CAPACITIVE SENSOR

Abstract

A capacitive sensor that includes: a sensing electrode having a capacitance to be measured; an alternating voltage source, configured to apply an alternating voltage to the sensing electrode; a capacitive first transfer device; a measurement circuit configured to measure the capacitance of the sensing electrode; and a switching arrangement. The switching arrangement is configured to alternately, in a first switching state, connect the first transfer device to the sensing electrode to enable a charge transfer from the sensing electrode to the first transfer device and, in a second switching state, connect the first transfer device to the measurement circuit to enable a charge transfer from the first transfer device to the measurement circuit.

Claims

1. A capacitive sensor comprising a sensing electrode for capacitively coupling to a counterelectrode to form a capacitance to be measured; an alternating voltage source, configured to operatively coupling an alternating voltage to the sensing electrode; a capacitive first transfer device; a measurement circuit configured to measure the capacitance; and a switching arrangement configured to alternately in a first switching state, connect the first transfer device to the sensing electrode to enable a charge transfer from the sensing electrode to the first transfer device; and in a second switching state, connect the first transfer device to the measurement circuit to enable a charge transfer from the first transfer device to the measurement circuit.

2. A capacitive sensor according to claim 1, wherein the first transfer device comprises a first transfer capacitor with a first terminal and second terminal; the alternating voltage source is configured to generate an alternating voltage at a first node; the measurement circuit is connected to a second node and a third node; and the switching arrangement is configured to alternately in the first switching state, connect the first terminal to the sensing electrode and the second terminal to the first node; and in the second switching state, connect the first terminal to the second node and the second terminal to the third node.

3. A capacitive sensor according to claim 1, wherein the measurement circuit (20) comprises an integration capacitor (C.sub.i) connected to the second node and the third node; and a transimpedance amplifier connected to the second node and the third node.

4. A capacitive sensor according to claim 1, wherein the alternating voltage source is configured to generate a sinusoidal voltage.

5. A capacitive sensor according to claim 4, wherein the switching arrangement is configured to be in the first switching state when the voltage is rising and is to be in the second switching state when the voltage is dropping.

6. A capacitive sensor according to claim 2, wherein the third node is connected to ground.

7. A capacitive sensor according to claim 2, wherein the third node is connected to a constant voltage source.

8. A capacitive sensor according to claim 2, wherein the switching arrangement is configured to connect the sensing electrode to the first node in the second switching state.

9. A capacitive sensor according to claim 1, wherein it further comprises a capacitive second transfer device and that the switching arrangement is further configured to in the first switching state, connect the second transfer device to the measurement circuit to enable a charge transfer from the second transfer device to the measurement circuit; and in the second switching state, connect the second transfer device to the sensing electrode to enable a charge transfer from the sensing electrode to the second transfer device.

10. A capacitive sensor according to claim 9, wherein the second transfer device comprises a second transfer capacitor having a third terminal and a fourth terminal and that the switching arrangement is configured to connect the third terminal to the third node and the fourth terminal to the second node in the first state; and connect the third terminal to the sensing electrode and the fourth terminal to the first node in the second switching state.

11. A capacitive sensor according to claim 1, wherein said counterelectrode is formed by ground and wherein said alternating voltage (V.sub.1) of said alternating voltage source is applied to said sensing electrode.

12. A capacitive sensor according to claim 1, wherein said counterelectrode is a transmitting antenna electrode for emitting an alternating electric field in response to an alternating signal from said alternating voltage source applied to said counterelectrode, and wherein said alternating voltage is induced in said sensing electrode by said alternating electric field coupling into said sensing electrode.

13. Method for capacitive sensing with a sensing electrode capacitively coupling to a counterelectrode to form a capacitance to be measured, a capacitive first transfer device and a measurement circuit, the method comprising: operatively coupling an alternating voltage to the sensing electrode; alternately in a first switching state, connecting the first transfer device to the sensing electrode to enable a charge transfer from the sensing electrode to the first transfer device; and in a second switching state, connecting the first transfer device to the measurement circuit to enable a charge transfer from the first transfer device to the measurement circuit; and the measurement circuit measuring the capacitance of the sensing electrode;

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0031] FIG. 1 is a schematic view showing a first embodiment of an inventive capacitive sensor;

[0032] FIG. 2 is a schematic view showing a second embodiment of an inventive capacitive sensor;

[0033] FIG. 3 is a diagram showing a time evolution of an alternating voltage; and

[0034] FIG. 4 is a schematic view showing a third embodiment of an inventive capacitive sensor.

