In the case of PLCs, PID control is implemented in two ways: In the first case, the PID algorithm
is integrated in the programming software of the PLC (software controller) and is called as a sub-
routine, which means as an FB that has been developed by the manufacturer of the PLC. The PID
algorithm exists in the relevant library, so that it only requires the declaration of variables and the
parameters of the controlled process. The values of these parameters, which are necessary for the
operation of the PID FB, are stored in a corresponding DB. In some PLCs, the PID algorithm
is integrated in the CPU, and the programming environment provides the user with a menu in
order to communicate with the controller and to set the required parameters. The physical magni-
tude that controls a PID FB is taken as a variable from one of the analog inputs of the PLC, and
subsequently, the PID FB regulates the value of an analog output of the PLC. Depending on the
capabilities of the PID algorithm developed as software from the manufacturer of the PLC, the
user can select any kind of a control configuration from the P, PI, PD, and PID controllers that
is suitable for the controlled process while, depending on the form of the selected PID controller,
the necessary gains need to be tuned. For example, in the case of a PI controller, the D gain is
automatically zeroed and the engineer needs to define the P and I gains only.
Concerning the declaration of parameters for a PID FB controller, the reader, who will encoun-
ter such an application in practice, should be prepared for a much larger number of parameters
PID Control in the Industry
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377
than the three described in classical PID control theory. For example, Figure 9.2 shows the graphic
symbol of the PID FB’s call in FBD language of Siemens Step7 software, with more than 20
parameters that may be declared in accordance with the desired function of the PID controller.
After programming a PID FB, it will run along with the rest of the automation program
within the time horizon of the scan cycle. Depending on the controlled process, this may result in
an applied PID control that is not satisfactory because of the duration of the scan cycle. The latter
has, of course, always a short duration that is satisfactory for the execution of the ON-OFF con-
trol, but which may not be sufficient for the action of the PID control, especially for fast processes.
In this case, a second method of implementing a PID controller is followed.
This second approach consists of supplying a separate PID control module that was mentioned in
Section 6.7, which has its own autonomous microprocessor and its own independent analog inputs
and outputs. In this way, the PID control module carries out the function of control, regardless of
the scan cycle of the CPU of the PLC, but communicates with the PLC to exchange the values of the
parameters and variables. In addition, the independent module of the PID control has much more
features than the built-in FB to the software algorithm. Furthermore, this module has the ability to
implement more than one PID control loop, depending on the number of pairs of analog inputs and
outputs. Figure 9.3 shows the block diagram of four PID control loops that can implement a PID
control module with four analog inputs and outputs. The bars between the controllers and the analog
inputs and outputs represent the ability of each PID controller to be combined with any (one or more)
from the analog inputs and outputs. The four PID controllers can operate independently of each other
or combined in different ways, as shown in Figures 9.4 and 9.5. The combined or individual operation
of controllers is independent of the order of execution of operations on each controller. This is specified
by the manufacturer, and is sequential from controller 1 to controller 4. This means that the conver-
sion of analogue input 1 and the PID controller algorithm 1 will be performed first, and then the
“TCONT-CP”
EN
PV-IN
PV-PER
DISV
INT-HPOS
INT-HNEG
PV
Select
LMN
Cycle
LMN-PER
Cycle-P
QPulse
SP-INT
QLMN-HLM
Man
QLMN-LLM
COM-RST
QC-ACT
Man-ON
ENO
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