GB2166710A

GB2166710A - Gripping assembly

Info

Publication number: GB2166710A
Application number: GB08527373A
Authority: GB
Inventors: David Christopher Munday
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1984-11-07
Filing date: 1985-11-06
Publication date: 1986-05-14
Also published as: GB8527373D0; GB2166710B; GB8428105D0

Abstract

A gripping assembly for a robot arm comprises at least one finger assembly (which may be a parallel linkage 6 or a set of serially articulated finger elements for example) provided with a touch sensor 23 for generating a control signal, first and second drive means coupled to the finger assembly, the second drive means 24, 25 being relatively slow and powerful in comparison with the first drive means 14, 15 and control means 1 arranged to selectively activate the first and second drive means in dependence upon the control signal. Preferably the first drive means remains friction-locked when deactivated, so that it can be used to rapidly locate the finger assembly round an object and can then be deactivated whilst the second drive means is activated to firmly grip the object. <IMAGE>

Description

SPECIFICATION Gripping assembly The present invention relates to gripping assemblies, of the type adapted to manipulate objects in an automated production line for example.

Such gripping assemblies must rapidly locate and firmly grip the object to be manipulated. Accordingly, known gripping assemblies generally employ high-power, high-speed drive mechanisms and are therefore massive in construction and highly expensive.

The present invention stems from an appreciation of the fact that the functions of mechanically locating and then gripping an object may be advantageously performed by separate drive mechanisms, one of which is light and rapid, and the other of which is relatively slow and powerful.

Thus, according to the present invention, a gripping assembly comprises a finger assembly provided with touch- sensitive means for generating a control signal, first and second drive means coupled to said finger assembly, the second drive means being relatively slow and powerful in comparison with the first drive means, and control means arranged to switch driving power from the first to the second drive means when the touchsensitive means indicates that the finger assembly has touched an object to be manipulated.

The touch-sensitive means may for example be a mechanical switch, or a photodetector arranged to detect reflected light, mounted on a gripping element of the finger assembly. The word "touch" is to be taken to include within its scope "close proximity" in the context of the present invention.

The touch-sensitive means may function indirectly by monitoring the condition of the first drive means - for example where the first drive means comprises an electric motor the touch sensitive means may monitor the motor current or voltage.

Preferably the first drive means remains frictionlocked when the first drive means is de-activated.

Preferably the second drive means remains friction-locked when de-activated.

Preferably the control means and second drive means are arranged to control the gripping force exerted by the finger assembly.

Preferably the second drive means acts on the mounting of the first drive means.

Preferably the first and second drive means comprise separate sources of motive power - for example the first and second drive means may be powered by respective torque motors. Alternatively however a common source of motive power may be used in conjunction with means for switching motive power between respective drive trains.

Preferably the finger assembly comprises an articulated assembly of finger elements. The touch sensitive means may then be arranged in conjunction with the control means to adjust both the position and orientation of the finger elements before the main gripping force is applied by the second drive means.

Preferably means are provided for sensing the gripping force exerted by the second drive means.

The control means may incorporate a force feedback loop for controlling the gripping force exerted by the second drive means. The gripping assembly may be provided with a slip detector arranged in use to maintain the gripping force at a level sufficient to prevent an object manipulated by the gripping assembly from slipping. The slip detector may for example be an ultrasonic or optical sensor and may be mounted either on or near to the finger assembly.

The gripping assembly preferably comprises two or more circumferentially spaced, radially acting finger assemblies.

Preferably the control means comprises a microcomputer programmed to coordinate the movement of the or each finger assembly so as to manipulate a gripped object in predetermined ways in response to appropriate commands.

The or each finger assembly may comprise a parallel-type linkage, one link of which constitutes the outermost finger element, the opposite link of which is a swinging link rotatably driven about a supporting pivot of the finger assembly by the first drive means so as to vary the orientation of said finger element, and another link of which is also pivotally supported and rotatably driven about said supporting pivot by the first drive means, so as to vary the position of the outermost finger element.

