How to use IMU?

A. Basic understanding of IMU, VRU, AHRS

1. What do 6 axis, 9 axis, IMU, VRU, AHRS refer to?

The concept of 6-axis and 9-axis is easy to understand: to put it bluntly, it means which and how many sensors are installed on the module

6-axis: three-axis (XYZ) accelerometer + three-axis (XYZ) gyroscope (also called angular velocity sensor) 9-axis: 6-axis + three-axis (XYZ) magnetic field sensor 6-axis module can form VRU (vertical reference unit) and IMU (inertial measurement unit), 9-axis module can form AHRS (heading and attitude reference system) IMU: Inertial measurement unit, which can output acceleration and angular velocity. Does not output other information such as attitude angle VRU: Built-in attitude calculation algorithm based on IMU, can output attitude information.

In the static state, the accelerometer can measure the gravity vector and use it as a reference, so the static pitch and roll angle will not drift and the accuracy is relatively high. However, because the heading angle is perpendicular to the gravity, there is no absolute reference, and the heading angle error in the horizontal direction will be As time gradually grows larger, it becomes more and more inaccurate.

When the module is in motion, the accelerometer measures not only gravity, but also other motion acceleration (harmful acceleration), so the gravity vector cannot be used as a reference to correct the pitch and roll angle during module motion. A simple conclusion is: if the module is in a state of large maneuvering for a long time, then the errors of the three Euler angles will become larger with time (increasingly inaccurate). Once stationary, the pitch and roll angles will be “pulled” back again. To the correct position, and the heading angle will not be corrected because there is no reference.

AHRS: Modified algorithm on the basis of VRU, can calculate the full attitude of the measured object, including absolute heading angle (angle with geomagnetic north pole), because the geomagnetic sensor is used, it must be a 9-axis module . In addition, due to the serious distortion of the geomagnetic field indoors, it is difficult for AHRS to obtain an accurate absolute heading angle indoors.

GPS: The United States’ Global Satellite Positioning System: Global Position System is called Global Positioning System.

GNSS: Global Positioning System, GPS, Beidou, GLONASS and other systems. Each system is called a “constellation”.

GNSS/INS: Satellite/Inertial Navigation System

B. Performance related

1. Performance and limitations of IMU, VRU and AHRS

IMU:

Inertial measurement unit, which can output acceleration and angular velocity. Does not output other information such as attitude angle

VRU:

Built-in attitude calculation algorithm based on IMU, can output attitude information.

In the static state, the accelerometer can measure the gravity vector and use it as a reference, so the static pitch and roll angle will not drift and the accuracy is relatively high. However, because the heading angle is perpendicular to the gravity, there is no absolute reference, and the heading angle error in the horizontal direction will be As time gradually grows larger, it becomes more and more inaccurate.

When the module is in motion, the accelerometer measures not only gravity, but also other motion acceleration (harmful acceleration), so the gravity vector cannot be used as a reference to correct the pitch and roll angle during module motion. A simple conclusion is: if the module is in a state of large maneuvering for a long time, then the errors of the three Euler angles will become larger with time (increasingly inaccurate). Once stationary, the pitch and roll angles will be “pulled” back again. To the correct position, and the heading angle will not be corrected because there is no reference.

AHRS:

Modify the algorithm based on the VRU to calculate the full attitude of the measured object, including the absolute heading angle (the angle with the geomagnetic north pole). Because the geomagnetic sensor is used, it must be a 9-axis module. In addition, due to the serious distortion of the geomagnetic field indoors, it is difficult for AHRS to obtain an accurate absolute heading angle indoors.

2. Can the module calculate the speed and position by integration?

The theory is OK, but the practice is not (not meaningful). If there is no other way to correct the deviation (such as GPS), then the position will diverge quickly, such as the HI226 module, the acceleration is integrated to get the speed, and the speed is integrated to get the position. In this way, after the second integration, even under static conditions, it will drift by tens of meters in 1 minute. It is also possible to move at high speeds/random drifting out of 1KM. Really pure inertial navigation solutions to obtain stable posture applications are high-end IMU (fiber optics, laser gyroscopes, etc.), which are generally of great value.

