Index Principles of Operation Optical Accelerometers Thermal Accelerometer Tunnel Accelerometer Piezoelectric Accelerometer Piezoresistive Accelerometer Capacitive Accelerometer This document aims to provide a review of the different operating principles of MEMS accelerometers. Initially the different types of acceleration sensing and their basic principles will be discussed as well as a brief overview of their manufacturing mechanism and finally the article will focus on the most commercialized and well-known accelerometer technique, namely the capacitive one. Furthermore, a comparative table of their performance will be shown based on acceleration sensor characteristics such as dynamic range, sensitivity, resolution and working temperature. finally, an evaluation of different MEMS accelerometer sensing techniques and conclusions conclude the paper. I. Introduction Acceleration sensors play a vital role in micromachined technology, moreover, the demand for new and high-performance accelerometers is increasing every day. The first industry to exploit the benefits of MEMS accelerometers was the automotive industry in 2000, using MEMS-Acc for car suspension and controllability systems and similarly for safety systems such as the airbag system. Nowadays the scope of application of accelerometers covers almost every aspect of engineering science. MEMS-Acc compared to conventional accelerometers have advantages of extremely small size and ability to be mass produced and, above all, lower production costs. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Accordingly, the application spectrum of these acceleration sensors is not limited to the automotive industry while they have opened up to the multitude of branches of science. For example, nowadays in the aviation and aerospace industry and after the emergence of modern autonomous unmanned aerial vehicle (UAV) technology, the demand for highly sensitive and low-cost accelerometers has increased significantly [2]. Furthermore, MEMS accelerometers are now the crucial part of spacecraft and rocket navigation systems. Furthermore, if we take a closer look at the consumer market of these accelerometers and based on the HIS-MEMS market tracker, the market portion of MEMS accelerometers is increasing rapidly and the main reason is that they are now an inseparable part of computer navigation. smart devices. and tracking systems. Similarly, in bioengineering, where the size of the sensor is much under magnification for researchers, MEMS accelerometers are used for health monitoring with the help of implanting sensors inside the body [3]. Based on the above-mentioned applications, different technologies and principles have been used for their fabrication and operation method until now, the vast majority of applications used capacitive and piezoresistive accelerometer since their transduction and fabrication mechanism is easier to operate, but There are more different operating principles which will be discussed in the next section of this document. Operating Principles As with any accelerometer, the basic operating principle is based on a fixed local inertial frame, a beam and, of course, the proof mass. When external forces are applied, the mass will be displaced relative to the local inertial system, the source of this force could be the constant gravitational force called static force or it could be caused by shocks or movements which can be called dynamic forces. With reference to the definitionof sensor, the acceleration sensor should convert the mechanical movement that has deviated the test mass, into a signal readable by the computer, for this reason there are different transduction mechanisms, some of them are more relevant such as capacitive or piezoresistive accelerometers and also others mechanisms such as optical, piezoelectric, thermal and tunneling, piezoelectric, electromagnetic, surface acoustic wave (SAW) accelerometers. Due to content restriction and less practical applications compared to other mechanisms, all of the above-mentioned principles except electromagnetic and SAW will be discussed in this document. Optical accelerometers The operating principle of optical accelerometers lies in the characteristics of a light beam. Compared to well-known capacitive accelerometers, optical accelerometers show better sensitivity and resolution, as well as higher thermal stability which make them applicable in hazardous environments. Optical accelerometers instead of measuring the displacement of the test mass measure the change in the characteristics of the light wave, for example by measuring the stress distribution between the test mass when it is deflected (photoelastic effect) or by determining the effect of different forces and mass shift on the phase of the optical signal (phase modulation). Phase modulation is normally used when a higher dynamic range is required. The other methods are intensity modulation, which is simple to implement but highly dependent on high-quality light sources, versus wavelength modulation which is completely independent of the deviation of the light source and is extremely accurate and sensitive. The outstanding advantage of optical accessories is their immunity against electromagnetic interference (EMI). Figure 1 shows the Optical-Acc sensor based on wavelength modulation