MICROELECTROMECHANICAL DEVICE HAVING AN OSCILLATING MASS, AND METHOD FOR CONTROLLING A MICROELECTROMECHANICAL DEVICE HAVING AN OSCILLATING MASS

• US 20110146403A1
• Filed: 12/21/2010
• Published: 06/23/2011
• Est. Priority Date: 12/21/2009
• Status: Active Grant

• Current Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Rizzo Piazza Roncoroni, Alessandra Maria, Caminada, Carlo, Prandi, Luciano
• Granted Patent: US 8,474,317 B2
• Time in Patent Office: Days
• US Class Current: 73/504.12
• CPC Class Codes: G01C 19/56 Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces
• CPC Class Codes: H03B 5/30 with frequency-determining element being electromechanical resonator
• CPC Class Codes: H03L 3/00 Starting of generators

Abstract

A microelectromechanical device includes a body, a movable mass, elastically connected to the body and movable in accordance with a degree of freedom, and a driving device, coupled to the movable mass and configured to maintain the movable mass in oscillation at a steady working frequency in a normal operating mode. The microelectromechanical device moreover includes a start-up device, which is activatable in a start-up operating mode and is configured to compare a current oscillation frequency of a first signal correlated to oscillation of the movable mass with a reference frequency, and for deciding, on the basis of the comparison between the current oscillation frequency and the reference frequency, whether to supply to the movable mass a forcing signal packet so as to transfer energy to the movable mass.

Claims

1. A microelectromechanical device comprising:

a body;

a movable mass, elastically connected to the body and oscillatably movable with respect to the body according to a degree of freedom;

a driving device coupled to the movable mass so as to form a microelectromechanical loop and configured to keep the movable mass in oscillation with a steady operating frequency in a normal operating mode; and

a start-up device activatable in a start-up operating mode and configured to compare a current oscillation frequency of a first signal correlated to oscillation of the movable mass with a reference frequency and to selectively transfer energy to the movable mass by providing the movable mass with a forcing signal packet, based on a comparison between the current oscillation frequency and the reference frequency.

2. The device according to claim 1, wherein the start-up device is configured to provide the movable mass with one forcing signal packet in response to determining that the current oscillation frequency is outside an acceptable range including the reference frequency.

3. The device according to claim 2, wherein:

the driving device is configured to provide the first signal having the current oscillation frequency;

the start-up device comprises a start-up stage and an oscillator configured to provide a second signal having the reference frequency; and

the start-up stage is coupled to the driving device and to the oscillator for receiving respectively the first signal and the second signal and is moreover configured to compare the current oscillation frequency with the reference frequency.

4. The device according to claim 3, wherein the start-up device comprises a forcing stage for providing the movable mass with forcing signal packets in response to a start-up signal from the start-up stage.

5. The device according to claim 4, wherein the start-up stage is configured to provide the start-up signal when the current oscillation frequency is outside an acceptable range relative to the reference frequency.

6. The device according to claim 3, wherein the start-up stage comprises:

a first counter, coupled to the driving device for receiving the first signal;

a second counter, coupled to the oscillator for receiving the second signal;

enabling means, configured to enable the second counter when a counting value stored in the first counter is lower than a control value, and a disabling value when the control value is reached; and

a counting comparator, configured to fetch the control value from the first counter and a final counting value from the second counter, when the first counter reaches the control value, and to compare the control value and the final counting value.

7. The device according to claim 6, wherein the counting comparator is configured to provide a clock lock signal having a lock value, when the control value and the final counting value comply with a lock condition, and an asynchronous frequency value otherwise.

8. The device according to claim 7, wherein the lock condition is met when

9. The device according to claim 2, wherein the driving device comprises a PLL circuit.

10. The device according to claim 9, wherein the start-up stage is configured to disable the PLL circuit when the current oscillation frequency is outside the acceptable range relative to the reference frequency and to enable the PLL circuit otherwise.

11. The device according to claim 1, wherein the forcing signal packets have a controlled duration.

12. A method for controlling a microelectromechanical device having a body and a movable mass, elastically connected to the body and movable with respect to the body according to a degree of freedom;

the method comprising;

comparing a current oscillation frequency of a first signal correlated to oscillation of the movable mass with a reference frequency; and

selectively transferring energy to the movable mass by providing the movable mass with a forcing signal packet, based on a comparison between the current oscillation frequency and the reference frequency.