3. Will the module be interfered by strong magnetic fields such as motors?

6-axis will not be possible at all, 9-axis will certainly be, and it is very large. Therefore, the 9-axis mode is generally not suitable for applications where there are magnetic materials around such as robots.

4. Explain the phenomenon of heading angle drift?

The heading angle drift of the 6-axis module is inevitable, but the degree is different. The algorithm cannot solve the limit of chip performance. It should be noted that all attitude modules need to be powered on and stand still for about 1s to obtain the gyro zero offset, otherwise the heading angle drift will be more serious, please refer to the description of the product manual for details.

The 9-axis module needs to be configured in 9-axis mode, and the geomagnetism has been calibrated, and there is no interference from the geomagnetism space distortion to output a stable and drift-free heading angle. In indoor environments: around desks, workshops, laboratories, instruments and equipment The spatial magnetic field distortion in the area next to it is very serious. The heading angle northing accuracy in the 9-axis mode is generally poor. You can test the performance of the module outdoors for the first time and compare it indoors.

5. Without professional equipment such as turntables, how to qualitatively evaluate the dynamic accuracy simply and quickly?

A simple qualitative analysis method:

Place the module horizontally, and when it stabilizes, pick up the module to perform random maneuvers (slowly do not move too violently, do not exceed the range of the gyro), and return to the horizontal position after a certain period of time (1min). At this time, you will find pitch and roll The angle has a slow “returning” process.

This is because the accelerometer in motion no longer only measures the gravity vector, so it cannot provide an absolute reference for the pitch and roll angle. It can only rely on gyro integration to recursive attitude. As time goes by, pure gyro integration attitude will inevitably have errors .

After repositioning it horizontally, the module is in a static state, and the accelerometer measures only the gravity vector, so it can continue to provide absolute reference for the pitch and roll angles, so there is a “return to normal” process. Therefore, the gyroscope performance of this product can be simply compared from the degree of “returning” (not the speed).

About wireless Hi221 node / Hi221 dongle

1. Will wireless receivers interfere with each other?

The wireless receiver receives all the nodes online in the set GWID frequency band, and distinguishes the nodes according to their own ID.

If the user has set different wireless frequency bands (GWID) according to the manual, and the distance between each set of receivers is more than 5 meters, the chance of interference is small. A few suggestions:

1. It is not recommended to use more than 2 sets of wireless receivers within 5 meters
2. It is recommended that the distance between each group of receivers and the partner node is close. For example, the receiver with GWID=1 is the closest to the node, avoiding the closest node with GWID=2.

2. What is the wireless receiver’s setting and maximum receiving frame rate?

Depends on the set baud rate. When the baud rate of the receiver is set to the maximum of 921600, 8 Hi221 at 100Hz and 16 Hi221 at 50Hz can be connected.

3. The sensitivity of the gyroscope to vibration

Very sensitive. Theoretically speaking, the gyroscope should be insensitive to acceleration (one measuring angular velocity and one measuring acceleration), but in fact MEMS devices are not (perfect). The top is also sensitive to acceleration (vibration) and is called “gravity sensitivity or G sensitivity”. These indicators are actually more important than the bias stability. For vibration occasions, the performance of low-cost IMUs is much worse than that of fiber optic gyroscopes and high-end MEMS (when other indicators are the same). In fact, fiber optic gyroscopes are because The measurement principle is different, there is no such vibration sensitivity index at all). For example: there is a vibration source (fan) around the IMU, which will greatly affect the accuracy of the output data of the IMU.

4. What should I do if the baud rate is configured incorrectly?

The module baud rate is only possible: 9600, 115200, 256000, 460800, 921600, 1000000. Entering any other baud rate is invalid. If you forget the previous baud rate, just try one by one.