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I'm trying to understand how a MEMS gyroscope can give you orientation. The question may not make any sense, I may have some misunderstandings about the different topics...

From the Wikipedia page about gyroscope:

A gyroscope is a device for measuring or maintaining orientation.

If we have a gyroscope that can measure rotation around one axis, I interpret that statement as saying the gyroscope can also tell you what your orientation is relative to some position. Is this right? And what is the orientation relative to?

I used this article to try to understand how a MEMS gyro works: http://sensorwiki.org/doku.php/sensors/gyroscope I can see how such a device can measure changes in angular velocity, but once the frame of reference stops spinning, how does it know the orientation? What is the position it is using to find the orientation?

If you put these gyroscopes in a phone (like the iPhone has), how does it know the orientation of the phone? Again, I understand how it could calculate rotational velocity as the phone moves, but how does it know the final orientation?

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I'm not entirely sure that Physics.SE is the right place for this. Not that it is a bad question, mind. If you are interested in how to build a circuit to do this Electronics.SE seems better; if you are programming a micro-controller to handle it in software Stack Overflow is probably best; if the math has you stumped maybe Math.SE would be better. –  dmckee Sep 5 '11 at 20:25
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4 Answers

Integration?

That is, if the instrument provides a continuous signal proportional to $\frac{dx}{dt}$, you can find the total change in $x$ of $[t_1,t_2]$ from $$ \Delta x = \int_{t_1}^{t_2} dt \frac{dx}{dt} \approx \sum_{i=0}^n \Delta t \left. \frac{dx}{dt} \right |_i . $$

Of course, as anyone working on inertial guidance knows, that approximation is makes for real difficulty as does the matter of noise and calibration drifts on the instrument. But if you can sanity check the result from time to time (say by comparing with GPS) this will work quite well.

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Depends what you mean by gyroscope.

Most cheap systems in phones etc are really accelerometers and so can only give position by integrating movements.

There are true or north seeking gyros, which can detect true heading by using the fact that the earth rotates and so find north. Traditionally these used spinning wheels, modern systems use pulses of light in coils of optical fibre, MEMS versions of this use Coriolis forces to make a mechanical pulse spin around a mechanical ring.

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MEMS gyros aren't usually employed for long-term orientation tracking. You'd have to poll them constantly to integrate results properly, and this would drain the phone's battery too quickly. (Ballistic rockets calculate coordinates through gyroscope data integration, but, once launched, they need not last as long as a phone.)

The MEMS gyros may help you play games, but once you've deactivated the app that uses them, they're off.

Note that some devices have separate angular and linear accelerometers for all 3 axes; I suppose that only angular accelerometers are MEMS gyros.

Linear accelereometers constantly feel Earth's gravitational field, so it's easy to rotate the screen correctly without tracking the position constantly.

The arrows which show your "absolute" direction on a smarthpone map is often based on a magnetic sensor (like a regular compass); in motion, you can calculate motion direction by GPS coordinates that the phone regularly receives. Presence of a magnetic sensor allows for 'metal seeker' apps for e.g. Android smartphones.

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MEM gyroscopes are not a specific type, but a class, of gyroscopes which are printed onto circuit boards using photolithography.A MEMS gyroscopes does not look like a normal gyroscope. Actually MEMS gyroscope is not a rotating structure but a vibrating structure. Each type of MEMs gyroscope has some form of oscillating component from where the acccleration, and hence direction change, can be detected. This is because, as per the conservation of motion law, a vibrating object likes to continue vibrating in the same plane, and any vibrational deviation can be used to derive a change in direction.You can get a comparison of MEMS Gyro and North Seeking Gyro work.

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protected by Qmechanic Mar 1 at 10:08

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