METHOD AND APPARATUS FOR CLOSED LOOP OFFSET CANCELLATION

• US 20080178671A1
• Filed: 01/25/2007
• Published: 07/31/2008
• Est. Priority Date: 01/25/2007
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Miller, Todd F., Kraver, Keith L., Fuhrmann, Marco
• Granted Patent: US 7,665,361 B2
• Time in Patent Office: Days
• US Class Current: 73/488
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

An apparatus (100, 200) and method (300) for sensing acceleration are provided. The method includes producing (305) a first signal in response to an acceleration sensed by a transducer, producing (310) a second signal based on the first signal, and actuating (315) the transducer in response to the second signal to remove offset in the transducer. The first signal represents the acceleration, and the second signal represents a low frequency component associated with an offset in the transducer. The apparatus (100) includes a transducer (102) producing a capacitance in response to the acceleration, a sensing system (104, 106, 108) producing a first signal from the capacitance representing the acceleration, and a compensation system (112, 110) coupled between the sensing system and transducer. The compensation system produces a second signal based on the first signal for substantially removing an offset of the transducer.

Claims

1. A method for operating a transducer to sense acceleration, the method comprising the steps of:

producing a first signal proportional to the acceleration sensed by the transducer;

producing a second signal based on the first signal, the second signal representing a low frequency component associated with an offset in the transducer; and

actuating the transducer in response to the second signal to substantially compensate for the offset in the transducer.

2. A method according to claim 1, wherein said step of producing a first signal comprises:

producing a capacitance in response to the acceleration sensed by the transducer;

producing a voltage responsive to the capacitance; and

converting the voltage to a digital signal to produce the first signal.

3. A method according to claim 1, wherein the transducer comprises a micro-electro-mechanical system (MEMS) device having a moveable element, wherein the method further comprises sensing a displacement of the moveable element in response to the acceleration, and wherein said step of producing a first signal comprises producing the first signal in response to the displacement.

4. A method according to claim 1, wherein said step of producing a second signal comprises filtering the first signal through a range of about 0.01 Hz to about 10 Hz to produce the low frequency component of the acceleration sensed by the transducer.

5. A method according to claim 1, wherein the transducer comprises a moveable element;

and wherein said step of actuating comprises;

applying an electrostatic voltage to the moveable element of the transducer; and

displacing the moveable element of the transducer in response to the electrostatic voltage.

6. A method according to claim 1, wherein the first signal comprises a digital signal, and wherein said step of producing a second signal comprises:

filtering the low frequency component from the digital signal;

converting the low frequency component to a voltage; and

producing an electrostatic force from the voltage.

7. A method according to claim 1, wherein said step of producing a second signal comprises:

filtering the low frequency component from the first signal;

converting the low frequency component to a third signal having a voltage; and

producing an electrostatic force from the voltage of the third signal.

8. A method according to claim 1, wherein said step of producing a first signal comprises:

producing a bit stream in response to the acceleration sensed by the transducer;

mapping the bit stream to a digital value;

low-pass filtering the digital value; and

applying a trim to the digital value, the first signal comprising the digital value.

9. An apparatus for sensing an acceleration comprising:

a transducer having an input and an output, said transducer configured to produce a capacitance in response to the acceleration;

a sensing system having an input coupled to said output of said transducer and having an output, said sensing system configured to produce a first signal from said capacitance, wherein said first signal is proportional to the acceleration; and

a compensation system having an input coupled to said output of said sensing system and having an output coupled to said input of said transducer, said compensation system configured to;

produce a second signal based on said first signal, said second signal indicating an offset of said transducer; and

supply said second signal to said transducer, said transducer compensating for said offset in response to said second signal.

10. An apparatus according to claim 9, wherein said sensing system comprises:

a capacitance-to-voltage converter having an input coupled to said output of said transducer and having an output, said capacitance-to-voltage converter configured to produce a voltage proportional to said acceleration; and

an analog-to-digital converter (ADC) having an input coupled to said output of said capacitance-to-voltage converter and having an output coupled to said input of said compensation system.

11. An apparatus according to claim 9, wherein

said sensing system comprises:

a modulator having an input coupled to said output of said transducer and having an output, said modulator configured to produce a bit stream based on said capacitance;

a sinc filter having an input coupled to said output of said modulator and having an output, said sinc filter configured to map said bit stream to a digital value, said digital value being proportional to said acceleration;

a first low pass filter (LPF) having an input coupled to said output of said sinc filter and having an output; and

a trim circuit having an input coupled to said output of said first LPF and having an output; and

said compensation system comprises;

a second LPF having an input coupled to said output of said trim circuit and having an output; and

a digital-to-analog converter having an input coupled to said output of said second LPF and having an output coupled to said input of said transducer.

12. An apparatus according to claim 9, wherein

said sensing system comprises:

a switched capacitance converter having an input coupled to said output of said transducer and having an output, said switched capacitance converter configured to convert said capacitance to a voltage;

a first low pass filter (LPF) having an input coupled to said output of said switched capacitance converter and having an output; and

a trim circuit having an input coupled to said output of said first LPF and having an output; and

said compensation system comprises;

a second LPF having an input coupled to said output of said trim circuit and having an output; and

a digital to analog converter having an input coupled to said output of said second LPF and having an output coupled to said input of said transducer.

13. An apparatus according to claim 9, wherein said first signal has a low frequency component;

and

wherein said compensation system is further configured to produce said second signal based on said low frequency component of said first signal.

14. An apparatus according to claim 13, wherein said low frequency component is less than or equal to about 10 Hz.

15. An apparatus according to claim 13, wherein said low frequency component is from about 0.01 Hz to about 10 Hz.

16. An apparatus according to claim 9, wherein said transducer comprises a micro-electro-mechanical system (MEMS) device having a moveable element configured to:

displace in response to the acceleration; and

displace in response to said second signal, independent of the acceleration, to compensate for said offset.

17. A method for operating a transducer to sense an acceleration, the method comprising:

sensing a capacitance representing an acceleration sensed by the transducer;

producing a digital signal based on the capacitance;

filtering a low frequency component from the digital signal, the low frequency component representing an offset in the transducer;

producing an electrostatic force from the low frequency component; and

supplying the electrostatic force to the transducer, the transducer compensating for the offset in response to the electrostatic force.

18. A method according to claim 17, wherein said step of producing a digital signal comprises:

producing a voltage responsive to the capacitance, the voltage proportional to the acceleration;

converting the voltage to the digital signal;

low-pass filtering the digital signal; and

trimming the digital signal.

19. A method according to claim 18, wherein said step of filtering comprises filtering the digital signal through a range from about 0.01 Hz to about 10 Hz.

20. A method according to claim 17, wherein the transducer comprises a micro-electro-mechanical system (MEMS) device having a moveable element;

and wherein the method further comprises centering the moveable element in response to the electrostatic force.

21. A method according to claim 17, wherein the offset is due to a temperature effect and a stress effect on the transducer.