Quaternions And How to Really Use Them

By | February 20, 2016

Quaternions are often used in attitude and heading systems to represent orientation. This post describes quaternions and how to use them in orientation and navigation systems.

Quaternions

If you type ‘quaternion’ into google, it’ll tell you something like this

a complex number of the form w + xi + yj + zk, where w, x, y, z are real numbers and i, j, k are imaginary units that satisfy certain conditions.

That’s a pretty frightening description, but with any luck, you’re a persistent type of person because once you’ve masterd quaternions (at least, unit quaternions used to represent orientation) you’ll probably find they are actually nowhere near as difficult as you may have been led to believe. For a start, you can ignore the imaginary components. They’re just a figment of your imagination, anyway.

From here on, we’ll use a representation with only four components. These are w, x, y and z.

Normalising

These four components make quaternions similar to four-dimensional vectors in a lot of ways. For a start, normalising a quaternion is done in the same way.

In pseudo-code

The quaternion library in ETK has magnitude and normalise functions

Multiplying

One of the most import quaternion operation is multiplication. It’s used to add the amount of rotation stored in one quaternion to another. For example, quaternion A represents straight, level and east facing ( 90 degrees). Quaternion B is straight, level and facing north east ( 45 degrees ). Multiplying A and B will give a quaternion that has a heading of 135 degrees – because multiplying A by B adds the rotation of B to A. Make sense?

Here’s the proof

The output from this program is 135 0 0.

One very important thing to note is that quaternion multiplication is non-communicative. That means

The Conjugate

For unit quaternions, the conjugate is the same as the inverse. This is great because the conjugate is a lot easier to calculate than the inverse. To find the conjugate, simply invert the x, y and z components.

etk::Quaternion has a conjugate function

The conjugate is great because it allows you to unrotate or substract a rotation from another quaternion. Multiplying quaternions adds rotation but multiplying by the conjugate subtracts it.

Rotating Vectors

Quaternions can be used to rotate vectors. This is really useful for converting vectors to a different frame of reference. Example:

A pilot experiences acceleration, but a large component of this is due to gravity. We know that gravity always measures 9.8m/s/s upwards in the world frame of reference and we know the orientation of the aircraft. We can use the aircrafts orientation to rotate a gravity vector into the aircraft frame of reference, then simply subtract it from total acceleration felt by the pilot.

Angular Rates To Quaternion

Converting angular velocity to a quaternion is easily done with ETK. Simply use the function fromAngularVelocity().

If angular velocity is regular velocity, then a quaternion represents position. It simply doesn’t make sense to convert velocity into a position unless you have a time delta. Velocity equates to a change in distance over a period of time. fromAngularVelocity() requires angular velocity in radians per second, and the sample rate of the gyroscopes in seconds. It will calculate a quaternion that represents that change in angular position.

You can also convert the quaternion back into an angular rate using toAngularVelocity().

Converting To Other Orientation Forms

etk::Quaternions can be converted to and from NED orientation matrices, axis angles and euler angles.

Helpful Resources

etk::Quaternion source

 

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