The first drive means may be mounted for rotation about said supporting pivot and acted upon by the second drive means to cause the outermost finger element to exert a gripping force. The second drive means preferably comprises a linear actuator (preferably screw driven) which acts upon the first drive means, via a sliding cam or a toggle link for example. Alternatively the second drive means may be a rotary actuator and may act on the first drive means via a rotary cam, for example. It will be appreciated that only a very small travel will generally be required of the second drive means, since in use the finger elements will be positioned in advance by the first drive means.The first drive means may comprise a pair of screw-driven linear actuators on a common mounting which act respectively on the swinging link and the other link of said linkage which is pivoted about said supporting pivot. The position of the gripping element may be monitored by position sensors (such as linear potentiometers for example) directly coupled to the two actuators of the first drive means.

The or each finger assembly may comprise two or more (preferably three) serially articulated finger elements, the innermost finger element being pivotally supported from the body of the gripping assembly, the finger elements being rotatably driven (preferably independently) about their articulated joints by the first drive means. Thus the first drive means may comprise a plurality of linear actuators, each coupled between a respective pair of adjacent finger elements of the finger assembly. The linear actuators may be screw driven and may be driven directly or via rotary drive cables by motors mounted on or near the body of the gripping assembly. The positions of the finger elements may be monitored by monitoring the relative orientations of the intermediate and outermost finger elements of the finger assembly, by means of potentiometers or example.

Two embodiments of the invention will now be described, by way of example, with reference to Figures 1 to 5 of the accompanying drawings, of which: Figure 1 is a side elevation, partly in section, of one gripping assembly in accordance with the invention; Figure 2 is an end elevation, partly in section, taken in the direction II on Figure 1; Figure 3 is a side elevation, partly in section, showing in more detail the driving arrangement for the finger assembly of Figure 1; Figure 4 is a side elevation, partly in section, of another gripping assembly in accordance with the invention, and Figure 5 is an end elevation, partly in section, taken in the direction V on Figure 4.

Referring to Figures 1 and 2, the gripping assembly shown is controlled from a microcomputer 1 and comprises three radially-acting finger assem blies (6, 8) located in respective circumferentiallyspaced mountings 2, 3 and 4 on a supporting bracket 5 (Figure 2). For the sake of clarity, only one finger assembly is shown, and comprises a parallel linkage 6 which is pivoted at a fixed supporting pivot 7. The outermost link of linkage 6 constitutes a finger element 8 and the opposite link is a generally triangular swinging link 9, which controls the orientation of element 8. The other link, 10, which is pivoted at pivot 7, constitutes a further finger element and is provided with an offset driving pivot 11 so as to control the radial position of finger element 8, as shown by the chaindotted lines.

A mounting comprising a pair of linked, generally triangular, parallel side plates 12, 13 is pivoted at. support pivot 7 and a high-speed driving assembly comprising two screw-driven linear actuators 14, 15 is supported on this mounting. Actuator 14 (which is powered by a D.C. torque motor 16) is pivotally mounted between side plates 12, 13 at a pivot 17 and is connected to swinging link 9 at a pivot 19 to control the orientation of gripping element 8. The other actuator 15 is similarly mounted between side plates 12, 13 at a pivot 20, is powered by a D.C. torque motor 18 and is connected by an offset pivot 11 to link 10 to control the position of gripping element 8. The instantaneous extension of actuators 14 and 15 are monitored by linear potentiometers 21, 21' (to which they are connected by links 22).Actuators 14 and 15 are controlled via controller 1 in response to a signal from touch sensors 23 (which may be microswitches for example) mounted on the gripping faces of the finger elements.

