Stepper and Servo Motor Controllers

Stepper motor and servo motor controllers are devices that provide or facilitate accurate control of stepper motors and servo motors, respectively.

Stepper Motor Controllers

A stepper motor is a motor that converts digital pulses into mechanical shaft rotation. These pulses control the rotation of the shaft in small angular 'steps', hence the name 'stepper motor'. Stepper motors come in a wide range of angular resolution, i.e., from 90 degrees per step to as low as 0.72 degrees per step. The speed and torque of a stepper motor are determined by the amount of current through its windings. Stepper motors are simple, low-cost, yet highly reliable motors that can operate in almost any environment.

A stepper motor rotates in discrete 'steps' because its movement is achieved by aligning certain 'teeth' of the rotor with certain poles of the stator (depending on which windings are energized and which are not) at any given time. As such, there are only specific equilibrium points at which the rotor can 'rest.' Every time a new set of pulses is delivered, the rotor rotates to the next 'equilibrium point', and its angular position with respect to the stator is locked in place until a new set of pulses arrives.

There are three stepping modes in which a stepper motor can be operated, namely, full stepping, half stepping, and micro-stepping. Every step taken under full stepping mode results in a rotation equal to 100% of the angular displacement specified for a single step. Half-stepping results in only 50% of this, so it would take twice as many steps under the half-stepping mode as what it would take under the full-stepping mode to cover the same angular displacement. Micro-stepping further reduces the angular displacement equivalent to a single step of the motor.

A stepper motor controller must be able to handle the generation and conditioning of pulses needed to operate the stepper motor easily even in complex applications. To accomplish this, a typical stepper motor controller IC consists of three basic elements: 1) an indexer, which generates low level signals that correspond to step pulses and direction signals (collectively referred to as 'indexer commands') needed to control the stepper motor; 2) a motor driver circuit, which translate the indexer commands into power that energizes the appropriate windings of the stepper motor; and 3) an interface that would allow it to be controlled more conveniently by a computer, PLC, or microcontroller.

Characteristics or features that designers take into consideration when choosing a stepper motor controller IC include the following: 1) the ability to put a stepper motor in continuous 'run' mode at various speed profiles, or in 'step' mode with precision control; 2) directional control; 3) available stepping modes for resolution control; 4) programmability; 5) specialized I/O controls; 6) feedback mechanisms about the state of the stepper motor; 7) effective and efficient energization of the stepper motor; 8) industry-standard user interfaces; and 9) ease of use.

Servo Motor Controllers

A servo motor is a motor whose angular displacement at any one time is determined by a coded signal, which is usually the width of the pulse applied to its control terminal. It is operated in a closed loop, i.e., it requires some form of analog feedback (usually provided by a potentiometer) to let it know the current rotor position. Thus, the repeatability of a servo motor's positioning depends greatly on the stability of the potentiometer and other components used in the feedback circuit. Since a stepper motor operates without feedback, a servo motor is a better choice than a stepper motor if monitoring of the rotor position at any given time is important.

Since a servo motor operates on the widths of the control pulses it receives, a servo motor controller must be capable of pulse width modulation (PWM). The servo motor expects to see a pulse regularly, say, every 20 milliseconds. The pulse width influences the amount of power delivered to the motor and, therefore, its angular displacement as well, i.e., the longer the pulse, the larger the rotation will be.

Examples of features offered by servo motor controller IC's in the market include : 1) ability to support multiple motors; 2) velocity and trapezoidal profiling; 3) directional control; 4) programmability; 5) specialized I/O controls; 6) stable feedback mechanisms; 7) overcurrent and power failure protection; 8) industry-standard user interfaces; and 9) ease of use.

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