by which light passes through the photonic crystal (PhC) and then enters the photodetector to measure the acceleration, when, and external forces applied to the test mass will move on its (y) axis which will cause a change in the output wavelength. As a result, the magnitude and direction of the acceleration would be measured based on the wavelength difference that occurred. Thermal Accelerometer Thermal accelerometers, compared to other techniques mentioned above, do not use a proof mass to detect acceleration, but use the phenomenon of thermal convection. Thermal-Accs typically consist of a silicon-etched SNx heater with two temperature sensors on both sides, within the thermally insulated encapsulated cavity. The heater reduces the density of the surrounding air (liquid), so when there is no acceleration two temperature sensors will detect the same temperature.2(A). By applying acceleration, the dense bubble will move in the direction of the applied acceleration, causing an asymmetric temperature profile for the detectors figure 2(B), consequently, this temperature difference will be detected and amplified to be converted into a digital signal according to the Wheatstone principle. bridge. [6] The manufacturing process of this accelerometer is simple, which means lower production costs compared to other mechanisms. Since there is no proof mass, the thermal accelerometer has extremely good shock resistance and compared to capacitive sensors it has higher sensitivity, on the other hand, the dynamic range is limited and the low frequency range makes it unsuitable for instant shock measurements or fall detection. Tunneling-Acc tunnel accelerometers typically consist of a metal tip connected to a proof mass that has distances of a few nanometers from a counter electrode and theoperating principle lies in the tunneling of quantum electrons. To activate the sensor a small bias voltage (approximately 100 mV) is required to be applied to this voltage, as a result a small current is created between the metal-coated tip and the counter electrode. When an acceleration is applied, the movement of the test mass will cause the sub-angstrom displacement of the tip which causes the current in the tunnel to change. The aim of this method is to keep the tunnel current (1nA) constant over time, therefore, feedback forces were applied to return the mass to its rest position, consequently, the magnitude of the acceleration could be measured by a closed loop detector circuit and with the help of the variation of the deflection voltage. The design and manufacturing of Tunnel-Accs vary from when they were introduced, including Cantilevered, Lateral, and Bulk-micromachined. Tunneling accelerometers have a low driving voltage supported by a wide frequency bandwidth and higher sensitivity than capacitive ones. On the other hand, with reference to Nanoscale Gap, they have a complicated manufacturing process and higher production costs. Piezoelectric accelerometer This type of access. exploit the intrinsic piezoelectric effect of the materials. A piezoelectric acc. as shown in figure.4 Usually, it consists of a piezoelectric material that is typically thin ZnO or PZT, sandwiched between two electrodes and deposited on a cantilever beam of silicon. [8] On one side the beam is fixed to the frame and on the other there is a test mass. In the presence of acceleration, the mass displacement causes the deformation of the beam, in the same way, the piezoelectric material undergoes compression or traction. The acceleration could therefore be measured by calculating the potential difference that occurred. PZT has a higher piezoelectric constant and sensitivity but cannot be integrated or miniaturized. On the other hand, ZnO has lower sensitivity but is embeddable, it is also compatible with new manufacturing technology, and its sensitivity could be improved by miniaturization. Overall piezoelectric-Acc. It has high sensitivity and compared to capacitive, lower power consumption and temperature dependence, as well as higher bandwidth. Piezoresistive Accelerometer The first MEMS accelerometer was piezoresistive and was developed in 1979[5]. It took twenty years for an automotive company to market the first MEMS accelerometer for its safety systems. The backbone of this method is based on the change in resistivity of a material under stress. Early piezoresistive acc designs have [9] which holds the test mass and is supported by a fixed frame, furthermore, the piezoresistors were placed at the special point of the beam where maximum deformation and stress occurs (usually edges) and the reading their circuit is based on the Wheatstone bridge principle. the acceleration and displacement of the test mass will cause the beam to deform and as a result, the resistivity of the piezoresistors will change, the change in resistance will lead to changes in the output voltage. Piezoresistive accelerometers are highly reliable and simple to fabricate, but integration is not simple. Almost all articles so far focus on improving performance and sensitivity by changing the geometric design and sensing mechanism or using different manufacturing technologies. For example, adding more beams instead of bending or using a cantilever with asymmetric gaps or ion etching of resistors onto the beam instead of diffusion.