Page 113 - Programmable Logic Controllers, Fifth Edition

(a) Digital signal (b) Analog signal Analog output
Valve
Level sensor

Analog ADC Binary PLC
Analog I/O
signal output Module
input 10 01 1 Processor
Level
Preset
(c) Analog-to-digital converter (ADC)
Point
Figure 5-53 Digital and analog signals.
Analog input

Figure 5-54 Analog control for a tank-filling process.
may produce significant errors as the signal is processed.
Analog signals must be coded into digital signals before
they can be processed by the PLC. An analog-to-digital • The valve is initially open 100%.
converter (ADC) converts analog input signals to digi-
tal signals. A digital-to-analog converter (DAC) converts • As the fluid level in the tank approaches the level
digital output signals to analog signals. preset point, the processor modifies the output to
Analog applications are present in many forms. degrade, closing the valve to 90%, 80%, etc., adjust-
Figure 5-54 shows a typical use of analog control for a ing the valve to maintain a set point.
tank-filling process. The operation of the circuit can be
summarized as follows:
• The processor controls the amount of fluid placed
in a holding tank by adjusting the percentage of the
valve opening.

94 Chapter 5 Basics of PLC Programming

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