Reject conveyor tracking records the failed inspection decision and follows the associated product to the reject point. Fixed-speed lines may use a validated time or sensor sequence; variable-speed lines often use encoder movement. Independent reject confirmation then checks the physical outcome rather than only the command.
Keep the decision with the product
Why does a reject system remove the wrong pack?
Common causes include speed changes after a fixed time delay was set, inconsistent product pitch, pack slip, missed sensor edges, uncontrolled accumulation and an inspection-to-reject distance that was not measured under real conditions. A sound design separates the inspection decision, tracking method, reject actuation and outcome confirmation into defined stages.
Time-based versus encoder-based tracking
A stable conveyor running at one validated speed may use elapsed time between inspection and rejection. If the speed changes, the same delay no longer represents the same travel distance. Encoder tracking instead advances the recorded product position with measured conveyor movement.
Encoder tracking is not automatically sufficient: products can slip relative to the belt, transfers can change spacing and accumulation can break the relationship between belt movement and pack movement. Photoelectric sensors and controlled conveyor zones may be used to re-establish position.
Product queues and consecutive rejects
The PLC can retain a queue of product decisions as packs move through the line. The system must define how close products can run, what happens when two failed decisions are adjacent and whether the reject mechanism can recover before the next target arrives.
Recipe changes should update timing, sensor windows and mechanism settings in a controlled way. Clear fault messages help operators distinguish an inspection failure from a tracking, actuator or confirmation fault.
Reject command is not reject confirmation
An output proves that the controller requested an action; it does not prove the failed product entered the reject destination. Independent sensing at the chute, bin or transfer lane can confirm physical removal. A missed confirmation then initiates the fault response agreed for the application.
Information needed for controls integration
Provide an I/O list or interface description from the inspection equipment, conveyor drive details, speed range, distances, sensor types, product pitch and the required line handshakes. This allows the tracking method and responsibility boundaries to be defined before build.
Key project considerations
- Fixed and variable-speed conveyors
- Photoelectric pack sensing
- Encoder distance tracking
- PLC product decision queues
- Consecutive reject logic
- Independent outcome confirmation