The present invention relates to a radar sensor and to a robot in which the radar sensor is used. Radar sensors are widely used in automotive tech nology for detecting persons and objects in the vi cinity of a vehicle. For instance, DE 10 2013 010993 A1 describes a radar system which is mounted to the rear of a vehicle body and which triggers an automatic opening of the vehicle boot if it detects the user approaching.

In robotics, it is a general practice to install sensors in the vicinity of a robot that are capable to detect the presence of a person, in order to stop a movement of the robot if the person is close enough to be injured if hit by the robot.

An inconveniency of radar sensors is that the transceiver is inherently sensitive to the radar wave emitted by it, and that the intensity of the radar wave being emitted is larger by several or ders of magnitude than any radar echo reflected off some object in the vicinity of the radar sensor. Therefore, the transceiver is sensitive to an echo only while it is not transmitting an impulse that might "leak" into the receiving channel. Therefore, a radar sensor is surrounded by a so-called leakage region in which objects cannot be detected. The ra- dius of this leakage range is about one or two or ders of magnitude greater than the wavelength of the radar signal, i.e. typically at least approxi mately 30 cm.

While this may be not inconvenient in automobile applications, it is in robotics. Here, a person can only be reliably protected against being hit by the robot if the minimum allowed distance between the person and the robot is substantially larger than the leakage range.

One object of the invention is, therefore, to pro vide a radar sensor which can provide precise and reliable distance measurements at close range.

This object is achieved by a radar sensor compris ing a transceiver unit for emitting a radar beam in a first direction and receiving radar radiation from said first direction, a reference object spaced from said transceiver unit in said first di rection by a distance larger than a leakage range of the transceiver unit, and a first mirror spaced from said transceiver unit in said first direction so as to reflect at least part of the radar beam in a second direction.

Using the reference object spaced from said trans ceiver unit by at least said leakage range, it is possible to ascertain that the radar sensor is in good working order. If the transceiver unit detects the reference object, it can be assumed to operate correctly; if it doesn't, appropriate safety measures should be taken.

Due to the first mirror, the leakage range doesn't have to extend from the transceiver unit towards an object to be detected, but can be "folded" into the radar sensor itself, so that, although the dimen- sion of the radar sensor in the second direction may be much less than the leakage range, objects just outside the radar sensor can be detected. The angle between the first and second directions should be between 45° and 135°; the dimension of the radar sensor in the second direction is minimal if the angle is perpendicular. The radar sensor can be made compact if the radar beam is reflected several times within the radar sensor. To this effect, mirrors can be located up stream or downstream of the first mirror. I.e. if the transceiver unit comprises an antenna for emit- ting a radar beam in a third direction, it may fur ther comprise at least a second mirror for reflect ing said radar beam propagating in the third direc tion into said first direction. Alternatively, the radar sensor may comprise at least a third mirror for deflecting the radar beam propagating in the second direction.

In order to be exposed to the radar beam propagat ing in the first direction, the reference object may be located on the first mirror or between the first mirror and the transceiver unit. In the lat ter case, the distance between the reference object and the mirror should be small, since the distance between the transceiver and the reference object is at least equal to the leakage range, a large dis tance between the reference object and the first mirror would only add to bulkiness of the radar sensor . In an alternative embodiment, in said first direc tion, the first mirror is located between the transceiver unit and the reference object, and part of the radar beam propagates in the first direction beyond the first mirror. For this purpose, a semi transparent mirror can be used, or the mirror may be placed so that part of the beam passes by it. In that case, the mirror may be located within the leakage range of the transceiver unit, and there may be a range in the vicinity of the radar sensor in which objects cannot be detected reliably, but this range is smaller than the leakage range by the distance between the transceiver unit and the first mirror .

The radar sensor of this alternative embodiment is particularly appropriate for use on a link of a ro bot arm: While the transceiver unit and the refer ence object can be spaced far apart from each oth er, at opposite ends of the link, the mirror can be located at an arbitrary position in between, so as to detect approaching objects all along the link.

The radar sensor may further comprise a drive means for displacing the reference object.

The reference object may be displaceable in the first direction. Such a variation would change the distance of the reference object that can be de rived from the output of the radar sensor, and by establishing a relation between the displacement of the reference object and the derived distance, the reliability of the judgment of the operation condi tion of the radar sensor is improved.

Alternatively, the reference object may be dis placeable between a position in which it reflects the radar beam, and a non-reflecting position. Thereby, too, defects may become apparent which wouldn't be visible if the reference object is sta- tionary. Moreover, in an apparatus in which several radar sensors are combined, such as the robot link mentioned above, one reference object may thus be used in several radar sensors at different times.

Further features and advantages of the invention will become apparent from the following description of embodiments thereof. Fig. 1 is a schematic view of a robot system in which the environment of the robot is moni tored by radar sensors.

Fig .2 is a schematic cross section of one of the radar sensors of Fig. 1;

Fig .3 is a schematic cross section of a link of the robot of Fig. 1;

Fig . 4 is a section along plane IV-IV of Fig. 3.

Fig. 1 is a schematic view of a manufacturing robot 1 comprising a stationary base 2, an end effector 3 and a plurality of elongate links 4 that are pivot- ably connected to one another, to the base 2 and the end effector 3 by joints 5. The environment of the robot 1 is monitored for the presence of per sons by radar sensors 6, 7. The radar sensor 6 is stationary and may be mounted on a workshop floor 8 in the vicinity of the robot base 2. The radar sen sors 7 are installed in the links 4.

