Proximity detector comprising capacitive sensor

0001The present invention relates to a proximity sensor capacitive sensor.

0002In many industrial applications it is necessary to detect and measure the proximity between a machine and an obstacle, whether it is another object or an individual, in order to deliver proximity distance information, warning signals and act Consequently.

0003By way of non-limiting example of industrial application of this type of detector, one can mention the management of an anti-collision function between mobile or fixed-station robots with an obstacle, the management of an anti-collision function. burglary, and more generally any management implementing a proximity detection.

0004In the medical field many robots used for auscultation of patients require to know the position of the patient in relation to the moving parts of the machine.

0005For example, for applications in radiology or imaging or for medical or surgical treatment it is essential to be able to provide the operator of equipment or automated control systems with information as accurate as possible about the position the patient to quickly and correctly position the auscultation elements.

0006In X-ray radiology systems, a real-time, millimetric knowledge of the position of a radiology equipment in relation to a patient and his or her immediate physical environment would increase the speed of movement of the machine. improve safety, and minimize X-ray exposure times.

0007Increasing the speed of movement of vascular positioners while ensuring a non-collision with the patient is one of the current wishes. However, the physiognomy of the patient and his position relative to the reference system of the machine is not known, the movement speeds of these robots are low so that the moving parts of the machine do not accidentally hurt the patient. Generally an emergency stop consisting of mechanical switches stops all movements when the detector or the transmitter X come into contact with the patient or another part of the equipment. However, the kinetics of objects in motion and the short stroke of the contactors impose low movement speeds. The increase of the speed of the robot n ' is then possible if a non-contact device detects the patient at a distance, said high threshold, sufficient to slow down the movements before coming into contact with the patient. A minimum distance, called the low threshold, makes it possible to perform the anti-collision emergency stop function.

0008There is, therefore, currently a real need for contactless proximity sensors providing accurate distance information and usable in specific environments such as medical imaging. The documents<patcit id="pcit0001" dnum="US4987583A"><text>US 4,987,583</text></patcit>, <patcit id="pcit0002" dnum="WO9730633A"><text>WO 9730633</text></patcit> and <patcit id="pcit0003" dnum="WO9719638A"><text>WO 9719638</text></patcit> disclose proximity sensors suitable for this type of application.

0009The document <patcit id="pcit0004" dnum="US5952835A"><text>US 5,952,835</text></patcit> discloses a contactless proximity sensor. The electronics used is an on-off detector operating with the aid of a FET transistor oscillator connected to an unguarded measuring electrode.

0010The document <patcit id="pcit0005" dnum="US5442347A"><text>US 5,442,347</text></patcit> discloses a capacitive proximity sensor with double controlled guard, operating in differential phase measurement by exploiting RC constants generated with reference resistors. A guard is created by reproducing the sensor signal using a buffer. However, a major problem of principle appears in this concept because the buffer adds a parasitic capacity to the capacity to be measured. This parasitic capacitance is much greater than the capacity to measure which gives rise to measurement errors and important instabilities.

0011The document <patcit id="pcit0006" dnum="US5554973A"><text>US5,554,973</text></patcit> discloses to him a capacitive type electrostatic detector operating according to a principle of operation with switched capacitors, without guard.

0012The document <patcit id="pcit0007" dnum="US6348862B"><text>US6,348,862</text></patcit> discloses a proximity sensor including a detection electrode and a plurality of control electrodes located near a spatial region in which an object to be detected is located.

0013Finally, the document <patcit id="pcit0008" dnum="US5651044A"><text>US 5,651,044A</text></patcit> discloses a detector according to the preamble of claim 1.

0014A main objective of the present invention is to provide a capacitive sensor proximity sensor which provides an accurate measurement (typically millimetric) of the position of an object at a measuring range (typically decimetric) greater than that allowed by the detectors of proximity of the prior art, with the particular effect of enabling the speed of movement of radiology machines to be increased and of providing a patient's topography for the purpose of evaluating its thickness in order to optimize the emission power of the beams X and thus to minimize the level of radiation necessary to make an image.

0015This objective is achieved with a capacitive proximity sensor according to claim 1.

0016One can thus achieve the equivalent of a pixel camera in which each pixel is constituted by an electrode. This camera moved along the body of a patient will allow to perform a topography of this patient to obtain a measurement of its thickness.

