Method and device for filtering signals produced by an accelerometer of the piezo-electric type, and application to a portable object such as a watch

• US 20050071133A1
• Filed: 09/29/2004
• Published: 03/31/2005
• Est. Priority Date: 09/29/2003
• Status: Active Grant

• Current Assignee: EM Microelectronic - Marin SA (The Swatch Group AG)
• Original Assignee: EM Microelectronic - Marin SA (The Swatch Group AG)
• Inventors: Peguiron, Mathias, Galli, Reto, Jeannet, Nicolas, Godat, Yves
• Granted Patent: US 7,143,012 B2
• Time in Patent Office: Days
• US Class Current: 702/193
• CPC Class Codes: G01R 19/1659 to indicate that the value is within or outside a predetermined range of values (window) G01R19/16514, G01R19/16519, G01R19/16528 and G01R19/16533 take precedence
• CPC Class Codes: G04G 21/02 Detectors of external physical values, e.g. temperature
• CPC Class Codes: H03K 5/1254 specially adapted for pulses generated by closure of switches, i.e. anti-bouncing devices debouncing circuits for electronic time-pieces G04G5/00

Abstract

The present invention concerns a method for filtering signals (V_IN) generated by a piezo-electric type accelerometer (34) in response to detection of a shock applied to an object to which the accelerometer is connected, this method being characterised in that it is ensured, during a first step, that the signal (V_IN) generated by the accelerometer (34), after having increased and exceeded a first threshold value (V_TH,ON) decreases and becomes lower than a second threshold value (V_TH,OFF) lower than the first threshold value (V_TH,ON) after a period of time (T_P,ON) greater than a minimum period of time (T_P,ON,MIN) and less than a maximum period of time (T_P,ON,MAX). The invention also concerns a device for filtering the signals generated by an accelerometer in response to detection of a shock applied to an object to which the accelerometer is connected. Finally, the invention concerns a portable object such as a wristwatch including a detection device of this kind.

Claims

1. A method for filtering signals (V_IN) generated by a piezo-electric type accelerometer in response to detection of a shock applied to an object to which the accelerometer is connected, wherein it is ensured, during a first step, that the signal (V_IN) generated by the accelerometer, after having increased and exceeded a first threshold value (V_TH,ON) decreases and becomes lower than a second threshold value (V_TH,OFF) lower than the first threshold value (V_TH,ON) after a period of time (T_P,ON) greater than a minimum period of time (T_P,ON,MIN) and less than a maximum period of time (T_P,ON,MAX):

2. The method according to claim 1, wherein it is ensured, during a second step following the first step, that the signal (V_IN) generated by the accelerometer remains lower than the first threshold value (V_TH,ON) for a period of time that is at least equal to a third predetermined value (T_P,OFF,MIN).

3. The method according to claim 1, wherein, if the signal (V_IN) generated by the accelerometer remains higher than the second threshold value (V_TH,OFF) after a period of time (T_P,ON) less than the minimum period of time (T_P,ON,MIN), or if the signal (V_IN) remains higher than the second threshold value (V_TH,OFF), but for a period of time (T_P,ON) greater than the maximum period of time (T_P,ON,MAX), the shock is not recognised as valid.

4. The method according to claim 2, wherein, if the signal (V_IN) generated by the accelerometer remains higher than the second threshold value (V_TH,OFF) after a period of time (T_P,ON) less than the minimum period of time (T_P,ON,MIN), or if the signal (V_IN) remains higher than the second threshold value (V_TH,OFF), but for a period of time (T_P,ON) greater than the maximum period of time (T_P,ON,MAX), the shock is not recognised as valid.

5. The method according to claim 2, wherein if the signal (V_IN) after the first step, becomes higher than the first threshold value (V_TH,ON) again, after a time less than the period of time (T_P,OFF,MIN), the shock is not recognised as valid.

6. The method according to claim 3, wherein if the signal (V_IN) after the first step, becomes higher than the first threshold value (V_TH,ON) again, after a time less than the period of time (T_P,OFF,MIN), the shock is not recognised as valid.

