GB2248571A - Computer controlled work treating robot - Google Patents

Abstract

A multi-axis robot 1 is linked by an interface 20 to a three-dimensional CAD/CAM system 19, 20 the interface being arranged to provide instructions to the robot from the CAD/CAM system, in order for the robot to carry out multi axis machining, welding, spraying or inspection of a three-dimensional work surface 22. <IMAGE>

Description

MULTI-AXIS ROBOT AND USE THEREOF The invention relates to a multi-axis robot and the use thereof, especially in the treatment of a workpiece. Examples of treatment usefully carried out by the invention include welding, spraying of paint, metal spraying, spraying glass fibre, the removal of excess material, e.g. by milling or machining, machining, inspection, e.g. by traversing an ultrasonic probe or other device across a surface.

In broad concept the invention provides a multi-axis robot in combination with a three-dimensional CAD/CAM system, the robot being linked to the system by an interface arranged to provide instructions to the robot srom the CAD/CAM system in a form intelligible to the robot.

In another aspect the invention provides apparatus for treating a workpiece, the apparatus comprising a multi-axis robot having an articulated limb, a tool attached to the free end of the limb, robot control means to control the movement of the articulated limb, computer means for generating data representative of a continuous path for the tool of a C.N.C. machining centre to follow across a surface spaced from or coincident with a workpiece from data representing the intended. shape of the surface; and interface means for operatively connecting the robot computer control means for automatically generating the data representative of the continuous path, in a similar manner to that of a C.N.C. machining centre.

In another aspect the invention provides a method of treating one or more workpieces using a multi-axis robot having an articulated limb, a tool being located at the free end of the limb, the method comprising the steps of; generating data representing the intended shape of a surface for the tool to move over, spaced from or coincident with the workpiece and storing the data in computer means; operating the computer means to compute from that shape representative data, data representing a path for the tool to follow across the surface; and performing the following steps at least once: locating a workpiece adjacent to the robot; communicating the path representative data to robot control means and controlling the robot control means and controlling the robot to move the tool automatically along the path across the surface.

The tool may be offset from the surface, e.g. for spraying, welding or inspection, or may be coincident with the surface as when machining it to shape.

In the known methods, the limb is caused to move along a path that has been taught to the robot by guiding it through a first travel by an operator. A few hundred spatial co-ordinates are memorized by the control system of the robot which moves along a path joining those co-ordinates together. Such a method of control is useful when the robot is required to perform relatively simple tasks such as moving components, or say, spraying or spot welding a vehicle body. However when the tasks become more complex, more time and effort is required to teach the robot.In contrast to this known technique, we have discovered that when the robot is controlled so as to move the tool along a continuous path that has been automatically generated on a computer means from data representing the intended shape of the workpiece, then the robot can be caused to perform relatively complex tasks automatically, accurately and rapidly, and the need for intervention by an operator is reduced.

A suitable means for generating the path representative data has been found to be that of the type used to control a computer numerical control (C.N.C.) machining centre, i.e. a 3 D CAD/CAM software package. Commercially available packages which can be used in their present form or with modification include "DUCT" available from Delcam International Ltd; "UNIGRAPHIX" available from McDonald Douglas, Inc, "CNC" available from Cadcentre Ltd, "CATIA" available from IBM; and EUCLID available from Matra Datavision of France.

In another, more specific aspect the invention provides a method of making one or more articles each having a predetermined three dimensional shape by removing excess material from a block of material having a low to medium density using a multi-axis robot having an articulated limb, a material removal tool being attached to the free end of the limb, the method comprising the steps of; generating data representing the intended shape of the article and storing the data in computer means; operating the computer means to compute from that shape representative data, data representing a continuous path for the removal tool to follow across the block; locating a block of material adjacent to the robot; communicating the path representative data to robot control means and controlling the robot automatically to move the removal tool substantially continuously along the path across the block.

