Inertial measurement unit

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Apollo Inertial Measurement Unit Apollo IMU at Draper Hack the Moon exhibit.agr.jpg
Apollo Inertial Measurement Unit
Apollo IMU, where Inertial Reference Integrating Gyros (IRIGs,Xg,Yg,Zg) sense attitude changes, and Pulse Integrating Pendulous Accelerometers (PIPAs,Xa,Ya,Za) sense velocity changes Apollo Inertial Measurement Unit.png
Apollo IMU, where Inertial Reference Integrating Gyros (IRIGs,Xg,Yg,Zg) sense attitude changes, and Pulse Integrating Pendulous Accelerometers (PIPAs,Xa,Ya,Za) sense velocity changes

An inertial measurement unit (IMU) is an electronic device that measures and reports a body's specific force, angular rate, and sometimes the orientation of the body, using a combination of accelerometers, gyroscopes, and sometimes magnetometers. When the magnetometer is included, IMUs are referred to as IMMUs. [1]

Contents

IMUs are typically used to maneuver modern vehicles including motorcycles, missiles, aircraft (an attitude and heading reference system), including uncrewed aerial vehicles (UAVs), among many others, and spacecraft, including satellites and landers. Recent developments allow for the production of IMU-enabled GPS devices. An IMU allows a GPS receiver to work when GPS-signals are unavailable, such as in tunnels, inside buildings, or when electronic interference is present. [2]

IMUs are used in VR headsets and smartphones, and also in motion tracked game controllers like the Wii Remote.

Operational principles

Inertial navigation unit of French IRBM S3 Centrale-intertielle missile S3 Musee du Bourget P1010652.JPG
Inertial navigation unit of French IRBM S3
IMUs work, in part, by detecting changes in pitch, roll, and yaw Flight dynamics with text.png
IMUs work, in part, by detecting changes in pitch, roll, and yaw

An inertial measurement unit works by detecting linear acceleration using one or more accelerometers and rotational rate using one or more gyroscopes. [3] Some also include a magnetometer which is commonly used as a heading reference. Some IMUs, like Adafruit's 9-DOF IMU, include additional sensors like temperature. [4] Typical configurations contain one accelerometer, gyro, and magnetometer per axis for each of the three principal axes: pitch, roll and yaw.

Uses

IMUs are often incorporated into Inertial Navigation Systems, which utilize the raw IMU measurements to calculate attitude, angular rates, linear velocity, and position relative to a global reference frame. The IMU equipped INS forms the backbone for the navigation and control of many commercial and military vehicles, such as crewed aircraft, missiles, ships, submarines, and satellites. IMUs are also essential components in the guidance and control of uncrewed systems such as UAVs, UGVs, and UUVs. Simpler versions of INSs termed Attitude and Heading Reference Systems utilize IMUs to calculate vehicle attitude with heading relative to magnetic north. The data collected from the IMU's sensors allows a computer to track craft's position, using a method known as dead reckoning. This data is usually presented in Euler vectors representing the angles of rotation in the three primary axis or a quaternion.

In land vehicles, an IMU can be integrated into GPS based automotive navigation systems or vehicle tracking systems, giving the system a dead reckoning capability and the ability to gather as much accurate data as possible about the vehicle's current speed, turn rate, heading, inclination and acceleration, in combination with the vehicle's wheel speed sensor output and, if available, reverse gear signal, for purposes such as better traffic collision analysis.

Besides navigational purposes, IMUs serve as orientation sensors in many consumer products. Almost all smartphones and tablets contain IMUs as orientation sensors. Fitness trackers and other wearables may also include IMUs to measure motion, such as running. IMUs also have the ability to determine developmental levels of individuals when in motion by identifying specificity and sensitivity of specific parameters associated with running. Some gaming systems such as the remote controls for the Nintendo Wii use IMUs to measure motion. Low-cost IMUs have enabled the proliferation of the consumer drone industry. They are also frequently used for sports technology (technique training), [5] and animation applications. They are a competing technology for use in motion capture technology. [6] An IMU is at the heart of the balancing technology used in the Segway Personal Transporter.

