Radar Display Is Stable but the PLC Value Jumps: A Diagnostic Guide

When the radar display is stable but the PLC value jumps, the measurement echo is not the first suspect. The fault is more likely between the transmitter output and the value used by the control program. Diagnose the chain in order: local measurement, configured output, field wiring, input card, raw PLC count, engineering scaling and application logic.

Prove that the radar value is genuinely stable

Observe distance, level, echo quality and device status over the same period as the PLC disturbance. A heavily damped local display can look stable while the raw measurement changes, so review diagnostics or a high-resolution trend rather than relying on one screen. Save the echo curve if lost-echo or false-echo events appear.

Confirm that local distance and configured level use the expected datum. If both remain stable while the reported output changes, continue downstream instead of remapping echoes or changing antenna parameters.

Compare output current with PLC raw counts

Measure loop current at an approved test point or read the transmitter's output diagnostic. Trend it beside the PLC raw analog count. Stable current with changing raw counts points toward the input channel, wiring near the cabinet, reference grounding or card configuration. Changing current with stable local level points toward output settings, power margin or device fault.

Do not judge only from scaled engineering units. Raw counts reveal whether a software conversion, range limit or data-type problem creates the apparent jump.

Check power, resistance and shielding

Measure voltage at the transmitter terminals under normal and high-current conditions. Include barriers, isolators, indicators, cable resistance and input resistance in the loop budget. Marginal voltage can cause resets or current limiting when the output rises.

Inspect polarity, loose terminals, moisture, shield continuity and routing near variable-frequency drives or motor cables. Follow the site's grounding design; adding an unplanned second shield bond can create circulating noise rather than remove it.

Verify the PLC input and program

Confirm that the channel is configured for active or passive 4–20 mA as required, not voltage or another range. Check update rate, digital filtering, broken-wire detection and channel diagnostics. Swap to a proven channel only under a controlled change and preserve the original configuration.

Review scaling endpoints, integer or floating-point conversion, alarm substitutions and moving averages. A program may replace bad quality with zero, reuse stale data or overflow an integer even when the electrical loop is healthy.

Reproduce and document the fault

Use a calibrated loop source to drive 4 mA, intermediate values and 20 mA into the PLC channel. Separately simulate the transmitter output if supported. This divides the system into testable sections and prevents replacing a good radar.

Record timestamps, local value, loop current, raw input count, scaled value and equipment operating state. Include motor starts and cabinet temperature where the symptom is intermittent. Acceptance is complete when the repaired loop remains stable through representative disturbances.

Engineering checklist

• Trend local distance, level, echo quality and output diagnostic.
• Compare measured current with PLC raw counts at the same timestamp.
• Verify terminal voltage and total loop resistance.
• Inspect cable routing, shield continuity, grounding and moisture.
• Confirm input type, range, filtering and diagnostics.
• Test the PLC channel with a calibrated current source.

Frequently asked questions

Should radar damping be increased?

Only if the actual process measurement is unstable. Damping cannot correct an electrical or PLC input fault and may delay alarms.

Why does the jump occur when a motor starts?

Investigate supply dips, electromagnetic coupling, grounding and cable routing. Compare raw current and PLC counts during the event.

Can swapping the input card prove the cause?

It can help, but first record the fault and configuration. A controlled current-source test usually separates card, wiring and transmitter more clearly.

Worked example: isolate a cabinet-side disturbance

Consider a tank radar whose local value remains at 62.4%, while the PLC trend briefly moves between 59% and 66% whenever a large motor accelerates. The first useful test is simultaneous recording, not parameter adjustment. Capture radar distance, configured output current, measured loop current at the cabinet, input-card raw count and scaled PLC value with synchronized timestamps.

If the transmitter reports and the meter confirms a steady 13.98 mA while the raw count moves, the fault is downstream of the field device. Inspect the cabinet-side shield termination, analog input common, channel mode and cable separation from the motor drive. If measured current also moves but radar distance remains steady, inspect loop power, isolation and the transmitter output stage. If radar distance moves first, return to echo and process diagnostics.

After correcting the cause, repeat the motor start at several loop outputs. One successful test at mid-scale is not sufficient because a marginal voltage budget may fail only near 20 mA. Record the before-and-after raw counts and confirm that the PLC alarm logic does not substitute a default value during a short quality fault.

Acceptance evidence for the signal chain

A useful handover sheet lists the transmitter range, output mode, fault current, supply voltage, total series resistance, cable and shield termination, input-card type, raw-count endpoints and PLC engineering scale. Attach the loop-calibrator results at 4 mA, 12 mA and 20 mA and note the observed values at the local display and operator interface.

When the symptom is intermittent, state the observation period and the operating events included. A loop proven only while nearby equipment is stopped has not demonstrated immunity under production conditions.

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