GB2102763A - Three-axes actuator assembly - Google Patents

Abstract

A three-axis actuator assembly comprises pitch, yaw, and roll hydraulic actuators (1, 2, 3) supported indirectly by corresponding brackets (4, 5, 6) in series relation. Each bracket is supported for rotation about the axis of the prior actuator in the bracket for the prior actuator so that the brackets, rather than the actuators, carry the loads other than torques. The torque reaction forces imposed on each actuator casing are carried by wide thin torque reaction bands (25) connected between the casing and the bracket for that actuator. Each band has an offset segment located between the actuator and the bracket to accommodate by flexure tolerance variations without imparting any significant side loads to the actuator. The bracket (4) for each actuator includes a manifold plate carrying a servovalve (PSV) that communicates through orifices (200, 202, 204, 206) and couplings (33) with the ports of the actuator (1). Anti-backlash coupling assemblies may connect the wing shafts of the actuators with the bracket for the next actuator or with the tool holder, in the case of the last actuator. <IMAGE>

Description

SPECIFICATION Three-axes actuator assembly The present invention is a three-axes actuator assembly that provides pitch, yaw and roll motions and is intended primarily for industrial uses involving moving a tool of some sort into various positions relative to a work piece. It might well be called "a mechanical wrist" because its movement simulate those of a human anatomical wrist.

Many factors of technological and economic origin (e.g. the development of the microprocessor, increases in labour costs, problems with quality control on production lines and the need for greater productivity) have led to a steadily increasing resort to "robots" for a multitude of industrial operations.

Modern robots can be programmed to move with great precision and speed between numerous work positions for performance of operations by a tool carried by the robot's "hand". For example, a robot can rapidly and precisely make a series of spot welds on an automobile body.

And important part of a robot is the "wrist" that connects the "arm" to the "hand", a tool holder. Like a human anatomical wrist, the robot's wrist provides motion about three axes so that the tool can be applied to the work piece in a variety of orientations.

These motions are pitch (up and down pivoting), yaw (side to side pivoting) and roll (rotation about an axis perpendicular to the axes of pitch and yaw).

An ideal device for performing each wrist movement of a robot is a vane-type hydraulic actuator, a well-known device that has long been in widespread use. Such actuators comprise a cylindrical hydraulic chamber subdivided into reversible pressure and return sides by a fixed semi-diametrical abutment and a rotor having a semi-diametrical vane carried by a rotatable shaft and sealed to the abutment and to the chamber walls. Hydraulic fluid supplied to one side of the chamber rotates the rotor and expels (or returns) fluid from the other side of the chamber.

Precision pumps and servo controls directed by the computer enable the hydraulic actuators of industrial robots to move rapidly and precisely to programmed positions. These wrists have considerable strength, agility, speed, and, of course, endurance and dedication, great advantages over their human anatomical counterparts.

In a three-axes actuator assembly movement about each of the three axes is performed by one actuator, so the assembly has three actuators. In currently available assemblies, the first actuator is carried by a bracket on the end of the robot's arm or other suitable support, the second on a bracket carried by the rotor (often called the "wing shaft") of the first and the third by a bracket carried by the wing shaft of the second. However, instead of bolting the casing of each actuator directly to the bracket for that actuator and then supporting the bracket that carries the next actuator in the series from the shaft of the preceding actuator, each bracket has journals which support the next bracket.This arrangement of the brackets, namely, journalling the second and third brackets in the first and second bracket, respectively, and journalling the tool holder in the third bracket, permits the loads carried by the wrist to be transmitted through the series of brackets. The alternative arrangement of bolting each casing to a bracket would involve transmitting loads through the actuators themselves from the tool holder back to the arm. In the known arrangement, it is also easier to maintain tolerances among the brackets and the actuators by having the respective brackets and actuators coaxial where they are joined together.

