Understanding Triboelectric Noise

Definition: What is Triboelectric Noise?

Triboelectric noise is a form of electrical interference that can contaminate vibration signals, particularly those from piezoelectric accelerometers. It is a low-frequency noise signal that is generated when the sensor cable itself is subjected to mechanical motion, such as bending, flexing, or being impacted. This noise is not related to the actual vibration of the machine being measured and can therefore lead to significant measurement errors if not properly controlled.

The Cause: The Triboelectric Effect

The noise is generated by the triboelectric effect, a phenomenon where static charge is created when two different materials are brought into contact and then separated. A typical coaxial accelerometer cable consists of a central conductor, a dielectric (insulating) layer, and an outer shield.

When the cable is flexed, the dielectric and the outer shield rub against each other, generating a small amount of static charge. This charge creates a voltage that is picked up by the highly sensitive amplifier connected to the piezoelectric sensor. The result is a low-frequency “rumble” or spurious voltage spike in the vibration signal that has nothing to do with the machine’s vibration.

Characteristics of Triboelectric Noise

• Low Frequency: The noise is predominantly a low-frequency phenomenon, typically occurring below 10 Hz.
• Spurious Spikes: It often appears as random, high-amplitude spikes in the time waveform.
• “Ski-Slope” Spectrum: In an FFT spectrum, it can contribute to a “ski-slope” effect, where the noise floor is very high at the low-frequency end of the spectrum and then drops off.
• Not Repeatable: The noise is not synchronous with the machine’s rotation and is not repeatable between measurements.

Why is it a Problem?

Triboelectric noise can mask the real low-frequency vibration signals from a machine. This is a significant problem when analyzing slow-speed machinery, where important diagnostic information (like unbalance and misalignment at 1X RPM) is located at these low frequencies. The noise can either hide these real signals or be mistaken for them, leading to a misdiagnosis.

How to Prevent Triboelectric Noise

Preventing triboelectric noise is achieved through proper cable selection and installation techniques:

1. Use High-Quality, Low-Noise Cable: Reputable sensor manufacturers offer “low-noise” cables. These cables have a special conductive graphite layer applied between the dielectric and the outer shield. This layer acts as a drain, preventing the buildup of static charge and dramatically reducing the triboelectric effect.
2. Secure the Cable: This is the most important practical step. The cable should be firmly tied down or glued to the machine surface as close to the sensor as possible. This ensures that the cable vibrates *with* the machine instead of flexing and moving independently. Any loose loops or dangling sections of cable are prime sources of noise.
3. Avoid Cable Impacts: Ensure that the cable is routed away from any sources of impact or whipping, such as rotating shafts or other moving machine parts.
4. Proper Grounding: While not directly related to the triboelectric effect, ensuring the sensor and cabling are properly grounded according to the manufacturer’s instructions is essential for preventing other forms of electrical noise, such as ground loops.

By using the correct type of cable and, most importantly, ensuring it is properly secured, the effects of triboelectric noise can be almost completely eliminated, leading to cleaner and more reliable low-frequency vibration data.

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