An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertial frame of reference). Proper acceleration is different from coordinate acceleration, which is acceleration with respect to a given coordinate system, which may or may not be accelerating.
For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity straight upwards of about g ≈ 9.81 m/s2. The reason for the appearance of a gravitational offset is Einstein's equivalence principle, which states that the effects of gravity on an object are indistinguishable from acceleration.
When held fixed in a gravitational field by, for example, applying a ground reaction force or an equivalent upward thrust, the reference frame for an accelerometer (its own casing) accelerates upwards with respect to a free-falling reference frame. The effects of this acceleration are indistinguishable from any other acceleration experienced by the instrument so that an accelerometer cannot detect the difference between sitting in a rocket on the launch pad, and being in the same rocket in deep space while it uses its engines to accelerate at 1 g.
For similar reasons, an accelerometer will read zero during any type of free fall. This includes use in a coasting spaceship in deep space far from any mass, a spaceship orbiting the Earth, an airplane in a parabolic "zero-g" arc, or any free-fall in a vacuum. However, this does not include a (non-free) fall in which air resistance produces drag forces that reduce the acceleration until constant terminal velocity is reached. At terminal velocity, the accelerometer will indicate 1 g acceleration upwards.
For the practical purpose of finding the acceleration of objects with respect to the Earth, such as for use in an inertial navigation system, a knowledge of local gravity is required.
Read also: Individual Martial Arts Instruction
Apple suspended in an upward-moving elevator.
How Accelerometers Work
A basic mechanical accelerometer is a damped proof mass on a spring. When the accelerometer experiences an acceleration, Newton's third law causes the spring's compression (or extension) to adjust to exert an equivalent force on the mass to counteract the acceleration. Since the spring's force scales linearly with the length change (according to Hooke's law) and because the spring constant and mass are known constants, a measurement of the spring's compression (or extension) is also a measurement of acceleration. The system is damped to prevent oscillations of the mass and spring interfering with measurements.
Many animals have sensory organs to detect acceleration, especially gravity. In these, the proof mass is usually one or more crystals of calcium carbonate otoliths (Latin for "ear stone") or statoconia, acting against a bed of hairs connected to neurons. The hairs form the springs, with the neurons as sensors. The damping is usually by a fluid. Many vertebrates, including humans, have these structures in their inner ears. Most invertebrates have similar organs, but not as part of their hearing organs.
Mechanical accelerometers are often designed so that an electronic circuit senses a small amount of motion, then pushes on the proof mass with some type of linear motor to keep the proof mass from moving far. The motor might be an electromagnet or in very small accelerometers, electrostatic. Since the circuit's electronic behavior can be carefully designed, and the proof mass does not move far, these designs can be very stable (i.e. they do not oscillate), very linear with a controlled frequency response.
Stop Annoying Ring Alerts !! How to customize Ring Doorbell Motion Sensitivity Zones 2021
In mechanical accelerometers, measurement is often electrical, piezoelectric, piezoresistive or capacitive. Piezoelectric accelerometers use piezoceramic sensors (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline). They are unmatched in high frequency measurements, low packaged weight, and resistance to high temperatures. Piezoresistive accelerometers resist shock (very high accelerations) better. Capacitive accelerometers typically use a silicon micro-machined sensing element.
Read also: The Jade Fortress Martial Arts System
Modern mechanical accelerometers are often small micro-electro-mechanical systems (MEMS), and are often very simple MEMS devices, 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. 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 tunnelling is also used; this requires a dedicated process making it very expensive.
Another MEMS-based accelerometer is a thermal (or convective) accelerometer. It contains a small heater in a very small dome. This heats the air or other fluid inside the dome. The thermal bubble acts as the proof mass. An accompanying temperature sensor (like a thermistor; or thermopile) in the dome measures the temperature in one location of the dome. This measures the location of the heated bubble within the dome. When the dome is accelerated, the colder, higher density fluid pushes the heated bubble.
