Micro-Electro-Mechanical Systems Inertial Measurement Units
Gijsbert M. Ewout van Bekkum – L01 Autonomous Convoy Group
Introduction
Tracking one’s location can be difficult without a spatial reference. However, tracking can still be
accomplished through dead reckoning. Dead reckoning is the process of determining one’s position based on the
direction, velocity, and time since the previous position. To perform dead reckoning an Inertial Measurement Unit
(IMU) can be used. An IMU measures velocity, orientation, and gravitational forces. IMUs with conventional
gyroscopes and accelerometers have previously been out of reach for many civilian and military uses due to bulky
sizes and high costs; but with recent advances in Micro-Electro-Mechanical Systems (MEMS) the prices and sizes
have shrunk significantly. MEMS IMUs can now be found in many systems—from smart phones for the masses
to smart munitions for the military.
Commercial Applications
Civilian
In recent years the auto industry has adopted IMUs to aid anti-lock brake systems, traction control
schemes, and triggering air bags [1]. Even entertainment systems such as the Nintendo Wii now contain IMUs.
The Wii system originally only used two ADXL330 accelerometers from Analog Devices [2]; this system could
not detect the rotation of the controllers well. However, recently Nintendo added the MotionPlus, with the IDG600 gyroscope from InvenSense [3], to implement a true IMU. The cost of the ADXL330 is less than eight dollars
when purchased in bulk and the IDG-600 is estimated to cost two dollars [4]. Personal GPS navigation companies
have also started to integrate MEMS IMUs into their products. When the GPS reception becomes temporarily
unavailable, these devices can revert to the IMU to calculate the approximate location and movement of the
device [5]. TomTom GO 920 is such a device and costs less than 200 dollars—only 30 dollars more compared to
models without the IMU feature, such as the TomTom GO 720.
Military
Honeywell, along with Rockwell Collins, produce an Integrated Guidance System (IGS) for the future
generation “projectiles, strike weapons, missiles and small unmanned systems” of the military [6]. This product
combines GPS with an IMU to create a robust system that has a spatial error of less than 4 meters, is difficult to
jam, can withstand more than 20,000 Gs, and weighs a little over half a kilogram [7]. Honeywell also makes
MEMS IMUs for military dead reckoning without GPS integration. One example is the HG1930 [8] which weighs
less than half a pound, measures two inches in diameter by one and a half inches tall and can survive the shell
shock of being fired out of an artillery gun [9].
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Underlying Technology
Operation
A MEMS IMU consists of multiple axes MEMS accelerometers and gyroscopes. A common type of
MEMS accelerometer is based on the piezoresistive effect. In these accelerometers, cantilevers are attached to a
piezoresistor base. As the cantilever experiences acceleration, stress is applied to the piezoresistor base causing a
change in the resistance of the base that can be measured [10]. Three important performance factors for the
accelerometer are the amount of noise of the sensor—which defines the resolution of the sensor, the maximum Gs
it can withstand, and the limits of the range in which the sensor can measure acceleration. A common type of
MEMS gyroscope is the tuning fork resonator gyroscope. Tuning fork gyroscopes contain masses which oscillate
in opposite directions. If the sensor is rotated, the Coriolis force causes the masses to vibrate orthogonal to the
oscillation. The orthogonal vibrations can be measured to determine the rotation [11]. It is important to know the
measurement range, the fastest rotation, of a gyroscope which is measured in degrees per second. The resolution,
like the accelerometer, is mainly defined by the noise of the sensor. The output of the accelerometer and
gyroscope are typically filtered with a low pass filter to remove noise from high frequency oscillations. While
many IMUs have analog outputs, some IMUs come equipped with analog to digital converters and even
microcontrollers or processors to further process the data to increase accuracy, functionality, and connectivity.
Recent Advances
The costs of MEM IMUs are steadily decreasing in price. New IMUs such as the Razor IMU[12] now
come with three axis magnetometers in addition to its multiple axis gyroscopes and accelerometers.
Magnetometers in the past were bulky and required a lot of power; new MEMS magnetometers such as the [13]
now require as low as a one milliamp supply current and have a resolution up to one microtesla. Previously, many
MEMS IMUs had separate modules for the gyroscope and accelerometers, sometimes even for the different axes;
now more and more of the MEMS IMUs are being integrated into the same package [14].
