Capacitance sensing circuit for a pressure sensor

• US 7,516,669 B2
• Filed: 01/16/2008
• Issued: 04/14/2009
• Est. Priority Date: 10/18/2004
• Status: Expired due to Fees

• Current Assignee: Precision Mechatronics Pty Ltd
• Original Assignee: Silverbrook Research Pty Limited
• Inventors: Silverbrook, Kia, Mallinson, Samuel George
• Primary Examiner(s): Allen, Andre J
• Assistant Examiner(s): JENKINS, JERMAINE L
• Application Number:
  US12/015,390
• Publication Number:
  US 20080110270A1
• Time in Patent Office: 454 Days
• Field of Search: 73700-756, 3612831-2834
• US Class Current: 73/724
• CPC Class Codes: B60C 23/0408 transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
• CPC Class Codes: G01L 15/00 Devices or apparatus for measuring two or more fluid pressure values simultaneously
• CPC Class Codes: G01L 19/0636 using particle filters
• CPC Class Codes: G01L 9/0047 Diaphragm with non uniform thickness, e.g. with grooves, bosses or continuously varying thickness
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

The invention provides for a capacitance sensing circuit for a pressure sensor. The pressure sensor includes a sensor membrane and a compensation membrane. The circuit includes a controller arranged in signal communication with a charge amplifier, a charge injector and two switches. The charge amplifier is connected to the switches via a sensor capacitor in parallel with a reference capacitor in parallel with a parasitic capacitance to ground. The charge injector and charge amplifier are arranged in parallel connection between the capacitors and the controller. The controller operates the circuit to determine a charge imbalance indicative of a pressure difference between the sensor and compensation membranes.

Claims

1. A capacitance sensing circuit for a pressure sensor having a sensor membrane and a compensation membrane, the circuit comprising

a controller;

and

a charge amplifier, a charge injector and two switches, in signal communication with the controller, the charge amplifier connected to the switches via a sensor capacitor C_s in parallel with a reference capacitor C_r in parallel with a parasitic capacitance C_p to ground, the charge injector and charge amplifier arranged in parallel connection between the capacitors and controller, the controller configured to determine a charge imbalance indicative of a pressure difference between the sensor and compensation membranes.

2. The sensing circuit of claim 1, having a power supply arrangement configured to supply constant reference voltages V1 and V2 to the respective switches.

3. The sensing circuit of claim 2, wherein voltages V1 and V2 are alternately switched onto capacitors C_r or C_s to charge said capacitors with respective voltages V_r and V_s.

4. The sensing circuit of claim 3, wherein the voltages V1 or V2 applied to respective capacitors C_r or C_s are fed through the capacitors C_r or C_s to charge amplifier as voltage V_in, the charge amplifier amplifying the voltage V_in to a voltage V_out.

5. The sensing circuit of claim 4, wherein voltage V_in provides a measure of charge imbalance in the circuit when capacitor C_r is switched to voltage V2, capacitor C_s is switched to voltage V1, the charge amplifier is reset to force voltage V_in to a fixed voltage V_r and the charge injector is set to known charge state Q_I1, the total stored charge in the capacitors Q₁ determined by

Q₁=C_r(V2−

V_r)+C_s(V1−

V_r)+Q_I1.

6. The sensing circuit of claim 5, wherein capacitor C_r is connected to voltage V1, capacitor C_s is connected to voltage V2, and an output from the charge amplifier is monitored by the controller with feedback applied to the charge injector such that the total charge is balanced by the charge injector injecting a charge of Q_I1−

Q_I1 to force voltage V_in equal to voltage V_r, so that the total stored charge in the capacitors Q₂ is given by

Q₂=C_r(V1−

V_r)+C_s(V2−

V_r)+Q_I2.

7. The sensing circuit of claim 6, wherein the controller applies feedback to the circuit to force Q₁=Q₂, with a digital output from the controller configured to output a charge difference result of

Q_I1−

Q_I2=(V1−

V2)·

(C_r−

C_s).

8. The sensing circuit of claim 7, wherein the controller performs an iterative procedure to determine a required output to obtain the charge difference.