The measured temperature changes. The temperature measurement is interpreted as acceleration. The fluid provides the damping. Gravity acting on the fluid provides the spring. Since the proof mass is very lightweight gas, and not held by a beam or lever, thermal accelerometers can survive high shocks. Another variation uses a wire to both heat the gas and detect the change in temperature. The change of temperature changes the resistance of the wire.
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 another out-of-plane device, three axes can be measured. Micromechanical accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of g's.
Read also: Deep Dive into Court Martial Films
Applications of Accelerometers
Highly sensitive accelerometers are used in inertial navigation systems for aircraft and missiles. In unmanned aerial vehicles, accelerometers help to stabilize flight. Micromachined micro-electromechanical systems (MEMS) accelerometers are used in handheld electronic devices such as smartphones, cameras and video-game controllers to detect movement and orientation of these devices. Vibration in industrial machinery is monitored by accelerometers.
When two or more accelerometers are coordinated with one another, they can measure differences in proper acceleration, particularly gravity, over their separation in space-that is, the gradient of the gravitational field. A single-axis accelerometer measures acceleration along a specified axis. An accelerometer at rest relative to the Earth's surface will indicate approximately 1 g upwards because the Earth's surface exerts a normal force upwards relative to the local inertial frame (the frame of a freely falling object near the surface).
Accelerometers can be used to measure vibration on cars, 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. Accelerometers are also increasingly used in the biological sciences.
High frequency recordings of bi-axial or tri-axial acceleration allows the discrimination of behavioral patterns while animals are out of sight. Furthermore, recordings of acceleration allow researchers to quantify the rate at which an animal is expending energy in the wild, by either determination of limb-stroke frequency or measures such as overall dynamic body acceleration. Such approaches have mostly been adopted by marine scientists due to an inability to study animals in the wild using visual observations, however an increasing number of terrestrial biologists are adopting similar approaches.
Accelerometers are widely used in structural health monitoring (SHM) of buildings, bridges and other civil infrastructure to record the dynamic response under ambient and forced loads (e.g., wind, traffic, machinery and earthquakes). From these vibration records, engineers estimate modal properties-natural frequencies, damping ratios and mode shapes-often using operational modal analysis (OMA) techniques for in-service structures.
In seismic regions, arrays of accelerometers installed in buildings and other structures provide strong-motion data for rapid post-event assessments and long-term performance studies. Instrumentation and data-quality practices for building vibration measurements are guided by international standards. Choice of accelerometer technology depends on frequency range and amplitude.
Piezoelectric accelerometers are common for higher-frequency, higher-amplitude measurements, whereas low-noise MEMS accelerometers have become attractive for low-frequency building and bridge monitoring and for dense or wireless deployments due to cost and power advantages. Networked and wireless smart-sensor approaches allow distributed monitoring at scale. Accelerometers are often fused with other sensors to improve displacement and drift estimation, especially for large or flexible structures.
Beyond permanently instrumented assets, indirect and crowdsourced approaches using smartphone accelerometers have been explored, particularly for bridges. Research has shown that modal frequencies-and in some cases spatial vibration characteristics-can be estimated from accelerometer data collected by vehicles crossing bridges, offering a complementary, low-cost screening tool for large inventories.
Long-term case studies illustrate large-scale deployments. Hong Kong's Wind and Structural Health Monitoring System (WASHMS) has instrumented the Tsing Ma Bridge since 1997; subsequent publications report decades of monitoring for load and response in service. SHM data are used for continuous condition tracking, event-triggered assessments (e.g., after earthquakes), and to support asset management decisions.
Accelerometers have been used to calculate gait parameters, such as stance and swing phase. An inertial navigation system is a navigation aid that uses a computer and motion sensors (accelerometers) to continuously calculate via dead reckoning the position, orientation, and velocity (direction and speed of movement) of a moving object without the need for external references. An accelerometer alone is unsuitable to determine changes in altitude over distances where the vertical decrease of gravity is significant, such as for aircraft and rockets.
