Mission 62 · PRO

The cylinder that never arrives: PLC programming exercise

Give every move three seconds, and stop safely if one runs out.

Run the A+ B+ B− A− cycle, and watch every move with a three second . A move that runs out of time stops the cycle with FaultLamp ON, and the cylinders come home in a safe order.

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The 3D electro-pneumatics trainer this mission runs on

Level: Advanced
Time: About 35 min
Reward: +500 XP
Rig: 3D electro-pneumatics trainer
Siemens editors: Ladder (LAD), SCL
Rockwell editors: Ladder, Structured Text
Beckhoff editors: Structured Text
Stage: 33. Pneumatics lab: cylinders, valves and reed switches

Required behaviour

The cylinder that never arrives: what your code must do

A sequence that waits for a switch waits for ever if the switch never comes: a jammed cylinder, a failed reed switch, a kinked hose. Keep AirOn ON, and run the same cycle as before: Start with AirOK, A0 and B0; A out (ExtendA), B out (ExtendB), B back (RetractB), A back, each on its reed switch, with RunLamp ON. Now time each move. If any one move takes more than 3 seconds, stop the cycle: RunLamp OFF and FaultLamp ON. Then come home safely: bring B back first (RetractB ON until B0) with A still held out, and only then let A go (ExtendA OFF). FaultLamp stays ON until Reset is pressed with both cylinders home (A0 and B0). While FaultLamp is ON, Start does nothing. A slow cylinder is not a fault: a move that finishes inside 3 seconds must not trip.

Cycle: A out, B out, B back, A back, on the reed switches.
Time limit: More than 3 s for any one move is a fault.
On a fault: B back first, then A. FaultLamp ON, RunLamp OFF.
Reset: Only with A0 and B0. Start does nothing until then.
How it's checked: The checks press the buttons and switch the jam on, then watch the trainer itself: its reed switches and its counts of finished strokes.

Tag table

The cylinder that never arrives: inputs and outputs

Your program reads 7 inputs and sets 6 outputs. 4 more inputs drive the simulation or show readings, for you to test with. The names are the same in every maker’s editor.

Tag	Data type	Your code	What it means on the machine
`Start`	`BOOL`	ReadsAn input your code needs	Green Start pushbutton
`Reset`	`BOOL`	ReadsAn input your code needs	Blue Reset pushbutton
`AirOK`	`BOOL`	ReadsAn input your code needs	Pressure switch 0B1: ON once the air is above 4.5 bar
`A0`	`BOOL`	ReadsAn input your code needs	Reed switch 1B1: cylinder A fully back
`A1`	`BOOL`	ReadsAn input your code needs	Reed switch 1B2: cylinder A fully out
`B0`	`BOOL`	ReadsAn input your code needs	Reed switch 2B1: cylinder B fully back
`B1`	`BOOL`	ReadsAn input your code needs	Reed switch 2B2: cylinder B fully out
`SlowA`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: wind in the flow control on A's out stroke, so A goes out slowly
`SlowB`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: wind in both flow controls on B, so B is slow both ways
`JamA`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: cylinder A jams part way along its stroke
`JamB`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Try it: cylinder B jams part way along its stroke
`AirOn`	`BOOL`	WritesAn output your code sets	Dump valve 0V2: ON lets the air in, OFF lets it out
`ExtendA`	`BOOL`	WritesAn output your code sets	Valve 1V1, spring return: ON sends A out, OFF brings it back
`ExtendB`	`BOOL`	WritesAn output your code sets	Valve 2V1, out solenoid: sends B out, and the valve stays there
`RetractB`	`BOOL`	WritesAn output your code sets	Valve 2V1, back solenoid: brings B back, and the valve stays there
`RunLamp`	`BOOL`	WritesAn output your code sets	Green lamp: a cycle is running
`FaultLamp`	`BOOL`	WritesAn output your code sets	Red lamp: fault

In Siemens SCL, write a block’s own tags with # in front, as TIA Portal does: #Start. Rockwell and TwinCAT use the name on its own: Start.

Automatic tests

The cylinder that never arrives: what the tests check

A healthy cycle runs with no fault, even with a slow cylinder.
A move that takes more than 3 seconds stops the cycle: RunLamp OFF, FaultLamp ON.
After a fault, B comes back first, and A lets go only when B is home.
The fault holds until Reset is pressed with both cylinders home.
Start does nothing while FaultLamp is ON.

Try it Use the buttons below the model and watch the rig respond.
Test solution Every check runs on your code in one press.
Rewind PRO (needs Pro) If one fails, step back through it one scan at a time to see why.

Stuck on this mission? Ask the members in the Locker Room.

Why it matters

Where you’d see this on site

Every real sequence has this watchdog. On an operator screen it reads as 'Step 20 timeout: B not out', which tells the fitter exactly where to look.

Without a time limit, a jammed cylinder leaves the machine sitting silently in the middle of a cycle with the clamp closed. People reach in to see what's wrong.

Where you'd find thisMove things with air, and know where every cylinder is

Most machines that clamp, push, lift or eject do it with air cylinders. A solenoid valve sends air to one side of the piston and lets it out of the other, and a small switch on the barrel tells the controller when the piston has arrived. Colleges teach it on a board like this one, because the same few parts make up almost every pneumatic machine.

Air supplyFiltered, set to 6 bar
ValvesOne 5/2 valve a cylinder
CylindersOut and back on airYou are here
Reed switchesProve each stroke

On a real site: places, parts and safety

Assembly machines that clamp a part, then press or drill it
Packing lines that push, lift and eject cartons
College and apprentice training workshops

Compressed air is stored energy. Shut the isolator and let the air out before touching pipework: a hose that comes off under pressure whips.

Letting the air out isn't always the safe state. A cylinder holding a load upright drops when the air goes, so real machines are assessed one cylinder at a time.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

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