Inertial Measurement Unit (IMU)

The VPU includes a built-in Inertial Measurement Unit (IMU) which is always mapped to the port port 6. The IMU measures the rotation rates - gyroscope [rad/sec] and acceleration - accelerometer [m/sec²] in three-dimensional space. The IMU is IIM-42652, and the full datasheet can be found on the official TDK website. Users can access the IMU data in firmware versions 1.4.30 and higher.

Configuration

A few of the IMU parameters can be configured in the JSON configuration:

Parameter	Description
`state`	Configure the IMU state.
`mode`	Only one mode is available: `imu_1_khz`
`data/availablePCICOutput`	The `buffer_id` of the PCIC output.

The default mode, imu_1_khz, configures the IMU with the following settings, which cannot be edited by the user:

The output rate is 1 KHz for both the gyroscope and acceleration.
The full-scale range is set to ± 4 g for accelerometer and to ± 500 degrees per second for gyroscope.
For both accelerometer and gyroscope, the low-noise mode is enabled.
The timestamp resolution is set to 16 µs.

Output

Each frame received from the IMU contains the following output.

Description	Type
The current version of IMU	`int`
The list of IMU samples. The number of elements in list depends on `pollrate`.	`Array`
The IMU sample data	`IMUSample`
Number of samples received	`int`
Extrinsic calibration parameters IMU with respect to `User`	`ExtrinsicCalibrationStructure`
Extrinsic calibration parameters IMU with respect to `VPU`	`ExtrinsicCalibrationStructure`
Receive timestamp	`int`

The IMU output is unfiltered. Additionally, the IMU data is not impacted by the extrinsic calibration set for port6. Any transformation to a custom coordinate system has to be performed by the user.

IMUSample

Description	Type
The hardware timestamp, as reported by the IMU	`uint16`
Acquisition timestamp: hardware timestamps scaled to OVP time.	`uint64`
IMU temperature	`float32`
Linear acceleration along X-Axis (m/sec²)	`float32`
Linear acceleration along Y-Axis (m/sec²)	`float32`
Linear acceleration along Z-Axis (m/sec²)	`float32`
Angular velocity along X-Axis (rad/sec)	`float32`
Angular velocity along Y-Axis (rad/sec)	`float32`
Angular velocity along Z-Axis (rad/sec)	`float32`

ExtrinsicCalibrationStructure

Description	Type
Translation parameter along X-Axis(m)	`float32`
Translation parameter along Y-Axis(m)	`float32`
Translation parameter along Z-Axis(m)	`float32`
Rotation parameter along X-Axis (m)	`float32`
Rotation parameter along Y-Axis (m)	`float32`
Rotation parameter along Z-Axis (m)	`float32`

Example

Accessing the IMU data is possible with the ifm3d API version 1.5.3 and above. As of ifm3d API 1.5.3, a custom deserializer has to be implemented and only a Python example is available.

Please find the Python example code that accesses and prints the IMU data at ifm3d-examples

The output looks like:

$ python3 imu_data.py 
IMU version: 1

Number of Samples: 51

First sample:
    Hardware timestamp: 19591
    Acquisition timestamp: 1651191293281612492
    Temperature: 32.729469299316406
    Acceleration: 
        x: -0.07661445438861847 
        y: -0.17717093229293823 
        z: -9.770736694335938
    Gyroscope: 
        x: 0.008788431994616985 
        y: -0.006391586735844612 
        z: -0.001065264455974102

Extrinsic IMU to User: 
 rot_x: 0.0 
 rot_y: 3.1415927410125732 
 rot_z: 3.1415927410125732 
 trans_x: 0.05280037596821785 
 trans_y: 0.01630011759698391 
 trans_z: 0.020900148898363113 
 
Extrinsic IMU to VPU: 
 rot_x: 0.0 
 rot_y: 3.1415927410125732 
 rot_z: 3.1415927410125732 
 trans_x: 0.052799999713897705 
 trans_y: 0.016300000250339508 
 trans_z: 0.020899999886751175 
 
Receive Timestamp: 1651191293330924608