How to Identify a Good Magnetic Field Environment

For motion capture devices, the core consideration is the consistency of the magnetic field direction (specifically heading). Heading refers to the direction parallel to the ground, i.e., the cardinal directions (north, south, east, west).

Sometimes, there may be minimal magnetic interference, but it can still have a slight impact on direction. There may also be specific areas with strong interference that do not affect the compass direction, such as when the magnetic field direction of the interference source is perpendicular to the ground or coincides with the Earth's magnetic field direction.

Therefore, all measurements should be based on direction, with magnetic field strength being an auxiliary means of judgment. Additionally, the Earth's magnetic field varies at different latitudes; the closer to the poles, the stronger the magnetic field, and the closer to the equator, the weaker the magnetic field. Therefore, if you see some bloggers' videos referencing magnetic field strength, it is generally not very meaningful because your latitude may differ significantly from theirs. For example, the geomagnetic field strength in Beijing, China, is about 55uT, while in Shenzhen, China, it is about 35uT. Moreover, the magnetic field is also influenced by geographical factors, so magnetic field strength information is generally for reference only. Magnetic field strength in different regions is not comparable; only the magnetic field strength within the same room is comparable!

Sources of Magnetic Field Interference

The above information is for reference only! Please refer to actual measurements!

Use Physical Compass for Measurement

Use a physical compass to measure multiple points, check the magnetic field direction, and check the consistency of the physical compass needle direction. It is recommended to place the physical compass on a large rectangular cardboard box to ensure the consistency of the compass direction as much as possible. Read the degree difference of the compass needle to determine the quality of the magnetic field environment.

In a good magnetic field environment, the directions at various points are consistent. If only the magnetic field direction near the floor at a height of 10~20cm is inconsistent, but other directions are consistent, the magnetic field environment can still be considered good. (Our software has direction constraints on the ankles, so it is immune to magnetic field interference on the foot sensors).

However, if the directions of the thigh height and waist magnetic field sensors are inconsistent, it can easily cause a small angle of self-rotation in the waist after sitting down, which is commonly referred to as tilting. (This refers to not using anti-magnetic algorithms and not in 6-axis working mode).

Generally, we consider a direction difference within 5 degrees to be good magnetic field consistency, depending on the user's sensitivity to the direction of their virtual skeleton.

Use Electronic Compass for Measurement

This section only introduces the use of a mobile phone compass for measurement

The mobile phone compass is relatively less accurate compared to the physical compass and also needs calibration. It is also recommended to place the mobile phone on a large rectangular cardboard box to prevent changing the phone's direction and affecting the readings. For specific measurement and result explanations, please see Use Physical Compass for Measurement.

[danger] Precautions for Using Mobile Phone Compass

The compass in the mobile app only adopts magnetic field sensor data when it is first opened. Subsequently, it uses the gyroscope to maintain compass operation. Therefore, any magnetic field interference will cause the compass to always point in one direction.

The correct measurement method is to kill the compass process after measuring each point, and then reopen the compass app!

Use Mobile Sensor Recording App for Measurement

It is recommended to download the phyphox app for measurement. Here is the Chinese version Android download. You can also download it from the official website.

After installing the app, open it and follow the instructions below to start recording. Then walk around and record the three-axis magnetic field strength at various locations. Make sure the phone always points in one direction!!! You can place it on a large square cardboard box to ensure the consistency of the phone's direction.

If the result shows three flat curves (ensure the phone was moved between positions, a flat curve at a single point is meaningless), it indicates a good magnetic field environment. Otherwise, the magnetic field direction needs to be carefully diagnosed. It is recommended to measure different heights simultaneously.

[info] Tip

This type of continuous three-axis magnetic field recording is greatly affected by personal movement. The direction is likely to change during movement, and any change in direction will cause peaks. Therefore, this method is generally not recommended. Some users may keep changing their direction during measurement, making the results even less reliable!!!

Use rebocap Sensor for Measurement

After calibration, the rebocap sensor has higher measurement accuracy than various smartphones. Therefore, if you have rebocap equipment, use the rebocap sensor for measurement. You can place one of the sensors in the charging box with the charging contacts facing up, then select a node to view the 3D rotation of that node.

