleration)






When the velocity and acceleration are each 0.4 (only 40% of their maximum values) the gain is reduced to 0.36. This gain reduction can be relatively high for velocity and acceleration values that do not approach the maximum allowed values.





FIG. 2B

shows a table wherein the values within the table are given by the equation:






sqrt ((1-actual velocity)*(1-actual acceleration))






For example, when the actual velocity and actual acceleration values are equal to 0.4, the gain calculation from the above equation is 0.6. Taking the square root prevents the gain numbers from being reduced by a relatively large amount as the actual velocity and actual acceleration increases.




In the final step of the gain reduction algorithm, the joystick integer value is multiplied by the resulting y-value of the square root function and then divided by the value 262,144. This final result is the gain used to drive the valve


32


.




The micro-processor that can be used to implement this algorithm is a 8 bit, 6.144 Mhz Z80180. The microprocessor can be formed as part of the controller


40


. The microprocessor has a 8×8 multiply instruction, and allows all three of the aforementioned steps to execute in approximately 2 to 4 msec. Preferably, the algorithm executes once every 6.25 msec.




The actual use of the gain algorithm demonstrates a disadvantage of not using a closed loop control algorithm, which can lead to poor velocity regulation. Given a predetermined maximum allowed velocity acceleration, a light load can still accelerate more quickly than a heavy load. In an alternate embodiment, the control system


10


includes a load selector


52


, as shown in FIG.


5


. The load selector


52


can be used to aid in the speed regulation of the hoist. The load selector


52


can be located on the actuator


38


as an additional control, as shown, or can be located separately from the actuator


38


. The load selector


52


allows a user to select the approximate weight or mass of the load


30


attached to the cable


28


. This selection is sent to the control system


40


by a load selection data line


54


. The selection implements various maximum velocity or acceleration values for the hoist


15


. The load selector


52


can include selections for either light


56


, medium


58


and heavy


60


loads or for a light and a heavy load. Such a selection improves speed regulation of the hoist


10


for a wide range of loads. The different selections implement different maximum velocities and accelerations to improve velocity and acceleration regulation over a large range of loads.




When a user is causing the hoist


15


to lift a load, the load lifts because the pressure inside the hoist


15


or balancer is greater than the pressure exerted by the load


30


itself. When the user requests the lifting to stop, the magnitude of the difference between these two pressures can add an overshoot error to the position where the user wants the balancer


15


to stop. A similar problem exists when lowering a load. A method of reducing the overshoot error is to pulse the valve


32


in a direction opposite that of its current motion to reduce the difference between the pressure within the balancer


15


, and the pressure of the load


30


suspended by the balancer


15


.




To implement this concept, the actuator


38


can also produce a deadband value. As shown in

FIG. 6

, the actuator controls


50


of the actuator


38


include a first directional or an up control


72


and a second directional or down control


74


. The controls


72


,


74


produce a voltage in the range of 0V to 5V, for example. The deadband value is equal to the voltage produced when neither control is engaged. For example, when neither control


72


,


74


is engaged, a deadband value of 2.5V is produced by the actuator


38


. Preferably, the deadband value is within the range of 2.4V and 2.6V, when neither control


72


,


74


is engaged. To allow the deadband value to fall within a range of values helps to minimize the potential effect of voltage drift on the voltage produced by the controls. The software for the control system


10


implements an algorithm such that whenever the control input transitions from an up


72


or down


74


control position and into the deadband range, the valve


32


is pulsed in a direction opposite to that of its current motion.




A pulse magnitude algorithm can be used in conjunction with the deadband value to adjust the magnitude and duration of a pulse on the valve. When the user lifts with the balancer


15


at a slow rate, a fixed magnitude pulse on the valve


32


in the opposite direction can be too large and cause the balancer to drop the load some distance because the pressure in the balancer


15


was reduced by a relatively large amount. Preferably, a pulse magnitude algorithm is used to scale a moving average of the valve magnitude as the hoist


15


lifts or lowers. The pulse magnitude is based upon the position of the valve


32


when the actuator controls


50


of the actuator


38


are disengaged and the actuator produces or engages a deadband value. Thus, if a user is lifting a load at a slow rate, the valve


32


is open a relatively small amount. Therefore, a pulse on the valve to a second position in the opposite direction, a second direction, is also relatively small. Conversely, if a user requires a large velocity, the valve


32


is open a relatively large amount. Therefore, a pulse on the valve to a second position in the opposite direction, a second direction, is also relatively large.




As described, the gain reduction algorithm is used as part of an open loop control system. Alternately, the gain reduction algorithm can be used in conjunction with a closed loop control system such as a position or pressure control loop. Once a load


30


has been lifted by the balancer


15


, a closed loop control on the balancer's position or on the balancer's pressure can be performed. By implementing a closed loop position control, once the load


30


has been lifted and positioned, the hoist


15


can hold a bucket or barrel of liquid, for example, in a constant location as it is filled or emptied. By implementing a closed loop pressure control, once the load


30


has been lifted, a user can grab and adjust the position of the load


30


by lifting or leaning on the load


30


itself.




The use of a closed loop system with a hoist


15


can create oscillations of the hoist


15


caused by position overshoot of the load


30


. The automatic gain reduction algorithm can be utilized in conjunction with the closed loop algorithm to minimize the oscillations created by use of the closed loop control algorithms. For example, a conventional proportional type control algorithms (including PID) can be used in the control system


10


. Prior to sending the results of the PID algorithm to the valve


32


, the gain reduction algorithm can be implemented. The gain reduction is activated whenever acceleration or velocity approaches a predetermined maximum value. The use of the gain reduction process in conjunction with the proportional control algorithm helps to prevent oscillations of the hoist. If the process does become unstable, the gain reduction algorithm maintains the oscillating process within its maximum allowed velocity and acceleration, resulting in small amplitude, controlled oscillations, rather than large amplitude, uncontrolled oscillations.




