Automating Manufacturing Systems - Process Control and ...

plc states - 12.18
Defined state variables:
ST1 = state 1 - green NS
ST2
ST3
ST4
=
=
=
state 2 - yellow NS
state 3 - green EW
state 4 - yellow EW
The state entrance and exit condition equations:
ST1 = ( ST1 + ST4 ⋅ TON 2 ( ST4,
4s)
) ⋅ ST1 ⋅ S1 ⋅S2
+ FS
ST2 = ( ST2 + ST1 ⋅ S1 ⋅ S2) ⋅ ST2 ⋅ TON 1
( ST2,
4s)
ST3 = ( ST3 + ST2 ⋅ TON 1
( ST2,
4s)
) ⋅ ST3 ⋅S1 ⋅S2
ST4 = ( ST4 + ST3 ⋅ S1 ⋅ S2) ⋅ ST4 ⋅ TON 2
( ST4,
4s)
Note: Timers are represented in these equations in the form TONi(A, delay). TON indicates
that it is an on-delay timer, A is the input to the timer, and delay is the timer
delay value. The subscript i is used to differentiate timers.
Figure 12.20
State Equations for the Traffic Light Example
The equations in Figure 12.20 cannot be implemented in ladder logic because of
the NOT over the last terms. The equations are simplified in Figure 12.21 so that all NOT
operators are only over a single variable.
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