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Accelerometers: A comprehensive guide to applications and techniques in vibration measurements

Accelerometers are essential transducers used to measure vibration and acceleration in various applications, including automotive safety, consumer electronics, and industrial machinery. This guide covers the selection, mounting, calibration, data collection, and analysis techniques necessary to effectively use accelerometers for precise vibration measurements.

What is an accelerometer?

An accelerometer is a transducer that measures vibration in terms of the acceleration of motion of a structure. It operates by converting vibration into a proportional electrical signal, based on Newton’s Second Law of Motion, which states that acceleration (m/s²) is directly proportional to the net force (Newton) acting on a body and inversely proportional to its mass (gram). Essentially, an accelerometer measures acceleration indirectly by detecting the force applied to one of its axes. A common sensing approach used in accelerometers is capacitance sensing, where acceleration is related to the change in capacitance of a moving mass. The transduction method depends on the accelerometer type, with different mechanical constructions for piezoelectric and MEMS accelerometers.

accelerometer
mems-accelerometer
hand-arm-vibration-accelerometer

What are the types of accelerometers?

The main types of accelerometers include:

  • Piezoelectric accelerometers use a sensing crystal to which a seismic weight is attached. When the sensor experiences acceleration, the weight exerts force on the crystal, generating an electrical charge proportional to the force and, thus, the acceleration. This high-impedance signal can be measured directly or amplified and conditioned by electronic circuits. They are widely used in machine vibration measurements.
  • MEMS (Microelectromechanical systems) accelerometers are built using microfabrication technology, consisting of tiny mechanical components on a silicon chip. A common sensing mechanism involves capacitance sensing, where changes in the position of a micro-machined mass cause variations in capacitance. When the body of the accelerometer is moved by an externally applied force, the motion of the seismic mass is detected by differential capacitive sensing elements. The signal produced is amplified, conditioned, and filtered by circuit components mounted inside the same IC package. MEMS accelerometers are widely used in consumer electronics, for measuring human vibration, and for low-frequency ground vibration.
  • Piezoresistive accelerometers use materials that change electrical resistance when subjected to mechanical stress. They work similarly to strain gauges, measuring stress. A force applied to the piezoresistive material deforms it, changing its resistance. This is measured using resistive elements arranged in a bridge configuration. Piezoresistive accelerometers have high bandwidth and low sensitivity, making them suitable for high-frequency, short-duration measurements, such as crash or weapons testing. They are essential in automotive crash test applications.
  • Strain gauge accelerometers use strain gauges attached to a structure that deforms under acceleration. This deformation changes the resistance of the strain gauges, which is measured using a bridge circuit. A modern strain gauge accelerometer typically uses a silicon or foil strain gauge deposited on or bonded to an element that flexes with the movement of a seismic mass. These accelerometers can measure accelerations down to zero Hertz, making them suitable for static measurements. They are widely used in the automotive industry for safety applications such as anti-lock braking systems, airbag deployment, and traction control systems.

Accelerometers are important because they provide precise measurements of acceleration and vibration, which enhance safety, improve navigation and control, enable advanced functionalities, facilitate predictive maintenance, support seismic monitoring, and assist in medical applications. These capabilities make accelerometers indispensable across various industries, ensuring efficient, reliable, and safe operations in the automotive, aerospace, consumer electronics, industrial machinery, structural health, and healthcare sectors.

A vibration accelerometer can measure:

  • Acceleration: The rate of change of velocity, which is the primary measurement provided by accelerometers.
  • Velocity: By integrating the acceleration data, it is possible to derive the velocity of the vibrating object.
  • Displacement: By further integrating the velocity data, the displacement of the vibrating object can be calculated. This is particularly useful in assessing the extent of vibration in mechanical systems.
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What are the main applications of accelerometers?

Accelerometers are crucial in designing, operating, and maintaining systems across various industries. Different types of accelerometers are selected based on their specific capabilities and the requirements of the application:

  • Designing: In the R&D phase, accelerometers are used to test and validate designs. For automotive safety systems, MEMS and piezoresistive accelerometers test components like airbags, anti-lock braking systems (ABS), and traction control systems. Piezoresistive accelerometers handle high-frequency impacts in crash testing. In consumer electronics, MEMS accelerometers enable features like screen rotation, motion gaming, and fitness tracking, providing critical data for designing user interfaces. In aerospace and defense, piezoelectric and MEMS accelerometers help design navigation systems, stability control, and structural health monitoring by measuring high-frequency vibrations and inertial navigation parameters.
  • Operation: During operation, accelerometers provide real-time functionality and safety features. In automotive applications, MEMS accelerometers detect rapid deceleration to deploy airbags. In consumer electronics, MEMS accelerometers rotate screens in smartphones and tablets, and measure body vibrations for health and ergonomic assessments. For seismic activity monitoring, MEMS and capacitive accelerometers detect and record ground movements, providing high sensitivity and wide frequency range measurements. In industrial machinery, piezoelectric accelerometers monitor and analyze vibrations for operational stability, detecting imbalances and potential failures.
  • Maintenance: For ongoing maintenance, accelerometers monitor the health and performance of systems. In predictive maintenance, piezoelectric and strain gauge accelerometers analyze vibrations to predict and prevent equipment failures, scheduling maintenance before catastrophic failures occur. In structural health monitoring, strain gauge and capacitive accelerometers assess the integrity of structures like bridges, buildings, and dams by measuring low-frequency vibrations and deformations. In vehicle health monitoring, MEMS and piezoresistive accelerometers continuously monitor vibrations and shocks, identifying issues requiring maintenance, such as imbalances or worn-out components.
vibraration-acceleration-measurement

How to use accelerometers for vibration measurements?

