The control system basically breaks down into two primary systems – the PLC and the motion hardware, when using a PLC in non-interpolated motion applications. Usually, motion hardware are consists of drives, cables and motors. The type of drive necessary for the application is determined by the type of motor, stepper or servo.
Stepper drives and motors are providing the lowest cost motion solution. The stepper drive sends signals to the motor to move a certain number of pulses in stepper applications. Stepper based systems are open loop motion systems, for instance stepper motors rarely provide feedback to the control system. If the motor can not respond correctly to pulses from the drive, the position of the stepper motor relative to what the controller commanded is lost.
In the servo systems, the servo motor usually has a built-encoder. Because the encoder is wired back to the drive, a servo system is considered to be a closed loop motion system. If the motor doesn’t move the distance requested by the PLC, the drive would generate a fault condition, which in turn can be communicated back to the PLC. The PLC can be programmed to clear the fault and start a sequence to a known position, which is typically a limit or proximity switch for the faulted axis.