An accelerometer is a measuring instrument able of detecting and/or measuring acceleration (or the gravitational force), calculating the force measured with respect to the mass of the object (force per unit of mass). Therefore the operating principle of an accelerometer is based on the detection of the inertia of a mass when it is subjected to an acceleration.
It usually employs a concentrated mass suspended on an elastic element, while a sensor detects its displacement with respect to the fixed structure of the device (supporting frame or container). In the presence of acceleration, the mass (which has its inertia) moves from its rest position in proportion to the acceleration detected. The sensor transforms this displacement into an electrical signal that can be acquired by modern measurement systems.
Modern accelerometers are typically micro-machined silicon sensors that are based on the capacitive, piezo-resistive, or optical detection of the deflection a small mass experiences when the sensors are subjected to acceleration.
- Measurement range – The level of acceleration supported by the sensor’s output signal specifications, typically specified in ±g. This is the greatest amount of acceleration 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 at 4g, the output may rail. Note that the breaking point 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 found out by analyzing the properties of the quartz crystal used and also the resonance frequency of the device.
- Accelerometer grounding – Grounding can be in two modes. One is called the Case Grounded Accelerometer which has the low side of the signal connected to their core. This device is susceptible to ground noise. Ground Isolation Accelerometer refers to the electrical device kept away from the case. Such a device is prone to ground produced noise.
- Resonant frequency – It should be noted that the resonant frequency should be always higher than the frequency response.
- Temperature of operation – Temperature sensitivity defines how the sensitivity of the accelerometer shifts with temperature. Accelerometers are mechanical systems so the temperature will impact the device’s mechanical properties and thus the sensitivity of the accelerometer. Temperature sensitivity is typically defined as a percentage shift per degree Celsius (%/°C). An accelerometer has a temperature range between – 50 degrees Celsius to 120 degrees Celsius. This range can be obtained only by accurate installment of the device.
- Sensitivity – The ratio of change in acceleration (input) to change in the output signal. This defines the ideal, straight-line relationship between acceleration and output. The device must be designed in such a way that it has higher sensitivity. That is, even for a small accelerative force, the electrical output signal should be very high. Thus a high signal can be measured easily and is sure to be accurate. Transverse sensitivity defines how sensitive the accelerometer is to accelerations 90 degrees (or orthogonal) to the sensitive axis of the sensor. This parameter is expressed as a percentage. Ideally, it should be 0% but due to manufacturing tolerances, the transverse sensitivity is often 5% or 10%. Sensitivity is specified at a particular 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 specified in a range (min, typ, max) or as a typical figure and % deviation. For analog-output sensors, sensitivity is ratiometric to supply voltage; doubling the supply, for example, doubles the sensitivity. Sensitivity change due to Temperature is generally specified as a % change per °C. Temperature effects are caused by a combination of mechanical stresses 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 measurement of deviation from a perfectly constant sensitivity, specified as a percentage with respect to either full-scale range (%FSR) or ± full scale (%FS). Typically, FSR = FS+FS. The nonlinearity of Analog Devices accelerometers is low enough that it can most often be ignored.
- Axis – Most of the industrial applications require only a 2-axis accelerometer. But if you want to go for 3D positioning, a 3-axis accelerometer will be needed. Higher-end accelerometers are typically only a single axis that may require to purchase and install 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 relates to the percentage of the output measured in the primary axis of vibration which is actually due to vibration applied to the accelerometer from a cross-axial direction. Typically less than 5% this parameter should always be checked at recalibration.
- Cross-axis sensitivity – A measure of how much output is seen on one axis when acceleration is imposed on a different axis, typically specified as a percentage. The coupling between two axes results from a combination of alignment errors, etching inaccuracies, and circuit crosstalk.
- Analog/Digital Output – You must take special care in choosing 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 integral electronics amplifier circuit operates at when powered. This will vary from one manufacturer to another 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 given for digital output accelerometers or systems that incorporate an analog to digital converter. The resolution will typically be specified as bits which can then be used to calculate the resolution in acceleration units.
- Noise – Noise levels can be defined in several different ways. Some accelerometers will define residual noise as a broadband RMS value, usually with the units of µV or µg. The random deviation from the ideal output and 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.
