Abstract
A sensor system for the capacitive detection of obstacles, having a capacitive sensor with conductive elements and a control circuit connected thereto. The control circuit has a bridge circuit, and a first end of the bridge branch is connected to a conductive element of the sensor positioned upstream in the direction of detection and a second end of the bridge branch is connected to a conductive element of the sensor positioned downstream in the direction of detection. A control signal is generated by a control section of the control circuit and the sum of impedances of the bridge circuit connected to the first end of the bridge branch is less than the sum of impedances of the bridge circuit connected to the second end of the bridge branch. An electronic evaluation unit is provided to evaluate a voltage difference between the first and second ends of the bridge branch.
Claims
1. A sensor system for the capacitive detection of obstacles, having a capacitive sensor with at least two conductive elements and a control circuit connected to the conductive elements, wherein the control circuit has a bridge circuit, wherein a first end of the bridge branch is connected to a conductive element of the sensor positioned upstream in the direction of detection and a second end of the bridge branch is connected to a conductive element of the sensor positioned downstream in the direction of detection, wherein a control signal is generated by a control section of the control circuit and wherein the sum of the impedances of the bridge circuit which are connected to the first end of the bridge branch is less than the sum of the impedances of the bridge circuit which are connected to the second end of the bridge branch, wherein the control signal is fed into both conductive elements and an electronic evaluation unit is provided to evaluate a voltage difference between the first end and the second end of the bridge branch.
2. The sensor system as claimed in claim 1, wherein that the two conductive elements are designed as conductors running continuously in the longitudinal direction of a switching strip profile or as flat electrodes or grid electrodes of a capacitive area sensor.
3. The sensor system as claimed in claim 1, wherein the control section has a first impedance, wherein the first end of the bridge branch is disposed between a second and a third impedance and the second end of the bridge branch is disposed between a fourth and a fifth impedance, wherein the first impedance is less than the sum of the second and the third impedance and the sum of the second and the third impedance is less than the sum of the fourth and the fifth impedance.
4. The sensor system as claimed in claim 1, wherein an adjustable impedance is provided in parallel with at least one of the impedances of the bridge circuit in order to effect an equalization of the voltage difference between the first end and the second end of the bridge branch via a change in the adjustable impedance.
5. The sensor system as claimed in claim 1, wherein the electronic evaluation unit has an adjustable impedance in order to effect an equalization of the output signal of the electronic evaluation unit.
6. The sensor system as claimed in claim 5, wherein adjustable impedance is provided in the feedback branch of an amplifier of the electronic evaluation unit,
7. The sensor system as claimed in claim 1, wherein a voltage level of the control signal amounts to twice to fifteen times, in particular ten times, the supply voltage of the electronic evaluation unit.
8. The sensor system as claimed in claim 1, wherein the control signal is designed as a sinusoidal signal.
9. The sensor system as claimed in claim 1, wherein a voltage deviation of between 20 volts and 40 volts, in particular 30 volts.
10. The sensor system as claimed in claim 8, wherein the control circuit has an oscillating circuit.
11. The sensor system as claimed in claim 10, wherein the sum of the impedances of the oscillating circuit is less than the sum of the impedances of the bridge circuit which are connected to the first end of the bridge branch.
12. The sensor system as claimed in claim 10, wherein the oscillating circuit is partially formed by the impedances of the bridge circuit which are connected to the first end of the bridge branch.
13. A method for the capacitive detection of obstacles with a sensor system as claimed in claim 1, the method including evaluation of a voltage difference between the first end and the second end of the bridge branch.
Description
[0025] Further features and advantages of the invention can be found in the claims and in the following description of preferred embodiments of the invention in conjunction with the drawings. Individual features of the different embodiments that are shown and described can be combined in any given manner without exceeding the scope of the invention. In the drawings:
[0026] FIG. 1 shows a schematic diagram of a switching strip system according to the invention according to a first embodiment,
[0027] FIG. 2 shows a schematic representation to explain the measurement principle of the switching strip system according to the invention,
[0028] FIG. 3 shows a schematic diagram of a second embodiment of the switching strip system according to the invention,
[0029] FIG. 4 shows a schematic diagram of a third embodiment of the switching strip system according to the invention,
[0030] FIG. 5 shows a schematic diagram of a fourth embodiment of the switching strip system according to the invention,
[0031] FIG. 6 shows a schematic diagram of a fifth embodiment of the switching strip system according to the invention,
[0032] FIG. 7 shows a schematic diagram of a sixth embodiment of the switching strip system according to the invention,
[0033] FIG. 8 shows a schematic diagram of a seventh embodiment of the switching strip system according to the invention, wherein different possible positions of one or more adjustable impedances are shown by dotted lines, and
[0034] FIG. 9 shows a schematic diagram of an eighth embodiment of the switching strip system according to the invention.
