The present invention relates to a mechanism and method of operating the mechanism. The mechanism is intended particularly, but not exclusively, as an end-effector or a "wrist" for use on a robot arm or other automatic process-tool support. The invention has particular application in the field of welding.

Standard welding process (e.g. welding) robots and automated process (e.g. welding) machines follow a predetermined path set by software or by hardware respectively. The tool (e.g. a Weld Torch) is held in a fixed position with respect to the tool-holder of the machine, so that there is no possibility of adjusting the position of the tool with respect to the workpiece except by manual intervention. In practice however the workpiece is subject to movement, to distortion and to changes in size and fit-up as consequences of changes in the ordinary process parameters, e.g. heating, wear and handling and of the planned and unplanned tolerances in producing piece parts. In a wide variety of process work the potential combined extent of these variations and movements are of the same order of size as the acceptable tolerances in the dimensions of the process action, e.g. weld width, to ensure acceptable quality. There is therefore a value in

providing such automated and robot process machines with the facility to detect and make automatic adjustment for these variations. The invention is designed in one emodiment to provide this facility in a particularly practical and effective manner.

The invention makes use of a limited form of sensor, e.g. vision system using light sensitive solid state electronic devices and a system of analysis undertaken by dedicated computers. There are known to the applicants a number of possible method of detecting, with varying degrees of success, the absolute or relative position of the intended process path.

Some of these methods may be based upσn measurement of process (e.g. eld) parameter variations, or upon ^' tactile sensing of process track (e.g. weld groove location), or upon eddy current detection or upon visual and/or optical sensing methods.

In one of the latter systems a modified process tool is used with both a laser projector and a solid state camera. The projector projects from a position adjacent to the tool centre line, a sheet of laser light angled towards the camera centre line and falling transversely across the process path. When at the correct relative position, and stand-off from the workpiece, the projected light sheet, falling at an angle upon the workpiece,

produces a bright line 2D image in the camera with a discontinuity at the process path. Movement from the expected central position of this discontinuity and from the symmetrical shape of the image indicates error in the relative position and shape of the workpiece.

The laser/camera system needs to be correctly orientated transversely to the process path, and the device includes a means of .rotating the integral sensor/process tool about its own _. centre-line to achieve this. The system is integrated into the robot controller, and a dedicated robot control system is provided to achieve this. The system can only be fitted to particular makes and types of robots for which the necessary interfacing hardware and software have been generated and cannot be fitted to automated machines that lack robot-like control. Using a 2D image requires a high processing overhead and produces a lower update frequency and therefore resolution.

The invention is particularly suited to automated welding operation but can equally be applied to such diverse processes as sewing, adhering and many others.

According to one aspect of the present invention, there is provided a mechanism comprising a holder for holding a process tool, means for rotating the holder about a first axis and means for pivoting the holder about a second axis transverse to the first axis.

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In an advantageous embodiment of the invention the mechanism is intended for use as an end effector (or "wrist") on a robot arm engaged in robot-controlled process such as welding. A camera or comparable sensor is mounted on the holder to survey the process (e.g. welding) line. Means are provided for detecting errors between the actual process line and the desired process line and for correcting those errors. The means for rotating the holder can comprise a stepper motor. Operation of this motor either in a complete rotational mode or in a reciprocal rotational mode can enable the camera to scan the process (e.g.welding) line. The means for pivoting the holder enables the orientation of the process tool disposed, in operation, in the holder to be varied as desired. These means for pivoting can comprise a stepper motor and a gi bal which is pivoted when the motor is rotated.

According to another aspect of the present invention, there is provided a method of controlling a process involving the travel of a process tool along a desired path including the steps of conducting the tool along the desired process path, storing the coordinates of points along the path, surveying the process during operation to determining errors between the actual path and the desired path and correcting the errors so determined.

The invention also includes workpieces produced by the method or mechanism defined above.

In order that the invention may be more clearly understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 shows a side elevational view in partial section of a mechanism intended as an end-effector or "wrist" for use in robot controlled welding.

