GB2132274A - Rotary hydraulic actuator - Google Patents

Abstract

An actuator 1, 6, or 8 in or for a robot arm has a rotary-position sensing means e.g. a potentiometer 46, producing a feedback signal for a motor-control means, the sensing means being operated by way of an internal torsionally-rigid and transversely-flexible shaft e.g. a piece of piano wire 31, to obviate mechanical damage owing to axial misalignment. The actuator may also have a piston vane 13 secured to a drive shaft 12, located thereon by radially-extending means e.g. a peg 27, and furnished with a radial seal(s) 21. <IMAGE>

Description

SPECIFICATION Robot wrist assembly The present invention relates to hydraulic robot arms and particularly to hydraulic wrist assemblies thereof. By robot is meant any programmed or remotely controlled machine capable of performing mechanical operations. Such arrangements are rapidly being brought into use for welding, paint spraying and similar operations and are generally controlled by a microprocessor via a servo loop. In this manner intricate welding and paint spraying operations can be carried out relatively rapidly and reliably even in highly unpleasant environments. However it has been found difficult to construct robot arms capable of reaching relatively inaccessible parts of confined spaces (for example car bodies) which arms are reasonably simple in construction and reliable in use.

An object of the present invention is to provide such a robot arm.

According to a first aspect of the present invention, a rotary hydraulic torque motor for a robot arm is provided with rotary position feedback means for supplying a motor control signal, the rotary axis of said feedback means being nominally aligned with the axis of said torque motor, wherein said feedback means is connected to said torque motor by a control shaft housed within said motor, which control shaft constitutes a rotationally rigid driving connection which is sufficiently flexible in the transverse direction to accommodate any slight misalignment of said axes. Preferably the torque motor incorporates a drive shaft which is hollow along at least part of its length and accommodates a substantial part of the length of said control shaft. The rotary position feedback means may suitably be a high precision rotary potentiometer.

Hitherto, position feedback potentiometers for hydraulic vane motors have generally been connected to the drive shaft by a belt drive, which takes up any misalignment of the potentiometer and drive shaft axes and prevents the potentiometer from being unduly strained, although direct connection is possibly when components of very close mechanical tolerance are used. Belt drives need to be protected particularly when the robot arm is used for paint spraying and similar operations, and the resulting arrangements can inhibit manoeuvrability. We have found that a thin flexible shaft, for example of piano wire, approximately 50 mm long and 1 mm in diameter, is sufficiently rigid rotationally to drive a potentiometer without backlash and sufficiently flexible transversely to accommodate considerable transverse or angular misalignment.Thus by using an internal flexible shaft in accordance with this aspect of the invention, one of the constraints on the allowable dimensional tolerances of the components is removed. This advantage may be enhanced and utilised in accordance with a second aspect of the invention, according to which a rotary hydraulic torque motor for a joint in a robot arm comprises a drive shaft and a part cylindrical piston vane mounted on said shaft for rotation in a rotary hydraulic cylinder, wherein said piston vane is a discrete member located on said shaft by radially extending positive location means and is sealed against leakage in the circumferential direction of working fluid by at least one radially acting resilient seal. Said seal may be located between the drive shaft and vane or between the vane and cylinder wall or both.

Preferably said seal is in the form of a ring which completely surrounds the vane in a substantially axial plane. Said positive location means may comprise a dowel or peg which engages a recess in the vane or shaft. In comparison with prior art rotary hydraulic torque motors, in which the vane and drive shaft have been made in one piece (necessitating a considerable amount of accurate machining) such constructions have considerable advantages. In particular, machining difficulties are reduced, and the use of a vane in conjunction with a ring seal (which tightly grips the vane) reduces leakage in comparison with constructions employing "C" seals. Slight radiai machining tolerances in the vane (of the order of 0.01 mm) can be accommodated by the seal or seals.By using a sliding fit peg (and particularly a sliding fit discrete dowel member) for connecting the vane and drive shaft the vane is allowed a slight axial freedom of movement over its travel, so that any slight deviations from flatness of the surfaces of the rotary cylinder in which the vane rotates can be accommodated.

A preferred embodiment of the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying drawings, of which: Figure 1 is an elevation of a three jointed robot wrist in accordance with the invention, and Figure 2 is a part sectional view on Il-Il of Figure 1.

Similar parts are designated by like references, but it should be understood that the orientations of the components are not necessarily completely consistent between the two Figures.

Referring to the drawings, the wrist assembly shown comprises a "rotate" joint assembly 1, a "tilt" joint assembly 6 and a "fan" joint assembly 8, each provided with a hollow drive shaft 1 2. The casing of "fan" joint assembly 8 is connected to the robot arm at holes 22. The shaft 12 of joint assembly 28 carries joint assembly 6, whose shaft 12 in turn carries the "rotate" joint assembly 1 from bracket 4. A welding head, spray nozzle or other implement may be attached to shaft 12 of joint assembly 1. The axis of joint assembly intersects the axes of joint assemblies 6 and 8.

Each drive shaft 12 is splined at one end 25 where it is gripped by a split bracket 4 in conjunction with a clamping bolt 7. Alternatively the brackets 4 may be secured to the shafts 12 by tapered locking hubs. The body of each joint assembly 1, 6 and 8 comprises a central rotary cylinder section 11 clamped between end caps 9 and 10 by bolts 59. Each joint assembly body is axially located by the stepped central section of its associated drive shaft 12 and a bearing ring 1 7 fixed to a bracket 5, and aligned by an annular support 1 6 which runs on the bearing 17 and is bolted to an end cap by bolts such as 63.