In navigation

Modern inertial measurement unit for spacecraft IMU-28 inertial measurement unit.jpg
Modern inertial measurement unit for spacecraft

In a navigation system, the data reported by the IMU is fed into a processor which calculates altitude, velocity and position. [7] A typical implementation referred to as a Strap Down Inertial System integrates angular rate from the gyroscope to calculate angular position. This is fused with the gravity vector measured by the accelerometers in a Kalman filter to estimate attitude. The attitude estimate is used to transform acceleration measurements into an inertial reference frame (hence the term inertial navigation) where they are integrated once to get linear velocity, and twice to get linear position. [8] [9] [10]

For example, if an IMU installed in an aeroplane moving along a certain direction vector were to measure a plane's acceleration as 5 m/s2 for 1 second, then after that 1 second the guidance computer would deduce that the plane must be traveling at 5 m/s and must be 2.5 m from its initial position (assuming v0=0 and known starting position coordinates x0, y0, z0). If combined with a mechanical paper map or a digital map archive (systems whose output is generally known as a moving map display since the guidance system position output is often taken as the reference point, resulting in a moving map), the guidance system could use this method to show a pilot where the plane is located geographically in a certain moment, as with a GPS navigation system, but without the need to communicate with or receive communication from any outside components, such as satellites or land radio transponders, though external sources are still used in order to correct drift errors, and since the position update frequency allowed by inertial navigation systems can be higher than the vehicle motion on the map display can be perceived as smooth. This method of navigation is called dead reckoning.

One of the earliest units was designed and built by Ford Instrument Company for the USAF to help aircraft navigate in flight without any input from outside the aircraft. Called the Ground-Position Indicator, once the pilot entered in the aircraft longitude and latitude at takeoff, the unit would show the pilot the longitude and latitude of the aircraft in relation to the ground. [11]

Positional tracking systems like GPS [12] can be used to continually correct drift errors (an application of the Kalman filter).

A major disadvantage of using IMUs for navigation is that they typically suffer from accumulated error. Because the guidance system is continually integrating acceleration with respect to time to calculate velocity and position (see dead reckoning), any measurement errors, however small, are accumulated over time. This leads to 'drift': an ever-increasing difference between where the system thinks it is located and the actual location. Due to integration a constant error in acceleration results in a linear error growth in velocity and a quadratic error growth in position. A constant error in attitude rate (gyro) results in a quadratic error growth in velocity and a cubic error growth in position. [13]

Performance

9-DoF IMU SiP breakout board SparkFun 9DoF-IMU-Breakout LSM9DS1 13284-03.jpg
9-DoF IMU SiP breakout board

A very wide variety of IMUs exists, [14] depending on application types, with performance ranging:

To get a rough idea, this means that, for a single, uncorrected accelerometer, the cheapest (at 100 mg) loses its ability to give 50-meter accuracy after around 10 seconds, while the best accelerometer (at 10 μg) loses its 50-meter accuracy after around 17 minutes. [15]

The accuracy of the inertial sensors inside a modern inertial measurement unit (IMU) has a more complex impact on the performance of an inertial navigation system (INS). [16]

Gyroscope and accelerometer sensor behavior is often represented by a model based on the following errors, assuming they have the proper measurement range and bandwidth: [17]

All these errors depend on various physical phenomena specific to each sensor technology. Depending on the targeted applications and to be able to make the proper sensor choice, it is very important to consider the needs regarding stability, repeatability, and environment sensitivity (mainly thermal and mechanical environments), on both short and long terms. Targeted performance for applications is, most of the time, better than a sensor's absolute performance. However, sensor performance is repeatable over time, with more or less accuracy, and therefore can be assessed and compensated to enhance its performance. This real-time performance enhancement is based on both sensors and IMU models. Complexity for these models will then be chosen according to the needed performance and the type of application considered. Ability to define this model is part of sensors and IMU manufacturers know-how. Sensors and IMU models are computed in factories through a dedicated calibration sequence using multi-axis turntables and climatic chambers. They can either be computed for each individual product or generic for the whole production. Calibration will typically improve a sensor's raw performance by at least two decades.

