Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 329

development of the two software components mentioned above. The practical
elements of the implementation are given in Chap. “ Experimental case study ” , for

the real application considered as case study.
The PLC Application is structured tasks which, in turn, contain program mod-
ules and routines. In designing the PLC Application one must consider a number of
elements, which leads to a general structure of subfolders, such as:

Controller Application contains tags (variables) visible at the PLC level

(Controller Tags), the logic of handling major errors from PLC (Controller Fault
Handler), and control logic for powering the PLC (Power-Up Handler);

—

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Tasks include tasks forming the process control logic. Each task consists of

programs which in turn contain variables (tags) and routines;

—

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Motion Groups — contains instructions for coordinating movements on multiple

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axes;

Add-On Instructions — contains instructions written by the user;

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Data Types — contains prede ned and user de ned data types;

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￿ Trends — contains charts for online monitoring of variables;

—

￿ I/O Con guration contains information about the project hardware con gu-

-

output modules for the
acquisition of process variables and sending of commands to process.
The PLC Application uses a number of variables for acquisition and processing
of input

ration. PLC hardware structure contains a set of input

output process variables, process parameterization, and implementation of
logical control sequences, and to ensure the communication with higher levels of
supervision of the installation. These variables can be of two types:

-

Global — declared at the higher level of the application (Controller Tags);

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Local — declared within each program module of the application tasks (Program

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Tags).

Local variables are visible only to that program module, while global variables
are visible inside any task, program or routine. Also, only global variables may be
transferred outside of PLC, for example, to the Human

- Machine Interface or to the

higher control level.
Human Machine Interface should provide a schematic representation of the

plant (block diagram) and should offer for users clear options regarding the control
and monitoring of the system elements. HMI runs on the engineering station and
can have the following functions:

-

allows real-time visualization of the main variables from the plant;

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￿ allows selecting the operating mode of the plant;

￿ allows the operator to apply direct commands to the plant;

￿ allows graphical display of the evolution of certain variables from the plant;

allows to store the values of the key variables for possible further processing.
The HMI application is usually done using the development tools provided by
the hardware manufacturer, but there are other general software tools for developing
HMIs (e.g., IntegraXor, IGSS).

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