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Physically, acceleration is a vector quantity having both direction and magnitude that is defined as the rate at which an object changes its velocity with respect to time. It is a measure of how fast speed changes . An object is accelerating when its velocity is changing. [1.ref11 ] In order to measure acceleration, an acceleration sensor called accelerometer is used. Accelerometer measures in units of g. A g is the acceleration measurement for gravity which is equal to 9.81 m/s². However, depending on altitude, this measurement can be 10 m/s² in some place.
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where f is natural frequency in Hz
k is spring constant in N/m
m is seismic mass in kg
If there was no friction in the spring-mass system, the mass would oscillate forever. This is, however, not the case in reality. The friction that causes the system to rest is defined by a damping coefficient that has a unit of s^-1. The effect of oscillation is described by periodic damped signal which has the equation as follows. [2,2.]
where XT(t) is transient mass position
μ is damping coefficient
f is natural frequency in Hz
Now two parameters that affect the accelerometer have been described. If the spring-mass system is exposed to a vibration, then the resultant acceleration is given by
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Figure 2 a. Response of spring-mass system to vibration compared to w^2 prediction b. Effect of various peak motion [2,3]
Reprinted from Process Control Instrumentation Technology book
Define f as the applied frequency, and fN is the natural frequency of the accelerometer. When f < fN , the natural frequency has little effect on the operation of the accelerometer. when f > fN, the accelerometer output is independent of the applied frequency. An important point worth noting is that with the applied frequency much larger than the natural frequency, the accelerometer becomes a measurement of vibration displacement . At near the resonance of accelerometer's natural frequency, the output of the accelerometer becomes high non-linear.
A rule of thumb is that with f < fN, the safe maximum applied frequency should be f < 2/5 fN.With f > fN, the minimum applied frequency should be f > 5/2 fN.[2,4.] So to best avoid the severe effect of resonance, the accelerometer should not be used near the resonance of their frequency because the output will become non-linear. Furthermore, the accelerometer should not be used with the applied frequency larger than the natural frequency if the measurement system is meant to measure acceleration.This has an important implication in the selection of the accelerometer for an application.
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Until now, all the basic characteristics of an accelerometer have been explained. One important characteristic is the sensitivity and the question is how to improve the sensitivity of the accelerometer. For analog-output sensors, sensitivity is ratiometric to supply voltage. Therefore, the sensitivity can be improved by increasing the supply voltage for the sensor as long as it does not exceed the maximum rating power supply specified by the datasheet. So this means that doubling the supply, for example, doubles the sensitivity. [9.]
Interface electronic circuits
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Unlike the three types of accelerometers mentioned above, Piezoresistive accelerometers do not use a spring. Instead of that, the mass is attached to cantilever beam which in turn is sandwiched in between strain gages.
Figure 9. Piezoresistive accelerometer [4,2]
Piezoresistive accelerometer's working principle is based on piezoresistive effect. As far as the piezoresistive effect is concerned, applied mechanical stress changes the resistivity of a semiconductor. The force exerted by the seismic mass changes the resistance of the strain gages. Piezoresistive accelerometers are used in high shock applications, and they can also measure accelerations down to zero Hz or up to ±1000g. But, the disadvantage is that they have limited high frequency response. [6.]
MEMS-Based Accelerometers
MEMS stands for Micro-Electro Mechanical System. It is the technology which is based advanced technologies used to form small structures with dimensions in micrometer scale. MEMS technology is now being employed to manufacture state-of-the-art MEMS-based accelerometers.
Initially, MEMS accelerometers was designed using piezoresistors. Since piezoresistors are less sensitive than capacitive detection, the majority of MEMS accelerometers nowadays use capacitive sensing principle. MEMS-based accelerometer typically consists of a proof mass with plates attached through a mechanical suspension system to a reference frame. Movable plates (part of the seismic mass) and the outer plates in stationary state form differential capacitor. Because of the forces due to acceleration, the seismic mass deflects; the deflection is measured in terms of capacitance change. [4,3.]
Figure 10. MEMS-based accelerometer structure [4,3]
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- Size and mass: Since the size and mass of an accelerometer have an effect on the object being tested, the mass of the accelerometers should be greatly smaller than that of the system to be monitored. [4,4.]
In addition to understanding some of the important specifications just mentioned, it should be noted that if the measurement is meant to measure acceleration, then the applied frequency f should be less than the natural frequency fN of the accelerometer.
As a rule of thumb, in low-frequency applications (having a bandwidth on orders from 0 to 10 Hz), position and displacement measurements generally provide good accuracy. In the intermediate-frequency applications (less than 1 kHz), velocity measurement is usually favored. In measuring high-frequency motions with appreciable noise levels, acceleration measurement is preferred. [7,327.]
Applications of accelerometers
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