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.
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 thin sheets, on which strain gauges connected as Wheatstone bridge are fixed.
In the presence of acceleration the mass moves, flexing the laminations and consequently, the strain gauges undergo an elongation. With a voltmeter, it is possible to read an unbalancing voltage of the Wheatstone bridge proportional to the acceleration.
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.
Often in these instruments, the mass is suspended on a plastic membrane, on which the piezoresistive elements have been attached.
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. A 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 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 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.
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.
The laser accelerometer is a particular type of the 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 conceptually different from those described above and 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.
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.
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.
The gravimeter is a particular type of accelerometer specifically designed to measure the acceleration of gravity. According to the equivalence principle of general relativity, the effects of gravity and acceleration are the same; therefore, an accelerometer cannot distinguish between the two cases.
As gravimeters, it is possible to use improved versions of accelerometers for static measurements, in which sensitivity, precision, and stability characteristics have been uniquely appointed. In fact, in this application, it needs to detect extremely small acceleration variations.
Where, for scientific purposes, it is necessary to carry out extremely precise measurements, an instrument is used that works with the same principle as the laser accelerometer: in this case, the acceleration of the fall of a grave is detected in a vacuum chamber, using a laser interferometer for measuring displacement, and an atomic clock for measuring fall time.
The detection of gravitational acceleration, besides being of interest in the scientific field (especially in physics and geology), is a practice of the mining industry (especially for the search of oil fields).