Ⅰ.  What Is An Accelerometer?

An accelerometer is a device that detects a structure's vibration or motion acceleration. The piezoelectric material is "squeezed" by vibrations or changes in motion (acceleration), resulting in a charge proportionate to the force applied to it. Charge is related to acceleration because charge is proportional to force and mass is constant. Accelerometers are now found in almost all cellphones. You may use it to see if the phone is moving in particular directions, and it's also how the automatic screen brightening function works when the phone is flipped over. Accelerometers can help engineers comprehend machine stability while detecting forces or vibrations that shouldn't be present in industrial environments.

 

Ⅱ.  How Does An Accelerometer Work?

Accelerometers are used to determine whether a force is static or dynamic. A constant force exerted on an object, such as gravity or friction, is known as static acceleration. These factors are largely predictable and uniform. For example, because gravity's acceleration is constant at 9.8 m/s, gravity is nearly the same everywhere on Earth.

Vibration and shock are two examples of non-uniform dynamic acceleration forces. Dynamic acceleration is well-exemplified by a car accident. In comparison to the preceding state, the acceleration change here is sudden. Accelerometers work on the principle of detecting acceleration and converting it to a measurable quantity, such as an electrical signal.

 

Ⅲ.  How Many Types Of Accelerometers?

Accelerometers are divided into four types according to their working principles:

1. Piezoelectric accelerometer

Figure. 1

The piezoelectric effect of piezoelectric crystals is used in piezoelectric accelerometers.

When some crystals are deformed in a specific direction by a force, polarization occurs within, and opposite-sign charges are created on their two surfaces; when the external force is removed, the crystals return to their uncharged state. The "piezoelectric effect" is the name given to this phenomena.

Piezoelectric crystals are crystals that have a "piezoelectric effect." Quartz, piezoelectric ceramics, and other piezoelectric crystals are commonly utilized.

Figure. 2

The force exerted by the mass on the piezoelectric element changes when the accelerometer vibrates. The force change is proportional to the recorded acceleration when the measured vibration frequency is substantially lower than the accelerometer's natural frequency.

Figure. 3 Structure of Piezoelectric Accelerometer 

The letter S stands for spring. M stands for mass. The letter B stands for base. R is the clamping ring, while P is the piezoelectric element.

Figure a shows a compression type with a centrally positioned piezoelectric element-mass-spring system mounted on a circular central pillar that is attached to the base. The resonance frequency of this structure is quite high. When the base B is linked to the test object, however, deformation of the base B will directly affect the vibration pickup's output. Changes in the test object and ambient temperature will also affect the piezoelectric element and alter the preload force, resulting in temperature drift.

The annular shear type is seen in Figure b, where the piezoelectric element is clamped on the triangular center column by a clamping ring. The piezoelectric element is subjected to shear stress when the accelerometer encounters axial vibration. This construction provides great base deformation and temperature change isolation, as well as a high resonance frequency and strong linearity.

Figure c shows a triangle shear form with a simple construction that can be turned into a tiny high-resonance-frequency accelerometer. The annular mass is bonded to the central pillar's annular piezoelectric element. Because the binder softens as the temperature rises, the maximum operating temperature is limited.

Large dynamic range, wide frequency range, sturdiness and longevity, reduced external interference, and the piezoelectric material itself generates a charge signal without any external power supply are all features of piezoelectric accelerometers. It's the most common vibration sensor on the market.

Although the piezoelectric accelerometer has a simple structure and a long history of commercial use, its performance indicators are closely related to material properties, design, and processing technology, so the actual parameters of performance and stability and consistency of similar sensors on the market are very different. The major disadvantage of piezoelectric accelerometers over piezoresistive and capacitive accelerometers is that they cannot measure zero-frequency impulses.

 2. Piezoresistive acceleration sensor

The first silicon micro-accelerometer was the piezoresistive accelerometer (based on MEMS silicon micromachining technology). The elastic element of a piezoresistive accelerometer is usually made consisting of a silicon beam and a mass block that is supported by a cantilever beam and rests on it. To measure bridges, make resistors  and connect them. The mass block travels up and down due to inertial force, and the of the resistance on the cantilever beam changes with the action of the stress, causing the output voltage of the measuring bridge to vary, allowing acceleration to be measured.

