Inductive angular position sensor
An inductive sensor may track an angle of a movable element. In some cases, it is desirable to operate the inductive sensor using battery power so that turns are tracked properly even when power is lost. The disclosed inductive sensor includes circuitry to conserve power, such as a non-resonant driver that allows for fast measurements without wasting energy and a difference encoder that can estimate the angle within a range without the need for digitization and complicated processing.
1. An inductive sensor comprising:
an exciter coil;
a non-resonant driver coupled to the exciter coil, the non-resonant driver configured to switch the exciter coil alternately between a positive terminal and a negative terminal of a voltage source to generate an excitation current in the exciter coil, the excitation current having a triangular-shaped amplitude;
a plurality of receiver coils;
a rotor coil mechanically coupled to a movable element, the rotor coil configured to couple a magnetic field generated by the excitation current to the plurality of receiver coils according to an angle-dependent inductive coupling between the rotor coil and the plurality of receiver coils, the magnetic field generating a plurality of coil voltages at the plurality of receiver coils; and
a difference encoder configured to compare the plurality of coil voltages to determine an angle measurement of the movable element.
2. The inductive sensor according to claim 1 , wherein:
the non-resonant driver is configured to switch the exciter coil alternately between the positive terminal and the negative terminal of the voltage source during a measurement period; and
the difference encoder is configured to compare the plurality of coil voltages to determine the angle measurement of the movable element at any time during the measurement period.
3. The inductive sensor according to claim 1 , wherein the non-resonant driver is activated from a sleep condition to switch the exciter coil alternately between the positive terminal and the negative terminal at a start of a measurement period, the plurality of coil voltages being ready for measurement immediately after the non-resonant driver is activated.
4. The inductive sensor according to claim 1 , wherein the non-resonant driver is configured to switch the exciter coil alternately between the positive terminal and the negative terminal of the voltage source at a switching frequency that is not based on a resonant frequency of the excitation current.
5. The inductive sensor according to claim 1 , wherein substantially all of the excitation current in the exciter coil is returned to the voltage source after the angle measurement of the movable element is determined.
6. The inductive sensor according to claim 1 , wherein the non-resonant driver is an H-bridge circuit that includes:
a first switch coupled between the positive terminal of the voltage source and a first terminal of the exciter coil;
a second switch coupled between the negative terminal of the voltage source and a second terminal of the exciter coil, the first switch and the second switch configured in an ON state by a switching signal to increase the excitation current in the exciter coil;
a third switch coupled between the positive terminal of the voltage source and the second terminal of the exciter coil; and
a fourth switch coupled between the negative terminal of the voltage source and the first terminal of the exciter coil, the third switch and the fourth switch configured in the ON state by the switching signal to decrease the excitation current in the exciter coil.
7. The inductive sensor according to claim 6 , wherein the H-bridge circuit does not include a capacitor coupled to the first terminal or the second terminal of the exciter coil.
8. The inductive sensor according to claim 1 , wherein the plurality of receiver coils include:
a first receiver coil, the magnetic field coupled to the first receiver coil generating a first coil voltage at the first receiver coil;
a second receiver coil, the magnetic field coupled to the second receiver coil generating a second coil voltage at the second receiver coil; and
a third receiver coil, the magnetic field coupled to the third receiver coil generating a third coil voltage at the third receiver coil.
9. The inductive sensor according to claim 8 , wherein:
the first receiver coil, the second receiver coil and the third receiver coil are identically shaped, symmetric around a common center, and planar on a common plane; and
the first receiver coil, the second receiver coil and the third receiver coil are rotated at angles with respect to each other around the common center.
10. The inductive sensor according to claim 9 , wherein:
the angle-dependent inductive coupling between the rotor coil and the first receiver coil is −120 degrees out of phase with the angle-dependent inductive coupling between the rotor coil and the second receiver coil; and
the angle-dependent inductive coupling between the rotor coil and the first receiver coil is +120 degrees out of phase with the angle-dependent inductive coupling between the rotor coil and the third receiver coil.
