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Inertial Measurement Unit

The Inertial Measurement Unit of a REV Expansion Hub is an electronic device that measures the following physical attributes:

  • Absolute Orientation
  • Angular Velocity
  • Linear Acceleration
  • Magnetic Field Strength
  • Gravity
  • Temperature

Warning

Although the IMU is capable of measuring linear acceleration, it is not optimal for measuring displacement due to inaccuracies. Consider a scenario where a robot stays at rest. The measured linear acceleration is supposed to be 0, but it may fluctuate around 0 due to signal noise, causing minor changes in velocity that in turn causes the calculated displacement to deviate from the resting position at a steady rate. When the robot accelerates, inaccuracies like these add up over time, reducing the usefulness of the displacement measurements. Do not rely on linear acceleration measurements from the IMU for displacement feedback.

The IMU is useful for rotation feedback, so you can use its angular orientation measurements to help your robot execute perfect turns during autonomous. You can also use the IMU to enable field-centric movement control during TeleOp, which may reduce driver cognitive workload for certain applications and improve scoring performance.

Dimensions

The following video from FIRST demonstrates how each rotational axis (heading, roll, and pitch) corresponds to the physical dimensions of the REV Expansion Hub.

Usage

Initialization

The IMU behaves slightly differently compared to other sensors—you need to explicitly initialize it in addition to retrieving the object from hardwareMap before reading measurements. Check out this function definition from ARC Lightning 2019-2020, where we call imu.initialize(parameters) to prepare it for normal operation. For reference, here is a more comprehensive example of how to initialize your IMU. Ideally, you should do it when getting all hardware devices from hardwareMap.

// Retrieve the IMU object from the hardware map
// Hint: you usually do not need to add this entry into your hardware configuration. It should already be there.
BNO055IMU imu = hardwareMap.get(BNO055IMU.class, "imu");
// `imu` is a variable in this example, but it is most likely a field in your code. Omit the type (`BNO055IMU`) in order to assign to the field.

// Make a new parameters object to store your preferences
BNO055IMU.Parameters params = new BNO055IMU.Parameters();

// Set the unit of all angles retrieved from the IMU.
// !: If you want to integrate with Road Runner, use RADIANS instead of DEGREES
params.angleUnit = BNO055IMU.AngleUnit.DEGREES;

// Initialize the IMU with your parameters
imu.initialize(params);

Reading Angles

Before beginning to read angles from the IMU, you need to determine the rotational axis to which the robot's heading corresponds. According to the video above, the robot's heading corresponds to the Z-axis if the hub is mounted with the words "EXPANSION HUB" facing up, and it corresponds to the X-axis if the hub is mounted with the words facing horizontally.

To read angular orientation measurements from the IMU, we use the getAngularOrientation method. It is possible to call this method with no parameters, but to make your code less ambiguous, we recommend supplying the three parameters.

  1. The first parameter should be AxesReference.INTRINSIC, as found in the official usage example.
  2. The second parameter depends on the rotational axis to be read. If you intend to read the Z-axis, use AxesOrder.ZYX. If you intend to read the X-axis, use AxesOrder.XYZ.
  3. The third parameter depends on how you intend to use the retrieved values. If you will use trigonometric functions with them, it is a good idea to use AngleUnit.RADIANS. Otherwise, it might be more intuitive to use AngleUnit.DEGREES.

Once you have retrieved the angular orientation data in an Orientation object, you can access the heading of the robot with the firstAngle field. The range of values for this field is \([-180, 180]\) for degrees and \([-\pi, \pi]\) for radians. The initial orientation of the hub will take on a value of 0, and the positive direction corresponds to counter-clockwise rotation. Examine the video above for more intuition.

// Obtain the orientation from the hub. The first angle shall be the X-axis reading. The unit of the first angle value should be degrees.
Orientation orient = imu.getOrientation(AxesReference.INTRINSIC, AxesOrder.XYZ, AngleUnit.DEGREES);

// `angle` is the heading of the robot in degrees
double angle = orient.firstAngle;

Warning

Since the angle output is constrained to \([-180, 180]\) or \([-\pi, \pi]\), the reading will wrap around the possible range when crossing an extremum. For example, if you were to keep track of the angle reading when the robot performs a full clockwise rotation, the reading would reach -180 degrees and immediately jump to 180 degrees. It is very possible that the robot might cross this heading when executing autonomous turns, so it is important to take the wraparound into consideration when programming.

Info

Q: Why do we have to use firstAngle to refer to the heading? If we need to access the Z-axis, couldn't we have AxesOrder.XYZ and use thirdAngle?

A: We have chosen firstAngle as our convention because of Road Runner. Road Runner Quickstart's FTC integration classes implement the getExternalHeading method by accessing the firstAngle of the input Orientation. It is a good idea to share the BNO055IMU instance used by Road Runner, since it is redundant to initialize another instance during a match. To minimize ambiguity and hide the implementation details of Orientation, you are free to wrap the access logic in your own class.

For a complete reference of the BNO055IMU class, check out the following source file from OpenFTC. Since the class is technically not part of the official org.firstinspires packages, you cannot find it in the official JavaDocs. Instead, you can read the inline JavaDoc source located in its extracted source file below:

{% embed url="https://github.com/OpenFTC/OpenRC-Turbo/blob/324ed36fa4c1adb305c026f52b746acd4692e88a/Hardware/src/main/java/com/qualcomm/hardware/bosch/BNO055IMU.java" caption="" %}

Check Your Understanding

  1. What IMU measurement is the most useful in FTC? How could you use this measurement to improve performance?
  2. If you are currently working on a robot, which axis (\(X\), \(Y\), or \(Z\)) of the Expansion Hub corresponds to the robot's heading?
  3. Given an initialized BNO055IMU instance assigned to a field named imu, retrieve the X-axis angular orientation reading in degrees and normalize it to the range \([0, 360]\).