METHOD FOR SETTING A SCANNING FREQUENCY OF A CAPACITIVE TOUCH-SENSITIVE SWITCH, OPERATING METHOD FOR A CAPACITIVE TOUCH-SENSITIVE SWITCH AND CAPACITIVE TOUCH-SENSITIVE SWITCH

Abstract

A method for operating a capacitive touch-sensitive switch having a capacitive sensor element and a sensor circuit includes initially setting a scanning frequency of the capacitive touch-sensitive switch with a good signal-to-noise ratio and then operating the capacitive touch-sensitive switch at a scanning frequency which has been set to detect switch actuation. The process of setting the scanning frequency includes operating the touch-sensitive switch using a first measurement method and at a selected scanning frequency, detecting the measurement signals from the sensor circuit and checking whether the detected measurement signals contain or could contain a critical alias effect. If no critical alias effect and no possibility of a critical alias effect are detected during the check, the scanning frequency can be set as the selection frequency for detection operation of the touch-sensitive switch. A capacitive touch-sensitive switch is also provided.

Claims

1. A method for adjusting a scanning frequency of a capacitive touch-sensitive switch having a capacitive sensor element and a sensor circuit, the method comprising: operating the touch-sensitive switch by using a first measurement method and at a scanning frequency having been selected from a group of available scanning frequencies; detecting measurement signals of the sensor circuit; checking whether the detected measurement signals contain or could contain a critical alias effect; and setting the scanning frequency as a selection frequency for a detection operating mode of the touch-sensitive switch when no critical alias effect and no possibility of a critical alias effect are identified during the check.

2. The method according to claim 1, which further comprises: carrying out the detection of the measurement signals by including a step of generating an amplitude/time graph; and carrying out the check as to whether the detected measurement signals contain or could contain a critical alias effect by including a step of converting the generated amplitude/time graph into an amplitude/frequency graph and checking whether the amplitude/frequency graph contains a peak.

3. The method according to claim 1, which further comprises, when an alias effect is identified during the check as to whether the detected measurement signals contain or could contain a critical alias effect, carrying out a check as to whether the identified alias effect would be critical in the detection operating mode of the touch-sensitive switch.

4. The method according to claim 2, which further comprises carrying out the check as to whether the identified alias effect would be critical in the detection operating mode of the touch-sensitive switch by including a step of comparing a frequency at which the peak in the amplitude/frequency graph is identified with a threshold value.

5. The method according to claim 3, which further comprises carrying out the check as to whether the identified alias effect would be critical in the detection operating mode of the touch-sensitive switch by including a step of comparing a frequency at which the peak in the amplitude/frequency graph is identified with a threshold value.

6. The method according to claim 2, which further comprises not setting the scanning frequency as the selection frequency when no peak is identified during the check as to whether the amplitude/frequency graph contains a peak.

7. The method according to claim 3, which further comprises not setting the scanning frequency as the selection frequency when no peak is identified during the check as to whether the amplitude/frequency graph contains a peak.

8. The method according to claim 4, which further comprises not setting the scanning frequency as the selection frequency when no peak is identified during the check as to whether the amplitude/frequency graph contains a peak.

9. The method according to claim 1, which further comprises: repeating the operation of the touch-sensitive switch, the detection of the measurement signals of the sensor circuit and the checking of the measurement signals, for a plurality of different scanning frequencies; and setting one of the plurality of scanning frequencies during the checking of the measurement signals of which neither a critical alias effect nor a possibility of a critical alias effect is identified, as the selection frequency.

10. The method according to claim 6, which further comprises, when no critical alias effect and no possibility of a critical alias effect are identified in each case during the checking of the measurement signals of a plurality of scanning frequencies, setting a scanning frequency of the plurality of scanning frequencies being best suited as the selection frequency, as the selection frequency.

11. The method according to claim 1, which further comprises selecting the first measurement method from a measurement method having a charge-charge cycle or a discharge-discharge cycle.

12. An operating method for a capacitive touch-sensitive switch, the method comprising: adjusting a scanning frequency of the capacitive touch-sensitive switch according to claim 1; and operating the capacitive touch-sensitive switch at the adjusted scanning frequency for detecting a switch actuation.

13. The operating method according to claim 12, which further comprises performing the adjustment of the scanning frequency of the capacitive touch-sensitive switch in an initialization procedure at least one of when the capacitive touch-sensitive switch is first brought into operation or periodically during the operation of the capacitive touch-sensitive switch.

14. The operating method according to claim 12, which further comprises performing the operation of the touch-sensitive switch for detecting a switch actuation by using a second measurement method differing from the first measurement method.

15. The operating method according to claim 13, which further comprises performing the operation of the touch-sensitive switch for detecting a switch actuation by using a second measurement method differing from the first measurement method.

