Method for measuring and displaying the distance between a motor vehicle and an obstacle, and capacitive measuring instrument for motor vehicles.

The invention relates to a method for measuring and displaying the distance of a motor vehicle from an obstacle by measuring the capacitance between a motor vehicle fixed capacitor plate and the environment by means of a capacitance sensing and to a capacitive proximity sensor for motor vehicles with a capacitance measuring for measuring the capacitance between at least one motor vehicle firm capacitor plate and the surroundings and having a display device, which is supplied with the output signal of the measuring device.

Such a proximity sensor is described in DE-OS 2 O44 79O. Hiebei to eliminate a bridge circuit disturbing or parasitic capacitances. While it is possible to eliminate with the help of certain AC bridges even large parasitic capacitance, but such a bridge when changing the impedances in their branches on the other hand very quickly and strongly detuned so that it ultimately reaches beyond the available balance area. It should be remembered first of all that when used in motor vehicles constantly changing environmental conditions occur. For this purpose is also for example, the change in the capacitance of the motor vehicle against the ground at different loadings of the vehicle. These changes are often an order of magnitude over the capacitance to be measured changes of the capacitor plate. Parasitic resistances occur and constantly change their value, for example, must be thought of saltwater influences in winter in this regard. Overall, the well-known bridge circuit is well suited for compensating the parasitic capacitance of a shielded cable which connects the transmitter to the bridge that this bridge circuit, however, does not consider the ever changing parasitic resistances and capacitances outside of the motor vehicle.

The aim of the invention is to provide a measuring method and a capacitive proximity sensor that takes into account the occurring in practice and interfering parasitic capacitances in a special way.

This goal can be achieved, in which the present invention the output of the capacitance sensing is fed to an amplifier, which includes a negative feedback loop with a memory element and a switch, and by this means the output of the amplifier is kept constant during normal driving and that with a method of the type mentioned to opens measuring the switch and by this means the negative feedback loop and closed again after finishing the measurement.

The object is furthermore achieved as mentioned above with a proximity sensor, wherein the fixed capacitor plate is formed as a panel, in particular as an indicator panel on the bodywork of the motor vehicle or as an electrically conductive layer before or after such a panel, the measuring means comprises a negative feedback control loop, a memory for storing the respective feedback signal (deviation) is provided and that the control loop is interrupted by a switch which is directly or indirectly, for example when inserting a reverse gear, actuated by the driver.

Thus, the invention assumes that when a board as a condenser plate, which is advantageously is the license plate, the output of the measuring device is commonly regulated by a negative feedback loop to a determined value, wherein the respective final value of the control deviation (corresponding to the feedback signal ) is stored. Only then, when the proximity sensor should actually be used, ie for example when parking and shifting into reverse gear, the control loop is interrupted by a switch. Now variations of the capacity of the most recently determined value are displayed, they may for example read directly from a meter or be fed to a limit comparator. After completion of the parking process, the control loop is closed by operating the switch.

The inventive solution should not be confused with a bridge zeroing, as it is for example mentioned in the DE-OS 27 37 11O, page 4, 5th paragraph.. Such zeroing to compensate eg environment capacity is not comparable to a negative feedback control and storage of the last measurement respectively.

An advantageous development of the invention is characterized in that the display device comprises a limit comparator for the output signal of the measuring device.

The invention together with further advantages and features is explained in the following example wise to hand embodiments, which are illustrated in the drawing. Therein, FIG. 1a to FIG. 1e in schematic plan view of various possibilities of arranging a capacitor plate on motor vehicles, FIG. 2 is a block diagram of the electronics of the proximity sensor, Fig. 3a and b drawn up in the schemati side view, the geometric or capacitive ratios between the capacitor plate and its surroundings, Fig. 4 shows the time course of the output signal of the proximity sensor before and during the measurement, Fig. 5 is a circuit of FIG. 2 in greater detail, and Fig. 5a two signal diagrams.

In Fig. 1a a capacitor plate 1 of the proximity sensor according to the invention is attached to the rear of a motor vehicle 2. As a condenser plate 1 is, advantageously, the rear license plate is used which is fixed for this purpose with respect to the isolated body. Fig. 1b shows a respective capacitor plate 1 or 1 'at the rear and front of a motor vehicle 2, Fig. 1c shows a capacitor plate 1 or 1' at the rear and front of a truck 3 with a trailer 4, wherein a capacitor plate 1 to the rear side of the trailer 4 and a capacitor plate 1 'located at the front of the truck. 3

As capacitor plate or plates not only the license plates but also other boards, such as billboards, etc. can be used. The capacitor plate may alternatively be formed by a thin conductive surface coating on the rear or the front of a panel, wherein said coating should be transparent or translucent in the latter case.

The block diagram of FIG. 2 reveals that the capacitor plate 1 is connected to the input of a capacitance sensing. 5 This measuring device 5 corresponds to a capacitance measuring essentially any of the prior art and provides an output measuring signal s_c (Eg voltage or current) that is a function of measured between the capacitor plate 1 and the area (mass) capacity.

The measurement signal s_can input of an amplifier 6 is supplied to its output at which a signal s_A occurs is connected to one input of a comparator 7th At the other input of this comparator 7 is a fixed threshold signal s_G, The comparator output is guided to a display device 8, make an optical or acoustic signal is as capable.