DETAILED DESCRIPTION

[0035] FIG. 1 by way of example illustrates a first embodiment of a capacitive sensor 1 according to the invention. The sensor 1 could be arranged in a vehicle seat as an occupancy sensor, in a vehicle bumper as a smart trunk opener, in a handle of a vehicle door as a smart door opener, in a mobile electronic device as part of a touch screen or in other known applications of capacitive sensors. It comprises a sensing electrode 2, which is associated with a capacitance C.sub.X relative to ground. The capacitance C.sub.X is unknown and varies with the presence of an object approaching or touching the sensing electrode 2. The object could be the body of a person, a person's finger or the like.

[0036] The sensing electrode 2 is connectable via a first switch 10 to an alternating voltage source 3. The alternating voltage source 3 applies a preferably sinusoidal voltage V.sub.1 (shown in FIG. 3) to a first node 7. In a first switching state (indicated by the number 1 in a circle), the first switch 10 is open so that there is no direct connection between the sensing electrode 2 and the alternating voltage source 3. In the second switching state (indicated by the number 2 in a circle), the first switch 10 is closed so that the sensing electrode 2 is connected to the first node 7 and thus to the alternating voltage source 3.

[0037] A first transfer capacitor C.sub.t1 is connected with a first terminal 5 to a second switch 11 and with a second terminal 6 to a third switch 12. The first transfer capacitor C.sub.t1 basically constitutes a first transfer device 4. In the first switching state, as shown in FIG. 1, the first terminal 5 is connected by the second switch 11 to the sensing electrode 2 and the second terminal 6 is connected by the third switch 12 to the first node 7. In the second switching state, the first terminal 5 is connected to a second node 8 and the second terminal 6 is connected to a third node 9. The second node 8 and the third node are 9 input nodes to a measurement circuit 20, which comprises an integrating capacitor C.sub.i and a transimpedance amplifier 21. The transimpedance amplifier 21 is constructed in a known way using an operation amplifier 22 and an impedance Z, which may be realized by a resistor and a capacitor connected in parallel. The operation amplifier 22 generates at its output a voltage V.sub.out which is indicative of the capacitance C.sub.X of the sensing electrode 2.

[0038] In the embodiment shown, the third node 9, which is connected to the non-inverting input of the operation amplifier 22, is connected to ground via a direct voltage source 15, which generates a constant voltage V.sub.2 as a bias point for the measurement circuit 20. Alternatively, the third node 9 could also be directly connected to ground.

[0039] The first, second and third switch 10, 11, 12 are parts of a switching arrangement and change from the first switching state to the second switching state (as indicated in FIG. 1) in a synchronized way. Moreover, they are synchronized to the time evolution of the alternating voltage V.sub.1 as indicated by the numbers in FIG. 3. When the voltage V.sub.1 is rising, the switching arrangement assumes the first switching state, and when the voltage V.sub.1 is dropping, the switching arrangement assumes the second switching state.

[0040] In the first switching state, as shown in FIG. 1, the first transfer device 4 is separated from the measurement circuit 20. The first transfer capacitor C.sub.t1 is connected in series with the unknown capacitance C.sub.X, wherefore a current flows between these two elements.

[0041] In the second switching state, the first transfer device 4 is disconnected from the alternating voltage source 3 and the sensing electrode 2 and is instead connected to the measurement circuit 20. At this point, the first transfer capacitor C.sub.t1 has been charged to a certain amount, which depends on the capacitance C.sub.X. Since the capacitance of the integrating capacitor C.sub.i is chosen to be much larger than the capacitance of the first transfer capacitor C.sub.t1, the latter is largely discharged in favor of the integrating capacitor C.sub.i. Also in the second switching state, the sensing electrode 2 is connected directly to the alternating voltage source 3 by the first switch 10.

[0042] Since the measurement circuit 20 is at no time connected to the sensing electrode 2, it is protected from electromagnetic noise which could be received by the sensing electrode 2. Therefore, the capacitive sensor 1 is relatively robust under electromagnetic compatibility (EMC) aspects. Furthermore, the measurement circuit 20 is at no time exposed to the alternating voltage source 3 and its alternating voltage V.sub.1. It rather is only connected to the first transfer capacitor C.sub.t1, which, for a given capacitance C.sub.X, is always charged to a voltage with the same amount and polarity.

[0043] Furthermore, since the sensing electrode 2 is only charged with a sinusoidal voltage, which (at least ideally) does not contain any upper harmonics with higher frequencies, its electromagnetic emissions are largely reduced with respect to e.g. a rectangular voltage.

[0044] FIG. 2 shows a second embodiment of an inventive capacitive sensor 1a, which also comprises a sensing electrode 2, an alternating voltage source 3 and a measurement circuit 20. These components are identical to the embodiment shown in FIG. 1 and therefore will not be described again.

