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Automating Manufacturing Systems - Process Control and ...

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plc timers - 9.12<br />

I/1<br />

I/2<br />

T4:1/DN<br />

T4:2/DN<br />

T4:3/DN<br />

T4:4/DN<br />

O/1<br />

0 5 10 15 20 25 30 35 40<br />

time<br />

(sec)<br />

Figure 9.12 A Timing Diagram for Figure 9.11<br />

Consider the short ladder logic program in Figure 9.13 for control of a heating<br />

oven. The system is started with a Start button that seals in the Auto mode. This can be<br />

stopped if the Stop button is pushed. (Remember: Stop buttons are normally closed.)<br />

When the Auto goes on initially the TON timer is used to sound the horn for the first 10<br />

seconds to warn that the oven will start, <strong>and</strong> after that the horn stops <strong>and</strong> the heating coils<br />

start. When the oven is turned off the fan continues to blow for 300s or 5 minutes after.

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