Figure 2 shows a plan view of the mechanism of Figure

1,

Figure 3 is a detail view of part of the mechanism of Figures 1 and 2 , and

Figure 4 shows a modification of the embodiment shown in Figures 1 to 3.

Referring to Figures 1 and 2, the end-effector comprises a machined metal chassis 1 in which two stepper motors 2 and 3 are mounted. One end of the chassis includes a flange 4 which is formed for connection to the robot arm with which the end-effector is to be used. At the other end of the chassis is disposed a mounting 5 for a weld torch. This mounting 5 includes an annular support 6

which forms part of the chassis 1. This support 6 is internally screwthreaded and a complementary externally screwthreaded mounting ring 7 is screwed into it. A gimbal carrier 8 is mounted in the ring 7 through a bearing 9 comprising an annular race in which a plurality of ball bearings are mounted.

The lower end of the ring 7 is also externally screwthreaded, the pitch of this latter screwthread being somewhat greater than that at the upper end of the ring 7. A guide ring 10, which is made of synthetic plastics material is internally screwthreaded in complementary manner to this lower end screwthread of the ring 7 and is screwed onto this lower end. The guide ring 10 is formed with a circular internal groove 11 for a purpose which will be described later.

The gimbal supported by the gimbal carrier 8 comprises a gimbal outer 12 and a tubular gimbal inner 13. The gimbal outer 12 comprises two downwardly dependent arms each of which are drilled to accommodate respective pivots disposed on opposite sides respectively of the gimbal inner 13. This gimbal inner 13 is in the- form of a split ring. A threaded bolt, which extends through the material on both sides of the slit, enables the inner gimbal to be tightened around a welding torch extending through it. Referring ^' additionally to Figure 3, a rider sp-ring 14 extends between the .inner 13 on which it is mounted -'and the guide

-1- ring 10 where it locates in the groove 11. A miniature solid state camera and associated circuitry (not shown) is attached to one of the downward arms of the gimbal outer 12.

The upper end of the gimbal carrier 8 is toothed at.15 and the external surface of the upper part of the guide ring 10 is also toothed at 16. The stepper motors 2 and 3 carry respective toothed pulleys 17 and 18 on their outputs shafts which are drivably connected with the gimbal carrier 8 and guide ring 10 by respective toothed belts 19 and 20. By operating the stepper motor 3 , the angular orientation of the gimbal inner 13 can be adjusted and by operating the stepper motor 2, the angular inclination of the gimbal inner 13 with respect to the central axis of the guide ring can be adjusted. This latter is achieved by the ^■ action of the guide ring 10 on the rider spring 14. As the ring moves up and down, the degree of tension in the spring alters and this causes the gimbal inner to move accordingly. It will be appreciated that movement of the holder about each axis can be effected independently of movement about the other axis.

In the modification illustrated in Figure 4, which is a sectional view, that part of the section on the right hand side of the centre line being in a plane at 90° to that part on the right of the centre line instead of the screwthreaded arrangement of Figures 1 to 3, a ball screw

40 is employed and instead of the spring rider and guide ring a drilled ball 41 and peg 42 is employed. Annular seals 43 combine with annular bellows 44 to prevent or reduce ingress of particles into the main bearings 45.

To carry out a welding action, for example, employing the above described end effector, it is first necessary to go through a set up phase with the effector connected to the robot arm which is to be used. With the welding torch centred in the end effector gimbal inner, the robot is taught so that the torch follows the desired welding path. The robot progressively stores the positions of its component links in order to be able to reproduce the coordinates of the path. The miniature camera (not shown), which is mounted on the end effector is roughly aligned with the desired weld path. The extension and robot arm are initially synchronised and orientation of the torch in the extension is progressively requested and stored electronically in the extension.

A rehearsal phase is then conducted in which the robot arm follows the weld path. The robot arm reproduces the rough positions stored during the set up phase and uses the camera output to refine camera position for correct orientation so that the centre point of the camera scan corresponds with the centre point of the weld.

