Driving a brushless motor requires control electronics for precise commutation. However, this is possible only if the control electronics “know” the exact position of the rotor at all times. Traditionally, this information was provided by sensors, e.g. Hall sensors, installed inside the motor. But it can be done differently. Sensorless control methods use current and voltage information from the motor to determine the rotor position.
The motor speed can then be derived from changes in the rotor position, and this information can be used for speed control. More advanced sensorless control methods can even control the current (torque) and the position. Leaving out the sensors has a range of benefits, such a lower cost and space savings, because cables, connectors, and sensitive electronic circuits become unnecessary.The EMF method with determination of the zero crossing uses induced voltage (or EMF) in the non-powered phase during block commutation. The zero crossing happens in the middle of the commutation interval (fig. 1). The time delay to the next commutation point can be estimated from the preceding commutation steps.The EMF method with zero crossing works only when the speed is high enough, because EMF becomes zero at standstill.
Starting up the motor requires a special process, similar to step motor control, and must be configured separately. True sensorless commutation is possible only with motor speeds of 500–1000 rpm and up. The commutation step frequency is used for speed control.The limited feedback information puts some constraints on the motor dynamics, although this can be improved by integrating estimation methods into the control algorithm (observer, Kalman filter.
The EMF method with zero crossing also has a range of benefits: It works for all brushless motor models, and it’s robust and cost-effective. The approach is used in many standard products, such as the maxon ESCON Module 50/4 EC-S. Observer or model-based EMF methods use information about the motor current to determine rotor position and speed. The model-based approach yields a much higher resolution of the rotor position. This enables sinusoidal commutation (or FOC, field-oriented control), with all its benefits: Higher efficiency, lower heat generation, less vibration and noise. However, the observer-based EMF method also requires a minimum speed of several 100 rpm to function properly.