What level do you want them to learn at?
The usual examples
- Elevator
- Parking Lot
- Traffic Light
have the property that they are all things that people "already know how they work". This is a usually desirable property for a student project. They don't need to be told the sequence. But they get experience in breaking up the sequence into the "correct" control modules.
They also allow for "scope creap" ("add a zebra crossing", "Add access to the roof, but only selectable via keyswitch")
Occationally we get asked by "5th year engineering studends" for some PLC project ideas. I typically send them to the kitchen. Automate a refigerator or an oven (zoned control of analog systems (PID), operator interfaces (including the light that comes on when the door is opened)). Toasters and microwaves are good too.
A whole-house control system has potential, in that you can start with a simple switch-turns-on-the-lamp control, and add on multiple switches, motion-sensors, photoeyes, timers, etc. This can be a semester-long project, which gets built up as they progress. The flip-flop problem is naturally introduced. Analog control can come in via the air-conditioning system, or even just a dimmer switch. But in this house, everything would be wired through the PLC.
If you are looking for more advanced programming (like what we find in the real world, where we have to be told what the machine is supposed to do), then the common list is:
- Pick & Place
- Batching Tank
- Packaging Line
.
For a good Pick-and-Place, see
Steve Bailey's Example in the Archives
For a batching tank, just draw a simple one with, say 4 solenoid valves feeding in raw materials (with one valve being a double-block-and-bleed arrangement), flow meters on each of the lines. Add a steam jacket (control valve on the infeed, solenoid valves on the return and drain), an agitator (VFD driven (enable, run DO's, running DI, and speed control and reference AO/AI), a discharge valve and pump (single speed). You can keep on adding bells & whistles (a recirc loop, going through, or bypassing, a heat exchanger for cooling), pressure control, nitrogen blankets, etc) until the system is as complex as you want. (Extra credit - programmable recipes).
For a packaging line, it's the same sort of thing: 10 conveyor motors. Cascaded start/stop. No upstream feeding to a failed conveyor. Jam-detect photoeyes. Add in other machines (bottle unscrambler, filler, capper, checkweigher, reject station, palletizer, shrinkwrapper) until again it's as complex as you want.
Best of luck to you (and your students).