Suitable applications for such a method include the production of polystyrene, foamed polystyrene, fibre board or like material for making patterns used for making moulds for metal castings, or say, the pre-production models made by vehicle manufacturers for wind tunnel testing. In the past, such articles have been either made by hand, or by using a large, rigid and expensive programmable multi-axis C.N.C. machining centre of the type used in the machining of metal components. When the cutting forces are low, i.e. when material of low to medium density is being removed, then the use of such a machining centre adds greatly to the cost of the finished product. We have discovered that the use of a robot, usually available at much lower cost, can produce articles of the required accuracy.A robot has a further advantage in that because its working envelope is relatively large, then correspondingly large articles, life-size vehicle replicas for example, can be made at low cost.

Due to the robots inherent lack of rigidity, a problem can arise when the path changes direction rapidly in that the tool tends to "overshoot" and deviate from the path. In a preferred aspect the rate of movement of the treatment tool is reduced prior to a change in direction of the path. In this way the tendency for the tool to overshoot is reduced.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows schematically a system for use in treating a workpiece; Figure 2 shows an enlarged perspective view of part of the system of Figure 1; Figure 3 shows a representation of the intended shape of an article; and Figure 4 shows a path for the treatment tool to follow to produce the article shown in Figure 3. and Figures 5 and 6 are flow diagrams of the steps involved in operating the robot.

The system shown in Figure 1 comprises a multi-axis robot 1. As shown, the robot is of the six-axis type, but must be at least capable of moving a tool along the three mutually orthogonal axes. One particularly suitable robot is the T3-776 model, made by Cincinnati Milacron, of Birmingham, England. The robot 1 has two elongated limb portions 2,3 pivotally joined about a horizontal axis at an elbow 4. The lowermost limb portion 2 is pivotally joined about a horizontal axis by a shoulder 5 to a turntable 6 adapted for rotation about a vertical axis. A wrist joint 7, as best shown in Figure 2, is present at the end of the furthermost limb portion 3. The wrist joint 7 is adapted to move the tool mounting plate 8 about an axis co-axial with that of the limb portion 3, as shown by arrow 9. Arrow 10 shows the direction of movement which the plate can take from side to side.

Arrow 11 is a third direction of movement in which the mounting plate 8 can rotate about its own axis. The directions of movement at the various joints of the limbs are indicated by arrows 12. The limbs may be controlled electrically, e.g. by stepper motors or, say, by air or hydraulic pistons 13. A material removal tool, as shown, a milling head 14 is attached to the free end of the limb portion 3 beyond the wrist joint 7. The milling head comprises an electrically driven spindle or motor 15 having a collet chuck 16 in which is mounted a milling cutter 17.

The spindle is preferably of the high speed type, preferably water-cooled. Another suitable material removal tool may be a heated wire.

An article to be made may be designed on a computer aided design (C.A.D) system, represented at 18, of the type used to design articles for manufacture by a C.N.C. machining centre. One especially suitable type of available software useful as computer means for generating data for the continuous path for the tool of a C.N.C. machining centre is available with a computer aided manufacture (CAM) package, together known as DUCT 5 and produced by Delcam Systems Ltd of Birmingham, England. Other software packages are also available for C.N.C. machining centres. The shape of an article designed on such a system is shown, by way of example only, in Figure 3, represented as a so-called wire frame model.The C.A.D system generates data representing the intended shape of the article which may then be stored in the computer1 s memory, on one or more discs, or any other suitable storage means. Data representative of a path that a machine tool can follow to reproduce that shape is then generated using a C.A.M system, represented separately for illustrative clarity at 19.

The C.A.M system is arranged to generate the path representative data from the C.A.D data that represents the shape of the article, and from other data such as the size of milling cutter and the surface detail and tolerances required. The path will normally be a non-analytical path, that is to say a path not bounded by a solvable equation, but instead a path defined by many thousands of spatial co-ordinates and passing to and fro across the shape in the manner of a raster-type pattern. An example of a path for reproducing the shape of Figure 3 is shown in Figure 4. The C.A.M system may generate two or more paths, firstly a "roughing" path, by which a relatively large milling cutter can produce a coarse precursor of the intended shape, and secondly one or more "finishing" paths to be made by relatively smaller milling cutter to define the surface detail of the shape.

