Application

AHRS and IMU Comparison

High Performance Navigation MEMS IMU

The attitude reference system is also AHRS. The AHRS is composed of accelerometer, magnetic field meter and gyroscope to measure the motion parameters and azimuth of the actual target. Through processing and calculating the measured values, the position and motion state of the target are tracked and monitored in real time. The real reference of AHRS comes from the earth's gravity field and the earth's magnetic field. In other words, if AHRS leaves the earth, there is gravity and When the magnetic field environment is not working properly, the more orthogonal the general magnetic field and the gravity field, the better the attitude measurement effect, that is, if the magnetic field and the gravity field are parallel, such as in the geomagnetic north and south poles. The magnetic field here is downward, that is, the same direction as the weight field. At this time, the route can not be measured, which is the defect of the attitude system. At high latitudes, the error of the route angle will become larger and larger.

The IMU is also an inertial measurement unit is a device that uses an acceleration sensor and gyroscope to measure the acceleration and rotational angular velocity of a target system to obtain the target's motion and status in the inertial reference frame. All motions can be decomposed into a linear motion and a rotary motion. Therefore, the inertial measurement unit measures the two motions. The linear motion can be measured by an accelerometer, and the rotary motion is passed through the gyro.

Reference system distinction

From the definitions,it’s easy to understand that the reference frame chosen by AHRS for measurement is the earth itself, and the location of its measurement object is also a specific geographical location. The IMU inertial measurement unit is different in that it measures the position and motion state relative to a specific inertial reference frame, which can be a fixed point such as a house, a building, or a system of uniform motion. Conceptually, inertial measurement units are more widely used because the Earth, the reference point of AHRS, is also an inertial reference frame (not absolutely, Earth-only). Using the Sun as the inertial reference frame in the solar system).

System composition distinction

Although the measurement elements of AHRS and IMU are basically the same, the reference systems of AHRS are different. AHRS has one more magnetic field sensor than IMU. Why is AHRS lower than IMU and depends on gravity field and magnetic field? This is determined by the sensor device architecture. AHRS sensor is usually a low cost MEMS sensor. The noise of the gyroscope and accelerometer of the sensor is relatively large. Taking the planar gyro as an example, the integration of the gyro gyroscope will drift about 2 degrees for one minute. If there is no magnetic field and gravity field to correct the three axes. In the case of the gyro, then the actual attitude and the measured output attitude of the object are completely changed after 3 minutes, so the field vector must be used for correction under the framework of such a low-cost gyroscope and accelerometer.The IMU is actually the same. Only the relatively accurate sensor, the IMU's gyroscope uses fiber optic gyro or mechanical gyro. The cost of this gyro is very high, and the accuracy is relatively high precision compared with the mems gyro.

Measurement accuracy

When the AHRS is monitoring the motion trajectory and condition, due to the zero drift of the gyroscope, the error will become larger and larger as time goes by when integrating the rotation Angle during the movement. Therefore, the magnetometer needs to calibrate the geographical azimuth Angle of the movement in time. The AHRS selects the Earth as the reference frame and calibrates the position by measuring the electromagnetic field with a magnetometer, which is often more accurate than an inertial measurement unit containing a magnetometer. The calibration of the geographical azimuth Angle through the magnetometer can improve the accuracy to a certain extent, but the magnetometer's own error and the existence of factors such as measuring time drift and complex magnetic fields.

If you want to get more details about IMU,pls visit https://www.ericcointernational.com/inertial-measurement-units/

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