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Motion capture and ergonomics

Full body motion tracking is a technology that allows XR Twin to capture and record the movements of a person's body in three-dimensional space. This is typically done using sensors that are placed on the body parts. The sensors capture in real time the body posture and send the data to XR Twin, which then processes the information and creates a 3D model of the person's movements. The digital human model is then used to calculate the ergonomic score according to the relevant standards (usually RULA).

Hardware Setup

LS Group has created a tracking suit using 5 devices (5 Vive trackers) placed on both arms, legs and around the belt. This suit is named "LS Suit".

To use the LS Suit:

  • Plug in a USB hub with the 5 dongles.
  • Wear the suit as shown above and pay attention to the labels of each device.
    • The arm trackers should face upward in "T-Pose" and be placed 5 cm below the elbow.
    • The leg trackers should face backward and be placed 5 cm below the knee.
  • Switch on both Vive controllers.
  • Switch on the 5 Trackers in this specific order:

    1. Right Arm
    2. Left Arm
    3. Right Leg
    4. Left Leg
    5. Belt

Auto trackers assignment

You can now switch on your Vive trackers in any order. The trackers role assignment will occur during the calibration phase.

  • Make sure SteamVR successfully recognizes the trackers and also check that they display a solid tracker image (blinking would mean that tracker signal is lost or disturbed).

All LED trackers should be green

If not, right click on SteamVR icon, then Device > Pair Controller and press the Vive Tracker button until the LED blinks. Wait for pairing. LED become green when paired.

Using the Vive trackers in XR Twin

When creating a project, select VR as display, keep VR Controllers as hand tracking device, click + button on the User silhouette (or Next arrow), then select LS Suit as body tracking device (see the user configuration page):

This will create an avatar in T-Pose in the scene:

User settings

To achieve an accurate posture estimation and body tracking, it is important to specify the user morphology. Select the user in the hierarchy and change the Height (size) and Weight according to the person wearing the LS suit, this will update the avatar accordingly.

You can also toggle the Visualization of the avatar (display as Skin or Ergo).

Calibration

When you first launch the simulation, follow the calibration instructions:

  • Face towards the panel.
  • Stand in T-Pose (with legs together not split, hands palms facing down and arm trackers upward).
  • Press the right controller trigger or click on the Calibrate button in the user properties panel.
  • Wait for the instructions to disappear.

If the procedure is successful, your avatar should move according to your body movements. If not, click on Detach in the user properties and restart the calibration procedure.

Ergonomic assessment

XR Twin provides a real time calculation of the user's ergonomics score following the RULA methodology (see RULA worksheet). The RULA score calculation is also accessible for Replays using pre-recorded motion capture (see Watching your records).

RULA: a survey method for the investigation of work-related upper limb disorders

RULA (Rapid Upper Limb Assessment) is a survey method developed for use in ergonomics investigations of workplaces where work-related upper limb disorders are reported.
Read full article to learn more on this assessment method from McAtamney and Corlett, 1993.

If you wish to use your own custom assessment method, please contact us at: xrtwin-support@ls-group.fr.