Capacitance difference detecting circuit and MEMS sensor

• US 20050253596A1
• Filed: 09/16/2004
• Published: 11/17/2005
• Est. Priority Date: 05/14/2004
• Status: Active Grant

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Fujitsu Limited
• Inventors: Sumi, Hideki, Kitano, Mayo, Moribe, Tsuyoshi
• Granted Patent: US 7,119,550 B2
• Time in Patent Office: Days
• US Class Current: 324/658
• 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: G01D 5/24 by varying capacitance
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass

Abstract

Oscillators have capacitors, respectively, whose capacitances change according to an external force and generate first oscillating signals according to the capacitances. Each of the capacitors is disposed, for example, between a substrate and a mass body that is movably disposed to face the substrate and oscillates in a direction perpendicular to the substrate. A detecting unit detects a relative difference between the capacitances of the capacitors as a difference between frequencies of the first oscillating signals. An angular speed or acceleration applied in a horizontal direction of the substrate is calculated according to the frequency change detected by the detecting unit. Therefore, a capacitance difference detecting circuit and a MEMS sensor that detect a minute change in the capacitances of the two capacitors caused by the external force are formed.

Claims

1. A capacitance difference detecting circuit comprising:

a plurality of oscillators having capacitors whose capacitances change according to an external force and generating first oscillating signals according to the capacitances, respectively; and

a detecting unit detecting a relative difference between the capacitances as a difference between frequencies of the first oscillating signals.

2. The capacitance difference detecting circuit according to claim 1, wherein:

at least one of said oscillators comprises a variable capacitance diode whose capacitance is variable; and

a control terminal which adjusts the capacitance of said variable capacitance diode; and

the at least one of said oscillators having said variable capacitance diode generates the first oscillating signal according to the capacitances of one of said capacitors and of said variable capacitance diode.

3. The capacitance difference detecting circuit according to claim 1, wherein:

said plurality of oscillators are two oscillators in pair; and

said detecting unit comprises a mixer receiving a pair of first oscillating signals and outputting a frequency difference signal with a frequency equal to a difference between frequencies of the received pair of first oscillating signals.

4. The capacitance difference detecting circuit according to claim 3, wherein

said detecting unit comprises a frequency/voltage converting unit that converts the frequency of the frequency difference signal to a detection voltage indicating the relative difference between the capacitances.

5. The capacitance difference detecting circuit according to claim 4, wherein

said frequency/voltage converting unit comprises a smoothing unit that smoothes a positive voltage component of the frequency difference signal to output the smoothed voltage as the detection voltage.

6. The capacitance difference detecting circuit according to claim 4, further comprising

a mask control unit that activates a mask signal when the difference between the frequencies of the pair of first oscillating signals is smaller than a preset value, wherein

while receiving the activated mask signal, said frequency/voltage converting unit outputs the detection voltage indicating that the difference between the capacitances is zero.

7. The capacitance difference detecting circuit according to claim 3, wherein:

the capacitances of said capacitors constituting said oscillators are different from each other in an initial state in which the external force is not applied; and

in the initial state said mixer outputs the frequency difference signal indicating a preset initial frequency.

8. The capacitance difference detecting circuit according to claim 3, further comprising

a frequency comparator comparing the frequencies of the first oscillating signals in magnitude.

9. The capacitance difference detecting circuit according to claim 3, further comprising

a mask control unit that activates a mask signal when the difference between the frequencies of the first oscillating signals is smaller than a preset value, wherein

while receiving the activated mask signal, said mixer outputs the frequency difference signal indicating that the difference between the frequencies of the first oscillating signals is zero.

10. The capacitance difference detecting circuit according to claim 3, further comprising

a pair of PLL circuits disposed between said oscillators and said mixer, respectively, receiving the first oscillating signals from said oscillators to generate second oscillating signals having frequencies that are higher than the frequencies of the first oscillating signals, respectively, and outputting the generated second oscillating signals to said mixer as the first oscillating signals.

