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Two tracks provide two output signals, but their usefulness for fault detection depends on the relationship expected between them and the faults the surrounding system can observe. The card drawing should define both functions against the same mechanical coordinate, while the receiver defines how to compare them under supply, temperature, loading and timing variation. A correlation window is therefore a system-derived tolerance, supported by paired measurements and a documented diagnostic model.
Key design decisions
- Specify each channel's output function, valid range and mechanical index before choosing a comparison rule.
- Separate ordinary tracking variation from faults, supply effects and acquisition timing errors.
- Assign diagnostic thresholds and reactions through the equipment's system analysis; dual tracks alone do not establish functional-safety compliance.
Define both channels against one travel coordinate
Write channel one and channel two as functions of the same angle or displacement. They may increase together with different slopes, or one may decrease while the other increases. State the endpoints, offsets, valid electrical range and expected behavior outside the active travel. Avoid describing the pair only as redundant without defining the relationship.
Include terminal assignment, excitation and output loading for each channel. If the channels use separate supplies or references, normalize them appropriately before comparison. A shared mechanical coordinate does not imply identical electrical scaling. The receiver must know which transformation makes the two outputs comparable and which measured quantity each channel actually represents.
Compare inferred position or a defined signal residual
For a linear channel, normalized position can be calculated by subtracting the low-travel output and dividing by the output span. Apply the corresponding transformation to the other channel, preserving the sign of a decreasing span. Their difference is a position-correlation residual. Nonlinear functions require the specified inverse mapping or a direct paired-output relation.
Consider hypothetical channel functions V1 equal to 0.5 plus four times normalized travel, and V2 equal to 0.25 plus twice normalized travel. Both infer the same position when their own offset and span are used. Comparing raw voltages would always show a difference and would not be a useful fault criterion. These values demonstrate scaling only and are not a pedal specification.
x1 = (V1 − a1)/b1; x2 = (V2 − a2)/b2; e = x1 − x2
- ai is the channel output at the defined zero position.
- bi is the signed output span over normalized travel from zero to one.
- e is the normalized position disagreement between channels.
Each channel follows the stated linear mapping and bi is nonzero; nonlinear, clipped or disconnected outputs need separate handling.
Build the correlation window from differential errors
Include curve conformity, mechanical registration, contact behavior, receiver accuracy, supply measurement and temperature effects. Some errors move both inferred positions together and may cancel in the residual. Others affect one channel differently and enlarge it. Preserve these relationships when constructing the budget rather than simply summing every absolute specification.
Measure the residual over travel, direction, temperature and relevant electrical states. A constant window may be convenient, but a position-dependent limit can better represent regions with different slopes or tolerances if the system design supports it. The chosen limit must leave an appropriate distinction between normal variation and the faults the system is required to detect; that distinction cannot be derived from nominal curve shape alone.
Control the time difference between channel samples
Two channels sampled at different times can disagree during motion even when both are correct. The approximate position difference is travel speed multiplied by sample-time separation. At an assumed normalized speed of two full travels per second, a five-millisecond separation creates a one-percent-travel difference.
Use synchronized acquisition or a justified timing model, and include filtering delay. Different filter responses can create a transient residual when motion accelerates or reverses. Record raw and processed channel signals during dynamic evaluation so the origin of a diagnostic event can be traced. Increasing a threshold to hide acquisition skew may also reduce sensitivity to a real channel error.
Map faults to observable signals
Signal conversion and redundancy monitoring have separate roles: one scales the primary output, while the other compares a second input against defined fault criteria. For a passive resistor card, define the actual receiver and fault cases before deciding what information the paired outputs can provide.
| Condition to evaluate | Potential observation | Additional information needed |
|---|---|---|
| One output opens | Output may move toward a receiver-defined bias | Receiver pull resistors, leakage and valid-range limits |
| One output shorts to a supply or return | Channel may saturate outside its intended range | Actual supply levels and input protection behavior |
| Channels short together | Raw relationship may collapse or become deceptively similar | Channel functions, output impedances and shared wiring |
| One track develops local discontinuity | Brief or position-specific residual excursion | Motion speed, bandwidth and contact observation |
| Shared supply changes | Both signals may shift together | Measured excitation and normalization architecture |
| Mechanical coupling shifts both channels together | Correlation may remain plausible | Independent system information and mechanical fault analysis |
Identify what the two tracks still share
Two printed tracks may share a substrate, mechanical support, wiper carrier, connector or environmental exposure. A common displacement or contamination event can influence both. If both outputs remain mutually consistent while representing the wrong mechanical state, a channel-correlation test alone may not detect the problem.
Document those shared elements in the system review. The card design can provide defined separation, terminal arrangement and paired-output behavior, while the equipment designer evaluates the complete sensing and reaction architecture. Do not equate the number of tracks with independence. The meaningful question is which faults lead to distinguishable information under the actual electrical and mechanical configuration.
Retain paired measurements throughout the validation sequence
Capture both channels at the same coordinate and time reference during forward and reverse sweeps. Record absolute channel errors as well as their correlation residual. A small residual can coexist with a common error in both channels, so reporting the residual alone is insufficient to characterize the card.
Repeat the sequence after relevant environmental and wear exposures, preserving the original pairing. Include receiver loading and supply conditions representative of the intended interface. If diagnostic fault insertion is part of system validation, use a controlled test setup and documented electrical models so the observed response can be attributed to the intended fault. The outcome should identify observed behavior and coverage limits, not assume every fault is detectable.
Keep component outputs and system reactions distinct
The card specification should define output functions, tolerances, shared mechanical reference, terminal arrangement and the conditions used to verify them. The receiver specification should define valid ranges, comparison logic, timing, persistence and response to diagnostic states. These documents need consistent units and revisions.
For quotation, supply the two curves and the receiver model along with the relevant system requirements. Thresholds, fault-detection coverage and the equipment's safe response require the responsible system engineering process. A well-defined resistor card provides measurable inputs to that process; it does not establish a universal correlation percentage or functional-safety rating merely by providing two tracks.
Send both channel functions and the receiver model
Provide the paired-output definition so electrical tracking and diagnostic inputs can be reviewed together.
- Channel curves, endpoint ranges, mechanical index and positive travel direction.
- Separate or shared excitation, return paths and receiver input circuits.
- Paired sweep data with position, timestamps, temperature and loading.
- Required dynamic behavior, sampling interval and filtering delays.
- System-defined fault cases, correlation requirements and validation ownership.
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