Using accelerometers for vibration measurements requires specific knowledge and expertise. Typically, these devices are used by engineers, technicians, and researchers who have experience in handling and interpreting data from accelerometers.

What devices use accelerometers?

Accelerometers are part of a measurement chain of vibration meters that includes powering, cables, and connectors. Proper powering of the accelerometer is essential to obtain accurate readings. The use of appropriate cables and connectors ensures reliable signal transmission and minimizes data loss. Vibration meters can be single or multichannel, and therefore, accelerometers are available in single and three-axis versions.

How to choose the right accelerometer?

When selecting an accelerometer for a given application, consider the following key characteristics:

  1. Frequency Range (Hz): Indicates the range of vibration frequencies to which the accelerometer responds. For example, the measurement frequency range of the vibration severity measuring instrument shall be from 10 Hz to 1,000 Hz, but can include other ranges. (In some parts of ISO 20816, a lower cut-off frequency of 2 Hz or even lower is used.)
  2. Dynamic Range (g): The range between the smallest detectable amplitude and the largest amplitude before distorting or clipping the output signal.
  3. Sensitivity (mV/g or mV/ms²): Defines the minimum detectable signal or the change in output electrical signal per change in input mechanical change. Usually provided at one frequency; the sensitivity is related to the operating range.
  4. Temperature: The operational temperature range the accelerometer can withstand.
  5. Mass: The weight of the accelerometer, which can affect its mounting and the measurement accuracy.
  6. Mounting Type: The method by which the accelerometer is attached to the measurement surface.
  7. Compatibility with the vibration meter: Ensuring the accelerometer works seamlessly with the vibration meter being used.

How to ensure accelerometer accuracy?

To ensure precise measurements, accelerometers need an initial calibration and verification in a laboratory every two years. This process involves setting a baseline and adjusting the sensor’s output to match known reference values. Regular in-situ checks are also necessary to maintain accuracy over time, typically using handheld calibrators like the SV110.

How to mount the accelerometers?

Accurate vibration measurements depend on correct mounting techniques. Accelerometer manufacturers typically describe the following mounting methods:

  • Stud mounting
  • Adhesive mounting (including cement and wax mounting)
  • Magnetic mounting

Properly securing the accelerometer to the machinery or structure minimizes interference and ensures that the sensor accurately captures the vibrations.

accelerometer accuracy

How to collect vibration data from the accelerometers?

Setting up a data acquisition system involves configuring the equipment (vibration meter) to record vibration data accurately. Selecting the appropriate sampling rate and data logging parameters is crucial for capturing the necessary details and avoiding data loss. It’s essential to select results like PEAK, RMS, MAX, PEAK-PEAK, VDV, or frequency spectrum analyses such as FFT or 1/3 octaves, and recording raw data (WAV recording).

Data processing and analysis

Once data is collected, it needs to be processed and analyzed. Specialized software, such as SvanPC++ tools, apply filters and algorithms to the raw data, extracting meaningful information about vibration patterns, frequencies, and amplitudes.

Interpreting data and making adjustments

The final step involves interpreting the processed data to understand the condition of the machinery or structure. Based on this analysis, engineers and technicians can make necessary adjustments to improve performance, prevent failures, and ensure safety.

Key Takeaways

  1. Accelerometers are versatile transducers critical for measuring vibration and acceleration across various applications, from automotive safety and consumer electronics to aerospace, industrial machinery, and structural health monitoring.
  2. There are different types of accelerometers, such as piezoelectric, MEMS, piezoresistive, and strain gauge accelerometers. Selecting the right type depends on the specific requirements of the application, including frequency range, dynamic range, sensitivity, and compatibility with the measurement system.
  3. Using accelerometers effectively requires specific knowledge and expertise in mounting techniques, calibration, and data acquisition. Proper installation and calibration ensure accurate measurements, while advanced data processing and analysis techniques are essential for interpreting vibration data and making informed adjustments.
  4. Accelerometers enhance safety and operational efficiency by providing real-time data for predictive maintenance, structural health monitoring, and detecting rapid changes in motion, which can trigger safety mechanisms like airbags in vehicles.
  5. The accuracy and reliability of vibration measurements depend on a comprehensive measurement chain that includes proper powering, suitable cables and connectors, and robust data acquisition systems. Regular calibration and in-situ checks are necessary to maintain the accuracy and reliability of the measurements.

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