CAPACITATIVE SENSOR SYSTEM

• US 20110304576A1
• Filed: 11/18/2009
• Published: 12/15/2011
• Est. Priority Date: 11/18/2008
• Status: Active Grant

• Current Assignee: Microchip Technology Germany GmbH (Microchip Technology Incorporated)
• Original Assignee: Microchip Technology Germany GmbH (Microchip Technology Incorporated)
• Inventors: Fasshauer, Peter
• Granted Patent: US 10,003,334 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G06F 3/011 Arrangements for interaction with the human body, e.g. for user immersion in virtual reality blind teaching G09B21/00
• CPC Class Codes: G06F 3/017 Gesture based interaction, e.g. based on a set of recognized hand gestures interaction based on gestures traced on a digitiser G06F3/04883
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe
• CPC Class Codes: H03K 2217/960715 Rc-timing; e.g. measurement of variation of charge time or discharge time of the sensor
• CPC Class Codes: H03K 2217/96072 Phase comparison, i.e. where a phase comparator receives at one input the signal directly from the oscillator, at a second input the same signal but delayed, with a delay depending on a sensing capacitance
• CPC Class Codes: H03K 2217/960765 Details of shielding arrangements

Abstract

The invention relates to a capacitive sensor system, in particular for detecting the approach of objects and in particular also for gesture recognition. The problem of the present invention is to provide a capacitively operating sensor system, which can be implemented with low component complexity and thus low costs and space requirements and is also characterized by low power consumption, so as to operate with batteries having a low charge capacity and/or a long operating time. Said problem is solved according to the invention by a circuit configuration for generating an output signal correlating with an approximation process based on changes in the dielectric properties of the surroundings of a sensor electrode, having a sensor electrode which is adjacent to an observation area in at least some sections, a microcontroller circuit (μC) for output of an alternating voltage, a voltage divider circuit for achieving an adjustment of the level of the alternating voltage output by the microcontroller (μC), and a field effect transistor (FET) in the function of an impedance converter, wherein the field effect transistor is incorporated into the circuit configuration in such a way that the voltage output by the voltage divider circuit is present at the gate input thereof and at the same time at the sensor electrode (ES).

Claims

1. A circuit arrangement for generating an output signal correlated with an approach based on changes of the dielectric properties of the environment of a sensor electrode, the circuit arrangement comprising:

a sensor electrode that at least partly juxtaposed with an observation area;

a microcontroller outputting an a/c voltage;

a voltage divider for adjusting a level of the a/c voltage outputted by the microcontroller; and

a field-effect transistor (FET) operating as an impedance converter and integrated into the circuit arrangement in such a way that the voltage outputted by the voltage divider is applied to its gate terminal and at the same time to the sensor electrode.

2. The circuit arrangement according to claim 1, wherein the field-effect transistor is integrated into the circuit arrangement as a source follower as an input stage.

3. The circuit arrangement according to claim 2, wherein the effect of parasitic capacitors formed by the gate terminal capacitance of the FET, the electrode capacitance relative to ground and structurally conditioned circuit capacitors, is used as a capacitor device C1 for the sensor function.

4. The circuit arrangement according to claim 3, wherein a charging resistance is present that located upstream of the capacitor device C1 and the gate terminal.

5. The circuit arrangement according to claim 1 that wherein the square-wave voltage delivered by the microcontroller is in the frequency range from 80 to 120 kHz.

6. The circuit arrangement according to claim 1 wherein the capacitor device is charged over a time interval t1 until reaching a determined threshold value u_S1 and discharged after half a cycle duration T of the square signal.

7. The circuit arrangement according to claim 6, wherein the switching times are coordinated in such a way that the following applies:

t1=RC1 ln(1−

u_S1/u₀)t1=−

RC1ln(u_S1/u₀)

8. The circuit arrangement according to claim 1 wherein the approach is detected on the basis of a time difference.

9. The circuit arrangement according to claim 1 wherein the charging resistance is determined in such a way that it is at a maximum each time during a half period T/2 of the square signal until an almost complete charge and discharge at the gate terminal of the field-effect transistor occurs.

10. The circuit arrangement according to claim 9, wherein a coupling electrode E_G connected with ground is provided.

11. The circuit arrangement according to claim 10, wherein the coupling electrode E_G, in the form of a coupling capacitance between E_S and E_G, delivers a further contribution to C1.

12. The circuit arrangement according to claim 11, wherein a shield electrode is provided between the signal electrode E_S and the electrode E_G connected to ground.

13. The circuit arrangement according to claim 12, wherein this shield electrode is connected to the output of a source follower and lies on almost equal potential as the gate terminal, by which the coupling E_S and E_G is reduced.

14. The circuit arrangement according to claim 1 wherein the drain terminal of the FET is used, in case of more distanced connection of the signal electrode E_S, for driving the braid of a coaxial cable, in order to reduce the cable capacitance.

15. The circuit arrangement according to claim 1 wherein, for the evaluation of the time shift of the charge and discharge process in case of approach, a XOR gate terminal is used whose the inputs are switched by integrated Schmitt triggers.

16. The circuit arrangement according to claim 15, wherein the time difference is represented by a DC voltage obtained by a low-pass filter connected to the XOR output.

17. A circuit arrangement for generating an output signal correlating with an approach of the dielectric properties of the environment because of changes of a sensor electrode, the circuit arrangement comprising:

a sensor electrode at least partly juxtaposed with an observation area;

a microcontroller outputting an a/c voltage;

a voltage divider for adjusting a level of the a/c voltage outputted by the microcontroller; and

a field-effect transistor operating as an impedance converter and configured in such a way that an approach is detected on the basis of an event correlating with the change of the ratio of charging time to discharging time.