11. The capacitance difference detecting circuit according to claim 10, wherein

each of said PLL circuits has an input terminal that receives a set value for changing a frequency division ratio of a frequency divider in order for said mixer to receive the second oscillating signals with equal frequencies to each other in an initial state in which the external force is not applied, the frequency divider being built in each of said PLL circuits.

12. The capacitance difference detecting circuit according to claim 11, wherein

said frequency divider has a fractional dividing function.

13. The capacitance difference detecting circuit according to claim 3, further comprising:

a PLL circuit having a frequency divider disposed between one of said oscillators and said mixer, receiving one of the first oscillating signals from the one of said oscillators, generating a second oscillating signal with a frequency that is higher than a frequency of the one of said first oscillating signals, and outputting to said mixer the generated second oscillating signal as the one of said first oscillating signals; and

a dividing control unit that changes a frequency division ratio of said frequency divider in order to make a frequency of the second oscillating signal coincide with a frequency of the other one of said first oscillating signals in an initial state in which the external force is not applied.

14. The capacitance difference detecting circuit according to claim 13, further comprising

a frequency comparator that compares in magnitude the other one of said first oscillating signals outputted from the other one of said oscillators with the frequency of the second oscillating signal, wherein

said dividing control unit changes the frequency division ratio of said frequency divider according to a result of the comparison by said frequency comparator.

15. The capacitance difference detecting circuit according to claim 13, wherein:

said PLL circuit includes a frequency divider; and

said frequency divider has a fractional dividing function.

16. The capacitance difference detecting circuit according to claim 1, further comprising:

a substrate;

a mass body displaceably disposed to face said substrate and oscillating in a direction perpendicular to said substrate, wherein;

said plural oscillators are two oscillators in pair;

the capacitor of each of said oscillators is constituted of a first electrode formed on said substrate and a second electrode formed on said mass body to face the first electrode; and

each of said oscillators generates a first oscillating signal having a frequency determined by the capacitance of each of said capacitors, the capacitances being changed when said mass body is displaced relative to said substrate by the external force.

17. A capacitance difference detecting circuit comprising:

a pair of oscillators having capacitors whose capacitances change according to an external force and generating first oscillating signals according to the capacitances, respectively;

a pair of PLL circuits having voltage controlled oscillators, and receiving the first oscillating signals from said oscillators to output second oscillating signals from the voltage controlled oscillators, respectively; and

a detecting unit that receives input voltages to be inputted to the voltage controlled oscillators of said PLL circuits, respectively, the input voltages being adjusted according to frequencies of the first oscillating signals and the second oscillating signals, and that detects a difference between the received input voltages as a relative difference between the capacitances.

18. A MEMS sensor comprising:

a substrate;

a pair of first electrodes formed on said substrate;

a mass body displaceably disposed to face said substrate, oscillating in a direction perpendicular to said substrate, and displaced by an angular speed or acceleration applied in a direction perpendicular to the direction of the oscillation;

second electrodes formed on said mass body at positions to face said first electrodes, respectively;

a pair of oscillators having a pair of capacitors each formed of the first electrode and the second electrode facing to each other, and generating the first oscillating signals according to capacitances of the capacitors, respectively;

a detecting unit that detects a relative difference between the capacitances as a change in frequencies of the first oscillating signals; and

a main control unit that calculates the angular speed or acceleration applied in a horizontal direction of said substrate according to the change in the frequencies detected by said detecting unit.

19. The MEMS sensor according to claim 18, wherein:

at least one of said oscillators comprises a variable capacitance diode whose capacitance is variable; and

a control terminal which adjusts the capacitance of said variable capacitance diode; and

the at least one of said oscillators having said variable capacitance diode generates the first oscillating signal according to the capacitances of one of said capacitors and of said variable capacitance diode.

20. The MEMS sensor according to claim 18, wherein

said detecting unit comprises a mixer receiving a pair of first oscillating signals and outputting a frequency difference signal with a frequency equal to a difference between frequencies of the received pair of first oscillating signals.

21. The MEMS sensor according to claim 20, wherein

said detecting unit comprises a frequency/voltage converting unit that converts the frequency of the frequency difference signal to a detection voltage indicating the relative difference between the capacitances.