While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.



Claims
  • 1. A hoist comprising:a housing having a first end wall and a second end wall, the housing, first end wall and second end wall forming a chamber, an inlet mechanism attached to the housing the inlet mechanism allowing the passage of a fluid into the chamber; a slidable piston mounted within the chamber; a cable wrapped on the piston for attachment to a load; a valve connected to the inlet mechanism, the valve controlling the amount of fluid entering the chamber; a pressure sensor attached to the chamber and in fluid communication with a fluid within the chamber; a position measuring device connected to the housing and in positional communication with the piston; an actuator in electrical communication with the valve, the actuator controlling the positioning of the valve; and a control system in electrical communication with the valve, the pressure sensor, the position measuring device and the actuator, the control system having a variable gain between the actuator and the valve, the gain being reduced as the velocity or acceleration of a load attached to the piston approaches a preset maximum value.
  • 2. The hoist of claim 1 wherein the variable gain of the control system comprises a gain reduction algorithm.
  • 3. The hoist of claim 2 wherein the gain reduction algorithm comprises a square root function.
  • 4. The hoist of claim 1 wherein the actuator comprises a plurality of actuator control inputs wherein engagement of an actuator control input pulses the valve in a first direction.
  • 5. The hoist of claim 4 wherein the actuator comprises a deadband value wherein engagement of the deadband value after engagement of an actuator control input pulses the valve in a second direction, opposite to the first direction, thereby stopping the motion of the piston.
  • 6. The hoist of claim 5 wherein the control system comprises a pulse magnitude algorithm wherein the pulse magnitude algorithm scales a moving average of a valve magnitude based upon the first direction of the valve.
  • 7. The hoist of claim 1 wherein the hoist further comprises a load selector wherein the load selector adjusts a maximum allowed velocity value and a maximum allowed acceleration value of the hoist with respect to a load attached to the hoist.
  • 8. The hoist of claim 1 wherein the control system further comprises a closed loop control system.
  • 9. The hoist of claim 8 wherein the closed loop control system comprises a position control.
  • 10. The hoist of claim 8 wherein the closed loop control system comprises a pressure control.
  • 11. A method for adjusting the velocity of a load attached to a pneumatic hoist comprising the steps of:providing a control system for a pneumatic hoist having a maximum allowed velocity value and a maximum allowed acceleration value for a load attached to the hoist; measuring a velocity or acceleration of the load attached to the hoist; comparing the measured velocity or acceleration of the load against the maximum allowed velocity value or maximum allowed acceleration value; and adjusting a gain between an actuator and a valve of the control system in response to the results of the comparison in order to adjust the velocity or acceleration of the load, such that the gain of the control system is reduced as the measured velocity or acceleration of the load approaches the preset maximum allowed velocity value or the maximum allowed acceleration value.
  • 12. The method of claim 11 further comprising the steps of:providing a load selector attached to the hoist; and selecting a load level on the load selector corresponding to the load attached to the hoist, the load level corresponding to a preset maximum allowed velocity value or a maximum allowed acceleration value.
  • 13. The method of claim 11 comprising the step of using a square root function to reduce the gain of the control system.
  • 14. The method of claim 11 comprising the step of using an updated peak velocity or peak acceleration value of the load attached to the hoist to reduce the gain of the control system.
  • 15. A method for stopping the motion of a load attached to a hoist comprising the steps of:providing a hoist having a piston, a valve and an actuator, the actuator having a plurality of piston directional control inputs and a deadband input; engaging a piston directional control input, thereby causing the piston to rotate in a chosen direction and causing the valve to pulse in a first direction; engaging the deadband input, thereby causing the valve to pulse in a direction opposite to the first direction; scaling a magnitude of the pulse from the valve to correspond with the velocity of the piston; reversing the direction of rotation of the piston; and stopping the motion of the load.
  • 16. A method for stopping the motion of a load attached to a hoist comprising the steps of:providing a hoist having a piston, a valve and an actuator, the actuator having a plurality of piston directional control inputs and a deadband input; engaging a piston directional control input, thereby causing the piston to rotate in a chosen direction and causing the valve to pulse in a first direction; engaging the deadband input, thereby causing the valve to pulse in a direction opposite to the first direction; scaling a duration of the pulse from the valve to correspond to the moving average of a valve magnitude based upon the first direction of the valve; reversing the direction of rotation of the piston; and stopping the motion of the load.
RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 60/179,268, filed on Jan. 31, 2000. The entire teachings of the above application(s) are incorporated herein by reference.

US Referenced Citations (6)
Number Name Date Kind
3758079 Workman et al. Sep 1973 A
3933080 Corrie Jan 1976 A
4044651 Warrick Aug 1977 A
4372534 Hansson Feb 1983 A
5439200 Braesch et al. Aug 1995 A
5915673 Kazerooni Jun 1999 A
Foreign Referenced Citations (5)
Number Date Country
19727194 Jan 1999 DE
2 738 808 Mar 1997 FR
1379246 Mar 1988 SU
1682639 Oct 1991 SU
WO 9519316 Jul 1995 WO
Provisional Applications (1)
Number Date Country
60/179268 Jan 2000 US