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A resistor card's output curve is meaningful only when its travel coordinate corresponds to the installed mechanism. A card can meet its bench curve and still produce the wrong system reading if its mounting origin, rotation axis or wiper path is displaced. A datum strategy connects the substrate, printed track, fixture and assembly through one inspectable coordinate system. It should control both where contact occurs and which mechanical position corresponds to each electrical point.
Key design decisions
- Choose locating features that represent the installed assembly and can be measured without damaging the card.
- Define mechanical zero, electrical index point and travel direction separately, then state their relationship.
- Propagate location and angular errors through the local output slope before assigning datum tolerances.
Connect the substrate coordinates to the mechanism
Identify the substrate features used to locate the card in its holder and the features that locate the holder in the mechanism. The printed track is a third part of that chain. A hole-to-edge dimension may locate the substrate accurately while leaving the track registration insufficiently defined. Include the track's position relative to the functional locating features.
For a rotary card, identify the axis of wiper rotation and the reference ray for angle. For a linear card, identify the travel axis and origin. State which view defines positive movement. A mirrored drawing or an assembly viewed from the opposite side can reverse the apparent direction, so the electrical curve should include a clear increasing-travel convention.
Choose datums that match the actual constraints
A datum scheme should represent how the card is supported and located during operation. A broad support surface may establish the plane, while a hole, slot or edge establishes in-plane location and orientation. The specific arrangement depends on the design; do not prescribe a generic hole pattern without knowing the real substrate geometry.
Avoid overconstraining the card with competing locating features. If the assembly permits controlled movement for thermal expansion or adjustment, the drawing and fixture should preserve that intent. Clamping that distorts the substrate or changes wiper force can produce an electrical difference even when the nominal in-plane coordinates appear correct. Include support and clamp locations in the review.
Distinguish electrical alignment from curve shape
An independent best-fit curve can describe shape while allowing its offset and slope to move. An absolute curve fixes the relationship to a mechanical index. Coupling alignment changes the realized relationship between sensor output and the installed mechanical index. For a custom card, define which relationship the customer needs before interpreting a curve report.
A curve that is shifted horizontally but otherwise matches the target may indicate a zero-reference error rather than a printing error. Conversely, aligning one index point does not prove that the rest of the track follows the required function. Retain both absolute-position comparison and shape residuals, so an assembly correction does not conceal a local electrical deviation.
Convert a datum error into an output error
For a small travel error, the resulting output error is approximately the local curve slope multiplied by the coordinate error. The same mechanical offset can matter more in a steep part of a nonlinear curve than in a shallow part. Use the local relationship rather than dividing only by total travel.
Suppose an assumed rotary output changes by four ohms per degree near an important point. An angular location error of 0.2 degree contributes approximately 0.8 ohm there. If the allowed error at that point is one ohm, little allowance remains for card variation, wiper contact and measurement. This is a hypothetical tolerance allocation, not a proposed manufacturing capability.
ΔRposition ≈ (dR/dx) × Δx
- x is the defined linear travel or angular coordinate.
- dR/dx is local resistance slope in the corresponding units.
- Δx is a small error between the intended and actual contact coordinate.
The curve is locally smooth and the error is small enough for a linear approximation; contact loss or path departure requires separate analysis.
Match the locating feature to the error it controls
A useful drawing assigns each critical relationship to a measurable feature. The fixture should reproduce those features and avoid silently replacing them with a more convenient edge. The table identifies the questions to resolve when different parts of the coordinate chain move.
| Relationship | Error to control | Verification approach |
|---|---|---|
| Track to substrate locator | Print registration changes contact position | Measure track coordinates from the functional locating feature |
| Card plane to wiper support | Height or tilt changes contact force and footprint | Inspect support plane and assembled wiper geometry |
| Rotary axis to track center | Eccentricity changes the swept path | Compare the actual axis with measured track geometry |
| Mechanical stop to electrical index | Zero shift changes output at a named position | Measure the index in the installed datum system |
| Fixture coordinate to assembly coordinate | Bench curves do not transfer consistently | Use a shared reference artifact and remounting study |
| Travel direction to terminal assignment | Output function is reversed or mirrored | Verify endpoints and sign with the controlled drawing view |
Review the whole wiper footprint through travel
The wiper touches an area, not an infinitely small point. Its contact footprint, lateral movement and permitted deflection should remain within the intended track region over the full motion. A correct centerline at one angle does not establish that the footprint stays on the track at both limits.
Evaluate axis offset, card rotation and assembly tolerances together using the actual geometry. Inspect where the contact approaches conductor transitions or track edges. If a tolerance combination leaves the active region, electrical interpolation cannot repair the loss of contact. The physical path must be corrected or the usable travel redefined through the agreed system design.
Verify remounting and transfer between fixtures
Measure a card repeatedly without removal, then remove and reinstall it using the intended locating sequence. Compare electrical index positions and curve residuals separately. A stable mounted sweep with shifting remount results points toward location or clamping repeatability rather than intrinsic curve noise.
When two fixtures are used, measure the same cards in alternating order and preserve each fixture's coordinate calibration. Compare actual mechanical positions, not merely the commanded motor steps. A fixture can repeat its own offset very well. Agreement should be assessed against the shared datum definition and an independent travel reference where the required uncertainty justifies it.
Place the datum and curve on the same revision
The released drawing should link the mechanical coordinate system, travel direction, electrical index and output table. State the wiper geometry or its controlled interface drawing, support plane and permitted mounting adjustment. If adjustment is used during assembly, define its range and verification rather than leaving it to informal operator judgment.
For quotation, send both the card drawing and the mechanism interface. Include the most sensitive output regions so datum tolerances can be reviewed where they matter. This allows printing registration, substrate features and assembly location to be evaluated together. A precise resistor curve supports the system only when the installed wiper follows the coordinate path used to define that curve.
Provide the card-to-mechanism datum chain
Send the physical locating scheme and electrical index together so the curve can be tied to installed travel.
- Card and holder drawings showing functional support and locating features.
- Rotary axis or linear travel origin, positive direction and drawing view.
- Output curve with mechanical index points and local error limits.
- Wiper footprint, force geometry and permitted path variation.
- Fixture coordinate definition and repeated mounting measurements where available.
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