Electronic Design

Tri-Axis Inertial-Measurement Units Combine Performance And Low Cost

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

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