Patent Application
20060266146
This invention relates to motion control or actuation devices in general, and in particular, to direct drive electromechanical rotary-to-linear motion actuators.
Electromechanical actuators are rapidly displacing hydraulic devices in a wide range of industries, including aviation. Improvements in solid state switching devices, their digital control and in the performance of magnetic materials have all contributed to an increased interest in electric actuators. Electromechanical linear actuators are particularly well suited to flight control applications, as well as a multitude of industrial uses, particularly in production automation. Automotive and other vehicle applications also abound, as may be found, for example, in the variable transmissions and caliper or disk brake actuators described in, e.g., U.S. Pat. Nos. 6,837,818 and 6,626,778 to Kapaan et al., and 6,367,597 and 6,318,512 to De Vries et al.
Flight applications, such as actuation of control surfaces, helicopter rotor blades, lift enhancement devices, landing gear deployment and braking, door opening, and the like, are all best handled with linear actuators. Rotary-to-linear conversions, correctly engineered, can provide backlash-free operation, high stiffness, high slew rates, good overall efficiency and high frequency response, which properties are all needed in combination for an ideal flight control actuator.
Concomitant with their widespread adoption, certain shortcomings in various mechanical aspects of the devices have also become apparent, and therefore merit attention. These problems include backlash, wear, complexity and cost and life limitations.
Accordingly, there is a long-felt but as yet unsatisfied need in a number of fields for linear actuators that overcome the backlash, rapid wear, complexity, high cost and limited life problems incident to prior art actuators, and that provide backlash-free operation, higher stiffnesses, slew rates and frequency responses, and better overall efficiency.
In accordance with the various exemplary embodiments of the invention described herein, direct-drive electromechanical rotary-to-linear actuators are provided that address and overcome many of the above and other shortcomings of prior art actuators.
In one exemplary embodiment thereof, a direct drive electromechanical rotary-to-linear actuator includes an elongated housing and an electric motor, including a stator fixed in the housing and a rotor supported for rotation relative to the stator, coupled to a planetary drive unit. The drive unit comprises an elongated central shaft coupled to the rotor for conjoint rotation therewith and has a plurality of helical threads on an external surface thereof. A planetary nut having a plurality of helical threads on an internal surface thereof is disposed concentric to the shaft, and a plurality of planetary rollers, each having a helical thread on an external surface thereof that is complementary to and in engagement with both a thread of the shaft and a thread of the nut, is disposed concentrically between the shaft and the planetary nut.
In one advantageous variation thereof, the central shaft of the actuator is made integral to the rotor of the motor and constructed of a magnetically permeable material, e.g., an alloy steel. The stator can comprise high-cobalt-content laminations and the rotor can comprise neodymium-iron magnets.
In another variation, the central shaft can include a hollow center, with a linear variable displacement transducer (“LVDT”) disposed therein for measuring the absolute position of an output end of the actuator relative to a fixed end thereof.
In another variation, a plurality of axial grooves can be provided in the housing adjacent to the planetary nut, and a plurality of axial splines can be provided on the nut, each of which is slidably disposed in a respective one of the grooves to prevent rotation of the nut relative to the housing and thereby react any torque on the nut to adjacent machine structure.
In another variation, the elongated rotor is of common material to the planetary nut housing which is coupled to the annular shaft within the annulus thereof to conserve actuator length and provide a double-ended actuator if desired. An absolute-angular-position encoder can be rotatably coupled to the central shaft for detecting the absolute linear position of the shaft.
In another exemplary embodiment, the actuator can include a second electric motor, e.g., a backup motor, including a second stator fixed in the housing and an elongated annular second rotor supported for rotation relative to the second stator. The associated planetary drive unit of this embodiment comprises a planetary nut that is coupled to the annular second rotor within the annulus thereof for both conjoint rotation and relative axial sliding therein. As above, a plurality of threaded planetary rollers is disposed concentrically between the central shaft and the planetary nut, and means are provided for selectably locking the respective rotors of the two motors against rotation, thereby providing a “fail operational” mode of the actuator in the case of a malfunctioning main motor.
In one possible alternative implementation of this embodiment, at least one of the two motors, e.g., the backup motor, comprises a “pancake motor,” and the malfunctioning main motor locking means can comprise a double-armature, double-disc, solenoid clutch brake operable to selectably lock/unlock respective ones of the two rotors.
