9 Motor Position Sensing

A motor rotor position sensor measures the angular position of the rotor shaft. Motor position sensors are very important for speed feedback loop control meeting safety requirements in EV applications. For position control, the sensors enable a known (safe) position, motor speed and the positions throughout motion, and also provides feedback to the torque control loop.

A variable reluctance resolver sensor implements the principle of a rotating transformer. The transformer has a single primary winding and two secondary windings positioned at a right angle from each other. Applying an excitation voltage (V_EXC) to the primary winding (generated through an excitation amplifier such as TI’s ALM2403-Q1 or TAS5431-Q1) results in current that generates the magnetic flux (Φ). The flux distributes through secondary windings with respect to the rotor angle (ϴ) and induces VSIN and VCOS accordingly. The feedback signals are converted from differential signals to the single-ended output for the ADC. A safety MCU calculates ϴ from the voltage ratio on the resolver secondary windings.

The inductive position sensors implement magnet-free technology that can be used for high-speed motor position sensing. The sensors use the principle of eddy currents to detect the position of a metallic target that is moving above a set of coils. The position sensor interface IC converts the input signals from the RX coils into differential sine and cosine output signals, processed from the MCU.