Motion Control is defined here as the control of an electromagnetic motor and the associated systems which are required to make it work in a system when positional control is required. There is many different types of electric motors to be controlled. Here we focus on those that are of most interest in the home and light industrial environments and the issues that result from this.
- ● Stepper motors - low power applications
- ● BLDC motors - fractional horsepower
- ● PMSM motors - fractional horsepower
Today, the focus is on BLDC and PMSM electric motors because they offer lower lifecycle cost and more efficient operation. This has changed in the last few years as rare earth magnets actually reduced the size of these motors to make them smaller and therefore less costly than competitive motors.
Motion Control is different from motor control in that positional control is required for motion control which leads to several differences in the approaches taken. These differences are:
- ● A profile is used to provide gradual changes in positional commands (and hence velocity, acceleration and jerk) to minimize equipment wear and tear.
- ● Three nested PID or PDFF loops which are separately tuned are used.
- ● The inner loop does not control velocity but torque or current instead to provide better performance.
- ● There is a velocity and a position loop in addition to the torque loop.
- ● Sensorless operation is not possible - all motion control axis require a position feedback sensor of some kind.
Motion Control for these motors is unique because commutation of the motor is electronic and the drive circuits must be electronically controlled to produce a constantly dithered approximation to the required position. This alignment is always at 90 degrees between the field and the rotor to maximize torque at all times regardless of the the loading of the motor.
Motion Control does not pay particular attention to energy efficiency. The reason for this is that the dithering required to maintain position creates a natural energy inefficiency. Future designs may offer improvements in this area.
DSPnano and UNISON are both capable of supporting motion control server software including limit switches and even mems based accelerometers. Please consult the factory to discuss current options for six step (BLDC) and vector (PMSM) control versions.
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