PHOTO 5
PHOTO 6
February and March 2005 issues of Nuts & Volts that can
get you started.
Figure 2 shows the connections necessary from the
pneumatic circuit to the PLC. Photo 5 shows the PLC
connected to the pneumatic board. Figure 3 shows the
complete ladder logic program that is being run in the
PLC to control the pneumatic circuit. This program was
written in RSLogix Starter for 10 point Micrologix 1000
software, and can be downloaded from http://ab.com.
The PLC used in this demonstration is from Allen-Bradley
(see Photo 6).
FIGURE 1
42 SERVO 02.2009
The PLC Program
The ladder logic program has several features in it that
I’ll use to illustrate industrial types of situations that may be
encountered, in an industrial setting. The system operates
as follows: First, the operator must press and hold the
green start button for three seconds to get the system
started. While holding the start button in, a buzzer sounds
and an amber light comes on. The purpose of this is to
warn people that the system is about to move. If the start
button is released before the three second delay has
completed, the timer will reset and the button must be
held in for a full three seconds again. After this warning
delay, the PLC piston output will be on as well as a red
light; the piston will extend until it hits the extended limit
switch. At this point, the output from the PLC to the
cylinder will turn off and the cylinder will return to its
retracted position. When the retracting piston hits the
retracted limit switch, the PLC piston output will be turned
on and continue the cycle over again. However, there is a
counter keeping track of every time the extended limit
switch is hit (rung #5 in the program). When the counter’s
accumulated value equals the preset value (currently 5),
the system stops, resets the counter, and goes back to its
original position waiting for an operator to hold in the start
button again. Pressing the red stop button at any time
will turn off all PLC outputs, reset the counter, and put the
PLC back into its original state waiting for a three second
start button press.
Industrial Control 101
Let's go through the ladder logic program one rung
at a time. Rung 0 (see Figure 4) waits for three seconds
while the start button is pressed. It turns on the buzzer
and amber light. Rung 1 (see Figure 5) sets a bit in memory
when the three second delay has completed. This bit in
FIGURE 2
