GB2218963A - Compliant link and positioning device for tool on robot arm - Google Patents

Abstract

A mechanism for coupling a tool G to a robot arm A and for aligning the tool orthogonally with respect to a curved or even double curved work surface S. There is a first compliant arrangement C connected to the robot arm and a second compliant arrangement 20 connected between the first and the tool. The first allows linear displacement of the arm with respect to the tool, e.g. by mutual sliding of cylinders 10, 11 against the bias of spring 14. The second has multiple contact probes 60 extending parallel to the tool allowing relative displacement of the tool until all probes 60 and the tool are in contact with the work surface. This aligns the tool orthogonally to the surface. It is preferred that three probes are provided and there may be a lock 40 to prevent relative displacement of arrangements C, 20, except when one probe and the tool are contacting the surface. Second arrangement 20 is a gimbal device allowing linear and/or rotational movement along or about one or two mutually orthogonal axes. <IMAGE>

Description

COUPLING MECHANISM This invention relates to a mechanism for coupling a tool, for example drill, to a robot arm, and for aligning the tool orthogonally with respect to a curved work surface.

Particularly in the aerospace industry, but also in other areas as well, there is a need to provide an industrial robot capable of carrying out a drilling task (especially for subsequent rivetting etc.) on double curvature panels and solids in which the hole must be drilled normal to the surface.

Because of the manufacturing tolerances of the panels and the limited precision of the robot, it is not usually possible for these operations to be accomplished by dead reckoning.

Therefore, it is usual to provide seme form of sensor in order to allow the robot to adapt to the curvature of the work surface. Conventionally, this may take the form of an arrangement of three linear displacement transducers (or something similar). The robot brings the drill into contact with the panel, and then the orientation of the end effector is adjusted until the outputs of the three transducers are equal.

At this point, the drill must be truly normal to the surface.

This arrangement does work, but is slow in operation and requires the use of relatively expensive transducers. It is therefore generally not cost-effective, particularly for the aerospace industry which has requirements for large numbers of drilling operations.

It is therefore an object of the present invention to provide a mechanism which at least alleviates the above-mentioned problem.

Accordingly there is provided a mechanism for coupling a tool, for example a drill, to a robot arm and being effective to align said tool orthogonally with respect to a curved work surface, the mechanism comprising first compliant means for connection to the robot arm and second compliant means for coupling the tool to said first compliant means, wherein said first compliant means has a longitudinal axis and is effective, in use, to allow displacement, along said longitudinal axis, of the arm with respect to the-tool, said second compliant means has at least two elongate contact probes which extend parallel to a said tool, attached to the second compliant means, and is arranged to allow a further degree of relative displacement provided any one of the contact probes, in addition to the tool, is in contact with a work surface, whereby all said contact probes may be brought into contact with said work surface thereby aligning the tool orthogonally with respect to that region of the work surface with which the tool is in contact.

Preferably there are three such contact probes arranged symmetrically around said tool.

Said further degree of displacement, provided by the second compliant means may consist of displacement in a linear direction along and or a rotational direction about first and or second mutually orthogonal further axes.

The second compliant means may be associated with locking means arranged to prohibit said further degree of freedom unless said any one of the contact probes, in addition to the tool, is in contact with the work surface, and the locking means may be responsive to a contact-indicating signal generated by contact sensors provided at the ends of said elongate contact probes.

In order that the invention may be carried readily into effect an embodiment thereof is now described, by way of example only, by reference to the accompanying drawings of which, Figures la and lb show longitudinal, cross-sectional views of a coupling mechanism in mutually orthogonal planes containing axes XX and YY respectively, and Figure 2 shows, a longitudinal, cross-sectional view corresponding to that of Figure lb, but in an unlocked condition.

The invention described hereinafter by reference to the accompanying drawings is based on a compliant coupling device as described in our European Patent No. 191246.

Accordingly, the present invention exhibits many of the attributes of that device, and, in particular, allows displacement of a robot arm witb respect to a tool, attached to the arm, when the tool is in contact with a work surface. In addition, the present invention provides the facility to align the tool orthogonally with respect to a curved work surface, and the invention is especially, though not exclusively, adapted to drilling tasks which need to be carried out on double curvature panels or solid panels as encountered, for example, in the aerospace industry.

Referring to Figures la and Ib, the mechanism, shown generally at C, connects a robot arm, represented at A, to a tool which is represented at G, and which in this example is a drill, for example a self-feeding drill. Both the arm and the drill may be of conventional form.

The mechanism C embodies a compliant arrangement which is so configured as to allow displacement of the tool, with respect to the arm, along a longitudinal axis shown at ZZ. Such displacement may occur in response to a force acting on the tool when it is brought into contact with a work surface, shown generally at S. As will be described in greater detail hereinafter, the mechanism is so arranged as to provide additional compliance, allowing displacement of the tool in a linear direction along, and or a rotational direction about, two other axes XX and YY which, in this example, are arranged orthogonally in relation to one another, and to axis ZZ. As will be described in greater detail hereinafter, a locking device 40 is provided to release this additional compliance, when required.

The coupling mechanism, shown in Figures 1, includes a piston arrangement 10 connected to the arm and a gimbal arrangement 20 coupled, via a support block 30, to the tool.

The gimbal arrangement is provided with three, elongate contact probes (of which only one, 60, is shown in the Figures) arranged parallel to, and symmetrically around, the longitudinal axis of the tool.

