Successful Instrumentation and Control Systems Design, Second Edition by Michael D. Whitt

Author:Michael D. Whitt
Language: eng
Format: epub
Publisher: International Society of Automation

FIGURE 2-55. SEQUENTIAL FUNCTION CHART WASHING MACHINE SEQUENCE CONTROL APPLICATION

6.Continuous Function Chart (CFC)

A CFC is a program fragment that runs continuously. Most analog logic fits well as a CFC. PID (proportional-integral-derivative) control functions are examples of CFCs. APT is a program by Siemens that combines the SFC with a continuous function chart. This is a powerful combination since the CFC is executed continuously while the SFC is sequenced. A PID control block that controls water temperature, for example, would be executed within a CFC, while the washing machine sequence is handled within an SFC.

Figure 2-56 presents an example of a CFC. In this example, TT-14 is the water temperature signal. It is sampled by an analog input function block that converts the input signal to a digital integer value and then scales it to engineering units in degrees Fahrenheit. This value is fed to a PID function block and to two comparators. The PID block compares the actual temperature process variable (PV) to the external set point (SP) value. The PID block generates an output, which is fed to the heating element TY-14 through an analog output block. The two comparators compare the actual temperature to a high or low set point, and, if the relationship is true, set the proper analog alarm, which alerts the operator that the water temperature is out of bounds.

g.Recommended Program Structure

There are many ways to approach a PLC programming task. One approach that works well is to program by final control device. This method treats each final control device as a separate entity. Each pump, control valve, alarm horn, on/off valve, and so on, will have a distinct program structure that is reused as many times as there are similar devices. Interlocks, start/stop commands, and other variables are then configured to make each program block unique and specific to its associated final control device.

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