Here's another device which helps you answer that eternal question of "where am I, and where am I going?" Targeted at applications in vehicles, guidance, antenna and camera orientation, among others, the Analog Devices ADIS16355 iSensor Inertial Measurement Unit (IMU) provides MEMS-based sensing along three orthogonal angular rates axes (roll, yaw, pitch) and three acceleration axes, for full six-degrees-of-freedom (6DOF) triaxial motion measurements. The vendor claims this unit, in a cube less than one inch on each side has sensor accuracy which is 50 times better than, and 1/10 the cost of, previous solutions.
[Unlike GPS units, which require access to overhead satellites in the GPS constellation, an IMU is entirely self-contained and "self-knowing" about its motion. This one-inch cube IMU can be used independently or to supplement a GPS-based system, providing updates during GPS blackout periods.]
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Measurement range is ±300 degrees/second, with 14-bit resolution for the gyroscope and ±10 g, also 14 bits, for the accelerometer (and 2000 g powered shock survivability). In-run bias stability is 0.015°/second, and bias temperature stability is 0.005°/sec/°C. Gryo axis non-orthogonality is ±0.05°. The single supply unit (nominal 5V) has 350 Hz bandwidth.
One long-standing problem with lower-cost IMUs is that the end-user must calibrate for temperature, voltage, vibration, and cross-axis effects, which requires specialized test equipment, know-how, and time. The ADI part is fully calibrated at the factory for these effects (note that a lower-cost unit which is calibrated only at 25°C, the ADIS16350, is also offered).
User I/O is via an SPI interface port for data output as well as initial set-up, control, and operation, including loop filtering bandwidth, sampling rate, power management, self test, and status and alarms. The device is complete and includes all sub-functions needed for calibration, power management, alarms, and even has an auxiliary 12-bit ADC and DAC plus digital I/O, see block diagram figure.
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The ADIS16355 is priced at $359, in 1000-unit quantities; the ADIS16350 is $275. More information is available at www.analog.com/pr/ADIS16355. --Bill Schweber
[Tutorial note: although we now commonly accept the use of IMUs for Earth-surface and space-based navigation, in the early part of the 20th century there was a raging debate on whether inertial position sensing for Earth navigation was even possible. Since it is not possible to inertially distinguish between acceleration due to an IMU's movement along the Earth's surface, versus acceleration due to the Earth's downward gravity as you moved along that surface, many prominent scientists argued that a black-box IMU was not possible at all. This was known as "The Problem of the Vertical." Einstein spoke about this earlier in the century, when he explained how he came to his thinking on special relatively, noting that for an observer in a closed box, there was no way to distinguish between gravitational mass and inertial mass.
To find out more about the problem of the vertical, and how it was overcome, go to http://fdab.gsfc.nasa.gov/live/Home/Symposium/Session1_No6.pdf or get a copy of Inventing Accuracy: A Historical Sociology of Nuclear Missile Guidance by Donald MacKenzie, MIT Press, 1990; it's a fascinating book.]