[0035] The representation in FIG. 1 shows a schematic diagram of a switching strip system or sensor system according to the invention according to a first embodiment. A switching strip profile 10 has a conductor 14 positioned upstream, as seen in a direction of detection 12, and a conductor 16 positioned downstream, as seen in the direction of detection 12. The direction of detection 12 merely represents the midline of a detection area which may extend over a greater angular range. The two conductors 16, 14 are merely shown schematically and may have a different geometric shape than the geometric shape illustrated. For the sake of simplicity, only two conductors 14, 16 are shown. The switching strip profile 10 may have more than two conductors according to the invention, for example a conductor 14 positioned upstream in the direction of detection 12, and three conductors 16 positioned downstream in the direction of detection 12. According to the invention, a capacitive sensor with at least two conductive elements can generally also be provided, for example an area sensor.
[0036] The switching strip system has a control circuit with a control section 18 and an electronic evaluation unit 20. In the schematic diagram shown in FIG. 1, the evaluation unit 20 is presented in the form of a single operational amplifier 22, but may obviously have a plurality of amplifiers, microcontrollers or the like, provided that they are capable of evaluating a voltage difference between the first conductor 14 and the second conductor 16.
[0037] The control section 18 has a bridge circuit 24 with four impedances Z.sub.2, Z.sub.3, Z.sub.4 and Z.sub.5. A bridge branch is defined between the points P.sub.1 and P.sub.2 and the conductor 14 of the switching strip profile 10 positioned upstream in the direction of detection 12 is connected to the first end P.sub.1 of the bridge branch and the conductor 16 of the switching strip profile 10 positioned downstream in the direction of detection 12 is connected to the second end P.sub.2 of the bridge branch. The two impedances Z.sub.2 and Z.sub.3 are connected to the first end P.sub.1 of the bridge branch. Z.sub.3 connects the first end P.sub.1 to ground. Z.sub.2 connects the first end P.sub.1 of the bridge branch to an oscillating circuit 26.
[0038] The second end P.sub.2 of the bridge branch is connected to the impedances Z.sub.4 and Z.sub.5. Z.sub.5 connects the second end P.sub.2 of the bridge branch to ground. Z.sub.4 connects the second end P.sub.2 of the bridge branch to the oscillating circuit 26.
[0039] The oscillating circuit 26 has a first impedance Z.sub.0 and a second impedance Z.sub.1. The impedances Z.sub.2 and Z.sub.4 are connected to a point between the two impedances Z.sub.0 and Z.sub.1. Z.sub.1 is connected, on the other side, to ground. The representation of the oscillating circuit 26 is merely schematic, the oscillating circuit 26 being excited in such a way that a sinusoidal signal is formed at the point between the impedances Z.sub.0 and Z.sub.1.
[0040] The sum of the impedances Z.sub.0 and Z.sub.1 which form the impedance of the oscillating circuit 26 is less than the sum of the second impedance Z.sub.2 and the third impedance Z.sub.3. The sum of the impedances Z.sub.2 and Z.sub.3 is in turn less than the sum of the fourth impedance Z.sub.4 and the fifth impedance Z.sub.5.
[0041] The impedances Z.sub.2, Z.sub.3, Z.sub.4 and Z.sub.5 are selected in such a way that a desired voltage level is present in each case on the upstream conductor 14 and the downstream conductor 16. In the operation of the switching strip system, a sinusoidal signal is thus present on both conductors 14, 16, wherein the voltage amplitudes and the voltage levels of these sinusoidal signals may be different, but, depending on the intended purpose of the application, may also be identical.
[0042] The two inputs of the operational amplifier 22 or generally the two inputs of the electronic evaluation unit 20 are connected to the first end P.sub.1 of the bridge branch or the second end P.sub.2 of the bridge branch and therefore also to the upstream conductor(s) 14 or the downstream conductor(s) 16. The operational amplifier 22 or the electronic evaluation unit 20 thus evaluates a voltage difference between the first end P.sub.1 of the bridge branch and the second end P.sub.2 of the bridge branch and, concomitantly, the voltage difference between the two conductors 14, 16.
[0043] In the idle state, i.e. when no obstacle is located downstream of the switching strip profile 10, seen in the direction of detection 12, the electronic evaluation unit 20 recognizes a constant voltage difference between the two conductors 14, 16. If an obstacle then approaches the switching strip profile 10, the capacitances between the upstream conductor 14 and ground and between the downstream conductor 16 and ground change. The reason for this is that an obstacle, for example a human hand, forms a capacitance between itself and each of the two conductors 14, 16 and additionally also a capacitance between itself and ground. The approach of an obstacle will therefore also change the signal on the two conductors 14, 16. Since the sum of the capacitances Z.sub.2 and Z.sub.3 differs from the sum of the capacitances Z.sub.4 and Z.sub.5, the approach of an obstacle will influence the signal on the upstream conductor 14 differently than the signal on the downstream conductor 16. A voltage difference will thus form between the first end P.sub.1 and the second end P.sub.2 of the bridge branch and can be detected by means of the electronic evaluation unit 20 and evaluated so that an obstacle is detected if a predefined limit value is exceeded. Following the detection of an obstacle, the drive of an electrically driven tailgate, for example, can be stopped or reversed.
