The present invention relates to a robot system comprising an articulated robot arm and an accessory that is attached to the robot arm. A robot arm to which a cushion is attached for protecting a person in case of collision with the robot is disclosed in DE 202021 100 817 U1. In this conventional system, the cushion is a tube made from a yielding material, the tube having longitudinally extending slits in its wall that allow it to be widened and to be mounted on a robot arm by pulling it over a distal end thereof. While this conventional cushion is easy to manufacture and to adapt to robot arms of varying sizes, it has a drawback in that it is difficult to keep from moving along the robot arm in operation, and in that when the cross section of the robot arm housing on which the cushion is mounted is not circular, the cushion tends to accumulate in regions of the housing that are close to a central axis of the robot arm, so that regions far from the axis, which tend to have a higher risk of collision than the former, may be left bare, so that collision protection is least efficient for those regions of the robot arm where it is needed most.

Fixing cushions or other accessories by means of screws is possible, but has the disadvantage that screw sockets must be designed into the robot arm, adding to its complexity and manufacturing cost, and creating openings in a housing of the robot arm that need to be looked after while no screw is installed in them. Further, screw heads that protrude from the robot arm while holding cushions add to the risk of injury unless appropriately cushioned themselves, so that screwed cushions tend to be bulky and can even reduce the mobility of the robot arm. When the accessory is not a cushion but some rigid device, there is even the risk of the robot arm being damaged if, during operation of the robot arm, the accessory collides with an obstacle.

It is therefore an object of the invention to provide a robot system comprising a robot arm and an accessory where the accessory is easy to install on the robot arm, but does not expose the robot arm to an increased risk of damage in case of a collision. in which regions of the robot arm far from the central axis can be reliably protected.

This object is achieved by a robot system comprising an articulated robot arm and one or more accessories attached to said robot arm, characterized in that the robot system comprises at least one pair of magnetically interacting elements, of which one is associated to the a first one of these accessories and the other to the robot arm or to at least a second one of the accessories, the at least one pair being effective to hold the one or more accessories removably attached to the robot arm by magnetic attraction.

I.e. the first accessory can be attached directly to the robot arm by said at least one pair of magnetically interacting elements, or magnetic attraction of the accessories among themselves may hold them attached to the robot arm.

Typically, the first accessory is a cushion or an environment sensor for monitoring the environment of the robot arm, such as a camera or a proximity sensor. ln a preferred embodiment, the first accessory has an attachment surface whose shape is complementary to that of a support surface of the robot arm or of the second accessory. A user can thus tell from the shapes of these surfaces where a first accessory is meant to be installed on the robot arm or on the second accessory, and can install it by simply placing it there and releasing it. The magnetic attraction between the surfaces can even help placing the first accessory precisely; when in a stable configuration in which the surfaces snugly fit each other, a straight line extending from one interacting element of said at least one pair to the other intersects the attachment surface and the support surface orthogonally, magnetic attraction can guide the way to said stable configuration.

The robot arm may have at least two identically shaped support surfaces. By shaping support surfaces identically, the number of different types of first accessories needed for the robot arm can be kept small, reducing the manufacturing cost per piece.

The interacting element of the first accessory can be a permanent magnet. Thus, the interacting element of the robot arm or the second accessory can be some component of ferromagnetic metal. Such a component is likely to be present in a legacy robot arm, allowing such a robot arm to be upgraded into a robot system of the invention by merely providing a matching cushion.

When the robot arm is specifically designed according to the present invention, it may be advantageous to provide a solenoid as the interacting element of the robot arm, or as a component of the interacting element. On the one hand, a voltage induced in the solenoid by the first accessory approaching or being removed from the robot arm or a measurement of inductivity of the solenoid allow to detect the presence or absence of the first accessory, on the other, the amount of magnetic attraction may be controlled, if desired, by causing a current to flow through the solenoid. Placing the first accessory can be facilitated by the first accessory having an at least n-fold axis of symmetry, n being an integer >1 , since when the shape of the first accessory or at least of its attachment surface is invariant under a rotation around the axis of symmetry by an angle of 27t/n, there are n orientations in which the first accessory fits the support surface. The position of the at least one interacting element associated to the first accessory should be invariant under the rotation, typically by n interacting elements being evenly distributed on a circle centered on the symmetry axis, since then the magnetic attraction will be the same in each of the n orientations.

