GB2465248A

GB2465248A - Measuring electrode arrangement, device and method for capacitive distance measurement

Info

Publication number: GB2465248A
Application number: GB0914190A
Authority: GB
Inventors: Rudolf Merkel; Uwe Zimmermann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2008-08-28
Filing date: 2009-08-13
Publication date: 2010-05-19
Also published as: DE102008041635A1; GB0914190D0; FR2935479A1

Abstract

The invention comprises a method of capacitive distance measurement, including a step of receiving (102, fig 1) a first capacitance value provided by a first measuring electrode 202, a step of receiving (104) a second capacitance value provided by a second measuring electrode 204, and a step of determining (106) a distance value 212, 214 on the basis of the first capacitance value, the second capacitance value and a predetermined distance 216 between the first measuring electrode and the second measuring electrode.

Description

Measuring electrode arrangement, device and method for capacitive distance measurement

Prior art

The invention concerns a measuring electrode arrangement, a device and a method for capacitive distance measurement, a corresponding computer program and a corresponding computer program product.

Although the following description essentially deals with determination of a distance of an object from a body in the form of a vehicle, the invention is not restricted to this, but is suitable for all kinds of bodies, even immovable ones.

Devices and methods for measuring the distance between vehicles and objects are known. For instance, the distance can be measured by means of capacitive sensors by an electrode system on the own vehicle, the capacitance of this electrode arrangement being affected by an approaching object.

Let "own vehicle" be understood to mean the vehicle which carries the distance measuring device which is considered in more detail below. Let "object" be understood to mean any object in the environment of the own vehicle which can be captured by the capacitive distance measurement. In particular, let it be understood to mean other vehicles ("external vehicles") and stationary objects in the immediate or wider region of the own vehicle.

A capacitive sensor is known from GB 24 044 43, for instance.

From DE 195 01 642 54, a capacitive sensor which extends over the whole rear bumper is known. The capacitive sensor is in the form of an electrode, which is connected to the input of an amplifier and via a resistor to a sine wave generator. The output voltage of an amplifier and the voltage of a sine wave generator are fed to a phase comparison circuit. The phase difference depends on the capacitor which is formed by the electrode against earth. This capacitor is affected by an object near it, moving towards it or moving away from it. DE 195 01 642 B4 discusses further the use of a screening or shield grid electrode in a capacitive distance sensor.

To determine the distance of an object from the own vehicle using a capacitive distance sensor, its capacitance or capacitance change is determined. For this purpose, usually the capacitance of a measuring electrode against the own vehicle is determined. The measurable capacitance is affected by a nearby object. By using a screening electrode between the measuring electrode and the own vehicle, an improvement of sensitivity can be achieved. In a similar way as for a plate capacitor, the measured capacitance depends, among other things, reciprocally on the distance d between the measuring electrode and the object, and on the effective area A, the electric field constant (Eo 8.85 1012 F/rn) and the relative permittivity Er of the material between them.

Modern application fields in automotive engineering require detection of objects in the environment of an own vehicle at different distances. For instance, for driver assistance functions, e.g. "pre-crash" technology, it is important to detect, for instance, relatively distant objects, in particular external vehicles, whereas for parking or rnanoeuvrirlg assistance functions, it can be important to recognise objects which are in the immediate neighbourhood of the own vehicle and could damage the vehicle during parking and/or manoeuvring.

As well as the dependence -which is decisive for distance measurement -of the capacitance on the distance d, the capacitance also depends on the effective electrode area and the dielectric constant or relative permittivity. Because of the high relative permittivity of water, the capacitance of the arrangement is greatly effected by weather effects, e.g. fluctuating atmospheric humidity or temperature. There is therefore a need for improved capacitive distance measuring devices.