Assembly

Apollo IMU stable member Apollo IMU at Draper Hack the Moon exhibit detail 1.agr.jpg
Apollo IMU stable member

High performance IMUs, or IMUs designed to operate under harsh conditions, are very often suspended by shock absorbers. These shock absorbers are required to master three effects:

Suspended IMUs can offer very high performance, even when submitted to harsh environments. However, to reach such performance, it is necessary to compensate for three main resulting behaviors:

Decreasing these errors tends to push IMU designers to increase processing frequencies, which becomes easier using recent digital technologies. However, developing algorithms able to cancel these errors requires deep inertial knowledge and strong intimacy with sensors/IMU design. On the other hand, if suspension is likely to enable IMU performance increase, it has a side effect on size and mass.

A wireless IMU is known as a WIMU. [18] [19] [20] [21]

See also

Related Research Articles

<span class="mw-page-title-main">Gyroscope</span> Device for measuring or maintaining the orientation and angular velocity

A gyroscope is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rotation is free to assume any orientation by itself. When rotating, the orientation of this axis is unaffected by tilting or rotation of the mounting, according to the conservation of angular momentum.

<span class="mw-page-title-main">Attitude indicator</span> Flight instrument which displays the aircrafts orientation relative to Earths horizon

The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions.

A guidance system is a virtual or physical device, or a group of devices implementing a controlling the movement of a ship, aircraft, missile, rocket, satellite, or any other moving object. Guidance is the process of calculating the changes in position, velocity, altitude, and/or rotation rates of a moving object required to follow a certain trajectory and/or altitude profile based on information about the object's state of motion.

<span class="mw-page-title-main">Accelerometer</span> Device that measures proper acceleration

An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration of the object relative to an observer who is in free fall. 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. By contrast, an accelerometer that is in free fall will measure zero acceleration.

<span class="mw-page-title-main">Missile guidance</span> Variety of methods of guiding a missile

Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its Probability of Guidance (Pg).

A vibrating structure gyroscope (VSG), defined by the IEEE as a Coriolis vibratory gyroscope (CVG), is a gyroscope that uses a vibrating structure to determine the rate of rotation. A vibrating structure gyroscope functions much like the halteres of flies.

An attitude and heading reference system (AHRS) consists of sensors on three axes that provide attitude information for aircraft, including roll, pitch, and yaw. These are sometimes referred to as MARG sensors and consist of either solid-state or microelectromechanical systems (MEMS) gyroscopes, accelerometers and magnetometers. They are designed to replace traditional mechanical gyroscopic flight instruments.

<span class="mw-page-title-main">Fibre-optic gyroscope</span> Gyroscope that uses fiber optics

A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre, which can be as long as 5 kilometres (3 mi).

GPS/INS is the use of GPS satellite signals to correct or calibrate a solution from an inertial navigation system (INS). The method is applicable for any GNSS/INS system.

A PIGA is a type of accelerometer that can measure acceleration and simultaneously integrates this acceleration against time to produce a speed measure as well. The PIGA's main use is in Inertial Navigation Systems (INS) for guidance of aircraft and most particularly for ballistic missile guidance. It is valued for its extremely high sensitivity and accuracy in conjunction with operation over a wide acceleration range. The PIGA is still considered the premier instrument for strategic grade missile guidance, though systems based on MEMS technology are attractive for lower performance requirements.

<span class="mw-page-title-main">Guidance, navigation, and control</span> Branch of engineering

Guidance, navigation and control is a branch of engineering dealing with the design of systems to control the movement of vehicles, especially, automobiles, ships, aircraft, and spacecraft. In many cases these functions can be performed by trained humans. However, because of the speed of, for example, a rocket's dynamics, human reaction time is too slow to control this movement. Therefore, systems—now almost exclusively digital electronic—are used for such control. Even in cases where humans can perform these functions, it is often the case that GNC systems provide benefits such as alleviating operator work load, smoothing turbulence, fuel savings, etc. In addition, sophisticated applications of GNC enable automatic or remote control.