Fields of application of the accelerometers
Accelerometers are used in many modern applications both in the domestic, industrial, and professional fields. An accelerometer is mainly used to measure the vibrations and oscillations that can develop on machinery and in industrial plants, it is often used also for the development of new products.
By determining the relationship between phase and amplitude of vibrations at various points of a structure, it is possible to obtain important information on the integrity of a system. The accelerometer can provide data for the following vibration parameters: acceleration, speed, and displacement. With all this information it is possible to identify precisely the characteristics of the vibration.
The accelerometer can be portable or fixed, just as it can also have a memory to store the measured data. Usually, the accelerometer is delivered with a factory calibration certificate, and optionally it is possible to request an ISO certification in order also to give legal value to the measurements.
The measurement of vibrations of buildings and bridges allows to establish deterioration as a result of earthquakes, or in the impact tests; the accelerometers are used to establish the levels of impact.
Finally, aeronautical and space applications are of particular importance. In the past, a widely used accelerometer was the differential transformer accelerator consisting of an LVDT (linear variable differential transformers) equipped with a spring and shock absorber to which a known mass was anchored. However, the presence of mobile masses of considerable importance results for these accelerometers a reduced bandwidth (typically of 100 Hz), a reduced measurement range (less than 100 g) together with reduced reliability.
Types and classification of accelerometers
The accelerometers can be classified according to the type of measurement they are going to perform, that is: for static acceleration measurements or dynamic acceleration measurements.
The accelerometers for static acceleration measurements are able to detect from constant and static accelerations (i.e., input magnitude with a frequency of 0 Hz) up to accelerations that vary with low frequencies (generally up to 500 Hz). They, therefore, have a pass-band with a low pass characteristic. This characteristic is typical of accelerometers made with the extensometric, LVDT, or capacitive principles. Application examples for these instruments are gravitational acceleration, centrifugal acceleration measurements of a moving vehicle in inertial guidance.
Accelerometers for dynamic acceleration measurements are devices that are unable to detect static accelerations (for example gravitational acceleration) but are able to detect accelerations that vary over time, for example, those generated by objects that vibrate or those that are generated in shocks. The bandwidth of these instruments can range from a few Hz to 50 kHz. They have a bandpass feature. Typical accelerometers of this type are those made with piezoelectric technology.
The accelerometers can also be classified according to the operating principle of the position sensor. The accelerometers currently most used are those of the piezoelectric and MEMS type.
Strain gauge accelerometers
The strain gauge accelerometer uses the same principle of the load cells as the detection principle, i.e., the resistance variation of a strain gauge due to the variation of its length. In these devices, a mass is suspended on a cantilever beam (fixed to the housing of the instrument), on which strain gauges connected as Wheatstone bridge are fixed.
In the presence of acceleration the mass moves, flexing the cantilever beam and consequently, the strain gauges undergo an elongation. Damping is provided by a viscous fluid-filled inside the housing. With a voltmeter, it is possible to read an unbalancing voltage of the Wheatstone bridge proportional to the acceleration.
Strain gauge accelerometer operative process: the accelerometer is fitted on to the structure whose acceleration is to be measured.
- Due to the vibration, vibrational displacement of the mass occurs, causing the cantilever beam to be strained.
- Hence the strain gauges mounted on the cantilever beam are also strained and due to this their resistance change.
- Hence a measure of this change in resistance of the strain gauge becomes a measure of the extent to which the cantilever beam is strained.
- But the resulting strain of the cantilever beam is proportional to the vibration/acceleration and hence a measure of the change in resistance of the strain gauges becomes a measure of vibration/acceleration.
- The leads of the strain gauges are connected to a wheat stone bridge whose output is calibrated in terms of vibration/acceleration.
The LVDT accelerometer uses an LVDT (Linear Variable Differential Transformer) integrated into the structure of the accelerometer as a principle for detecting mass displacement. In these devices, the mass itself constitutes the ferromagnetic core of the LVDT sensor and flows (suspended on springs or other elastic elements) inside a channel, around which the coils destined to detect the position of the mass are wound.
A circuit detects the position of the core relative to the coils and generates an electrical signal proportional to the displacement with respect to the rest position.
The capacitive accelerometer uses, as the operating principle for detecting mass displacement, the variation of the electric capacity of a capacitor to vary the distance between its armatures.
