BACKGROUND ART An industrial robot is composed of interconnected robot parts such as, for example, a stand, robot arms, an arm housing, a wrist and a turn disc. Two adjoining robot parts are connected to each other so as to be rotatable in rela- tion to each other around an axis of rotation or are line- arly displaceable in relation to each other.

An embodiment of an industrial robot according to the prior art comprises a stationary foot, which is mounted to a base. The foot supports a stand that is rotatably arranged in relation to the foot about a first axis. The stand sup- ports a first robot arm. The first robot arm is rotatable in relation to the stand about a second axis. The first robot arm supports an arm housing. The arm housing is ro- tatable in relation to the first robot arm about a third axis. The arm housing supports a second robot arm. The second robot arm is rotatable in relation to the arm hous- ing about a fourth axis, which coincides with the longitu- dinal axis of the second robot arm. The second robot arm comprises a wrist housing. The wrist housing supports a wrist. The wrist is rotatable in relation to the wrist housing about a fifth axis. The wrist supports a turn disc, which is rotatably arranged about a sixth axis. The turn

disc comprises a toolholder. The toolholder is adapted for attachment of a tool.

The wrist housing comprises two drive units which substan- tially determine the dimensions of the wrist housing. The wrist housing is determined by its length and turning ra- dius. By the length of the wrist housing is meant its ex- tent in the longitudinal direction, parallel to the centre axis of the wrist housing. By the turning radius of the wrist housing is meant its extent perpendicular to the centre axis of the wrist housing. The space within which the robot is able to move with all physical positions is referred to as operating space.

The drive units of the wrist housing comprise electric motors. The transmission means that is present between the motor and the driven unit is designated driving rope. A driving rope thus comprises gears, chains or drive belts that transmit the motor rotation to the desired rotation around each axis of rotation. Arranging the motors for driving the rotary motions of the wrist and the turn disc inside the robot arm or in the wrist housing is a known design. The arm of an industrial robot must often pass through narrow passages and work in narrow spaces. The longer and thicker a wrist housing is, the worse will be the accessibility. It is thus desirable to limit the length and the turning radius of the wrist housing. To limit the extent of a wrist housing in space, the motor that drives the movement of the wrist is often mounted at the opposite end of the robot arm.

The motors are driven by electric current via electric ca- bling and are controlled via signals from signal cabling.

These cablings are often disposed internally of the robot arms. The tool and the functions specific for the tool are driven and controlled via process cabling. The process cabling comprises cables for power and signals, conduits for fluids and gases, and sometimes an empty bendable pro-

tective casing, which is arranged for laying one or more conduits and cables. Process cabling transports process media, such as electricity, water and air, to the tool.

Process cabling is space-demanding and stiff and is often disposed outside of the robot.

Process cabling that is disposed outside of the robot is easily subjected to impacts and extreme bending. To avoid such damage, it is desirable to dispose the process cabling inside the robot. In this way, the cables are above all protected against damage caused by squeezing. In this way, the cables are also protected from extreme twisting that may lead to rupture. At the same time, loose cables are prevented from encroaching upon the operating space. Such cables make it difficult to access and perform work in narrow spaces.

It is known that motors and gears are centrally located in the wrist housing. Motors and gears are thus located in such a way as to make it impossible to dispose the process cabling internally.

WO 97/47441 describes an industrial robot with a robot arm comprising motors. From this publication it is known to place motors in a wrist housing. The object is to achieve a tight cable entry of motor cabling to the motor that con- trols the wrist. However, this publication only concerns motor cabling. In addition, the motors in the wrist housing are centrally placed.

WO01/39933 describes an industrial robot with a hollow robot arm. The robot arm is rotatable around its longitu- dinal axis. From this publication, an internal arrangement of process cabling is known. The object is essentially to bring about a controlled cable entry to the drive pulley.

However, this publication does not describe how the drive motors for the wrist and the drive pulley are placed.

EP 1128937 describes a further industrial robot with a hollow robot arm. From this publication it is known to dispose cabling internally of a robot arm. The publication also relates to a control device for controlling said ca- bling. The object is to protect the cabling by an internal arrangement of the cables in the robot arms. However, this publication only concerns the arrangement of cabling in the interior of a robot arm.

Thus, there is a need in industry to be able to utilize an industrial robot which has a compact wrist housing compri- sing drive units and which is able to work in small spaces.

