DRIVE MODE AND SENSE MODE RESONANCE FREQUENCY MATCHING

• US 20190055116A1
• Filed: 08/21/2017
• Published: 02/21/2019
• Est. Priority Date: 08/21/2017
• Status: Active Grant

• Current Assignee: Invensense Incorporated (TDK Corporation)
• Original Assignee: Invensense Incorporated (TDK Corporation)
• Inventors: SHIRVANI, Alireza, KUMAR, Sagar
• Granted Patent: US 10,538,427 B2
• Time in Patent Office: Days
• CPC Class Codes: B81B 2201/025 Inertial sensors not provided for in B81B2201/0235 - B81B2201/0242
• CPC Class Codes: B81B 3/0072 For controlling internal stress or strain in moving or flexible elements, e.g. stress compensating layers
• CPC Class Codes: B81B 7/008 MEMS characterised by an electronic circuit specially adapted for controlling or driving the same B81B7/0087 takes precedence; arrangements for starting, regulating, braking, or otherwise controlling an actuator H02N; control arrangements or circuits for visual indicators G09G3/00
• CPC Class Codes: G01C 19/5656 the devices involving a micromechanical structure
• CPC Class Codes: G01C 19/5762 the sensing mass being connected to a driving mass, e.g. driving frames
• CPC Class Codes: G01C 19/5776 Signal processing not specific to any of the devices covered by groups G01C19/5607 - G01C19/5719
• CPC Class Codes: G01P 15/0975 by acoustic surface wave resonators or delay lines
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: H01G 5/40 Structural combinations of variable capacitors with other electric elements not covered by this subclass, the structure mainly consisting of a capacitor, e.g. RC combinations
• CPC Class Codes: H01L 21/76898 formed through a semiconductor substrate
• CPC Class Codes: H03H 3/0075 Arrangements or methods specially adapted for testing microelecro-mechanical resonators or networks
• CPC Class Codes: H03H 9/02259 Driving or detection means

Abstract

In some embodiments, a micro electro mechanical system (MEMS) includes a proof mass, sense electrodes, sense circuitry, and a frequency matching circuitry. The proof mass is configured to move responsive to stimuli. The sense electrodes are configured to generate a signal responsive to the proof mass moving. The sense circuitry is coupled to the sense electrodes. The sense circuitry is configured to receive the generated signal and further configured to process the generated signal. The frequency matching circuitry is configured to apply a DC voltage to the sense electrodes. The DC voltage is configured to change a stiffness of a spring of the proof mass. According to some embodiments, the change in the stiffness of the spring matches a resonance frequency between a sense mode and a drive mode. According to some embodiments, the sense electrodes are a comb structure.

Claims

1. A micro electro mechanical system (MEMS) comprising:

a proof mass configured to move responsive to stimuli;

sense electrodes configured to generate a signal responsive to the proof mass moving;

sense circuitry coupled to the sense electrodes, wherein the sense circuitry is configured to receive the generated signal and further configured to process the generated signal; and

a frequency matching circuitry configured to apply a direct current (DC) voltage to the sense electrodes, wherein the DC voltage is configured to change a stiffness of a spring of the proof mass and wherein the change in the stiffness of the spring matches a resonance frequency between a sense mode and a drive mode.

2. The MEMS as described in claim 1, wherein the sense electrodes are a comb structure.

3. The MEMS as described in claim 1, wherein the frequency matching circuitry comprises:

a diode;

a capacitor coupled to the diode; and

an NMOS switch, wherein the diode is configured to charge the capacitor when the NMOS switch is open, and wherein the NMOS switch is configured to electrically connect the capacitor to the sense electrodes and to the sense circuitry when the NMOS switch is closed, wherein the DC voltage is a voltage from the capacitor being applied to the sense electrodes when the NMOS switch closes, and wherein the diode is electrically disconnected from the capacitor when the NMOS switch is closed, and wherein the NMOS switch is protected against high voltage stress by opening the NMOS switch when the capacitor is being charged by the diode and by closing the NMOS switch when the capacitor is not being charged by the diode.

4. The MEMS as described in claim 3, wherein a circuitry for generating a signal for closing and opening the NMOS switch comprises:

another diode;

a level shifter configured to receive a signal having a first voltage value or a second voltage value and wherein the level shifter is configured to generate a shifted signal having the first voltage value or a third voltage value; and

another capacitor coupled between the diode and the level shifter, wherein a voltage associated with the another capacitor is the DC voltage being applied to the sense electrodes, and wherein the another capacitor is charged using the another diode and output of the level shifter, and wherein the DC voltage is a sum of the first voltage value and a voltage being applied by the diode or a sum of the third voltage value and the voltage being applied by the another diode.

5. The MEMS as described in claim 1, wherein the frequency matching circuitry is coupled to a capacitor that is coupled to the sense circuitry, wherein the capacitor masks the sense circuitry from the DC voltage being applied.

