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Accelerometer

Ideally an accelerometer has zero size and zero mass, but in actuality the mass of the unit does load the test structure. Simply pushing on a test location with a finger can sometimes help 'get a feel' of the structure's local compliance. This rough estimate of the compliance can be useful in assessing at what frequency test errors result from the weight of the accelerometer. A piezoelectric accelerometer generates its signal by taking mechanical energy from the test specimen and converting some of that energy into an electrical output. In attempting to attain high sensitivity, high resonance frequency, and a low transducer mass- all desirable qualities- one is limits by the efficiency of the conversion of mechanical energy into electrical energy.

 

What is Accelerometer?

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Can accelerometer is used to measure the acceleration of a vibrating body. If the natural frequency of the instrument is very high compared to the frequency, which is to be measured, the ratio is very small (very much less than 1), Since the natural frequency of the instrument is very high, it is very light in construction.  Some crystals like barium titanate, lead zirconate have very high natural frequencies, e.g, 10 kHz or above. An accelerometer can easily integrated once or twice with the help of modern electrical circuits and, thus, obtain the velocity or displacement of the system. There is no low cut off frequency for an accelerometer as in case of seismometer, but there is an upper limit up to which the accelerometer has linear response. Because of its small size and usefulness for determining velocity and displacement besides acceleration, it is very widely used as a vibration measuring device and is termed high frequency transducer. The voltage signals obtained from an accelerometer are usually very small which can be pre-amplified to see them bigger in size on oscilloscope. For getting velocity and displacement, double integration device may be used and the results are obtained on screen.

How does it works?

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An accelerometer contains a mass on a spring. When the device moves, the mass starts compressing the spring until it reaches the point where the mass moves at the same speed as the accelerometer. When that point is reached, the displacement of the mass is measured and that value is the read out of the sensor. 

A conventional accelerometer requires charge amplifier to amplify the induced acceleration by conventional accelerometer. Such an amplifier usually requires a supply voltage higher than that of an ambedded system.

Uses of Accelerometer

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Recently the interest about the services in the ubiquitous environment has increased. These kinds of services are focusing on the context of the user's activities, location or environment. There were many studies about recognizing these contexts using various sensory resources. To recognize human activity, many of them used an accelerometer, which shows good accuracy to recognize the user's activities of movements, but they did not recognize stable activities which can be classified by the user's emotion and inferred by physiological sensors. In this includes an accelerometer and physiological sensors, we used them with a fuzzy Bayesian network for the continuous sensor data. The fuzzy membership function uses three stages differed by the distribution of each sensor data. Experiments in the activity recognition accuracy have conducted by the combination of the usages of accelerometers and physiological signals. For the result, the total accuracy appears to be 74.4% for the activities including dynamic activities and stable activities, using the physiological signals and one 2 axis accelerometer. When we use only the physiological signals the accuracy is 60.9%, and when we use the 2 axis accelerometer the accuracy is 44.2%. We show the using physiological signals with accelerometer is more efficient in recognizing activities.

Types of Accelerometer

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There are different types of accelerometers based on the different displacement measuring techniques. Based on the type of spring element and the form of damping used, there are number of types of accelerometer.
Potentiometric Type Accelerometer:
It is the simplest form of accelerometer. The seismic mass is attached to the wiper arm of the potentiometer. The change in position of mass relative to accelerometer case is converted directly into change in resistance at the output terminals. By using different signal conditioning circuits, change in resistance can be converted into corresponding voltage or current signal. In this type of accelerometer, damping is provided by either filling the space inside the case with a viscous fluid of providing  dashpot. 

LVDT Accelerometer:
In LVDT accelerometer, the core of LVDT itself acts as a seismic mass. The core is attached to two spring steel one at top and other at bottom with the help of rods. The spring steels are attached to the case firmly. With the help of this arrangement null position of the core of LVDT is maintained. Both the spring steel provide necessary spring action. As compared to the potentiometric type accelerometer, the natural frequency of LVDT accelerometer is higher and it is around 80 Hz. It offers lower resistance to the motion, so the resolution is better compared with the potentiometric type accelerometer. No errors due to the moving contacts as there are no sliding contacts in the instrument. 

Piezoelectric Accelerometer:
The piezoelectric accelerometer is based on a property of certain crystals that when it is subjected to stress, a voltage is generated across the crystal. In the piezoelectric accelerometer, a spring loaded crystal is placed touching the seismic mass. When accelerometer is subjected to an acceleration, the seismic mass stress the crystal by a force equal to (F=ma). This generates voltage across crystal which is directly proportional to the acceleration. The main advantage of the piezoelectric accelometer is that the crystal acts as a spring and damper in the instrument. The natural frequency of the crystal is very high, hence it can be used for very high frequency vibration measurements. 

Strain Gauge Accelerometer:
Similar to the piezoelectric crystals, strain gauge elements also act as spring element and mass displacement measuring element. The main advantage of this fact is that, the construction of the strain guage accelerometer is simple. This type accelerometer is called cantilever beam type accelerometer. When the accelerometer is subjected to acceleration, the beam bends producing strains in the beam which are directly proportional to the acceleration. Then the displacement of mass with respect to case is directly proportional to the force acting on mass. Eventhough this type of accelerometer is simple in construction, its natural frequency is very low because of limitations of the beam size on which gauges are mounted. To increase bandwidth, fluid filled damping is provided. The standard wire, foil or semiconductor strain gauges are used in wheatstone's bridge, with two gauges in compression and remaining two in tension. To have higher frequency response, generally a cylinder type accelerometer is used.

Accelerometer Sensor

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There are many different types of accelerometers, ranging from strain gauge devices to those that use electromagnetic induction. For example, the force that causes the acceleration may be converted into a proportional displacement using a spring element, and this displacement may be measured using a convenient displacement sensor. Piezoelectric accelerometer is used as an accelerometer sensor, which uses a piezoelectric element to measure the inertia force caused by acceleration.

Accelerometer Applications

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By using accelerometer of any type, the acceleration of a body of known mass due to the applied force can be measured. Then by using Newton's law of mass acceleration, the applied unknown force can be calculated. But practically use of accelerometers for force measurement is limited because the forces are not free but they are the part of system. It is very difficult to decouple these forces from the system. Also many systems, the body on which force acting is not free to accelerate. Still this technique is useful in measuring some transient forces. The advanced application of this technique is in calibration of forces produced by thrust motors in the space vehicles.  
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