Shielded capacitive load cell apparatus responsive to weight applied to a vehicle seat
First Claim
1. Apparatus for measuring a weight applied to a seating surface of a vehicle seat, comprising:
- a load cell disposed in said seat including first and second conductive plates separated by a compressible insulator;
a conductive shield disposed between said seating surface and said load cell; and
circuitry coupled to said load cell and said conductive shield, including means for maintaining said shield at a reference voltage, an operational amplifier having a first input coupled to said first conductive plate and an output coupled to said second conductive plate, current supply elements for supplying a constant current to said load cell, and means for determining a capacitance of said load cell based on a rate of change in voltage at the output of said operational amplifier, said capacitance providing an indication of the weight applied to said seating surface.
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Accused Products
Abstract
The capacitance of a shielded capacitive load cell is determined so as to minimize the effect of stray or parasitic capacitance between the load cell and other objects including the shield. The load cell conductors are coupled across input and output terminals of an operational amplifier that is tied to a reference voltage. A constant current is applied to the load cell, and the resulting rate of change in voltage at the amplifier output is measured as a representation of the load cell capacitance. In a vehicle seat sensor application including an electromagnetic interference shield between the load cell and the seating surface, the amplifier output is coupled to the load cell electrode furthest from the shield, the amplifier maintains the other load cell electrode at a virtual reference voltage, and the shield is tied to the reference voltage.
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Citations
15 Claims
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1. Apparatus for measuring a weight applied to a seating surface of a vehicle seat, comprising:
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a load cell disposed in said seat including first and second conductive plates separated by a compressible insulator;
a conductive shield disposed between said seating surface and said load cell; and
circuitry coupled to said load cell and said conductive shield, including means for maintaining said shield at a reference voltage, an operational amplifier having a first input coupled to said first conductive plate and an output coupled to said second conductive plate, current supply elements for supplying a constant current to said load cell, and means for determining a capacitance of said load cell based on a rate of change in voltage at the output of said operational amplifier, said capacitance providing an indication of the weight applied to said seating surface. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A capacitive load cell, comprising:
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first and second conductive plates separated by a compressible insulator; and
circuitry coupled to said first and second conductive plates, including an operational amplifier having a first input coupled to said first conductive plate and an output coupled to said second conductive plate, current supply elements for forcing a constant current between said first and second conductive plates, and means for determining a capacitance between said first and second conductive plates based on a rate of change in voltage at the output of said operational amplifier. - View Dependent Claims (10, 11, 12, 13, 14, 15)
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Specification