Sinusoidal Control
- Avani Joshi (Deactivated)
The basic premise of the sinusoidal drive is to provide each motor winding with currents that vary sinusoidally, based on the rotor position. These currents are phase-shifted by 120 degrees from each other, relative to its corresponding Hall sensor. In BLDC motor control, the drive signals that are used require variable voltages that change with respect to the speed and position of the motor. This variable voltage is provided using the PWM technique. By providing sinusoid-based signals through the PWM modules to the MOSFET driver, the current is generated on each of the motor windings. Due to the gradual changing of the applied voltage, the sinusoidal drive’s torque ripple is somehow lower in comparison with the trapezoidal drive.
Pure sinusoidal drive voltages are rarely used in practice because they are inefficient for each motor terminal with respect to ground. This is done by varying the PWM duty cycle relative to ground using a “saddle” profile, rather than sinusoidal. The resulting phase current driving the motor then follows the pure sine wave variation of the phase-to-phase voltage.
The result of using this approach is a higher torque and speed for a given voltage and improved efficiency.
The saddle profile technique offers two advantages - first, the max differential voltage generated is higher than can be produced with a pure sinusoidal signal and speed for a given input. Second, each terminal output is zero for 1/3rd of the time, further reducing switching losses in the power stage.
A drawback of this method is that the interpolation of motor speed between Hall sensors is likely to be inaccurate during the startup phase when the motor is accelerating quickly - this can cause a jerky torque response.
Another design challenge is the phase lag between the drive voltage and the resultant sine-wave current for a given phase that naturally occurs in an uncompensated BLDC motor. The motor will operate satisfactorily, but with a reduced efficiency, which would defeat much of the purpose of implementing a sinusoidal control scheme in the first place. The source of this inefficiency is not the phase lag between drive voltage and phase current, rather the phase lag between phase current and sinusoidal back EMF. Fortunately, many drive chips allow the designer to introduce a phase-angle advance to the sinusoidal drive current to ensure its peak coincides with that of the back EMF. The phase-angle advance is typically set to increase linearly with the input voltage, which determines the motor speed.
