Capacitance difference detecting circuit and MEMS sensor

• US 7,119,550 B2
• Filed: 09/16/2004
• Issued: 10/10/2006
• 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
• Primary Examiner(s): Lee, Diane
• Assistant Examiner(s): Kramskaya, Marina
• Application Number:
  US10/942,127
• Publication Number:
  US 20050253596A1
• Time in Patent Office: 754 Days
• Field of Search: 324/658, 324/686, 735/041.2, 735/041.3, 735/041.4, 735/041.5, 735/041.6, 735/140.1, 735/140.2, 735/142.9, 735/143.2, 73488-551
• 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 pair of oscillators having capacitors whose capacitances change according to an external force and generating first oscillating signals according to the capacitances, respectively;

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

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

said detecting unit includes 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, and a frequency/voltage converting unit that converts the frequency of the frequency differences signal to a detection voltage indicating the relative difference between the capacitances and

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

2. A capacitance difference detecting circuit comprising:

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

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

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

said detecting unit includes 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.

3. A capacitance difference detecting circuit comprising:

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

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

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

said detecting unit includes 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, and

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.

4. A capacitance difference detecting circuit comprising:

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

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

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, wherein

said detecting unit includes 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.

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

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

said frequency divider has a fractional dividing function.

7. A capacitance difference detecting circuit comprising:

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

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

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, wherein

said detecting unit includes 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.

8. The capacitance difference detecting circuit according to claim 7, 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.

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

said PLL circuit includes a frequency divider; and

said frequency divider has a fractional dividing function.

10. 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.

11. 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 chance in the frequencies detected by said detecting unit; and

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

said detecting unit includes 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, and a frequency/voltage converting unit that converts the frequency of the frequency differences signal to a detection voltage indicating the relative difference between the capacitances and

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

12. 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;

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; and

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

said detecting unit includes 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.

13. 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;

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; and

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

said detecting unit includes 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, and

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.

14. 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;

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

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, wherein

said detecting unit includes 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.

15. The MEMS sensor according to claim 14, 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.

16. The MEMS sensor according to claim 15, wherein

said frequency divider has a fractional dividing function.

17. 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 chance 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 chance in the frequencies detected by said detecting unit; and

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, wherein

said detecting unit includes 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.

18. The MEMS sensor according to claim 17, 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.

19. The MEMS sensor according to claim 17, wherein:

said PLL circuit includes a frequency divider; and

said frequency divider has a fractional dividing function.

20. 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.