While the cushion is advantageous in that in reduces the risk of injury to persons in case of a contact with the robot, it is disadvantageous in that it hinders dissipation of heat from the robot arm. As a compromise between these two requirements, it is preferred that in a cross section plane extending through the support surface and through surface portions of the movable link adjacent to the support surface and not covered by the cushion or another type of first accessory, a radius of curvature of the support surface is smaller than a radius of curvature of the adjacent surface portions, since in this way the first accessory will cover those surface portions of the robot arm that are rather likely to come into contact with a person, while those close to internal heat sources of the robot arm are not covered and can dissipate heat freely.

Like the tube of DE 20 2021 100 817 U1, the first accessory may have a central opening, but in contrast to DE 20 2021 100 817 U1, the robot arm is not meant to extend through the opening. Rather, the first accessory can be attached to edges of a an end face of a protruding portion of the robot arm such as a lid attached to a housing opening of the arm; as long as a central portion of the end face does not project beyond the first accessory, there is no need for the first accessory to cover the central portion, too, so that by leaving it uncovered, dissipation of heat is promoted. Further, in a robot system comprising an articulated robot arm and a first accessory adapted to be attached to said robot arm, a sensor may be provided for detecting whether the first accessory is attached to the robot arm or not, and a controller may be connected to the sensor and adapted to impose a maximum allowed speed for the robot arm depending on whether the sensor detects the first accessory to be attached or not. In case of the first accessory being a cushion, since an operating speed that would be judged as dangerous to a person in case of contact with the naked robot arm may be safe in case of contact with the cushion, a maximum allowed speed may be set higher for the robot arm bearing a cushion than for the robot arm without a cushion; i.e. by providing the cushion, productivity of the robot arm can be increased. In case that the first accessory is a sensor, it may also be justified to allow the robot arm to move at higher speed when the sensor is mounted, since the speed can be reduced in time if the sensor detects that there is a risk of collision. The first accessory can be attached by magnetic interaction as described above, but other kinds of attachment are appropriate, too.

It should be noted that while a maximum allowed speed can be set globally for the robot arm, so that no element of the robot is allowed to exceed it, it may be more effective to set it individually for a given element of the robot arm or for the first accessory applied to it. Typically, the maximum allowed speed for a proximal element of the arm, close to a base and having a rather high inertia, will be lower than for a distal element, close to an end effector and having a rather low inertia.

If the sensor detects removal of the first accessory while the robot arm is operating, it is likely that the removal is due to an unforeseen collision. In that case, it is preferable not merely to reduce the maximum allowed speed to the value it would have without a first accessory installed, but to stop the robot arm altogether. ln case of the first accessory being a cushion, the effectiveness of the cushion may vary depending on its dimensions and design, on characteristics of cushioning materials that may vary with age and/or conditions of use, etc. Therefore, detecting the presence of a cushion may not be sufficient to allow an increase of the maximum allowed speed, but it must be ensured that the right cushion is installed. To this end, the cushion preferably comes with a communication interface for communication with the controller, and the controller is adapted to set the maximum allowed speed depending on information received from the communication interface. Similarly, if the first accessory is a sensor, it is important to ascertain its type to take into account its capabilities. Therefore, also in this case it is useful for the controller to set the maximum allowed speed depending on information received from the communication interface.

When a first accessory has been installed or removed, an operator should be able to tell whether the controller has taken account thereof. Therefore, preferably, the robot arm comprises an indicator for indicating whether the sensor detects the first accessory to be attached or not, or whether the controller has changed the maximum allowed speed subsequent to a change of condition of the first accessory.

