Capacitive weight sensor

• US 6,218,632 B1
• Filed: 06/30/2000
• Issued: 04/17/2001
• Est. Priority Date: 12/08/1998
• Status: Expired due to Fees

• Current Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: McCarthy, Michael C., Lemense, Thomas J., Fullerton, Michael G.
• Primary Examiner(s): Gibson, Randy W.
• Application Number:
  US09/608,727
• Time in Patent Office: 291 Days
• Field of Search: 177/136, 177/144, 177/210.C, 738/626.26, 738/625.2, 361/287, 361/292, 180/271, 180/273, 280/735, 340/667, 340/562, 324/660, 324/661, 324/662, 324/686, 324/670, 324/672, 324/679
• US Class Current: 177/144
• CPC Class Codes: G01G 19/4142 for controlling activation of safety devices, e.g. airbag systems electrical circuits for triggering safety arrangements in case of vehicle accidents B60R21/015
• CPC Class Codes: G01G 7/06 by electrostatic action

Abstract

The present invention is directed to a weight sensing apparatus (10) for a vehicle seat (22). The weight sensing apparatus (10) includes a differential capacitor (150) including a first capacitor (152) and a second capacitor (176). The first capacitor (152) includes first and second opposed plate assemblies (154 and 156). The second capacitor (179) includes a third plate assembly (180) and the second plate assembly (156). Each plate assembly (154, 156, and 180) includes a plurality of electrically conductive elongated members that extend in an axial direction. The second plate assembly (156) is interposed between the first and third plate assemblies (154 and 180) and is movable in an axial direction relative to the first and third plate assemblies (154 and 180) in response to a load being applied to a vehicle seat. A source of alternating current supplies electrical energy to the differential capacitor (150) and an output circuit is electrically connected with the differential capacitor (150).

Claims

1. A weight sensing apparatus for a vehicle seat comprising:

a differential capacitor including;

a first capacitor including first and second opposed plate assemblies;

a second capacitor including a third plate assembly and said second plate assembly;

said second plate assembly being interposed between said first and third plate assemblies and axially adjoining said first capacitor to said second capacitor;

each plate assembly including a conductive plate, said conductive plate of said first plate assembly extending parallel to and being fixed relative to said conductive plate of said third plate assembly, said conductive plate of said second plate assembly extending parallel to and being moveable in an axial direction relative to said first and third plate assemblies in response to a load being applied to the vehicle seat;

each plate assembly having a plurality of elongated members which extend in a substantially axial direction, each elongated member being at least partially formed from a conductive member which is electrically connected to said conductive plate of the respective plate assembly;

said plurality of elongated members of said first and third plate assemblies extending toward said second plate assembly;

said second plate assembly having a plurality of elongated members extending toward said first plate assembly and a plurality of elongated members extending toward said third plate assembly, said plurality of elongated members extending from said second plate assembly terminating in gaps created by said elongated members of said first and third plate assemblies, said plurality of elongated members of said second plate assembly being movable relative to said plurality of elongated members of said first and third plate assemblies;

a source of alternating electrical energy which provides electrical energy to said differential capacitor; and

an output circuit electrically connected with said differential capacitor which provides an output signal indicative of the load applied to said weight sensing apparatus, the output signal of said output circuit varying as a function of the capacitance of both said first capacitor and said second capacitor.

2. The apparatus of claim 1 wherein said plurality of elongated members of the respective plate assemblies is a plurality of parallel, concentric cylindrical members.

3. The apparatus of claim 1 further including a dielectric fluid material interposed between and in communication with said first and second plate assemblies;

and

additional dielectric fluid material being interposed between and in communication with said second and third plate assemblies.

4. The apparatus of claim 1 wherein each of said first and second capacitors provides a corresponding output signal to said output circuit, said output circuit determining a difference in phase shift between the output signal of said first capacitor and the output signal of said second capacitor, the difference in phase shift being proportional to the applied load.

5. The apparatus of claim 1 further including means for monitoring a phase shift between an input signal to the first capacitor supplied by said source of electrical energy and an output signal of said first capacitor and for monitoring a phase shift between an input signal to said second capacitor supplied by said source of electrical energy and an output signal of said second capacitor, a difference between the monitored phase shift of the respective output signals of said first and second capacitors being functionally related to the applied load.

6. The apparatus of claim 5 wherein said output circuit further comprises a controller to which said means for monitoring phase shift provides a first output signal indicative of the monitored phase shift for said first capacitor and a second output signal indicative of the monitored phase shift for said second capacitor, said controller comparing the first and second output signals from said means for monitoring phase shift to determine a value indicative of the applied load.

7. The apparatus of claim 5 wherein said means for monitoring phase shift includes a pair of Exclusive-OR gates each having two inputs, one input of each of said pair of Exclusive-OR gates being coupled to said source of electrical energy for receiving the input signal, the other input of each of said pair of gates being couple to an output of a corresponding one of said first and second capacitors.

8. The apparatus of claim 7 further comprising a controller, each of said Exclusive-OR gates coupled to said controller for providing an output signal, said controller comparing said output signals from each of said pair of Exclusive-OR gates to determine a value indicative of the applied load which is a function of the differential capacitance of said first and second capacitors.

9. The apparatus of claim 1 wherein said first and second capacitors are electrically connected in series across said source of alternating electrical energy, an electrical node located at a juncture between said first and second capacitors being electrically connected to said output circuit, said output circuit determining a value indicative of the applied load as a function of the voltage at the electrical node, the voltage at the electrical node varying as a function of the differential capacitance of said first and second capacitors in response to the applied load.

10. The apparatus of claim 9 wherein said source of electrical energy provides a high frequency signal to said first capacitor of said differential capacitor, the voltage at the electrical node being a function of the differential capacitance between the first and second capacitors according and a function of the high frequency signal being provided by said source of electrical energy.

11. The apparatus of claim 10 wherein said monitoring circuit further includes an amplifier circuit electrically connected to the electrical node between said first and second capacitors, said amplifier circuit providing an amplified output signal indicative of the differential voltage at the electrical node.