The output signal s_A of the amplifier 6 is fed back via a negative feedback loop 9 to a second input of the amplifier. 6 In the loop, 9 is a switch 1O and a storage member 11 are provided. The switch 1O is directly or indirectly operated by an actuator 12 of, said actuator for example be formed from the switch for the reverse gear or a separate switch may be on the dashboard. The negative feedback via the loop 9 comprises a negative feedback factor to one, so that when the switch is closed 1O The output signal S_A a constant value, independent of the measurement signal S_c, Maintains. As indicated by dashed lines in Fig. 2, the negative feedback can if necessary act on the measuring device. 5

Before the function of the circuit will be explained, reference is made to Figs. 3a and b. The capacitor plate 1, for example, the license plate, has a height a, a length b and a thickness c on. When mounting the plate 1 shown in Fig. 3b and c, the plate having an area X = b. c against the bottom 13 and with an area Y = a. b to the rear (or front), for example against an obstacle 14. It is understood that the area Y is much greater because of the relatively small thickness c of the plate 1 as the base 13 facing surface X.

Between the panel 1 and located at a distance d barrier 14 (eg, another vehicle) is present during an approaching strongly changing capacitance C₁. The motor vehicle 2 has a capacitance C₂ against the bottom 13 and between the plate 1 and the vehicle 2 is a (relatively large) capacitance C₃ ago. is an undesired parasitic capacitance C₄ whose value varies with the ground clearance of the motor vehicle 2 between the panel 1 and the bottom. there may be a capacitive coupling between the C₅ obstacle 14 and the floor. 13 It should be noted that the capacitances C₂, C₃, C₄ and C₅ change with a change in the distance d is not or only minimally, whereas the essential for measuring capacitance C₁ is directly dependent on the distance d.

The proximity sensor according to the invention operates as follows. In normal operation of the motor vehicle, the switch 1O remains closed. Although the measuring device 5 provides a constantly changing measurement signal s_c, But the output signal remains s_A the amplifier 6 unchanged. The respective size of the feedback signal, that is, the control error is, however, stored in a storage capacitor to the memory element 11, for example.

If, for example when parking, the proximity sensor is to come into operation, the switch 1O and thus the negative feedback loop is opened. This can be done for example in reverse gear is engaged via the switch for the reversing light or on the driver's direct command. At this moment changes the output signal s_A not because the negative feedback input of the amplifier 6 is the last authentic i cherglied 11 stored value. However, each of this, taking place especially by approaching the obstacle 14 amending covered by the panel 1 capacity now changes the output signal s_AAs shown in Fig. 4, in which with t_m the time of opening of the switch is referred to 1O.

Unless a comparator 7 is provided, is by exceeding or falling below the fixed limit signal s_G outputted via the display device 8, a signal for the driver is notified of the fact that he has dropped below a certain distance to the obstacle. After closing the switch 1O (time t_e in Fig. 4) is again a closed loop (negative feedback) loop in front and the donor is for the next measurement.

Although the capacitance C₃ between the panel 1 and the body of motor vehicle 2 in the order of 1OOpF, to be detected change in the capacitance C₁, however, is of the order of O, 1pF, thanks to the invention, a reliable evaluation of this change is possible, whereby substantially is that the actual measuring is limited to the short period during parking, so that long-term changes of the remaining capacity, especially the capacity C₂ and C₃, not significant. For example, a bending of the license plate (Plate 1) lead to a change in the capacitance C₃ and higher loading of the motor vehicle to a magnification of C₂; the operational capability of the encoder is by this means, however, not be adversely affected.

In Fig. 5 is a circuit of Fig. 2 is shown in greater detail and described below. The measuring device 5 comprises a resistor R₁, which is partly due to a DC voltage and on the other hand connected to the fixed panel first The capacitor C₁ is charged up via the resistor R₁, R₂ to the driven via Schmitt trigger switches, wherein the switch-over voltage is determined by the resistors R₂ and R₃. Now C₁ discharges through the diode D₁. The measurement signal s_c thus shows a variable, dependent on the size of the capacitor C₁ duty cycle.

The amplifier 6 includes an operational amplifier OP₁ with integration by the RC Gied C₆, R₆. The gain is set by the resistors R₄ and R₆. The output signal s_A the amplifier is a voltage, the duty cycle of the signal S from the_c depends.

Via the controlled switch 1O, the output signal s_A the memory member 11 is fed, and that the inverting input of an operational amplifier OP₃. At the non-inverting input is a reference voltage U_ref, The output of the amplifier is connected to its inverting input via a capacitor C₈, corresponds to the voltage of the control deviation, so that voltage that is necessary to - fed via R₅ - to bring the amplifier OP₁ in a state in which a constant output signal s_A present. The non-inverting input of the amplifier OP₁ is also located on the RC element R₇, C₇ a reference voltage U_ref,

When closing the switch S₁ (in the actuator 12), here the switch for the reversing light, the voltage of the car battery B to the reversing light is R_y down and by this means open the controlled switch S₂.

The comparator 7 compares the signal by an amplifier OP₂ s_A, Which is at the inverting input, with the threshold voltage S_GWhich is located at the non-inverting input and which is adjustable by a potentiometer P₁.

The output of the comparator 7 operated a controlled switch S₃ of the display device, so that, for example, an audio frequency generator G with a loudspeaker L or - only indicated by dashed lines - an indicator lamp L_A be connected to a voltage.

The circuit shown in Figure 5. Represents merely one embodiment of the invention which can be realized also in other ways using known electronic assembly or using micro computers and appropriate software.