When the first accessory is attached to the robot arm by magnetic attraction, it is convenient for the sensor to be a magnetic field sensor.

Further features and advantages of the invention will become apparent from the subsequent description of embodiments, referring to the appended drawings.

Fig.1 illustrates a robot system according to the invention, comprising an articulated robot arm, various cushions adapted to be mounted on the robot arm, and a controller; Fig. 2 is an axial section of part of the robot arm and a cushion mounted to it;

Fig. 3 is an axial section illustrating various embodiments of cushions;

Fig. 4 is a plan view of round and elongate cushions;

Fig. 5 is a perspective view of accessories for attachment on an elongate link of the robot arm;

Fig. 6 is a perspective view of an accessory for attachment on an end cap of a link.

Fig. 1 illustrates a robot system comprising an articulated robot arm 1 and a controller 2 for controlling movement of the robot arm 1 and operations of an end effector, not shown, mounted at a distal end 3 of the robot arm 1. Typically, operation of the controller 2 comprises inducing the robot arm 1 to move the end effector along a predetermined path and executing, at given positions of the path, operations by the end effector.

The robot arm 1 has a stationary link or base 4-1 of substantially cylindrical shape, which accommodates a motor for rotating the other links 4-2 to 4-6 around a vertical axis 5-1. Link 4-2 is also substantially cylindrical and houses a motor for rotating all more distal links 4-3 to 4-6 around an axis 5- 2. Link 4-3 is elongate in a radial direction with respect to axis 5-2. Link 4-4 has a substantially cylindrical portion in which a motor is coupled to more proximal link 4-3, for rotating links 4-4 to 4-6 around axis 5-3. A motor in link 4-5 drives rotation around an axis 5-4. Link 4-6 has two intersecting cylindrical portions, one of which houses a motor for rotating link 4-6 around axis 5-5 and the other, for rotating the end effector, not shown, around an axis 5-6.

The cylindrical portions of the links 4-2, 4-4, 4-6 have a substantially tubular housing, to one end face of which an adjacent link is rotatably connected, and the other end face of which is closed by a cap 6. In the embodiment shown the cap 6 is attached to the tubular housing by screws 27; other types of connection, e.g. a bayonet coupling, can be used instead.

When the robot arm 1 moves in operation, the risk of colliding with a person is larger for one of the caps 6 than for most other regions of the arm 1 , and there is the possibility that, with one of the motors running at full speed, one of the links might reach a speed that might cause harm to a person in case of a collision. In order to reduce this risk, controller 2 controls the motors so that no part of the arm 1 will move faster than an allowed maximum speed, and cushions 7-1, 7-2 are provided that can be fitted onto the caps 6 and other portions of the arm that are likely to reach a high speed. Fig. 1 shows these cushions 7-1, 7-2 slightly offset from those portions of the robot arm 1 on which they are to be mounted. All caps 6 have identical dimensions, so that one type of cushion 7-1 can be mounted indiscriminately on any of them.

Fig. 2 is a cross section of a cap 6 and of the cushion 7-1 attached to it. The cap 6 has a flat or slightly convex front wall 8, a tubular portion 9 and a strongly curved region 10 in between. The cushion 7-1 is in the form of a ring having an L-shaped or arcuate cross section extending over a support surface 11 of the cap 6, comprising the strongly curved region 10 and adjacent parts of the front wall 8 and the tubular portion 9.

The cushion 7-1 has a concave attachment surface 12 whose cross section closely matches the support surface 11 , so that while both are in contact, the only possible movement of cushion 7-1 is a rotation around axis 5 of the cap 6.

