Coil actuated sensor with sensitivity detection

Abstract

A magnetic field sensor includes a first coil responsive to a first AC coil drive signal having a first frequency, a magnetic field sensing element responsive to a sensing element drive signal and configured to simultaneously detect a directly coupled magnetic field generated by the first coil and a reflected magnetic field generated by an eddy current induced in a conductive target by the first coil, the conductive target disposed proximate to the magnetic field sensing element, the magnetic field sensing element further configured to generate a magnetic field signal, a second coil responsive to a second AC coil drive signal having a second frequency that is the same as the first frequency and current sensing circuitry configured to measure a magnitude of the second AC coil drive signal that causes the magnetic field signal to be approximately zero.

1 . A magnetic field sensor, comprising:
 a first coil responsive to a first AC coil drive signal having a first frequency;   a magnetic field sensing element responsive to a sensing element drive signal and configured to simultaneously detect a directly coupled magnetic field generated by the first coil and a reflected magnetic field generated by an eddy current induced in a conductive target by the first coil, the conductive target disposed proximate to the magnetic field sensing element, the magnetic field sensing element further configured to generate a magnetic field signal;   a second coil responsive to a second AC coil drive signal having a second frequency that is the same as the first frequency; and   current sensing circuitry configured to measure a magnitude of the second AC coil drive signal that causes the magnetic field signal to be approximately zero.   
     
     
         2 . The magnetic field sensor of  claim 1 , wherein the second coil does not induce an eddy current in the conductive target. 
     
     
         3 . The magnetic field sensor of  claim 1 , further comprising a coil driver configured to generate the first AC coil drive signal and the second AC coil drive signal. 
     
     
         4 . The magnetic field sensor of  claim 3 , further comprising processing circuitry responsive to the magnetic field signal and configured to generate a feedback signal coupled to the coil driver. 
     
     
         5 . The magnetic field sensor of  claim 4 , wherein the processing circuitry comprises an amplifier and a low pass filter. 
     
     
         6 . The magnetic field sensor of  claim 1 , further comprising output signal generation circuitry configured to generate an output signal of the magnetic field sensor indicative of the reflected magnetic field based on the measured magnitude of the second AC coil drive signal that causes the magnetic field signal to be approximately zero. 
     
     
         7 . The magnetic field sensor of  claim 6 , wherein the output signal generation circuitry comprises an amplifier. 
     
     
         8 . The magnetic field sensor of  claim 1 , wherein the second AC coil drive signal is 180 degrees out of phase with respect to the first AC coil drive signal. 
     
     
         9 . The magnetic field sensor of  claim 1 , further comprising a magnetic field sensing element driver configured to generate the sensing element drive signal. 
     
     
         10 . The magnetic field sensor of  claim 1 , wherein the second coil is smaller than the first coil. 
     
     
         11 . The magnetic field sensor of  claim 1 , wherein the current sensing circuitry comprises a shunt resistor in series with the second coil. 
     
     
         12 . A method of generating a magnetic field sensor output signal comprising:
 driving a first AC current through a first coil;   driving a second AC current through a second coil;   detecting with a magnetic field sensing element a directly coupled magnetic field generated by the first coil and a reflected magnetic field generated by an eddy current induced in a proximate conductive target by the first coil;   adjusting the second AC current to cause the magnetic field signal to be approximately zero; and   measuring the adjusted second AC current.   
     
     
         13 . The method of  claim 12 , further comprising amplifying the measured second AC current to generate the magnetic field sensor output signal. 
     
     
         14 . The method of  claim 12 , wherein adjusting the second AC current to cause the magnetic field signal to be approximately zero comprises generating a feedback signal in response to the magnetic field signal and coupling the feedback signal to a coil driver. 
     
     
         15 . The method of  claim 12 , further comprising generating the first AC current and the second AC current with a coil driver. 
     
     
         16 . The method of  claim 15 , wherein generating the first AC current and the second AC current comprises generating the second AC current to be 180 degrees out of phase with respect to the first AC current.