Three-Axis Inertial Measurement Unit

Implement three-axis inertial measurement unit (IMU)

Library

GNC/Navigation

Description

The Three-Axis Inertial Measurement Unit block implements an inertial measurement unit (IMU) containing a three-axis accelerometer and a three-axis gyroscope.

For a description of the equations and application of errors, see the Three-Axis Accelerometer block and the Three-Axis Gyroscope block reference pages.

Parameters

Units

Specifies the input and output units:

Units	Acceleration	Length
Metric (MKS)	Meters per second squared	Meters
English	Feet per second squared	Feet

IMU location

The location of the IMU, which is also the accelerometer group location, is measured from the zero datum (typically the nose) to aft, to the right of the vertical centerline and above the horizontal centerline. This measurement reference is the same for the center of gravity input. The units are in selected length units.

Update rate (sec)

Specify the update rate of the accelerometer and gyroscope. An update rate of 0 will create a continuous accelerometer and continuous gyroscope. If noise is selected and the update rate is 0, then the noise will be updated at the rate of 0.1. The units of update rate are seconds.

If you:

• Update this parameter value to 0 (continuous)

• Configure a fixed-step solver for the model

you must also select the Automatically handle rate transition for data transfer check box in the Solver pane. This check box enables the software to handle rate transitions correctly.

Second order dynamics for accelerometer

Select to apply second-order dynamics to acceleration readings.

Accelerometer natural frequency (rad/sec)

The natural frequency of the accelerometer. The units of natural frequency are radians per second.

Accelerometer damping ratio

The damping ratio of the accelerometer. A dimensionless parameter.

Accelerometer scale factors and cross-coupling

The 3-by-3 matrix used to skew the accelerometer from body-axis and to scale accelerations along body-axis.

Accelerometer measurement bias

The three-element vector containing long-term biases along the accelerometer axes. The units are in selected acceleration units.

Accelerometer lower and upper output limits

The six-element vector containing three minimum values and three maximum values of acceleration in each of the accelerometer axes. The units are in selected acceleration units.

Gyro second order dynamics

Select to apply second-order dynamics to gyroscope readings.

Gyro natural frequency (rad/sec)

The natural frequency of the gyroscope. The units of natural frequency are radians per second.

Gyro damping ratio

The damping ratio of the gyroscope. A dimensionless parameter.

Gyro scale factors and cross-coupling

The 3-by-3 matrix used to skew the gyroscope from body axes and to scale angular rates along body axes.

Gyro measurement bias

The three-element vector containing long-term biases along the gyroscope axes. The units are in radians per second.

G-sensitive bias

The three-element vector contains the maximum change in rates due to linear acceleration. The units are in radians per second per g-unit.

Gyro lower and upper output limits

The six-element vector containing three minimum values and three maximum values of angular rates in each of the gyroscope axes. The units are in radians per second.

Noise on

Select to apply white noise to acceleration and gyroscope readings.

Noise seeds

The scalar seeds for the Gaussian noise generator for each axis of the accelerometer and gyroscope.

Noise power

The height of the PSD of the white noise for each axis of the accelerometer and gyroscope.

Inputs and Outputs

Input	Dimension Type	Description
First	Three-element vector	Contains the actual accelerations in body-fixed axes, in selected units.
Second	Three-element vector	Contains the angular rates in body-fixed axes, in radians per second.
Third	Three-element vector	Contains the angular accelerations in body-fixed axes, in radians per second squared.
Fourth	Three-element vector	Contains the location of the center of gravity, in selected units.
Fifth	Three-element vector	Contains the gravity in body axis, in selected units.

Output	Dimension Type	Description
First	Three-element vector	Contains the measured accelerations from the accelerometer, in selected units.
Second	Three-element vector	Contains the measured angular rates from the gyroscope, in radians per second.

Assumptions and Limitations

Vibropendulous error, hysteresis affects, anisoelastic bias and anisoinertial bias are not accounted for in this block. Additionally, this block is not intended to model the internal dynamics of different forms of the instrument.

Examples

See asbhl20 for an example of this block.

Reference

Rogers, R. M., Applied Mathematics in Integrated Navigation Systems, AIAA Education Series, 2000.