The data quality of the MVN systems can be optimized by taking some important factors into account: posture, environment and body dimensions. If these factors are accounted for during calibration, like avoiding magnetic environments, you can expect a very accurate system without any drift in orientation. After a good calibration, you can even go into challenging environment to perform your measurements without trouble.
Also enter the body dimensions of your subject. These are used to determine in-body distances and they eventually aid in global positioning. Please refer to the following video on how to accurately measure body dimensions.
Measuring body dimensions
Quality calibration posture:
The Npose is the recommended calibration procedure, since this pose relates closely to a natural standing position. Next to watching the tutorial video about calibration (for MVN Link or MVN Awinda) the next few statements will help you improve your data quality even further:
- The joint center of the ankles should be located straight below the respective hip joints. As a rule of thumb you can use the method as shown in the picture below to get the correct orientation and approximate distance in between the feet:
- Extend the knees straight, while making sure not to overstretch/overextend.
- Stand up straight like you normally do. Do not overextend your spine (pushing your chest forward). After calibration if you see the avatar bending a bit forward while you stand normally, you were standing a bit overextended during calibration.
- The thumb is pointing forward and the center of the wrist joint straight underneath the center of the shoulder joint. Make sure that the arm is straight in all axes: prevent overstretching/overextending and sideways (lateral) bending in the elbow. The anatomy of the elbow in some people can cause a natural offset in a sideways bend. Try to minimize this effect as much as possible.
As a rule of thumb for calibrating an 'average person' you can touch the hips slightly with the wrist. Commonly, this aligns the arms with a straight and vertical line from the shoulders. This will also help with keeping the arms still for a moment while calibrating.
- Have the palm of the hand in a straight line with the forearm.
Quality environment during calibration:
During calibration it is important to avoid magnetic interference. Magnetic interference can cause the trackers close to this magnetic field to output an offset/drift in orientation. Commonly magnetic interference comes from the floor. This will have the result like in the picture below. After a successful high quality calibration, the system can go into almost any environment.
- The first thing to pay attention to is the colored spheres around the hands/feet/pelvis. These are magnetic field indicators. These indicators give you an approximation about whether there is magnetic interference present, and close to which body segment. Strong magnetic fields can also cause the other indicators to change color. Walk around in the room until you find a spot with all indicators solid green and perform the calibration there.
- If the indicators around the ankles are not turning completely green, you can calibrate while standing on top of the MVN suitcase to create more distance between the magnetic interference from the floor and the feet.
- If you have trouble with finding a good spot, you can analyze the magnetic field data in a real-time graph and walk around to find a good spot (MVN Studio BIOMECH feature only!).
Follow the next steps for locating a good spot for your calibration:
- Wear the MVN system and connect to MVN Studio, but don't calibrate yet.
- Make a graph and open the tab Segments > Foot > Sensor Data > Magnetic Field. Drag and drop this parameter in the graph area.
- Disregard the X, Y and Z lines and only look at the black line: the magnetic norm. The value of the magnetic norm is different in each country. The norm is 1 for the magnetic field in Enschede, the Netherlands. The norm should be between 0.8 and 1.1.
- Start walking around. The mag. norm will be stable if there is no magnetic interference. The mag. norm will change if there are other magnetic fields around (a non-homogeneous magnetic environment).
- Stop walking when you think you found a good spot. While analyzing the mag. norm lift up your foot and put it down again. You are in a spot free of magnetic interference when the mag. norm does not change when you do that movement. See the picture below for an example of a graph of a 'good' and 'bad' environment for a calibration in terms of magnetism. This data is produced while repetitively lifting up a foot. The left graph is an example where there is no magnetic interference. This location is suitable for a good quality calibration. The right graph does show magnetic interference, proving this location is not suitable for a calibration.
Quality data throughout the recording:
The MVN system will perform very well after a high quality calibration. To make this high data quality last throughout all sessions there are a few steps you can do.
- Warm up the data filters. The systems work with Kalman filters. They use data from the (near) past as a reference to correct and predict recent and future data. Immediately after calibration the filters start building their data memory. Feeding the trackers for the first 30 seconds after calibration with low-dynamic 2D/3D data and free of magnetic interference will secure a high quality and stable performance of your MVN system.