Electricfil Automotive rotor position sensor for EVs is based on eddy-current technology

December 02, 2011 // By Christoph Hammerschmidt
Automotive electronics manufacturer Electricfil has developed the EMPOS rotor position sensor for electric vehicles that offers several unique features: Its operating principle, based on edy current technology, makes cheaper than comparable solutions without compromising its exactness. And it is extremely immune to low-frequency magnetic fields.

Permanent magnet synchronous motors (PMSMs) offer high efficiencies and high power densities for electric and hybrid vehicles. However, to operate efficiently and provide smooth continuous torque at all speeds, the alternating current used to generate the magnetic fields in the stator windings must always be “synchronous” with the rotor. For this reason, the motor's ECU needs to know the angular position of the rotor.

In industrial motors, incremental encoders or electromagnetic resolvers are used to measure the rotor position. However, in the EV world, constraints like robustness, manufacturing costs, integration and functional reliability are not easy to meet with traditional sensor technologies.

With this in mind, Electricfil Automotive have developed a completely new sensor called EMPOS (Electric Motor POsition Sensor), based on eddy-current technology. A primary winding and two secondary windings are printed on the sensor PCB attached to the stator, replacing the bulky and more expensive wound coils of resolvers. An aluminum trigger wheel is fixed on the rotor. A high frequency AC current is injected into the primary winding, generating an alternating magnetic field that causes eddy currents to flow in the teeth of the trigger wheel. The eddy currents in return generate a magnetic field that is proportional to the AC voltages measured in the secondary windings. The ratio of the two secondary voltages determines the target position.

An application specific integrated circuit (ASIC) generates the excitation signal and handles the whole signal treatment chain, including analog and digital processing. The ASIC also provides diagnostic functions to avoid false position information in case of any single failure.

The ASIC is soldered directly on the sensor PCB, ensuring short signal paths for high frequency analog signals. The position signal and a diagnostic signal are transmitted to the motor controller via a digital serial interface ensuring no loss of accuracy due to analog-digital conversion accuracy, noise and EMI.

In this way, EMPOS offers +/- 1° accuracy (electric), largely sufficient