Page 245 - Programmable Logic Controllers, Fifth Edition - Mobile version

BCD conversion value
Data
Analog output module bus

0
0 1
1
Variable voltage 2 1
or current input 3 0
Thermocouple
1
Analog A/D 4 0
input c 5 1
L1 device o 6 0
n 7
v
e 1
L2 r t 9 8 1
e 10 1
r 11 0
1
12 0
13 0
14
15 1

Figure 10-41 Analog input interface module.

Analog
output
Word Digital Voltage
data to Isolation Amplifier and Servo Load
from analog current motor
CPU converter driver

Figure 10-42 Analog output interface module.

device. Figure 10-42 illustrates how an analog output in- 10.6 Closed-Loop Control
terface operates. The operation of this output module can
be summarized as follows: In open-loop control, no feedback loop is employed
and system variations which cause the output to deviate
• The function of the analog output module is to ac- from the desired value are not detected or corrected. A
cept a range of numeric values output from the PLC closed-loop system utilizes feedback to measure the ac-
program and to produce a varying current or voltage tual system operating parameter being controlled such as
signal required to control a connected analog output temperature, pressure, flow, level, or speed. This feedback
device. signal is sent back to the PLC where it is compared with
• Data from a specific register or word location in the the desired system set-point. The controller develops an
CPU memory are passed through the controller’s error signal that initiates corrective action and drives the
data bus to the digital-to-analog (D/A) converter. final output device to the desired value.
• The analog output from the D/A converter is then PLC set-point control in its simplest form compares
used to control the analog output device. an input value, such as analog or thumbwheel inputs, to
• The level of the analog signal output is based on the a set-point value. A discrete output signal is provided
digital value of the data word supplied by the CPU if the input value is less than, equal to, or greater than
and manipulated by the control program. the set-point value. The temperature control program
• These output interfaces normally require an external of Figure 10-43 is one example of set-point control.
power supply that meets certain current and voltage In this application, a PLC is to provide for simple off/
requirements. on control of the electric heating elements of an oven.

226 Chapter 10 Data Manipulation Instructions

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