Three faults account for most zero-pressure accumulation problems: a sensor that sees the wrong thing, a zone that is the wrong length for the pallet, and a timer set by guesswork. All three are commissioning faults, not equipment faults.

The principle, stated plainly

Each zone has a drive, a sensor at its downstream end and a controller that knows whether the next zone is occupied. A pallet enters zone 3 and stops at the sensor. Zone 3 will not run again until zone 4 is clear. The result is a queue of pallets with a gap between each one, and no pressure on the first pallet in the queue.

Everything else is detail about how the sensor sees the pallet and how long the controller waits before it believes what it sees.

Zone length against pallet length

The rule that prevents most problems: a zone should be at least 1.2 times the longest pallet and no more than 2.5 times the shortest. Outside that range the logic cannot distinguish a pallet from a gap.

A 600 mm zone with 400 mm and 900 mm pallets is workable. A 900 mm zone with the same pallets is not: the short pallet occupies less than half the zone, and the sensor may see the gap behind it as a clear zone.

Where the mix is too wide, use a sensor pair rather than a single photoeye, or split the conveyor into two zone lengths.

Sensor position

Mount the sensor so that it detects the pallet body, not a fork pocket, a label or a shrink-wrap overhang. The standard position is 50 to 80 mm upstream of the zone end, at a height that stays inside the pallet silhouette for every variant.

Diffuse sensors are convenient and unreliable on dark or glossy pallets. Through-beam sensors cost more to install and remove most of the false triggers. On lines handling black plastic totes, through-beam is the correct choice.

Field noteA distribution centre in Foshan had zones releasing in pairs. The cause was a reflective pallet wrap that triggered the upstream sensor from 900 mm away. Repositioning the sensor 15 mm lower solved a problem that had been blamed on the PLC for three weeks.

Timing settings that matter

Three timers do most of the work, and all three should be set from measurement rather than habit.

  • Block time (10 to 30 ms) filters vibration and short shadows. Too long and the line reacts late; too short and a passing fork pocket stops the zone.
  • Release delay (100 to 300 ms) prevents two pallets entering the same zone during deceleration. It should be longer than the drive's stop time.
  • Jam timer (5 to 15 s) flags a pallet that has not reached the sensor. Set it from the actual travel time plus 50 percent, not from a default.

Commissioning sequence

  1. Run the line empty and confirm every zone starts and stops from its own sensor.
  2. Load one pallet and walk it through, zone by zone, checking that each zone stops it at the sensor.
  3. Fill the line completely and confirm the gaps are consistent from end to end.
  4. Block the discharge and confirm the line fills without contact, then clear it and confirm it releases in order.
  5. Repeat with the shortest and the heaviest pallet in the mix.
  6. Record the settings on the electrical drawing; the next technician will need them.

When zero-pressure is the wrong choice

Zero-pressure logic costs more per metre than slug accumulation and needs more sensors. It earns that cost when products can be damaged by contact, when the queue is long, or when the discharge rate is variable. For a short line feeding a single machine at a constant rate, slug mode with a mechanical stop is simpler and adequate.

The honest test: if the products can touch without damage and the queue never exceeds four units, do not pay for zone control.