Once each path has been generated, it must then be translated into instructions suitable for causing the servo motors of the robot to move by the appropriate amount for the tool to reproduce the path. Such translation is known as "post processing". The post processing stage is represented by the interface 20 in Figure 1 between the C.A.M. system 19 and the robot's own computer control system, represented at 21. The interface 20 translates the path into control data which is transmitted to the robot control means by a D.N.C. (direct numerical control) link.

In other words the movement needed for the tool to travel between each point on the path is converted into instructions which produce appropriate movement of the servo motors so as to move the tool along the x, y and z axes.

As shown in Figures 5 and 6 the CAM system is designed to generate co-ordinates at points defining the shape of the surface across which the tool is intended to pass. The list of coordinates defines the cutter path and that information, together with additional information such as the feed rate, is translated into a format which broadly follows that of an International Standards Organisation (ISO) standard cutter location data file.

That information is then converted into a form appropriate to the specific robot by the post processing program which reads the tool path information and writes a file of instructions which will be understood by the selected robot. The final phase of the process is to load these instructions into the control unit for the robot. When translating the cutter location file into the instructions for the robot it is necessary to adjust the instructions to suit the selected robot and specify the robot axes for each machining move, together with the velocity from the programmed feed rate. The post processing system can accept cutter paths prepared for both 3 and 5 axes machining. For 3 axes machining the tool orientation will remain constant throughout but for a 5 axes machining process the cutting tool may be angled to gain better access to the workpiece.For this reason one may need to define the orientation of the tool and the position of the workpiece relative to the robots origin.

As is common, the robot will only accept a limited list of instructions, 999 in the particular example, which list is far too short to enable the definition a complicated surface designed on a CAD system. The incompatibility is resolved by sending short package of instructions to the robot, the transfer being initiated by the robot, which then performs the instructions before initiating transfer of another package. The instructions may define movements, wait during tool changes, or finish sequences performed before requesting another package of instructions.

As discussed previously, due to the inherent lack of rigidity in a robot, there can be a tendency for the tool to 'lovershoot" when the path changes direction rapidly (when the path turns through a right angle for example). This can lead to inaccuracies in the finished article. At the post processing stage, appropriate instructions are generated which cause the robot to reduce the rate of movement of the tool along the path prior to such changes of direction. In this way, the tendency of the tool to "overshoot" is reduced. Because the path has already been generated, the instructions relating to the reduction in the rate of movement can be looped back to effect such reduction before the path changes direction.This is to be contrasted with a possible situation where a sensed change in direction causes the robot to slow down, by which time deviation from the path might already have occurred. The speed may be adjusted to a constant value between consecutive points on the path or the tool may be accelerated between consecutive points, and then decelerated in time to reduce to a desired low speed by the time the point at which the large change of direction occurs. It is to be understood that once the shape has been designed and the tool path created, that information can be stored, e.g. on disc, for repeated use by the robot's control system 21 to reproduce the shape many times.

The workpiece 22 to be treated is mounted upon a stand 23. As shown, the workpiece 22 comprises a block of material of low to medium density, say between 15-1000 kg/m3. Examples of such materials are expanded polystyrene, polyurethane, fibre board, Ureol plastic, Plaster of Paris and the like. The block of material, positioned at a predetermined location relative to the robot, is being milled away by the milling cutter 17. The robot 1 is moving the milling cutter 17 substantially continuously and automatically along a predetermined path that has been created by the C.A.M system from the C.A.D data to define the shape, until the treatment has been completed.

The system as discussed above can be made available at relatively low cost, and is suitable for use by foundries, especially those of small to medium size, for producing patterns in-house.

