Most of the modern devices use sensors to monitor and control various physical quantities like pressure, temperatures, humidity, light intensity, direction, etc. One of such sensors, used to measure the acceleration of the devices, is called the accelerometer sensors.
Once upon a time, you would have found sensors like this only in advanced machines such as space rockets or jumbo jets. Now they are in virtually every smartphone, laptop, car, and gaming console. Let’s dig deeper and find out what they are, how they work, and what are they used for?
What Is An Accelerometer?
Definition: An accelerometer is an electromechanical tool that measures acceleration (the rate of change of velocity). The acceleration could be static, like acceleration due to gravity, or it could be dynamic like movement and vibrations caused by an external factor.
By measuring the amount of gravitational acceleration, the tool can figure out the angle it is tilted at with respect to the Earth. For instance, an accelerometer resting on the Earth’s surface will measure an acceleration of 9.81 m/s2 in a straight upward direction.
By measuring the amount of dynamic acceleration, it can figure out how fast and in what direction the device is moving. A three-axis accelerometer, for example, can detect magnitude and direction (in all three axes) of acceleration as a vector quantity.
Accelerometers are used in various industries and scientific research. They are mostly used in electronic devices to sense orientation, coordinate acceleration, shock, and vibration.
Accelerometers integrated into smartphones, for example, figure out when to switch the screen layout from landscape to portrait. The data provided by these sensors can help determine whether a device is going uphill or falling downward.
Highly sensitive accelerometers are integrated into inertial navigation systems of missiles and jets. Unmanned aerial vehicles also use such devices to stabilize flight.
How Does An Accelerometer Work?
The mechanical accelerometer consists of a spring attached with a mass. This spring is usually suspended inside an outer casing. When the whole device accelerates, the casing moves off immediately in the same direction. The mass, however, stays at its position (for a short duration), stretching the spring with a force that corresponds to the acceleration.
Working principle of a Mechanical accelerometer
By measuring the length of a stretch spring, we can determine the acceleration. This can be done in various different ways. A seismometer, for example, uses the same principle to measure earthquakes.
When an earthquake occurs, it shakes the seismometer cabinet but a mass takes longer to move. A pen is attached to the mass to trace its movement on a paper chart.
The modern accelerometers generate electric or magnetic signals, instead of using a pen trace on a paper.
Most Common Types of Accelerometer
Most of the commercial devices are equipped with capacitive, piezoresistive, and piezoelectric instruments to transform the mechanical motion into an electrical signal.
1. Piezoelectric accelerometers employ the piezoelectric effect of specific materials to measure acceleration, vibration, or mechanical shock. These materials accumulate electric charge (piezoelectricity) in response to applied mechanical stress.
Working principle of a piezoelectric accelerometer
A piezoelectric material, such as lead zirconate titanate, is attached to the mass. When the accelerometer moves, the mass puts a mechanical pressure on this material. As a result, the material generates a tiny electric voltage, which can be decoded to figure out the corresponding acceleration.
2. Piezoresistive Accelerometers work on a similar principle. They utilize resistance change of piezoresistive materials to convert mechanical strain to a DC output voltage. These types of accelerometers are suitable for impact measurements where g level and frequency range are significantly high.
Endevco 727 | a lightweight piezoresistive accelerometer ideal for measuring shock during drop testing
Piezoelectric components, on the other hand, are unmatched in terms of their high-temperature range and low packaged weight.
3. Capacitive accelerometers rely on a change in electrical capacitance in response to acceleration. They contain two components: a primary (stationary) plate attached to the housing and a secondary plate connected to the mass, which is free to move inside the housing.
The capacitance changes with the distance between two metal plates, and by measuring the capacitance, one can determine the applied acceleration. These types of accelerometers can measure constant as well as slow transient and periodic acceleration.
A triaxial capacitive accelerometer
Modern accelerometers come in all three forms. They are often micro-electro-mechanical systems (MEMS) containing several components, each between 1 and 100 micrometers in size. The accelerometers integrated into tablets and smartphones are usually smaller than 100 millimeter-square.
A micromechanical accelerometer is sensitive only to a single direction in the plane. A two-axis accelerometer is built by integrating two devices perpendicularly, and a three-axis accelerometer can be made by adding another out-of-plane device. The integrated module could be much more accurate than three discrete devices merged after packaging.
To achieve ultra-high sensitivity, one can use quantum tunneling. However, the process is extremely difficult and expensive.
With existing technologies, we can measure accelerations up to thousands of g’s. The engineers and manufacturers have to compromise between the maximum measurable acceleration and device sensitivity.
Accelerometers are used in various fields, from engineering and consumer electronics to biology and medical applications. Below are the most common uses of accelerometer sensors.
The inertial navigation system (also known as inertial reference platform) uses a computer and accelerometers to continuously measure the location, orientation, and speed of a moving object without requiring any external reference.
Accelerometers are widely used for measuring vibration on machines, vehicle engines, and buildings. In the automobile sector, high g accelerometers are used for detecting vehicle crashes and deploying airbags at just the right time.
They are also used for monitoring machinery health and reporting vibration of rotating instruments such as compressors, turbines, which, if not maintained, can lead to expensive repairs. Some accelerometers are specifically configured (integrated into gravimeters) to measure gravitational forces.
In spacecraft, accelerometers are used to detect apsis, the point in the orbit of a satellite at which it is furthest from the Earth.
They are used in almost all laptops, mobiles, and cameras to detect the position and orientation of the device and display content in an upright position on screens. Gaming consoles, such as PlayStation’s DualShock remote, uses a three-axis accelerometer to make steering more realistic in racing games.
Many laptop manufacturers use accelerometers to protect hard drives from damage. If the sensor detects a sudden drop, the heads of the hard drive are parked to avoid disk damage and data loss.
Biology and Medical Applications
The use of accelerometers is increasing in the biological sciences. The data provided by highly sensitive three-axis accelerometers allows scientists to discriminate the behavioral pattern of animals while they are out of sight.
Many automated external defibrillators contain an accelerometer to determine the depth of CPR chest compression.
Several companies manufacture watches for athletes, which consist of accelerometers to measure speeds and distances covered by runners. Modern sleep-phase alarm clocks are also integrated with accelerometric sensors so that they can detect movement of a sleeper and wake the person in the Non-REM sleep cycle.