Page 273 - Programmable Logic Controllers, Fifth Edition - Mobile version

domestic washing machine is another example of the use
of a sequencer, as are dryers and similar time-clock con-
Motor trolled devices.
An example of the wiring and timing chart for a dish-
washer that uses a cam-operated sequencer, commonly
known as the timer, is shown in Figure 12-5. A synchro-
nous motor drives a mechanical train that, in turn, drives a
series of cam wheels. The operation of this sequencer can
be summarized as follows:
• The timer motor operates continuously throughout
the cycle of operation.
• The cam advances in time increments of 45 seconds
in duration.
Contacts • The data timing chart shows the sequence of opera-
Figure 12-4 Dishwasher timed sequencer switch. tion of the timer.
• A total of sixty 45 second steps are used to com-
sequencer switch used in dishwashers to pilot the ma- plete the 45 minute operating cycle.
chinery through a wash cycle (Figure 12-4). The cycle is • Numbers in the active devices column refer to
always the same with a fixed routine of actions at each control devices active during each step of the
step for a specific time to complete its specified task. The cycle.

L1 L2
Ground Timer Active
Machine function increment devices
Timer Off 0–1
L1
Drain 2 1 2 4
Fill 3 1 3 4 5
First prerinse
1 Line Rinse 4–5 1 4 5 6
switch Drain 6 1 2 4 5
Fill 7 1 3 4 5
Prewash Wash 8–10 1 4 5 6
Drain Drain 11 1 2 4 5
2 pump
Fill 12 1 3 4 5
MTR Second prerinse Rinse 13–15 1 4 5 6
Drain 16 1 2 4
Safety water
level switch 3 Fill Fill 17 1 3 4
valve Wash Wash 18–30 1 4 5 6
Drain 31 1 2 4 5
Timer Fill 32 1 3 4 5
4 motor First rinse Rinse 33–34 1 4 5 6
Drain 35 1 2 4 5
MTR
Fill 36 1 3 4 5
Second rinse Rinse 37–41 1 4 5 6
Circulating Drain 42 1 2 4 5
5 motor
Dry 43–58 1 4 6
MTR Dry Drain 59 1 2 4 6
Dry 60 1 4 6

6 Heater

Figure 12-5 Dishwasher wiring diagram and timing chart.

254 Chapter 12 Sequencer and Shift Register Instructions

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