On the other hand the known arrangement, as heretofore practiced, presents another problem; inasmuch as each actuator transmits rotation about its axis to the succeeding actuators in the series, it is necessary for the casing of each actuator to be restrained against rotating with the shaft of the actuator. In presently known designs this has been done by bolting the casing of each actuator to the bracket that supports it, such bolting being designed primarily to carry the torque reaction of the casing but not to support the load of any succeeding brackets, actuators or the tool downstream of that actuator.The problem with this arrangement is that the tolerances between the journals that carry the shafts of the actuators and the tolerances of the bolts, pins or other fasteners providing torque reaction restraint between the casing and the bracket must be maintained very close, lest a side load be introduced into the journals, thereby causing undesirable side load forces that offset the working forces of the actuator shaft and inducing undesirable stresses in the assembly.

Presently known wrist assemblies also have a problem in the hydraulic system for operating the actuators. Conventionally, the servovalves for all of the actuators have been carried on a manifold on the arm, and each servovalve is connected up to the associated actuator by hoses or pipes. This introduces a degree of imprecision into the motion of each actuator inasmuch as the hoses or pipes expand and contract depending upon the pressure of the hydraulic fluid which, in turn, depends on the load on the particular time. Moreover, the compressibility of the fluid, which is also a function of the pressure in the fluid at any particular time, changes the volume of fluid in the line. The two effects just mentioned are cumulative and affect, as mentioned above, the precision of motion.The servo-valve may be commanded to deliver a certain amount of fluid to the actuator, but the actual amount of fluid that reaches the actuator is not the same as that commanded because of the cumulative affect of compression or expansion of the fluid itself and compression or expansion of the line in which that fluid is conducted from the servovalve to the actuator.

Not only is the effect of expansion and contraction of the piping and fluid cumulative with respect to each fluid line, but, more importantly, the errors resulting from such expansion and contraction are cumulative among all of the actuators insofar as affecting the precision of location of the tool holderwith respect to the location of the arm or other support is concerned. In other words, the effect of imprecise roll, pitch and yaw motions of the respective actuators is additive with respect to the actual position of the tool, relative to the desired position. A correction for this variation can be programmed into the computer for the robot, but that introduces a complication i-hat might well be avoided.

Presently known wrist assemblies for robots have yet another problem. In accordance with usual machine design practices the output shafts of the actuators are splined and are connected by splined collars to the bracket of the next actuator or to the tool holder. The splined connections are susceptible to backlash which, like the problem of the hydraulic system, affects the precision of motion of the wrist.

A three-axes actuator assembly, that improves upon the state-of-the-art assemblies, comprises first, second and third hydraulic actuators supported indirectly by first, second and third brackets in series relation in that order with their shafts oriented on three difference axes. The first and second brackets have journals that support the second and third brackets for rotation about the axes of the first and second actuators, respectively. The third bracket has a journal that supports a tool holder. The shafts of the first and second actuators are affixed to and supported by the second and third brackets, respectively, and the third actuator is affixed to and supported by the tool holder.Thus, each actuator is supported substantially solely by its shaft, and the loads carried by the assembly are transmitted principally through the tool holder and the brackets.

Only the torque loads are carried by the respective actuators. As a solution to the first problem, namely, side loads on the shafts due to the torque reaction restraint bolt connections between the casings and the brackets, in accordance with a primary aspect of the invention, the casing of each actuator is connected to the bracket for that actuator solely by at least one, and preferably two, wide thin torque retention bands that restrain the casing ofthe actuator against rotation about the axis of its shaft.

Each band includes an offset segment located between the actuator and the bracket to accommodate by fixture of the band tolerance variations between the bracket and the wing shaft of the actuator without imparting any significant side loads to the actuator. Preferably, the reaction band is generally L-shaped in end profile, and the offset is located generally at the juncture between the legs of the "L".

One leg is affixed to one end of the actuator casing, and the other leg is affixed to a portion ofthe bracket laterally of the circumferential wall of the casing.