In another alternative implementation, e.g., in a high power actuator application, the rotor locking means can comprise a clutch plate mounted on the rotor of the main motor for conjoint rotation and positioned such that it rotates between the jaws of a caliper brake. A plurality of rocking levers having rollers disposed at respective first ends thereof are arranged to move between a first orientation, in which the rollers engaged in respective slots in an end of the rotor of the backup motor, and a second orientation in which the rollers are disengaged from the slots. A plurality of first springs biases the rocking levers into the first orientation, such that the backup motor is locked against rotation during normal operation. A second spring, e.g., a belleville spring, biases the jaws of the caliper brake together and against the clutch plate on the main rotor, to thereby clamp the rotor against any rotation during a malfunction. A spool is moveable between a first position compressing the second spring and thereby relieving the bias of the second spring on the caliper brake jaws during normal operation of the main motor, and a second position locking the main motor and urging the rocking levers against the bias of the first springs and into the second orientation thereof, thereby unlocking the backup motor for rotation.
Means are provided for releasably holding the spool in the first position during normal operation of the actuator. In a simple yet reliable implementation, these means can comprise a plurality of ball bearings disposed in apertures in the spool and held captive in an adjacent circumferential groove by an arm of a solenoid. The same solenoid can be used to selectably release the spool from the first position for movement to the second position, e.g., in response to a malfunction of the main motor, whereupon the actuator rapidly switches to a “fail operational” mode in which the backup motor takes over operation of the actuator from the malfunctioning main motor.
In yet another exemplary embodiment that is advantageously adapted to miniaturized actuator applications, the actuator can comprise an electric motor that includes a stator supported for axial movement in a stator housing, and a rotor supported in the housing for conjoint axial movement with, and rotation relative to, the stator. The planetary drive unit of this embodiment comprises an elongated, narrow, unthreaded cylindrical spindle having a long axis and coupled to the rotor for conjoint rotation therewith. In the place of a planetary nut, a drum having a thin, cylindrical sidewall with a plurality of helical corrugations therein is disposed concentric to the spindle. In the place of elongated, planetary rollers, a plurality of disc-like rollers is disposed in a radially symmetrical array about the spindle. Each of the rollers is mounted on the stator housing for rotation about an axis that is skewed at an angle equal to the pitch angle of the drum corrugations, and each has a circumferential surface with convolutions that correspond to the corrugations in the drum. The circumferential surface of each of the rollers is disposed in frictional engagement with both the spindle a respective one of the corrugations of the drum.
In an alternative embodiment, the rollers can be located in adjacent planes perpendicular to the spindle, each plane containing a radially symmetrical array of two or more rollers. In another advantageous variation, the wall of the drum can be strained from a cylindrical shape to a trochoidal shape incorporating a plurality of longitudinal zones having a smaller radius alternating with a plurality of longitudinal zones having a larger radius. The resulting beam-bending thereby imposed in the wall of the drum, coupled with the stiffening effect of the corrugations therein, provides an inward-directed restoring force that engages the rollers in diametral compression between the walls of the drum and the motor spindle.
A better understanding of the above and many other features and advantages of the present invention may be obtained from a consideration of the detailed description of the exemplary embodiments thereof below, particularly if such consideration is made in conjunction with the appended drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures therein.
This invention discloses various exemplary embodiments of novel, direct-drive, electromechanical, rotary-to-linear actuators. As variously illustrated in the figures, the exemplary mechanism that converts rotation to translation in the various embodiments is a helically threaded, planetary-nut-and-shaft arrangement of a type such as is described in U.S. Pat. No. 3,884,090 to L. I. Dock. Unlike a conventional nut/thread combination, such an arrangement comprises three elements, viz., 1) a central shaft, or lead screw, containing one or more external helical threads; 2) a grouped set of rollers or “planets” incorporating a corresponding external helical thread; and, 3) a “nut,” or housing containing a plurality of corresponding internal helical threads circumscribing the former two elements. These elements can be respectively analogized to those of a conventional concentric planetary gear arrangement of a type that includes a “sun gear,” a set of “planetary gears,” and a “ring gear” circumscribing the two former elements. However, as discussed below, such planetary gear trains have certain drawbacks that are overcome by the present invention. Further, as will be appreciated by those of skill in the art, other types of nut arrangements, such as a ball-screw-and-nut, acme-thread-and-nut, recirculating-ball-nut, or plain thread and nut can also function to some effect, and may be substituted for the above planetary-nut-and-shaft arrangement in some of the embodiments described herein.
