Inertial measurement units (IMUs)

01

Set range, bandwidth and noise

VectorNav's IMU specification guide defines range as the input span a sensor can measure, bandwidth as the frequency where response falls to half power (−3 dB), distinct from sample rate, and noise as random variation under constant input. Because averaging reduces measured noise, datasheets give noise density, and integrating a noisy rate produces random walk: angle random walk (ARW) for gyroscopes and velocity random walk (VRW) for accelerometers. A noise density of X °/s/√Hz equals an ARW of X °/√s, or 60X °/√h, and 1 mg ≈ 0.01 m/s², so X mg/√Hz ≈ 0.01X m/s/√s. [1]

Bosch's BMI088 combines a 16-bit triaxial gyroscope and accelerometer in a 3.0 × 4.5 × 0.95 mm LGA for drones and robotics: accelerometer ranges of ±3, ±6, ±12 and ±24 g, output data rate 12.5–1,600 Hz, bandwidth 5–280 Hz depending on settings, and noise density of 160 µg/√Hz (X and Y) and 190 µg/√Hz (Z); gyroscope ranges of ±125 to ±2,000 °/s. It is designed to suppress vibration above a few hundred hertz. [2]

Editorial calculation: by VectorNav's conversions, the BMI088's 160 µg/√Hz (0.16 mg/√Hz) accelerometer noise equals a velocity random walk of about 0.0016 m/s/√s; a gyroscope with 0.01 °/s/√Hz noise density would have an angle random walk of 0.6 °/√h. [1] [2]

Give the supplier the motion ranges (g and °/s), bandwidth and output rate, required drift or accuracy over your mission time, temperature range and vibration and shock environment, interface (SPI, I²C, UART, CAN), supply, size and weight limits, and whether AHRS or GNSS/INS outputs are needed.

02

Check bias stability, scale factor and alignment

VectorNav calls in-run bias stability (bias instability) a critical specification, since it sets the floor on how accurately bias can be estimated; turn-on bias stability (repeatability) varies from one power-up to the next, and bias over temperature can be calibrated within the limits of in-run stability. Scale-factor error and nonlinearity are given in ppm or percent, orthogonality errors as cross-axis sensitivity and misalignment, and gyroscopes also show g-sensitivity and g²-sensitivity (vibration rectification). [1]

The BMI088 datasheet lists, for example, accelerometer zero-g offset of 20 mg with temperature drift below 0.2 mg/K, cross-axis sensitivity of 0.5% and alignment error of 0.5° relative to the package; gyroscope zero-rate offset of ±1 °/s, offset drift of ±0.015 °/s per K, g-sensitivity of 0.1 °/s/g and nonlinearity of ±0.05 %FS. Its absolute maximum ratings include shock of 10,000 g for up to 200 µs and 2,000 g for up to 1.0 ms, and supply of 2.4–3.6 V with 1.2–3.6 V I/O over SPI or I²C. [2]

Murata's SCHA600, a 6-DOF automotive component, shows what a higher-stability MEMS part states: Allan variance bias instability down to 0.9 °/h, gyro RMS noise below 0.007 °/s, cross-axis error better than 0.3° over temperature from component-level dynamic cross-axis calibration, gyro ranges of ±125 or ±300 °/s with sensitivity of 160 or 80 LSB/°/s, an accelerometer range of ±6 g at 4,905 LSB/g, selectable amplitude response of 13, 20, 46 or 300 Hz, typical offset temperature dependency of ±0.06 °/s (Z) and ±0.25 °/s (XY) for the gyro and ±3 mg for the accelerometer, −40 to +110 °C operation, a 3.0–3.6 V supply, SPI output, AEC-Q100 qualification, ISO 26262 compliance for systems up to ASIL-D, and a 32-pin SOIC of 19.71 × 12.15 × 4.6 mm. Murata's page also states that the SCHA600 series is not recommended for new designs and is scheduled for discontinuation. [4]

Editorial calculation: 0.9 °/h ÷ 3,600 s/h = 0.00025 °/s, about 1/4,000 of the BMI088's ±1 °/s zero-rate offset; the two figures measure different things (in-run stability against initial offset), so ask each supplier for both in-run bias stability and offset over temperature, and for the product's lifecycle status before design-in. [4] [2] [1]

03

Compare a consumer-industrial MEMS IMU's ranges and noise

TDK InvenSense's product page for its ICM-42688-P 6-axis IMU lists gyroscope noise of 2.8 mdps/√Hz and accelerometer noise of 70 µg/√Hz, 6-axis current of 0.88 mA in low-noise mode, user-selectable gyro ranges from ±15.6 to ±2,000 dps and accelerometer ranges of ±2, 4, 8 or 16 g, programmable digital filters, a 0.91 × 2.50 × 3.00 mm package, and 19-bit (gyro) and 18-bit (accel) data in the FIFO. It states that an accurate external clock input helps reduce system sensitivity error and the ODR's sensitivity to temperature and part-to-part variation. [3]

Set these against the Bosch BMI088 figures and the VectorNav definitions above: a noise density only becomes an angle or velocity error once integrated over your bandwidth and time, and bias stability, which this page does not list, sets the long-term drift floor. [3] [2] [1]

Editorial calculation: 2.8 mdps/√Hz equals 0.0028 °/s/√Hz; converting as VectorNav describes (X °/s/√Hz = 60X °/√h) gives about 0.17 °/√h angle random walk.

Ask for noise density and bias stability (in-run and over temperature) together, the ranges and bandwidth you will run, and whether an external clock or temperature calibration is supported.

04

Compare what suppliers offer

The suppliers linked to this page list inertial measurement units; their cards link to the pages reviewed.

Ask any supplier for ranges, bandwidth and output rates, noise density or random walk, in-run and turn-on bias stability, bias and scale factor over temperature, misalignment, g-sensitivity, calibration method, interface and supply, shock and vibration ratings, enclosure and connector, and test data.

05

Trace the IMU parts

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your system

An IMU quotation needs values for your application. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the motion, accuracy and environment data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Motion: [acceleration range; rate range; bandwidth]
Accuracy: [drift over mission time; noise]
Environment: [temperature; vibration; shock]
Interface: [SPI; I2C; UART; CAN; output rate]
Package: [chip; module; enclosed; size; mass]
Outputs: [raw IMU; AHRS; INS]

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