Quick answer
Feed spikes during a loss-in-weight feeder refill are usually caused by the transition between gravimetric and refill control rather than by the screw, belt or rotary element alone. Diagnose the refill event by trending feeder weight, refill-valve state, feeder speed, commanded rate, actual rate, hopper pressure and material behavior on the same time base. The objective is to determine whether the spike comes from refill timing, refill mass flow, poor filtering/tuning, hopper pressurization, material compaction or a mechanical disturbance.
Problem / job
Actual feed rate jumps or dips every time the loss-in-weight hopper refills, disturbing kiln, mill or dosing stability.
Evidence
Capture hopper weight, feeder rpm/speed, refill-valve command, actual feed rate, refill duration, material level, hopper vent condition and controller mode.
Decision
Separate control-transition error from excessive refill flow, aeration/pressure effects, material compaction, weighing disturbance or mechanical drag.
As-found evidence capture
Do not begin by changing PID values. First record several complete refill cycles. Trend feeder gross or net weight, commanded rate, measured rate, feeder drive speed, refill-valve or gate command, refill start/stop time and any mode flag showing when the controller leaves normal gravimetric operation. If the hopper is vented, record vent-filter condition or pressure where instrumentation exists.
Compare a disturbed refill with a stable historical cycle. Note whether the spike appears at refill start, during the refill itself, when the valve closes, or when the controller returns to gravimetric mode. The timing sharply narrows the cause family.
Seven-step diagnostic workflow
1. Identify the exact point of disturbance
If the rate spike starts at refill initiation, investigate the mode transition and sudden load applied to the weighing system. If it occurs at valve closure, check impact, vibration and controller handover. If the disturbance continues after refill, inspect material compaction or poor weight-signal recovery.
2. Verify the weight signal
Check load-cell signal stability, mounting freedom, flexible connections, cable condition and mechanical contact around the hopper. Refill pipework, dust extraction or a stiff flexible connector can transmit force into the weighing vessel and create a false apparent mass change.
3. Check refill flow rate and duration
An aggressive refill can shock the hopper and change bulk density. Record refill duration and mass added. Compare with the intended refill band. If the refill valve is oversized, sticky or opening too far, reduce the cause rather than hiding the signal with excessive filtering.
4. Inspect hopper venting and pressure
Incoming material displaces air. Restricted venting can pressurize the hopper, influence load-cell readings, disturb material flow and increase dust release. Verify the vent path and any filter or breather serving the weigh hopper.
5. Review control logic and filtering
Confirm how the feeder handles refill: volumetric fallback, frozen speed, feed-forward correction or another strategy. Check whether the weight filter, refill compensation and handover timing were changed during recent control work. Use the feeder supplier’s logic and tuning method rather than copying settings from another machine.
6. Check material condition and compaction
Fine powders may compact differently after a rapid refill. Cohesive material can bridge then release, producing a real feed surge after the apparent control event. Compare refill behavior with moisture, temperature, product grade and upstream aeration status.
7. Prove the correction over repeated cycles
After correcting the dominant cause, observe multiple refills at representative feed rate. Acceptance should be based on repeatable rate stability, normal weight recovery and no growing deviation after each cycle.
Evidence-to-decision table
| Evidence | Likely direction | Next check |
|---|---|---|
| Spike begins exactly when refill valve opens | Mechanical/weight-signal disturbance or control handover | Load-cell trend, pipe forces, refill logic |
| Weight signal oscillates but drive speed is steady | Weighing disturbance | Load cells, flexible connections, hopper contact |
| Drive speed changes sharply during refill | Control/tuning issue | Mode transition and refill compensation |
| Rate remains high after refill ends | Material density/compaction or real flow surge | Hopper condition and material behavior |
| Long refill with unstable hopper pressure | Refill restriction or poor venting | Valve, refill line and vent path |
Return-to-service acceptance criteria
Accept the correction only when repeated refill cycles complete without unacceptable feed-rate spikes, the weight signal settles predictably, feeder speed follows the intended control strategy, refill time is consistent and no abnormal hopper pressure or mechanical contact is observed. Save the successful trend as the new reference trace.
Common mistakes
Do not tune the controller before verifying the weighing hardware. Do not over-filter the weight signal until the process looks smooth while hiding a real mechanical disturbance. Do not assume all refill spikes are software problems. Do not ignore venting, flexible connectors or refill-valve dynamics. Finally, do not judge one refill cycle; intermittent problems require several repeatable cycles.
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