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Accelerometer Measurement

Understanding Accelerometer Sensors and Selecting the appropriate Data Acquisition Hardware

by Leo Cordaro
Sr. Systems Engineer at MAS

This article will highlight some key aspects of the sensor and some of the important attributes to look for when selecting the data acquisition hardware. It is also intended to provide the reader with a basic understanding of accelerometers and the data acquisition hardware used to acquire their signal. At the end of the article, you will find additional resources which will provide further details.

Accelerometers can be used for a variety of test applications, for example product validation, or part of industrial automation monitoring system. Machine condition monitoring (such as factory automation), noise-vibration-harshness testing, and structural testing are some examples of where a vibration sensor can be used. While these test applications span a vast area, the basic principles remain the same.

As a side note, many everyday consumer electronics contain accelerometers. Most smartphones make use of accelerometers so that the phone “knows” its’ orientation. These sensors are typically referred to as MEMS accelerometers (Micro-Electro-Mechanical-Systems) and can range from a single axis accelerometer to three axis accelerometers (triaxial). Below is an example of a MEMS Accelerometer that might be found in your smartphone.

Piezoelectric accelerometers (or sometimes referred to as piezoelectric transducers) generate an electrical signal that is proportional to acceleration. The production of electricity by applying a mechanical stress to certain crystals is generally referred to as the piezoelectric effect. Electrodes in the sensor collect the total accumulated charge generated by a mass stressing a crystal. This all occurs within the accelerometer housing. Wires from the electrodes transmit the signal to a signal conditioner first, then to data acquisition hardware (which might contain additional signal conditioning). Voltage mode sensors contain the signal conditioner built into the sensor, and are classified as Integrated Electronics Piezoelectric (IEPE). Charge mode sensors require external signal conditioning. Below is an image of a typical accelerometers.

The construction of the accelerometers can be divided up into several categories, the most common being the Shear Mode, followed by Flexural Mode. Trying to keep the physics of these sensor constructions to a minimum, just note that:

  • Shear mode sensors: the mass applied to the crystal occurs in a vertical fashion and as acceleration is applied, the mass shears the crystal.
    • Ideally, these sensor are used for high-frequency, high acceleration measurements
  • Flexural mode sensors contain a beam-shaped sensing crystal which is supported by a fulcrum. As acceleration is applied to the sensor, a strain is created on the crystal.
    • Ideally, these sensors are used for low-frequency, low acceleration type of measurements.

A general purpose IEPE accelerometer will have the following specifications:

  • Sensitivity is usually expressed in mV / g, such as 10 mV/g is generated by the sensor for every 1 G-force. This is not really a force, but a force per unit mass and a measurement of acceleration felt as a weight. 1 g-force is equal to the force of gravity, or 9.8 meters / sec2. The g-force on an object is the acceleration relative to free fall.
  • Measurement range defines the peak g-force that can be detected by the sensor, for instance, ± 500 g. Knowing the sensitivity, you can determine that this sensor will generate a ± 5 volt signal at full scale.
  • Frequency range is the spectrum over which the sensor can detect, such as 0.5 – 10,000 Hz.

The data acquisition hardware for accelerometers can provide some of the signal conditioning, therefore providing a unified hardware solution, rather than separate modules. For example, in order to properly read the acceleration of an IEPE type of accelerometer, a current must be applied to the sensor (typically 2ma). Additionally, the acquisition hardware can provide low pass filters before digitizing the accelerometer signal to avoid any high frequency noise interference.

  • Dynamic Range in dB = 20*Log10( Vmax / Vmin)

For example, suppose our data acquisition hardware specification for dynamic range is 102 dB, and a maximum input of 5V. Then the smallest possible voltage that the device can detect is 40 µV. Having high dynamic range allows for the data acquisition hardware to detect signals ranging from 40 µV to 5 V. This is especially apparent in the frequency domain. A high dynamic range will allow the system to detect the large and small amplitudes of frequencies and provide a much better picture of harmonics.

Please visit the additional resources below and feel free to contact us for any additional questions, or application specific questions.

Resources

Use accelerometers for product validation testing

Equipment condition monitoring can be used to minimize down time by performing vibration analysis.

Links