Pedestrian crossing

What you will build

Work order MC-2606. Millbrook Council, School Road crossing.

Millbrook Council (traffic and civil) needs a controller program for School Road crossing. Run a push-button crossing through amber, walk and clearance on timers, never showing Walk to pedestrians while cars can move, and ignoring presses during the cycle.

The council wants a push-button crossing on the busy road outside a school. Car_Green, Car_Amber and Car_Red are the traffic lights, and Walk and Dont_Walk are the pedestrian signals. Request_PB is the push button on the pole; the buttons on both sides are wired in parallel. Normally the cars have green and pedestrians see Dont_Walk. As soon as somebody presses the button, the cars get amber for s. Then the cars get red and pedestrians get Walk, with Dont_Walk off, for s. Then Walk goes off and Dont_Walk comes back on for a s clearance while the cars stay on red, and then the cars get green again. Exactly one car light is on at a time. Walk must never be on while the cars have green or amber. Presses during the cycle are ignored: the crossing does not remember them, so after the cycle the cars stay on green until somebody presses again. Holding the button down, or leaning on it, counts as one press and must not make the cycle repeat.

What you will learn

• Sequential control with timers
• One-shot (ONS) edge detection
• Safety interlocking of outputs
• State thinking
• Read a short user requirement specification (URS) and turn it into ladder logic
• Prove your program against a numbered factory acceptance test (FAT)

How it goes

1. Read the work order and the URS.
2. Build the ladder program against the tag table.
3. Run the plant and watch it respond.
4. Run the FAT and fix whatever fails.

Tools

• Ladder editor in the browser

  Draw contacts, coils, timers and counters, then run the program.

• A working simulated plant

  Watch the machine respond as your logic runs.

• A tag table

  1 inputs and 5 outputs: Request_PB, Car_Green, Car_Amber, Car_Red, Walk, Dont_Walk.

• URS and FAT documents

  The paperwork a real project starts from, with numbered tests.

Questions

Is it free?

Yes. The lab is free to use with a free account. There is nothing to buy to open this scenario.

Do I need to install anything?

No. It runs in your browser with the ladder editor and a simulated plant.

What will I do in PLC-06 Pedestrian crossing?

You build and test a program, then check your work against an acceptance test.

How long does it take?

A first attempt takes about 75 minutes.

Do I need experience?

No. Each work order says what to do step by step, and hints are there if you get stuck.