Sort a picking wave on a conveyor: read each code and send the item to its order

The warehouse system sends the wave to the PLC before it starts: which codes each order needs and which lane holds each order. Then the PLC runs the same six steps on every item.

1. Infeed. A short belt at the start releases the items with a gap between them, so the sensors can see where one item ends and the next one starts.
2. Scan. A photo sensor at the scanner sees the front of the item and opens a read window. The scanner sends the code on RS-232. The window closes when the back of the item passes.
3. Look up. The PLC looks for an open order that still needs this code. That order gives the lane. No code, two codes or a code that no order needs means exceptions.
4. Track. The PLC stores the encoder count at the scan. The item moves with the belt, so the count tells where it is at any moment, even if the belt stops and starts again.
5. Divert. When the count reaches the diverter of its lane, the PLC opens the valve. The arm swings across the belt and guides the item into the lane, then returns.
6. Confirm. A sensor at the entry of the lane must see the item within 1.2 s. Only then does the PLC count the item in its order. When an order has all its items, its lane light turns on for the packer.

Run the Normal wave scenario and watch how the order cards fill up while the items arrive grouped by code.

It counts the pulses of an encoder on the belt, and keeps a list of the items between the scanner and the end of the belt. Each entry holds the encoder count at the scan, the length of the item and its lane.

• Encoder: in the example, 1 pulse per mm. At 0.5 m/s that is 500 pulses per second, well inside the 20 kHz of the optoisolated inputs of the M-Duino family. Use an interrupt input, so the program does not miss a pulse while it talks to the scanner or the network.
• Diverter 1 at 2.0 m: the scanner is at 0.8 m, so the item is 1,200 pulses away. The PLC opens the valve 60 pulses earlier, because the arm needs time to cross the belt, and closes it when the back of the item has passed.
• The gap: the photo sensor sees two items that touch as one long item, and the scanner reads two codes in one window. The PLC cannot tell which code belongs to which item, so both go to exceptions. Items that are too close for the arm to swing back also go to exceptions.

A timer instead of an encoder works only while the belt speed never changes. Here the belt stops for faults and for full lanes, so the PLC follows the encoder count, not the time.

Set Infeed gap to Not metered and run the Normal wave again. Some items arrive touching and go to exceptions.

The item goes to the exceptions chute at the end of the belt, and the PLC writes the reason in its record. It never guesses a lane.

• No read: a damaged or folded label. An operator fixes it and puts the item back on the belt. The order waits for it.
• Code not in the wave: a picking error. The item goes back to stock, and the warehouse system finds out which order is short.
• No open order needs it: one item too many of a valid code. Every order with that code is already complete.
• Lane full: a sensor near the top of the lane stays covered. The PLC checks it when the item reaches the diverter. Pushing another item into a full lane jams it.

The read rate on the panel is the share of items with exactly one code in the window. A falling read rate often points to dirty scanner glass, a moved scanner or a new label printer, before it becomes a problem in the orders.

Run the No-reads and extras scenario, then try the Jammed lane scenario with both options of When a lane is full.

Because a valve output that is on does not prove that the item is in the lane. A diverter can stick, lose air or arrive late. Without a check, the PLC counts the item in its order anyway. The light says the order is complete, and the packer finds an item missing.

With the lane sensor, the PLC counts the item only when it sees it enter. A missed item stays open in its order and goes to exceptions, where the operator can re-induct it. Two misses in a row on the same diverter stop the belt: a single miss can be a badly placed item, two in a row point to the diverter.

Run the Diverter fault scenario with Confirm with the lane sensor, then again with Trust the diverter, and compare order 3 at the end.

The belt, the diverters and the safety of people are separate questions. The emergency stop and the guards go to a safety relay that removes the power of the belt motor and the air of the diverters, without going through the PLC (IEC 60204-1, ISO 13850). The PLC reads the state of the safety circuit on one input and stops the sorting logic. It does not replace it. See how a PLC reads the state of interlocks and the e-stop.

1. A metered infeed

The infeed belt stops and starts to leave a minimum gap before each item. It costs a little throughput, and it saves most double reads. Without it, the gap depends on how the pickers drop the items.

2. Confirm or trust the diverter

A sensor at each lane entry costs one input per lane. It turns a silent error at the packing table into an exception on the sorter, with the lane and the diverter in the record.

3. A full lane: exceptions or hold

Sending to exceptions keeps the other orders moving, and adds work for the exceptions operator. Holding the infeed sends nothing to exceptions that a free lane could take, and stops every order until the packer empties the lane. Items already on the belt for the full lane still go to exceptions.

4. Which order gets an item

When two open orders need the same code, the PLC must choose. The lowest order number is simple to follow. The order closest to complete finishes orders earlier, so packers can start sooner. Compare the time of each complete order in the log.

5. What lives in the PLC and what lives in the warehouse system

The PLC keeps only the current wave: codes, quantities and lanes. The M-Duino 42+ has 8 KB of RAM, enough for a wave of about a hundred lines with short codes. The catalogue, the stock and the history stay in the warehouse system. If the network stops, the PLC can finish the wave it has, keep the records on its microSD card and send them when the link is back.

This simulator puts together patterns from several of our cases: picking and packing online orders, sorting packages by their barcode, storing and shipping boxes, tracing packs on a packaging line, and driving a conveyor belt. Each case describes its own plant. This simulator shows the sorting logic they share.

• E-commerce pick and pack with an ESP32 PLC: orders, presence sensors and two scans for grouped orders
• Logistics automation with a Raspberry PLC: a barcode read at the entry and sorting on the belt
• Automated warehouse with an M-Duino: QR codes, storage by weight and dispatch to the docks
• Packaging line traceability with ESP32 PLCs: a barcode scanner, batches and cases
• Packaging line retrofit with an ESP32 PLC: line, phase and product data from an old line
• Conveyor belt speed control with an M-Duino 42+: the inverter that moves the belt

What you see here is a general pattern. The codes, orders, lanes, distances, times and faults are examples, not data from those cases.

To sort by weight instead of by code, try the weight sorting simulator.

These tutorials and examples cover the parts of this sorter on an Arduino based PLC.

• How to use interrupt inputs with industrial Arduino boards, for the belt encoder
• How to use RS-232 on an industrial Arduino based PLC, for the barcode scanner
• MQTT between an M-Duino PLC and a Raspberry PLC, to receive the wave and send each record
• Modbus RTU speed control of a VFD with an M-Duino PLC (PLC Solution Library), for the belt drive

More examples: PLC Solution Library · all simulators.

Example I/O for this sorter

This example uses 21 of the 26 inputs and 13 of the 16 outputs. The encoder goes to an interrupt input. The digital outputs give up to 70 mA and the analog/digital outputs up to 20 mA, so valve coils, lamps and the run signals go through interposing relays or a valve terminal. For the diverter valves, which switch on every item, prefer solid-state relays or a valve terminal. The datasheet gives 0 to 24 Vdc for the analog/digital inputs in digital mode, so 24 V buttons can connect there. For more lanes, add a second PLC on Ethernet or choose a larger M-Duino. Check your loads and levels against the M-Duino family datasheet. Full specs: M-Duino 42+ · M-Duino family.