During a welding operation itself (or production)

any displacement from the desired stored path as refined during the rehearsal phase is treated as an error and corrected for after an appropriate delay either by rotating the torch or pivoting the gimbal using the stepping motors 2 and 3. camera could also measure weld width and, if that width changed from the desired width, the torch could be moved from side to side to adjust the width of the resulting weld bead to that desired or appropriate for the conditions encountered, with, if desired, continuous adjustment also and weld parameters.

During the first (set-up) phase the tool is centralised within the mechanism (wrist) and secondly the tool is guided along a desired operational path using the movement of the host alone, i.e. without moving ^' axes of the mechanism and positions of component links of the mechanism (wrist) supporting the tool are stored in memory. The tool supporting mechanism (or wrist) has a memory separate from a memory of the host (i.e. robot-arm or other tool support), and movement of the host and wrist are synchronised during the set-up phase. The sensor (camera) on the mechanism is initially approximately aligned with a desired process path, during the rehearsal phase and coordinates of the mechanism stored in memory during progress along the desired process path. Synchronisation is effected, after the set-up phase, by replaying the host programme and, at each turning point, stopping the host,

moving the wrist to align the sensor with the process line and storing the rough mechanism coordinates generated at each turning point. During the second (rehearsal) phase, the tool is caused to follow the process path, the wrist mechanism reproduces the rough position stored during the set-up phase and uses the sensor output to refine sensor and tool position so that a centre point of the sensor scan corresponds with a centre point of the process line to generate refined coordinates defining an expected process path.

The above described end effector and method enables errors arising during an actual welding operation to be detected and corrected whilst permitting ready access by the weld torch to the workpiece.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For example, in relatively large flexible structures, or in components made by imprecise means, like casting, the normal operation of a robot without feedback from sensors cannot satisfactorily locate with the 'desired accuracy. Usually either a physical feature or edge can be used as a datum for location, or guiding marks or rails can be provided in an earlier operation, marks being the more quickly and cheaply applied guidance. These guiding features and marks are then detected by tactile, ultrasonic

or visual sensors. As with the problem of weld path tracking, tactile sensors though accurate on certain types of feature are vulnerable. Ultrasonic and optical sensors avoid this vulnerability.

The tool holder into the area of the edge or physical feature or mark to be used as the first location point with the scan at right angles to the feature to be detected. Th robot will have a facility to allow it to advance in the programmed "direction until a selected sensor goes positive: This feature would be used to advance the tool holder until aligned with the desired feature. The next step would be to index in from this first datum by a fixed . distance to the required operating point. Finally the swing axis would be adjusted either for instance to bring the tool holder perpendicular to the workpiece surface, or to correct for the observed error with respect to another reference mark or feature roughly at right angles to the first.

By these means comparable results with fewer expensive sensors could be achieved in a more compact and less vulnerable configuration, and probably with reduced cycle times. Means can be provided to enable a second movement component, such as a reciprocation, to be applied to a tool as it moves along a process line. When the wrist (mechanism) is for robot-controlled welding, the second component of movement is a zig-zag reciprocation to cause

weaving at the weld. Means can be provided on the movable parts of the mechanism to enable 'zero' positions of the parts to be ascertained automatically. Such means can be constituted by a radiation or proximity sensor, such optical radiation sensing being connected to a process of the mechanism.

The wrist processor can be programmed .to vary process parameters if a departure from a norm is detected by the sensor. For example, weld tool parameters as voltage, current, wire feed, can be altered simultaneously with movement of the weld head. Further, the mechanism (wrist) processor and circuitry can be adapted to be capable of initiating/deactivating operation of the process tool in response to signals from the sensor, which is adapted to detect the start/finish of a process path.

Synchronisation of the host and wrist can be effected the host (arm) processor and a mechanism (wrist) processor ^" having separate programmes and memories, the host being programmed at each turning point, to emit an activation signal to the wrist processor and, after having done so, to remain inactive until it receives a "finished" signal from the wrist processor, and the wrist processor, upon receipt of an activation signal commences and carries out an action stored in its own memory for that turning post, and, having done so, sends a "finished" signal to the host processor, the wrist thereafter remains inactive until it receives a

next activation signal.

Many other variations are possible within the scope of the invention.