The high-speed driving assembly described above (which is pivotally mounted on supporting pivot 7 in common with finger assembly 6, 8) is itself acted upon by means of a sliding cam 24 driven by a relatively powerful, slow acting screwdriven linear actuator 25. Cam 24 runs in inclined slots in side plates 12, 13 and serves to tilt the entire high -speed driving assembly and finger assembly (6, 8) about pivot 7 to cause finger elements 8 and 10 to exert a gripping force on any object which they touch. Actuator 25 is driven by a high-power D.C. torque motor 27 via a flexible coupling 28 and a screw 29. The instantaneous extension of actuator 25 is monitored by a linear potentiometer 30 (to which it is connected by link 31) and the gripping force is monitored by a pressure sensor 32 (which may be a piezo-electric element for example).A vibration sensor 33 is provided on gripping element 8 and acts as a slip sensor.

The operation of the above gripping assembly is as follows. In response to an appropriate command (which may be generated in a program in controller 1, entered manualiy in controller 1 or generated by an external process) actuators 14 % 15 move the finger elements into contact with the surface of the object (not shown) until de-activated by controller 1 in response to signals from touch sensors 23 mounted on the gripping faces of the finger elements. The number and position of the contact points on the object will depend on the object shape. The other two finger assemblies (not shown) are similarly controlled. Actuator 25 is then actuated and controlled by pressure sensor 32 to exert a controlled gripping force on the gripped object. The object may then be manipulated in accordance with a program loaded in controller 1 or separate controller.The position potentiometer 30 provides controller 1 with information on the actuator position so as not to allow the actuator to over travel. The gripping force is increased in response to any signals received from slip sensor 33.

Figure 3 shows in more detail the high-speed drive assembly of Figures 1 and 2, some parts having been omitted for the sake of clarity. Each linear actuator 14, 15 is provided with a piston 35 driven by a drive screw 34, the pitch of the drive screws being such that they lock in their respective pistons when their associated torque motors are switched off and a reaction is generated at the surface of the gripped object and transmitted to them by linkage 6. This reaction is absorbed by a pair of thrust bearings 36 disposed on either side of a supporting washer 37. Any lateral misalignment between the motor and the actuator is absorbed by a flexible coupling 38.

The gripping assembly shown in Figure 4 comprises four radially acting finger assemblies 6 (only one of which is shown) spaced at 90" intervals around the surface of a supporting plate 5. Each finger assembly comprises three links 8, 39, 40 supported from a pivot 41 in a pair of linked mounting plates 12', 13' and serially articulated at intermediate pivots 42 and 43. Each link of the finger assembly is independently driven by an associated D.C. motor 44, 45 or 46 and pivoted screwdriven linear actuator 47, 48, 49. Motors 45 and 46 are free to slide axially in keyed supports 51 and 50 respectively and drive their associated actuators by Bowden cables 52. Keyed support 51 is pivoted from plate 12 at pivot 53 and motor 44 (which is rigidly connected to its associated actuator 47) is similarly pivoted at pivot 54. The above driving assembly is relatively light and quick acting, and serves to locate the finger elements around the surface of an object (not shown) to be manipulated in dependence upon the outputs of sensors 23.

Further position sensors (not shown) mounted on the Inear actuators 47, 48, 49 enable the position of gripping element 8 to be calculated by the controller 1. The required gripping force is generated by a high-power D.C. motor 64 which tilts the finger assembly and associated drive about pivot 44 via a rotary cam system, 55, which is supported from mount 56. Motor 64 is controlled in dependence upon a signal from pressure transducer 32 and its angular position is monitored by a potentiometer 65. It will be appreciated that the operation of the above gripping assembly will be generally similar to that described above with reference to Figures 1 to 3.

Claims

1. A gripping assembly comprising a finger assembly provided with touch-sensitive means for generating a control signal, first and second drive means coupled to said finger assembly, the second drive means being relatively slow and powerful in comparison with the first drive means, and control means arranged to switch driving power from the first to the second drive means when the touchsensitive means indicates that the finger assembly has touched an object to be manipulated.