Accelerometers are also being used in Intelligent Compaction rollers. One of the most common uses for MEMS accelerometers is in airbag deployment systems for modern automobiles. In this case, the accelerometers are used to detect the rapid negative acceleration of the vehicle to determine when a collision has occurred and the severity of the collision.
Another common automotive use is in electronic stability control systems, which use a lateral accelerometer to measure cornering forces. A free-fall sensor (FFS) is an accelerometer used to detect if a system has been dropped and is falling. It can then apply safety measures such as parking the head of a hard disk to prevent a head crash and resulting data loss upon impact. This device is included in the many common computer and consumer electronic products that are produced by a variety of manufacturers. It is also used in some data loggers to monitor handling operations for shipping containers.
Some smartphones, digital audio players and personal digital assistants contain accelerometers for user interface control; often the accelerometer is used to present landscape or portrait views of the device's screen, based on the way the device is being held. Apple has included an accelerometer in every generation of iPhone, iPad, and iPod touch, as well as in every iPod nano since the 4th generation. Automatic Collision Notification (ACN) systems also use accelerometers in a system to call for help in event of a vehicle crash.
Prominent ACN systems include OnStar AACN service, Ford Link's 911 Assist, Toyota's Safety Connect, Lexus Link, or BMW Assist. Many accelerometer-equipped smartphones also have ACN software available for download. Accelerometers are used in vehicle Electronic stability control systems to measure the vehicle's actual movement. A computer compares the vehicle's actual movement to the driver's steering and throttle input.
The stability control computer can selectively brake individual wheels and/or reduce engine power to minimize the difference between driver input and the vehicle's actual movement. Nintendo's Wii video game console uses a controller called a Wii Remote that contains a three-axis accelerometer and was designed primarily for motion input. Users also have the option of buying an additional motion-sensitive attachment, the Nunchuk, so that motion input could be recorded from both of the user's hands independently.
A microphone or eardrum is a membrane that responds to oscillations in air pressure. Conversely, carefully designed sounds can cause accelerometers to report false data. A number of 21st-century devices use accelerometers to align the screen depending on the direction the device is held (e.g., switching between portrait and landscape modes). Such devices include many tablet PCs and some smartphones and digital cameras. The Amida Simputer, a handheld Linux device launched in 2004, was the first commercial handheld to have a built-in accelerometer.
Camcorders use accelerometers for image stabilization, either by moving optical elements to adjust the light path to the sensor to cancel out unintended motions or digitally shifting the image to smooth out detected motion. Some stills cameras use accelerometers for anti-blur capturing. The camera holds off capturing the image when the camera is moving. When the camera is still (if only for a millisecond, as could be the case for vibration), the image is captured. An example of the application of this technology is the Glogger VS2, a phone application which runs on Symbian based phones with accelerometers such as the Nokia N96.
Many laptops feature an accelerometer which is used to detect drops.
Structural Health Monitoring System of the Hardanger Bridge.
Understanding Motion Sensitivity
Stephanie Yip is one of our vestibular therapists here at Ladner Village Physiotherapy. In Part 1 of her two part series on motion sensitivity, Stephanie explains what motion sensitivity is, why some people suffer from it and what we can do about it. As a kid growing up, cars were my nemesis. I still have vivid memories of throwing up on pretty much every family road trip, and even worse, throwing up in my friend’s dad’s brand-new car on the way to a soccer game. Planes were even worse.
One of my earliest memories is of me as a 5-year-old, non-stop projectile vomiting on a 12-hour flight, with everyone around us handing us their puke bags since I had used up all of my own. How many of you can relate to this? After sharing with friends, I discovered that there are so many of us out there, struggling with cars, buses, boats, and thinking that this is something we just have to live with. Well, I am here to share with you all the greatest revelation which has completely turned my world upside down.
