The present invention relates to a collision detec tor for detecting a collision of two objects. Such a collision detector has applications in the field of robotics for detecting collisions between a robot and a human, between robots, etc.

EP 1 810 795 A1 discloses a collision detector com prising a cushion body which is mounted on a robot arm and which has a sealed cavity the pressure of which will rise if the cushion body is compressed in contact with a human. An increase of pressure in the cavity may be due to other reasons, and it is nec essary to distinguish between an increase due to contact and an increase due to other causes. In order to account for pressure changes due to a variation of atmospheric pressure, a further pressure sensor can be provided outside the cavity. Two pressure sensors located at different places of the cavity are supposed to allow more robust detection by meas uring pressure gradients.

Although a local impact may cause a pressure gradient in the cavity of this conventional collision detec tor, no direct conclusion can be drawn from the ex istence of a gradient, since its characteristics de pend on the location of the impact relative to the pressure sensors. Changes in atmospheric pressure may have an influence on the pressure detected in the cavity, but this influence tends to be small when compared e.g. to thermal effects such as fluid in the cavity expanding under the influence of heat generated by the robot arm in operation.

An object of the present invention is therefore to provide a more reliable collision detector.

This object is achieved by a collision detector for detecting a collision of two objects, comprising a cushion body which is adapted to be mounted on a first one of said objects and has an internal cavity that is compressible by an impact of the sec ond object, and

a pressure sensor which is responsive to a pres sure inside said internal cavity,

characterized in that the cushion body has a pinhole by which the internal cavity communicates with a space which is the environment or a second cavity.

Due to the pinhole, slow variations such as atmos pheric pressure variations or thermal effects will not have an effect on the pressure difference between the internal cavity and the space, whereas transient pressure variations are reliably detected.

Of course, the pinhole must be small enough to ensure a reliably detectable pressure increase in case of a collision with a person. Since the volume flow through the pinhole will be substantially directly proportional to the pressure difference between the internal cavity and the space, the pressure difference Ap can be expected to decrease exponen tially with time t, Ap ~ e ^~ ' ^h , to being a time constant of pressure equilibration between the cavity and the space. Since collisions occur on a time scale down to appr. 10 ms, the time constant should be in the same range in order to ensure that a pressure in crease due to the collision is clearly detectable.

On the other hand, in order to suppress an influence of temperature changes and the like, the pinhole should be large enough for the time constant to be not more than 1 s .

The cushion body should be resilient, in order to resume its original shape after a collision. Resil iency can be due to a skin of the cushion having bending flexibility; in can also be provided by a permeable elastic stuffing of the cavity. When an elastic membrane is provided between said cavity and said space, the pressure sensor can be adapted to detect a deformation of said membrane.

The output of the collision detector can be raw data of the sensor. Preferably, and in order to minimize the transmission bandwidth required for its opera tion, the collision detector may further comprise a data processing unit for deciding whether a colli sion has occurred or not based on output of the sensor.

The data processing unit may be adapted to decide that a collision has occurred when the pressure detected by the pressure sensor exceeds a predeter mined threshold.

The risk for a person to be injured by collision with a robot is related not so much to the pressure p reigning in the internal cavity, but to the amount of energy involved in the collision, which is pro portional to J V dp, V being the (pressure-dependent) volume of the internal cavity. Since the volume of internal cavities may vary from one collision detec tor to another, a given collision energy may be as sociated to different pressure increases in differ ent collision detectors. Therefore, if a robot has more than one collision detector according to the present invention, these preferably employ threshold values that are the larger the smaller the volume of the associated internal cavity is. Specifically, the thresholds may be inversely proportional to the vol umes of the associated internal cavities.

The collision detector may further comprise a stor age for recording data derived from output of the pressure sensor. While for controlling the movement of the robot, the only concern is whether a pressure increase exceeds the threshold or not, the recorded data should pref erably comprise a peak pressure detected by the sen sor, since therefrom the collision energy can be derived, and if a collision has occurred, it can be demonstrated that the collision energy did not ex ceed the permissible limits. In order to allow an unambiguous association of a detected peak pressure to an incident, the detection time of the peak pres sure can also be recorded in the storage.

Installation of the collision detector is facili- tated by the collision detector further comprising a wireless interface for transmitting output of the pressure sensor or data derived therefrom.

A collision detection system may comprise, in addi- tion to the impact detector described above, a robot which is the first one of the above-mentioned ob jects, and a motion controller for controlling mo tion of the robot, wherein the motion controller is adapted do at least one of braking, stopping and reversing a movement of the robot when an impact is detected .