Figure. 4 Piezoresistive accelerometer schematic

Piezoresistive silicon micro-acceleration sensors come in a variety of structural configurations, including cantilever beams, double-arm beams, 4-beams, and double-island-5 beams. The sensitivity, frequency response, range, and other properties of the sensor are determined by the structural form and size of the elastic element. The mass block can increase the tension on the cantilever beam when the acceleration is reduced, improving the sensor's output sensitivity.

Under high acceleration, the mass block's action may cause the cantilever beam's stress to surpass the yield stress, causing the cantilever beam to break due to excessive deformation. As a result, the structural form of single-arm beam and double-arm beam with equal mass block and beam thickness is proposed for high gn value acceleration, as indicated in the picture.

Figure. 5 Double arm beam structure

The piezoresistive accelerometer is based on the world's premier MEMS silicon micromachining technology and has the advantages of small size and low power consumption. It's simple to integrate into a variety of analog and digital circuits, and it's commonly used in automotive crash tests, test instruments, and equipment vibration monitoring, among other applications.

The strain piezoresistive accelerometer's sensitive core is built of a called a resistance measuring bridge, and its structural dynamic model is still a spring mass system.

With the advancement of current microfabrication manufacturing technology, piezoresistive form-sensitive cores may now be designed with significant flexibility to meet a variety of measuring requirements. Piezoresistive accelerometers range in sensitivity and range from low-sensitivity and high-range shock readings to high-sensitivity low-frequency observations.

Simultaneously, the piezoresistive accelerometer's measurement frequency range can range from a signal to a high frequency measurement with high stiffness and a measurement frequency range of tens of kilohertz. Piezoresistive sensors are likewise notable for their ultra-miniaturized design. It should be noted that, while piezoresistive sensing cores have a lot of versatility in terms of design and application, their scope of applicability for a single design is often narrower than piezoelectric sensors.

Another problem of piezoresistive acceleration sensors is that they are highly impacted by temperature, which necessitates temperature adjustment in most practical sensors. In terms of cost, piezoresistive sensors used in large quantities have a low cost of production, while sensitive cores for unique applications will have a significantly higher manufacturing cost than piezoelectric acceleration sensors.

 

Figure. 6 Piezoresistive Accelerometer  

 3. Capacitive accelerometer

Capacitive accelerometers use the capacitance concept to provide polar-distance-variable capacitive sensors. One electrode is stationary, while the other is an elastic diaphragm that changes. The capacitance changes as the elastic diaphragm is displaced by an external force (air pressure, hydraulic pressure, etc.). This sensor can measure the airflow (or liquid flow) vibration velocity (or acceleration), as well as the pressure.

Figure. 7 Capacitive accelerometer schematic

Simple circuit structure, wide frequency range of around 0450Hz, linearity less than 1%, high sensitivity, reliable output, limited temperature drift, small measurement error, steady-state response, low output impedance, and output power equivalent to The relational formulation of vibration acceleration is simple, convenient, and straightforward to calculate, with a wide range of applications.

However, because the signal's input and output are nonlinear, the range is limited, and it is impacted by the cable's capacitance, and the capacitive sensor is a high-impedance signal source, the capacitive sensor's output signal frequently needs to be improved by succeeding circuits. Capacitive accelerometers are generally employed for low-frequency measurement in practical applications, their adaptability isn't as excellent as piezoelectric accelerometers, and their cost is substantially greater.

Figure. 8 Capacitive Accelerometer

Capacitive accelerometers/capacitive accelerometers are a type of acceleration sensor that is widely used. In some industries, such as airbags, mobile phones, and mobile devices, there is no equivalent. Microelectromechanical systems (MEMS) are used in capacitive accelerometers/capacitive accelerometers. It becomes cost-effective in mass production, ensuring decreased costs.