In these accelerometers, the mass (made with conductive material) constitutes an armature, while the other is realized on the fixed structure of the device, in the immediate proximity of the mass. The mass is suspended on a relatively rigid elastic element (typically a membrane). A particular circuit detects the capacitance of the capacitor thus created and generates an electrical signal proportional to the position of the mass. This type of accelerometer is made for typical applications such as air-bags and mobile technological devices, with Micro-Electro-Mechanical Systems (MEMS) technology. Manufacturing technology with high volume processes and therefore lower production costs.
The capacitive accelerometers are of low cost with a signal-to-noise ratio and non-optimal dynamic response. An intrinsic feature of all capacitive elements is the internal clock circuit. The frequency of this circuit is high (about 500 kHz) and is an integral part of the current detection circuit, always present in the output signal. The noise present is at high frequency and in general outside the acceleration measurement range. Thanks to its built-in amplifier/IC, the 3 wires (or 4 wires for differential output) are the connection to a stable voltage source.
The capacitive accelerometer bandwidth is limited to a few hundred Hertz due to the gas damping that reacts to the element due to the damping effect. The structure of the capacitive sensor element favors the low acceleration measurement range. The maximum range is generally limited to less than 100 g. Apart from these limitations, modern capacitive accelerometers, in particular, high-quality devices, offer excellent linearity and high stability of the output signal.
Capacitive accelerometers are more suitable for monitoring applications. They are ideal for measuring low-frequency movement where level g is also low, such as vibration measurements in civil engineering.
The piezoresistive accelerometer is a variant of the strain gauge accelerometer, where piezoresistive sensors are used instead of strain gauges. These sensors behave similarly to strain gauges, but allow more extended elongation and sensitivity, despite having some stability problems with the temperature variation.
A piezoresistive accelerometer has a stable structure composed of a silicon chip created by micromachining and semiconductor production technology. A mass and a beam on which a set of the piezo-resisters are created on a silicon chip. A set of electrical bridged is formed by such piezo-resistive resisters to generate signals proportional to the applied acceleration.
The key benefits of piezoresistive accelerometers are:
- measurements are possible over the wide frequency range covering down from the ultra-low up to the high frequency as of several kHz
- compact and lightweight
- high sensitivity
- highly acceleration resistant
Instead of sensing the capacitance changes in the seismic mass, a piezoresistive accelerometer takes advantage of the resistance change of piezoresistive materials to convert mechanical strain to a DC output voltage. Most of the Piezoresistive designs are either MEMS type (gas damped) or bonded strain gauge type (fluid damped) and they are suitable for impact measurements where frequency range and g level are considerably high.
Piezoresistive accelerometers are widely used in automobile safety testing including anti-lock braking systems, safety air-bags, and traction control systems as well as weapon testing and seismic measurements. Besides, micromachined accelerometers are available and used in various applications, such as submillimeter piezoresistive accelerometers in extremely small dimensions used for biomedical applications.
The piezoelectric accelerometer uses, as a principle for detecting mass displacement, the electrical signal generated by a piezoelectric crystal (quartz or ceramic crystals) when subjected to mechanical stress. This effect is exploited by placing a known mass in contact with the crystal, also known as the seismic mass or test mass which constitutes both the sensor and the elastic element so that it exerts a force.
Like other transducers, piezoelectric accelerometers convert one form of energy into another and provide an electrical signal in response to the condition, property, or quantity. Acceleration acts upon a seismic mass that is restrained by a spring or suspended on a cantilever beam and converts a physical force into an electrical signal.
In the presence of acceleration, the mass (which has certain inertia) compresses the crystal with force directly proportional to the acceleration, which will generate an electrical signal directly proportional to the compression force to which the sensor is subjected. Considering that the elastic element is a crystal, the characteristics of these devices are peculiar:
- they have a relatively low sensitivity;
- they can detect very high accelerations without being damaged (even 1000 g);
- they cannot detect constant accelerations over time.
A particularly important consideration lies in the fact that the crystals generally used in the construction of the elastic element have a very high value of the elastic constant, as well as high stability and repeatability, which has a profound influence on the differential equation that governs the phenomenon vibratory which involves the instrument system.