There is also a need in industry to be able to utilize pro- cess cabling that has a long service life and that does not encroach upon the operating space.

SUMMARY OF THE INVENTION The object of the invention is to provide an industrial ro- bot that has a robot arm comprising drive unit and that permits the arrangement of process cabling inside the robot arm. A further object is to provide a wrist housing that is simple and cost-effective and that is easy to install or reach or replace during servicing.

According to the invention, these objects are achieved by a robot arm that exhibits the characteristic features descri- bed in the characterizing portion of claim 1, and by a method that exhibits the features described in the charac- terizing portion of claim 9, and by an industrial robot that exhibits the features described in the characterizing portion of claim 13.

By placing the motors in the wrist housing eccentrically in relation to the centre axis of the wrist housing, a space is formed in which is arranged a duct for receiving process cabling. The wrist housing thus comprises a through-duct

and a motor space that surrounds the channel and in which the motors are placed.

The wrist housing is rotatable about a first axis, the cen- tre axis of the wrist housing, which coincides with the longitudinal axis of the second robot arm and is thus the same as the previously described fourth robot axis. The wrist housing is rotatable about a second axis, which coincides with the previously described fifth robot axis.

The turn disc and hence the toolholder are rotatable about a third axis, which is the same as the previously described sixth robot axis. The wrist housing further comprises a first motor and a first driving rope adapted to drive the rotation of the wrist about said second axis. The wrist housing also comprises a second motor and a second driving rope adapted to drive the rotation of the turn disc about the third axis. The first motor in the wrist housing is arranged spaced from the centre of the wrist housing. The drive shaft of the first motor crosses the longitudinal axis of the robot arm. Also the second motor is arranged in the wrist housing spaced from the centre of the wrist hous- ing. The drive shaft of the second motor crosses the longi- tudinal axis of the robot arm. In the wrist housing, a con- tinuous duct is arranged between the motors. The duct is arranged essentially along the centre axis of the arm. Said duct is adapted to receive process cabling. Arranging a robot arm as described below permits process cabling to be placed centrally.

In one advantageous embodiment, the drive shaft of said first motor and the drive shaft of said second motor are arranged in planes parallel to the centre axis of the wrist housing.

In another advantageous embodiment, the drive shaft of said first motor and the drive shaft of said second motor form an angle with each other.

In a further advantageous embodiment, the drive shaft of said first motor and the drive shaft of said second motor are perpendicular to the longitudinal axis of the wrist housing.

In an additional advantageous embodiment of the invention, straight gears are used, which is made possible by a loca- tion of motors in a wrist housing according to the present invention. By a straight gear is meant a gear with cylin- drical gear wheels and parallel shafts. Straight gears are easily mounted and constitute a cost-effective design. In addition, the size and the weight of the robot arm are reduced.

In yet another embodiment, the process cabling is adapted to pass through the centre of the turn disc out to the tool. The advantage of this embodiment of the invention is that the process cabling is adapted to pass through the centre of rotation of the second axis. By pulling the pro- cess cabling through the centre of rotation, the risk of rupture of the cables is reduced. If the process cabling is not pulled through the centre of rotation, stretching of the cabling arises when bending the robot arm in one direc- tion, and compression of the cabling arises when bending in the other direction.

One advantage of the invention is that a compact wrist housing is obtained. A compact wrist housing facilitates the accessibility for a robot arm in small and narrow wor- king spaces while at the same time the cabling does not physically encroach upon the operating space. It is thus important that the motors in the wrist housing be placed so that the dimensions of the wrist housing are minimized.

According to a preferred embodiment, the motors are placed in a common plane on both sides of the duct. With this em- bodiment, the length of the wrist housing is minimized. The turning radius of the wrist housing is limited by the loca-

tion of the two motors. For a wrist housing designed accor- ding to the invention, this results in a physically large dimension but a reduced operating space.

According to another preferred embodiment, the motors are placed in a common plane on the same sides of the duct.

Admittedly, this embodiment results in a longer wrist hous- ing but permits the wrist housing to be optimized to opera- te in narrow passages.

The mechanics of a wrist housing according to the invention is arranged for the purpose of achieving a long service life and good mobility. The space in the wrist housing is limited and, in addition, the motors comprised therein re- quire space. Motors and gears are arranged in the wrist housing in such a way as to accommodate internally disposed process cabling.

