BACKGROUND OF THE INVENTION There are many industries in which robotics can be used to improve productivity, consistency of production and profitability. One such industry is the aerospace industry.

The process of assembling parts of the wings of aircraft is time consuming, costly and involves the use of large dedicated assembly jigs. Automating the manufacture of these parts, using robots, not only increases production but also increases flexibility of the process if dedicated jigs can be replaced. Drilling, reaming, countersinking, deburring, fastener insertion and sealant application are a few of the tasks where robots can be utilised. Drilling, however, is the most critical where an accuracy in the order of a 100 Am between drilled holes is typically necessary. Some aerospace assemblies require tens of thousands of drilled holes over several metres to this high level of accuracy. With a combination of drilling reaction forces and the inherent flexibility within robot joints, the accuracy required is often not achievable using a standard robot alone.

In robotic drilling the forces generated when drilling a 5mm diameter hole in carbon composite can be as high as 200 N normal to the surface and 60 N in either the

X and Y planes perpendicular to the drill bit. These forces are significant enough to generate deflections and vibrations in a manipulator that are detrimental to the quality and accuracy of the drilled hole. Published papers in this art shown that the key parameters for robotic drilling are the drill rotational speed and thrust force.

Previous research in robotic drilling has used the robotic manipulator to provide the linear drill feeding whereby a conventional drill, coupled to a force sensor, is attached to the last link. This work utilises force feedback to regulate the thrust force generated by both variable spindle speed and linear feed. Force feedback aims to minimise any reaction forces the manipulator has to provide, but highlights the problem that arises from hard-on-hard contact as the drill tip contacts the work surface. Thus, through quantisation, a loss of information from the collocated joint encoders affects the kinematic placement of the tool tip resulting in a subsequent loss of accuracy. This problem is exacerbated by low manipulator stiffness, joint wear, tool wear and can be problematic even at relatively low drilling forces. Some systems, previously developed for robotic drilling, have"clamped"the drill to the right position and angle before the start of drilling. By doing this, the overall stiffness of the manipulator is improved during drilling but the static positioning of the drill tip after clamping is compromised and made dependent on the manipulators positional accuracy. The drill tip thus requires a further micro-positioning movement after clamping, to remove the positional error.

Table 1 below indicates the typical accuracy requirements for aerospace drilling and those achieved by manual and automated means. It can be seen that an articulated robot alone is not sufficient to provide the

level of desired repeatability, even in an unloaded capacity. The manufacturers specified accuracy of a robot is not included and is generally determined by a number of factors specific to the application that it is used. Positional Accuracy Imperial Metric Conventional manual drilling with jigs 0. 008" 0. 200mm and fixtures Worlds best practice 0. 005"0. 125mm Manufacturers specifications of a 6 DOF articulated robot repeatability >0. 004">0. 100mm (unloaded) Desirable accuracy <0. 003"<0. 075mm Table 1.-Typical drilling and positional accuracy.

A second approach is to acquire less accurate robots, but rely on a high accuracy metrology system that could help compensate for robot inaccuracies. This method measures all three position and three orientation components without physical contact with the robot. The manipulator is then continually calibrated in real time significantly improving the accuracy of the robot.

Accuracy of two thousands of an inch (0.05mm) have been reported using a drilling end effector coupled with a laser subsystem.

It is these issues that have brought about the present invention.

SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided an end effector for attachment to a robotic manipulator, the end effector comprising a frame adapted to support means to drive a tool and means to displace the tool in X, Y, and Z planes, the end

effector being adapted to be used with a tracking system that provides a signal indicative of the error between the correct tool position and the actual tool position, and means to compute the error to cause the displacement means to displace the tool to the correct position in the X and Y planes.

According to a further aspect of the present invention there is provided an end effector for attachment to a robotic manipulator, the end effector comprising a frame adapted to support means to drive a tool and means to displace the tool in X, Y, and Z planes, the end effector being adapted to be used with a tracking system that provides a signal indicative of correct tool position, the end effector including position sensitive detectors to sense the variation of the actual tool position from the correct tool position, and means to compute the variation to cause the displacement means to displace the tool to the correct position in the X and Y planes.

According to a further aspect of the present invention there is provided a method of carrying out a machining step using a robotic manipulator that terminates in an end effector, the method comprising: a. ) macro positioning the tool through movement of the robotic manipulator, b. ) using a tracking system to provide a signal indicative of the correct tool position, c. ) computing the error between the correct tool position and actual tool position, d. ) passing a signal representing the error to displacement means, and micro positioning the tool to the correct position in the X and Y planes.

DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described by way of example only in which: Figure 1 is a side elevational view of an end effector attached to a robot manipulator, Figure 2 is a front view of the end effector, Figure 3 is a plan view of the end effector, Figure 4 is a bottom view of the end effector, Figure 5 is a left hand side elevational view of the end effector, Figure 6, is a schematic cross sectional view of a position sensitive detector, and Figure 7 is a block diagram of a control strategy.

DESCRIPTION OF THE PREFERRED EMBODIMENT The accompanying drawings illustrate an end effector 10 for use in robotic drilling. The end effector is bolted to the end of a robotic manipulator RM that forms part of a six axis robot (not shown). As shown in Figure 1, the robotic manipulator RM positions the end effector in a desired position over a work piece W. The end effector 10 essentially comprises a frame structure 12 that defines a base 13 with upstanding legs 14 supporting a robot adapter plate 15. The robot adapter plate 15 has an array of apertures 17 positioned around a central aperture 16 to enable the plate to be bolted to the end of the robotic manipulator RM as shown in Figure 1. The frame 12 supports a drill spindle 20 that is coupled to a drill feed 21 to drive the drill tip 22 that is secured to the drill spindle about a chuck 23.