Each cylindrical portion of link 4-2, 4-4 and 4-6 houses a motor 13. Between the motor 13 and the cap 6, a circuit board 14 is provided which supports a plurality of magnet sensors 15 arranged at regular intervals on a circle around axis 5. The magnet sensors can be of any known type; in Fig. 2 they are schematically shown to comprise solenoids 16, optionally with a ferromagnetic core 17, in which a voltage is induced when the sensors 15 are exposed to a time-varying magnetic field, or the inductance of which varies depending on the amount of magnetic or ferromagnetic material in the vicinity. The sensors 15 further comprise evaluation circuitry 18 connected to the solenoids 16, which, based on an induced voltage or on an inductance measurement, is adapted to judge whether one of a plurality of permanent magnets 19 embedded in the cushion 7-1 is close to each sensor 15 or not. The magnets 19 are also evenly spaced on a circle around axis 5, so that as long as the cushion is intact, each sensor 15 faces a magnet 19 or none does.

While the shapes of the cap 6 and the cushion 7-1 would allow one to rotate with respect to the other as explained above, magnetic attraction between the magnets 19 and the solenoids 16 will lock the cap 6 in an orientation in which the distance between each magnet 19 and an associated solenoid 16 is minimized, i.e. where a straight line 20 extending between the magnet 19 and the solenoid 16 intersects the support and contact surfaces 11 , 12 orthogonally.

According to an embodiment, the controller 2 is adapted use a signal from the sensors 15, indicating that a magnet 19 is close to each of these, as an indication that the cushion 7-1 has been mounted. When the cushion 7-1 is not mounted, or if not all support surfaces 11 that should bear a cushion 7- 1 or 7-2 do so, there is the possibility of a person being hit by a naked support surface 11 , and the controller 2 must limit the speed of movement of the robot arm 1 to a value where a person is certain not to be injured by such a hit. Obviously, with the cushions mounted, such a safe speed is higher than without cushions. When it is detected that cushions have been mounted at all required locations, the controller 2 therefore allows the robot arm 1 to move at a higher speed, and operates an indicator 21 in order to inform a user or a person in the vicinity of the robot arm 1 of the fact. In the embodiment shown, the indicator 21 is a lamp, e.g. an LED, mounted inside link 4-6, the light of which is visible through transparent windows 22 of the cylindrical housing. The indicator 21 may be controlled to blink for a limited time whenever the controller 2 detects that a cushion has been placed or removed, so that the user, when installing the cushions, can know for each one whether it has been detected and will be taken into account properly by the controller 2.

When the maximum allowed speed has been increased in response to the detection of a cushion, it will be decreased again when the controller 2 detects that the cushion has been removed. However, if the removal is detected while the robot arm 1 is moving, it can be assumed that the cushion was knocked off by the arm 1 hitting an obstacle, which might have been a person. Therefore, in such a case, movement of the robot arm 1 is stopped altogether, and the controller 2 waits for an operator to reset this error condition.

According to a preferred embodiment, detection of the magnets 19 by the sensors 15 is a necessary, but not a sufficient condition for the controller 2 to increase the maximum allowed speed of the robot arm 1. In this embodiment, the cushion is provided with a data carrier 23 and a communication interface 24, e.g. an embedded RFID tag, which is adapted to be read remotely by a transceiver 25 of the robot arm 1. Information that can thus be conveyed from the cushion 7-1 to the controller 2 can comprise an identifier of the cushion 7-1 , based on which the controller 2 can judge whether the cushion is a genuine spare part that can be relied upon to be safe at the increased maximum allowed speed, and can refuse the increase if genuineness cannot be ascertained. Further, past operating conditions of the cushion 7-1 such as operating duration, operating temperatures, past impacts etc. can be recorded in the controller 2 or written back to the data carrier 23 via transceiver 25, allowing the controller 2 to judge whether the cushion is still safe or whether it should be replaced, and, eventually, to refuse an increase of the allowed maximum speed if safety of the cushion can no longer be guaranteed.

Fig. 3A-C illustrates various possible modifications of the cushion 7-1. As shown in Fig. 1 , the caps 6 can be attached to their respective links by means of screws 27, the heads of which are hidden in recesses 26 of the caps. As shown in Fig. 3A, when these screws 27 are ferromagnetic, the magnets 19 of the cushion 7-1 can be located so as to attract the screws 27. Efficiency of the cushion 7-1 can be improved by the magnets 19 being located inside the recesses 26, and not projecting beyond the support surface 11 .

