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Coordinate Transformations in Robotics

In robotics applications, many different coordinate systems can be used to define where robots, sensors, and other objects are located. In general, the location of an object in 3-D space can be specified by position and orientation values. There are multiple possible representations for these values, some of which are specific to certain applications. Translation and rotation are alternative terms for position and orientation. Robotics System Toolbox™ supports representations that are commonly used in robotics and allows you to convert between them. You can transform between coordinate systems when you apply these representations to 3-D points. These supported representations are detailed below with brief explanations of their usage and numeric equivalent in MATLAB®. Each representation has an abbreviation for its name. This is used in the naming of arguments and conversion functions that are supported in this toolbox.

At the end of this section, you can find out about the conversion functions that we offer to convert between these representations.

Robotics System Toolbox assumes that positions and orientations are defined in a right-handed Cartesian coordinate system.

Axis-Angle

Abbreviation: axang

A rotation in 3-D space described by a scalar rotation around a fixed axis defined by a vector.

Numeric Representation: 1-by-3 unit vector and a scalar angle combined as a 1-by-4 vector

For example, a rotation of pi/2 radians around the y-axis would be:

axang = [0 1 0 pi/2]

Euler Angles

Abbreviation: eul

Euler angles are three angles that describe the orientation of a rigid body. Each angle is a scalar rotation around a given coordinate frame axis. The Robotics System Toolbox supports two rotation orders. The 'ZYZ' axis order is commonly used for robotics applications. We also support the 'ZYX' axis order which is also denoted as “Roll Pitch Yaw (rpy).” Knowing which axis order you use is important for apply the rotation to points and in converting to other representations.

Numeric Representation: 1-by-3 vector of scalar angles

For example, a rotation around the y-axis of pi would be expressed as:

eul = [0 pi 0]

Note: The axis order is not stored in the transformation, so you must be aware of what rotation order is to be applied.

Homogeneous Transformation Matrix

Abbreviation: tform

A homogeneous transformation matrix combines a translation and rotation into one matrix.

Numeric Representation: 4-by-4 matrix

For example, a rotation of angle α around the y-axis and a translation of 4 units along the y-axis would be expressed as:

tform =
 cos α  0      sin α  0 
 0      1      0      4
-sin α  0      cos α  0
 0      0      0      1

You should pre-multiply your homogeneous transformation matrix with your homogeneous coordinates, which are represented as a matrix of row vectors (n-by-4 matrix of points). Utilize the transpose (') to rotate your points for matrix multiplication. For example:

points = rand(100,4);
tformPoints = (tform*points')';

Quaternion

Abbreviation: quat

A quaternion is a four-element vector with a scalar rotation and 3-element vector. Quaternions are advantageous because they avoid singularity issues that are inherent in other representations. The first element, w, is a scalar to normalize the vector with the three other values, [x y z] defining the axis of rotation.

Numeric Representation: 1-by-4 vector

For example, a rotation of pi/2 around the y-axis would be expressed as:

quat = [0.7071 0 0.7071 0]

Rotation Matrix

Abbreviation: rotm

A rotation matrix describes a rotation in 3-D space. It is a square, orthonormal matrix with a determinant of 1.

Numeric Representation: 3-by-3 matrix

For example, a rotation of α degrees around the x-axis would be:

rotm =

     1     0         0
     0     cos α     -sin α
     0     sin α     cos α

You should pre-multiply your rotation matrix with your coordinates, which are represented as a matrix of row vectors (n-by-3 matrix of points). Utilize the transpose (') to rotate your points for matrix multiplication. For example:

points = rand(100,3);
rotPoints = (rotm*points')';

Translation Vector

Abbreviation: trvec

A translation vector is represented in 3-D Euclidean space as Cartesian coordinates. It only involves coordinate translation applied equally to all points. There is no rotation involved.

Numeric Representation: 1-by-3 vector

For example, a translation by 3 units along the x-axis and 2.5 units along the z-axis would be expressed as:

trvec = [3 0 2.5]

Conversion Functions and Transformations

Robotics System Toolbox provides conversion functions for the previously mentioned transformation representations. Not all conversions are supported by a dedicated function. Below is a table showing which conversions are supported (in blue). The abbreviations for the rotation and translation representations are shown as well.

 Converting ToAxis-Angle (axang)Euler Angles (eul)Numeric Quaternion (quat)Quaternion Object (quaternion)Rotation Matrix (rotm)Homogeneous Transformation Matrix (tform)Translation Vector (trvec)
Converting From 
Axis-Angle (axang)——axang2quatquaternionaxang2rotmaxang2tform—
Euler Angles (eul)——eul2quatquaternioneul2rotmeul2tform 
Numeric Quaternion (quat)quat2axangquat2eul—quaternionquat2rotmquat2tform 
Quaternion Object (quaternion)—eulercompact— — 
Rotation Matrix (rotm)rotm2axangrotm2eulrotm2quatquaternion—rotm2tform 
Homogeneous Transformation Matrix (tform)tform2axangtform2eultform2quat—tform2rotm—tform2trvec
Translation Vector (trvec)—————trvec2tform—

The names of all the conversion functions follow a standard format. They follow the form alpha2beta where alpha is the abbreviation for what you are converting from and beta is what you are converting to as an abbreviation. For example, converting from Euler angles to quaternion would be eul2quat.

All the functions expect valid inputs. If you specify invalid inputs, the outputs will be undefined.

There are other conversion functions for converting between radians and degrees, Cartesian and homogeneous coordinates, and for calculating wrapped angle differences. For a full list of conversions, see Coordinate Transformations.

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