As shown with particular reference to Figures 2 and 4, the underside of the base plate 13 of the frame 12 supports four vacuum generated suction cups 30 that are

bolted to the base plate as shown in Figure 2.

The end effector 10 essentially combines a clamping mechanism, laser metrology system and a micro positioning drill head to compensate for robot inaccuracy.

The end effector 10 relies on the robot manipulator RM to macro position the tool tip 22 to the inherent accuracy of the robot. After clamping, any measured offset is corrected by repositioning the drilling head independent to the manipulator by a micro-positioning stage.

The end effector frame 12 including the base plate 13 and adapter plate 15 are constructed from aluminium to reduce the overall weight of the mechanism.

To enable seamless control of the drill rotational speed and spindle feed rate, two independent BaldorX AC servodrives with quadrature control are used.

AC servodrives offer the controllability necessary to minimise the drilling forces generated from the drill tip.

The drill spindle 22 has a rotational speed capacity of that similar to conventional air drills used in a manual drilling process. The spindle motor 20 is attached via a table to a miniature ballscrew and linear bearing assembly, as illustrated in Figure 2, to allow linear translation of the spindle motor in the vertical deviation.

Four 75mm diameter suction pads 30 are attached to the base plate 15 of the end effector to increase the static stiffness of the robotic manipulator RM during the drilling cycle. The suction pads 30 are arranged in a pattern that ensures that at least three pads remain in contact with the workpiece surface for each hole drilled.

The suction pads are coupled to a vacuum pump, not shown, and various tests were undertaken to ensure

that the suction force was sufficient to withstand the drilling forces and to ensure that the suction did not introduce further undesirable deflections in the robotic manipulator. A mechanical hard stop 28 was positioned adjacent the base of the end effector 10 to guarantee orthogonal drilling while providing a stiff coupling interface between both the work piece and end effector.

The large scale manipulator, to which the end effector is attached, macro-positions the work tool to a nominal positional accuracy anywhere in the large scale work area and, using the suction pads, vacuum attaches itself to the work piece for rigidity. Two solid state lasers located independently are projected to the end effector in both the X and Y planes respectively. The lasers are either statically mounted around the perimeter of the work area or dynamically moved to pre set locations by linear translation. The lasers fall upon two Position Sensitive Detectors 40,41 (PSD's) mounted on the end effector, near the drill tip, in the respective planes. As shown in Figure 6, the surface of each detector on which incident laser light falls is dissected and the position of the incident laser falling on the PSD creates two currents directly proportional to the linear offset from its median. Using a simple mathematics function, the current is converted to an error voltage and amplified to represent a positional error of the tool tip with respect to the laser alignment within 5 Am.' This error is then used by a micro-positioning stage to improve the accuracy of the tool tip position in both the X and Y planes respectively. By securing the hard stop'28 against the workpiece W, the end effector 10 becomes positional to a high level of accuracy in all three axes (x, y, z). This relatively inexpensive metrology device can in specific circumstances exceed the capability of more expensive laser tracking devices using

interferometry techniques.

The drill spindle 22 is mounted on an X-Y linear stage table 29 using four linear bearings attached to the baseplate of the end effector. Two closed loop piezoelectric linear actuators 43,44 link the stage and allow linear movement in each respective two-dimensional plane of the drill tip. Each actuator has a stroke of 180 gm with an incremental resolution 1.8 nm and an unloaded resonant frequency of 2 kHz ( 20%). This relatively large bandwidth gives the micro-position stage the capability of a fast reactive response to high frequency vibrational forces. Push/pull force of the actuators is 4500/500 N respectively. Particular attention has been given to the available magnitude of actuator driving force in order to provide sufficient reaction force to the drill tip disturbances and accommodate for any inertial forces generated from high speed manipulator motion. A dedicated high voltage amplifier drives each piezoelectric actuator by using the amplified 0-10 volt error signal generated by the PSD's. Figure 7 illustrates the block diagram of the actuator position control strategy.

By applying a classical proportional integral derivative controller to the error signal and using a suitable time constant, positional errors less than + 90 p, m can be corrected before or during the drilling stage.

Figure 6 illustrates the simple control architecture used for each axis. Theoretically, the frequency response of the actuators should attenuate vibrations and positional disturbances caused by drilling up to 2 kHz.

The end effector described above is capable of achieving a level of accuracy that has a tolerance of an order of magnitude better than a conventional robot manipulator over large work areas.

The metrology measurement device employed is a simple and inexpensive alternative to some other researched laser tracking systems of substantially higher cost (2 orders of magnitude more expensive). By coupling this with high accuracy piezoelectric actuators, positional errors can be controlled and holes of the desired accuracy can be drilled. By designing compliance into the end effector, less emphasis is placed on the need for high accuracy manipulators creating a flexible environment whereby less expensive, dated or worn robots could be employed. More importantly, by creating a drilling head capable of machining holes with accuracy greater than that specified by aerospace manufacturers, expensive and cumbersome jigs and fixtures could possibly be made obsolete.

Although the preferred embodiment uses a laser tracking system and position sensitive detectors on the end effector it is possible to use other tracking systems that provide an error signal between the correct tool position and the actual tool position. The error is then transmitted to the end effector which computes the error to displace the tool to the correct position. In this embodiment there is no need for position sensitive detectors in the end effector and the error signal is computed by the tracking system. In systems of this kind a transceiver or reflector is positioned on the end effector to reflect the signal back from the tracking system to allow computation of the error.

It is understood that the end effector described above is not restricted to drilling and can be adapted to conduct other operations such as countersinking and riveting reaming or fastener location insertion reaming.

It is further understood that the invention is not restricted to the use of lasers. The end effector can

be designed to interpret any signal that positively dictates the position of the work piece or tool. A variety of drives can also be utilised to correct the error.