Electronic sensing circuit using piezoresistors

An electronic sensing circuit for replacing an LVDT device, comprising:

• a Wheatstone bridge sensing circuit responsive to applied pressure and having piezoresistive strain gauges where at least one gauge in said bridge varies resistance for a force applied in one direction and another gauge in said bridge varies resistance for a force applied in an opposite direction, wherein said Wheatstone bridge includes two bridge legs, each with two piezoresistive elements in series, said bridge legs being connected in parallel between a first pair of bridge points, with a second pair of bridge points being formed by respective connection points between the piezoresistor elements of the respective bridge legs, wherein a piezoresistor element in one bridge leg and an opposite piezoresistor element in the other leg are arranged together to vary their resistances in response to a force applied to said elements in said one direction, and the other piezoresistor elements are arranged together to vary their resistances in response to a force applied in said opposite direction;
• first and second input terminals for receiving an AC operating voltage applied therebetween, coupled to said first pair of bridge points wherein first and second voltages are provided at respective first and second ones of said second pair of bridge points, wherein an applied force in either direction produces changes in said first and second voltages;
• and
• first and second operational amplifiers having first and second output terminals, respectively, each having non-inverting and inverting terminals;
• said first one of said second pair of bridge points being coupled to said non-inverting terminal of said first amplifier, and to said inverting terminal of said second amplifier, said second one of said second pair of bridge points being coupled to said inverting terminal of said first amplifier and to said non-inverting terminal of said second amplifier;
• a third output terminal coupled to said second input terminal of said sensing circuit and forming a common ground therewith;
• said first and second voltages being converted to third and fourth output voltages, respectively, at said first and second output terminals, with said third and fourth output voltages equal for a zero force applied to said bridge and with said third and fourth output voltages increasing and decreasing correspondingly for said force output applied in either direction, whereby the sum of said third and fourth voltages is constant for any direction and value of applied force.