As an additional safety measure, the motion control ler can be adapted to limit the speed of a member of the robot bearing the cushion body such that the stopping distance of said member is less than the dimension of the cushion body measured in the direc tion of motion. In that way, if a collision is de tected, the robot can be brought to a standstill before the cushion body is fully collapsed, whereby the risk of injury for a person is reduced substan tially.

Further features and advantages of the invention will become apparent from embodiments thereof de scribed below referring to the appended drawings. Fig. 1 is a schematic view of a robot equipped with collision detection system according to the invention; Fig. 2 is a cross section of a collision detector

Fig. 3 is a cross section of an alternative col lision detector;

Fig. 4 is a cross section of another alternative collision detector; and

Fig . 5 is a diagram illustrating an output signal from a pressure sensor of the collision detector during a collision.

Although the collision detection system of the pre sent invention will be described herein specifically with respect to the robot illustrated in Fig.l; it should be borne in mind that is applicable with any kind of articulated industrial robot. The robot 1 of Fig. 1 has a stationary, e.g. floor-mounted, base 2 on which a first joint 3 is mounted rotatable around a vertical axis. A proximal link 4 is connected to joint 3 and is pivotable around a horizontal axis. A distal link 5 is pivotably connected to the prox imal link 4 by another horizontal axis, and is ro tatable around its longitudinal axis. An end effec tor mount 6 has at least one, preferably two or three degrees of rotational freedom with respect to the distal link 5 carrying it. A cushion body 7 is shown, by way of example only, to envelop most of distal link 5. Similar cushion bodies, not shown, can be provided on other displaceable members of the robot 1; preferably any surface of the robot 1 that is capable of colliding with a person 8 has a cushion body applied to it. Further the single cushion body 7 of Fig 1 might be subdivided in the longitudinal and/or circumferential directions of link 5 into plural cushion bodies, each of which is part of one collision sensor.

Fig. 2 is a schematical cross section of a cushion body 7 of a collision sensor. The cushion body 7 has a shell 9 which encloses an internal cavity 10. At least an outer portion of the shell 9 is formed by an elastic membrane 11 that, while being self-sup porting, yields easily to touch, and will elas- tically revert to its original shape when no longer touched. An inner portion 12 of shell 9 is rigid, so as to practically not change its shape when the cush ion body 7 is installed on the robot 1, and is shaped so as to mate the surface portion of the robot 1 where it is to be installed. The inner portion 12 supports or comprises a circuit board 13 on which are mounted a pressure sensor 14 designed for sensing a pressure difference between the cavity 10 and the environment, a data processing unit 15, a storage 16, a wireless interface 17 and a status indicator

18, such as a LED or an assembly of LEDs shining in different colours.

The pressure sensor 14 can be mounted so as to com- municate with both the cavity 10 and the environment and to directly measure a pressure difference be tween both. Else it can be a barometric pressure sensor, and the data processing unit 15 is further connected to a second pressure sensor 14' for detecting barometric pressure of the environment and calculates the difference between both pressures.

The pressure sensor 14' can be common to a plurality of collision detectors mounted on a same robot. Al ternatively, it may be a pressure sensor of some other collision sensor which is unlikely to be sub ject to an impact at the same time as cushion body 7, e.g. by being provided on a side of link 5 oppo- site to that of cushion body 7.

Besides supporting communication with outside cir cuitry such as a motion controller 19 controlling robot 1, the wireless interface 17 may also serve for harvesting operating power from radio waves transmitted to it; alternatively the collision sen sor might comprise a thermoelectric element for har vesting energy from a temperature gradient between the robot 1 attached to the inner portion 12 and the environment, on the outer side of membrane 12.

In an alternative embodiment, the pressure sensor 14 is not a component on circuit board 13, but is lo cated on the membrane 11, formed e.g. by a strain gauge that is sensitive to the deformation the mem brane 12 undergoes in case of a pressure change.

The cavity 10 communicates with the environment through a pinhole 20 of shell 9. The size of the pinhole 20 is chosen so that the time constant of pressure equilibration between the cavity 9 and the environment is between 10 ms and 1 s. The cushion body 7 of Fig. 3 is similar to that of Fig. 2 in all respects except one: In Fig. 3, the shell 9 lacks a pinhole for communication with the environment. Instead, the shell 9 encloses two cav- ities, the internal cavity 10 and a second cavity 21 which communicate via a pinhole 22, and the pressure sensor is a differential pressure sensor for sensing a pressure difference between the cavities 10, 21, e.g. based on the displacement of a membrane 23 ex- tending between the cavities 10, 21. The internal cavity may in part be confined by rigid walls 24 that do not yield to a pressure change, so that the membrane 23 is elastically expanded by an increase of pressure in cavity 10, and a pressure difference between cavities 10, 21 is due to the elastic dis placement of the membrane 23.