The Tamam Division of the Systems Missiles and Space Group of the Israel Aerospace Industries (IAI) is a manufacturing plant in the development and production of high performance Inertial and Electro-Optic (EO) products and systems and maritime drones.

<span class="mw-page-title-main">Inertial navigation system</span> Continuously computed dead reckoning

An inertial navigation system is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity of a moving object without the need for external references. Often the inertial sensors are supplemented by a barometric altimeter and sometimes by magnetic sensors (magnetometers) and/or speed measuring devices. INSs are used on mobile robots and on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Older INS systems generally used an inertial platform as their mounting point to the vehicle and the terms are sometimes considered synonymous.

<span class="mw-page-title-main">LN-3 inertial navigation system</span>

The LN-3 inertial navigation system is an inertial navigation system (INS) that was developed in the 1960s by Litton Industries. It equipped the Lockheed F-104 Starfighter versions used as strike aircraft in European forces. An inertial navigation system is a system which continually determines the position of a vehicle from measurements made entirely within the vehicle using sensitive instruments. These instruments are accelerometers which detect and measure vehicle accelerations, and gyroscopes which act to hold the accelerometers in proper orientation.

Spacecraft attitude control is the process of controlling the orientation of a spacecraft with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc.

<span class="mw-page-title-main">Pressure reference system</span> Flight instrument

Pressure reference system (PRS) is an enhancement of the inertial reference system and attitude and heading reference system designed to provide position angles measurements which are stable in time and do not suffer from long term drift caused by the sensor imperfections. The measurement system uses behavior of the International Standard Atmosphere where atmospheric pressure descends with increasing altitude and two pairs of measurement units. Each pair measures pressure at two different positions that are mechanically connected with known distance between units, e.g. the units are mounted at the tips of the wing. In horizontal flight, there is no pressure difference measured by the measurement system which means the position angle is zero. In case the airplane banks (to turn), the tips of the wings mutually change their positions, one is going up and the second one is going down, and the pressure sensors in every unit measure different values which are translated into a position angle.

<span class="mw-page-title-main">Hemispherical resonator gyroscope</span> Type of gyroscope

The hemispherical resonator gyroscope (HRG), also called wine-glass gyroscope or mushroom gyro, is a compact, low-noise, high-performance angular rate or rotation sensor. An HRG is made using a thin solid-state hemispherical shell, anchored by a thick stem. This shell is driven to a flexural resonance by electrostatic forces generated by electrodes which are deposited directly onto separate fused-quartz structures that surround the shell. The gyroscopic effect is obtained from the inertial property of the flexural standing waves. Although the HRG is a mechanical system, it has no moving parts, and can be very compact.

The terminology quantum compass often relates to an instrument which measures relative position using the technique of atom interferometry. It includes an ensemble of accelerometers and gyroscope based on quantum technology to form an inertial navigation unit.

<span class="mw-page-title-main">Pose tracking</span> Detection of a virtual/augmented reality users pose

In virtual reality (VR) and augmented reality (AR), a pose tracking system detects the precise pose of head-mounted displays, controllers, other objects or body parts within Euclidean space. Pose tracking is often referred to as 6DOF tracking, for the six degrees of freedom in which the pose is often tracked.

Levitation based inertial sensing is a new and rapidly growing technique for measuring linear acceleration, rotation and orientation of a body. Based on this technique, inertial sensors such as accelerometers and gyroscopes, enables ultra-sensitive inertial sensing. For example, the world's best accelerometer used in the LISA Pathfinder in-flight experiment is based on a levitation system which reaches a sensitivity of and noise of .