As shown herein, when driven by an electric motor, or motors, several advantageous embodiments incorporating such planetary-nut-and-shaft arrangements are possible, and provide useful mechanical or reliability improving outcomes not previously found in prior art actuators. This invention is thus directed to non-geared arrangements of components, sometimes referred to as “direct drive” actuators. The use of direct drive avoids the backlash, complexity, efficiency and reliability issues associated with prior art actuators that incorporate gear trains. Thus, while some gear tooth forms may appear in some of the embodiments described and illustrated herein, it should be understood these are used for alignment purposes only, and not for power transmission purposes. For example, the rollers in a planetary nut may have small gears disposed at each end that mesh with corresponding internal sets of teeth at corresponding ends of the nut. It should be understood that these function only to maintain the rollers parallel to the shaft and nut during operation of the device, and are not used to transmit either torque or thrust.
Turning now to the figures, a first exemplary embodiment of a direct drive electromechanical linear actuator 100 in accordance with the present invention, comprising a single motor, single-ended device, is illustrated in the cross-sectional side elevation view of
In use, the housing 106 of the actuator 100 is attached to adjacent machine structure (not illustrated), and the output end 120 is attached to the component to be actuated (not illustrated). Optionally, a linear variable displacement transducer (“LVDT”) 125, comprising a magnetically permeable rod 126 disposed concentrically within a pair of electromagnetic coils 127 contained in the central shaft 112, is mounted for relative axial movement within the coils to sense the absolute position of the output end of the actuator relative to the other, fixed end 121 thereof. In this, the simplest of actuator configurations, the torque developed by the motor 102 and transmitted to the output end 120 via the central shaft and nut 122 is reacted by adjacent structure (not illustrated), and optionally, at the output end by the actuated device, such that the nut is prevented from rotating, and hence, limited to only translational motion along the long axis of the device.
A close examination of the actuator 100 of
A second exemplary embodiment of a direct drive electromechanical linear actuator 200 in accordance with the present invention, comprising a single motor, single-ended device with internal torque reaction, is illustrated in the cross-sectional side elevation view of
As a result of this arrangement, the position-to command fidelity of the actuator 200 is not affected by any free movement that might be present in another type of arrangement, e.g., a gear train arrangement. This embodiment of the actuator can thus be mounted on ball joints (e.g., as a pin-jointed link) at either or both ends 220 and 221 thereof, and can be used, e.g., between machine parts that move along different axes, because all torque reactions occur internally of the actuator. Optional elastomeric anti-jamming travel stops 236 and a sliding seal 238 disposed around the tubular output extension 218 can also be provided, as shown in
The difference in the material bulk moduli between the low-friction sliders 232 and the complementary housing tracks 234 within which they slide ensures that the relative movement of the fixed and moving parts of the actuator 200 is accomplished with low frictional losses and a complete avoidance of any clearance over a long life span. This enables backlash-free operation of the actuator and the achievement of high-frequency motion.
A third exemplary embodiment of a direct drive electromechanical linear actuator 300 in accordance with the present invention, comprising a single motor, and either a single- or double-ended device with enhanced travel range, is illustrated in the cross-sectional side elevation view of
As illustrated in
As may be seen by reference to
It may be further appreciated in connection with the third exemplary embodiment of actuator 300 that, by combining the nut 322 feature of a planetary roller screw arrangement directly with a hollow rotor portion 308 of an electric motor, and further, by combining the nut as an integral part of the magnetic material of the rotor, as illustrated in the alternative cross section of the rotor half-portion below the centerline of the actuator of
A fourth exemplary embodiment of a direct drive electromechanical linear actuator 400 in accordance with the present invention, comprising a dual motor, single output, internal-torque-reacting actuator with a substantial degree of redundancy, is illustrated in the cross-sectional side elevation view of
As may be seen in
During normal operation of the fourth embodiment of actuator 400, the main motor 402A has the lower inertia of the two motors and is therefore preferred for high-frequency operation. By contrast, the backup motor 402B is preferably configured in a “pancake” form, i.e., one having a relatively shorter length and a relatively larger diameter, so that the planetary nut 422 and sliders 432 can operate within its inside diameter, thereby conserving actuator length. In normal operating mode, the clutch brake 442B of the backup motor 402B is spring loaded to engage and lock the rotor. The slider grooves 434 of the nut anti-rotation element 446 are thus held static, and nut torque is normally reacted through the engaged backup motor clutch. The clutch brake for the main motor is spring loaded to be normally disengaged, and is thus normally out of contact with the main rotor.
