Table of ContentsPrinciples of OperationOptical AccelerometersThermal AccelerometerTunnel AccelerometerPiezoelectric AccelerometerPiezoresistive AccelerometerCapacitive Accelerometer This article aims to provide a review of the different operating principles of MEMS accelerometers. First, we will discuss the variety of acceleration sensors and their basic principles, as well as a brief overview of their manufacturing mechanism. Finally, the article will focus on the most commercialized and well-known accelerometer technique, namely the capacitive technique. Additionally, a comparative table of their performance based on acceleration sensor characteristics such as dynamic range, sensitivity, resolution and operating temperature will be presented. finally, an evaluation of the different MEMS-Accelerometer detection techniques as well as the conclusion conclude the article. I. Introduction Acceleration sensors play a vital role in micromachined technology. Moreover, the demand for new high-performance accelerometers is increasing every day. The first industry to take advantage of the benefits of MEMS accelerometers was the automotive industry in 2000 using MEMS-Accs for suspension systems and car controllability and similarly for safety systems such as the dash system. airbags. Nowadays, the application scope of accelerometers covers almost all aspects of engineering sciences. Compared to conventional accelerometers, MEMS-Accs have the advantages of extremely small size and ability to be mass-produced, as well as significantly lower manufacturing costs. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get an original essay Therefore, the application spectrum of these acceleration sensors is not limited to the automobile industry as they have paved their way in a multitude of scientific branches. . 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 inexpensive accelerometers has increased sharply [2]. Additionally, MEMS accelerometers are now a crucial part of spacecraft and rocket navigation systems. Moreover, if we take a closer look at the mainstream market of these accelerometers, and based on the HIS-MEMS market tracking, the market share of MEMS accelerometers is increasing rapidly, the main reason being that they are now an inseparable part navigation of smart devices. and monitoring systems. Similarly, in bioengineering where the size of the sensor is highly underestimated for researchers, MEMS accelerometers are used for health monitoring using the implantation of sensors inside the body [3 ]. Based on the above-mentioned applications, different technologies and principles have been used so far for their manufacturing and operation method. The vast majority of applications used capacitive and piezoresistive accelerometers because their transduction mechanism and fabrication are easier to use, but there are more different operating principles which will be discussed in the next section of this article. Operating principles As with any accelerometer, the basic operating principle is based on a fixed local inertial reference frame, a beam and of course the test mass. When external forces apply the mass will be displaced relative to the local inertial frame, the source of this force could be a constant force of gravity called a static force orit could be caused by a shock or a movement which can be called dynamic forces. Referring to the sensor definition, the acceleration sensor must convert the mechanical movement that deflected the test mass into a readable computer signal. For this reason, there are several transduction mechanisms, some of which are more relevant, such as capacitive or piezoresistive accelerometers and also others. mechanisms such as optical, piezoelectric, thermal and tunneling, piezoelectric, electromagnetic and surface acoustic wave (SAW) accelerometers. Due to content restriction and less practical applications compared to other mechanisms, in this article all the principles mentioned above except electromagnetic and SAW will be discussed. Optical accelerometers The operating principle of optical accelerometers lies in the characteristics of a beam of light. Compared to well-known capacitive accelerometers, optical Accs feature better sensitivity and resolution as well as higher thermal stability, making them applicable in hazardous environments. Optical accelerometers, instead of measuring the displacement of the proof mass, measure the variation in characteristics of light waves, such as measuring the stress distribution among the proof mass when it is deflected (photoelastic effect) or determining the The effect of different forces and mass displacements on the phase of the optical signal (Phase modulation). Phase modulation is normally used when a higher dynamic range is required. Other methods are intensity modulation which is simple to manufacture but highly dependent on high quality light sources, compared to wavelength modulation which is completely independent of light source deviation and is very precise and sensitive. The outstanding advantage of Optical-Accs 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 mass test, it will move on its (y) axes which will cause a change in output wavelength. Therefore, the magnitude and direction of the acceleration would be measured based on the wavelength difference that occurred. Thermal accelerometer Compared to the other techniques mentioned above, thermal accelerometers do not use a proof mass to detect acceleration, they use the phenomenon of thermal convection. Thermal accessories typically consist of a silicon-etched SNx heating element with two temperature sensors on both sides, inside the thermally insulated encapsulated cavity. The radiator reduces the density of the surrounding air (liquid). Therefore, when there is no acceleration, two temperature sensors will detect the same temperature figure.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). Therefore, this temperature difference will be detected and amplified to be converted into a digital signal by the Wheatstone principle. bridge. [6] The manufacturing process of this accelerometer is simple, which means lower manufacturing cost than other mechanisms. Since there is no proof mass, the thermal accelerometer has very good shock resistance and, compared to capacitive sensors, it has more sensitivity. On the other hand, the dynamic range is confined and the low frequency range does not make it suitable formeasurements of instantaneous shocks or falls. detection. Tunneling-Accs accelerometers generally consist of a metal tip connected to a proof mass which has a few nanometer distances to a counter electrode and the working principle lies in the tunneling effect of quantum electrons. In order to activate the sensor, a small bias voltage (approximately 100 mV) must be applied. This voltage therefore creates a small current between the metal-coated tip and the counter electrode. When acceleration is applied, the movement of the proof mass will cause a sub-angstrom displacement of the tip, which will cause a change in the tunnel current. The goal of this method is to keep the tunnel current (1 nA) constant over time. Therefore, feedback forces were applied to return the mass to its resting position. As a result, the magnitude of the acceleration could be measured in a closed loop. detection circuit and using the variation of the deviation voltage. The design and manufacture of Tunnel-Accs has varied since their introduction, cantilevered, lateral and bulk micro-machined are some of them. Tunneling accelerometers have a low drive voltage supported by a wide frequency bandwidth as well as higher sensitivity than capacitive accelerometers. On the other hand, when it comes to the nanoscale gap, they have a complicated manufacturing process and higher production costs. Piezoelectric accelerometer This type of access. take advantage of the piezoelectric effect inherent in the materials. A piezoelectric acc. as shown in Figure 4. It is usually a piezoelectric material which is usually thin ZnO or PZT, sandwiched by two electrodes and deposited on a silicon cantilever beam. [8] The beam is attached to the frame on one side and on the other side there is a test mass. In the presence of acceleration, the mass displacement causes deformation of the beam, in the same way, the piezo material undergoes compression or tension. The acceleration could then be measured by calculating the potential difference that occurred. PZT has higher piezoelectric constant and sensitivity, but it cannot be integrated or miniaturized. On the other hand, ZnO has lower sensitivity but can be further integrated with new technology manufacturing compatibility and its sensitivity could be improved by miniaturization. Global piezoelectric access. Has high sensitivity and compares 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 the first MEMS accelerometer to be commercialized by an automobile manufacturer for its safety systems. The backbone of this method is based on the variation in resistivity of a material under stress. Early piezoresistive acc designs have [9] which maintains the proof mass and is supported by a fixed frame. Additionally, the piezoresistors were located at the special place on the beam where maximum deformation and stress occurs (usually the edges) and display Their circuit is based on the Wheatstone bridge principle. acceleration and displacement of the test mass will cause deformation of the beam and hence the resistivity of the piezoresistors will change, the variation in resistance will cause changes in the output voltage. Piezoresistive accelerometers are very reliable and simple to manufacture but integration is not simple. So far, almost all articles focus on improving performance and.