In another embodiment, shown in Fig. 4, two cushion bodies 7, Ί ' of the type shown in Fig. 2 have their pinholes 20 connected to each other so that if one of these cushion bodies is compressed by a collision while the other is not, air will flow from the in ternal cavity 10 of compressed cushion body 7 into that of the uncompressed one 7', and the pressure difference can be obtained by one of the data pro cessing units 15, 15' calculating the difference be tween pressure data output by the respective pres sure sensors 14, 14' of these two cushion bodies 7, 7' .

In both of these two latter embodiments, the pressure in cavity 10 is increased permanently while the cush ion body 7 is compressed in contact with person 8, which facilitates close range braking of the robot

1.

When operation of the robot 1 starts, motion con- troller 19 transmits a wakeup signal to the collision detectors that are mounted on the robot 1, while the robot 1 is still at rest. This causes the data pro cessing units 15 of the collision detectors to dis play a not-ready state using their respective status indicators 18. The not-ready state may be repre sented by the status indicator 18 LEDs being on or shining in a specific colour, so that a person 8 can tell from the hue of the membrane 11 of a specific collision detector whether the detector is in the not-ready state or not. The person is invited to press the membrane 11 of each collision detector in the not-ready state. If the pressure sensor of a collision detector detects a pressure increase, the collision detector is operative, and its status is changed to "ready", which can be seen e.g. from the status indicator 18 going off or changing its colour. It is thus easy for the person 8 to see whether all collision detectors have been tested in this way. If all collision detectors have communicated their ready status to motion controller 19, the robot 1 is safe to operate, and motion controller 19 starts executing a predefined operation program.

In the above starting phase, while the robot 1 is standing still, every pressure increase can be as sumed to be due to the person 8 deliberately touching the robot. In contrast, in the operation phase, when the robot 1 moves under control of the motion con troller 19, pressure changes are likely to be caused by accidental contact. Regardless of whether the collision detector is of the Fig.2 or Fig. 3 type, what can be observed in the output of the pressure sensor 14 is a sharp increase in pressure while the internal cavity 10 is being compressed, and a slow decrease when the movement of the robot has been stopped, and the pressure difference gradually de creases via the pinhole 20 or 22. The energy of the collision can be estimated as proportional to the volume of the cavity 10 multiplied by the pressure increase. Therefore it is possible to define for any volume of the cavity 10, a threshold Ap _thr of the pressure increase Dr which corresponds to a collision energy far below those that might cause injury, e.g. 100 mJ, so that whenever a collision sensor detects a pressure increase which exceeds the threshold Ap _thr , e.g. at time tl in Fig. 5, a message is output to the motion controller 19 which causes the latter to brake, preferably to stop or even reverse, the displacement if the robot 1.

The motion controller 19 can be programmed to cal culate, at any time, by what distance d the shell 9 of any one of the collision detectors can be com- pressed in the current displacement direction e (see

Fig. 2) of the robot 1, before its membrane 11 hits inner portion 12. By limiting the displacement speed of the robot in function of this distance d, the motion controller 19 can ensure that when a person 8 at rest is hit by the membrane 11, the robot 1 can be stopped before the membrane 11 touches inner por tion 12. In that way, the membrane 11 will at all times be soft to the touch, and the person 8 can never get squeezed between the robot and an outside obstacle, whereby the risk of injury is greatly re duced .

At least after the threshold Ap _thr has been exceeded at time tl, the data processing unit 15 starts dig itizing the pressure difference data of the curve of Fig. 5 and recording these in storage 16, preferably associated by a time stamp identifying tl or the instant in time when a specific pressure difference data was obtained. If an accident has occurred, a user can accede to storage 16, eventually identify data belonging to a specific incident based on their time stamp, evaluate the data of the curve and cal culate the energy of the collision. This is done by multiplying the peak pressure Ap _max or the integral of the curve with a respective proportionality fac tor that is selected dependent on the size of the pinhole 20 or 22, in order to verify if collision energy limits as imposed by Table A.4 of ISO/TS 15066 have been met or exceeded.

PI 81487

Reference numerals

1 robot

2 base

3 joint

4 link

5 link

6 end effector mount

7 cushion body

8 person

9 shell

10 internal cavity

11 membrane

12 inner portion

13 circuit board

14 pressure sensor

15 data processing unit

16 storage

17 wireless interface

18 status indicator

19 motion controller

20 pinhole

21 second cavity

22 pinhole

23 membrane

24 wall