Introduction to Industrial Automation

6.7.2   Stepper Motor Control Modules

Stepper motors are utilized in applications of micropositioning of objects, mainly due to their 

accuracy during their rotation that, in most cases, is altered to a corresponding linear movement. 

The stepper motors can rotate in very small steps (e.g., 1

o

 or even smaller) and with high accu-

racy in both directions of rotation. The power supply and driving unit of these stepper motors 

requires the application of pulses as control signals. For each input pulse, the motor produces an 

equivalent amount of rotation, the so-called “step”, as described in Section 2.1.1. The frequency 

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  Introduction to Industrial Automation

of this pulse train defines the speed of motor rotation. A second pulse input is utilized to select 

the direction of rotation, while a third one defines the full or half step rotation. Such control 

signals are not usually produced by most PLCs, and thus it is necessary to utilize specific stepper 

motor control modules. Based on the program’s logic at the PLC, when the stepper motor should 

be moved, a small block of data related to the magnitude, the speed, and the direction of the 

rotation is transmitted to the control module of the stepper motor that handles the generation of 

proper signals for driving the motor. The overall control system is of an open-loop architecture, 

which means that the PLC has no information (feedback) of the real movement that the stepper 

motor has executed.

The complex structures of micropositioning with 3–6° axis of movement (one stepper motor 

for each motion axis) are usually equipped with sensors of position measurements (e.g., encoders) 

that transmit information concerning the real position of the motor, which allows for the further 

application of control signals for tuning the performed movement to be equal to the desired one 

(closed-loop control architecture). In Figure 6.28, a closed-loop control system, where the stepper 

motor control module is connected to the main processing unit of the PLC and to the controlled 

process, is indicated. The stepper motor control module receives commands from the main pro-

cessing unit of the PLC that are related to the rotation of the stepper motor, and transmits posi-

tioning information back to the PLC when it is asked. The stepper control module receives the real 

position of the controlled moving object from a position encoder, compares it to the desired posi-

tion (in the form of a pulse train) that has been transmitted to the motor controller, and creates a 

corresponding positioning error. Based on this error and the applied controller, a corresponding 

corrective action is produced to control the movement. Depending on the number of motors that 

one module can control, different multiple combinations of movements can be generated that can 

be stored in the stepper’s control module, either in the form of a movement’s library or as alterna-

tive motion programs. The central processing unit of the PLC can activate a program of move-

ments by transmitting a specific control signal to the stepper motor control module.