[0044] The representation in FIG. 1 clearly shows that the approach of an obstacle influences both the signal on the upstream conductor 14 and the signal on the downstream conductor 16. The evaluation of the voltage difference between the upstream conductor 14 and the downstream conductor 16 and between the first end P.sub.1 and the second end P.sub.2 of the bridge branch thus has the fundamental disadvantage that the approach of an obstacle is partially compensated. Surprisingly, however, the evaluation of the voltage difference between the first end P.sub.1 and the second end P.sub.2 of the bridge branch has the substantial advantage that interfering influences on the two conductors 14, 16 are also automatically compensated. If, for example, the switching strip 10 is located in the field of an electromagnetic radiation, both conductors 14, 16 are hereby influenced and the signal on the two conductors 14, 16 will therefore also reflect the influence of this electromagnetic interference. However, this electromagnetic interference is automatically eliminated in the determination of the difference between the voltages on the two conductors 14, 16. The same applies, for example, to interference due to temperature influence, for example if the switching strip 10 changes its temperature. As a result, the resistances of the conductors 14, 16 and, where relevant, also a capacitance between the two conductors 14, 16 and, where relevant, also a capacitance of the two conductors 14, 16 to ground will also inevitably change. However, in such a case also, the signals on the two conductors 14, 16 will be influenced so that these interfering influences will be eliminated by the evaluation of the voltage difference between the first end P.sub.1 and the second end P.sub.2 of the bridge branch. The fundamental disadvantage in the evaluation of the voltage difference between the two conductors 14, 16 will therefore be more than compensated by the substantial advantage of a very low sensitivity to interference.
[0045] The representation in FIG. 2 serves to explain the measurement principle with the switching strip system according to the invention, wherein the measuring circuit itself is not shown in the drawing for the sake of clarity. The two conductors 14, 16 of the switching strip profile 10 in each case form a capacitance to ground. A capacitance between the conductor 14 positioned upstream, seen in the direction of detection 12, is denoted Z.sub.31, a capacitance between the conductor 16 positioned downstream in the direction of detection 12 and ground is denoted Z.sub.51. A hand 30 forms an obstacle to be detected by means of the switching strip system. The hand 30 has a capacitance to ground Z.sub.62, representing the human body with its discharge to ground. The hand 30 forms a capacitance Z.sub.60 with the conductor 16 positioned downstream in the direction of detection 12 and a capacitance Z.sub.61 with the conductor 14 positioned upstream in the direction of detection 12. The voltage signals on both conductors 14, 16 are thus influenced by the hand 30. As a result of the presence of the hand 30, the voltages on the first end P.sub.1 of the bridge branch and on the second end P.sub.2 of the bridge branch thus change in the same direction, but with different strengths, since, as explained, the impedances Z.sub.2 and Z.sub.3 are different from Z.sub.4 and Z.sub.5. This results in a voltage difference which can then be evaluated. A dimensioning of the impedances Z.sub.2, Z.sub.3, Z.sub.4 and Z.sub.5, see FIG. 1, in such a way that the ratio
[00001]
is equal to the ratio of the sum of the impedances in the first and the second bridge branch, i.e.
[00002]
has been found to be advantageous.
[0046] The aim is that, as far as possible, no phase shift occurs in the input signals on the inputs of the operational amplifier 22.
[0047] The representation in FIG. 3 shows a schematic diagram of a switching strip system according to the invention according to a further embodiment of the invention. Only the features differing from the embodiment shown in FIG. 1 will be explained.
[0048] In the embodiment shown in FIG. 3, an oscillating circuit is integrated into the bridge circuit 24. The oscillating circuit is formed by the impedances Z.sub.0, Z.sub.12 and Z.sub.13. The first end P.sub.1 of the bridge branch is located between the impedances Z.sub.12 and Z.sub.13. The impedance Z.sub.4 is connected to a point between the impedances Z.sub.0 and Z.sub.12. Through the integration of the oscillating circuit into the branch of the bridge circuit which is connected to the first end P.sub.1 of the bridge circuit, this side of the bridge circuit can be designed as having a lower impedance. The impedance Z.sub.13 can therefore be selected as less than the impedance Z.sub.3 according to FIG. 1.