Disciosure of the invention Against this background, with this invention a method of capacitive distance measurement, and also a computing unit which uses this method, and a corresponding computer program and computer program product, are proposed according to the independent claims. Advantageous versions are given in the appropriate subclaims and in the following descriptionS The invention is based on the recognition that in the case of capacitive distance measurement, weather dependencies can be eliminated by means of capacitance measurement at different distances. According to the invention, by using a suitable geometrical arrangement of electrodes on a body, e.g. the own vehicle, the distance to other objects such as vehicles can be determined as far as possible irrespective of weather. For this purpose, as the sensor system an arrangement consisting of at least two electrodes, each of which is at a different distance from the measurement object, is proposed.

The device according to the invention has at least two measuring capacitors, each of which essentially comprises at least one measuring electrode and one counter-electrode, the counter-electrode usually being formed by the body to which the measuring electrode arrangement is attached, e.g. the own vehicle. In this case, a measuring capacitor comprises the measuring electrode and the counter-electrode, which is attached to the body or formed by the body itself, and the measuring capacitor has a capacitance which is raised by the approach of an object. The capacitance of the measuring capacitor is affected by objects which are in the environment of the electrodes. In this way a movement of an object towards or away from the electrodes can be established in the form of a capacitance change, and/or the distance of an object from the electrode, or magnitudes derived from it, can be determined. In particular, the method according to the invention uses a device according to the invention. In another version, which is also suitable for the invention, the counter-electrode is formed by the object.

If the counter-electrode is formed by the own vehicle, to improve the measurement characteristics, a screening electrode can additionally be provided between the measuring electrode and the counter-electrode, and for instance is at the same potential as the measuring electrode, via a high-resistance impedance converter according to normal circuit engineering. In this way, between the screening electrode and the measuring electrode a field-free region can be created. By this arrangement, an improvement of the measurement quality is achieved, the course of the lines of force between measuring electrode and counter-electrode at a lower absolute value of the capacitance being proportionally more strongly raised by an approaching object than would be the case without a screening electrode. In this case, i.e. in the absence of a screening electrode, the capacitance would be essentially determined by the measuring electrode being directly adjacent to the vehicle, but less by more distant objects.

Advantageously, the approach according to the invention provides an inexpensive method of eliminating weather effects, e.g. atmospheric humidity, on the measurement of distances using capacitive distance sensors.

This invention creates a method of capacitive distance measurement, a first capacitance value provided by a first measuring electrode and a second capacitance value provided by a second measuring electrode being determined. A distance value is determined on the basis of the first capacitance value, the second capacitance value and a predetermined distance between the first measuring electrode and the second measuring electrode. In particular, the predetermined distance is greater than zero in the direction of a surface normal. Preferably, the electrcde planes are thus arranged essentially parallel to each other at a predetermined distance. The method according to the invention makes it possible to eliminate environmental effects on capacitive distance measurement by measuring at different distances.

According to one version, the first measuring electrode has a first distance to a measurement object, the second measuring electrode has a second distance, which is different from the first distance, to the measurement object, and the predetermined distance corresponds to a difference between the first distance and the second distance.

The distance value can advantageously be determined without taking account of a relative permittivity of the material between object and measuring electrodes. In particular, the distance value can be determined depending on a ratio of the two capacitance values.

Advantageously, the distance value can be determined irrespective of the relative permittivity, since through formation of a ratio the latter no longer enters the distance calculation.

The first capacitance value and second capacitance value can be capacitance values which are provided independently of each other. Advantageously, in relation to a precision which is required for distance measurement, the first capacitance value and second capacitance value are at least approximately simultaneously captured capacitance values. In this way, imprecision in the distance determination, e.g. because of a moving measurement object, can be reduced. Advantageously, in relation to the distance measurement, the first measuring el'ectrode and second measuring electrode see at least approximately the same effective electrode area.

According to one version, to determine the distance value an analogue/digital conversion of the first and second capacitance values can take place. Thus the capacitance measurement values can be further processed in digital form.

