Accelerometer and vibration sensors in NVH testing

In NVH testing, accelerometers are one of the key devices for collecting vibration data on structures. From modal analysis and vibration measurement during vehicle operation to determining vibration pathways, the signal quality from the sensor directly affects the analysis results.

A suitable sensor needs to have more than just the right measurement range. Sensitivity, frequency range, sensor weight, installation method, and environmental conditions must all be considered from the measurement design stage.

1. What is an accelerometer and why is it important? 

An accelerometer is a device that measures acceleration at a point on a structure. In NVH testing, the acceleration signal is used to assess vibration levels, determine frequency components, and track the path of vibration from the excitation source to the user's perceived location.

Each vibration source on a vehicle, such as the engine, transmission, suspension, or electric motor, has different vibration characteristics. By taking measurements at multiple locations and synchronizing the data, engineers can identify the vibration source, transmission path, and structural location with a high response.

2. Operating principle

Most accelerometers used in NVH (Non-Vehicle-Based Vehicles) operate on the piezoelectric principle. When the structure vibrates, the piezoelectric element inside the sensor generates an electrical signal proportional to the applied acceleration. This signal is fed into a data acquisition system for amplification, modulation, and conversion into digital data.

In addition to piezoelectric sensors, MEMS sensors are also used in a number of applications due to their small size, low weight, and ability to be deployed in large quantities.

The choice of sensor technology should be based on the frequency range, vibration level, environmental conditions, and specific measurement method.

3. Some notable sensor lines from Kistler

3.1. General-purpose Triaxial Accelerometer (IEPE) Sensors

The triaxial sensor measures simultaneously in three directions: X, Y, and Z.

In NVH testing, triaxial sensors are often used when a comprehensive assessment of the vibration response of a single measurement point is required. Compared to using multiple single-axis sensors, this configuration reduces the number of sensors and installation time.

This is also a suitable option when analyzing complex vibration patterns or structures that do not have a single vibration direction.

• - Kistler 8763B : The most popular 3-axis (X, Y, Z) vibration sensor from the company. It features a lightweight, hermetically sealed titanium housing for excellent water resistance. According to Kistler's test data, the 8763B boasts extremely low background noise (as low as 0.00029 g), making it ideal for measuring micro-vibrations on thin, lightweight structures.
• - Kistler 8764B : This version features an integrated central threaded hole. This design allows engineers to freely rotate the sensor body 360° to orient the output cable flow as desired, offering exceptional flexibility.

3.2. High Temperature Sensor Series

When vibration testing is required in engine combustion chambers, exhaust systems, or harsh industrial environments, Kistler utilizes PiezoStar crystalline material with superior heat resistance.

Kistler 8768A : A 3-axis IEPE accelerometer specifically designed for high-temperature environments. Thanks to PiezoStar crystal technology combined with a hybrid circuit, the device maintains absolute sensitivity stability and data integrity even with sudden temperature changes.

3.3. Low-frequency capacitive sensors (MEMS Capacitive K-Beam)

For applications involving gravitational acceleration (DC) and very low-frequency vibrations, such as in automotive and train dynamics testing or bridge and road structure monitoring, conventional piezoelectric sensors are inadequate. Kistler solves this problem with MEMS technology.

Kistler 8396A : A high-end 3-axis accelerometer sensor utilizing proprietary K-beam capacitive MEMS microelectromechanical technology . The device operates stably over long periods at low frequencies (down to 0 Hz), enabling three-dimensional motion analysis with low noise and superior linearity.

3.4. General Purpose / Multipurpose Single-Axis Sensors

Single-axis sensors measure acceleration in only one direction – typically used when the direction of vibration is predetermined, for example, measuring vertical vibration on the vehicle floor or axial vibration on the cardan shaft.

This is a line of sensors designed for basic vibration measurements in machinery, rotating motors, or frequency response analysis of structures.

Kistler 8702B / 8704B : Single-axis accelerometer sensors designed according to K-Shear technology . Extremely durable stainless steel housing, wide measurement range (25g to 500g), and side connector for optimal installation height.

4. Parameters to consider when selecting a sensor

Selecting a sensor based solely on a single parameter, such as frequency range or sensitivity, can lead to measurement inaccuracies. Several parameters should be considered, including:

• - Sensitivity: Measured in mV/g or pC/g - highly sensitive sensors are suitable for measuring small oscillations, but easily saturate when measuring large vibrations.
• - Frequency range: The frequency range of the phenomenon being measured must be sufficient - for example, measuring engine vibration requires a low frequency range (below 500 Hz), while measuring brake squeal requires a frequency range up to tens of kHz.
• - Measurement range: In grams - a suitable measurement range should be selected to match the actual oscillation amplitude, avoiding signal clipping or loss of resolution.
• - Sensor mass: This is especially important when measuring on lightweight structures such as thin sheets, plastic parts, or furniture components. Sensor mass can alter the vibration characteristics of the structure.
• - Heat and environmental resistance: For measurements near the engine or braking system, the sensor needs to withstand high temperatures and oily environments.

5. Sensor applications according to NVH source groups

6. The role of accelerometers in modal analysis

In experimental modal analysis, the accelerometer sensor captures the structural response when the system is excited by a hammer impact or a vibratory device.

The data obtained was used to determine:

• - Natural frequency of the structure
• - Damping coefficients of oscillation modes
• - Oscillation waveform at each frequency

The number and location of measurement points directly affect the ability to describe the vibration pattern. If too few points are used, complex vibration patterns may not be fully identified. Conversely, too many measurement points will increase the time required for setup, data collection, and processing.

Therefore, sensor placement should be designed in conjunction with the measurement model and analysis objectives from the outset.

7. Common problems when using accelerometer sensors

• - Incorrect installation technique: Using wax mounting for high-frequency measurement is wrong, as it reduces the sensor's actual frequency response range.
• - Neglecting periodic calibration: Sensor sensitivity can shift over time if not calibrated, leading to accumulated errors throughout the measurement series.
• - The effect of mass loading is not taken into account: This is especially serious when measuring on plastic or thin sheet materials in the car's interior.
• - Electromagnetic signal interference: Particularly important when measuring near electric motors or power converters on electric vehicles - shielded cables and sensors with good interference rejection capabilities are necessary.

GPower Vietnam - Partner providing specialized accelerators for NVH testing.

In practical testing, there is no single accelerometer sensor that is suitable for all measurements.

A sensor used for modal analysis on an interior panel will have different requirements than a sensor placed near the engine. Similarly, measuring high-frequency vibrations of the braking system or electric motor will require a different configuration than measuring low-frequency body vibrations.

Therefore, the selection process should begin with the question of what phenomenon needs to be measured, at what location, under what conditions, and to what extent analysis is required. Only then can the type of sensor, measurement range, frequency range, sensitivity, installation method, and data acquisition configuration be determined.

GPower Vietnam provides and advises on sensor solutions for NVH testing, from sensor selection and measurement system configuration to integration with data acquisition systems and analysis software.

Contact GPower Vietnam for advice on configuring sensors to suit your measurement requirements and actual test conditions.

Refer to related articles on NVH testing:

• - NVH anechoic chamber

• - Noise and vibration measurement equipment for NVH testing

• - Head Acoustics acoustic camera system

• - Dytran accelerometer

• - Kistler multi-axis force and torque sensors

• - NVH testing