7. The method according to claim 4, wherein if the signal (V_IN) after the first step, becomes higher than the first threshold value (V_TH,ON) again, after a time less than the period of time (T_P,OFF,MIN), the shock is not recognised as valid.

8. A device for filtering signals (V_IN) generated by a piezo-electric type accelerometer in response to detection of a shock applied to an object to which the accelerometer is connected, wherein this device includes:

comparison means for comparing the level of the signal (V_IN) generated by the accelerometer to a first and a second threshold level (V_TH,ON;

V_TH,OFF);

means for switching on a time base, which is actuated when the level of the signal (V_IN) from the accelerometer supplied to the comparison means is higher than the first threshold level (V_TH,ON);

a generator of reference time periods controlled by the time base, and

means for checking over time the output level (OUT(COMP)) of the comparison means in relation to the reference periods of time provided by the generator to determine whether the shock applied to the object was valid or not.

9. The device according to claim 8, wherein the reference time period generator generates the following period of times:

a minimum period of time (T_P,ON,MIN) and a maximum period of time (T_P,ON,MAX) between which there must be comprised a period of time (T_P,ON) after which the signal (V_IN) becomes lower than a second threshold value (V_TH,OFF) after having exceeded the first threshold value (V_TH,ON);

a third predetermined value (T_P,OFF,MIN) during which the signal (V_IN) must not exceed the first threshold value (V_TH,ON) again;

a first time delay (T_OUT) during which a function remains activated or during which a digital sort of checking circuit remains applied to an input of a microcontroller, and

a second time delay (T_MUTE) during which the filtering device does not accept any new shocks.

10. The device according to claim 9, wherein:

the period of time (T_P,ON,MIN) has a value of 0.488 ms;

the period of time (T_P,ON,MAX) has a value of 5.127 ms;

the period of time (T_P,OFF,MIN) has a value of 5.127 ms, and

the period of time (T_OUT) has a value of 46.8 ms.

11. The device according to claim 8, wherein it includes an integrated circuit.

12. The device according to claim 11, wherein a capacitor (C1) is mounted in parallel across the terminals of the accelerometer outside the integrated circuit such that the comparison means always see the same signal regardless of the application for which the filtering device is intended.

13. The device according to claim 8, wherein the comparison means include a hysteresis comparator having two threshold values, respectively (V_TH,OFF) and (V_TH,ON).

14. The device according to claim 13, wherein a resistor (R_IN) is mounted in parallel to the terminals of the comparator, so that, when the filtering device is at rest, the terminals of the comparator are at the same potential.

15. The device according to claim 8, wherein the time base includes an RC oscillator.

16. The device according to claim 15, wherein the clock frequency of the RC oscillator is fixed at 65 kHz.

17. The device according to claim 8, wherein the time period generator includes a chain division which counts down time.

18. The device according to claim 8, wherein it is also a function of the checking means to indicate to the comparison means which of the two high or low levels (V_TH,ON;

V_TH,OFF) has to be compared to the voltage (V_IN) generated by the accelerometer.

19. The device according to claim 8, wherein it includes fuses which allow the thresholds (V_TH,OFF;

V_TH,ON) and the clock frequency of the time base to be adjusted.

20. The device according to claim 11, wherein it includes a follower circuit connected to a reference voltage source which generates a voltage comprised between the negative and positive supply potentials (V_SS) and (V_DD) of the integrated circuit, said voltage being used to polarise one of the terminals of accelerometer and of the comparator.

21. The device according to claim 20, wherein the voltage generated by the follower circuit is fixed in proximity to 700 mV.

22. The device according to claim 8, wherein the threshold value (V_TH,OFF) corresponds to a potential difference of the order of 0V+/−

5 mV, and in that the threshold value V_TH,ON can be fixed in proximity to 30 mV+/−

2 mV.

23. A portable object such as a wristwatch including a filtering device according to claim 8, wherein, if the checking means determine that the shock applied to the object is deliberate, they generate a control signal for switching on a function of the portable object.