Principles of magnetic sensor calibration
(1) Why is calibration necessary?
When you measure a magnetic field with a magnetic sensor, changes in the magnetic field (physical phenomena) may show as changes in voltage output from the sensor.
When a sensor detects a magnetic field, the relationship to the output voltage has linear relationship.
For example, when a magnetic field of +h1 is applied to the sensor, output of voltage v1 comes from the sensor.
Stated conversely, when we see a voltage of v1, we know that a magnetic field h1 is applied to the sensor.
Next, consider the example of several sensors (3).
According to each sensor's individual characteristics, the straight line for each sensor will also vary.
These lines can be expressed in the form, y =ax + b, where x is the magnetic field, y is the voltage, a is the slope, and b is the y-intercept.。
When compared with sensor 1, sensor 2 has a smaller "b" and sensor 3 has a smaller "a."
In looking for the particular line for each sensor, we can find the correct magnetic field from the voltage. This is called "calibration".
(2) Process of calibration
The process of calibration is shown below.
In seeking the line y=ax + b, look for the line who's equation passes through the two points (-h,v1) and (+h,v2).
(Finding the 2 points is explained later)
The calculation method is as shown below.
When we transform the equation y=ax + b to equal x, it becomes x=(y-b)/a. As such, to find the magnetic field from the sensor's voltage,
Magnetic field = (sensor's voltage - b) / a
Calculate using the above equation.
(3) Process for creating external magnetic fields for calibration
The above calibration method uses two data points, +h and -h.
In order to measure this data, an external magnetic field must be applied to the sensor.
You can use the Aichi Micro Intelligent Corporation's "PalmGauss" which generates a standard magnetic field.
Also, you can use a Helmholz coil to create a standard magnetic field.
In this case, you must first adjust the standard magnetic field, which may not match the zero point due to the earth's magnetic field.
Through the Aichi Micro Intelligent Corporation's "Magnetic Shield Case", you can make it match the zero point through a separate calibration.