External charger for an implantable medical device having at least one sense coil concentric with a charging coil for determining position

1. An external charger for wirelessly providing energy to an implantable medical device (IMD), comprising:

a charging coil configured to produce a magnetic field to wirelessly provide energy to the IMD; and

a first sense coil comprising one or more turns at a constant first radius and a second sense coil comprising one or more turns at a constant second radius larger than the constant first radius, wherein the first sense coil is configured to be induced by the magnetic field with a first induced signal with a first magnitude, and wherein the second sense coil is induced with a second induced signal with a second magnitude, wherein the first and second sense coils are concentric; and

control circuitry configured to determine from the first and second magnitudes the position of the charging coil with respect to the IMD.

2. The external charger of claim 1 , further comprising an electronics module and a charging coil assembly coupled to the electronics module by a cable,

wherein the charging coil and the first and second sense coils are within the charging coil assembly, and

wherein the control circuitry is within the electronics module.

3. The external charger of claim 1 , further comprising a circuit board, wherein the first and second sense coils are formed in one or more traces in the circuit board.

4. The external charger of claim 3 , wherein the charging coil comprises a wire winding, wherein the charging coil is placed on a side of the circuit board, or wherein the circuit board is circumscribed by the wire winding of the charging coil.

5. The external charger of claim 1 ,

wherein the control circuitry is configured to determine from the first magnitude whether the charging coil is centered with respect to the IMD, and from the second magnitude whether the charging coil is aligned with respect to the IMD,

wherein centered comprises a condition in which the coupling between the charging coil and the IMD is higher than a first coupling value, and

wherein aligned comprises a condition in which the coupling between the charging coil and the IMD is higher than a second coupling value lower than the first coupling value.

6. The external charger of claim 5 ,

wherein the control circuitry further includes a first threshold and a second threshold, and

wherein the control circuitry is configured to determine whether the charging coil is centered by comparing the first magnitude to the first threshold, and whether the charging coil is aligned by comparing the second magnitude to the second threshold.

7. The external charger of claim 1 , further comprising a user interface configured to indicate to a user the determined position of the charging coil with respect to the IMD.

8. The external charger of claim 1 , wherein the charging coil comprises a third radius, and wherein the constant first radius and the constant second radius are smaller than the third radius.

9. An external charger for wirelessly providing energy to an implantable medical device (IMD), comprising:

a charging coil configured to produce a magnetic field to wirelessly provide energy to the IMD;

first and second sense coils, concentric with the charging coil, wherein the first sense coil comprises a circle comprising one or more turns at a constant first radius and is configured to be induced by the magnetic field with a first induced signal having a first magnitude affected by a position of the charging coil with respect to the IMD, and wherein the second sense coil comprises a circle comprising one or more turns at a constant second radius larger than the first radius and is configured to be induced by the magnetic field with a second induced signal having a second magnitude affected by a position of the charging coil with respect to the IMD; and

control circuitry configured to determine from the first and second magnitudes the position of the charging coil with respect to the IMD.

10. The external charger of claim 9 , further comprising an electronics module and a charging coil assembly coupled to the electronics module by a cable,

wherein the charging coil and the at least one sense coil are within the charging coil assembly, and

wherein the control circuitry is within the electronics module.

11. The external charger of claim 9 , further comprising a circuit board, wherein the at least one sense coil is formed in one or more traces in the circuit board.

12. The external charger of claim 11 , wherein the charging coil comprises a wire winding, wherein the charging coil is placed on a side of the circuit board, or wherein the circuit board is circumscribed by the wire winding of the charging coil.

13. The external charger of claim 9 ,

wherein the control circuitry is configured to determine from the first magnitude whether the charging coil is centered with respect to the IMD, and from the second magnitude whether the charging coil is aligned with respect to the IMD,

wherein centered comprises a condition in which the coupling between the charging coil and the IMD is higher than a first coupling value, and

wherein aligned comprises a condition in which the coupling between the charging coil and the IMD is higher than a second coupling value lower than the first coupling value.

14. The external charger of claim 13 ,

wherein the control circuitry further includes a first threshold and a second threshold, and

wherein the control circuitry is configured to determine whether the charging coil is centered by comparing the first magnitude to the first threshold, and whether the charging coil is aligned by comparing the second magnitude to the second threshold.

15. The external charger of claim 9 , further comprising a user interface configured to indicate to a user the determined position of the charging coil with respect to the IMD.