The task on which the invention is based can also be achieved quickly and efficiently by the variant embodiment of the invention in the form of a computing unit. A computing unit according to the invention for capacitive distance measurement has, in particular, a device to receive a first capacitance value provided by a first measuring electrode via an interface, a device to receive a second capacitance value provided by a second measuring electrode via the interface, and a device to determine a distance value on the basis of the first capacitance value, the second capacitance value and a predetermined distance between the first measuring electrode and the second measuring electrode.

This invention also creates a sensor system for capacitive distance measurement, with a first measuring electrode and at least one second measuring electrode, the first measuring electrode and the at least one second measuring electrode being arranged so that they are at different distances from a measurement object and/or a counter-electrode. Advantageously, integration of the measuring electrodes into a multi-coat paint system of a vehicle, or building them behind protective mouldings or trim strips, is possible. Such a sensor system is suitable for providing measured capacitance values which can be evaluated by the method according to the invention. The method according to the invention can be executed by a controller, for instance.

The invention also concerns a computer program with program code means to carry out all steps of a method according to the invention, if the computer program is executed on a computer or a corresponding computing unit, in particular a controller (e.g. ECU).

The computer program product which is provided according to the invention, with program code means which are stored on a computer-readable data medium, is in a form to carry out all steps of a method, if the computer program is executed on a computer or a corresponding computing unit, in particular a controller (e.g. ECU) . Suitable data media are, in particular, diskettes, hard disks, flash memories, EEPROMs, CD-ROMs, DVDs and others. Downloading a program via computer networks (Internet, intranet etc.) is also possible.

Other advantages and versions of the invention are given in the

description and the attached drawings.

It is understood that the above-mentioned features, and those still to be explained below, can be used not only in the stated combination, but also in other combinations or alone, without leaving the framework of this invention.

The invention is shown schematically in the drawings on the basis of an embodiment, and is described in detail below with reference to the drawings.

Brief description of the drawings

Fig. 1 shows a. flowchart of amethod according to the invention; Fig. 2 shows a schematic representation of an electrode arrangement according to an embodiment of this invention; and Fig. 3 shows a schematic representation of an electrode arrangement according to a further embodiment of this invention.

Embodiment(s) of the invention Identical or similar elements can be provided in the following figures by identical or similar reference symbols. The figures of the drawings, their description and the claims also include numerous features in combination. It is clear to a person skilled in the art that these features can also be considered individually, or can be combined into further combinations which are not described explicitly here.

Fig. 1 shows a flowchart of a method of capacitive distance measurement according to an embodiment of this invention. In a first step, a first capacitance value, which is provided by a first measuring electrode, is determined 102. This capacitance value can, in particular, be received via an interface. The interface can be an interface between the first measuring electrode and a device to carry out the method according to the invention. In a second step, a second capacitance value, which is provided by a second measuring electrode, is determined 104.

The determining or receiving steps can be executed simultaneously or successively in time. In a third step, a distance value is determined 106, on the basis of the first capacitance value, th'e second capacitance value and a predetermined distance between the first measuring electrode and the second measuring electrode. The predetermined distance can be provided by a memory device, for instance.

According to an embodiment, the measuring electrodes can be arranged so that they have different distances to a measurement object. In this case, the predetermined distance can correspond to a difference between the different distances of the measuring electrodes to the measurement object. To determine the distance value, a computing device, which is in a form to determine the distance value on the basis of the first and second capacitance values and the predetermined distance, can be provided. For simpler processing, the first and second capacitance values can previously be subjected to analogue/digital conversion. The analogue/digital conversion can be carried out by means of a suitable analogue/digital converter. The computing device can be in a form to determine the distance value on the basis of a ratio of the first and second capacitance values.

Ideally, the first and second capacitance values are provided irrespective of different measuring electrodes. Capture of the first and second capacitance values should take place simultaneously or at least closely in time, so that a change of distance to the measurement object does not affect the measurement precision disadvantageously. The measuring electrodes can also be implemented and arranged so that they see the same or at least approximately the same effective electrode area.

Fig. 2 shows an embodiment to eliminate weather effects by means of two measuring electrodes at a predetermined distance. In particular, Fig. 2 shows a version of a measuring electrode arrangement 201 according to the invention, which thas two measurement distances and is suitable for use with the device according to the invention for capacitive distance measurement.