0017The floating capacitive bridge may advantageously be of the type disclosed in the document <patcit id="pcit0009" dnum="FR2756048"><text>FR2756048</text></patcit>. It is also possible to use a capacitive measuring chain of the type described in the document<patcit id="pcit0010" dnum="FR2640373"><text>FR2640373</text></patcit>which implements a bias voltage source and a triaxial transformer.

0018The proximity sensor according to the invention, consisting of a plurality of measurement electrodes oriented along several axes in order to cover all the useful zones, can be made of several detection antennas.

0019In an advantageous embodiment of a proximity sensor according to the invention, the detection antenna further comprises a single guard for all the measuring electrodes of the antenna.

0020But one can also provide a configuration in which the detection antenna further comprises several guards each provided for part of the set of measuring electrodes of the antenna.

0021The detection antennas can be made using a rigid or flexible circuit and connected to the electronic means.

0022The electronic means and the digital control and calculation means can cooperate to measure a distance successively on each electrode of an antenna, in a predetermined and modifiable order.

0023The detection antennas preferably comprise a test track placed at the rear or near the electrodes (guard plane side), which, in normal operation, is at the potential of the guard, and, in test, is set to mass.

0024Under these conditions, each electrode sees a parasitic capacitance simulating the presence of an object, in order to verify the integrity of the proximity detector.

0025The electronic means and the digital control and calculation means cooperate to deliver an alarm signal indicating an incoherent measurement or a malfunction of the digital control and calculation means.

0026It can also be provided that the electronic means comprise one or more reference capacitors for controlling the calibration of the electronics or performing an automatic recalibration.

0027In a particular configuration of a proximity detector according to the invention, it is possible to place, in the vicinity of the measurement electrodes, guard or ground surfaces arranged to modify the field lines of these electrodes. It is thus possible to create particular shapes of equivalent surface of these measurement electrodes.

0028In a particular embodiment, the proximity sensor according to the invention is disposed on the inner or outer surface of a cover or housing, for example, the imaging detector X or the transmitter X.

0029The electronic means and the digital control and calculation means cooperate to deliver proximity detection threshold alarm signals. The distances measured between the electrodes and the detected objects are delivered in digital and analogue form.

0030To enslave the movements according to the six degrees of freedom, antennas are for example arranged on five sides of the housing or cover.

0031In the case of a proximity detector implemented in X-ray radiology equipment comprising a device for transmitting an X beam intended to irradiate an object or a body, an antenna, called antenna X, is then less partially traversed by the beam X.

0032In a simple configuration, the antenna X may for example comprise a bore allowing the passage of the beam X. This zone is then non-measuring because not provided with electrodes.

0033To overcome this drawback, it is then possible to provide that the antenna X is, in the zone of the beam X, made of materials at least partially transparent to the X-ray. This embodiment is possible from a flexible printed circuit composed of a insulation is metallized on both sides by a very thin layer of chromium constituting the bonding layer of a copper layer, this copper layer is removed by etching on the zone which corresponds to the passage of the beam X to leave on the insulator as the thin layer of chromium in which bonding tracks, capacitive electrodes, test track and guard are realized (<figref idref="f0003">Fig. 5</figref>). The transmitter X may also be provided with antennas known as X.

0034This latter configuration allows the electrode detector to be completely covered to increase the efficiency of this detector.

0035The proximity detector according to the invention differs in particular from the detector disclosed in the document <patcit id="pcit0011" dnum="US5952835A"><text>US 5,952,835</text></patcit> in that in the present invention, the electronics operates in amplitude measurement with a guard, and that the oscillator has constant characteristics independent of the ability to measure. In addition, the detector according to the invention operates in amplitude measurement and not in differential phase measurement.

0036This proximity detector device makes it possible to increase the speed of movement of current radiology machines, to detect safety (anti-collision), to make a coarse three-dimensional image of the patient, to evaluate the thickness of a patient in order to optimize the power of X-rays to achieve images with minimal radiation, and improve image quality.

0037The capacitive proximity sensor according to the invention makes it possible to control the approach of a vascular positioner for medical application, using several antennas, equipped with a multitude of capacitive electrodes, housed in the detector. This device performs real-time measurement of several absolute distances (a distance per electrode) separating the surface of the detector cover and the surrounding objects such as a patient or the table.