22. The MEMS sensor according to claim 21, wherein

said frequency/voltage converting unit comprises a smoothing unit that smoothes a positive voltage component of the frequency difference signal to output the smoothed voltage as the detection voltage.

23. The MEMS sensor according to claim 21, further comprising

a mask control unit that activates a mask signal when the difference between the frequencies of the pair of first oscillating signals is smaller than a preset value, wherein

while receiving the activated mask signal, said frequency/voltage converting unit outputs the detection voltage indicating that the difference between the capacitances is zero.

24. The MEMS sensor according to claim 20, wherein:

the capacitances of said capacitors constituting said oscillators are different from each other in an initial state in which the external force is not applied; and

in the initial state said mixer outputs the frequency difference signal indicating a preset initial frequency.

25. The MEMS sensor according to claim 20, further comprising

a frequency comparator comparing the frequencies of the first oscillating signals in magnitude.

26. The MEMS sensor according to claim 20, further comprising

a mask control unit that activates a mask signal when the difference between the frequencies of the first oscillating signals is smaller than a preset value, wherein

while receiving the activated mask signal, said mixer outputs the frequency difference signal indicating that the difference between the frequencies of the first oscillating signals is zero.

27. The MEMS sensor according to claim 20, further comprising

a pair of PLL circuits disposed between said oscillators and said mixer, respectively, receiving the first oscillating signals from said oscillators to generate second oscillating signals having frequencies that are higher than the frequencies of the first oscillating signals, respectively, and outputting the generated second oscillating signals to said mixer as the first oscillating signals.

28. The MEMS sensor according to claim 27, wherein

each of said PLL circuits has an input terminal that receives a set value for changing a frequency division ratio of a frequency divider in order for said mixer to receive the second oscillating signals with equal frequencies to each other in an initial state in which the external force is not applied, the frequency divider being built in each of said PLL circuits.

29. The MEMS sensor according to claim 28, wherein

said frequency divider has a fractional dividing function.

30. The MEMS sensor according to claim 20, further comprising:

a PLL circuit having a frequency divider disposed between one of said oscillators and said mixer, receiving one of the first oscillating signals from the one of said oscillators, generating a second oscillating signal with a frequency that is higher than a frequency of the one of said first oscillating signals, and outputting to said mixer the generated second oscillating signal as the one of said first oscillating signals; and

a dividing control unit that changes a frequency division ratio of said frequency divider in order to make a frequency of the second oscillating signal coincide with a frequency of the other one of said first oscillating signals in an initial state in which the external force is not applied.

31. The MEMS sensor according to claim 30, further comprising

a frequency comparator that compares in magnitude the other one of said first oscillating signals outputted from the other one of said oscillators with the frequency of the second oscillating signal, wherein

said dividing control unit changes the frequency division ratio of said frequency divider according to a result of the comparison by said frequency comparator.

32. The MEMS sensor according to claim 30, wherein:

said PLL circuit includes a frequency divider; and

said frequency divider has a fractional dividing function.

33. A MEMS sensor comprising:

a substrate;

a pair of first electrodes formed on said substrate;

a mass body displaceably disposed to face said substrate, oscillating in a direction perpendicular to said substrate, and displaced by an angular speed or acceleration applied in a direction perpendicular to the direction of the oscillation;

second electrodes formed on said mass body at positions to face said first electrodes, respectively;

a pair of oscillators having a pair of capacitors each formed of the first electrode and the second electrode facing to each other, and generating the first oscillating signals according to capacitances of the capacitors, respectively;

a pair of PLL circuits having voltage controlled oscillators, and receiving the first oscillating signals from said oscillators to output second oscillating signals from the voltage controlled oscillators, respectively; and

a detecting unit that receives input voltages to be inputted to the voltage controlled oscillators of said PLL circuits, respectively, the input voltages being adjusted according to frequencies of the first oscillating signals and the second oscillating signals, and that detects a difference between the received input voltages as a relative difference between the capacitances; and

a main control unit that calculates the angular speed or acceleration applied in a horizontal direction of said substrate according to the change in the frequencies detected by said detecting unit.