Since an industrial robot with a robot arm according to the present invention has process cabling disposed internally, such a robot has a well-defined outer contour that is sim- ple to define graphically. An efficient simulation of such an industrial robot in a virtual computer environment is simpler to perform than of an industrial robot with a robot arm equipped with external process cabling. This is utili- zed, for example, to visualize stages of the operation or to control the industrial robot.

Advantageous further developments of the invention will become clear from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail, by des- cription of embodiments, with reference to the accompanying drawings, wherein:

Figure-1 is a perspective view of one embodiment of the invention, Figure 2 is a perspective view of another embodiment according to the invention, Figure 3 is a perspective view of the embodiment accor- ding to the invention shown in Figure 2, as viewed from the opposite side, and, Figure 4 is a perspective view of an additional embodi- ment according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description relates both to the method and to the device.

Figure 1 shows a perspective view of the outer part of the second robot arm of an industrial robot (not shown in its entirety) of the previously described type. The outer part of the second robot arm comprises a wrist housing 1. The wrist housing supports a wrist 2 and a turn disc 3. The wrist housing 1 comprises a fork comprising two fork ele- ments 4a, 4b. The wrist 2 comprises a sheath part 2a com- prising two lugs. The wrist is connected to the wrist hous- ing 1 with said lugs via said fork elements. The wrist housing further comprises a first drive unit comprising a first motor 5 and a first driving rope 7, comprising a first gear means 7a, 7b and a first torque transmission member 7c. The wrist housing also comprises a second drive unit with a second motor 6 and a second driving rope 8, comprising a second gear means 8a, 8b and a second torque transmission member 8c. In addition, the wrist housing com- prises a space, a through-duct 9. Said duct is intended to receive cabling. Surrounding the duct 9 is a motor space 20, in which the motors are placed.

The wrist housing is rotatable in relation to an arm hous- ing (not shown) about a first axis A, the centre axis of the wrist housing, which coincides with the longitudinal axis of the second robot arm and is thus the same as the previously described fourth robot axis. The wrist is con- figured to rotate relative to the wrist housing about a second axis B, which is the same as the previously descri- bed fifth robot axis. The second axis B is oriented at right angles in relation to the first axis A. By rotation of the wrist about the second axis B, a rotation relative to the wrist housing and hence relative to the first axis A is achieved. The turn disc 3 is configured to rotate rela- tive to the wrist about a third axis C, which is the same as the previously described sixth robot axis. The third axis C is shown in Figure 1 in such a way that it coincides with the first axis A. However, it is to be understood that this is only one of the positions assumed by the third axis C when the wrist rotates about the second axis. By rotation of the turn disc about the third axis, a rotation of the turn disc both relative to the wrist and relative to the wrist housing is achieved. The third axis is preferably perpendicular to the first axis. Further, the turn disc is suitably so arranged that it is rotatable about the third axis in both directions, in one or more turns.

The two fork elements 4a, 4b are fixedly mounted in the wrist housing. The wrist 2 is mounted between the two fork elements 4a, 4b in the wrist housing and rotatably journal- led in each of the fork elements. The wrist housing has a primary side and a slave side. In the figure, the primary side is shown facing away from the viewer and the slave side facing the viewer. The wrist is rotated by torque transmission from the first motor 5 via the first gear means 7a, 7b to the first torque transmission member 7c of the wrist housing. The first gear means is arranged on the primary side of the wrist housing. The turn disc is adapted for attachment of a tool to the toolholder. The toolholder is thus rotatable between different positions by torque

transmission from the second motor 6 via the second gear means 8a, 8b to the second torque transmission member 8c of the wrist housing. The second gear means is arranged on the slave side of the wrist housing. The torque transmission members are arranged in a manner known within the technical field. The motors are chosen, for example as in this case, as ordinary compact servo motors. The first gear means is in this case a toothed gearing. Also the second gear means is in this case a toothed gearing.

The end of the wrist housing shown in Figure 1, which end is opposite to the wrist housing and is intended to be mounted to a robot arm, forms a rectangle with the primary side and the slave side of the wrist housing as two sides.

