Previously, selecting high-precision rate and acceleration sensors with six
degrees of freedom involved painful cost/performance tradeoffs and elaborate
implementation and calibration processes. Now, Analog Devices' ADIS16355 inertial
measurement unit (IMU) combines three axes of angular rate sensing and three
axes of acceleration sensing with 50 times more accuracy than other off-the-shelf
inertial sensors. And it comes pre-calibrated.
One version comes pre-calibrated for
–40°C to 85°C. The other is pre-calibrated for room temperature. That doesn't
mean they come with a table of correction values, though. By the time the data
from the IMU appears on the output bus,
it already has been corrected. In 1000unit lots, the temperature-calibrated version costs $359, and the room-temperature version costs $275.
Target applications include vehicle-mounted cameras and antennas, commercial
aircraft guidance units, robotics, and prosthetics. In aircraft, ships, truck
fleets, agricultural equipment, and other vehicles that rely on GPS satellite
navigation to maintain accurate positional information, the IMU compensates
for GPS signal loss or vehicle-induced signal irregularities.
The basic IMU is a cube measuring a little shy of an inch per side (see
the figure). The mounting feet add about a quarter inch in one direction.
The 24-pin connector and its flex circuit add a little over half an inch in
the other direction.
Each of the three gyros has a ±300°/s dynamic measurement range.
Each accelerometer has a ±10-g measurement range. Output resolution is
14 bits. Although the maximum dynamic range is ±300°/s, the IMUs
provide ±75°/s and ±150°/s ranges as well. The lower dynamic
range settings limit the minimum filter tap sizes to maintain resolution as
the measurement range decreases.
Each sensor's signal-conditioning circuit has an analog bandwidth of approximately
350 Hz. The IMU provides a Bartlett Window finite impulse response (FIR) filter
with programmable step sizes for additional noise reduction on all of the output
data registers.
In addition to the full "six degrees of freedom" (6DOF) set of calibrated motion
measurements, the IMU measures power supply and temperature and provides an
auxiliary 12-bit analog-to-digital converter (ADC) channel. This output data
updates internally, regardless of user read rates. Output data can be either
12 bits or 14 bits in length. If it's 12 bits, bits 12 and 13 are assigned "don't
care" status.
Additionally, an auxiliary 12-bit successive-approximation ADC makes it possible
to digitize other system-level analog signals. An auxiliary 0- to 2.5-V output
digit al-to-analog converter (DAC) provides a 12bit level-adjustment function.
Data is stored in registers. I/O is via a
simple serial peripheral interface (SPI)
port. A complete data frame contains
16 clock cycles. Because the SPI port
operates in full duplex mode, it supports simultaneous, 16-bit receive and
transmit functions during the same
data frame.
The IMU's accelerometers are oriented along the axis of rotation for each gyroscope.
An aluminum structure that provides tight force and motion coupling holds the
whole assembly together. An ADC samples each sensor's output signal. The ADC's
output is fed into a proprietary DSP that applies correction tables to each
sensor's output, manages the I/O function, and offers other features that simplify
designs.
Resonator gyros supply the rate information. According to the data sheet, two
polysilicon sensing structures contain a dither frame, which is electrostatically
driven to resonance. This provides the necessary velocity element to produce
a Coriolis force during rotation. At two of the outer extremes of each frame,
orthogonal to the dither motion, movable fingers are placed between fixed fingers
to form a capacitive pick-off structure that senses Coriolis motion. The resulting
signal is fed to a series of gain and demodulation stages that produce the electrical
rate-signal output.
To sense acceleration, the core acceleration sensors are surface-micromachined polysilicon structures built on top
of the silicon base. Polysilicon springs
suspend the structure over the surface
of the wafer and provide a resistance
against acceleration forces.
Structural deflection is measured
using a differential capacitor that consists of independent fixed plates and
central plates attached to the moving
mass. An acceleration will deflect the
beam and unbalance the differential
capacitor, resulting in a differential output that is fed to a series of gain and
demodulation stages that produce the
electrical rate-signal output.
Factory-calibration is the real enabler that makes it possible to design-in
the ADIS16355 without painstaking measurements in the target-system environment.
It corrects for initial sensor bias and sensitivity, power-supply variation,
axial alignment, and, for the gyros, linear acceleration.
Analog Devices
www.analog.com