A controller 9 is connected to the radar sensors 7, 6 and is programmed to slow down or possibly stop the robot 1 if the distance between the robot 1 and a person drops below a predetermined threshold. A schematic cross section of the stationary radar sensor 6 is shown in Fig.2. A transceiver unit 10 of radar sensor 6 comprises an antenna 11 for transmitting and receiving radar signals. If neces- sary, a lens 12 may be provided for shaping the ra dar waves from antenna 11 into a beam propagating in a direction. In the present case, a mirror 13 may also be regarded as part of the transceiver unit 10. Transceiver unit 10 thus produces a radar beam that propagates in upward direction 14.

This beam is deflected horizontally by a mirror 15. The mirror 15 carries on its surface a reference object 16. The reference object 16 can be of any nature and composition, provided that it is effec tive to reflect the radar beam back towards the an tenna 11. The reference object 16 is located beyond the leakage range of transceiver unit 10, so that a distance between the transceiver unit 10 and the reference object 16 can be derived from the re flected beam by controller 9. The controller 9 is programmed to determine a failure of the radar sen sor 6 if the intensity of the reflected beam is be low a given threshold, if the distance of the ref- erence object 16 cannot be derived from the re flected beam or if the derived distance of the ref erence object 16 deviates significantly from an ex pected value. If such a failure is found, it cannot be guaranteed that a person in the vicinity of the robot 1 will be detected, and the controller 9 stops the robot 1.

Most of the radar intensity that reaches mirror 15 is reflected further by mirrors 17, 18. The refer- ence object might be located on any of these mir rors, instead of mirror 15. Mirror 18 may be rotat able, so as to emit a horizontal beam which scans the surroundings of sensor 6 in a plane parallel to the floor 8 surface. Since the mirror 18 is outside the leakage range, objects in the immediate vicini ty of the sensor 6 can be detected.

In an alternative embodiment, one of the mirrors, such as mirror 15, is semi-transparent, and the reference object 16' is located behind it, so as to reflect radiation which has passed the semi- transparent mirror. The semi-transparent mirror may be located inside the leakage range, provided that the reference object 16' is not.

Since the path of the radar beam is folded in mul- tiple ways inside radar sensor 6, the dimensions of the radar sensor 6 can be a fraction of the leakage range .

Fig. 3 is a cross section of a link 4 of robot 1. The link is cylindrical, and several radar sensors

7 are evenly distributed over its circumference. Fig. 4 is a longitudinal section of the link 4, showing two of these sensors 7. Each sensor 7 comprises a transceiver unit 10 which is located at one end of the link and is designed to generate a radar beam which propagates in a first direction 19 towards the opposite end of the link 4. A reference object 16, 20 is provided at said opposite end. Between the transceiver unit 10 and the reference object 16, 20, there is a mirror

21 which deflects the beam in a radial direction 22. A lens 23 may be provided which widens the re flected beam in the axial and circumferential di- rections, so the radar waves spread from the link 4 in all directions and that beams from adjacent sen sors 7 overlap. While the reference object 16, 20 is located out side the leakage range of transceiver unit 10, the mirror 21 may be inside. In that case, there may be a region surrounding the link 4 which still belongs to the leakage range and in which objects cannot be detected. However, when compared to a conventional sensor in which a transceiver unit emits directly in the radial direction 22, the radius of this leakage range is decreased by the distance between the transceiver unit 10 and the mirror 21.

The reference object of the sensors 7 may be a sta tionary object 16, as in the sensor of Fig. 2, which reflects a radar echo to the transceiver unit 10, so that when the controller 9 evaluates the output of the transceiver unit 10 it detects the reference object 16 at a constant distance.

In the embodiment shown in Fig. 4, the reference object 16, 20 is made up of two parts, a stationary member 16 and a mobile member 20. Here, the mobile member 20 comprises a ring 24 which is driven to rotate around the longitudinal axis of link 4 by a motor 25, and which carries at least one lug 26. In the course of the rotation of the ring 24, the lug 26 traverses through the sensors 7 between mirror 19 and stationary member 16 and reflects the radar waves before these can reach stationary member 16.

If the lug 24 is inside the leakage range of the transceiver unit 10, the controller 9 will detect the stationary member 16 only while it is exposed to the radar waves. Therefore, the sensors 7 will be judged to operate correctly by controller 9 only if in each sensor 7, the echo from reference object 16 vanishes periodically. In the embodiment of Fig. 4, the width of the lug 24 is so that it can block only one sensor 7 at a time. Therefore, as an additional condition for correct operation of the sensors 7, the controller can check that the echo doesn't vanish in two mutu ally adjacent sensors 7 at a same time.

In a preferred embodiment, the lug 26 is outside the leakage range of the transceiver unit 10. In that case, its radar echo can be processed by the controller 9 just like that from the stationary member 16, and as a condition for correct operation of the sensors 7, the controller 9 verifies that whenever in one sensor 7 the echo from stationary member 16 vanishes, there is an echo from lug 26, instead .

Of course, radar sensors of the type shown in Fig. 2, in which the radar beam from transceiver unit 10 first reaches a reference object 16 and, after wards, a deflecting mirror 15, can also be used in a robot link 4 as described above. Vice versa, a radar sensor of the type of Fig. 4, in which the reference object 16 is located behind a semi transparent mirror, can also be used as the sta tionary sensor 6.

Further, the reference object 16 of the sensor 6 of Fig. 2 can also be moved by a motor, as described above referring to Fig. 3 and 4. Reference numerals 1 robot

2 base

3 end effector

4 link

5 joint

6 radar sensor

7 radar sensor

8 floor

9 controller

10 transceiver unit

11 antenna

12 lens

13 mirror

14 direction

15 mirror

16 reference object (stationary member)

17 mirror

18 mirror

19 direction

20 reference object (mobile member) 21 mirror

22 radial direction

23 lens

24 ring

25 motor

26 lug