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A linear position card can be printed straight while the installed contact travels sideways, pitches under cable load or reaches a mechanical stop before the specified electrical point. Guide straightness, rail-to-card parallelism, carriage play, wiper footprint, cable bend resistance and stop compliance all influence the sampled path. A credible interface definition therefore begins with the complete moving assembly. It separates cross-track containment from along-track calibration, and it treats the cable as a mechanical load path rather than an invisible electrical connection. Final machine protection and motion safety remain system responsibilities.
System boundary
The boundary covers a linear resistive card, card supports, guide or rail, carriage, wiper, end stops, moving cable or flex, connector, harness and receiving circuit. ChipSimple may review drawing-defined card features. Guide selection, cable routing, actuator control, guarding, diagnostics and system qualification remain with the integrator.
System integration decisions
- Define the functional line of travel and its datum relationship to the card.
- Reserve endpoint distance for stops, footprint and tolerance without reducing calibrated stroke silently.
- Measure cable forces throughout motion and route them away from the brittle substrate.
- Specify raw electrical acquisition before choosing filters or interpolation.
Define the functional travel line before locating the track
Establish a straight reference from the guide features that control the carriage, not from a convenient enclosure edge. Record axis direction, coordinate zero and positive travel. Relate that line to the card supports and printed-lane center using measurable datums. If the guide has two rails or bushings, state which features control translation and rotation. Include carriage play, preload direction and mounting sequence. The track location should be chosen after this installed transform is known; otherwise a centered artwork can be systematically displaced once the card is clamped.
Budget cross-track containment along the entire stroke
Parallelism error accumulates with travel, while local straightness and carriage yaw can change sign. Build an error envelope at several stations rather than checking only one midpoint. Include substrate placement, printed registration, lane width, wiper footprint, guide straightness, rail-to-card angle, carriage clearance and elastic deflection. The relevant pass condition is not whether the wiper center remains nominally inside the lane. The complete intended contact footprint must retain the agreed coverage without touching adjacent conductors, glaze boundaries or forbidden edge regions.
Calculate lateral displacement from offset and slope
A small-angle model shows how an apparently minor rail angle becomes important over a long travel.
y(x)=y_0+x tan(alpha)+e_g(x)+e_c(x); C(x)=C_nom-|y(x)|
- y_0 is guide-to-card lateral offset at the selected origin.
- alpha is relative guide-to-track angle.
- e_g(x) is local guide straightness deviation.
- e_c(x) includes carriage play and load-dependent deflection.
- C(x) is remaining nominal footprint coverage at station x.
Planar geometry, a consistently defined sign convention and no statistical cancellation. Wiper deformation and wear require separate evidence.
Quantify a guide-angle example
Consider an illustrative 100 mm stroke with 0.18 mm origin offset and 0.20 degree rail-to-track angle. The angular term at full travel is about 0.349 mm. Add a 0.10 mm adverse local guide deviation and 0.12 mm carriage deflection; total lateral displacement reaches about 0.749 mm. If nominal available coverage is 0.90 mm, only 0.151 mm remains. These figures are not capability data. They show why a short fixture indication near the origin cannot establish full-stroke containment and why the far endpoint can dominate the design.
Allocate physical stops and electrical endpoints separately
List, in order, the forbidden card edge, continuous-contact boundary, uncalibrated transition, calibrated endpoint, normal control range, deceleration region and hard stop. The order may differ at each end. Use the entire wiper footprint and the adverse stop position when computing separation. A compliant bumper can compress dynamically beyond a static dimension. Define whether output beyond the calibrated span must remain monotonic, stay electrically continuous or enter a diagnostic range. Never let a controller clamp conceal contact leaving a qualified region.