The primary side and the slave side then interconnect the other two sides of said rectangle, here referred to as a first and a second side. The first motor is placed at that end of the wrist housing that is opposite to the wrist. The first motor is arranged at a distance to the centre of the wrist housing towards the first side of said rectangle, with the drive shaft D of the motor at right angles to the first axis A. Also the second motor is arranged at a dis- tance from the centre of the wrist housing, but instead towards the second side of said rectangle, with the drive shaft E of the motor at right angles to the first axis A through the wrist housing. Long and narrow motors are utilized. A compact motor that is so short that it is possible to place the motor with the drive shaft crossing the axis of rotation of the robot arm, while at the same time the motor in its entirety is housed within the wrist housing, is utilized. By the above-mentioned location of motors and gears, a space in the centre is thus free from motors and gears. Said space extends through the whole wrist housing along the first axis A through the wrist housing. A through duct 9 is arranged in said space. The duct is adapted to receive process cabling. The wrist hous- ing further comprises a central opening adapted to allow the insertion of and the passage of said cabling. The wrist

also comprises at least one opening adapted to allow the introduction and passage of said cabling out to the turn disc. Since the wrist is provided with two lugs, said cab- ling passes through the third axis with a minimum amount of tension.

The first motor in the example shown and the second motor are thus symmetrically arranged around a space arranged for centrally located cabling, in this case process cabling.

One advantage is that such a wrist housing is easily accessible for service of the robot arm. The fact that the wrist housing comprises cabling, drive motors and gear means also means that this outer part of the robot arm in its entirety is simply removed and replaced by a new one without complicated adjustments of gear means having to be performed during assembly.

According to one embodiment of the invention, the motor of the first drive unit is arranged in the wrist housing in the manner shown in Figure 1, that is, with its drive shaft perpendicular to the first axis. Torque transmission from the drive shaft of the motor to the rotatable torque trans- mission member of the wrist occurs via the first gear means. The first gear means is arranged on the primary side of the wrist housing. The drive shaft of the first motor is adapted to directly drive a gear wheel for the first gear means with a straight gear. Gear mechanism is utilized for transmitting torsional force. With a straight shaft, the size and weight of the robot arm are reduced compared with a different solution according to the prior art, where torque transmission from the motor is performed with a bevel wheel. Straight gears are easy to mount. In addition, with straight gears it is easier to achieve high precision in comparison with bevel gear pairs. The second motor drives the turn disc via the second gear means. The second gear means is arranged on the slave side of the wrist housing.

Figure 2 shows a perspective view of an embodiment accor- ding to the invention of an outer part of a second robot arm comprising a wrist housing that supports a wrist. The figure shows driving ropes comprising gear wheels 11 or, alternatively, an intermediate gear 12.

Figure 3 shows a perspective view of the wrist housing, shown above in Figure 2, which supports a wrist viewed from the opposite side. The driving ropes shown comprise gear wheel 11 or, alternatively, intermediate wheel 13.

Figure 4 shows a perspective view of an embodiment accor- ding to the invention of an outer part of a second robot arm comprising a wrist housing supporting a wrist. The driving ropes shown comprise a compact gear 14 and a cog belt [also called positive drive belt].

The outer part of the first robot arm shown in Figures 1, 2,3 and 4 comprise wrist housings according to preferred embodiments of the invention, arranged to be connected, for example, to an industrial robot. An industrial robot with a robot arm according to the invention is not limited to six- axis industrial robots only but is intended to be utilized also in other types of industrial robots, such as Cartesi- an, anthropomorphic, polar, cylindrical robots or overhead travelling crane robots.

Also in other respects, the embodiments shown are to be regarded as non-limiting examples of the invention, which may be freely varied within the scope of the appended claims. Thus, different solutions to customer interface and suspension of the wrist and the turn disc may occur. Diffe- rent detailed solutions may be chosen which arrange the drive of the wrist and the turn disc while at the same time reducing the dimensions thereof.

For example, the shape of the wrist housing may be adapted in dependence on the application of the industrial robot.

Any kind of torque transmission member according to known or future technique may be used in connection with the invention. The gear ratio of the gear may also be changed.

Also, it is possible to omit the toolholder, in which case the first motor directly drives a tool, such as a drill.

Further, an industrial robot comprising a robot arm accor- ding to the invention may, of course, be provided with a larger as well as a smaller number of cooperating robot arms and also have a shape that differs from what has been described above.

In a further embodiment, compact gears are used and these are driven directly from a motor via a cog belt, which en- tails a saving in weight but also results in a wrist hous- ing with a design with somewhat large dimensions than the preferred embodiment.

In yet another embodiment, the wrist comprises a central opening adapted to permit the introduction of and the passage of said cabling out to the turn disc.