The second problem with state-of-the-art three- axes actuator assemblies, expansion and contraction of lines and of the fluid in those lines between the remote servovalves and the respective actuators, may be solved by providing a manifold plate for each actuatorthat is affixed to, and indeed may form a part of, the bracket for that actuator. Each manifold plate has a fluid supply passage and a fluid return passage. The servovalve for the respective actuator is affixed directytothe manifold plate relatively close to the actuator, and passages communicate the valve with the supply and return passages in the manifold and with ports opening from each plate in alingment with the ports of the corresponding actuator.Those ports communicate the servovalve with the actuator through couplings that connect the manifold ports with the actuator ports. In accordance with this aspect of the invention, the servovalve is nearby the actuator, and there is no hose or pipes and hence no expansion or contraction of piping and little expansion or contraction of the fluid that can affect the precision of motion. This, in turn, eliminates the need to program the robot to take into account the effect of piping expansion and contraction and fluid expansion and contraction between the servovalve and the respective actuator.

Another improvement now proposed is an antibacklash coupling assembly on the shaft of at least one of the actuators, and preferably on the shafts of all of the actuators. Such an anti-backlash coupling includes a sleeve surrounding the shaft and rotatable with the shaft within the associated journal and a collar having a portion adjacent the end nearest the casing of the actuator that is splined to the shaft and is split to enable deflection of segments toward the shaft. At least part of the split portion is externally tapered outwardly with respect to the shaft and away from the casing. An internally tapered surface on the sleeve matches and engages the tapered part of the collar, and a drive ring engages a shoulder on the collar that faces away from the casing.A drive bolt threaded into the shaft bears against the drive ring and forces the collar toward the casing so that the tapered surfaces of the collar and sleeve cam the segments of the split, splined portion ofthe collar into tight, backlash-free engagement with the splines of the actuator shaft. Thus, this third aspect of the invention solves the problem of backlash in the couplings between the actuator wing shafts and the bracket or tool holder, as the case may be, that the actuator moves.

Cumulatively, all of these new features contribute to an overall improvement in the precision with which the tool can be manipulated by the three-axes or "wrist" assembly.

For a better understanding of the invention, reference may be made to the following description of an exemplary construction, taken in conjunction with the figures of the accompanying drawings, in which: Figure lisa plan, parts of the assembly being broken away in cross section; Figure 2 is a side elevation portions being broken away in cross section substantially along the axis of the yaw actuator; Figure 3 is a plan of the yaw and roll units of the actuator, portions of the roll unit being shown in cross section taken substantially along the axis of the roll actuator; Figure 4 is a plan of one ofthetorque retention brackets and is exemplary of those used with all three actuators; Figure 5 is a side elevation of the bracket of Figure 4; Figure 6 is an end elevation of the bracket of Figures 4 and 5; and, Figure 7 is a fragmentary cross-section of the coupling used between the ports of the manifolds and the actuator ports, the view being taken generally along the line 7-7 in Figure 3 and in the direction of the arrows.

The three-axes actuator assembly shown in the drawings comprises a pitch unit designated generally by the letter "P", a yaw unit designated generally by the letter "Y", and a roll unit designated generally by the letter "R". The pitch unit includes an hydraulic rotary actuator 1 having end shafts la and 1b one at each end, that are supported indirectly within the inner races 71 and 73 of roller bearings.

The outer races 72 and 74 of the bearings are received in relatively large diameter holes in a pair of side plates 13 and 12 of a pitch unit bracket. The side plates 13 and 12 are connected to a combination cross piece and fluid supply return manifold 4 of the pitch unit by bolts 79 and 80. The manifold 4 has tapped holes (not shown) by which it can be mounted on the free end of an arm or on any other suitable support and is drilled with supply and return passages 200 and 202 to which the hoses of an hydraulic power unit are connected.A pitch servovalve PSV is mounted directly on the back face of the manifold 4 and communicates with the passages 200 and 202 through branch passages 204 and 206 and communicates with the supply and return ports of the actuator through passages, e.g. 208. The device for coupling the manifold to each actuator is described in greater detail below in connection with Figure 7 of the drawings.