This does not have to be our reality. There is a solution. Just like how we rehabilitate and strengthen our ankle after a sprain, we too, can rehabilitate and strengthen our vestibular system to improve our motion sensitivity!
What is Motion Sensitivity?
There are two main types of motion sensitivity. One is considered visually induced motion sensitivity in which you experience symptoms due to complex visual environments. Do you ever find yourself feeling sick at the grocery store as you scan the aisle for that one type of flour you need? Does scrolling on your phone too quickly make you feel loopy? If so, this may be the type of motion sensitivity you have.
The second type is the one we mostly think about when we think of motion sensitivity, and that’s why it’s called true motion sickness in which symptoms are caused by passive motion. Passive motion means that you are not actively moving but something is moving you i.e. being in a car or boat.
Common Symptoms of Motion Sensitivity
After sharing my experiences with friends, I’ve discovered that everyone experiences motion sensitivity quite differently, so this list of symptoms is definitely not all-encompassing. Personally, I get a weird background headache, followed by a woozy feeling in my head almost like I’m floating. Despite my earlier experiences of nausea and vomiting, I rarely feel sick to the stomach now unless I’m on a tiny boat with very choppy waves.
On the flip side, many of my friends have described nausea as their main symptom. Other common symptoms include fatigue, imbalance, increased sweating, disorientation, palor (aka looking quite pale), excessive production of saliva, and burping.
Why Does Motion Sensitivity Happen?
If you’ve been an avid reader of our blog, you will already know that our sense of balance comes from three main sources: the vestibular system, the visual system, and the somatosensory system. People who get motion sensitivity often rely too much on their visual system, which means their brains can easily be tricked. If you’re sitting in your parked car, and the truck next to you starts moving, your vestibular system is telling your brain that you’re not moving, but your visual system is saying the opposite.
If you’re someone who over relies on their visual system, that system will take over, convince the brain that you are indeed moving, and make you feel really sick. This can also be referred to as visual vestibular mismatch, or VVM.
Treatment for Motion Sensitivity
If you’re a lifetime sufferer of motion sensitivity, you’ve probably already tried Gravol, ginger pills, cracker nibbling, looking straight ahead when the car is moving, etc. etc. But all of these things are only band aid solutions to help you cope. What if you could cure your motion sensitivity?
Just like an ankle sprain, there is no magic wand that can cure you in an instant, but there are many exercises you can do to start training yourself so that you no longer experience motion sensitivity. There are two main elements you will need to work on:
- We need to train the brain to stop its over-reliance on the visual system, and to start relying more on the vestibular system instead. How? Take the other two systems away. Stand on your cushy couch, close your eyes, and don’t fall over.
- We need to gradually desensitize or habituate the brain so that it can tolerate these icky situations more and more. This means that yes, we do need to trigger those symptoms to train the brain, but only mildly. You should feel only mild symptoms that resolve within 5 minutes. What that may look like totally depends on the individual. For some, that may mean 1 minute in the car as a passenger. For others, it may mean getting on a roller coaster if that’s the only trigger they have.
For more exercises that you can easily incorporate into your everyday life, check out part 2 of our motion sensitivity series where I will be sharing my Top 10 Motion Sensitivity Exercises.
Accelerometer Types and Specifications
Different types of accelerometers offer varying performance characteristics. Here is a comparison of common accelerometer types:
| Type | Sensing Element | Frequency Measurement | Shock Resistance | Typical Applications |
|---|---|---|---|---|
| Piezoelectric | Piezoceramic or single crystals | High | Moderate | High-frequency vibration analysis |
| Piezoresistive | Piezoresistors in springs | Moderate | High | High-acceleration environments |
| Capacitive (MEMS) | Silicon micro-machined element | Low to Moderate | Moderate | Consumer electronics, low-frequency monitoring |
| Thermal (MEMS) | Heated gas | Low | High | High-shock environments |