Functionally, servomotors and steppers overlap. Both can rotate their shaft a portion of a turn and hold that position for any length of time as determined by an external controller. A step motor is one of several specific positioning motor constructions whereas servocontrol refers to a type of control that involves feeding back information about the position of the motor shaft and using it in the control of the motor operation. Several kinds of motors can be operated as servos, including the simplest brushed dc motor.
Three-phase, high-powered induction motors can, in a manner of speaking, operate as servos with certain variable-frequency drives (VFD) that incorporate encoder feedback; some newer VFDs will move an induction motor to a position and hold it, so they technically could be called servo controls. However, induction motors use a squirrel-cage type construction where both stator and rotor are of a wound type to create the magnetic flux. These motors create less magnetic flux density than permanent-magnet brush or brushless motors. Consequently, permanent magnet motors are preferred for servo use because they can accelerate or decelerate much more responsively than squirrel cage-type motors.
Despite being inexpensive and easy to implement, the stepper is not universally used in servo applications because if may fall out of synch if overloaded. That is because the stepper is usually operated open loop, with no feedback of motor shaft position that can be used to compensate for errors between the commanded position and real motor shaft location.
In the case of sync loss, the step motor must be backed up and there must be a reset of angular alignment in the controller, motor and load. The servo system, in contrast, with an optical, Hall effect or other sensor conveying the position of the load, provides continuous load-position feedback. If the motor shaft position lags the commanded position, error correction instantly ensures correction of this position error.
In the typical permanent-magnet servomotor, an external controller feeds electrical pulses to the field winding. Speed is controlled not by frequency, but by the pulse width or duty cycle.
The controller may communicate with the motor using various algorithms. The most widely used is proportional-integral-derivative (PID). It is a generic protocol, also widely used in non-electrical applications. Like many successful innovations, it surfaced in the final decade of the nineteenth century. In an effort to build an automatic steering device for maritime use, engineers observed and recorded the actions of expert helmsmen during stormy conditions at sea. Steering patterns were codified as the PID algorithm.
Proportional refers to present error (as in, if you see more error, apply proportionally more correction). Integral is a record of past errors. Derivative looks into the future, where error is seen as a function of the rate of change. Assigning weighted values applicable to the contemplated usage, the controller synthesizes a signal that is continuously modified so as to shape the electrical current conveyed to the motor. It is what a child does riding a bike.
In servomotor control, pulse-width modulation (PWM) is a great power-saving algorithm. The usual electrical cabling scheme is three wires connecting the controller to the motor. Color coding varies, but a common arrangement is red for power, black for ground and white for the pulse-width modulated square-wave control signal.
The width of the pulse width, not the amplitude or frequency, controls the motor speed. Direction of rotation and relocation to the center position are also controlled by the pulse. This information is conveyed in the control signal, not the power feed. Solid-state circuitry at the motor is necessary to interpret the PWM signal.
Filed Under: Test & Measurement Tips