Although as described above, the system is used to produce a shape from generated data, it is to be understood that the system may also be used to treat an article already having a predetermined shape which can be represented by data, for example by moving the tool along a path offset from the article. In this way the system may also be used, for example, to drive a welding tool along a continuous path across a complex surface, or spray paint onto vehicle bodies, spray glass fibre or metal into moulds or to inspect an article. In both cases a path can be generated in a rigorous or strictly methodical way using C.N.C. machine tool techniques from data representing the shape of the article being treated.

Claims (17)

1. A multi-axis robot in combination with a three-dimensional CAD/CAM system, the robot being linked to the system by an interface arranged to pass instructions from the CAD/CAM system to the robot in a form intelligible to the robot.

2. A robot according to Claim 1, including a control unit arranged to receive instructions from an interface post processing program arranged to translate instructions in ISO format received from the CAD/CAM system, which instructions comprise tool robot movement according to the co-ordinates defining the shape of a surface over which the tool of the robot is to pass and allied tool movement information.

3. A robot according to Claim 2, in which the robot control unit is arranged to receive instructions in short packages, each being transferred at the initiative of the robot.

4. Apparatus for treating a workpiece, the apparatus comprising a multi-axis robot having an articulated limb, a tool attached to the free end of the limb, robot control means to control the movement of the articulated limb, computer means for generating data representative of a continuous path for the tool of a C.N.C. machining centre to follow across a surface spaced from or coincident with a workpiece from data representing the intended shape of the surface; and interface means for operatively connecting the means for generating path representative data for the tool of a C.N.C.

machining centre to the robot computer control means.

5. A method of treating one or more workpieces using a multi axis robot having an articulated limb, a tool being coated at the free end of the limb, the method comprising the steps of; generating data representing the intended shape of a surface for the tool to move over, spaced from or coincident with the workpiece, and storing the data in computer means; operating the computer means to compute from that shape representative data, data representing a continuous path for the tool to follow across the surface; and performing the following steps at least once, locating a workpiece adjacent to the robot; communicating the path representative data to robot control means and controlling the robot to move automatically the tool along the path across the surface.

6. A method as claimed in Claim 5, wherein the surface represented by the data is the intended surface of an article.

7. A method according to Claim 5 or 6, wherein the shape representative data is initially created on a C.A.D. means.

8. A method according to any of Claims 5 to 7, wherein the path representative data represents many thousands of three dimensional spatial co-ordinates to define the path which is arranged, in use, to cross the workpiece repeatedly to and fro.

9. A method according to any of Claims 5 to 8, wherein the path representative data is generated within a 3 D CAD/CAM software package.

10. A method according to any of Claims 5 to 9, including the step of reducing the rate of movement of the tool prior to a change in direction of the path.

11. A method of making an article having a predetermined three dimensional shape by removing excess material from a block of material having a low to medium density using a multi axis robot having an articulated limb, a material removal tool being attached to the free end of the limb, the method comprising the steps of; generating data representing the intended shape of the article and storing the data in computer means; operating the computer means to compute from that shape representative data, data representing a continuous path for the removal tool to follow across the block; and performing the following steps at least once locating the block of material adjacent to the robot; communicating the path representative data to robot control means and controlling the robot to move the removal tool along the path across the block.

12. A method according to Claim 11, wherein the removal tool is adapted to remove excess material by milling away or by means of a heated wire.

13. A method according to Claim 11 or 12, wherein the material has a density of between 15-1000 kg/m3

14. A method according to any of Claims 11 to 13, wherein the material is selected from the list comprising polystyrene, polyurethane, fibre board, Ureol plastic, Plaster of Paris or the like.

15. A method according to any of Claims 9 to 12, wherein the article being made comprises a pattern for use in metal casting.

16. A multi-axis robot substantially as described and with reference to the drawings.

17. A method of treating one or more workpieces, substantially as described herein and with reference to the drawings.