Inductive temperature sensing

1. An apparatus for inductive temperature sensing comprising a coil configuration, a target disposed adjacent to the coil configuration, and an impedance measurement circuit, wherein

the target has a non-zero conductivity and susceptibility;

the impedance measurement circuit measures an apparent inductance of the coil configuration at a measurement frequency close to a crossover frequency to provide an indication of a temperature of the target;

the measurement frequency is the frequency at which the apparent inductance is measured; and

the crossover frequency is the frequency at which the apparent inductance of the coil configuration equals a known inductance of the coil configuration without the target for a given temperature.

2. An apparatus according to claim 1 , wherein the measurement frequency is between 70% of the crossover frequency and 130% of the crossover frequency.

3. An apparatus according to claim 1 , wherein the measurement frequency is between 90% of the crossover frequency and 110% of the crossover frequency.

4. An apparatus according to claim 1 , further comprising an inductive heating circuit disposed adjacent to the target for inductively heating the target.

5. An apparatus according to claim 1 , wherein the target is arranged to function as an induction heating target as well as a sensing target.

6. An apparatus according to claim 1 , wherein the impedance measurement circuit measures a resistive part of an apparent impedance of the coil configuration close to or exactly at a second crossover frequency, defined as the frequency at which the resistive part of the apparent impedance of the coil configuration is substantially independent of the temperature of the target.

7. An apparatus according to claim 1 , wherein the coil configuration comprises an inductive heating coil and a sensing coil.

8. An apparatus according to claim 1 , wherein the target is made of a material or a composite of materials which have a conductivity that decreases as temperature increases over the measured range.

9. An apparatus according to claim 1 , wherein the target is made of a material or a composite of materials which have a susceptibility that decreases as temperature increases over the measured range.

10. An apparatus according to claim 1 , wherein the target material is magnetic steel.

11. An apparatus according to claim 1 , wherein the coil configuration and target are substantially planar.

12. An apparatus according to claim 1 , wherein the coil configuration is wrapped around the outer perimeter of the target.

13. An apparatus according to claim 1 , wherein the coil configuration comprises a sensing coil and a heating coil separated from each other.

14. An apparatus according to claim 13 , wherein the sensing coil is substantially balanced with respect to the heating coil such that an inductive coupling factor is close to zero.

15. An apparatus according to claim 14 , arranged such that an induction heating from the heating coil can be interrupted in order to make a temperature measurement of the target.

16. An apparatus according to claim 15 , arranged such that the interval between temperature measurements can be varied using a control system.

17. An apparatus according to claim 1 , wherein the apparatus is configured to rotate machinery, and wherein the target is rotatable or movable with respect to the coil configuration.

18. An apparatus according to claim 1 ,

wherein the coil configuration further comprises an inductive heating coil, and

wherein the target is composed of a susceptible material for inductive heating of the target.