Treat the moving cable as a position-dependent spring
A cable loop changes curvature as the carriage travels. Its force can reverse direction, peak near an endpoint or introduce torsion through an offset connector. Document conductor count, insulation, shielding, minimum bend radius, unsupported length and restraint coordinates. Measure force and moment at relevant positions and temperatures because polymer stiffness changes with state. Route load into the carriage structure, not through card solder joints or ceramic. If a flex circuit is used, control crease orientation and repeated-bend zone rather than assuming it contributes no mechanical force.
Screen carriage pitch caused by cable moment
Suppose a measured cable force is 1.8 N and its line acts 22 mm above the guide reaction plane. The resulting illustrative moment is 39.6 N·mm. If the carriage angular stiffness about that axis is 800 N·mm per radian, the estimated pitch is 0.0495 radian, roughly 2.84 degrees. That would be large enough to require redesign or a more complete nonlinear model. Actual force and stiffness must be measured or provided by their owners. The example demonstrates that a modest-looking cable force can become significant when applied through a lever arm.
Record geometry, load and signal evidence together
Synchronized evidence is more diagnostic than a final pass mark.
| Question | Measurement | Responsible owner |
|---|---|---|
| Does the footprint remain contained? | Wiper-to-lane map at multiple travel stations | Mechanical and contact owners |
| Where does stop compression end? | Independent position during static and dynamic stop loading | Motion-system owner |
| How does the cable load the carriage? | Force, moment and routing photograph by position | Harness owner |
| Is output error mechanical or electrical? | Raw resistance, acquired signal and position time series | Electronics integrator |
| Do terminals carry residual strain? | Assembled displacement and continuity under cable manipulation | Assembly owner |
Preserve raw travel information through acquisition
Define excitation, return path, receiver impedance, sample interval, moving-average or digital filter and conversion law. Acquire direct card response during initial integration, then compare it with the controller channel. Direction-dependent difference may come from carriage clearance or wiper friction; periodic disturbances may follow cable motion; constant scaling may indicate loading or reference error. Keep raw data available when validating endpoint behavior. A display that looks smooth can still contain an unsafe momentary open, and a slow sample rate can miss rebound entirely.
Validate tolerance corners and representative motion cycles
Inspect the card to its substrate datums, then measure card placement relative to the guide. Map the footprint at the origin, midpoint and both endpoints under minimum and maximum cable force states. Perform slow bidirectional sweeps to expose clearance take-up, followed by representative speeds and stop encounters. Include relevant temperature and orientation states supplied by the integrator. Record independent position, raw electrical output, supply and carriage attitude. Acceptance for accuracy, dropout, wear, motion safety and service life belongs to the complete-system owner.
Use the spatial signature to direct investigation
A steadily growing lateral error indicates angular misalignment; a localized deviation suggests rail straightness or card seating. A discontinuity repeating at one carriage position can follow cable-loop inversion or guide damage. An electrical change that occurs at one printed coordinate in both directions suggests a local card/contact issue, while a direction-dependent offset points toward friction or clearance. Photograph the installed route and preserve as-found support conditions before loosening fasteners. These patterns prioritize measurements but are not proof without dimensional and electrical confirmation.
Control all parts that define the moving boundary
Reopen the interface review after changes to guide, carriage, bearings, wiper, spring, stop, bumper, card support, adhesive, artwork origin, lane width, connector, cable, restraint, actuator profile, acquisition or filter. Record the approved routing and datum definitions with part revisions. A supplier substitution that preserves pinout can still change cable stiffness; a housing revision can rotate the guide without changing nominal stroke. Configuration control must therefore include mechanical, electrical and software items that create the observed transfer.
Provide the guide, stop and moving-cable definition
A practical review needs installed motion and loading information as well as the card curve.
- Card drawing, printed-lane coordinates, substrate datums and required transfer.
- Guide and carriage geometry, straightness, parallelism, play and support method.
- Wiper footprint, force, compliance, stroke speed and reversal pattern.
- Stop positions, compression, overtravel, calibrated span and diagnostic behavior.
- Cable or flex construction, routing, restraints, connector, receiver and validation states.
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