[0045] Again, a first transfer capacitor C.sub.t1 of a first transfer device 4 is connected between two switches 11, 12, by which, in a first switching state, a first terminal 5 of the capacitor C.sub.t1 is connected to the sensing electrode 2 and a second terminal 6 is connected to the first node 7 connected to the alternating voltage source 3. In the second switching state, the first terminal 5 is connected to the second node 8 and the second terminal 6 is connected to the third node 9. Like in FIG. 1, the second node 8 is connected to the inverting input terminal of the operation amplifier 22 and the third node 9 is connected to the non-inverting input terminal. Unlike FIG. 1, the third node 9 is directly connected to ground.

[0046] The sensor 1a further comprises a second transfer device 4.1, which comprises a second transfer capacitor C.sub.t2 having a third terminal 5.1 and a fourth terminal 6.1, which are connected to two additional switches 13, 14. In the first switching state, as shown in FIG. 2, the third terminal 5.1 is connected to the third node 9 and the fourth terminal 6.1 is connected to the second node 8. In the second switching state, the third terminal 5.1 is connected to the sensing electrode 2 while the fourth terminal 6.1 is connected to the first node 7 and the alternating voltage source 3.

[0047] In this embodiment, too, the switches 11, 12, 13, 14 are part of the switching arrangement, which is synchronized with the alternating voltage V.sub.1 in the way indicated by the numbers in FIG. 3. In the first switching state, the first transfer device 4 is connected to the sensing electrode 2 and the alternating voltage source 3. In this state, the alternating voltage V.sub.1 is rising, which means that the first transfer capacitor C.sub.t1 and the capacitance C.sub.X are both charged by a positive current. At the same time, the second transfer capacitor C.sub.t2 is disconnected from the sensing electrode 2 and the alternating voltage source 3, but connected to the second and third node 8, 9, which are input nodes to the measurement circuit 20. The transfer capacitors C.sub.t1, C.sub.t2 have equal capacitance and the integrating capacitor C.sub.i has a capacitance that is much larger. Therefore, in the first switching states, the second transfer capacitor C.sub.t2 is discharged in favor of the integrating capacitor C.sub.i.

[0048] In the second switching state, when the voltage V.sub.1 is dropping, the first transfer capacitor C.sub.t1 is disconnected from the voltage source 3 and the sensing electrode 2 and is instead connected to the second and third node 8, 9. The second transfer capacitor C.sub.t2, on the other hand, is disconnected from the second and third node 8, 9 and is instead connected between the alternating voltage source 3 and the sensing electrode 2. Now the capacitance C.sub.X and the transfer capacitor C.sub.t2 are subjected to a negative current, wherefore the second transfer capacitor C.sub.t2 is charged with a polarity that is opposite to that of the first transfer capacitor C.sub.t1 in the first switching state. Since, however, the terminals 5.1, 6.1 of the second transfer capacitor C.sub.t2 are connected to the second node 8 and the third node 9 in an opposite way with respect to the terminals 5, 6 of the first transfer capacitor C.sub.t1, the integrating capacitor C.sub.i is charged with the same polarity in both switching states.

[0049] It should be noted that in the second embodiment, both the rising flank as well as the falling flank of the voltage V.sub.1 are used to charge one of the transfer capacitors C.sub.t1, C.sub.t2, both of which are used to charge the integrating capacitor C.sub.i. Therefore, the sensitivity of the capacitive sensor 1a according to the second embodiment is approximately twice as high as in the first embodiment. Like in the first embodiment, the measurement circuit 20 is at no time connected to the sensing electrode 2, wherefore it is protected from electromagnetic noise which could be received by the sensing electrode 2.

[0050] The embodiments of the invention shown in FIGS. 1 to 3 all relate to a capacitive sensor operating in the so-called “loading mode”. It should however be noted that the principle of the invention applies mutatis mutandis also to a capacitive sensor operating in “coupling mode”.

[0051] FIG. 4 shows an embodiment of a capacitive sensor in “coupling mode”, which is similar to the embodiment shown in FIG. 1. In the embodiment shown in FIG. 4, the counterelectrode is a dedicated transmitter electrode 16 and the capacitance to be determined C.sub.x is formed between the sensing electrode 2 and a dedicated transmitter electrode 16. The alternative voltage source 3 is in this embodiment coupled to the antenna electrode 16, while node 7 is coupled to ground. In operation, the transmitting antenna electrode 16 emits an alternating electric field in response to the alternating signal from the alternating voltage source 3 applied to said antenna electrode 16. In this case an alternating voltage is induced in said sensing electrode 2 by said alternating electric field coupling into said sensing electrode 2. In this embodiment, the measurement circuit determines the current or voltage that is induced in the sensing electrode when the transmitting antenna electrode is operating.

[0052] It should be noted that the operating of the coupling mode sensor of FIG. 4 with respect to the charge transfer is similar to the loading mode sensor of FIG. 1. It will however be appreciated, that the switching cycles will be inverted with respect to the time evolution of an alternating voltage of FIG. 3. It will finally be appreciated that the the coupling mode sensor could also be provided with a second transfer device in analogy to the embodiment of FIG. 2.