An unmanned weighbridge looks like a hardware project: load cells, cameras, an RFID reader, a boom barrier. In practice most of the work is software, and specifically one question: when is a weighment valid?
The event sequence
A single weighment is a small state machine:
-
VEHICLE_DETECTED: RFID read or number-plate match -
POSITION_OK: platform sensors confirm all axles are on the scale -
WEIGHT_STABLE: the indicator reports a stable value for N readings -
EVIDENCE_CAPTURED: camera snapshots stored with the transaction ID -
TRANSACTION_CLOSED: slip issued, barrier opened, record sent onward
If any state fails or times out, the transaction moves to EXCEPTION and a supervisor is alerted. Nothing is silently accepted.
Reading the indicator
Most weight indicators push a continuous string over RS-232 or TCP. The terminal parses it, checks the stability flag and keeps a short rolling window. The key design rule: the weight value never passes through a user-editable field.
Idempotent ERP posting
The weighment is posted to the ERP (often SAP) with the transaction ID as the key. If the network drops, the terminal retries, and the ERP rejects duplicates. This avoids the classic double goods-receipt problem.
Evidence storage
Photos are stored against the transaction ID with the time stamp and gross, tare and net weights. When someone disputes a weight months later, a single lookup returns everything.
Handling exceptions
The exception path deserves as much design as the happy path. Typical cases: unknown RFID tag, plate read with low confidence, weight outside the order tolerance, or a camera offline. Each case gets a clear rule: hold the barrier, alert a supervisor, or allow with a flag. Log the decision and who made it, so audits can follow the trail.
Offline mode
Plants lose network more often than anyone admits. The terminal should keep a local queue of transactions and evidence, continue weighing under the same rules, and sync when the link returns, using the same idempotent keys.
Monitoring
Each terminal sends a heartbeat. A dashboard shows queue length, average platform time and exception counts per site, which is where truck turnaround improvements actually become visible.
We wrote a non-technical overview of the same system for plant and port managers. It covers features and rollout planning: intelligent terminal for weighbridges.
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