ROBOT ARM

Abstract: A robot arm (10) is mounted at one end by means of a ball and socket joint (20, 22). The ball and socket joint permits angular movement of the arm whilst retaining a large area of contact between the ball and the socket. Angular movement of the arm is effected by means of driven links (24 and 26) which pull or push the arm in a desired direction. However, thrust loads on the arm during operation are transferred to the ball and socket joint and are not transmitted to the drive mechanism.

Description: Title: Improvements in Robot Arms Field of the invention This invention relates generally to robot arms and in particular to a robot arm for carrying a tool the use of which causes large mechanical forces to be transmitted back through the arm to its support. Background to the invention In the manufacture of vehicle bodies as an example, but also in many other fields, it is often not possible to use a conventional caliper welding tool. It is instead commonly required to carry out single point welding using a anvil or opposed tool on the reverse side of the parts to be welded in order to provide back-up against the applied thrust. Typical welding impact thrust forces are in the range of 500 to 2000 lbs (227 to 907 kg) at the point of impact and it is this large force which has to be transmitted back to the base support, via the robot arm itself and the bearing at which this arm is mounted for movement. A large impact force of the order of that mentioned above, if taken by roller or ball races incorporated in the drive transmission to the arm, rapidly cause indentation or other damage to the race, and where the arm consistently operates over a small arc of travel, the region of wear is concentrated, which is even more disadvantageous. The invention According to the invention, robot arm apparatus includes a robot arm adapted to carry a tool, the arm being mounted for movement by means of a ball and socket joint at which the ball is in direct contact with the interior surface of the socket over a spherically curved surface area which remains substantially constant irrespective of angular movement of the arm, and a drive coupling connected to the arm for effecting angular movement of the latter, the arrangement being such that, in use, load forces derived from the tool are substantially wholly absorbed at the ball and socket joint and are not transmitted through the drive coupling. In accordance with the invention, therefore, the path of load transmission is separated from the drive path. This enables the drive coupling to be of small dimensions as compared to the size which would be needed if the load thrust was to be transmitted therethrough. More generally, therefore, the invention avoids the requirement to use increasingly large and expensive drive transmissions according to the thrust load to.which the robot arm is to be subjected. A preferred arrangement employs a ball formed at the end of the robot arm, seated in a socket carried by a suitable support, which could be the moving end of a primary robot arm but more typically will be a carriage mounted for linear movement. The drive coupling preferably comprises two drive linkages, one for effecting angular movement of the robot arm in one plane and the other for effecting angular movement of said arm in a plane normal to said one plane. Each linkage may conveniently includes a link which at one end is pivotally connected to the robot arm about an axis normal to the length of the latter, ie. normal to the direction in which the thrust load is transmitted in use. The other end of each link is preferably connected, as by means of a rose joint, to an eccentric drivable, as by means of a power drive afforded by a d.c. servo and harmonic gear box, in a circular path centred at a spacing from the centre of the ball and socket joint by the length of the.link. Such a simple link thus causes the robot arm to be moved in a corresponding circular arc, whilst the link remains normal to the path of load transmission in all positions in the range of angular movement. The link pivot point on the robot arm is spaced from the ball and socket joint, but preferably by a distance which is only a small fraction of the length of the arm to the tool. The length of the path of angular movement of the tool is thus considerably greater than that of the driven eccentric. An important feature of the invention concerns the use of a socket (at the ball and socket joint) which is formed in parts which can be relatively adjusted to enable the ball either to rotate freely therein or to be clamped in a chosen position of angular adjustment. In operation, typically in the welding of a vehicle body, for example, the socket clamp will be released, the robot arm swung to the next welding point, and the socket clamp restored prior to the welding operation. In single point welding, either a shaped anvil or an opposed tool, preferably carried by a similar robot, will provide a back up for the applied thrust. The socket support will, in application to the welding of vehicle bodies, preferably be carried-by a rectilinear carriage system which enables linear adjustment of the robot arm in three dimensions. Such a carriage system is readily designed in turn to absorb the applied thrust derived from the ball and socket joint. The robot arm may alternatively be carried by a primary robot arm, in turn designed to transmit the derived thrust, for example by means of a ball and socket joint in accordance with the invention. Description of the drawings In the accompanying drawings: Figure 1 is a diagrammatic view of a robot arm spot welding apparatus in accordance with the invention; Figure 2 shows the apparatus in somewhat greater detail; Figure 3 is an enlarged view of the robot arm and its mounting; Figure 4 shows a robot arm mounted on a sliding carriage; and Figure 5 is a perspective view of the overall layout of a welding station, in particular for vehicle bodies, incorporating the apparatus of the invention. Description of embodiment Referring to Figures 1 to 8, a spot welding apparatus incorporates a robot arm 10 for a welding tool, which as indicated and as is conventional, comprises a gun cylinder 12 and piston/tool head 14 carrying the tool 16. Reference 18 denotes a power supply feeder. The arm 10 is formed, at the and remote from the tool head, with a ball 20 held within a socket 22 to form a ball and socket joint. The ball 20 is in direct contact with the interior of the spherical socket, and has a constant area of engagement therewith independently of angular movement of said robot arm 10. Angular movement of the robot arm is produced by drive couplings 24, 26 by which the arm can be swung in either of two mutually perpendicular planes. Each drive coupling 24 or 26 comprises a simple pivotal link 28 driven by an eccentric 30 to which the link is pivoted via a rose joint 32. The eccentric 30 is drivable along a circular arc to effect a corresponding rotational movement of the robot arm via the link. The drive for the eccentric 30 is produced by a d.c servo 34 associated with resolver 36, with transmission via harmonic gear box 38 (Figure 1). The transmission also includes brake 40. It will be seen from Figure 3 that the link 28 is pivotal to the arm 10 at an intermediate position in the length of said arm, of the order of 1/5 to 1/4 of the length of the arm from the ball and socket joint. As diagrammatically indicated in Figure 3, the part forming the socket of the ball and socket joint is formed in two relatively adjustable parts 42, 44. By means of the adjustable parts, the ball 20, and thus the robot arm 101 can be clamped tight in the socket and freed sufficiently for angular rotation by either one of the drive couplings. In use, therefore, the robot arm 10 is first unclamped, swung to a fresh operational position, and reclamped before the welding tool 16 is operated. It will be noted from Figure 2 that the spot welding tool 16 acts on the part 46 to be welded against back-up tooling 48, which in some circumstances may alternatively consist of an opposed welding tool. In use, operation of the tool involves an impact welding force of some 500 to 2000 lbs (227 to 907 kg) at the welding spot. With the above-described construction in accordance with the invention, the impact or thrust load is transmitted through the arm 10 directly to the ball and socket joint 20, 22, and the drive couplings 24, 26 play substantially no part in load transfer. Thus, from the ball and socket joint 20, 22, the load is transferred directly to the support 50 on which the socket 22 is mounted, typically a carriage mounted for linear movement on a slide 52. Such an arrangement is shown by way of example in Figure 4, where the slide 52 is carried on an overhead gantry 54. Other references in Figure 4 are the same as those employed in Figures 1 to 3. Figure 5 shows the layout of a welding station on an automatic guided vehicle production line. Reference 60 denotes the vehicle chassis, to which is being welded, for example, the transmission tunnel housing. The chassis units are conveyed longitudinally through the station, whereat they are temporarily unloaded from their conveying carriages 62 to enable positioning of back-up tooling on a tray 64 beneath the chassis, prior to a spot welding operation by welding apparatus 66 which is arranged in the manner previously described with reference to Figures 1 to 3, including a robot arm carried on a ball and socket joint itself carried by a linear carriageway system. The apparatus of the invention is especially useful also, for example, for welding inside an closed vehicle chassis (ie chassis plus roof body). Due to its compact size enabled by the ball and socket joint, it can be mounted on the end of a column mounted arm projecting into the closed chassis through a window opening. However, the invention can also be employed in fields other than welding of vehicle bodies, and even fields outside welding, in particular where a robot arm is required to carry a tool the use of which causes generation of high thrust loads which have to be transmitted through the robot arm to its support.