0038Proximity detector devices according to the invention may also be used in moving machines or robots, in particular machine tools, industrial robots, transport vehicles, etc., with the effect of increasing their speed. operating speed and improve safety.

0039Other advantages and features of the invention will appear on examining the detailed description of an embodiment which is in no way limitative, and the appended drawings in which:<ul><li>the <figref idref="f0001">figure 1</figref> is a schematic block diagram of an X-ray equipment incorporating two proximity sensors according to the invention;</li><li>the <figref idref="f0001">figure 2</figref> illustrates the arrangement of an antenna within a proximity detector according to the invention;</li><li>the <figref idref="f0002">figure 3</figref> illustrates an exemplary structure of an antenna of a proximity detector according to the invention;</li><li>the <figref idref="f0002">figure 4</figref> schematically represents the inputs and outputs of an electronic card fitted to a proximity detector according to the invention; and</li><li>the <figref idref="f0003">figure 5</figref> illustrates the structure of a flexible circuit used for producing an antenna within a proximity detector; and</li><li>the <figref idref="f0003">figure 6</figref> illustrates the structure of a measurement system fitted to a proximity detector according to the invention.</li></ul>

0040We will now describe, with reference to <figref idref="f0001">figure 1</figref>, an example of implementation of proximity sensors according to the invention in an X radiology machine, for a vascular positioner.

0041A first proximity sensor (1A) is disposed within a detector device X (5) fitted to a radiology machine (10), and has a plurality of antennas lining five of the inner or outer walls of the detector cover X, each antenna having a plurality of electrodes Ei, j. A second proximity sensor (1B) is disposed on the inner surface of the transmitter device X (2) of the machine (10). The X transmitter device (2) and the X detector device (5) are installed at both ends of a C-shaped mobile part rotatable around an examination table (4) on which a patient (3) is lying down.

0042With reference to the <figref idref="f0001">figure 2</figref>, a proximity detector according to the invention 1 comprises an antenna 20 disposed on the inner surface of a face of the cover 22 and side faces 21, 22. The antenna 20 consists of a plurality of electrodes arranged in a matrix , comprising electrodes Ei, j located integrally on one face, electrodes E'i, j disposed on the edge on two sides, and electrodes arranged entirely on the sides.

0043It should be noted that the antenna 20 could also be disposed on the outer surface of the detector cover.

0044We will now describe, with reference to <figref idref="f0002">figure 3</figref>, an exemplary embodiment of an antenna 30 in the form of a flexible circuit. This antenna 30 lines the inner surface of a main face of a cover with electrodes 31 and the inner surfaces of the side faces of the cover with electrodes 32, 33, 34. These electrodes are all connected to an electronic card via tracks. conductive (not shown).

0045The antennas equipping the proximity sensors according to the invention can be made using a multilayer technique, as schematically illustrates the <figref idref="f0003">figure 5</figref>. To make said antenna X can be used a flexible printed circuit 50 composed of an insulator I metallized on both sides of a thin layer of chromium Cr and a thick layer of copper Cu, the two layers of copper being removed on a zone ZX which corresponds to the passage of the beam X and in which connecting tracks, capacitive electrodes Ecr, a test track P, and a guard are made from the two layers of chromium.

0046A conductive layer G copper + chromium guard, electrodes Ecu + cr copper + chromium, and Ecr electrodes in chrome, are made according to an industrial process using flexible circuits multi-layer type "Adhesiveless", which have on a polyimide support a thin layer of chromium covered with copper. This industrial process is mastered by the manufacturers of flexible circuits.

0047An electronic card fitted to a proximity detector according to the invention comprises, with reference to the <figref idref="f0002">figure 4</figref>, 64 links to the electrodes of three detection antennas, a test input connected to a test electrode for each antenna, a Reset reset input, and a DC voltage supply input.

0048This electronic board provides a "Watchdog" alarm signal, five alarm threshold detection signals (objects or patients too close), an X transmitter detection signal, five analog output signals corresponding to the five faces of the transmitter. bonnet, an analog output detection signal of the transmitter X, an excitation signal of the test electrode, and a serial digital link to communicate with the central unit of the equipment.