A rotary piston vane 13 is attached by a dowel 27 to each shaft 12 and with a stop post 14 defines a pair of opposed cylinder chambers 43 and 44 which communicate with respective hydraulic supply pipes 38. It will be appreciated that the invention includes within its scope arrangements in which part 14 is attached to cylinder wall 11 by a radially extending peg 27, so that cylinder wall 11 constitutes a hollow drive shaft and part 14 constitutes a piston. It will furthermore be appreciated that an integral projection (not necessarily of circular crosssection) from the vane or shaft may be fitted to the shaft or vane as an alternative to the dowel connection.In use, high pressure hydraulic fluid (typically at 2 x 107 N/m2) is supplied to the pipes 38 and relatively rotates the shaft 12 and joint assembly body to a desired angular position, as will subsequently be described in more detail.

Circumferential leakage of the hydraulic fluid past the piston vane 1 3 is prevented by a recessed resilient ring seal 21 which is fitted tightly round the vane in an axial plane. The radial tolerances in the vane 1 3 and cylinder section 11 may be typically 0.02 mm for a cylinder radius of 25 mm.

A recessed resilient ring seal 1 8 of bronzeloaded p.t.f.e. seals the stop post 14 (which is fixed to the end caps by a dowel 28) in a similar manner to seal 21. Annular seals 23 which abut the seal 21 minimise circumferential leakage 5 between cylinder spaces 43 and 44. The dowel 27 is a sliding fit in the recesses in the vane 13 and shaft 12, and allows the vane to "float" very slightly on the seal 21 as it rotates, thereby absorbing machining tolerances in the opposed interior faces of the end caps 9 and 10. Annular seals 53 prevent leakage between the end caps and cylinder section 11, and two dual seals 54 prevent leakage past the shaft 12.Central annular leakage channels 57 communicate with a drain pipe (not shown) which returns any leaking fluid to a iow pressure hydraulic reservoir from which it is pumped to the high pressure hydraulic supply network (not shown). In this manner the outer seal of each pair 54 is isolated from high pressure leaking fluid.

The angular position of each joint assembly 1, 6 and 8 is controlled by a closed servo loop (not shown), which controls the flow of hydraulic fluid to chambers 43 and 44 in response to its angular position as measured by the resistance of an axially located precision potentiometer 46. The body of each potentiometer 46 is attached to the body portion of its associated joint assembly via the support ring 16. A length of piano wire 31 (approximately 50 mm long and 1 mm in diameter) connected at one end to a pan head screw 1 5 and at the other end to a connector 45 forms a backlash-free control shaft between the drive shaft 12 and potentiometer spindle 26.

Owing to its flexibility, the piano wire need not be aligned precisely on the axis of the shaft 12, which axis need not be aligned precisely with the spindle 26.

The invention includes within its scope robots and robot wrists in accordance with either the first or the second aspect of the invention.

Claims (16)

1. A rotary hydraulic torque motor for a joint in a robot arm, said motor being provided with rotary position feedback means for supplying a motor control signal, the rotary axis of said feedback means being nominally aligned with the axis of said torque motor, wherein said feedback means is connected to said torque motor by a control shaft housed within said motor, which control shaft constitutes a rotationally rigid driving connection which is sufficiently flexible in the transverse direction to accommodate any slight misalignment of said axes.

2. A torque motor as claimed in Claim 1 wherein said torque motor incorporates a drive shaft which is hollow along at least part of its length and accommodates a substantial part of the length of said control shaft.

3. A torque motor according to Claim 1 or Claim 2 wherein said feedback means is a potentiometer.

4. A torque motor according to any preceding Claim wherein said control shaft is a steel wire.

5. A torque motor according to Claim 4 as dependent on Claim 3 wherein said steel wire has a length approximately 50 times its thickness.

6. A torque motor as claimed in any preceding Claim wherein the body of said feedback means is attached to the body of the torque motor and the end of said control shaft distal from said feedback means is connected to the drive shaft of the torque motor.

7. A robot or a robot wrist incorporating a torque motor as claimed in any preceding Claim.

8. A robot or robot wrist as claimed in Claim 7 wherein said feedback means is incorporated in a control loop.

9. A robot wrist incorporating a torque motor as claimed in any of Claims 1 to 6 and substantially as described hereinabove with reference to Figures 1 and 2 of the accompanying drawings.

10. A rotary hydraulic torque motor for a joint in a robot arm, said torque motor comprising a drive shaft and a part cylindrical piston vane mounted on said shaft for rotation in a rotary hydraulic cylinder, wherein said piston vane is a discrete member located on said shaft by radially extending positive location means and is sealed against leakage in the circumferential direction of working fluid by at least one radially acting resilient seal.

11. A torque motor as claimed in Claim 10, wherein said seal is a ring seal which surrounds said vane in a substantially axial plane.

12. A torque motor as claimed in Claim 10 or Claim 11 wherein said means comprises a discrete peg or dowel member which is fitted into registering holes in said piston vane and shaft.

13. A torque motor as claimed in Claim 12 wherein said peg is a sufficiently loose fit for said piston vane to move in an axial plane relative to the drive shaft during its rotary travel and thereby accommodate any slight deviations from flatness of the surfaces of the rotary cylinder in which it rotates.

14. A torque motor as claimed in any of Claims 11 to 13 wherein at least one said seal is composed of bronze-loaded P.T.F.E.

1 5. A torque motor as claimed in any of Claims 10 to 14 comprising a central hollow cylindrical member held between two end caps which together define said rotary hydraulic cylinder in which said piston vane rotates.

16. A robot or a robot wrist incorporating a torque motor as claimed in any of Claims 10 to 1 5.

1 7. A robot wrist incorporating a torque motor as claimed in any of Claims 10 to 1 5 and substantially as described hereinabove with reference to Figures 1 and 2 of the accompanying drawings.