WHAT IS AN ACCELEROMETER?
An accelerometer is a device that measures the proper acceleration of the device. This is not necessarily the same as the coordinate acceleration (change of velocity of the device in space), but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame of reference of the accelerometer device. For an example of where these types of acceleration differ, an accelerometer will measure a value when sitting on the ground, because masses there have weights, even though they do not change velocity. However, an accelerometer in gravitational free fall toward the center of the Earth will measure a value of zero because, even though its speed is increasing, it is in an inertial frame of reference, in which it is weightless.
An accelerometer thus measures weight per unit of (test) mass, a quantity also known as specific force, or g-force. Another way of stating this is that by measuring weight, an accelerometer measures the acceleration of the free-fall reference frame (inertial reference frame) relative to itself.
Most accelerometers do not display the value they measure, but supply it to other devices. Real accelerometers also have practical limitations in how quickly they respond to changes in acceleration, and cannot respond to changes above a certain frequency of change.
Single- and multi-axis models of accelerometer are available to detect magnitude and direction of the proper acceleration (or g-force), as a vector quantity, and can be used to sense orientation (because direction of weight changes), coordinate acceleration (so long as it produces g-force or a change in g-force), vibration, shock, and falling (a case where the proper acceleration changes, since it tends toward zero). Micromachined accelerometers are increasingly present in portable electronic devices and video game controllers, to detect the position of the device or provide for game input.
Pairs of accelerometers extended over a region of space can be used to detect differences (gradients) in the proper accelerations of frames of references associated with those points. These devices are called gradiometers, as they measure gradients in the gravitational field. Such pairs of accelerometers in theory may also be able to detect gravity waves.
An accelerometer is an electromechanical device that will measure acceleration forces. These forces may be static, like the constant force of gravity pulling at your feet, or they could be dynamic - caused by moving or vibrating the accelerometer.
Thus An accelerometer is an instrument for measuring acceleration, detecting and measuring vibrations, or for measuring acceleration due to gravity (inclination). Accelerometers can be used to measure vibration on vehicles, machines, buildings, process control systems and safety installations. They can also be used to measure seismic activity, inclination, machine vibration, dynamic distance and speed with or without the influence of gravity.
HOW DOES AN ACCELEROMETER WORK?
Used for calculating acceleration and measuring vibrations, the accelerometer is capable of detecting even the slightest movements, from the tilting of a building to smallest vibration caused by a musical instrument. Inside the accelerometer sensor minute structures are present that produces electrical charges if the sensor experiences any movement.
Accelerometers need to be placed on the surface of the object in order to determine the vibrations. It is not capable of work in isolation or apart from the object it is required to assess, it must be firmly attached to the object in order to give precise readings.
Conceptually, an accelerometer behaves as a damped mass on a spring. When the accelerometer experiences an acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration.
In commercial devices, piezoelectric, piezoresistive and capacitive components are commonly used to convert the mechanical motion into an electrical signal. Piezoelectric accelerometers rely on piezoceramics (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline). They are unmatched in terms of their upper frequency range, low packaged weight and high temperature range. Piezoresistive accelerometers are preferred in high shock applications. Capacitive accelerometers typically use a silicon micro-machined sensing element. Their performance is superior in the low frequency range and they can be operated in servo mode to achieve high stability and linearity.
Modern accelerometers are often small micro electro-mechanical systems (MEMS), and are indeed the simplest MEMS devices possible, consisting of little more than a cantilever beam with a proof mass (also known as seismic mass). Damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not result in a lower sensitivity.
Under the influence of external accelerations the proof mass deflects from its neutral position. This deflection is measured in an analog or digital manner. Most commonly, the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple, reliable, and inexpensive. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process steps are needed during the fabrication sequence. For very high sensitivities quantum tunneling is also used; this requires a dedicated process making it very expensive. Optical measurement has been demonstrated on laboratory scale.
Another, far less common, type of MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor.
Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.
Micromechanical accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of g's. The designer must make a compromise between sensitivity and the maximum acceleration that can be measured.
KINDS OF ACCELEROMETER
The two kinds of basic accelerometers are:
1. ANALOG ACCELEROMETER
At times Inputs and output readings also matter especially when it comes to determining the kind of accelerometer that needs to be placed on a certain object. If the output is digital then a digital accelerometer must be placed and vice versa. The main feature of this accelerometer is that the output tends to change when there is even a slight change in the input.
The most common type of this accelerometer is used in airbags of automobiles, to note the sudden drop in the speed of the vehicle and to trigger the airbag release. Even laptops are now being equipped with accelerometers in order to protect the hard drive against any physical dangers, caused mainly due to accidental drops.
2. DIGITAL ACCELEROMETER
The digital accelerometer is more sophisticated than the analog. Here the amount of high voltage time is proportional to the acceleration. One of its major advantages is that it is more stable and produces a direct output signal. Accelerometers are now also used in aerospace and many military applications, such as missile launch, weapon fire system, rocket deployment etc. Many a times these accelerometers are used to protect fragile equipment during cargo transportation, and report any strain that might cause a possible damage. Some companies have also managed to develop a wireless 3-axis accelerometers which are not only low in cost but are also shock durable. This 3-axis accelerometer has sensors that are used to protect mobiles and music players. Also these sensors are used in some of the devices used for traffic navigation and control.
Types of accelerometer
- Piezoelectric accelerometer
- Shear mode accelerometer
- Surface micromachined capacitive (MEMS)
- Thermal (submicrometre CMOS process)
- Bulk micromachined capacitive
- Bulk micromachined piezoelectric resistive
- Capacitive spring mass base
- Electromechanical servo (Servo Force Balance)
- Strain gauge
- Magnetic induction
- Surface acoustic wave (SAW)
- Laser accelerometer
- DC response
- High temperature
- Low frequency
- High gravity
- Modally tuned impact hammers
- Seat pad accelerometers
- Pendulating integrating gyroscopic accelerometer
Depending upon the kind of work, the accelerometers vary in the way they are prepared and how they work. Some accelerometers use piezoelectricity, these are man-made. In such accelerometers the acceleration is calculated based upon the charges derived from the microscopic crystalline structures when they are accelerated due to motion.
Another kind works with the capacitance and the changes initiated within it as a result of some accelerative force. This technology is used from automotive industry to agriculture industry and from NASA to military researches and operations.
This device is used to measure strain in an object, which is detected by a foil strain element. If the object, to which the gauge is attached is some how deformed that creates electrical charges and is known as the gauge factor.
ACCELEROMETER IS USED IN:
Due to high demand and wide spread use of accelerometers in the automotive industry and new hi-tech technology, these sensors are now light weight and are available at low cost and reduced prices.
Microphones also carry accelerometers. That is how they are able to detect the minute frequencies.
The forces that can cause vibrations which are detected by the accelerometer can be static, dynamic or gravitational. Certain accelerometers are rated G. G stands for Gravity. Such accelerometers are used mostly in robotics. They are more sensitive to motion and can be triggered at the slightest changes in gravitational pulls.