16. The operating method according to claim 14, which further comprises selecting the second measurement method from a measurement method having a charge-discharge cycle or a discharge-charge cycle.

17. The operating method according to claim 15, which further comprises selecting the second measurement method from a measurement method having a charge-discharge cycle or a discharge-charge cycle.

18. A capacitive touch-sensitive switch, comprising: a capacitive sensor element; a sensor circuit connected to said capacitive sensor element; and a control unit connected to said sensor circuit for driving said sensor circuit, said control unit configured to execute the operating method according to claim 9.

19. A capacitive touch-sensitive switch, comprising: a capacitive sensor element; a sensor circuit connected to said capacitive sensor element; and a control unit connected to said sensor circuit for driving said sensor circuit, said control unit configured to execute the method for adjusting the scanning frequency of the capacitive touch-sensitive switch according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0027] FIG. 1 is a block diagram showing the configuration of a capacitive touch-sensitive switch in accordance with the present invention;

[0028] FIG. 2 is a flowchart of an operating method for a capacitive touch-sensitive switch in accordance with one exemplary embodiment of the present invention;

[0029] FIG. 3 is a flowchart of a method for adjusting a scanning frequency of a capacitive touch-sensitive switch in accordance with one exemplary embodiment of the present invention; and

[0030] FIGS. 4A and 4B are exemplary amplitude/time graphs and amplitude/frequency graphs for detected measurement signals at different scanning frequencies.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a basic configuration of a capacitive touch-sensitive switch shown in a very simplified way.

[0032] The capacitive touch-sensitive switch 10 has in particular a capacitive sensor element 12, for example in the form of an electrode which, together with, for example, a finger of a user and a touch panel acting as a dielectric, for example, of an operating faceplate in between, forms a capacitor having a capacitance which is variable corresponding to the actuation of the touch-sensitive switch 10, i.e. the touching or not-touching of the touch panel associated with the capacitive sensor element 12. The capacitive sensor element 12 is connected to a sensor circuit 14, which detects the changes in capacitance at the sensor element for example during charge or discharge phases of the capacitor.

[0033] A control unit 16 is connected to the sensor circuit 14. The control unit 16 drives the sensor circuit 14 in order to switch it on or off, for example, and to preset a scanning frequency for detecting the changes in capacitance for it. The measurement signals detected by the sensor circuit 14 are transmitted to the control unit 16, which evaluates the measurement signals in order to identify touching or not-touching of the touch panel at the capacitive sensor element 12 by a user.

[0034] FIG. 2 shows the sequence of an operating method according to the invention for such a capacitive touch-sensitive switch 10.

[0035] First, in a step S10, a suitable scanning frequency is adjusted for the sensor circuit 14 of the touch-sensitive switch 10, which scanning frequency offers a good signal-to-noise ratio and avoids critical alias effects as a result of noise signals. Then, in step S20, the capacitive touch-sensitive switch 10 is operated at the scanning frequency fixed in step S10 in order to detect a switch actuation by a user. If the operating duration T of the touch-sensitive switch 10 exceeds a preset limit value Tx (Yes in step S22), step S10 is repeated. In other words, the adjustment of the scanning frequency is repeated periodically at time intervals Tx. This is advantageous since noise signals can change over time and the scanning frequency is then correspondingly adapted. Step S10 of adjusting the scanning frequency is in addition preferably performed in an initialization procedure when the touch-sensitive switch 10 is first brought into operation.

[0036] FIG. 3 shows, by way of example, an exemplary embodiment of the adjustment method from step S10.

[0037] In the adjustment method S10, first, in a step S102, the touch-sensitive switch 10 or its sensor circuit 14 is operated at a first scanning frequency (No. x) and using a first measurement method A. The first scanning frequency No. x is selected from a group of available scanning frequencies; the first scanning frequency No. x is, for example, 111 kHz. The first measurement method A is preferably a measurement method having a charge-charge cycle or a measurement method having a discharge-discharge cycle. Then, in a step S104, the corresponding measurement signals are detected by the sensor circuit 14 over a predetermined time period (for example 64 measurements) and a corresponding amplitude/time graph is generated by the control unit 16, as is illustrated by way of example as the top graphs in FIGS. 4A and 4B.

[0038] Then, the control unit 16, in a step S106, converts the generated amplitude/time graph into an amplitude/frequency graph by using a fast Fourier transform (FFT), as is illustrated by way of example as the bottom graphs in FIGS. 4A and 4B. The control unit 16 then checks, in a step S108, whether the amplitude/frequency graph contains a peak. Such a peak is caused by an alias effect which is generated by a noise signal.

