Hydraulic pressure sensor noise in PLC inputs: Troubleshooting guide

In a lot of industrial hydraulic systems, the signal noise is generally due to grounding, wiring, or shielding interference rather than the PLC itself.

An example of a typical field can be an analog cable that is routed in parallel to the motor of a VFD inside this tray. The pressure pattern could appear uncertain even though the hydraulic system is constant. The reason is usually a problem with the cable routing, not a malfunctioning controller.

This article provides teams of maintenance and control teams with a method to identify the issue and determine the cause and then restore a stable measurement.

What is the noise of a pressure signal? It appears to be

Common signs

• Readings fluctuate at constant pressure in the hydraulic system.
• Value increases during motor start-up and valve switch-over, as well as closure of a contactor.
• Signal degradation in VFDs, servo drives, or solenoids
• A noisy or intermittent PLC trend curve
• An alarm that is intermittently activated in normal process operation

Why is it important?

Unstable readings can affect control accuracy and create false machine stops and make PID tuning ineffective and hide the actual hydraulic problems in the guise of electrical interference.

Learn the signal's path prior to troubleshooting

Every measurement chain has the pressure gauge and a cable and an analog input to a PLC. Noise may enter any time in the chain. The most commonly used signals include 4-20 milliamps, 0-10V, and low-level outputs of mV/V. Hydraulic automation loops with 4-20 mA are typically preferred since they can withstand electrical interference better than voltage signals that are spread over longer cables.

Start with the most basic checks first.

• Verify that the output type of the sensor corresponds to the input module for PLC setting.
• Verify the scaling of the PLC program. An incorrect engineering unit conversion may result in artificial drift.
• Compare the live reading to the range of sensor datasheets.
• Examine connectors, terminals, cables, and jackets of cable for loose screws or crushing damage.

Common reasons for high pressure in the hydraulic sensors the noise of PLC systems

If the initial tests are not able to solve issues, then the primary reason is typically one or one of the below: electromagnetic interference from VFDs, servo amplifiers, or contactors; inadequate protection of the ground; mistakes in grounding like shields that are left floating or not properly terminated and cable routing along power conductors; unstable supply voltage shared by inductive loads; or unbalanced output from the sensor with an input module for the PLC input module. A well-chosen sensor can help; however, the quality of installation affects the performance in the real world.

A sequence for troubleshooting that is practical

1. Identify the cause

Find out if the noise is continuous or occurs only at specific times during machine instances. Note the operating conditions along with the load's status as well as any nearby electrical activity.

Second step: Test the sensor in isolation

Check the output at the sensor's terminals with the aid of a multimeter. Then, measure it at the input of the PLC. An undistorted signal from the sensor and a noisy signal downstream at the PLC point.

Third step: Examine the installation and cable

Verify for damage to the mechanical and verify the shield's termination and trace the cable's route for long parallel runs alongside motor or power wiring.

Step 4: Review settings for the PLC configurations of the module's input.

Examine your input frequency, frequency of the filter, and the averaging options. Compare the raw count jitter with the expected stability of the signal for the particular application.

5. Check the power of your system and check for grounding

Check the voltage of the supply when the system is in use, and ensure that the return signal isn't being shared with a high-current inductive return. Examine possible differences between the sensor's grounded body and PLC ground.

Table of Diagnostics

How can you reduce noise after the reason is identified?

Enhance cabling and grounding as well as filtering

Utilize a shielded twisted-pair cable to keep it physically separate from power wiring and connect the shield at one point only, usually on the PLC cabinet ground bus. If the environment remains rough, the external signal isolation or conditioner could be used to provide galvanic isolation as well as additional filtering. The majority of PLC analog input modules provide adjustable low-pass filters that can help reduce noise at high frequencies but without significantly impacting control response.

For specific installation procedures, look up IEC 60204-1 and wiring instructions for the control panel provided by Siemens as well as Rockwell Automation.

Don't ignore the hydraulic aspect

Pressure pulsation caused by valve chatter, pumps, or lines that are poorly dampened produces a real pressure ripple that sensors will accurately report. Check with a mechanical gauge or a high-speed logger prior to accusing the electronic components.

If you suspect the PLC module or sensor itself

If the noise is detected at the sensor's terminals when the measurement is independent, either the device or the local source is a possible source of the problem. If the noise persists on one PLC channel even when sensors are swapped, then the input module or wiring path may be the issue.

If the sound is coming from the sensor, it could be the sensor. If it remains with the channel, you should suspect the wiring or module.

Preventive measures for longer-term stability of signals

• The output type of the sensor should be matched to that of the PLC input module prior to wiring.
• Make sure analog signal cables are physically separate from power wiring.
• Create and document the shielding and grounding scheme once, then shield it from accidental modifications.
• Review panel layouts during engineering, not immediately after commissioning.
• Document scaling, filtering, and wiring protocols to maintain teams.
• Check the cabinet bonding the shield clamps, as well as connector torque, when scheduled maintenance is planned.

Stable pressure readings don't result from a single element but are derived from an appropriately managed measurement chain. When you consider the noise as a systemwide issue and follow a logical diagnosis sequence, you can resolve it quicker and avoid blame on the PLC for the problems it hasn't caused. For the realm of hydraulic automation, well-planned wiring and grounding are just as crucial as a well-designed hydraulic design.