9 Motor Position Sensing

A motor rotor position sensor measures the angular position of the rotor shaft. Motor position sensors are vital for a closed-control loop control meeting safety requirements. For position control, the sensors enable a known position and the positions throughout motion. In general, two options can be explored, a resolver structure or inductive sensing, which simplifies the system design.

The resolver approach implements the principle of a rotating transformer, see Figure 9-1. The transformer has a single primary winding and two secondary windings positioned at a right angle. Applying an excitation voltage (V_EXC) to the primary winding using a PWM signal from the MCU, generated through an excitation amplifier such as TI’s ALM2403-Q1 device, results in a current that generates magnetic flux. The flux distributes through the secondary windings with respect to the rotor rotation angle and induces V_SIN and V_COS. The feedback signals are mostly single-ended, but can also be converted from differential signals to a single-ended output. A safety MCU recaptures the rotor angle from the voltage ratio on the resolver secondary windings.

Figure 9-1 Resolver Structure Featuring Excitation Coil L_EXC and Secondary Coils L_SEC1/2 to Retrieve Motor Rotation Angle

For inductive position sensing, TI’s LCD5072-Q1 inductive position sensor front end (see Figure 9-2) is paired with an excitation coil (black) and sensing coils (colored) to cover a conductive target fixed to the rotor. An LC oscillator is integrated in the LDC5072-Q1 device for generating an excitation current from 2.4MHz to 5MHz into the excitation coil. This elevates the simplicity of the system by eliminating the need for the external excitation structure. The target (made from Al or Cu) and PCB coils are placed parallel to each other, and there is an eddy current in the target excited by the excitation current, which also generates an induced sine and cosine envelope voltage into the sensing coils. The amplitude of the induced voltages is determined by the motor angle position. Then, the sensing signals are demodulated by the LDC5072-Q1 device and the outputs of the sine and cosine signals to the MCU for angle calculation.

Figure 9-2 Inductive Sensing Diagram Using LCD5072-Q1

The LCD5072-Q1 device supports very high rotation speeds with an angular accuracy < 1° at target speeds up to 480,000rpm. The device is designed in accordance to ISO26262 for functional safety requirements. The TLC5072Q1EVM evaluation module helps evaluate the operation and performance of the sensor front end in a quick and easy setup.