Apparatus for detecting the rotor position of an electric motor and related method
An apparatus for detecting a position of a rotor of a DC motor with N phases having a plurality of windings. The apparatus includes circuitry to couple at least two of the windings between a supply voltage and a reference voltage according to a first current path and allow the current stored in the two windings to be discharged through a second current path. The circuitry is configured to force the at least two windings at a short circuit condition in the second current path. The apparatus also includes a measurement circuit configured to measure the time period of discharging the current stored in the two windings and a rotor position detector for detecting the rotor position based on the measured time period.
1. An apparatus for an electric motor having a plurality of windings and a rotor associated therewith, the apparatus comprising:
a circuit configured to couple at least two of the plurality of windings between a supply voltage and a reference voltage according to a first current path and to discharge a current stored in the two windings according to a second current path, said circuit configured to force the at least two windings to a short circuit condition in the second current path;
a measurement circuit configured to measure a time period of discharge of the current stored in the two windings between a peak value of the current in the first current path and a portion of said peak value reached by the current in the second path; and
a rotor position detector configured to detect a rotor position based on the measured time period of discharge of the current in the second path.
2. The apparatus according to claim 1 , further comprising at least one sense element configured to detect the current circulating through the two windings in both the first current path and in the second current path.
3. The apparatus according to claim 2 , further comprising:
three half-bridges coupled between the supply voltage and the reference voltage, each one of the three half-bridges having a high side transistor and a low side transistor; and
said at least one sense element comprises an embedded sense element coupled to one of the low side transistors.
4. The apparatus according to claim 1 , further comprising:
three half-bridges coupled between the supply voltage and the reference voltage, each half-bridge having a high side transistor and a low side transistor; and
the electric motor having three phases and three windings, the three windings coupled to central points of said three half-bridges and configured to pass the current through two low side transistors of two half-bridges in said second current path.
5. The apparatus according to claim 1 , further comprising:
three half-bridges coupled between the supply voltage and the reference voltage;
each half-bridge having a high side transistor and a low side transistor and a recirculating diode associated to the low side transistor; and
the electric motor having three phases and three windings coupled to central points of said three half-bridges and configured to pass the current through one low side transistor of one half-bridge and one recirculating diode of a second half-bridge in said second current path.
6. The apparatus according to claim 1 , wherein the electric motor has three phases and three windings, and wherein the measurement circuit is configured to measure the time period of discharge of the current stored in the two windings for each pair of phases of the electric motor.
7. The apparatus according to claim 1 , wherein said portion is 50% of said peak value of the current in the second current path.
8. The apparatus according to claim 1 , wherein the circuit is configured to stop the electric motor to detect the rotor position.
9. A method for detecting a rotor position of an electric motor having a plurality of windings, the method comprising:
producing a current through two of the plurality of windings by coupling the two windings between a supply voltage and a reference voltage according to a first current path;
discharging the current stored in the two windings through a second current path by forcing the two windings to a short circuit condition;
measuring a time period of discharging the current stored in the two windings between a peak value of the current in the first current path and a portion of the peak value reached by the current in the second path; and
detecting the rotor position based on the measured time period of discharging the current in the second path.
10. The method according to claim 9 , wherein the portion is 50% of the peak value reached by the current.
11. The method according to claim 9 , further comprising stopping the electric motor to detect the rotor position.