The piston arrangement permits displacement, along axis ZZ, of the arm with respect to the support block, whereas the gimbal arrangement 20 allows additional displacement in a linear direction along and/or a rotational direction about axes XX and YY, provided that, in addition to the tool, any one of the contact probes 60 is in contact with the work surface. To that end, each contact probe has, at its tip, a contact sensor 61 (of conventional kind) and locking device 40 which is arranged to respond to a contact-indicating signal generated by any of the contact sensors. Until such contact is sensed locking device 40 maintains the gimbal arrangement in a locked condition (as shown in figures la and lb) so that the additional displacement with respect to axes XX and YY cannot take place.If, however, contact is sensed the locking device frees the gimbal arrangement allowing additional displacement along, and or about, the XX and YY axis directions.

In operation, as arm A approaches work surface S the drill tip makes contact therewith, contact being detected by means of a sensor, shown generally at 50 in Figure lb, which is responsive to relative displacement, along axis ZZ, of the arm and tool. Sensor 50 is described in greater detail in the afore-mentioned European Patent No. 191246. Upon slight, continued inwards movement of the arm one of the three contact probes 60 will also come into contact with the work surface and this is detected by a respective sensor 61 which causes the locking device 40 to free the gimbal arrangement.The mechanism is then free to adapt to the curvature of the work surface, whereby each of the three contact probes 60 is brought into contact with the work surface, thereby aligning the tool orthogonally with respect to that region of the work surface with which the tool is in contact, as illustrated in Figure 2.

The drill tip will normally be provided with a friction surface or pad which will ensure that the actual contact point of the drill does not move relative to the work surface, thereby maintaining the dimensional location of the hole to be drilled.

Referring in more detail to the drawings, piston arrangement 10 comprises a cylindrical plunger 11 which can move on axis ZZ within a close fitting cylindrical sleeve 12 connected at P to the arm. The plunger and sleeve are coupled together resiliently by a compression spring 13. which is connected, as shown, to a flange 14 mounted at the open end of the plunger. If the tool experiences a force directed along axis ZZ the plunger will be displaced with respect to the sleeve causing the spring 13 to be compressed.

The gimbal arrangement 20 comprises an inner, generally annular frame member 21 supported by an outer frame member 22, which is formed integrally with plunger 11. Support block 30 is mounted fixedly on a shaft 23 which is supported in bearings by the inner frame member 21. In the unlocked condition of the gimbal arrangement, the shaft 23 is free to slide and/or turn in the bearings so that the support block may be displaced in a linear direction along and/or a rotational direction about the longitudinal axis XX of the shaft as illustrated in Figure 2.

Similarly, the inner frame member is mounted fixedly on a pair of shafts 24, 25 supported freely, in bearings, by the outer frame member 22. Again, in the unlocked condition, the inner frame member, and so the support block, may be displaced in a linear direction along, and/or a rotation direction about, the common longitudinal axis YY of shafts 24, '25. The inner and outer frame members are provided with respective collars 26, 26' which provide a sufficient degree of frictional resistance to reduce any tendency for the shafts to move irregularly in their respective bearings. The locking device 40 includes a locking plate 41 connected to an additional piston member 42 which can move slidingly within the plunger 11.A coil spring 43, acting between the piston member and flange 14, as shown, urges the locking plate against a complementary surface 31 of support block 30 and holds the gimbal arrangement in the locked condition. A groove 44 and a complementary projection 32, formed respectively in plate 41 and on surface 31, maintain the plate and support block in centralised, interlocked relationship.

The piston member 42 defines a closed, substantially air-tight cavity 45 within plunger 11. This cavity is coupled, via a bore 46, to a source (not shown) of compressed air which can be actuated in response to a contact-indication signal generated by any one of the contact sensors 61. In these circumstances, the air pressure inside the cavity is caused to rise forcing the piston member back against the action of spring 43 and so lifting the locking plate away from the support block 30, thereby releasing the gimbal arrangement.

Another sensor 50, shown in Figure lb, is arranged to monitor displacement of the gripper on axis ZZ and to generate a control signal indicating that the tool has been brought into contact with the work surface, In the illustrated embodiment, the sensor 50 comprises a LED Light source and an associated photo sensitive detector which are both mounted on the outer frame member. If the gripper is displaced' with respect to the arm, the sensor will approach a probe 51 mounted on housing 12, as shown. The probe can be so positioned in relation to the sensor as to interrupt the light beam produced by the LED.

With a mechanism as described, the displacements involved are relatively small (generally less than 1 mm) and so the mechanism relies for successful operation on a prior knowledge of the geometry of the work surface so that the robot arm may bring the tool into approximate alignment with the work surface.

Claims (5)

1. A mechanism for coupling a tool, for example a drill, to a robot arm and being effective to align said tool orthogonally with respect to a curved work surface, the mechanism comprising first compliant means for connection to the robot arm and second compliant means for coupling the tool to said first compliant means, wherein said first compliant means has a longitudinal axis and is effective, in use, to allow displacement, along said longitudinal axis, of the arm with respect to the tool, said second compliant means has at least two elongate contact probes which extend parallel to a said tool, attached to the second compliant means, and is arranged to allow a further degree of relative displacement provided any one of the contact probes, in addition to the tool, is in contact with a work surface, whereby all said contact probes may be brought into contact with said work surface thereby aligning the tool orthogonally with respect to that region of the work surface with which the tool is in contact.

2. A mechanism according to Claim 1 in which there are three said elongate contact probes extending parallel to said tool.

3. A mechanism according to either Claim 1 or Claim 2 in which said further degree of relative displacement provided by the second compliant means consist of displacement in a linear direction or a rotational displacement or both, along or about first or second or both of mutually orthogonal further axes.

4. A mechanism according to any preceding claim in which the second compliant means is associated with locking means arranged to prohibit said further degree of freedom unless said any one of the contact probes in addition to the tool, is in contact with the work surface, and the locking means is responsive to a contact-indicating signal generated by contact sensors provided at the ends of said elongate contact probes.

5. A mechanism for coupling a tool to a robot arm, the mechanism being substantially as herein described with reference to the accompanying drawings.