Inductive spool displacement sensor

Abstract

An inductive spool displacement sensor for determining the position or displacement of a spool within a particular hydraulic valve including a sensor housing having a cavity formed therein adapted for mounting within the hydraulic valve, the sensor housing being positioned such that at least a portion of the valve spool is disposed to enter the sensor cavity during certain operations of the valve. The sensor further includes a sensing coil wound about the housing cavity and a reference coil wound about the housing and spaced from the sensing coil, the inductance of the sensing coil being responsive to movement of the spool within the cavity and the inductance of the reference coil being responsive to the temperature of the fluid passing through the hydraulic valve. Based upon eddy current principles and because eddy currents are a function of material conductivity, the sensing coil, when energized, generates a signal indicative of that portion of the spool present within the housing cavity. Electronic circuitry, including a microprocessor, are coupled to the coils for receiving signals therefrom, the microprocessor determining the position of the spool relative to the cavity, or some other predetermined reference point, and outputting a signal proportional to the position or displacement of the spool within the hydraulic valve. The reference coil is provided to compensate for changing inductance values due to changing fluid temperature.

What is claimed is: 
     
       1. A sensor for determining the position of a displaceable spool located within a hydraulic valve, said sensor comprising: (a) a housing having a cavity formed therein for receiving at least a portion of the spool of the hydraulic valve;   (b) a sensing coil wound about said housing cavity and disposed to generate electrical signals responsive to movement of the valve spool within said cavity, said electrical signals being representative of that portion of the spool located within the cavity in response to the change of the inductance of said sensing coil due to the position of the spool within the cavity;   (c) a reference coil wound about said housing and disposed to generate electrical signals responsive to the temperature of the valve fluid within the cavity, said electrical signals being independent of the position of the spool within said cavity; and   (d) circuit means coupled with the sensing coil and the reference coil for producing an output signal proportional to the axial displacement of the spool relative to said cavity.   
     
     
       2. The sensor, as set forth in claim 1, wherein said circuit means includes an oscillator selectively coupled with the sensing coil and the reference coil for producing an oscillating waveform, wherein the frequency of the oscillating waveform is a function of the induction of one of said coils. 
     
     
       3. The sensor, as set forth in claim 2, wherein said circuit means includes a multiplexer adapted to select one coil to energize at a given time. 
     
     
       4. The sensor, as set forth in claim 3, wherein said oscillator produces an oscillating waveform having a series of pulses, the frequency of which is a function of the one energized coil inductance. 
     
     
       5. The sensor, as set forth in claim 1, wherein said output signal is a signal having a pulse width modulated waveform indicative of that portion of the spool located within said cavity. 
     
     
       6. The sensor, as set forth in claim 1, wherein the spool is made of an electrically conducting material. 
     
     
       7. A spool displacement sensor for determining the axial displacement of a spool located within a hydraulic valve, said sensor comprising: (a) a housing having a cavity formed therein for receiving at least a portion of the valve spool;   (b) a first coil wound about said housing cavity and disposed to generate electrical signals responsive to the position of the spool within said cavity, said electrical signals being representative of that portion of the spool within the cavity in response to the change of the inductance of said first coil due to movement of the spool within the cavity;   (c) a second coil wound about said housing and disposed to generate electrical signals responsive to the temperature of the fluid medium flowing through the hydraulic valve and within said cavity, said electrical signals being independent of the position of the spool within said cavity; and   (d) a microprocessor coupled with the first and second coils and adapted to receive the electrical signals from said first and second coils, said microprocessor determining the axial displacement of the spool relative to a predetermined location within the hydraulic valve.   
     
     
       8. The sensor, as set forth in claim 7, wherein said microprocessor produces a detecting signal having a pulse width modulated waveform indicative of the axial movement of the spool relative to said predetermined location. 
     
     
       9. In a hydraulic valve having a body portion with an axially displaceable spool element located therewithin, and means for selectively positioning the spool element in the valve body, the improvement comprising an apparatus for determining the axial displacement of the spool element relative to the valve body, said apparatus comprising: (a) a housing having a cavity formed therein for receiving one end portion of the spool element;   (b) a sensing coil disposed adjacent said cavity wherein the induction of the sensing coil is responsive to movement of the spool element within said cavity;   (c) a reference coil disposed adjacent said cavity wherein the inductance of said reference coil is responsive to the temperature of the fluid within the hydraulic valve and wherein said induction is independent of the position of the spool element within said cavity; and   (d) circuitry coupled to said coils for producing an output signal proportional to the axial displacement of the spool element relative to the valve body.