Method to force-balance capacitive transducers

• US 6,496,348 B2
• Filed: 05/25/2001
• Issued: 12/17/2002
• Est. Priority Date: 03/10/1998
• Status: Expired due to Fees

• Current Assignee: Robert B. Mcintosh
• Original Assignee: Robert B. Mcintosh
• Inventors: McIntosh, Robert B.
• Primary Examiner(s): Tso, Edward H.
• Application Number:
  US09/866,351
• Publication Number:
  US 20010047689A1
• Time in Patent Office: 571 Days
• Field of Search: 361/115, 361/277, 361/278, 361/280, 361/283.3, 361/283.4
• US Class Current: 361/115
• CPC Class Codes: B81B 2201/0264 Pressure sensors
• CPC Class Codes: B81B 2203/0392 profiles not provided for in B81B2203/0376 - B81B2203/0384
• CPC Class Codes: B81B 3/0086 Electrical characteristics, e.g. reducing driving voltage, improving resistance to peak voltage
• CPC Class Codes: G01D 5/2417 by varying separation
• CPC Class Codes: G01L 11/008 electrostatic or electromagnetic counterbalancing forces
• CPC Class Codes: G01L 9/0072 using variations in capacitance
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

A method to maintain a movable electrode of a variable capacitor in fixed relationship with a stationary electrode by electrostatic force feedback when one capacitor electrode is grounded. The invention exploits the high quiescent capacitance and capacitance-load sensitivity of a variable-area capacitance transducer in combination with the advantage of controlling the flexible electrode with a low voltage compared to transducers with parallel-plate capacitors. The invention can be used to accurately control the position of a surface, stylus, inertial mass, valve, electrical contact, electrical component; or an optical component such as a mirror, lens, grating, filter, or holographic element.

Claims

1. A method to control a movable electrode of a variable capacitor by electrostatic force feedback comprising the steps of:

a. providing a capacitor with a movable electrode and a cooperating stationary electrode;

b. providing a capacitance detection circuit with an input coupling capacitor electrically connected to one said capacitor electrode;

c. detecting and amplifying a change in capacitance of said variable capacitor to generate a feedback control voltage;

d. applying said feedback control voltage across said movable and said stationary electrodes, whereby said movable electrode is maintained at a fixed position or predetermined generatrix.

2. The method as claimed in claim 1 wherein said capacitor is a variable-area capacitor.

3. The method as claimed in claim 1 wherein said capacitor is a capacitive transducer.

4. The method as claimed in claim 1 wherein said movable electrode is connected to a ground potential.

5. The method as claimed in claim 1 wherein said stationary electrode is connected to a ground potential.

6. The method as claimed in claim 1 wherein said capacitor at least in part is bulk micromachined.

7. The method as claimed in claim 1 wherein said capacitor at least in part is surface micromachined.

8. A method to control by electrostatic force feedback a moveable electrode located between two cooperating capacitor electrodes that form a first and a second capacitor element of a differential capacitor comprising the steps of:

a. providing a differential capacitor with a movable electrode and two stationary electrodes;

b. providing a capacitance detection circuit that includes two input coupling capacitors and a bridge network means actively nulled by current feedback;

c. providing an amplifying means with an output connected to a beam steering means;

d. differentially detecting and amplifying a capacitance change of said first and said second capacitor elements to generate an error signal proportional to said capacitance change;

e. amplifying said error signal to provide a feedback control voltage and steering said control voltage to one said capacitor element.

9. The method as claimed in claim 8 wherein said capacitor is a differential variable-area capacitor.

10. The method as claimed in claim 8 wherein said capacitor is a differential capacitive transducer.

11. The method as claimed in claim 8 wherein said movable electrode of said differential capacitor is connected to a ground potential.

12. The method as claimed in claim 8 wherein said capacitor at least in part is bulk micromachined.

13. The method as claimed in claim 8 wherein said capacitor at least in part is surface micromachined.

14. An electrostatically force-balanced capacitive transducer comprising:

a. a variable capacitor with a first electrode connected to a ground potential and a second electrode connected to a coupling capacitor connected to a first-side node of a bridge network means actively nulled by current feedback;

b. said first-side node connected to a first input of a differential integrating means and a second-side node of said bridge network connected to a second input of opposing polarity of said integrating means and an output of said integrating means connected to a voltage-controlled current sourcing means connected to said first-side node of said bridge network means, whereby a first negative feedback loop actively nulls said bridge network means;

c. said output of said integrating means connected to an input of an amplifying means with finite output impedance connected to said second electrode of said transducer, thereby a second negative feedback loop electrostatically force-balances said capacitive transducer.

15. The electrostatically force-balanced capacitive transducer of claim 14 further including a reference capacitor connected between said second-side node of said bridge network means and said ground potential, thereby forming a balanced full-bridge network.

16. The electrostatically force-balanced capacitive transducer of claim 14 further including a third electrode of said transducer connected to a second coupling capacitor connected to said second-side node of said bridge network means and said amplifying means connected to a feedback steering means with a first output terminal connected to said second capacitor electrode and a second output connected to said third capacitor electrode, thereby forming a differential electrostatically force balanced capacitive transducer.