Introduction to Industrial Automation

Figure 6.28  Closed-loop control of a stepper motor with a specific I/O module.

---

Basic Operating Principles of PLCs 

◾

 

225

as a subroutine. The user defines the desired parameters and the analog inputs/outputs for which 

this routine will be executed, and the latest is inserted in the main PLC program. Since the control 

algorithm includes mathematical operations, the scanning time of the PLC can be significantly 

increased, and thus it might cause improper control of the overall process. For these cases, PLC 

manufacturers provide independent special PID control modules, which contain their dedicated 

microprocessor and thus do not affect the overall scanning time of the PLC. These modules con-

tain their own analog inputs and outputs that are directly connected to the controlled process. 

From the central processing unit of the PLC, only the initial values, the desired set points, etc., 

are transmitted to the PID control module. Subsequently, the microprocessor of the control mod-

ule operates in parallel to the PLC to execute the desired control task. When the PLC needs the 

variables of some values which exist in the memory of the control module in order to perform the 

proper control of the overall process, these are transmitted from the I/O memory of the control 

module once during the PLC scan cycle. More information on the continuous analog control of 

three terms (PID) is provided in Chapter 9.