Patent Description:
Radar sensors are widely used in automotive technology for detecting persons and objects in the vicinity of a vehicle. For instance, <CIT> 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. <CIT> discloses a radar sensor which is installed in a wheel well and is used for monitoring tire characteristics based on reflection by a structural element embedded in the tire tread.

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. <CIT> describes an articulated robot whose links carry radar sensors for collision detection.

In the case of <CIT>, if the radar sensor isn't operating properly and fails to detect the approach of a person, the vehicle boot remains closed and has to be opened by hand. In a robot system, on the other hand, if the approach of a person goes undetected, the robot may be moving to the same place as the person, and if both collide, there is a serious risk of injury. Therefore, in robotic applications, it is necessary to detect a possible failure of the radar sensor, so that operation of the robot may be enabled only if it has been established unequivocally that the radar sensor is operating correctly and is capable of detecting the approach of a person.

A conventional way to do so is by detecting a radar echo from a reference object which is placed outside the leakage zone of the radar sensor, in a region where the radar beam from a transceiver of the sensor is expected to propagate.

A drawback of this approach results from the fact that the distance between the transceiver and the sample object must be smaller than the distance in which the person must be detected, since otherwise detection of the reference object might be thwarted by the presence of a person between the radar sensor and the reference object, but the smaller the distance is, the stronger is the radar echo of the reference object. So a situation may arise in which a strong echo from the reference object is detected, but a weaker echo from a person isn't, in which case the safety of the person cannot be ensured. Further, the presence of an echo from the reference object increases the noise from which the echo of a person has to be distinguished, so that it is desirable not to receive both echoes simultaneously.

Moveable components by which the radar beam can be directed selectively either on the reference object or into a region where a person is to be detected increase the probability of defects. XP000387448 presents precision measurements of the radar cross section of simple rod and cylinder targets. XP010597366 describes bistatic radar scattering experiments of parallel wire grids.

One object of the invention is, therefore, to provide a radar sensor whereof e correct operation can be established with higher reliability.

This object is achieved by a radar sensor as defined in claim <NUM>.

At least some of the reference objects should be located beyond a leakage range of the transceiver unit.

By making the reference objects small in at least one direction, the intensity of their echo can be reduced in spite of their potentially close proximity to the radar transceiver. When the size of a reference object is smaller than the radar wavelength, its radar cross section is proportional to the fourth power of the size; therefore the power of the echo of the reference objects can be adjusted to be smaller than that of the echo of a person, so that if the transceiver is sensitive enough to detect the echo of the reference objects, any person will be detected a fortiori.

The size of the reference objects in said first dimension may be smaller than half the wavelength, preferably smaller than a fifth of the wavelength, and still more preferably, smaller than a tenth of the wavelength of the radar beam.

Since due to the small size of the reference object, the power of its echo can be made as small as desired, and there is no need to remove the reference object while monitoring the environment for the presence of a person. Therefore the at least one reference object can be stationarily mounted with respect to the transceiver, so that it is exposed to the radar beam at all times.

Typically, the reference objects can be metallic wires.

Manufacture of the radar sensor is facilitated if each reference object is linear having first and second ends, and if the first and second ends of the reference objects are mounted in a frame.

In order to decrease the influence of possible inhomogeneity of the laser beam on the echo from the reference objects, a plurality of said reference objects should be distributed over the cross section of the radar beam, preferably in the form of one or more regular patterns.

Preferably, the reference objects or the patterns formed by these are arranged in at least one plane that intersects the beam.

If reference objects are arranged in first and second regular patterns, the first pattern extending in a first plane that intersects the beam, and the second pattern extending in a second plane which is parallel to the first plane, the radar transceiver can be continuously or discretely tuneable, so as to be selectively operable at at least a first and a second frequency. If the distance between the two patterns equals half the Talbot distance at said first frequency, the first pattern will generate a Talbot pattern in the plane of the second pattern, and the intensity of a radar echo from the second pattern depends on whether its reference objects are located in bright or dark zones of the Talbot pattern.

Specifically, when the transceiver is operated at said first frequency and the reference objects of the second regular pattern are located in dark zones of the Talbot pattern generated by said first regular pattern, they will produce no echo. In that way, although the radar beam propagates across said second regular pattern, formation of a radar echo from the second regular pattern can be avoided, so that radar reflections from other objects can be detected with a minimum of background noise.

Reflection of a radar echo from the first regular pattern cannot be avoided, but this radar echo will go undetected if the first regular pattern is located in the leakage zone of the radar transceiver.

The radar sensor may be mounted on a robot arm, in order to detect persons in the vicinity of the robot and to control movements of the robot based on this detection so as to avoid collisions between the robot and a person.

Further features and advantages of the invention will become apparent from the following description of embodiments thereof.

