The general definition of IMU says that any device that has accelerometer, gyroscope and magnetometer/GPS packed together is an IMU. Then why aren't other devices that have the same sensors called IMUs?
Electronic – the difference between IMUs and other devices that have accelerometers and gyroscopes (smartphones, tablets, sensewear etc.)
accelerometercellphonegyroscopeimumagnetometer
Related Solutions
They are three sensors that are useful for determining position and orientation, but they measure different things.
- A magnetometer measures magnetic fields. Because the earth has a significant magnetic field, the magnetometer can be used as a compass. As such it is useful to determine absolute orientation in the NESW plane.
- An accelerometer measures accelerations. This is useful to measure changes in velocity (directly, as the acceleration is the first time derivative of the velocity) and changes in position (by integrating the signal). They are usually used for measuring small movements. Also note that gravity acts like a continuous acceleration upward (via Einstein's equivalency principle), so a multiple-axis accelerometer can also be used as an absolute orientation sensor in the UP-DOWN plane.
- A gyroscope measures either changes in orientation (regular gyro or integrating rate gyro) or changes in rotational velocity (rate gyro).
The reason these sensors are combined is because they excel at different things. For example, for orientation, a magnetometer has poor accuracy for fast movement, but pretty much zero drift over time. An integrating scheme using gyros on the other hand reacts quickly and accurately to changes, but accumulates vast error over time. It also requires to start from a known orientation, as it only reacts to changes.
Combining the inputs to these sensors allows for quick and accurate position and orientation determination with a low amount of drift over time.
Together, these sensors are also referred to as an Inertial Measurement Unit, or IMU. You can find more information on the wikipedia page for this term. Note that the unit is not strictly speaking purely inertial when you add magnetometers, but this nomenclature is widely used.
The issue is primarily with the sensors themselves. The MEMS devices just do not have exceptionally good performance characteristics. The readout electronics are noisy and do not have very good dynamic range. The biasing will also vary with voltage and temperature, manifesting as an offset that results in drift. It's a classic engineering/economics trade-off. The devices are very small, low power, and relatively easy to mass-produce. If you want a 'real' gyro, then you need to get a high-precision mechanical gyro, or fiber optic gyro or a ring laser gyro. However, these are high precision devices that require expensive precision components and time-consuming alignment and calibration. They are also quite large and consume a lot of power. They're perfect for a commercial or military airplane, missile, or rocket, but outside of that they are far too expensive. (Not to mention probably ITAR controlled)
There is really no way to improve the accuracy of an IMU aside from either using better inertial sensors, putting the whole shebang in a constant temperature oven, and/or adding data from other, non-inertial sensors, such as a magnetometer or a motion capture system. Of course this is presuming you are already using all of the standard IMU sensors (3x gyro and 3x accelerometer) with a good sensor fusion algorithm. Adding a second IMU will likely not help much as they are just as likely to drift in the same direction as they are to drift in opposite directions.
Best Answer
Where did you find that definition?
It's true that most systems classified as IMUs have most of those sensors, but there are variations.
In general, an IMU is a device designed to track the position and orientation of an object — along with their derivatives such as velocity and angular rates.
The data is provided relative to some reference. If the reference is an absolute one, such as the orientation of the Earth, it is referred to as an absolute IMU. Otherwise, if the the reference is arbitrary (such as the position/orientation at power-up), then it is a relative IMU.
Accelerometers and rate gyros give relative information only, and are subject to drift in various forms. An IMU that includes only these sensors can only be a relative IMU. Magnetometers provide zero-drift readings of orientation, while GPS can give a zero-drift measurement of position. IMUs that include these sensors can provide absolute data.
It takes a significant amount of software to take the raw data from all of the available sensors and "fuse" it together to come up with the best possible estimate of position and orientation. The Kalman Filter is one popular approach.
Because "being an IMU" is not their primary function. They may not even include the appropriate software to combine the data from the various sensors into a full position+orientation solution.