Further, a cushion 7-1 can have different cross sections as shown in Fig. 3B and 3C. Evidently, the cushion 7-1 will be the more effective, the thicker it is. On the other hand, an excessively thick cushion may limit the freedom of movement of the robot arm 1. Therefore, the user can be provided with a set of cushions of different cross section to choose from according to his requirements. When these cushions are provided with the data carrier 23 and the communication interface 24 of Fig. 2, the controller 2 may automatically set an appropriate maximum allowed speed for a cushion installed by the user based on the characteristics it received from the data carrier 23 of the cushion. Fig. 4 is a schematic plan view of a round cushion 7-1 for installation on a cap 6, and an elongate cushion 7-2 for installation on link 4-3. The round cushion 7-1 has n=6 magnets equally spaced at angles 27t/n, so that there are n=6 orientations in which the cushion 7-1 is stably held with each magnet 19 facing one of the sensors 15 inside the cap 6. Of course n can take any integer value, e.g. when the magnets 19 are arranged so as to engage the recesses 26 of the caps as discussed with respect to Fig. 3, n =4.

In case of the elongate cushion 7-2, magnets 19 are substantially evenly spaced along the circumference of the cushion, but there are just two orientations, rotated by around axis 28, in which the cushion 7-2 fits the link 4- 3.

The operating safety of the robot arm 1 can be improved not only by mounting cushions but also by attaching appropriate sensors, typically proximity sensors adapted to sense the proximity of a human body or some other type of obstacle with which a collision should be avoided. A proximity sensor having a planar support designed to wrap around a robot arm and sensing electrodes on either side thereof has been known from WO 2016/050344 A1. The cushions described above might also serve as proximity sensors if e.g. provided with measuring and shielding electrodes as described in WO 2016/050344 A1 on inner and outer sides thereof.

Attaching accessories such as cushions or sensors to robot arm 1 is possible also if the robot arm 1 is completely non-magnetic or if there aren’t enough ferromagnetic elements in the robot arm for safe attachment. An example is provided in Fig. 5. A proximity sensor 29 is identical in shape to cushion 7-2 described above, and is designed for attaching to link 4-3, as illustrated in Fig. 1. Magnets 19 are arranged along the sensor body, similar to what is shown in Fig. 4. If there is no ferromagnetic material in link 4-3 for each magnet 19 to be attracted to, this lack can be overcome by mounting brackets 30 of ferromagnetic sheet metal around link 4-3 so that free ends 31 of each bracket 30 face two of the magnets 19 when the sensor 29 is attached to the link 4-3.

Fig. 6 illustrates a hemicircular accessory 32 for attachment to a cap 6. Magnets 19 at the two ends 33 of the hemi-circle enable the accessory to adhere magnetically to another accessory 32 of identical design, thus forming a full circle which, on its outside, may similar in shape to any of cushions 7-1 of Fig. 3. On an inner side of the circle, a circumferential contour, e.g. a rib 33, is formed which is designed to engage a complementary con- tour, not shown, of cap 6, so that as long as the two accessories adhere to each other and their contour engages that of the cap 6, they are immobilized in the axial direction of the cap 6.

Reference numerals

1 robot arm

2 controller

3 distal end

4 link

5 axis

6 cap

7 cushion

8 front wall

9 tubular portion

10 strongly curved region

11 support surface

12 attachment surface

13 motor

14 circuit board

15 magnet sensor

16 solenoid

17 ferromagnetic core

18 evaluation circuitry

19 magnet

20 straight line

21 indicator

22 window

23 data carrier

24 communication interface

25 transceiver

26 recess

27 screw 28 axis,

29 proximity sensor

30 bracket

31 end 32 accessory

33 end

34 rib