[0049] When an obstacle approaches, see FIG. 2, the impedance Z.sub.13 is located parallel to the capacitance Z.sub.31, since both the impedance Z.sub.13 and the capacitance Z.sub.31 connect the conductor 14 located upstream in the direction of detection and the first end P.sub.1 of the bridge branch to ground. However, compared with the embodiment shown in FIG. 1, the impedance formed by the parallel connection of the impedances Z.sub.13 and Z.sub.31 is less than the sum of the impedances Z.sub.61 and Z.sub.62, see FIG. 2. The impedances Z.sub.61 and Z.sub.62 represent the capacitance of the hand 30 to the upstream conductor 14 and between the hand 30 and ground. As a result, the switching strip system reacts more sensitively to the approach of a hand 30 and the range can be optimized.
[0050] The representation in FIG. 4 shows a further embodiment of a switching strip system according to the invention. The switching strip system shown in FIG. 4 has a first switching strip profile 10 and a second switching strip profile 10 ′. Further switching strip profiles can be connected in the same way. An electronic evaluation unit 20 or 20 ′ is allocated to each switching strip profile 10, 10 ′. A bridge circuit 24 or 24 ′ is also allocated to each switching strip profile 10, 10 ′. The oscillating circuit 26 feeds the generated sinusoidal signal not only into the bridge circuit 24 and therefore into the conductors of the switching strip profile 10 but also into the bridge circuit 24 ′ and therefore into the conductors of the switching strip profile 10 ′. In this way, two or more switching strip profiles 10, 10 ′ can be controlled with the same sinusoidal signal. For example, switching strips on different sides of a motor-driven tailgate can be controlled and evaluated in this way.
[0051] The representation in FIG. 5 shows a further embodiment of a switching strip system according to the invention. In this embodiment also, two or more switching strips 10, 10 ′ or a plurality of sensors are provided and each of these switching strips 10, 10 ′ or sensors is allocated to the electronic evaluation unit 20 or 20 ′. As in the embodiment explained with reference to FIG. 3, the oscillating circuit is integrated into the side of the bridge circuit which is connected to the first end P.sub.1 or P.sub.1′of the bridge branch. For example, two switching strips 10, 10 ′ can be combined with one or more capacitive area sensors.
[0052] FIG. 6 shows a further embodiment of a switching strip system according to the invention, wherein, compared with the switching strip system shown in FIG. 1, only one adjustable impedance Z.sub.V is provided. The adjustable impedance Z.sub.V, is provided in the feedback branch of the operational amplifier 22. The voltage difference between the two conductors 14, 16 and the voltage difference between the first end P.sub.1 and the second end P.sub.2 of the bridge branch can be set to a desired value with the adjustable impedance Z.sub.V. As a result, for example, slightly different installation conditions in series production can be compensated, and the same voltage difference is always present on the operational amplifier 22 or on the electronic evaluation unit in the idle state, i.e. without the presence of an obstacle. The adjustable impedance Z.sub.V is advantageously set immediately after the installation of the switching strip profile 10 and the switching strip system is thereby equalized. This can take place, for example, during production on the assembly line, for example when the switching strip profile 10 has been fitted in the area of the tailgate of a motor vehicle.
[0053] The representation in FIG. 7 shows a further switching strip system according to the invention, wherein, unlike in the representation in FIG. 6, the adjustable impedance Z.sub.V has now been provided in parallel with the impedance Z.sub.2 which connects the first end P.sub.1 of the bridge branch to the point between the impedances Z.sub.0 and Z.sub.1. Such an arrangement of the adjustable impedance Z.sub.V also allows an, in particular automatic, equalization of the voltage difference between the first end P.sub.1 and the second end P.sub.2 of the bridge branch.
[0054] The representation in FIG. 8 shows the switching strip system from FIG. 1, wherein possible positions of the adjustable impedance Z.sub.V are indicated by dotted lines. Each of these positions of the adjustable impedance Z.sub.V, indicated by dotted lines can be selected alone, but two or more adjustable impedances Z.sub.V are possible at the positions shown in order to provide an automatic equalization of the voltage difference between the first end P.sub.1 and the second end P.sub.2 of the bridge branch.
[0055] The representation in FIG. 9 shows a further sensor system according to the invention, wherein a further impedance Z.sub.E which interconnects the two conductors 14, 16 of the switching strip 10 is provided between the operational amplifier 22 and the two conductors 14, 16 of the switching strip 10. The impedance Z.sub.E is designed as an inductance and is appropriately provided at one end of the switching strip 10. The circuit can be made very narrowband by means of the impedance Z.sub.E, as a result of which a very high sensitivity is achieved in the area of the resonant frequency. In addition, the impedance Z.sub.E can be used for the diagnosis of the switching strip 10, i.e. to check whether the conductors 16 of the switching strip 10 are interrupted or short-circuited.