The last characteristic is to be remarked: as mentioned, the crystal generates an electrical signal proportional to the compression, but if the compression on the crystal remains, the generated signal tends to dissipate after a short period. As a result of this phenomenon, called leakage, these accelerometers are unable to detect a quasistatic acceleration; in fact, after a few seconds from the acceleration, the first signal “freezes” and then dissipates, and in output, there will be no signal. This is due to the high resistance of the accelerometer or, possibly, also to an incorrect setting of the lower limit frequency on the preamplifier.
These accelerometers are used in applications where dynamic accelerations such as those generated in vibrations and mechanical shocks must be detected. There are two types of piezoelectric accelerometers: high and low impedance. High impedance accelerometers have a charge output that is converted into a voltage using a charge amplifier or external impedance converter. Low impedance units use the same piezoelectric sensing element as high-impedance units, and incorporate a miniaturized built-in charge-to-voltage converter and an external power supply coupler to energize the electronics and decouple the subsequent DC bias voltage from the output signal.
The key benefits of piezoelectric accelerometers are:
- Extremely wide dynamic range
- Low output noise
- Wide frequency range
- No moving parts (suitable for shock and vibration measurement)
- Compact, non-contact design
- Excellent linearity over their dynamic range
- Acceleration signal can be integrated to provide velocity and displacement
- Highly sensitive
- Self-generating (no external power required)
Major applications of piezoelectric accelerometers include:
- Engine testing – Combustion and dynamic stressing
- Ballistics – Combustion, explosion, and detonation
- Industrial/factory – Machining systems, metal cutting, and machine health monitoring
- Original equipment manufacturer – Transportation systems, rockets, machine tools,
- engines, flexible structures, and shock/vibration testers
- Engineering – Dynamic response testing, shock and vibration isolation, auto chassis
- structural testing, structural analysis, reactors, control systems, and materials evaluation
- Aerospace – Ejection systems, rocketry, landing gear hydraulics, shock tube instrumentation, wind tunnel, and modal testing.
The laser accelerometer is a particular type of accelerometer family, used when it is necessary to carry out extremely precise measurements that cannot be obtained with other types of instruments. The operating principle is based on the physical principle that acceleration is a derivative of speed over time. In this device, a laser interferometer measures instant by instant the movement of the moving object, a computer connected to it performs the second derivative with respect to time, thus directly obtaining the acceleration value.
A laser accelerometer comprises a frame having three orthogonal input axes and multiple proof masses, each proof mass having a predetermined blanking surface. A flexible beam supports each proof mass. The flexible beam permits movement of the proof mass on the input axis. A laser light source provides a light ray. The laser source is characterized to have a transverse field characteristic having a central null intensity region. A mirror transmits a ray of light to a detector. The detector is positioned to be centered on the light ray and responds to the transmitted light ray intensity to provide an intensity signal. The intensity signal is characterized to have a magnitude related to the intensity of the transmitted light ray.
The proof mass blanking surface is centrally positioned within and normal to the light ray null intensity region to provide increased blanking of the light ray in response to transverse movement of the mass on the input axis. The proof mass deflects the flexible beam and moves the blanking surface in a direction transverse to the light ray to partially blank the light beam in response to acceleration in the direction of the input axis. Control responds to the intensity signal to apply a restoring force to restore the proof mass to a central position and provides an output signal proportional to the restoring force.
The problems with these devices are that they are expensive, slightly bulky, they require the interferometer to be mounted on the ground (or on a place to be considered fixed), and the laser must be pointed continuously towards the moving object.
The accelerometers realized with MEMS technology (Micro-Electro-Mechanic System) are nothing more than miniaturized accelerometers based on a mobile micromechanical structure, realized by engraving a silicon substrate with standard photolithographic methods.
The main advantages of these devices are the low production cost (in particular for two or three measurement axes) and the presence inside them of the conditioning circuit for the response at low frequencies extended up to the continuous. A MEMS accelerometer provides accurate detection while measuring acceleration, tilt, shock, and vibration in the performance-driven application.
The negative aspects of this production technology have repercussions on performance in terms of accuracy and stability currently lower than the best piezoelectric accelerometers. Furthermore, the coating containers typically used for such devices (the Dual in-line packages for integrated circuits) are not suitable for industrial measurements in hostile environments.