The respective shaft end portions 1a and 1b of the pitch actuator 1 are affixed to and supported within the aforementioned bearings in the pitch bracket by couplings associated with a yaw bracket 11. This part of the yaw bracket is generally U-shaped in plan and includes holes concentric to the shaft portions 1a and 1b of the pitch actuatorthat receive the shaft portions and are aligned and coupled to the shaft portions by couplings, the details of which are apparent from the drawings, are largely a matter of engineering and, therefore, need not be described in detail. In general, one end of the shaft 1b of the pitch actuator is splined to the yaw bracket 11 and the other end portion la is firmly affixed to the yaw bracket 11 by a tapered sleeve 18 driven home by a retaining nut 21.Thus, the yaw bracket 11 is supported within the bearings of the pitch bracket and is affixed to the wing shaft of the pitch actuator 1 so that upon rotation of the wing shaft of the pitch actuator the yaw bracket 11 pivots with the shaft upwardly or downwardly within the bearings received by the pitch bracket. The load of the struc tureoutboard of the yaw bracket 11 is carried by the pitch bracket, while torque and motion are transmitted to the yaw bracket 11 by the wing shaft of the pitch actuator 1.

It is, of course, essential that the casing of the pitch actuator 1 be restrained against rotation by a structure that is substantially rigid with respect to rotation about the axis of the wing shaft of the actuator. On the other hand, such torque retention structures for the casing of the pitch actuator should not be such as to produce a side load in any direction radially of the shaft which would tend to produce a substantial eccentric load on the bearings that support the yaw bracket and the actuator. Each actuator is restrained against rotation about the axis of its shaft in response to the torque reaction forces by a pair of widethintorque retention bands, each of which is affixed between the casing and the mounting bracket for the actuator. In the case of the pitch unit casing of the actuator 1, it is restrained by bands 25.As shown in Figures 4 to 6 of the drawings, each band 25 is a thin flat strip of metal, such as a one-sixteenth inch thick sheet of Type 301 stainless steel, half hard, formed into a generally L-shape (Figure 6). The torque retention band 25 includes a relatively longer leg 25a, a shorter leg 25b and a curved offset portion 25c located between the two legs in a position where it lies between the casing of the actuator and the adjacent member of the bracket supporting the actuator.

The longer leg 25a of each torque retention band is clamped to the end wall of the actuator between plates 82 and 30. Locating pins 97 position each torque retention band in the proper location on the end wall and bolts 81 fasten it to the end wall. The shorter leg of each retention band is cramped to the adjacent manifold using a clamp plate 38 with threaded holes that receive bolts installed through the manif old.

Because of its substantial width and the orientation of the bracket such that the torque loads are acting substantially in the widthwise direction, the band offers substantial resistance to torque. On the other hand, forces acting radially, with respect to the axis of the actuator, are transmitted only by the curved offset portion 25c and that portion offers very little ability to transmit such radial forces. Accordingly, any misalignment or looseness in the tolerances between the actuator and the bracket do not result in any significant or meaningful side load on the actuator, and the bearings are not subjected to an eccentric load and will operate freely and smoothly rather than possibly diminishing the capability of the three-axis actuator of supporting and moving a design load smoothly and with precision.