References

  1. ↑ Fang, Bin; Sun, Fuchun; Liu, Huaping; Liu, Chunfang (2018). "3D human gesture capturing and recognition by the IMMU-based data glove". Neurocomputing. 277: 198–207. doi:10.1016/j.neucom.2017.02.101 . Retrieved 2022-09-02.
  2. ↑ "GPS system with IMUs tracks first responders". Archived from the original on 2012-10-03. Retrieved 2011-06-16.
  3. ↑ Iosa, Marco; Picerno, Pietro; Paolucci, Stefano; Morone, Giovanni (2016). "Wearable inertial sensors for human movement analysis". Expert Review of Medical Devices. 13 (7): 641–659. doi:10.1080/17434440.2016.1198694. hdl: 11573/1478060 . ISSN   1743-4440. PMID   27309490. S2CID   205908786.
  4. ↑ "Adafruit 9-DOF Absolute Orientation IMU Fusion Breakout - BNO055". Adafruit. Retrieved May 21, 2024.
  5. ↑ "An IMU-based Sensor Network to Continuously Monitor Rowing Technique on the Water". ethz.ch. Archived from the original on 2014-03-21. Retrieved 2012-05-14.
  6. ↑ "The fascination for motion capture". Xsens 3D motion tracking. Archived from the original on 2019-01-22. Retrieved 2015-11-19.
  7. ↑ "GNSS/INS". Xsens 3D motion tracking. Archived from the original on 2019-01-22. Retrieved 2019-01-22.
  8. ↑ "OpenShoe". www.openshoe.org. Retrieved 2018-04-04.
  9. ↑ "GT Silicon Pvt Ltd". www.gt-silicon.com. Retrieved 2018-04-04.
  10. ↑ Nilsson, J. O.; Gupta, A. K.; Händel, P. (October 2014). "Foot-mounted inertial navigation made easy". 2014 International Conference on Indoor Positioning and Indoor Navigation (IPIN). pp. 24–29. doi:10.1109/IPIN.2014.7275464. ISBN   978-1-4673-8054-6. S2CID   898076.
  11. ↑ "Robot Navigator Guides Jet Pilots." Popular Mechanics, May 1954, p. 87.
  12. ↑ IV, Hyatt Moore. "Moore Stanford Research" (PDF). web.stanford.edu. Archived from the original (PDF) on 2021-01-25. Retrieved 2018-06-03.
  13. ↑ Siciliano, Bruno; Khatib, Oussama (20 May 2008). Springer Handbook of Robotics. Springer Science & Business Media. ISBN   9783540239574 – via Google Books.
  14. ↑ "Inertial Measurement Units, IMUs".
  15. ↑ Calculated from reversing S = 1/2·a·t2 into t = √(2s/a), where s = distance in meters, a is acceleration (here 9.8 times g), and t is time in seconds.
  16. ↑ von Hinüber, Edgar (2024-07-21). "If you intend to use an inertial measurement system" (PDF). FAQ for using inertial Technology. Retrieved 2024-07-21.
  17. ↑ Lawrence, Anthony (1998). "Gyro and Accelerometer Errors and Their Consequences". Modern Inertial Technology. Mechanical Engineering Series. New York, NY: Springer New York. pp. 25–42. doi:10.1007/978-1-4612-1734-3_3. ISBN   978-1-4612-7258-8.
  18. ↑ http://www.patentstorm.us/patents/5067084/description.html Archived 2009-12-13 at the Wayback Machine Description of IMU aiding from Roll isolated Gyro
  19. ↑ Inertial Navigation: 40 Years of Evolution - Overview at http://www.imar-navigation.de www.imar-navigation.de
  20. ↑ http://www.mathworks.com/access/helpdesk/help/toolbox/aeroblks/index.html?/access/helpdesk/help/toolbox/aeroblks/threeaxisinertialmeasurementunit.html Three Axis IMU
  21. ↑ http://www.starlino.com/imu_guide.html A Guide To using IMU (Accelerometer and Gyroscope Devices) in Embedded Applications
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