Page 193 - Programmable Logic Controllers, Fifth Edition - Mobile version

Optical
sensor
Light Wood
source

Cutter
control
Optical
encoder

Generated pulses Optical Rotary
disk encoder
Programmable
Pulses controller
Figure 8-32 Cutting objects to a specified length.
Lines

• The photoelectric sensor monitors a reference point
on the conveyor. When activated, it prevents the unit
Figure 8-31 Optical incremental encoder.
Source: Courtesy of Nidec Avtron Automation. from counting, thus permitting the counter to accu-
mulate counts only when bar stock is moving.
of random pieces of bar stock moved on a conveyor. The • The counter is reset by closing the reset button.
operation of the program can be summarized as follows:
• Count input pulses are generated by the magnetic 8.6 Combining Counter
sensor, which detects passing teeth on a conveyor
drive sprocket. and Timer Functions
• If 10 teeth per foot of conveyor motion pass the Many PLC applications use both the counter function and
sensor, the accumulated count of the counter would the timer function. Figure 8-34 illustrates an automatic
indicate feet in tenths. stacking program that requires both a timer and counter.
Sprocket
Magnetic
sensor Photoelectric
sensor

Conveyor
Reflector

(a)
Ladder logic program
Inputs
L1 Magnetic Photo 10 counts per foot
sensor sensor
CTU
Magnetic COUNT-UP COUNTER CU
sensor Counter C5:1
Preset 10 DN
Accumulated 0
Photo
sensor

Reset
C5:1
Reset RES

(b)
Figure 8-33 Counter used for length measurement. (a) Process. (b) Program.

174 Chapter 8 Programming Counters

pet73842_ch08_156-183.indd 174 03/11/15 3:58 PM