Sensor-Card Terminals: Cable Loading and Contact-Resistance Error

Draw the full signal path to the receiver

Include every terminal, joint, connector and cable conductor in the schematic used for interface analysis. Mark the excitation and return paths separately from the wiper-output path. A shared return can carry other currents, adding a voltage difference that is unrelated to the resistor curve.

Define the location of the acceptance measurement. An incoming card test at printed pads characterizes a different boundary from an assembled harness test at the controller. Both can be useful, but their limits should agree with the allocation of interface error. Without that boundary, two valid measurements can appear to contradict each other simply because they include different parts of the circuit.

Calculate series error in a resistance-reading connection

In a simple two-wire resistance measurement, lead and contact resistance add to the sensor resistance. Their fractional effect is largest where the sensor value is smallest. A constant added resistance therefore distorts a nonlinear level or position interpretation differently across travel.

For a hypothetical sensor value of 20 ohms and combined harness/contact resistance of one ohm, the added contribution is five percent of the sensor value. At 200 ohms it is 0.5 percent. Subtracting a fixed offset may help only if the contribution remains sufficiently stable with temperature, connector condition and motion. It does not remove variable contact resistance or a shared-return voltage.

Evaluate the wiper load in a voltage-divider connection

For a three-terminal divider, the receiver input creates a load on the lower and upper track portions according to the circuit connection. The wiper contact and output lead can add further series impedance. Their effect differs from the simple two-wire resistance sum, so use the actual loaded network rather than reusing a resistance-offset correction.

Wiper loading can degrade linearity and increase the effect of contact-resistance variation. A custom card needs a calculation using its own total resistance and receiver. No universal load ratio should replace that analysis. Include input bias, protection components and any multiplexed acquisition behavior that changes the load over time.

Check settling when the harness adds capacitance

A cable and receiver input add capacitance to the output node. Together with the position-dependent source resistance, this can slow the signal response. A card may appear accurate in a static reading while showing lag during motion or after an acquisition channel switches onto it.

For a simple assumed source resistance of ten kilohms and load capacitance of one nanofarad, the time constant is ten microseconds. The time needed for a particular accuracy is longer than one time constant and depends on the allowed residual error. Use the actual circuit and position-dependent impedance, then verify settling with the intended cable, receiver and sampling schedule.

Select measurements that locate the interface contribution

Compare the signal at successive boundaries while preserving loading as closely as possible. Adding a probe can itself change the circuit, so account for its input characteristics. The objective is to identify which interface introduces the discrepancy.

Keep cable force from becoming an uncontrolled terminal load

Identify how cable tension, bending and vibration reach the terminal. A strain-relief feature should transfer the intended mechanical loads into a suitable support rather than relying on the printed conductor or brittle substrate alone. The actual arrangement depends on the card and enclosure; do not infer a load capability from terminal appearance.

Show cable routing and the first support point on the assembly drawing. A short unsupported lead can apply a different bending moment from a longer flexible loop. Evaluate thermal movement and assembly handling as well as normal operation. If the connection process includes soldering, welding, adhesive or clamping, retain the relevant process and material identification in the interface review.

Observe electrical output during mechanical evaluation

A static pull or visual inspection can miss an intermittent electrical change. Where mechanical testing is part of the agreed validation, monitor continuity or output while applying the defined load and motion. Record the timing and position of any event so it can be connected to the physical interface.

Measure the baseline and final curve under the same conditions, and inspect the original terminal before rework. A connection that recovers after unloading may still have produced unacceptable interruptions during use. Conversely, a change in probe contact during the mechanical test can mimic a product event. Secure the measurement leads independently and include a fixture check that distinguishes these possibilities.

Specify the delivered interface and its error allowance

Provide terminal geometry, attachment process, harness length and conductor information with the target card curve. State receiver input behavior and the point where the system requirement is measured. Allocate permissible electrical contribution and mechanical loading to the interface using project-specific evidence.

For quotation, include photographs or drawings of the intended cable support and connector arrangement. If the harness or receiver is still undecided, preserve those as explicit design inputs to resolve before final validation. A resistor card can be characterized at its pads, but predictable installed output requires the surrounding electrical and mechanical path to be defined as well.