The unit Y comprises an actuator 2, the external shaft portions of which are affixed by a spline coupling sleeve 17 at one end and a cone coupling 19 at the other end to a bracket 39 for the roll unit R. The bracket 39, in turn, is journalled in roller bearings with races 75 and 76, in end plates 14 and 214 of the yaw unit bracket 5. Torque retention bands 26 at each of the casing of the actuator restrain the casing of the actuator from rotation, but the bearings 75-76 permit the actuator to provide yaw motion to the roll unit, the bracket 39 and all components carried by the bracket 39. The torque retention bands 26 are substantially identical in structure and identical in principle to those shown in Figures 4 to 6 of the drawings. Fluid is supplied to and returned from the actuator through passages drilled through the bracket 5 and then plugged. The yaw servovalve YSV is mounted on an arm 5a of the bracket that extends laterally outwardly. Hydraulic fluid is delivered to and returned from the yaw unit th rough hoses leading from the hydraulic unit and connected to couplings connected to the arm 5a. Passages drilled in the arm 5a communicates with ports that open to the servovalve YSV. The actuator 2 is connected to ports leading from drilled passages in the bracket 5 by couplings, e.g. 33. Accordingly, the extension 5a and a plate Sb of the bracket 5 constitute a manifold, as well as portions of the bracket, for the yaw unit.

The roll unit R comprises a tool holder 8 that is affixed to one end portion 3a of the wing shaft of a roll rotaryactuator3 byan anti-backlash coupling that is described in detail below. The tool holder 8 is supported within a roller thrust bearing 77-78 supported within a transverse end plate 7 of a roll unit mounting bracket 222. The inboard end (i.e. the end toward the yaw unit) of the roll rotary actuator 3 is unsupported, but the casing of the actuator 3 is coupled to an upper manifold plate 6 of the roll unit mounting bracket 222 by torque retention bands 27 that are identical in principle and substantially identical in construction and manner of attachment to those shown in Figures 4-6 and described above.

The manifold plate 6 of the bracket 222 has a series of drilled and plugged holes that provide passages 224 and 226 leading from threaded coupling receiving holes in the upper surface of the plate for attachment of hoses and communication of fluid to and from the roll unit servovalve RSV.The servovalve RSV, in turn, is connected by passages 230 and 232 in the manifold plate to ports opening adjacent the underside of the manifold plate 6. As shown in Figure 7 a coupling tube 33 spans the gap between the manifold plate 6 and casing 3a ofthe actuator3 and connects the passage 230 with an inlet port3b of the actuator 3. O-rings 236 and 238 seal the coupling tube 33 to bores 240 and 242 milled in the mounting-plate 6 and the casing 3a, respectively.

The chambers on either side of the wing shaft of each of the three hydraulic rotary actuators of the assembly are coupled to the corresponding manifold plate by the coupling arrangement (i.e. coupling tubes 33) shown in Figure 7.

The tool holder 8 of the roll unit is connected to the wing shaft portion 3a by an anti-backlash coupling.

The shaft portion 3a is splined and receives a correspondingly internally splined sleeve 20. The splined portion of the sleeve 20 is split, and the inner end portion has an external conical surface 20a that diverges away from the casing of the rotor and outwardly from the shaft portion 3a. One or more coupling pins 101 are received in semi-cylindrical slots milled in the outer wall of the sleeve 20 and provide positive position index control in case the cone friction connection slips. A sleeve portion 8a of the tool holder 8 has an internal conical surface that matches the conical portion 20a of the sleeve 20 and has semi-cylindrical slots for reception of the coupling pins 101. A retainer-drive ring 51 bears against the outer end of the sleeve 20 and is connected to the shaft portion 3a by a bolt 86.When the bolt 86 is tightened down, it drives the ring 20 toward the rotor casing, while a spacer ring 23 keeps the tool holder 8 in a predetermined position axially ofthe shaft. The matching conical surfaces of the sleeve 20 and the sleeve portion 8a of the tool holder 8 force the split, splined portion of the sleeve 20 into tight fit with the splines on the shaft 3a, thereby eliminating any possible looseness in the spline connection and any backlash that might otherwise exist in the coupling between the shaft and the tool holder.