Claims: CLAIMS 1. A robot arm arrangement including a robot arm (10) adapted to carry a tool, a ball and socket joint (21, 22) by means of which the arm is movably mounted and in which the ball is in direct contact with the interior surface of the socket over a spherically curved surface area which remains substantially constant irrespective of angular movement of the arm, and a drive (24, 26) coupling connected to the arm for effecting angular movement of the latter, the arrangement being such that, in use, load forces derived from the tool are substantially wholly transferred to the ball and socket joint and are not transmitted through the drive coupling. 2. An arm arrangement as claimed in Claim 1, wherein a ball (20) is formed at the end of the robot arM, and is seated in a socket carried by a suitable support (22). 3. An arm arrangement as claimed in Claim 2, wherein the support is the moving end of of a primary robot arm (10). 4. An arm arrangement as claimed in Claim 2, wherein the support is a carriage (50, 52) mounted for linear movement. 5. An arm arrangement as claimed in any preceding claim, wherein the drive coupling comprises two drive linkages (24, 26), one for effecting angular movement of the robot arm in one plane and the other for effecting angular movement of said arm in a plane normal to said one plane. 6. An arm arrangement as claimed in Claim 5, wherein each linkage includes a link (28) which at one end is pivotally connected to the robot arm about an axis normal to the direction in which the thrust load is transmitted along the arm, in use. 7. An arm arrangement as claimed in Claim 6, wherein the other end of each link is connected to an eccentric (30) drivable in a circular path centred at a spacing from the centre of the ball and socket joint by the length of the link. 8. An arm arrangement as claimed in Claim 7, wherein a power drive formed by a dc servo (34) and a harmonic gear box (38) is provided to drive the eccentric. 9. An arm arrangement as claimed in any one of Claims 6 to 8, wherein the link pivot point on¯the robot arm is spaced from the ball and socket joint by a distance which is less than half the length of the arm. 10. An arm arrangement as claimed in any preceding claim, wherein the socket is formed in parts which can be relatively adjusted to enable the ball either to rotate freely therein or to be clamped in a chosen position of angular adjustment.

Inventors: PIGOTT, NORMAN BRIAN
Applicants: LITTON U.K. LIMITED
Application year: 1985
Main classification: B25J18
IPC codes: B25J9, B23K11, B23Q1, B25J11, B25J17