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A jagged position trace may be electrical noise, but it may also be genuine stick-slip. Packing friction stores actuator force until the stem jumps; linkage clearance delays feedback reversal; bracket compliance lets the card move relative to the valve. Filtering can make the plot smooth while leaving unstable mechanics. Diagnosis needs an independent valve coordinate, actuator force or pressure, contact coordinate and raw electrical channel on one timebase. Process containment and safe actuation remain with the valve owner.
System boundary
The boundary includes valve motion reference, packing or seal friction, actuator, linkage, feedback mechanism, resistive card, harness and controller. ChipSimple may review the passive card drawing. Valve integrity, actuator sizing, control stability and functional safety remain customer-owned.
System integration decisions
- Measure valve-side motion independently from feedback.
- Preserve opening and closing directions and load.
- Resolve event timing before applying filtering.
- Separate linkage loss from wiper or acquisition disturbance.
Observe four coordinates rather than one trace
Record controller demand, actuator output, independent valve position and feedback electrical value. Add an intermediate linkage coordinate when practical. Define axes, direction and sampling alignment. A jump in valve position with matching feedback is mechanical motion; a feedback-only jump narrows attention toward contact or acquisition. Command alone never proves motion. Preserve process load or bench-force state because friction changes under load.
Model the force threshold behind stick-slip
A simple force balance identifies the stored excess that can release into motion.
F_net=F_act-F_process-F_friction-F_spring; motion begins when F_net exceeds the local restraint
- F_act is actuator force.
- F_process is process-load contribution.
- F_friction includes packing and guide effects.
- F_spring represents return or compliant loads.
Quasi-static screening; dynamic inertia, fluid forces and nonlinear seals require system analysis.
Calculate an illustrative released displacement
If a bracket-linkage chain stores 90 N/mm and force rises 36 N above its sliding requirement before release, elastic displacement can reach 0.40 mm. When friction breaks away, part of that displacement appears as a jump. The numbers are explanatory only, not valve data. They show how real motion can resemble an electrical spike at inadequate sample resolution.
Compare opening, closing and small reversals
Traverse slowly in both directions and pause at defined points. Apply small reversals that reveal clearance take-up separately from packing breakaway. Record dwell because static friction can increase with time. Do not average opening and closing curves before plotting their separation. If hysteresis follows process load, investigate mechanics before adjusting the printed transfer.
Synchronize force, motion and signal events
Use a common trigger or verified clock. Sample fast enough to resolve the shortest suspected jump and electrical dropout. Record actuator pressure or current, independent position, feedback voltage and supply. A signal event preceding physical motion suggests electrical disturbance; a force ramp followed by simultaneous motion and feedback jump supports stick-slip. Correlation guides diagnosis but still needs inspection.
Classify signatures by coordinate
The matrix prevents every irregular trace being assigned to the card.
| Signature | Additional observation | Likely boundary |
|---|---|---|
| Valve and feedback jump together | Actuator force releases | Valve friction |
| Feedback moves before valve | Intermediate linkage moves | Linkage clearance |
| Feedback spikes without motion | Direct card signal changes | Contact or wiring |
| Controller only changes | Local signal stable | Acquisition or grounding |
| Opening/closing offset grows with load | Bracket displacement changes | Structural compliance |
Inspect wiper behavior at fixed mechanical position
Hold the mechanism safely at selected positions and compare direct resistance or voltage with controller acquisition. Manipulate only approved harness states. Map events to the printed coordinate and contact footprint. A repeatable local dropout differs from a stem jump that can occur at different positions as friction changes. Preserve surface and wiper condition before cleaning or adjustment.
Evaluate filtering after physical separation
Record filter, sample rate, rate limits and control gains. Filtering can delay feedback and contribute to oscillation. First validate the raw position chain under slow controlled motion, then assess closed-loop dynamics. Do not widen plausibility limits or increase damping merely to conceal an unexplained mechanical or electrical event. Control acceptance belongs to the integrator.
Validate across load and environmental states
Use safe representative process loads or justified fixtures, temperature states, packing adjustment and linkage tolerance. Acquire repeated opening, closing, dwell and reversals. Inspect mounting, wiper and wiring afterward. ChipSimple can review card transfer by drawing but cannot certify valve friction, actuator margin, process sealing or loop stability.
Reopen diagnosis after mechanical or software changes
Changes to packing, lubrication, actuator, bracket, coupler, linkage, wiper, card, harness, input, filter or control gains alter signatures. Bind configuration and load state to each record. A smoother trace after filtering is not evidence that stick-slip disappeared. Repeat the coordinate comparison when a chain element changes.
Keep process and actuator hazards controlled
Tests must respect isolation, pressure, stored energy, pinch and unexpected-motion controls established by the customer. This page does not define permissible motion or leakage. If independent position or safe load control is unavailable, do not infer root cause from an energized system. Numerical examples are models, not acceptance values.
Quantify event order and resolution
For each irregular event, calculate the delay between actuator-force change, independent valve displacement, linkage movement and electrical response. Report the time resolution and alignment uncertainty. When two events are closer than that uncertainty, classify their order as unresolved instead of inventing causality. Repeat at several ramp rates: frictional breakaway often depends on dwell and rate, whereas an artwork-local contact event follows track coordinate. This experimental discipline helps decide whether mechanical maintenance, linkage correction, card inspection or acquisition work should come next.
Provide the valve motion and feedback evidence chain
Review needs independent position, actuation load and raw electrical data.
- Valve coordinate, packing, process load and safe test state.
- Actuator command, pressure or current, force and speed.
- Linkage, pivots, compliance, adjustment and backlash.
- Wiper, card transfer, terminals, harness and receiver.
- Sampling, filtering, acceptance and validation ownership.
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