Inertial Measurement Unit (IMU)

The VPU includes a built-in Inertial Measurement Unit (IMU) which is always mapped to the virtual port port 6. The IMU measures the rotation rates - gyroscope [rad/sec] and acceleration - accelerometer [m/sec] in three-dimensional space w.r.t to the VPU coordinate frame (extrinsic calibration). The IMU sample rate is 1 KHz per default.

Users can access the IMU data in firmware versions 1.4.30 and higher.

Configuration

Parameter	Description
`state`	Configure the IMU state.
`mode`	Only one mode is available: `imu_1_khz`
`acquisition/pollrate`	The rate at which the data is polled from the IMU.
`data/availablePCICOutput`	The `buffer_id` of the PCIC output.

Changing the pollrate parameter to a higher value may affect the CPU load. Configuring this value lower than 8 can lead to diagnostic RECEIVED_IMPLAUSIBLE_IMU_DATA.

Note

To check the minimum and maximum limits, as well as the default value for each parameter, the user can refer to the schema.

For example, use the ifm3d API with:

$ ifm3d jsonschema | jq .properties.ports.properties.port6.properties.acquisition.properties.pollrate
{
  "attributes": [
    "conf"
  ],
  "default": 20,
  "description": "Pollrate of the IMU",
  "maximum": 200,
  "minimum": 1,
  "multipleOf": 1,
  "readOnly": false,
  "type": "integer"
}

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`

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