In particular, the electrode arrangement is suitable for providing the first and second capacitance values. The measuring electrode arrangement 201 has a first measuring electrode 202 and a second measuring electrode 204, for interaction with a counter-electrode 203. The counter-electrode 203 is usually formed by the own vehicle. The two measuring electrodes 202 and 204 have a predetermined distance 216 from each other in the direction of the counter-electrode 203 and/or of the object 206.

Because of the predetermined distance, the capacitance which the first measuring electrode 202 provides differs from the capacitance which the second measuring electrode 204 provides.

By means of the method according to the invention, distances such as the distance 212 or the distance 214, i.e. the distance between one of the electrodes 202, 204 and the object 206, can be determined.

The capacitance which the electrodes provide depends on, among other things, the distance of the object from the electrodes, and on the material between object and electrodes and/or its relative perrnittivity. Since the relative permittivity or dielectric constant depends very strongly on the atmospheric humidity (c of water 80) among other things, reproducible measurement of the distance, depending on the capacitance of the electrode arrangement, is highly error-prone.

According to the method according to the invention, from the known distance 216 and the measured capacitance values, for instance the distance 212 (d1) can be determined irrespective of the relative permittivity of the material between electrodes and object. The distance 214 (d2) could be determined corresponding1y.

The distances d1 and d2 can be approximated using a constant K, as:

K K

d,-d2,-, Ui Thus, the result for the distance d1 is the following roughly approximated relation: d2C2 M d1-,-C2 i.e. the distance to be determined d1 depends approximately only on the known distance of the electrodes d and the quotient of the measured capacitances. The weather-dependent relative permittivity r no longer enters the calculation.

The capacitances can be measured by means of known methods, e.g. by measuring the rise time. The quotient can advantageously be formed by dividing the A/D converted dimensions in an arithmetic unit.

According to this embodiment, the capacitances of the two arrangements are measured independently of each other. The aim is to measure the capacitance values as simultaneously as possible, in comparison with the dynamics of the setting. The measuring electrodes should also have the same effective electrode area as far as possible.

Fig. 3 shows an embodiment for attaching the measuring electrodes 202, 204 to a vehicle 301 (own vehicle) . A schematic representation of the vehicle 301 with one occupant is shown. On one side of the vehicle is the measuring electrode arrangement 201, comprising the first electrode 202 and the second electrode 204. The measuring electrode arrangement 201 is thus suitable for measuring the distance to an object 206. Corresponding to Fig. 1, the distance 212 (di) between the first electrode 202 and the object 206, and the distance 214 (d2) between the second electrode 204 and the object 206 are shown. The distance 216 (Ad) between the first electrode 202 and the second electrode 204 is also shown. The arrow 302 indicates a movement of the object 206 in the direction of the electrodes 202, 204.

For the arrangement according to Fig. 3, the counter-electrode is formed by the own vehicle 301. A movement of the external vehicle 206 results in a change of the distances 212, 214 between the external vehicle, the first measuring electrode 202 and the second measuring electrode 204, and thus to a capacitance change.

The electrodes 202, 204 can be arranged on the vehicle according to different integration strategies. A possible integration strategy is the integration of the electrodes 202, 204 into a multi-coat paint system. Alternatively, it is possible to build the electrodes 202, 204 behind protective mouldings or trim strips on the own vehicle. All arrangements which make a distance measurement to the object 206 possible are suitable.

The described embodiments are only chosen as examples, and can be combined with each other. Instead of the shown two different measuring electrodes, which have different measurement distances to the object, three or more measuring electrodes can be used.

In this way several measured capacitance values can be captured, and increase the measurement precision. The measuring electrodes can be in the form of plates or in a different suitable electrode form. The arrangement of the measuring electrodes can also be adapted to the conditions of the relevant vehicle. In particular, however, the invention is not restricted to use with vehicles.