13. The method according to claim 12, comprising providing the movable mass with a forcing signal packet when the current oscillation frequency is outside an acceptable range relative to the reference frequency.

14. The method according to claim 13, comprising terminating the supply of forcing signal packets when the current oscillation frequency is inside the acceptable range.

15. The method according to claim 12, wherein the forcing signal packets have a controlled duration.

16. A system, comprising:

a body;

a mass coupled to the body, and movable with respect to the body;

a driver capacitively coupled to the movable mass and configured to provide a signal to the movable mass so as to maintain oscillation movement of the movable mass, relative to the body, at a stable frequency; and

a start-up circuit configured to supply a first quantity of energy to the movable mass, and to determine, following the supply of the first quantity of energy, whether the movable mass is oscillating with sufficient energy to continue in stable oscillation controlled by the driver.

17. The system of claim 16 wherein the start-up circuit is configured to compare a detected oscillating frequency of the movable mass with a reference frequency, and to determine that the movable mass is oscillating with sufficient energy to continue in stable oscillation if the detected oscillating frequency of the movable mass is within a selected range relative to the reference frequency.

18. The system of claim 16 wherein the start-up circuit is configured to supply an additional quantity of energy to the movable mass if, following the supply of the first quantity of energy, the movable mass is not oscillating with sufficient energy to continue in stable oscillation.

19. The system of claim 16 wherein the first quantity of energy is selected to be less than a quantity that would send the movable mass into stable oscillation starting from a condition of rest.

20. The system of claim 16 wherein the start-up circuit is configured to supply successive quantities of energy to the movable mass until the movable mass is oscillating with sufficient energy to continue in stable oscillation.

21. The system of claim 20 wherein the start-up circuit is configured to determine, following the supply of each of the successive quantities of energy, whether the movable mass is oscillating with sufficient energy to continue in stable oscillation, and to terminate supplying successive quantities of energy when the movable mass is oscillating with sufficient energy to continue in stable oscillation.

22. The system of claim 16 wherein a value of the first quantity of energy is selected according to whether the system is starting from an off condition or from a power down condition.

23. A method, comprising:

sending an activation signal to a device that includes a mass that is configured to oscillate with respect to a body;

in response to the activation signal, supplying a first quantity of energy to the movable mass; and

after supplying the first quantity of energy, determining whether the movable mass is oscillating with sufficient energy to continue in stable oscillation.

24. The method of claim 23 wherein the determining comprises comparing a difference between a detected oscillating frequency of the movable mass and a reference frequency with a threshold value, and determining that the movable mass is oscillating with sufficient energy to continue in stable oscillation if the difference is less than the threshold value.

25. The method of claim 23 comprising, if the movable mass is not oscillating with sufficient energy to continue in stable oscillation, supplying an additional quantity of energy to the movable mass.

26. The method of claim 23 wherein the first quantity of energy is not sufficient to send the movable mass into stable oscillation starting from a condition of rest.

27. The method of claim 23, comprising:

resetting first and second counters;

while supplying the first quantity of energy to the movable mass, incrementing the first counter at a rate that corresponds to the current oscillating frequency of the movable mass;

while supplying the first quantity of energy to the movable mass, incrementing the second counter at a rate that corresponds to the reference frequency, until the second counter reaches a threshold value; and

when the second counter reaches the threshold value, stopping the supplying the first quantity of energy to the movable mass and stopping the incrementing the first counter, and

wherein the determining comprises, after stopping the supplying, comparing a value reached by the first counter with the threshold value.

28. The method of claim 27, comprising:

if the value reached by the first counter is within a selected margin of the threshold value, switching to a driver circuit for normal operation of the device; and

if the value reached by the first counter is not within the selected margin of the threshold value, supplying an additional quantity of energy to the movable mass, and repeating the resetting, the incrementing the first counter, the incrementing the second counter, and the determining.

29. The method of claim 23 wherein, if the activation signal is a start-up signal, then the supplying a first quantity of energy comprises supplying a quantity of energy of a first value, and if the activation signal is an exit from power-down signal, then the supplying a first quantity of energy comprises supplying a quantity of energy of a second value, lower than the first value.