Commissioning with the programmer

Commissioning is the moment of truth: the panel that has been built and the program that has been written meet the real machine for the first time. Things almost never all work first time, and that is fine if you work methodically. What separates a two-day commissioning from a two-week one is not luck: it is method, and above all how well the technician in the field and the programmer, often on the other end of the phone, understand each other.

The order of a commissioning

Every company has its own procedure, but the general outline is always the same: first everything is checked dead, then power is applied section by section, inputs and outputs are tested one by one, the safety functions are validated, and only then is the machine moved, first by hand and then in automatic.

1. Before applying power: dead checks

With the panel dead, you check the things that would be very costly to find later:

• Terminal tightness. A gentle tug on each wire finds the ones left loose. A loose terminal causes intermittent faults, the hardest to diagnose, and on power circuits it heats up.
• Earth connected in the panel, on the motors and on the metal parts.
• Correctly chosen protection. Circuit breakers of the rating on the schematic, and overload relays set to the rated current on each motor’s nameplate.
• No short circuits in the control circuit. A resistance measurement between +24 V and 0 V, with no power, must not read almost zero. If it does, there is a short circuit that must be found before connecting the power supply.
• Cleanliness. No offcuts of wire and no drilling swarf inside the panel: a metal chip on a module can cause a short circuit.

2. Apply power section by section

Never switch everything on at once. Power is applied to the main supply and measured: on a 230/400 V supply, about 400 V between phases and about 230 V phase to neutral. Then the circuits are closed one by one: the 24 V power supply (its output is checked), the PLC, the modules, the field devices. If something trips or smells burnt, you know exactly which circuit it was.

When the PLC is switched on, check its indicators: the CPU and the modules must show no errors. If the program is not loaded yet or the PLC is in STOP, the outputs do not switch anything on, which is exactly what you want at this stage.

3. Check the inputs, one by one

The key tool is the I/O list: a table with every signal, its PLC address, its name, what it physically is and which terminal it is on. The field technician operates each sensor or push button and the programmer confirms on screen that the right input changes. Each line is marked: correct, corrected or pending.

Remember how contacts behave: a stop push button, wired normally closed, has its input at 1 at rest and goes to 0 when pressed. If the programmer sees a permanent 0 on a stop, something is open: the push button itself, the wire or the common.

4. Check the outputs: forcing with permission

To test an output without running the whole program, the programmer switches it on from the computer. Two things must be kept apart:

• Modifying a variable means writing a value to it once. If the program writes it again in the next scan, the modified value disappears. In TIA Portal this is done from the watch table.
• Forcing means fixing a value that stays no matter what, until someone removes it. In TIA Portal it is done from the force table, and on the S7-1200 and S7-1500 only on inputs and outputs. While a force is active, the CPU shows it with a warning LED.

Forcing an output really moves the machine, bypassing the program logic, including any interlocks the programmer has written. That is why there are rules that are not up for discussion:

1. The field technician gives explicit permission for each force, after checking that the area is clear.
2. One output is forced at a time, and you say out loud which one.
3. When finished, all forces are removed, and you check that the warning LED has gone out.

Motor direction of rotation

The first start of each motor is a short bump, if possible with the motor uncoupled from the mechanics. On a three-phase motor connected directly to the mains, if it turns the wrong way you swap any two phases, always with the power off.

If the motor runs on a variable frequency drive, the order of the input phases does not affect the rotation: the drive generates its own output. The direction is changed with a drive parameter or by swapping two phases between the drive and the motor.

Valves and analog signals

• Solenoid valves: switch the output on and check that the cylinder moves the way the schematic says and that its limit switch turns on when it gets there. If it extends when it should retract, the tubes or the coils are usually crossed.
• Analog signals: compare what the PLC reads with what the instrument shows. In Siemens PLCs, the rated range of an analog input is converted into a value from 0 to 27648; with a 4–20 mA transmitter, 4 mA corresponds to 0 and 20 mA to 27648. If the value that arrives makes no sense, measure the loop current.

5. Validate the safety functions before moving the machine

Before running real cycles, each safety function is tested, one by one, and the result is recorded:

• Every emergency stop stops what it has to stop, without exception.
• Reset does not start the machine. Releasing the emergency stop and resetting only makes the machine ready; starting must be a separate, deliberate action.
• Doors and light curtains: when opened or crossed, hazardous movements stop.
• After a power cut, the machine does not start by itself when the power comes back.

6 and 7. Functional tests

1. Manual: each movement on its own, at slow speed, checking limit switches and end stops.
2. Automatic dry run: the full cycle with no product.
3. With product: slowly at first, then at the real rate.
4. Induced faults: cover a sensor, cut the air, open a door mid-cycle. The machine must stop safely and show a message people can understand.

Working with the programmer on the phone

Many commissionings are done with the programmer connected remotely or on the phone. Communication is half the job:

Remote access to the PLC must also be secure: only when needed, with the customer’s permission, over a protected connection, and disconnected afterwards. A machine with permanent, uncontrolled remote access is an open door to anyone.

8. Handover: the machine comes with paperwork

• Checked I/O list, with every signal ticked off.
• Updated schematics with every change made on site (“as built”).
• Backup of the final program, dated, and of the drive parameters.
• Signed record of the safety tests.
• Punch list: what is still to be resolved, with an owner and a date.

Typical mistakes

• Powering everything at once and not knowing which circuit failed.
• Checking inputs “by eye” without the I/O list.
• Forcing outputs without warning whoever is next to the machine.
• Leaving forces active at the end.
• Testing the safety functions at the end, or not at all.
• Changing wires in the field without marking it on the schematic.
• Handing over without a backup of the final program.

Summary

• Order: dead checks, power section by section, inputs, outputs, safety, manual, automatic and handover.
• Inputs are checked one by one against the I/O list, confirming the exact address.
• Modifying writes a value once; forcing holds it until removed. No force without permission from the field.
• A mains-fed three-phase motor that turns the wrong way is corrected by swapping two phases, with the power off.
• Safety functions are validated before the machine is moved in earnest, and they are never bridged.
• Handover comes with the I/O list, updated schematics, a program backup and a record of the tests.