The conveyor tracking drifts. The robot picks the part two inches off. You re-teach the position and it drifts again in a week. The encoder is fine. The coupling between the encoder and the pulley shaft is slipping.
Why a keyway is not enough
Mounting an encoder on a conveyor roller with a key sounds mechanical, but at the torque levels a belt conveyor generates, the key fits loosely. Over months of start-stop cycles, it rocks in the keyway. The encoder shaft doesn’t move with the roller. It moves when the backlash has been taken up, then sits there.
The fix is a flexible coupling that grips both shafts with a split clamp, not a key. A bellows coupling or a disk coupling takes the slight misalignment and transmits rotation without play. The encoder sees exactly what the shaft does.
The encoder never touches the drive pulley
If you mount the encoder on the drive pulley, you’re measuring the motor and gearbox, not the belt. The belt slips on the pulley, the gearbox windup under load changes, and the reading lies. Mount the encoder on the tail pulley or a driven shaft at the far end of the conveyor. That way, belt slip doesn’t show up as a position error.
For long conveyors, put the encoder on the driven end. If you must use a remote pickup wheel, spring-load it against the belt so it stays in contact through belt stretch and wear. A spring that loses tension over time is the second most common cause of lost pulses.
Cable strain breaks the encoder
The encoder cable flexes with every conveyor cycle. If it’s tied down solidly at both ends, the conductor breaks inside the jacket. The encoder still powers up, but the pulse channel is dead. You don’t get a fault; you get a slow drift as the channel drops pulses intermittently.
Use a high-flex cable rated for drag-chain applications. Loop it with a service loop, not a straight pull. Shield the cable and ground the shield at one end only. A grounded shield at both ends creates a ground loop that injects noise into the pulse count.
Counting pulses matters
A 1000 PPR encoder sounds precise, but after the gear ratio and pulley diameter, that might be 0.1 mm per count. For a 2000 mm conveyor, you need enough resolution that a stop position is repeatable within a fraction of a millimeter. If the encoder is too coarse, the PLC stops at the nearest pulse, which is two millimeters off. That’s enough for a robot to miss the part.
Don’t compensate by doubling the encoder PPR without checking the PLC input speed. A high-count encoder on a slow input channel misses pulses at speed. Use a high-speed counter module, or divide the count in the drive.
Bottom line
An encoder is only as good as its mechanical coupling. Use a zero-backlash clamp coupling, mount it on the driven end, and flex the cable properly. If the tracking drifts, check the coupling before you buy a higher-resolution encoder.
