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I've been messing around with hall effect style compass sensors, which need calibrated at their location to work properly. I noticed the rear view mirror in my car has some kind of compass indicator, and it just seems to work no matter where I am without recalibrating.

Does anyone know the type of sensors used in these?

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    \$\begingroup\$ It probably recalibrates automatically as you're making turns. \$\endgroup\$ Commented Oct 12, 2023 at 14:37
  • \$\begingroup\$ Also might be not magnetic, but GPS and dead reckoning based, i.e. it just pulls the heading from the nav system. The compass in my car is like that. \$\endgroup\$
    – user71659
    Commented Oct 12, 2023 at 19:11
  • \$\begingroup\$ How old is the electric compass? Or how old is the car? MEMS magnetometers are a recent development. Older cars used magnetic coils. I do not believe many commercial magnetometers uses Hall Effect (which BTW would not be considered MEMS as nothing moves in such a sensor). Instead they use MEMS in creative ways by employing the Lorentz force. \$\endgroup\$
    – st2000
    Commented Nov 10, 2023 at 13:41

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They usually use MEMS compass chips. For example, HMC5883L is widely used for detecting the heading. Although these kinds of chips are not accurate without calibration, they are acceptable if you are measuring approximate headings. In most of the cars, the heading accuracy needs to be less than 45 degrees. This is easily achievable using that chip even without calibration.

However, the chip can be calibrated while you are moving. When you are making a turn (360 degrees), the algorithm can make an estimate of a fixed magnetic field that is present in the car. This could come from any Iron part close to it. After making that estimation, the accuracy will increase.

There are some applications that need very high accuracy. In those applications, the GPS data also is fused with magnetic measurement. The GPS data will help to retrieve the expected earth's magnetic field from a stored database. Then, the device can work accurately all over the world. I don't think the car compass does that (even the cars with built-in GPS systems).

On top of GPS fusion, some more expensive devices use the accelerometer and gyro in data fusion. This will lead to a complete IMU. Using those sensors helps to have better estimates especially when you are changing the direction fast.

There is another method mentioned by @user71659 in the comment. It uses Purely GPS data without any magnetometer. This one calculates the direction after you start moving. The idea is it uses the current coordinates and the previous one (sampling every second or so) to find out your direction. This method might not be very accurate around the sharp corners (as it needs to wait for new coordinates to get updated). But on straight roads, it will be accurate.

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    \$\begingroup\$ The system in my car is purely GPS/dead reckoning based, there's no magnetic flux sensor anywhere. One characteristic is if you disconnect the battery, it won't tell you the direction until you start moving again. (It's smart enough to remember your last heading across key cycles) They need the GPS in all new cars for telematics/eCall anyway, then it's just a bit of software to make it work. \$\endgroup\$
    – user71659
    Commented Oct 12, 2023 at 19:16
  • \$\begingroup\$ "Although these kinds of chips (MEMS) are not accurate without calibration, they are acceptable if you are measuring approximate headings." I work with MEMS magnetometers and have found they are all completely useless without some magnitude and offset calibration. To be clear, none of them could be used off the shelf even if all you wanted were the 4 points of a compass. BTW, most cars I've seen display 8 points (N, NE, E, SE, S, SW, W & NW) so need to be accurate to 22.5 degrees. \$\endgroup\$
    – st2000
    Commented Nov 10, 2023 at 13:32

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