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Inertial Sensors

Overview

The glasses include inertial sensors managed through M2SensorsService (accessed from Evs.sensorsService):

Sensor Description
Gyroscope Measures orientation and angular velocity
Magnetometer Measures changes in the Earth's magnetic field
Accelerometer Measures acceleration

The raw inertial sensor base rate is 204 Hz. The quaternion (fused) base rate matches the display refresh at ~67 Hz.

All inertial sensors are disabled by default when the glasses are turned on.

The inertial sensors are also the foundation for the Line of Sight (LOS) augmented reality system, which uses the fused quaternion data to anchor virtual content to real-world positions.

Sensor Data

Using the SDK, the sensors can be enabled and consumed in the following ways:

Type Description
Base Sensors The sensor x,y,z values are corrected according to the glasses calibration parameters
Fused (Composite) All sensors are fused and the SDK supplies the calibrated Quaternion information

Sensor Rates

The sensors rate is configurable:

Rate Quaternion Hz Description
SensorsRate.Slow 1 Hz Low power
SensorsRate.Normal 2 Hz Default
SensorsRate.Medium 4 Hz Moderate
SensorsRate.Fast 33 Hz High rate
SensorsRate.VeryFast 66 Hz Full display-rate (matches on-board LOS)

Coordinate System

The sensors axes follow the standard right-hand coordinate system aligned with the glasses frame.

Sensors Rate and Rendering Rate

When the inertial sensors are active, they are capturing data at the requested SensorsRate.

To optimize the usage of the Bluetooth channel, the collected sensor information is sent from the glasses to the SDK in batches at the requested rendering interval.

As a result, when you register for sensor events, the event will be triggered before the rendering cycle, and it may be called multiple times to report each of the collected sensor data within the batch.

Base Sensors

In order to receive calibrated sensors you should:

  1. Register for the required sensor callbacks
  2. Enable the required sensors via Evs.sensorsService.enableInertialSensors(rate)
Evs.sensorsService.enableInertialSensors(SensorsRate.Normal)

Measurement units:

Sensor Units Description
Accelerometer m/s² meter/(second²)
Magnetometer mGauss milli-Gauss
Gyroscope rad/s radians/seconds

Don't forget to disable the sensor and to unregister the events when they are not required.

Fused (Composite)

The fused sensors provide orientation as a quaternion (M2Quaternion).

In order to receive the fused sensors information you should:

  1. Register to IM2SensorsEvents for quaternion events
  2. Enable the sensors fusion via Evs.sensorsService.enableInertialSensors(rate)

Enabling the sensors fusion will enable the glasses Gyroscope, Magnetometer, Accelerometer and will start the fusion algorithm process.

Quaternion Example

Evs.sensorsService.registerListener(object : IM2SensorsEvents {
    override fun onQuaternion(
        timestampMs: Long,
        quaternion: M2Quaternion,
        calibrationState: M2ImuCalibrationState
    ) {
        val w = quaternion.w
        val x = quaternion.x
        val y = quaternion.y
        val z = quaternion.z
    }
})

Evs.sensorsService.enableInertialSensors(SensorsRate.Normal)

class QuaternionHandler: IM2SensorsEvents {
    func onQuaternion(
        timestampMs: Int64,
        quaternion: M2Quaternion,
        calibrationState: M2ImuCalibrationState
    ) {
        let w = quaternion.w
        let x = quaternion.x
        let y = quaternion.y
        let z = quaternion.z
    }
}

Evs.shared.sensorsService.registerListener(listener: QuaternionHandler())
Evs.shared.sensorsService.enableInertialSensors(rate: .normal)

Cleanup

Don't forget to disable the sensor fusion and to unregister the events when they are not required:

Evs.sensorsService.disableInertialSensors()

Evs.shared.sensorsService.disableInertialSensors()

Sensor Calibration

The inertial sensors report their calibration state via M2ImuCalibrationState. Monitor it through the quaternion callback to know when calibration is needed:

Evs.sensorsService.registerListener(object : IM2SensorsEvents {
    override fun onQuaternion(
        timestampMs: Long,
        quaternion: M2Quaternion,
        calibrationState: M2ImuCalibrationState
    ) {
        when (calibrationState) {
            M2ImuCalibrationState.Ok -> { /* sensors calibrated */ }
            M2ImuCalibrationState.Calibrating -> { /* calibration in progress */ }
            M2ImuCalibrationState.Required -> { /* calibration needed - prompt user */ }
            M2ImuCalibrationState.Unknown -> { /* state not yet determined */ }
        }
    }
})

class CalibrationMonitor: IM2SensorsEvents {
    func onQuaternion(
        timestampMs: Int64,
        quaternion: M2Quaternion,
        calibrationState: M2ImuCalibrationState
    ) {
        switch calibrationState {
        case .ok:
            break // sensors calibrated
        case .calibrating:
            break // calibration in progress
        case .required:
            break // calibration needed - prompt user
        default:
            break
        }
    }
}

Evs.shared.sensorsService.registerListener(listener: CalibrationMonitor())

See Also