Hall-based commutation

There are three common techniques used to control brushless motors, Hall-based (also known as 6-step commutation), sinusoidal commutation, and field oriented control. Of these, Hallbased is the simplest to implement, and requires the use of 3 Hall-effect or optical position sensors. As they are most commonly used, these three binary sensors define six useable rotation states (000 and 111 are excluded states) and a simple table converts the input Hall state to the output motor drive signal for each winding.

Excitation schemes

But Hall-based commutation is far from ideal. As the rotor rotates, the electrical phase angle of the stator should be continuously adjusted to keep the Q at maximum. Halls break an overall electrical cycle into six 60 deg sections. And so in the worst case, the generated torque vector will be off by 30 degrees positive or negative with respect to the ideal angle. This has two undesirable consequences. The first is that energy is wasted. The second is that the motor torque will not be constant with rotor position.

Since angle error is up to +/- 30 deg for a 6-step Hall system, using Hall sensors there will be a continuous torque variation from +86.6% (at -30 degrees) to 100% (at 0 degrees) and back to +86.6% (at +30 degrees), and this cycle will be repeated every 60 degrees, or six times per electrical rotation. This is shown in Figure 2.

At low speed this torque ripple is generally not a problem, although if a constant torque on the load is required by the application, then Hall-based commutation is not a good option. For applications running at higher speed, the torque ripple may or may not be a problem. The torque ripple will be injected as cyclic energy into the load mechanic at the electrical rotation frequency, where it may be damped or amplified by the natural resonances in the load. This unwanted vibration may exhibit itself as excessive noise, positioning inaccuracy, or servo stability problems.