3 BEMF Measurement and Interpretation

Multiple peripherals like the AD converter (ADC) or the Analog Comparator (AC) can be used to sample the BEMF and detect Zero Crosses. In this method, the Analog Comparator samples the BEMF. All the PWM driving signals must synchronously be logically low, the coils are not driven and the signal measured after all the propagation delays is the motor’s BEMF.

The Analog Comparator is connected to the divided motor's phase signals on the positive input and the motor summing neutral to the negative input, as depicted in Figure 4-1. The neutral can be either reconstructed in software, or in hardware using resistors in a star configuration. The comparator output acquisition is timed to coincide with the dead time period and possible delays. The comparator output provides the BEMF position equivalent to the current position through the coils. Using this signal, it is possible to find the Zero Cross Detection and estimate the rotor's position. Figure 3-1 shows the Analog Comparator Output, and Figure 3-2 an overview of a Zero Cross point.