Accelerometer

An accelerometer is a measuring instrument capable of detecting and/or measuring acceleration (or gravitational force) by calculating the force measured in relation to the mass of the object (force per unit of mass). The operating principle of an accelerometer is therefore based on the detection of the inertia of a mass when it is subjected to an acceleration.

It usually uses a concentrated mass suspended from an elastic element, while a sensor detects its displacement with respect to the fixed structure of the device (support frame or container). In the presence of an acceleration, the mass (which has its inertia) moves from its rest position in proportion to the acceleration detected. The sensor converts this displacement into an electrical signal that can be acquired by modern measurement systems.

Modern accelerometers are typically micromachined silicon sensors based on capacitive, piezoresistive or optical detection of the deflection of a small mass when the sensors are subjected to acceleration.

Accelerometer specifications

  • Measurement Range → The amount of acceleration supported by the sensor’s output signal specifications, typically expressed in ±g. This is the largest amount of acceleration that the part can measure and accurately represent as an output. For example, the output of a ±3g accelerometer is linear with acceleration up to ±3g. If it is accelerated to 4g, the output may break. Note that the breakpoint is specified by the absolute maximum acceleration, NOT by the measurement range. A 4g acceleration will not break a ±3g accelerometer.
  • Frequency Response → This parameter can be determined by analyzing the characteristics of the crystal used and the resonant frequency of the unit.
  • Accelerometer Grounding → Grounding can be done in two modes. One is called the Case Grounded Accelerometer which has the low side of the signal connected to its core. This device is susceptible to ground noise. Ground Isolation Accelerometer refers to the electrical device that is kept away from the case. Such a device is susceptible to ground generated noise.
  • Resonant Frequency → Note that the resonant frequency should always be higher than the frequency response.
  • Operating Temperature → Temperature sensitivity defines how the sensitivity of the accelerometer changes with temperature. Accelerometers are mechanical systems, so temperature will affect the mechanical properties of the device and thus the sensitivity of the accelerometer. Temperature sensitivity is typically defined as a percentage change per degree Celsius (%/°C). An accelerometer has a temperature range of -50 degrees Celsius to 120 degrees Celsius. This range can only be obtained by accurately mounting the device.
  • Sensitivity → The ratio of change in acceleration (input) to change in output signal. This defines the ideal, linear relationship between acceleration and output. The device must be designed to have high sensitivity. This means that even for a small acceleration force, the electrical output signal should be very high. This way, a high signal is easy to measure and is sure to be accurate. Transverse sensitivity defines how sensitive the accelerometer is to accelerations that are 90 degrees (or perpendicular) to the sensitive axis of the sensor. This parameter is expressed in percent. Ideally, it should be 0%, but due to manufacturing tolerances, transverse sensitivity is often 5% or 10%. Sensitivity is specified at a given supply voltage and is typically expressed in units of mV/g for analog output accelerometers, LSB/g or mg/LSB for digital output accelerometers. It is usually given as a range (min, typ, max) or as a typical number and % deviation. For analog output sensors, sensitivity is ratiometric to the supply voltage; for example, doubling the supply voltage will double the sensitivity. Sensitivity change due to temperature is generally given as a % change per °C. Temperature effects are caused by a combination of mechanical stress and circuit temperature coefficients.
  • Nonlinearity → Ideally, the relationship between voltage and acceleration is linear and described by the sensitivity of the device. Nonlinearity is a measure of the deviation from perfectly constant sensitivity, expressed as a percentage of either full scale range (%FSR) or ± full scale (%FS). Typically, FSR = FS+FS. The nonlinearity of Analog Devices accelerometers is so small that it can be ignored in most cases.
  • Axis → Most industrial applications require only a 2-axis accelerometer. But if you want to go for 3D positioning, a 3-axis accelerometer is needed. Higher-end accelerometers are typically single axis, which may require purchasing and installing three units for testing, which can be both expensive and time consuming.
  • Cross-Axis Error A critical parameter when recalibrating accelerometers. The cross-axis error refers to the percentage of the output measured in the primary axis of vibration that is actually due to vibration applied to the accelerometer from a cross-axis direction. Typically less than 5%, this parameter should always be checked during recalibration.
  • Cross-Axis Sensitivity → A measure of how much output is seen on one axis when acceleration is imposed on another axis, typically expressed as a percentage. The coupling between two axes results from a combination of alignment errors, etching inaccuracies, and circuit crosstalk.
  • Analog/Digital Output → Special care must be taken when selecting the type of output for the device. Analog output will be in the form of small changing voltages and digital output will be in PWM mode.
  • Bias Voltage → Only relevant to integrated electronics Piezo-Electric (IEPE), this reflects the operating DC voltage that the integrated electronics amplifier circuit operates at when powered. This will vary from manufacturer to manufacturer and higher is not necessarily better than lower, the level is simply a feature of the electronics design.
  • Resolution → The resolution of an accelerometer is generally only specified for digital output accelerometers or systems that include an analog-to-digital converter. The resolution is typically specified in bits, which can then be used to calculate the resolution in acceleration units.
  • Noise → Noise levels can be defined in a number of ways. Some accelerometers define the residual noise as a broadband RMS value, usually in µV or µg units. The random deviation from the ideal output is equal to the multiplied product of the noise density and the square root of the noise bandwidth. The units for this parameter are typically mg-RMS.

Accelerometer applications

Accelerometers are used in many modern applications in the domestic, industrial and professional fields. An accelerometer is mainly used to measure the vibrations and oscillations that can occur in machinery and industrial equipment, and it is often used in the development of new products.

By determining the relationship between the phase and amplitude of vibrations at different points of a structure, it is possible to obtain important information about the integrity of a system. The accelerometer can provide data for the following vibration parameters: acceleration, velocity and displacement. With all of this information, it is possible to accurately identify the characteristics of the vibration.

The accelerometer can be portable or fixed, as well as have a memory to store the measured data. Usually the accelerometer is supplied with a factory calibration certificate, and optionally it is possible to request ISO certification to give legal value to the measurements.

Vibrations of buildings and bridges can be measured to determine the damage caused by earthquakes, or in impact tests, accelerometers are used to determine the level of impact.

Finally, aerospace applications are of particular importance. In the past, a widely used accelerometer was the differential transformer accelerometer, which consisted of an LVDT (linear variable differential transformer) equipped with a spring and shock absorber to which a known mass was anchored. However, the presence of moving masses of considerable importance results in a reduced bandwidth for these accelerometers (typically of 100 Hz), a reduced measuring range (less than 100 g) together with reduced reliability.

Types and classification of accelerometers

Accelerometers can be classified according to the type of measurement they are intended to perform, i.e. static acceleration or dynamic acceleration.

Static accelerometers are capable of detecting accelerations ranging from constant and static (i.e., input magnitude at a frequency of 0 Hz) to accelerations that vary at low frequencies (generally up to 500 Hz). They therefore have a low-pass frequency response. This characteristic is typical of accelerometers based on the extensometric, LVDT or capacitive principles. Examples of applications for these instruments are gravitational acceleration, centrifugal acceleration measurements of a moving vehicle in inertial guidance.

Accelerometers for dynamic acceleration measurements are devices that cannot measure static accelerations (such as gravitational acceleration), but can measure accelerations that vary with time, such as those generated by vibrating objects or those generated by shocks. The bandwidth of these instruments can range from a few Hz to 50 kHz. They have a band-pass characteristic. Typical accelerometers of this type are those made with piezoelectric technology.

Accelerometers can also be classified according to the operating principle of the position sensor. Currently, the most commonly used accelerometers are piezoelectric and MEMS.

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