On this page
The mechanical travel of a sensor mechanism can extend beyond the region where its resistor track follows the calibrated output function. Some end regions remain electrically continuous but nonlinear; others may permit contact loss. These behaviors must be defined separately. A travel-limit review connects the wiper footprint, track boundary and mechanical stop to an explicit output requirement, so the receiver sees the intended behavior throughout the permitted motion.
Key design decisions
- Distinguish calibrated electrical travel, continuous contact travel and mechanical travel on the same drawing.
- Evaluate the full contact footprint and tolerance combinations at both ends.
- Specify receiver behavior for each end region rather than extrapolating the central curve beyond its valid domain.
Name the different travel boundaries
The calibrated electrical region is where the output must follow its specified curve. A continuity region may extend farther while remaining conductive but no longer meeting that curve. Mechanical travel describes where the mechanism can physically move. Those ranges may overlap without being identical.
Distinguish electrical travel from mechanical travel, with boundaries defined for the custom card's actual track and wiper. Do not import a packaged potentiometer's angular ranges. Show the card's own endpoints, transition regions and mechanical-stop relationship in the installed coordinate system.
Inspect the transition from resistor to conductor
Near an end, the wiper may cross from a resistive region into a conductor or termination transition. Its footprint can span both at once, producing an output that differs from a point-contact model. The transition's location and width depend on actual printed geometry and registration.
Define which part of the footprint determines the usable boundary for the application. Inspect processed edges and the assembled path rather than relying only on a nominal centerline. If the contact partly leaves the intended region, local force and output continuity can change. An apparently generous centerline margin may be insufficient for a broad or tilted contact.
Combine stop, card and wiper location errors
The minimum end margin depends on card location, track registration, wiper position and mechanical-stop variation. Use a consistent coordinate and sign convention to combine them. A nominal gap between the stop and the track boundary is not the guaranteed margin when these contributions move in unfavorable directions.
For a hypothetical linear mechanism, suppose the nominal contact-center margin is one millimeter. A 0.2-millimeter card shift, a 0.15-millimeter track shift and a 0.25-millimeter stop overtravel could reduce it to 0.4 millimeter before the contact half-width is considered. This arithmetic illustrates a geometric review; it is not a recommended allowance or a manufacturing tolerance.
Mmin = Mnom − Σδtoward-edge − Hcontact
- Mnom is nominal center-to-boundary margin at the limiting mechanical position.
- δtoward-edge are the selected adverse location contributions.
- Hcontact is the contact extent toward that boundary; Mmin is remaining footprint margin.
The example uses a one-dimensional conservative stack; rotary, tilted or correlated geometry requires the appropriate spatial analysis.
Specify the expected output in every end region
State whether the output should plateau, continue changing within a wider tolerance, enter a defined diagnostic range or remain outside the permitted operating travel. If contact can open, the observed receiver voltage depends on its input circuit and bias. The card alone does not determine the disconnected voltage.
Do not extend a central linear equation into an end region without checking the physical path. A plateau can be acceptable for a defined function but provides little or no position information there. If the system needs to distinguish several positions near the stop, the active region and local slope must support that requirement.
Choose checks according to the end behavior
Test both ends independently because their conductor geometry, direction of approach and mechanical constraints may differ. A satisfactory result at one end does not establish the other.
| End-region behavior | Required definition | Verification focus |
|---|---|---|
| Output follows the calibrated curve to the stop | Valid domain and endpoint tolerance | Worst-case contact position and full-footprint coverage |
| Output plateaus before the stop | Plateau range and allowed resistance or voltage | Continuity and receiver interpretation through remaining travel |
| Output remains continuous but nonlinear | Transition region and permitted error | Dense position sampling across the transition |
| Contact may leave the track outside valid travel | Mechanical exclusion and receiver condition | Actual travel limit and input behavior on contact loss |
| Overtravel is possible during assembly or operation | Permitted mechanical range and contact response | Tolerance stack and physical inspection after the event |
| Reverse motion restores contact differently | Directional behavior near the boundary | Slow forward/reverse sweeps and raw signal capture |
Sweep the ends with sufficient spatial and temporal detail
Use slower controlled motion and denser position sampling through the transition regions than through a smooth central section where appropriate. Record raw output and position with synchronized timing. A brief interruption can disappear in an averaged reading, while a timing offset can shift the apparent boundary.
Approach each end from within the active region, dwell if relevant and reverse direction. Repeat at the permitted mounting and wiper-force extremes using a controlled fixture. Record the actual mechanical stop position rather than only its command. Preserve electrical and visual observations before changing the stop or re-aligning the card.
Separate a dead zone from contact or wiring faults
A stable plateau at a repeatable location may be an intended conductor region. An erratic output near the same location can indicate edge departure, contact unloading or contamination. A receiver saturation that occurs everywhere may instead come from a wiring or input-circuit issue.
Compare end-to-end track resistance, moving output and the receiver's defined open-input behavior where the test setup permits. Inspect the contact path at the position where the discrepancy begins. Avoid assigning every end-region jump to an inherent dead zone; the actual card geometry may be continuous while the installed wiper is misaligned.
Keep electrical travel and mechanical protection linked
The final interface drawing should show the calibrated region, continuity boundary, wiper footprint and mechanical stops in one coordinate system. Include tolerances and the output behavior through the permitted end travel. The receiver and mechanical design should use the same definitions.
For quotation, provide the required useful travel and any assembly or operating overtravel. Explain how the system uses the end output and which positions must remain distinguishable. This supports a focused card and fixture review. Defining continuity at the travel limits contributes to the system design, but it does not establish the mechanical system's safety or fault response on its own.
Send the complete travel-boundary drawing
Include the contact footprint and stop tolerances so end-region behavior can be evaluated.
- Calibrated electrical range, continuity range and mechanical travel in one coordinate.
- Wiper footprint, force geometry and processed track or conductor boundaries.
- Card location, registration, stop and overtravel tolerances.
- Required output behavior in each end region and receiver input circuit.
- Dense forward/reverse endpoint sweeps with synchronized position and raw output.
The drawing-upload form loads as you reach this section.