What is Centrifuge Control?

The motion control challenge involved with high-speed centrifuge control begins with the unusual nature of the motor itself. To achieve their high rotation rate these motors typically have 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 centrifuges 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. High switching amplifier 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 centrifuge 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 centrifuge control applications where current (torque) control is needed for the application. This would apply for the large percentage of centrifuges that drive a load that may vary - either because the velocity changes during the spinning cycle or because the payload inertia changes from one use cycle to the next.

An alternate control method can be used for centrifuges which operates the motor in voltage mode 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 is only appropriate for systems where the load on the motor is relatively constant. Industrial 'flow-through' centrifuges which continuously process materials and spin at the same speed may have this characteristic.

Traditional centrifuge controllers that begin from a standstill, bring the payload up to speed, and then return to a stand still must provide a velocity 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.

Regardless of the control method used, safety techniques to avoid overheating the motor are important in the centrifuge control application. One important method is called i2t current foldback. This is a technique that measures how much current (and therefore heat) is being injected into the motor above and beyond what it can naturally radiate or conduct away. A more accurate approach is for the controller to input a thermistor input signal located on the motor. In both cases the controller shuts down or limits current output if the motor heats up excessively.