In the design shown in the accompanying drawings, each of the actuators 1 and 2 is connected to the bracket that supports the next actuator by a conventional splined coupling, but it would be possible for the shaft of each of the actuators of the three-axis assembly to be connected to the bracket for the next actuator or the tool holder by an anti-backlash coupling substantially identical in construction and identi cal in principle to the coupling between the roll actuator 3 and the tool holder 8. The adaptation of the anti-backlash coupling to all the actuators is a matter of engineering.

A number of details of the design and construction of the three-axis actuator assembly that are shown in the accompanying drawings have not been described in detail. Among them are the coupling of the shaft of each actuator to a potentiometer type position monitor, such a monitor 43 associated with the pitch unit P (see the upper portion of Figure 1).

Each bracket assembly includes various spacer rings, bearing retainers, oil seals, cover plates and like elements that are widely used in machine design. Such elements are illustrated in the drawings and will be readily recognized by those skilled in the art.

Thus the invention, as embodied in the structure shown in the drawings and described above, improves over presently known three-axis actuator assemblies by substantially eliminating side loads on the shafts of the actuators, substantially eliminating the effect of expansion and contraction of hoses and tubing and the fluid carried therein between the servovalves and the respective actuators, and eliminates backlash in the couplings between the actuators and the brackets which they support. The effect of these improvements on more certain, more reliable and more precise function of the actuator is cumulative.

Claims (7)

1. A three-axes actuator assembly providing motion in pitch, yaw and roll comprising first, second and third hydraulic actuators supported indirectly by first, second and third brackets in series relation in that order with their shafts oriented on three different axes, the first and second brackets having journals supporting the second and third brackets for rotation about the axes of the first and second actuators, respectively, the third bracket having a journal supporting a tool holder, the shafts of the first and second actuators being affixed to and supported by the second and third brackets, respectively, and the shaft of the third actuator being affixed to and supported by the tool holder, and at least one widethintorque retention band affixed between the casing of each actuator and the bracket for that actuatorto restrain the casing of the actuator against rotation about the axis of its shaft, each band including an offset segment located between the actuator and the bracket to accommodate by flexure tolerance variations without imparting any significant side loads to the actuator.

2. An actuator assembly according to claim 1, wherein each torque retention band is generally L-shaped in end profile, the offset being located generally at the juncture between legs of the "L", and wherein one leg is affixed to one end of the respective actuator casing and the other is affixed to a portion of the corresponding bracket laterally of the casing.

3. An actuator assembly according to claim 1 or claim 2, wherein there are two torque retention bands for each actuator, one connected to each end of the corresponding casing.

4. An actuator assembly according to any one of the preceding claims, wherein the bracket for each actuator includes a manifold plate, each manifold plate having a fluid supply passage and a fluid return passage, a servovalve affixed to each plate, orifices in each plate communicating the valve with the passages, ports opening from each plate in alignment with the ports of the corresponding actuator for communicating the valve with the actuator, and a coupling connecting the ports.

5. An actuator assembly according to claim 4, wherein each coupling is a tube received in and sealed to the ports by annular sealing elements.

6. An actuator assembly according to any one of the preceding claims, and further comprising an anti-backlash coupling assembly on the shaft of at least one of the actuators, the shaft having a splined portion and the coupling assembly including a sleeve surrounding the shaft and rotatable therewith within the associated journal, a collar having a portion adjacent the end nearest the casing of the actuator that is splined to the shaft and is split to enable deflection of segments thereof toward the shaft, at least part of the split portion being externally tapered outwardly with respect to the shaft and away from the casing, an internal tapered surface on the sleeve matching and engagingthetapered portion of the collar, a drive ring engaging a shoulder on the collar that faces away from the casing, and a drive boltthreaded into the shaft, bearing against the drive ring and adapted to be tightened to drive the collar toward the casing so that the tapered surface of the collar and the sleeve cam the segments of the split portion of the collar into tight backlash-free engagement with the splines of the actuator shaft.

7. An actuator assembly, substantially as described with reference to the accompanying drawings.