<FIG> is a schematic view of a manufacturing robot <NUM> comprising a stationary base <NUM>, an end effector <NUM> and a plurality of elongate links <NUM> that are pivotably connected to one another, to the base <NUM> and the end effector <NUM> by joints <NUM>. The environment of the robot <NUM> is monitored for the presence of persons by radar sensors <NUM>, <NUM>. The radar sensor <NUM> is stationary and may be mounted on a workshop floor <NUM> in the vicinity of the robot base <NUM>. The radar sensors <NUM> are installed in the links <NUM>.

A controller <NUM> is connected to the radar sensors <NUM>, <NUM> and is programmed to slow down or possibly stop the robot <NUM> if the distance between the robot <NUM> and a person drops below a predetermined threshold.

A schematic cross section of the stationary radar sensor <NUM> is shown in <FIG>. A transceiver <NUM> is provided for transmitting and receiving radar signals. If necessary, a lens <NUM> may be provided for shaping, in particular collimating, the radar waves emanating from antenna <NUM> into a beam <NUM>, and for focussing a reflected radar echo onto the transceiver <NUM>.

A receiving channel of transceiver <NUM> is inherently sensitive to the radar wave emitted by a transmitting channel thereof. At the transceiver <NUM> the intensity of the radar wave being emitted is larger by several orders of magnitude than any radar echo reflected off some object in the vicinity of the radar sensor <NUM>. Therefore, in case of the transceiver emitting radar pulses at a single frequency, the transceiver <NUM> is sensitive to an echo only while it is not transmitting an impulse that might "leak" into the receiving channel. Alternatively, the transceiver <NUM> can be of the FMCW (frequency modulated continuous wave) type, i.e. it emits a continuous radar wave the frequency of which is continuously ramped, so that a frequency difference between the transmitted wave and the echo received at the same time is representative of the distance between the transceiver and the object from which the echo originates. In that case, the frequency difference between the outgoing wave and the received echo must exceed a certain threshold in order for the echo to be detectable. In either case, the transceiver <NUM> is surrounded by a so-called leakage range in which objects cannot be detected because their echo is made undetectable by the outgoing wave.

Within this leakage range, the radar beam <NUM> passes through a grid <NUM> formed of thin metallic wires <NUM> arranged parallel to each other in a regular pattern extending in a plane perpendicular to the propagation direction of beam <NUM>. The width of the beam <NUM> is sufficient to irradiate a plurality of said wires <NUM>. The length of the wires <NUM> should preferably be greater than the diameter of the beam <NUM>, so that ends of the wires <NUM> can be mounted on a frame <NUM> that doesn't block the beam <NUM>.

The diameter of the wires <NUM> is smaller than the wavelength of the radar beam <NUM>; e.g. in case of the radar beam having a mean frequency f<NUM> of <NUM>, corresponding to a wavelength λ<NUM> of <NUM>, the diameter of the wires is less than <NUM>, preferably less than <NUM>, and still more preferably, less than <NUM>, so that the wires do not cast a shadow at the downstream side of the grid <NUM> and do not reflect the radar beam <NUM>, but merely scatter it.

Since the grid <NUM> is located within the leakage range, radar waves that scattered back from it to the transceiver <NUM> are not detected.

A second grid <NUM> having the same structure as grid <NUM> is provided in the path of beam <NUM> outside the leakage range. The two grids <NUM>, <NUM> extend in parallel planes. The wires <NUM> of the two grids <NUM>, <NUM> are aligned with each other, i.e. when seen in the propagation direction of beam <NUM>, the wires <NUM> of one grid overlap with those of the other. The distance d between the two grids <NUM>, <NUM> equals d<NUM><NUM>/2λ<NUM>, so that if the wavelength of the radar beam is λ<NUM>, the first grid <NUM>, by Talbot effect, gives rise to an intensity distribution of the radar wave in the plane of the grid <NUM> which has the form of a line grid whose intensity minima coincide with the wires of the grid <NUM>. Therefore, when the transceiver <NUM> operates at the wavelength λ<NUM>, or is ramped in a small interval around λ<NUM>, the second grid <NUM> has no effect on the propagation of the radar beam <NUM>.

It has an effect, however, when the wavelength emitted by transceiver <NUM> is sufficiently different from λ<NUM> for the wires <NUM> of grid <NUM> to be exposed to a substantial amount of radar radiation. In that case the grid <NUM> contributes to the radar echo received at transceiver <NUM>, and since the grid <NUM> is outside the leakage range, this contribution is detected.