4. Servo accelerometer

When the mass block deviates from the static equilibrium position due to acceleration input through the accelerometer shell, the displacement sensor detects the displacement signal, amplifies it by the servo amplifier, and outputs a current, which flows through the electromagnetic coil and into the permanent magnet's magnetic field. The electromagnetic restoring force is generated in the center, forcing the mass block to return to its original static equilibrium position; the accelerometer is in a closed-loop state, and the sensor emits an analog signal proportionate to the acceleration value.

Figure. 9 Servo accelerometer schematic

A closed-loop test system with strong dynamic performance, a large dynamic range, and good linearity is a servo accelerometer  . Its operating idea is that the sensor's vibration system is made up of the "mk" system, which is similar to a conventional accelerometer but with an electromagnetic coil coupled to the mass m. The mass block deviates from the equilibrium position when the base is accelerated, and the displacement is defined by After being amplified by the servo amplifier, the displacement sensor detects it and converts it to a current output. Servo accelerometers work in a closed loop because the current passes through the electromagnetic coil and generates an electromagnetic restoring force in the permanent magnet's magnetic field, attempting to retain the mass block in its original equilibrium position in the instrument housing.

The anti-interference ability is improved, the measurement precision is improved, and the measurement range is expanded as a result of the feedback effect. Inertial navigation and inertial guidance systems, as well as high-precision vibration measurement and calibration, utilise servo acceleration measuring technology.

5. Three-axis accelerometer

It is likewise founded on the acceleration principle. Acceleration is a vector in space. On the one hand, the components on an object's three coordinate axes must be measured to accurately grasp its motion state; on the other hand, only the three-axis acceleration sensor is employed to detect the acceleration signal in the case of.

The three-axis accelerometer can achieve a double-axis plus and minus 90 degrees or a double-axis 0-360 degree inclination because it is also based on the theory of gravity. The accuracy of the single-axis accelerometer is higher after adjustment. The measuring angle is greater than 60 degrees.

The working principles of piezoresistive, piezoelectric, and capacitive three-axis accelerometers are used in the majority of modern three-axis accelerometers. The resulting acceleration is proportional to changes in resistance, voltage, and capacitance, and it is collected via amplification and filter circuits.

The three-axis accelerometer is tiny in size and light in weight (grams), and it can monitor acceleration in space as well as fully and precisely reflect object motion features. Aerospace, robotics, autos, and medicine all use it.

 

Ⅳ.  Main Applications Of Accelerometers

Accelerometers offer a wide range of uses in both life and industry.

Digital devices: Accelerometers in smartphones and digital cameras are used to rotate the display automatically based on the orientation in which the display is held.

Vehicles: Over the years, the invention of the airbag has saved millions of lives. During a violent shock, the sensor provides a signal, which the accelerometer can use to activate the airbag.

Drones: Accelerometers assist drones in maintaining their direction while flying.

Rotating machinery: Accelerometers detect undulating vibrations in rotating machinery.

Industrial Platforms: Check the stability or inclination of the platform.

Vibration Monitoring: Moving machinery produce vibrations that, if not monitored, can be harmful to the machine. Accelerometers are increasingly being utilized in industrial plants, turbines, and other places to detect vibration.

 

Ⅴ. Working Parameters Of Accelerometers

Sensitivity

Sensitivity is measured in millivolts per gram of output voltage, or mv/g. For example, 100mV/g implies that 1 gram of acceleration produces 100 millivolts of  voltage. The voltage will alternate at the vibration's frequency, the amplitude of the AC signal will correspond to the amplitude of the vibration being monitored, and the vibration signal spectrum will be presented at all times.

Frequency response

The maximum variance of sensitivity over a frequency range is represented by the frequency response parameter. It's worth noting that the sensor's nominal and real sensitivity are both evaluated at a specific frequency. Most industrial sensors use a calibration reference frequency of 159.15Hz. The frequency response parameter is presented as 3 dB or stated as a positive or negative percentage (for example, 5% or 10%). When the frequency range is within the provided percentage range, positive and negative percentages imply that the sensitivity will be within the specified percentage range. The 3dB range is commonly used for military or scientific demands; 3dB is approximately 30%, and 3dB is around 30%.

