Page 128 - Programmable Logic Controllers, Fifth Edition

Through-beam
From transmitter To receiver

From transmitter
Detecting the level Detecting transparent bottles
Retroreflective of chocolate
To receiver
4- to 20-mA
output

Inches
30
Inches
5
10
15
5
4 20 20
Figure 6-31 Fiber optic sensors. Output 25
Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com. (mA) 30
Level detection
of the symbol’s code. Figure 6-32 illustrates a typical
PLC application which involves a bar code module read- Figure 6-33 Ultrasonic sensor.
ing the bar code on boxes as they move along a conveyor Source: Courtesy Keyence Canada, Inc.
line. The PLC is then used to divert the boxes to the ap- to external control devices. The operation of this process
propriate product lines according to the data read from can be summarized as follows:
the bar code. • The 4- to 20-mA mA represents the sensor’s mea-

Ultrasonic Sensors surement span.
An ultrasonic sensor operates by sending high-frequency • The 4-mA set point is typically placed near the bot-
sound waves toward the target and measuring the time it tom of the empty tank, or the greatest measurement
takes for the pulses to bounce back. The time taken for distance from the sensor.
this echo to return to the sensor is directly proportional to • The 20-mA set point is typically placed near the top
the distance or height of the object because sound has a of the full tank, or the shortest measurement dis-
constant velocity. tance from the sensor.
Figure 6-33 illustrates a practical application in which • The sensor will proportionately generate a 4-mA
the returning echo signal is electronically converted to a signal when the tank is empty and a 20-mA signal
4- to 20-mA output, which supplies a monitored flow rate when the tank is full.

Diverter

Scanner/decoder

Programmable
controller

Figure 6-32 PLC bar code application.
Source: Courtesy Keyence Canada, Inc.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 109

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