PLC and HMI retrofit checklist for an old stretch wrapper

Decide before you swap the controller

Swap the controller only after you can describe the machine on paper. A sticking photo-eye, a worn film carriage guide, a loose terminal or a misadjusted cut timer all read as logic faults. New logic around the same mechanical fault behaves differently and the wrapper stays a nuisance.

First steps, in order:

• Write down today's complaint: which state the machine hangs in, which sensor it ignores, how often it happens.
• Lock out the machine and inspect: sensor faces, cable insulation, terminal tightness, actuator travel, and mechanical binding on the turntable and film carriage.
• Repair what the inspection finds, or record the fault as known before you plan the program around it.

One hard stop: if the wrapper is unsafe today, or its e-stop or guard circuit has a bypass in it to keep the line moving, stop there. Have the safety circuit assessed and rebuilt before the retrofit. A retrofit planned around a bypassed guard ships with the bypass.

Record every operating state

The write-up is a behavior specification, not a guess at the original ladder. Capture these states:

• Power-up: what the machine does when control power returns, and what the operator must do before motion is allowed again. The article on PLC bits in a safe state after a power outage covers the controller side of that first scan.
• Ready: load present, turntable home, film threaded.
• Cycle start: which sensor arms the start and which operator action fires it.
• Wrap: the turntable revolutions or carriage passes the machine actually uses, and how film tension is handled.
• Cut and clamp: what triggers the cut and what confirms it completed.
• Home and unload.
• Fault states: each fault, the sensor that reports it, and what clearing it requires. Write whether the cycle resumes on its own or waits for a new start command.

Watch at least 10 full cycles in each mode the operators use, plus one power-up. Write the input or operator action that permits every state change. If wrap settings or product changeovers change the sequence, write them as separate sequences. The check: hand the write-up to a second technician and ask them to name the permissive for each state change. If they can, the sequence is documented.

List every I/O point against its terminal

Trace field devices to terminals and build the I/O list before anyone assigns addresses. For each point record:

• Terminal and slot on the old controller, so the new drawings reuse the same field wiring.
• The field device and its label.
• Signal type and voltage. On a machine of this type expect 24 V DC sensors and 120 V AC control loads; read both from device labels and fuse markings, not from wire color.
• Normal state: powered on while the machine idles, or made only when the event happens.
• Function: where the point sits in the written sequence.

Mark every point you cannot explain and leave its conductor isolated and tagged until traced. List the loads too: contactor coils, the cut solenoid, film carriage directions, panel lamps, any heater, and any analog signal such as a film tension adjust. An analog input or a networked drive decides whether the spare rack needs a special module.

The check: every point in the sequence write-up matches a terminal and a device, and nothing on the list is a guess.

Keep the e-stop and guard circuits hardwired

The e-stop buttons, guard switches and safety relay do not move into the spare PLC. The machinery safety standard for safety-related parts of control systems requires each safety function to be defined from the machine's risk assessment and the built result to be validated. An ordinary PLC rung with no safety rating does not meet that requirement, however tidy the program is.

Keep the standard wiring: the e-stop and guard switches feed a safety relay on two channels, and the relay's output contacts drop the coils that power the turntable drive, the film carriage and the cut circuit. The new PLC reads one status contact from the relay so the HMI can report an open guard. The PLC is never the device that removes power from the hazardous motion.

If the existing safety circuit is undocumented, damaged or bypassed, that part of the job goes to a machinery-safety professional before the controller swap is quoted.

Check the spare hardware against the list

With the list closed, compare it to what you have:

PLC

• Point count with spares; the example below shows the arithmetic.
• Module types per signal group: DC input, relay output, and any analog or network module the loads demand.
• Chassis slots: I/O cards plus the processor, with one slot free if you can get it.

HMI

• A driver for the new PLC's protocol and connection. Check the panel driver list before you tune serial settings, and keep one protocol on one cable.
• Screen budget: most six-inch monochrome panels of that generation run 320 x 240 pixels. At 20-pixel text rows that is 12 rows per screen, so a header leaves about 10 lines. Plan every screen against that budget.
• The objects the sequence needs: indicators, momentary buttons, a numeric display for wrap counts, and the fault list.

Software

• Programming software for the PLC and configuration software for the HMI, with licenses in hand.
• A confirmed backup and restore path, and a project backup saved at every commissioning step.

The check: every documented point has a home on a module, and every planned screen fits the 12-row budget.

Example: a 24-input, 16-output wrapper to a module count

The machine here is illustrative; run your own list through the same arithmetic. The sequence is documented and the I/O list closed at 24 inputs and 16 outputs. The 24 includes the safety relay status contact. The 16 cover contactor coils, the cut solenoid, film carriage directions and panel lamps.

Inputs. Sensors at 24 V DC, so 16-point DC input cards. 2 x 16 = 32 points. 32 - 24 = 8 spare inputs.

Outputs. Coils and solenoids at 120 V AC, so 8-point relay output cards. 16 / 8 = 2 cards, which leaves zero spare. Zero spare is the wrong answer on a retrofit: one stuck contact or one added load forces a chassis change. Order three cards: 3 x 8 = 24 points. 24 - 16 = 8 spare outputs.

Chassis. 2 input cards + 3 output cards + 1 processor = 6 slots. A 7-slot chassis leaves one slot open for the analog or network module the load list may still add.

Fixed-unit check. 24 + 16 = 40 documented points. A fixed-I/O PLC with 40 points carries this machine with nothing spare. The rack above carries it with 16 spare points.

Screens. At 320 x 240 and 20-pixel rows, 240 / 20 = 12 rows per screen, 10 lines under a header. The plan: one wrap status screen with state, wrap count and film status; two fault screens for 15 documented faults, 10 rows and 5; and three raw I/O screens for commissioning, 16 bits per screen, 40 / 16 = 2.5, rounded up to 3. The pattern in 16 PLC input and output bits on one screen works screen by screen. Keep the raw I/O screens off the operator menu once the machine runs.

Decision: two input cards, three relay output cards, six slots, six screens. The documented machine fits the spare hardware with 16 spare I/O points. When your own list runs the same arithmetic to zero spares, add a card or a chassis before the first rung is written.

What to send when you ask for a quote

If an integrator writes the program, or you want a price on the modules you are missing, send:

• The operating state write-up and the mode list.
• The I/O list with terminal numbers and voltages.
• Nameplate photos of the PLC, the HMI and any drive.
• Photos of the panel interior and both sides of the terminal strips.
• The safety circuit description: devices, safety relay, and the loads it drops.
• The faults observed during the observation runs.

Say in the same request who writes the program, who commissions on site, and who signs the safety validation. Those three answers separate a quote for a working wrapper from a quote for a box of modules.

Recommendation: document first, swap second. Write the sequence and close the I/O list before you order anything, keep the e-stop and guard circuits hardwired through their safety relay, and size the new rack so at least one card of each type stays spare. If the documented safety circuit is incomplete, quote that repair separately and finish it before the controller swap.

Questions