[0039] If the control unit 16 identifies a peak in the amplitude/frequency graph (Yes in S108), the control unit then checks, in a step S110, whether this peak is in the critical range. For example, the control unit 16 checks whether the frequency fp of this peak is below a predetermined threshold value fs. The threshold value fs is, for example, a preset frequency value or half the present scanning frequency.

[0040] If the peak in the amplitude/frequency graph is in the critical range below the threshold value fs (Yes in step S110), as is illustrated by way of example in FIG. 4A, the presently tested scanning frequency is not suitable for the detection operating mode of the touch-sensitive switch since this low-frequency alias effect cannot be safely distinguished from a touch of the switch 10 in an evaluation of the measurement signals and is therefore critical. The presently tested scanning frequency is therefore not set as a selection frequency for the detection operating mode of the touch-sensitive switch; instead, the method in this case continues directly with a step S118, explained further below.

[0041] If, on the other hand, the peak in the amplitude/frequency graph is in the uncritical range above the threshold value fs (No in step S110), as is illustrated by way of example in FIG. 4B, the presently tested scanning frequency is suitable for the detection operating mode of the touch-sensitive switch since this high-frequency alias effect can be identified as such during an evaluation of the measurement signals and can be distinguished from a touch of the switch 10 which generates a low-frequency measurement signal. The method therefore continues in this case with a step S112.

[0042] In step S112, a check is performed as to whether a suitable scanning frequency has already been set as selection frequency in the scanning frequency adjustment method. If this is not the case (No in step S112) because, for example, no suitable scanning frequency has been found yet or because it is the first tested scanning frequency, the method continues with step S114 in order to set the present scanning frequency, which has been judged to be suitable in step S110, as the selection frequency for the detection operating mode of the touch-sensitive switch 10.

[0043] If, on the other hand, a selection frequency has already been set in the scanning frequency adjustment method (Yes in step S112), the method continues with a step S116. In this step S116, the control unit 16 compares the suitability of the present scanning frequency with the suitability of the set selection frequency. A scanning frequency is, for example, better suited to the detection operating mode of the touch-sensitive switch 10 when the uncritical alias effect has a higher frequency and therefore generates a peak at a higher frequency fp in the amplitude/frequency graph.

[0044] If the previously set selection frequency is better suited than the present scanning frequency (No in step S116), the method continues with the step S118 and the previously set selection frequency remains set as the selection frequency. If, on the other hand, the present scanning frequency is better suited than the previously set selection frequency (Yes in step S116), the method continues with the step S116 in order to reset the present scanning frequency as the selection frequency.

[0045] Then, the method (Yes in step S110, No in step S116 or after step S114) continues with the step S118. In this step S118, a check is performed as to whether now all of the available frequencies have been tested in the above-described way for their suitability as scanning frequency for the detection operating mode of the touch-sensitive switch 10. If not all of the available frequencies have yet been tested (No in step S118), the method continues with a step S120, in which the next scanning frequency No. x+1 is selected from the group of available scanning frequencies. The new scanning frequency No. x+1 is, for example, 80 kHz. The method then returns to the step S102 in order to perform the above-described suitability check for the new scanning frequency No. x+1 after step S102. If, on the other hand, all of the available frequencies have now been tested (Yes in step S118), the method continues with a step S122, in which the frequency that was set last as the selection frequency in step S114 is fixed as the scanning frequency for the detection operating mode of the touch-sensitive switch, which is preferably performed by using a second measurement method B, which differs from the first measurement method A.

[0046] As is illustrated in FIG. 3, the method in addition goes from step S108 directly to step S118 if no peak is identified in the amplitude/frequency graph. That is to say that although no critical alias effect is identified, the present scanning frequency is not judged as being suitable and in particular is also not set as the selection frequency. The reason for this is that, in this case, it cannot be ruled out that the measurement signals could contain a critical alias effect which, however, is not identifiable in the amplitude/frequency graph. Particularly critical are alias effects with a frequency value fp close to zero or equal to zero. If the alias frequency value fp is equal to zero, the amplitude/frequency graph is, however, flat over the entire frequency range and no peak is identifiable. That is to say that when the control unit 16 does not identify a peak in the amplitude/frequency graph in step S108, this can mean that actually no alias effect is present or alternatively that a critical alias effect is not identifiable.

[0047] As mentioned, in the suitability test method S10, a first measurement method A is used, which is preferably a measurement method having a charge-charge cycle or a discharge-discharge cycle. In the detection operating mode of the touch-sensitive switch S20 at the scanning frequency (S122) fixed in the suitability test method S10, a second measurement method B is preferably then used which differs from the first measurement method A. The second measurement method B is preferably a measurement method with a discharge-charge cycle or a charge-discharge cycle.