Rotor position sensor for EVs based on eddy-current technology
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 for EV and HEV applications, at speeds up to 20,000 rpm. It is insensitive to pollution, EMI, vibrations and positioning errors. It can measure the true absolute position at power-on and is available for through-shaft or end-of-shaft designs.
Electricfil Automotive has packed the circuit board and electronic components in a water- and oil-tight housing. The sensor is thin (<10 mm) and lightweight, containing no ferromagnetic parts.
The shape of the aluminum sensor wheel has been kept simple so that the wheel can be manufactured as a punched part, keeping costs down. The inside diameter of the sensor can be easily tailored to customer requirements.
Prototypes are now available from Electricfil Automotive for testing on customer facilities. Mass production is scheduled for 2012.
For further information: https://www.electricfil.com.
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