What is Spindle Control?

The motion control challenge involved with high speed spindle control begins with the unusual nature of typical spindle motors themselves. To achieve their high rotation rate these motors typically have very low coil resistances and electrical time constants. So when driven by a PWM (Pulse Width Modulation) switching amplifier a central consideration is that the bridge circuitry operate efficiently at high PWM rates.

Typical PWM rates for high speed spindles are 80 kHz or higher. If driven with a lower PWM rate the current ripple due to the switching waveforms will generate excess heat in the motor. Switching amplifier high PWM rates are best-accommodated by SiC (Silicon Carbide) MOSFETs due to their very low switching losses. Nevertheless Silicon MOSFETs remain popular and have improved in performance over time.

A second important consideration in high speed spindle control with Brushless DC motors is use of FOC (Field Oriented Control). FOC improves the available torque output for a given motor at high speed and reduces heat generation in the motor. FOC is recommended for all spindle control applications where current (torque) control is needed for the application. This would apply for spindles that drive a high inertia load such as centrifuges or cryo-pumps, or where the load may create variable levels of resistive torque such as routers and drills as they cut into the target material.

An alternate control method can be used for spindles which operates the motor in voltage made and uses a scheme called third-leg floating to drive the motor coils. Third leg floating helps minimize the speed-reducing impact of back-EMF and has the advantage of being simple to implement. This scheme does not provide current/torque control and so typically relies on the operator to avoid stalling the motor resulting in an over current situation. A common application is dental and surgical drills/cutters where the doctor/practitioner listens to the motor speed and applies more or less pressure to moderate the applied torque.

Most spindle controllers also provide a velocity control and profiling function. While traditional position PID servo loops can be used for velocity control (if you control position with time you also control velocity with time) use of a dedicated velocity PI servo loop may have benefits such as higher velocity tracking accuracy.