6. The MEMS as described in claim 1, wherein the frequency matching circuitry comprises:

a PMOS switch;

a capacitor coupled to the PMOS switch; and

a NMOS switch,

wherein the capacitor is configured to charge when the PMOS switch closes and when the NMOS switch opens, and wherein the capacitor is electrically connected to the sense electrodes when the NMOS switch closes and when the PMOS switch opens and wherein the DC voltage is applied to the sense electrodes when the capacitor is electrically connected to the sense electrodes, and wherein the NMOS switch is protected against high voltage stress by opening the NMOS switch when the capacitor is being charged and by closing the NMOS switch when the capacitor is not being charged.

7. The MEMS as described in claim 1 further comprising:

a force feedback circuitry configured to apply a charge associated with a sensed signal to the sense electrodes during a forcing time period to move the proof mass from a first position to a second position and wherein the sense circuitry is inactive during the forcing time period, wherein the force feedback circuitry and the sense circuitry are time multiplexed, and wherein the sense electrodes are used for sensing, for force feedback and for matching the resonance frequency between the sense mode and the drive mode.

8. A micro electro mechanical system (MEMS) comprising:

a proof mass configured to move responsive to stimuli;

sense electrodes configured to generate a signal responsive to the proof mass moving; and

a frequency modifying circuitry configured to apply a direct current (DC) voltage to the sense electrodes, wherein the DC voltage is configured to change a stiffness of a spring of the proof mass, and wherein the change in the stiffness of the spring changes a resonance frequency in a sense mode and in a drive mode.

9. The MEMS as described in claim 8, wherein the sense electrodes are a comb structure.

10. The MEMS as described in claim 8, wherein the frequency modifying circuitry comprises:

a diode;

a capacitor coupled to the diode; and

an NMOS switch,

wherein the diode is configured to charge the capacitor when the NMOS switch is open, and wherein the NMOS switch is configured to electrically connect the capacitor to the sense electrodes when the NMOS switch is closed, wherein the DC voltage is a voltage from the capacitor being applied to the sense electrodes when the NMOS switch closes, and wherein the diode is electrically disconnected from the capacitor when the NMOS switch is closed, and wherein the NMOS switch is protected against high voltage stress by opening the NMOS switch when the capacitor is being charged by the diode and by closing the NMOS switch when the capacitor is not being charged by the diode.

11. The MEMS as described in claim 10, wherein a circuitry for generating a signal for closing and opening the NMOS switch comprises:

another diode;

a level shifter configured to receive a signal having a first voltage value or a second voltage value and wherein the level shifter is configured to generate a shifted signal having the first voltage value or a third voltage value; and

another capacitor coupled between the diode and the level shifter, wherein a voltage associated with the another capacitor is the DC voltage being applied to the sense electrodes, and wherein the another capacitor is charged using the another diode and output of the level shifter, and wherein the DC voltage is a sum of the first voltage value and a voltage being applied by the diode or a sum of the third voltage value and the voltage being applied by the another diode.

12. The MEMS as described in claim 8, wherein the frequency modifying circuitry comprises:

a PMOS switch;

a capacitor coupled to the PMOS switch; and

a NMOS switch,

wherein the capacitor is configured to charge when the PMOS switch closes and when the NMOS switch opens, and wherein the capacitor is electrically connected to the sense electrodes when the NMOS switch closes and when the PMOS switch opens and wherein the DC voltage is applied to the sense electrodes when the capacitor is electrically connected to the sense electrodes, and wherein the NMOS switch is protected against high voltage stress by opening the NMOS switch when the capacitor is being charged and by closing the NMOS switch when the capacitor is not being charged.

13. The MEMS as described in claim 8 further comprising:

a force feedback circuitry configured to apply a charge associated with a sensed signal to the sense electrodes during a forcing time period to move the proof mass from a first position to a second position, and wherein the sense electrodes are used for sensing, for force feedback and for matching the resonance frequency between the sense mode and the drive mode.

14. A method comprising:

generating a signal, using sense electrodes, responsive to a stimuli displacing a proof mass;

applying a direct current (DC) voltage to the sense electrodes; and

responsive to the applying the DC voltage, changing a stiffness of a spring of the proof mass, wherein the DC voltage is selected to change the stiffness to match a resonance frequency between a sense mode and a drive mode.

15. The method as described in claim 14 further comprising:

masking a sense circuitry associated with the sense electrodes from the DC voltage being applied.

16. The method as described in claim 14 further comprising:

generating the DC voltage; and

protecting a switch for applying the DC voltage from a high voltage of the DC voltage.

17. The method as described in claim 16, wherein the protecting comprises:

raising a control voltage associated with closing/opening of the switch, wherein a difference between the control voltage and the DC voltage is smaller than a difference between the control voltage without a raised voltage and the DC voltage.

18. The method as described in claim 17, wherein the raising the control voltage comprises:

level shifting an input voltage; and

bootstrapping the level shifted input voltage to the DC voltage.

19. The method as described in claim 14 further comprising:

realizing high impedance associated with the DC voltage without using a poly resistor.

20. The method as described in claim 14 further comprising:

applying a charge associated with a sensed signal to the sense electrodes during a forcing time period to move the proof mass from a first position to a second position, and wherein the sense electrodes are used for sensing, for force feedback and for matching the resonance frequency between the sense mode and the drive mode.