11. The inductive sensor according to claim 8 , wherein the difference encoder includes:
a first comparator configured to generate a first comparison of the first coil voltage and the second coil voltage;
a second comparator configured to generate a second comparison of the second coil voltage and the third coil voltage; and
a third comparator configured to generate a third comparison of the third coil voltage to the first coil voltage.
12. The inductive sensor according to claim 11 , wherein the inductive sensor further includes a state machine configured to determine an angle state based on the first comparison, the second comparison, and the third comparison, the angle state corresponding to the angle measurement of the movable element.
13. The inductive sensor according to claim 12 , wherein the inductive sensor further includes a processor configured by software instructions to detect angle-state transitions of the state machine over time to adjust a turn-count.
14. The inductive sensor according to claim 12 , wherein the inductive sensor further includes a processor configured by software instructions to detect angle-state transitions of the state machine over time to estimate a speed of the movable element relative to a sampling rate of the angle measurement.
15. The inductive sensor according to claim 14 , wherein the processor is further configured by the software instructions to adjust the sampling rate of the angle measurement based on the speed of the movable element.
16. A method for measuring an angle, the method comprising:
moving a rotor coil of a transducer to the angle;
switching, using a non-resonant driver, an exciter coil of the transducer alternately between a positive terminal and a negative terminal of a voltage source during a measurement period to generate an excitation current having a non-sinusoidal amplitude;
generating a plurality of coil voltages at a plurality of receiver coils based on an angle-dependent inductive coupling between the rotor coil an each of the plurality of receiver coils; and
comparing, using a difference encoder, the plurality of coil voltages to measure the angle.
17. The method according to claim 16 , wherein the plurality of receiver coils includes a first receiver coil, a second receiver coil and a third receiver coil, and wherein comparing, using the difference encoder, the plurality of coil voltages to measure the angle includes:
generating, using a first comparator, a first comparison of a first coil voltage at the first receiver coil and a second coil voltage at the second receiver coil;
generating, using a second comparator, a second comparison of the second coil voltage at the second receiver coil and a third coil voltage at the third receiver coil; and
generating, using a third comparator, a third comparison of the third coil voltage at the third receiver coil and the first coil voltage at the first receiver coil.
18. The method according to claim 17 , further comprising:
determining an angle state of a state machine based on the first comparison, the second comparison, and the third comparison, the angle state corresponding to the angle.
19. The method according to claim 18 , further comprising:
taking repeated measurements of the angle at a sampling rate;
detecting angle-state transitions of the state machine based on the repeated measurements;
estimating a speed of the rotor coil relative to the sampling rate based on the angle-state transitions; and
adjusting the sampling rate based on the speed of the rotor coil.
20. The method according to claim 18 , further comprising:
taking repeated measurements of the angle at a sampling rate;
detecting angle-state transitions of the state machine based on the repeated measurements; and
adjusting a turn-count based on the angle-state transitions.
21. The method according to claim 16 , wherein the non-sinusoidal amplitude is triangular shaped and piecewise linear.
22. A system for measuring angular movement, comprising:
a controller;
a communication channel; and
an inductive sensor configured to capture angle measurements in a low-power mode using a battery power while not powered by a power supply, the inductive sensor in the low-power mode configured to:
switch an exciter coil alternately between a positive terminal and negative terminal of a voltage source to generate an excitation current in the exciter coil;
receive a plurality of coil voltages corresponding to a position of a rotor coil at a plurality of receiver coils; and
compare the plurality of coil voltages to determine a plurality of comparisons; and
determine an angle state of a state machine based on the plurality of comparisons, the angle state corresponding to an angle measurement of a movable element mechanically coupled to the rotor coil.
23. The system for measuring angular movement according to claim 22 , wherein the inductive sensor in the low-power mode is further configured to:
detect angle-state transitions of the state machine over time;
estimate a speed of the movable element relative to a sampling rate of the angle measurement based on the angle-state transitions over time; and
adjust a sampling rate of the angle measurement based on the speed of the movable element.
24. The system for measuring angular movement according to claim 22 , wherein the inductive sensor in the low-power mode is further configured to:
detect angle state transitions of the state machine over time; and
adjust a turn-count based on the angle state transitions.
25. The system for measuring angular movement according to claim 22 , wherein the excitation current has a non-sinusoidal amplitude.