Implementation
Costs vary widely depending on the application: from around $10 for the Wii to expensive, exotic
products from Honeywell for military applications [5]. The physical mounting is critical when implementing an
IMU in a system. The gyroscope can only measure rotation around itself; therefore, the IMU should be as close as
possible to the point of rotation. For example, for the Wii MotionPlus the gyroscope is located at on the bottom of
the controller near the user’s wrist, which is near the point of rotation. The IMU should also be mounted so that
the different axes of the IMU line up properly with the device, primarily for the accelerometer. Some IMUs might
require an A/D converter or a digital connection such as SPI or I2C. A processor and algorithm is needed to
convert the raw data from the IMU into the desired format. For example, if dead reckoning is the desired
application, the data must be converted with an algorithm into the estimated location. The requirements of the
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processor depend highly on the application and could require anything from a low cost microcontroller to a DSP
chip.
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References
[1]
S. Dussy. (2009, Sept.) ESA preparing ‘sugar-cube’ gyro sensors for future missions. [Online] Available:
http://www.esa.int/SPECIALS/Space_Engineering/SEMVXYUHYXF_0.html.
[2]
Analog Devices. “Small, Low Power, 3-Axis ±3 g iMEMS Accelerometer.” ADXL330 datasheet. 2007.
[Online] Available: http://www.analog.com/static/imported-files/data_sheets/ADXL330.pdf.
[3]
InvenSense. “Integrated Dual-Axis Gyroscope.” IDG-600 datasheet. 2008. [Online] Available:
http://www.invensense.com/jp/shared/pdf/IDG600_ProductBrief_English.pdf.
[4]
F. Caron. “Of gyroscopes and gaming: the tech behind the Wii MotionPlus.” arstechnica.com. Aug. 26,
2008. [Online] Available: http://arstechnica.com/gaming/news/2008/08/wii-motion-sensor.ars/1. [Accessed
Jan. 25, 2010]
[5]
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Available: http://www.tomtom.com/lib/doc/TTGO%20920%20New%20Spec.pdf.
[6]
Honeywell, et al. “Next generation in guidance systems.” Integrated Guidance Systems sales literature.
2007. [Online] Available: http://www.igsllc.com/docs/IGS%20brochure.pdf.
[7]
Honeywell, et al. “IGS-2xx series, deeply integrated guidance family for mortars and projectiles.” IGS-2xx
datasheet. 2009. [Online] Available: http://www.igsllc.com/docs/IGS-2xx%20Data%20Sheet%200509.pdf.
[8]
Honeywell. “HG1930 MEMS IMU.” HG1930 datasheet. June 2006. [Online] Available:
http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/MissilesMunitions/HG1930_Datasheet.pdf.
[9]
Honeywell. “Honeywell Aerospace: Inertial Measurement Units.” Honeywell.com. 2009. [Online]
Available:https://www.honeywell.com/sites/portal?smap=aerospace&page=Missles_Munitions3&theme=T
5&catID=C47E27B3A-58A6-A2E2-09FE-769AD775C886&id=H5AA6B1BD-95B2-AEA7-C62EC28BC3E3EF73&sel=4&c=n#MultipleDownload1.
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[10] M. Andrejašic, et al. (March 2008). MEMS Accelerometers. [Online] Available: http://mafija.fmf.unilj.si/seminar/files/2007_2008/MEMS_accelerometers-koncna.pdf.
[11] M. F. Zaman, A. Sharma, et al. “High Performance Matched-Mode Tuning Fork Gyroscope,” in Micro
Electro Mechanical Systems, 2006. MEMS 2006. Istanbul. 19th IEEE International Conference on, May
2006, pp. 66-69.
[12] Sparkfun Electronics. “9 Degrees of Freedom – Razor IMU – AHRS compatible” sparkfun.com. 2009.
[Online] Available: http://www.sparkfun.com/commerce/product_info.php?products_id=9623.
[13] J. S. Pulskamp, “PZT MEMS resonant Lorentz force magnetometer,” U.S. Patent 7 642 692, January 5,
2010.
[14] Electronics.ca publications. “MEMS Accelerometer, Gyroscope and IMU Market.” electronics.ca. June,
2009. [Online] Available: http://www.electronics.ca/publications/products/MEMS-Accelerometer,Gyroscope-and-IMU-Market.html.
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