Make sure to turn off the anti-magnetic mode; otherwise, the measurement results will be invalid!!!

Then check the consistency of the cube box direction at various points. If all are consistent, it indicates a good magnetic field environment. If not, during use, if it is in non-anti-magnetic mode, the virtual body skeleton near the corresponding measurement point will deviate.

We can also use the rebocap sensor, calibrated for magnetic field strength measurement (see here to determine if calibration is needed), to measure the interference degree of different interference sources. [The calibrated rebocap sensor has higher magnetic field strength measurement accuracy than using a mobile phone].

How to Identify If Magnetic Field Calibration is Needed

Overall Qualitative Check

If already worn and lit up, you can check the magnetic field strength in the hardware information list. Generally, if the overall strength is consistent, it will show all green. If it exceeds the average range significantly, it will show yellow or even red. Therefore, pay special attention to yellow and red nodes and perform single node detection.

However, there is also a situation where the interference source in some areas is indeed large. This can be tested again away from the interference source.

Another way to judge is that the magnetic field strength of the left and right trackers at the same height should be consistent. From the ground to the head, the magnetic field strength generally changes consistently. For example, both feet are 35, calves are around 39, thighs are around 45, and chest and waist are around 46. This is generally due to environmental factors rather than magnetic field deviation.

The above judgment methods are qualitative. Whether calibration is needed, specifically refer to the next section.

Single Node Detection

Open the node to check the magnetic field strength, then find a point in space, point a, move to point a, press the sensor switch to restart, and slowly rotate around the three axes while observing the magnetic field strength changes. For example, we can use 6 faces, stay for 1 second with each face facing up, and check the magnetic field strength. If the magnetic field strength is completely consistent, calibration is not needed; otherwise, calibration is needed.

We can also check the relative magnetic field strength. If the variation range is within 0.1, generally, recalibration is not needed. However, if you are not confident, you can perform magnetic field calibration to see if the consistency of the magnetic field strength improves after calibration.

[info] Principle Explanation

If there is a magnetic field offset, the magnetic field strength will be inconsistent when the sensor is in different directions. For example, suppose the x-axis has an offset of 15uT. If the magnetic field direction is consistent with the x-axis, the magnetic field strength is 45uT. When the magnetic field is in the same direction as the positive x-axis, the measured value is 60uT, and in the opposite direction, the measured result is 30uT. If there is no offset, the measured result will be 45uT.

However, the magnetic field strength in our room will always have some interference. The specific magnetic field in the room often shows small fluctuations. Therefore, a variation of about 1~3uT during the measurement process is normal, and the average value should be observed.

[warning] Tip

The detected spatial point must be far from interference sources, preferably half a meter away from the computer, and rotate around a point in space.

How to Choose Anti-Magnetic Mode

抗磁模式选择

how to choose magnet mode

耐磁模式选择

Other Common Issues

• After sleeping, the chest or waist is completely twisted

  If using 6-axis mode, this is caused by long-term drift. If the node is moved away from the chest to avoid fluctuations caused by breathing, our algorithm can suppress gyroscope drift.

  If using anti-magnetic mode, if the relative magnetic field size is not around 1.0 for a long time after lying down, the sensor essentially works in 6-axis mode.

• Using anti-magnetic mode, the waist still has slight bending when sitting down

  In anti-magnetic mode, it mainly depends on the magnetic field strength to choose whether to use 6-axis or magnetometer. If the magnetic field strength is consistent, but there is a slight angle deviation when standing and sitting, the anti-magnetic algorithm cannot recognize it, so there will still be a small angle deviation. You can consider using 6-axis mode without the magnetometer to solve this problem.

  The anti-magnetic algorithm mainly solves the problem of poor magnetic field at a certain point, but in most areas and most of the time, the magnetic field environment is good. For example, when the handle is close to the sensor or temporarily moved to the front of the computer, the anti-magnetic effect will be very obvious. However, for small angle magnetic field errors and consistent magnetic field strength, the anti-magnetic algorithm currently cannot achieve good recognition. Please wait for updates. ```