Temperature output sensitivity

Refers to the change in output voltage as a function of temperature change. The accelerometer circuit is independent from the temperature control circuit. The temperature circuit is powered by the same power supply as the accelerometer that is internally amplified. The temperature control circuit "biases" the supply voltage to a voltage that corresponds to the accelerometer case temperature.

Power, voltage

The user's maximum and minimum input voltages to the sensor are critical; an overvoltage power source can destroy the sensor. An amplifier's vibrating signal that exceeds the maximum peak amplitude stated above can overload the amplifier under voltage supply, resulting in poor amplifier performance and signal distortion.

Power supply, constant current

To prevent the amplifier from damage, the input current must be regulated, which is commonly done by a constant current (CCD) in the data collector or analyzer power supply. The"biases" the Biased Output Voltage (BOV) to a predetermined level from the input supply voltage; the normal range for a good sensor's BOV is usually the nominal value given on the datasheet, 2V.

Temperature range

The maximum and minimum storage temperatures, as well as the temperature range over which the sensor will operate, are specified. Temperatures that are higher than the stipulated temperature can cause lasting damage. Short-term exposure to temperatures outside the defined range will not harm the sensor under typical circumstances.

Resonance frequency

The resonance frequency is the frequency point in the frequency response of the accelerometer that corresponds to the highest sensitivity output, and the unit is Hertz (Hz), which is the result of the sensor's own mechanical structure's intrinsic resonance.

Electrical noise

The amplifier circuit generates this electronic noise. "Broadband" or "Spectral" noise are the two types of noise. Broadband measures are total noise energy measurements over a given bandwidth (usually 2 to 25000 Hz). The equivalent vibration unit "g" can be used to express spectral noise, which is noise measured at a certain frequency. The measured noise reduces with increasing frequency in general. Low-frequency noise, on the other hand, is more harmful than high-frequency noise since lower acceleration measurements are often associated with lower frequencies.

Peak amplitude

The highest amplitude vibration that the sensor can measure before the amplifier is distorted by overload is defined by the peak amplitude.

Sensor sensitivity, supply voltage, and sensor BOV all influence peak amplitude. This is true for all  type two-wire sensors.

 

Ⅵ.  Installation Of The Accelerometer

Using an accelerometer for measurement, in order to make the data accurate and easy to use, it can be installed using a variety of methods. There are several applications for you to choose.

 1. Screw installation

Its frequency response can be approximated to the original calibration frequency response via screw placement, which is known as stiff installation. On the measured object, the screw installation is used to drill holes and taps along the axis of the vibration source, allowing holes to be drilled.

2. Adhesive installation

When drilling is not possible, various adhesives, such as "502," epoxy resin glue, double-sided adhesive tape, and plasticine, can be utilized. It's worth noting that the first two ways' use frequency is similar to the rigid installation method, while the latter two are typically employed in low-frequency fields and will substantially reduce the frequency to be recorded. The bonding method is ineffective for measuring impact.

3. Magnetic base

The magnetic base has the advantage of not destroying the measured object and being portable. However, it should be noted that using the magnetic base test would limit the accelerometer's frequency response to less than one-third (the magnetic base should be removed from the short circuit component when in use!). When using, place the magnetic base on the object to be measured first, then screw on the sensor, or gently adsorb the two on the object. The sensor will acquire charge as a result of the shock, which will compromise the test accuracy.

4. Mica sheet/tetrafluoro film

Mica sheet installation serves two purposes: heat insulation and insulation. A mica sheet with a thickness of 0.1 mm can be utilized for high-temperature specimens, although the frequency response of the accelerometer will be slightly diminished.   and tetrafluoro are the ideal materials for insulating the specimen from the accelerometer.

5. Three-way sensor installation

For screw through hole installation, side thread for inspection or testing.