0049The "Watchdog" alarm signal is set low in case of inconsistent measurements, no electrode or software failure. Analog outputs are images of the minimum distances from the detector face, detector sides, or transmitter. The Reset signal is a reset signal of the microcontroller. The digital link provides the 64 measured distances and the operating status of the proximity switch. The sensor is directly connected to the SI central unit of the equipment without an interface card.

0050As illustrated <figref idref="f0003">figure 6</figref>, an antenna A of a proximity detector according to the invention is connected via a flexible connection cable CL to an electronic card 60 including an analog multiplexer allowing an input scan, a multi-channel capacitive floating bridge implementing a disclosed technology in the document <patcit id="pcit0012" dnum="FR2756048"><text>FR2756048</text></patcit> corresponding to French patent no.<patcit id="pcit0013" dnum="FR9613992"><text>96 13992</text></patcit> of November 15, 1996, an analog / digital conversion module, and a numerical module for calculating distances, for checking the operation and for communicating with the information and control system SI of the machine.

0051We will now describe the operation of a proximity sensor according to the invention, with reference to the above figures. The proximity sensor measures a distance successively on each electrode in an order that can be changed simply in the software.

0052The proximity sensor has a test electrode which in normal operation is at the potential of the guard and when it is grounded allows to test the proper functioning of the sensor and the good condition of the connector + antenna assembly .

0053By acting on the test command, it will also test the correct operation of the measurement chain for each sensor.

0054If a distance measured on one of the electrodes reaches a predefined low threshold, the logic output corresponding to the antenna which supports the electrode goes low, it will return to the high state when the distance will again exceed the threshold. The "watchdog" alarm output goes low if one of the n measurements is inconsistent (test failure) or if the microcontroller is blocked or faulty.

0055At first, the digital outputs are the images of the distances measured for each electrode, but the processing can then become more complex, it is therefore necessary to provide a reserve of computing power in the microcontroller (or DSP).

0056We will now describe a practical example of manufacturing a proximity sensor according to the invention.

0057In this practical example, the electrical module is disposed on a 160 mm length card and 100 to 160 mm wide, and comprises a connector for the analog outputs (shielded twisted), a connector for the logic inputs / outputs, a connector for the power supply, and several connectors for the electrode signals.

0058The antennas that occupy the edges of the detector will have half of their area on the side and the other on the large face. There are 33 electrodes distributed on the 4 antennas: 3 antennas for the detector and an antenna for the transmitter X. 13 electrodes are located on the side antennas of the detector, 16 on the antenna X and 2 on the transmitter X .

0059The range of the sensors is greater than 100 mm with a millimeter resolution, which optimizes the control of the approach speed of the detector towards the patient (maximum speed with minimal impact risk).

0060The cables that connect the electronics to the antennas on the side of the transmitter X undergo movements and must in practice accept a radius of curvature of 50mm in dynamics.

0061To allow the replacement of the antenna transmitter side X or electronics without removing the cable, it is for example equipped with a connector antenna side and a connector on the electronic side. It is also possible to provide a guard strip on the sides of the detector in order to modify the field lines of the electrodes to modify the equivalent surfaces of these electrodes and their measurement range and their range.

0062As this is a safety device, the detection distance must be very reliable and the system must be able to be notified in case of failure. In real conditions the surroundings of the equipment are very congested. The objects to be detected are of different natures: human body (patient lying on the mattress of the table or doctor standing next to the table), metal parts to the mass or not, non metallic parts but slightly conductive. Detection must be in any direction. The detection is done on the entire active surface of the detector and on its edges, which corresponds to five surfaces of a box.

0063The antenna X must be almost transparent to X-rays, which implies for the realization of the electrodes and the guard the use of a thin metal. Doctors usually install a lightweight plastic pad on the detector (charlotte). The order of magnitude of the time in which complete proximity is to be detected is 50 ms for an antenna of 64 electrodes. The size of the objects to be detected is variable: from the patient's abdomen to his hand, a finger or his nose.

0064Of course, the invention is not limited to the examples which have just been described and many adjustments can be made to these examples without departing from the scope of the invention. More generally, proximity sensors according to the invention can be implemented in any industrial application, when it comes to detecting complex shapes or presence using multi-electrode antennas. It is thus possible to provide proximity sensors according to the invention equipping mobile robots or transport vehicles, to improve the safety around these devices. Proximity sensors according to the invention can also be used in burglar-proof devices and in anti-collision devices.