Page 144 - Programmable Logic Controllers, Fifth Edition

EXAMPLE 6-2
A motorized overhead garage door is to be operated au- • Green door open light to signal when the door is fully
tomatically to preset open and closed positions. The field open.
devices include one of each of the following: • Yellow door closed light to signal when the door is fully
closed.
• Reversing motor contactor for the up and down
directions. The sequence of operation requires that:
• Normally open down limit switch to sense when the • When the up button is pushed, the up motor contac-
door is fully closed. tor energizes and the door travels upward until the up
• Normally open-held closed up limit switch to sense limit switch is actuated.
when the door is fully opened. • When the down button is pushed, the down motor
• Normally open door up button for the up direction. contactor energizes and the door travels down until
• Normally open door down button for the down the down limit switch is actuated.
direction. • When the stop button is pushed, the motor stops.
• Normally closed door stop button for stopping the The motor must be stopped before it can change
door. direction.
• Red door ajar light to signal when the door is partially Figure 6-72 shows the ladder logic program required for
open. the operation implemented using an SLC 500 controller.

Input devices Program Output devices
(shown in unactuated
condition) L2
L1
Down
Up limit Stop Up Down Intrlock Up limit Motor up Door ajar
I:3/0 I:3/7 I:3/4 I:3/5 O:4/4 I:3/0 O:4/3 O:4/0 R

Motor up
Down limit O:4/3
I:3/1
Door open
Up
Stop Down Up intrlock Down limit Motor down
I:3/7 I:3/5 I:3/4 O:4/3 I:3/1 O:4/4 O:4/1 G
Door up
I:3/4
Motor down Door
O:4/4
closed
Door down O:4/2 Y
Up limit Down limit Door ajar
I:3/5 I:3/0 I:3/1 O:4/0
Motor
contactor
Stop door
I:3/7 Up limit Door open O:4/3 UP
I:3/0 O:4/1

Down limit Door closed Motor
I:3/1 O:4/2 contactor
O:4/4 DN

Figure 6-72 Motorized overhead garage door PLC program.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 125

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