Capacitive force sensor

• US 7,891,253 B2
• Filed: 04/10/2008
• Issued: 02/22/2011
• Est. Priority Date: 04/11/2007
• Status: Expired due to Fees

• Current Assignee: Silverbrook Research Pty Limited
• Original Assignee: Silverbrook Research Pty Limited
• Inventors: Shipton, Gary
• Primary Examiner(s): NOORI, MAX H
• Application Number:
  US12/100,403
• Publication Number:
  US 20080250864A1
• Time in Patent Office: 1,048 Days
• Field of Search: 737/80, 738/626.26
• US Class Current: 73/780
• CPC Class Codes: G01D 2205/90 Two-dimensional encoders, i.e. having one or two codes extending in two directions
• CPC Class Codes: G01D 5/3473 Circular or rotary encoders
• CPC Class Codes: G01D 5/34746 Linear encoders
• CPC Class Codes: G01L 1/144 with associated circuitry G01L1/146 and G01L1/148 take precedence
• CPC Class Codes: G01L 1/146 for measuring force distributions, e.g. using force arrays G01L1/148 takes precedence
• CPC Class Codes: G06F 3/03545 Pens or stylus
• CPC Class Codes: G06F 3/0383 Signal control means within the pointing device

Abstract

A force sensor is provided which has a capacitive sensor circuit incorporating a sensor capacitor having variable capacitance C_sen arranged so that an output of the capacitive sensor circuit is proportional to 1/C_sen.

Claims

1. A capacitive sensor circuit, a reference capacitor connected to an input of the capacitive sensor circuit, a sensor capacitor having variable capacitance C_sen arranged within a capacitive feedback circuit of the capacitive sensor circuit which has an integrator such that one of the plates of the sensor capacitor is connected directly to an output of the capacitive sensor circuit, a feedback capacitor connected across the integrator, and a capacitor connected across the sensor capacitor for cancelling parasitic capacitance across the sensor capacitor, the arrangement of the capacitors resulting in the output of the capacitive sensor circuit being proportional to 1/C_sen.

2. A sensor circuit as claimed in claim 1, wherein the sensor capacitor is a parallel plate capacitor having one fixed plate and one movable plate.

3. A sensor circuit as claimed in claim 2, wherein the movable plate is configured to move relative to the fixed plate due to a pressure force exerted on the movable plate thereby altering separation of the plates and said capacitance C_sen.

4. A sensor circuit as claimed in claim 2, wherein a compression spring is connected to the movable plate.

5. A sensor circuit as claimed in claim 1, wherein the capacitive sensor circuit is a sigma-delta convertor circuit.