2. A gripping assembly according to Claim 1, wherein said touch-sensitive means is a mechanical switch mounted on a gripping element of the finger assembly.

3. A gripping assembly according to Claim 1, wherein said touch-sensitive means is a photodetector arranged to detect reflected light and mounted on a gripping element of the finger assembly.

4. A gripping assembly according to Claim 1, wherein said touch-sensitive means functions indirectly by monitoring the condition of the first drive means.

5. A gripping assembly according to Claim 4, wherein the first drive means comprises an electric motor and the touch-sensitive means is arranged to monitor the motor current or voltage.

6. A gripping assembly according to any preceding claim, wherein the first drive means is arranged to remain friction-locked when the first drive means is de-activated.

7. A gripping assembly according to any preceding claim, wherein the second drive means remains friction-locked when de-activated.

8. A gripping assembly according to any preceding claim, wherein the first and second drive means comprise separate sources of motive power.

9. A gripping assembly according to any of Claims 1 to 7, said first and second drive means comprise a common source of motive power in conjunction with means for switching motive power between respective drive trains.

10. A gripping assembly according to any preceding claim, wherein said finger assembly comprises an articulated assembly of finger elements.

11. A gripping assembly according to Claim 10, wherein the touch-sensitive means is arranged in conjunction with the control means to adjust both the position and orientation of the finger elements before the main gripping force is applied by the second drive means.

12. A gripping assembly according to any preceding claim, wherein means are provided for sensing the gripping force exerted by the second drive means.

13. A gripping assembly according to Claim 12 wherein the control means incorporates a force feedback loop for controlling the gripping force exerted by the second drive means.

14. A gripping assembly according to Claim 13, wherein the gripping assembly is provided with a slip detector arranged in use to maintain the gripping force at a level sufficient to prevent an object manipulated by the gripping assembly from slipping.

15. A gripping assembly according to Claim 14, wherein the slip detector is an ultrasonic or optical sensor and is mounted either on or near to the finger assembly.

16. A gripping assembly according to any preceding claim wherein the or each finger assembly comprises a parallel-type linkage, one link of which constitutes the outermost finger element, the opposite link of which is a swinging link rotatably driven about a supporting pivot of the finger assembly by the first drive means so as to vary the orientation of said outermost finger element, and another link of which is also pivotally supported and rotatably driven about said supporting pivot by the first drive means, so as to vary the position of the outermost finger element.

17. A gripping assembly according to Claim 16 wherein the first drive means is mounted for rotation about the supporting pivot and acted upon by the second drive means to cause the outermost finger element to exert a gripping force.

18. A gripping assembly according to Claim 16 wherein the second drive means comprises a linear actuator which acts upon the first drive means, via a sliding cam or a toggle link.

19. A gripping assembly according to Claim 16 wherein the second drive means is a rotary actuator and acts on the first drive means via a rotary cam.

20. A gripping assembly according to any of Claims 16 to 19 wherein the first drive means comprises a pair of screw-driven linear actuators on a common mounting which act respectively on said swinging link and said other link of said linkage which is pivoted about said supporting pivot.

21. A gripping assembly according to Claim 20 wherein the position of the outermost finger element is monitored by position sensors directly coupled to said linear actuators of the first drive means.

22. A gripping assembly according to Claim 1, wherein the or each finger assembly comprises two or more serially articulated finger elements, the innermost finger element being pivotally supported from the body of the gripping assembly and the finger elements being rotatable driven about their articulated joints by the first drive means.

23. A gripping assembly according to Claim 22, wherein the first drive means comprises a plurality of linear actuators, each coupled between a respective pair of adjacent finger elements of the finger assembly.

24. A gripping assembly according to Claim 23, wherein the linear actuators are screw driven and are driven directly or via rotary drive cables by motors mounted on or near the body of the gripping assembly.

25. A gripping assembly substantially as hereinbefore described with reference to Figures 1, 2 % 3 or Figures 4 % 5 of the accompanying drawings.