For this reason, in the embodiment contemplated here, the transceiver <NUM> is adapted to switch between two frequency ranges for ramping the frequency of the radar wave, the first one being centered around f<NUM>=c/ λ<NUM>, the other around a different frequency f<NUM>. When the radar sensor <NUM> of this embodiment starts to operate, the transceiver <NUM> first emits in the frequency range around f<NUM>, and a radar echo from grid <NUM> is detected by transceiver <NUM>. If the intensity of this echo has an expected non-vanishing intensity, it is concluded that the sensor <NUM> is functional, and the frequency of the transceiver <NUM> is switched over to a range around f<NUM>. In this way, although the radar beam <NUM> still passes through the grids <NUM> and <NUM> on its way from and to the transceiver <NUM>, the grids <NUM>, <NUM> leave no trace in the radar echo received, and contributions of objects and persons in the vicinity of the robot <NUM> can be detected with a minimum of background noise.

As shown in <FIG>, the sensor <NUM> can have a rotating mirror <NUM> or similar mobile element for redirecting the beam <NUM> and thus scanning the surroundings of the sensor <NUM>. The structure of the sensors <NUM> can be identical to that of the sensor <NUM>, except for the rotating mirror <NUM>, which isn't needed if, as shown in <FIG>, several sensors <NUM> are distributed along a circumference of a link <NUM>.

<FIG> gives schematic examples of echo signals detected by transceiver <NUM>. If the frequency of the radar beam <NUM> is ramped linearly, the frequency difference between outgoing and incoming radar signals is directly representative of the distance between the transceiver <NUM> and an object which is the source of an echo. Curve A of <FIG> is obtained with the sensor operating around f<NUM>; at a small frequency difference Δf<NUM>, just above the leakage range represented by hatched area C, there is the echo from grid <NUM>; at a larger difference Δf<NUM>, there is an echo from an object, e.g. from the robot <NUM> itself. When the sensor <NUM> is operating in the frequency range around f<NUM>, the radar beam <NUM> is unaffected by grid <NUM>, and only the object contributes to the radar echo, but not the grid <NUM>, as shown by curve B.

Due to the small diameter of the wires <NUM>, the radar echo from grid <NUM> can be limited to a low value which will not overshadow an echo from an outside object, even if this outside object is close to the grid <NUM>, and by appropriately choosing this diameter, the intensity of the radar echo from grid <NUM> can be set to any desired value. As shown in the diagram of <FIG>, when the diameter of a metallic sphere is much larger that the radar wavelength, i.e. at a relative frequency of <NUM> or above, the ratio between radar cross section and projected area of the sphere converges towards unity. On the other hand, when the diameter is smaller than the wavelength, this ratio is proportional to the fourth power of the frequency. A similar relation holds for the wires <NUM> of grids <NUM> and <NUM>. Therefore, the diameter of the wires <NUM> can be chosen so that although the grid <NUM> extends across the entire cross section of beam <NUM>, the echo that originates from the grid <NUM> is only slightly above the detection threshold of transceiver <NUM>.

When the intensity of the echo from grid <NUM> is set as low as this, the radar sensor <NUM> can be simplified by dispensing with grid <NUM>. In that case, the echo from the grid <NUM> is present continuously while the sensor <NUM> is operating, but this doesn't cause a problem, since this echo is too weak to conceal the echo of an outside object close to the leakage range that should be detected. Quite to the contrary, precisely because the echo from the grid <NUM> is weak, any malfunction of the sensor <NUM> is likely to cause it to drop below the detection threshold, whereby the malfunction is detected. An example of a typical radar echo signal according to this simplified embodiment is shown in <FIG> in a diagram analogous to that of <FIG>.

Claim 1:
A radar sensor (<NUM>, <NUM>) comprising a transceiver unit (<NUM>) for emitting a radar beam (<NUM>) along a beam path in an outgoing direction and receiving radar radiation along said beam path in an incoming direction, the radar sensor (<NUM>, <NUM>) further comprises a reference object (<NUM>) placed in said beam path for redirecting part of the outgoing radar beam (<NUM>) in the incoming direction, the reference object being one of a plurality of reference objects placed in the radar beam (<NUM>), and the size of the reference objects in at least one dimension being smaller than the wavelength of the radar beam (<NUM>),
wherein the size of the at least one reference object in a second dimension orthogonal to said first dimension is larger than the wavelength of the radar beam (<NUM>), and each reference object (<NUM>) is linear and has first and second ends,
characterized in that, the first and second ends of the reference objects are mounted in a frame (<NUM>), wherein the reference objects are arranged in first and second regular patterns, the first pattern (<NUM>) extending in a first plane that intersects the beam, and the second pattern (<NUM>) extending in a second plane which is parallel to the first plane, the radar transceiver (<NUM>) is selectively operable at at least a first and a second frequency (f<NUM>, f<NUM>), and the distance between the two patterns equals half the Talbot distance at said first frequency (f<NUM>).