Planar linear inductive position sensor having edge effect compensation
A planar linear inductive position sensor is formed on a substrate and includes at least one oscillating coil, a first sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along a linear axis along which a linear position of a conductive target is to be sensed, and a second sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along the linear axis. The first and second sensing coils have geometries selected such that equal opposing magnetic fields are induced in the first and second sensing coils in the presence of a magnetic field generated by the oscillating coil when no conductive target is proximate to the first and second sensing coils and unequal opposing magnetic fields are induced in the first and second sensing coils when the conductive target is proximate to the first and second sensing coils, a difference in the unequal opposing magnetic fields induced in the first and second sensing coils correlated to the position of the conductive target.
1. A planar linear inductive position sensor comprising:
a substrate;
at least one oscillating coil;
a first sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along a linear axis along which a linear position of a conductive target is to be sensed;
a second sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along the linear axis;
the first and second sensing coils having geometries selected such that equal opposing magnetic fields are induced in the first and second sensing coils in the presence of a magnetic field generated by the oscillating coil when no conductive target is proximate to the first and second sensing coils and unequal opposing magnetic fields are induced in the first and second sensing coils when the conductive target is proximate to the first and second sensing coils, a difference in the unequal opposing magnetic fields induced in the first and second sensing coils correlated to the position of the conductive target.
2. The planar linear inductive position sensor of claim 1 wherein the substrate is a printed circuit board.
3. The planar linear inductive position sensor of claim 1 wherein the oscillating coil is shaped as a rectangle, a long side of the rectangle aligned in the direction of travel of the conductive target to be sensed.
4. The planar linear inductive position sensor of claim 1 wherein:
the first sensing coil is formed as a first segment having the shape of a 360° cycle of a sine function sin x starting at 0° and a second segment having the shape of a 360° cycle of a sine function −sin x starting at 0°, both the first and second segments sharing a common axis, first ends of the first and second segments meeting at and electrically connected to each other at a first location along the common axis, and second ends of the first and second segments meeting at and electrically connected to each other at a second location along the common axis;
the second sensing coil is formed as a first segment having the shape of a 360° cycle of a cosine function cos x starting at 0° and an opposing second segment having the shape of a 360° cycle of a cosine function cos x starting at 0°, both the first and second segments of the second sensing coil sharing the common axis, first ends of the first and second segments of the second sensing coil electrically connected to each other, and second ends of the first and second segments of the second sensing coil electrically connected to each other; and
wherein a linear position of the conductive target is sensed as positions along the common axis.
5. The planar linear inductive position sensor of claim 1 wherein the at least one oscillating coil is formed as a center-tapped coil having two segments.
6. The planar linear inductive position sensor of claim 1 wherein the opposing edges of the first and second sensing coils each extend beyond opposing edges of the oscillating coil along the linear axis by an amount selected to cancel a no-target-present offset voltage induced at the opposing edges of the second sensing coil.
7. A planar linear inductive position sensor system comprising:
a substrate;
an oscillating coil;
a first sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along a linear axis along which a linear position of a conductive target is to be sensed;
a second sensing coil having opposing edges extending beyond opposing edges of the oscillating coil along the linear axis;
the first and second sensing coils having geometries selected such that equal opposing magnetic fields are induced in the first and second sensing coils in the presence of a magnetic field generated by the oscillating coil when no conductive target is proximate to the first and second sensing coils and unequal opposing magnetic fields are induced in the first and second sensing coils when the conductive target is proximate to the first and second sensing coils, a difference in the unequal opposing magnetic fields induced in the first and second sensing coils correlated to the position of the conductive target;
a capacitor coupled to the oscillating coil to form a resonant LC circuit at a resonant frequency;
a signal generator generating a signal at the resonant frequency coupled to the oscillator coil; and
sensor circuits coupled to the first and second sensing coils.
8. The planar linear inductive position sensor system of claim 7 wherein the substrate is a printed circuit board.
9. The planar linear inductive position sensor system of claim 7 wherein:
the first sensing coil is formed as a first segment having the shape of a 360° cycle of a sine function sin x starting at 0° and an opposing second segment having the shape of a 360° cycle of a sine function sin x starting at 0°, both the first and second segments sharing a common axis, first ends of the first and second segments of the first sensing coil meeting at and electrically connected to each another, and second ends of the first and second segments of the first sensing coil meeting at and electrically connected to each another;
the second sensing coil is formed as a first segment having the shape of a 360° cycle of a cosine function cos x starting at 0° and an opposing second segment having the shape of a 360° cycle of a cosine function −cos x starting at 0°, both the first and second segments of the second sensing coil sharing the common axis, first ends of the first and second segments of the second sensing coil electrically connected to each other, and second ends of the first and second segments of the second sensing coil are electrically connected to each other; and
wherein linear position of the conductive target is sensed as positions along the common axis.
10. The planar linear inductive position sensor system of claim 7 wherein the oscillating coil is formed as a center-tapped coil.
11. The planar linear inductive position sensor system of claim 10 wherein the capacitor coupled to the oscillating coil to form a resonant LC circuit at a resonant frequency comprises:
a first capacitor coupled to a first end of the center-tapped coil; and
a second capacitor coupled to a second end of the center-tapped coil.
12. The planar linear inductive position sensor system of claim 7 wherein the opposing edges of the first and second sensing coils each extend beyond opposing edges of the oscillating coil along the linear axis by an amount selected to cancel a no-target-present offset voltage induced at the opposing edges of the second sensing coil.
13. A method for detecting the linear position of a conductive target with respect to a substrate comprising:
providing an oscillating coil on the substrate;
providing first and second sensing coils on the substrate, the first and second sensing coils having geometries selected such that equal opposing magnetic fields are induced in the first and second sensing coils in the presence of a magnetic field generated by the oscillating coil when no conductive target is proximate to the first and second sensing coils and unequal opposing magnetic fields are induced in the first and second sensing coils when the conductive target is proximate to the first and second sensing coils, a difference in the unequal opposing magnetic fields induced in the first and second sensing coils correlated to the position of the conductive target, the provided first and second sensing coils having opposing edges extending beyond opposing edges of the oscillating coil along a linear axis along which the linear position of the conductive target is to be sensed;
exciting the oscillating coil with a generated oscillator signal;
measuring voltages induced in the first and second sensing coils from the generated oscillator signal; and
determining presence and position of the conductive target as a function of a difference in the voltages induced in the first and second sensing coils.
14. The method of claim 13 wherein providing an oscillating coil on the substrate comprises providing a resonant circuit including the oscillating coil and a capacitor, the resonant circuit having a resonant frequency; and
exciting the oscillating coil with a generated oscillator signal comprises exciting the oscillating coil with a generated oscillator signal at the resonant frequency.
15. The method of claim 13 wherein providing first and second sensing coils on the substrate comprises:
providing the first sensing coil formed as a first segment having the shape of a 360° cycle of a sine function sin x starting at 0° and an opposing second segment having the shape of a 360° cycle of a sine function sin x starting at 0°, both the first and second segments sharing a common axis, first ends of the first and second segments of the first sensing coil meeting at and electrically connected to each other, and second ends of the first and second segments of the first sensing coil meeting at and electrically connected to each other; and
providing the second sensing coil formed as a first segment having the shape of a 360° cycle of a cosine function cos x starting at 0° and an opposing second segment having the shape of a 360° cycle of a cosine function cos x starting at 0°, both the first and second segments of the second sensing coil sharing the common axis, first ends of the first and second segments of the second sensing coil electrically connected to each other, and second ends of the first and second segments electrically connected to each other.