12.05.2014 Views

Automating Manufacturing Systems - Process Control and ...

Automating Manufacturing Systems - Process Control and ...

Automating Manufacturing Systems - Process Control and ...

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

plc states - 12.13<br />

FOURTH STATE WAIT FOR TRANSITIONS<br />

state_4<br />

MCR<br />

t_st4<br />

RTO<br />

delay 4s<br />

t_st4.DN<br />

t_st4.DN<br />

t_st4.DN<br />

state_4<br />

U<br />

state_1<br />

L<br />

t_st4<br />

RST<br />

MCR<br />

Figure 12.15<br />

Ladder Logic for State Four<br />

The previous example only had one path through the state tables, so there was<br />

never a choice between states. The state diagram in Figure 12.16 could potentially have<br />

problems if two transitions occur simultaneously. For example if state STB is active <strong>and</strong> A<br />

<strong>and</strong> C occur simultaneously, the system could go to either STA or STC (or both in a poorly<br />

written program.) To resolve this problem we should choose one of the two transitions as<br />

having a higher priority, meaning that it should be chosen over the other transition. This<br />

decision will normally be clear, but if not an arbitrary decision is still needed.<br />

www.PA<strong>Control</strong>.com

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!