The main and backup motors 402A and 402B of the actuator 400 are provided with independent power, motor control and motor-encoding circuits. A malfunction of the main motor or its control will show as an incorrect or nonexistent response to a position command. This detection is extremely rapid. For example, the main motor could be shorted, stationary, moving slowly, dithering, or accelerating at full power to an un-commanded position. In all such malfunction scenarios, it is necessary that the malfunctioning motor be frictionally arrested as quickly as possible. To effect this, the solenoid clutch brake 442A is electrically activated and latched to a “backup” state, namely, malfunctioning motor locked, and backup motor freed. Accordingly, the clutch brake is sized to arrest the full torque of a powered motor. The solenoid windings of the brake are likewise sized to permit being energized constantly, and hence, to produce the coercive force needed for engagement of the appropriate friction plate 448A or 448B without overheating.
In this “fail operational” mode, the actuator 400 now operates with a rotating nut 422 riding on a static central shaft 412, and functions normally, although with reduced frequency response because of the inertia of the larger-diameter components in the “backup” path. Operation of the single LVDT 425 is unaffected by the drive path change. Notably, both the roller screw mechanism and the LVDT are considered to be of such high reliability as to not warrant redundancy. If, after investigation, the fault is found to be in the prime path motor controller, this embodiment of actuator will revert without any external attention (i.e., by absence of a solenoid command) to normal operation via the main motor 402A.
As may be seen from the foregoing, the fourth exemplary embodiment of the actuator 400 combines many of the benefits of the first, second and third embodiments described above, namely, freedom from backlash, high slew rate, high system stiffness and long life, with a substantial degree of redundancy by incorporating the space-saving benefits of a backup motor 402B, shaped as a “pancake,” with internal space used advantageously for the above planetary nut-and-slider arrangement. Also incorporated is a unique packaging arrangement in which the respective lamination cores of the solenoid windings (not illustrated) of the respective rotor clutch brakes are disposed back-to-back in a rigid, monolithic structure that is attached to the housing, and thus serves as a robust load path for transferring the substantial axial loads imposed by the main motor 402A and the thrust and backup motor bearings 444 and 414B to the housing, thereby enabling a substantial reduction in size to be achieved, with the result that the redundant actuator is only about 20% longer than the single-motor actuator 200 of the second embodiment described above.
An alternative implementation of the double-armature, double-disc solenoid clutch brake mechanism 442B of the fourth embodiment of actuator 400 is illustrated in the enlarged detail view of
The main motor rotor 408A is radially extended and splined to accept a rotating clutch plate 456 of a high-friction material disposed between the jaws of a caliper clutch brake 457. In contrast, the backup motor rotor 408B is axially extended to form a drum 458 having axial slots at an end thereof. The rotating clutch plate of the main rotor is alternately locked against, or unlocked for, rotation about the central axis by means of the compressive force of a belleville spring 460 acting against a moveable one of the jaws of the caliper clutch brake. In contrast, the backup rotor is alternately locked against, or unlocked for, rotation by the engagement or disengagement of rollers 462, disposed at the respective ends of a plurality of reciprocating rocking levers 464, in the respective axial slots in the end of the backup rotor drum extension in the following manner.
As shown in
Those of skill in the art will appreciate that, as the size, power and hence, torque rating of an actuator increases, the ability of a direct-acting solenoid clutch combined with a friction element (as in the alternative clutch brake embodiment of
By definition of the nut-reaction torque path, the rotor 408B of the backup motor 408B is a statically reacting torque. An effective, reliable method of releasing this torque is by means of the rollers 462 positioned with their respective rolling axes disposed normal to the plane of the applied force. As described above, the main motor 402A fault mode can be one of being stalled, moving slowly, dithering, or having a full torque speed excursion. In all such cases, the main rotor 402A is best arrested by using a clutch plate 456 having a high coefficient of friction (e.g., carbon-on-carbon), clamped between the jaws of the caliper brake 457 by a high-force spring. One advantageous characteristic of the belleville spring 460 is that its release force increases non-linearly with travel. This means that the force required to hold the main rotor in the unlocked state (effected through the mechanism of the spool 468, annular groove 470 and ball bearings 474) is relatively low, while the force available to lock the main rotor and to release the rollers 462, thereby unlocking the backup rotor, is relatively much higher. The large force margin (6:1) guarantees backup rotor rotation within milliseconds of the state-change command to the solenoid 454. The unlocked, or fail operational, configuration of the alternative clutch brake 442B, i.e., after the malfunctioning main rotor has been arrested and locked against rotation, and the backup rotor unlocked for rotation, is as illustrated in the half-portion of the actuator below the centerline of
Thus, making the state change from prime path motor drive to backup path motor drive of the alternative embodiment of the actuator 400 irreversible (except by subsequent intervention) enables not only the ability to use different clutch types that are best suited to the particular conditions of the respective rotor release or engagement, but also the ability to release and engage two clutches simultaneously using only one primary belleville spring 460 and a low-energy-signal solenoid 454. This alternative arrangement therefore offers extremely high mechanical reliability by virtue of its inert, prime-path-locked state.
A fifth exemplary embodiment of a direct drive electromechanical linear actuator 500 in accordance with the present invention, comprising a single-motor linear actuator with helical-roller stator and other features suitable for miniaturization of the actuator, is illustrated in the cross-sectional side elevation view of
The rollers 524 are mounted in ball-bearings 566 and their respective axes of rotation are mutually inclined in the same direction at a common helix angle φ, as illustrated in the enlarged detail view of
Convolutions 523 in the circumferential surface of each of the star rollers 525 engage in corresponding helical corrugations 574 in the thin-walled, cylindrical side wall of a hardened drum 522, wherein the helix angle φ matches the angle of inclination φ of the roller axes, as illustrated in the detail view of
The star roller mountings of the stator 508 are axially stiff, but radially are just sufficiently compliant to allow full contact between the rollers 524 and the motor spindle 512. This arrangement enables transfer of motor drive torque into the planetary system defined by the spindle, rollers and drum 522. A resistive film potentiometer (not illustrated) can be attached to the casing 506 and moved axially by connection with the translating drum for position sensing. Optionally, a motor drive control circuit board (not illustrated) can be mounted inside the casing. As described in connection with the first embodiment of actuator 100 above, the actuator 500 is mounted to external structure (not illustrated) in such a way as to be torque-reacting at the fixed end 521 opposite to the output end 520.
In operation, the rollers 524 of the fifth embodiment 500 are positioned in two planes of symmetry—therefore, the motor spindle 512 is always in force-balance and carries no journal loads, and hence, requires only a minimum thrust load to translate the motor 502 through small distances. This translation is given by d/D X linear distance moved by the drum 522. In the case of the 0.5 mm spindle and 10 mm diameter drum example above, this translation will be 1/20th of drum travel. The pressure angle of the roller convolutions is on the order of 20 degrees. The ultimate load carrying capacity of the actuator 500 is governed by the Hertzian stress at the eight symmetrical points of contact between the cylindrical portion on the top of the roller convolutions and the drive spindle. From calculations, it appears that, with Hertzian stresses of 125 ksi (i.e., approximately half the allowable), and with 4 rollers in a 10 mm diameter shell, axial forces of up to 4 lb. can be produced by the miniature actuator. This is accompanied by high stiffness, complete freedom from backlash and the potential for high frequency response, in a miniature package less than 0.5 inch in diameter.
Those of skill in the art will appreciate that, in the context of miniaturized actuators, such as the miniature actuator 500 described in the above example, it is difficult or practically impossible to achieve shallow helix angles (p with machined helical threads. However, as demonstrated above, they are readily achievable using directly contacting disc-rollers 524 skewed at a shallow angle φ, and further, when expressed to the larger diameter D of the drum 522, the desired helical structure is readily producible by forming discrete hemi-cylindrical corrugations 574 that are capable of reacting axial loads. In additional, the use of the strain energy of a corrugated thin-walled cylinder, when deflected, advantageously produces a diametral force that, when coupled with the prevailing friction between the rollers and the drive spindle 512, creates the torque required to operate the device, thereby opening the way to miniaturized planetary roller devices of high mechanical advantage.
By now, those of skill in this art will appreciate that many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of implementing the linear actuators of the present invention without departing from its spirit and scope. Accordingly, the scope of the present invention should not be limited to the particular embodiments illustrated and described herein, as they are merely exemplary in nature, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.
