[ros-users] tf and quaternions

[Rodrigo Baravalle]

Hello everyone!

I have already made a question about tf and synchronization which was
succesfully answered, so I will tell again briefly what I am doing:

I am currently working on building a 3D map of the environment of a
robot. I am using a swissranger SR4000 and a xsens MTI for the
rotation of the camera. The camera is attached to the robot a bit
displaced from the center. More precisely, 80cm back and 1.4m above,
so, when the robot turns, the camera describes a circle around the
center of the robot. For that, I have made this transformation:

...
swissranger.header.frame_id = "base_link";
swissranger.child_frame_id = "swissranger";
swissranger.transform.translation.x = x_scan_offset_-0.8; // offset in
x for the camera
swissranger.transform.translation.y = y_scan_offset_;
swissranger.transform.translation.z = 1.4; // height at which the
camera is placed
...

I have a 2D map of the same environment, and I use that map for
localization with amcl. When a new message from amcl arrives, I took a
pointcloud from the swissranger. I also use the transformation from tf
between /map and /swissranger to get the actual x-y-z of the
swissranger in the map:

...
tlistener.lookupTransform("/map", pc_current.header.frame_id,
                               pc_current.header.stamp, bswisstransform);

...

my first question is: does this sentence represent the actual position
of the swissranger in the map?:

bswisstransform.getOrigin();

I also have the values of the gyroscopes in a transformation called
stransform. I publish it as a transformation between "/map" and a new
frame which I named "/rot_swissranger", because I only need the
values. It is ok to do that in this way?

Finally, I want to use the position of the swissranger + the values of
the gyroscopes of the xsens to build a transformation in this way:


transform = tf::Transform(tf::Quaternion(stransform.getRotation()[0],
                                         stransform.getRotation()[1],
                                         stransform.getRotation()[2]-0.41,
                                         stransform.getRotation()[3]),
                tf::Vector3(bswisstransform.getOrigin()));

The "0.41" value is there because the gyroscope is put in a different
orientation to the direction that the robot moves. The problem is that
it works well (or at least, not too bad) when it moves horizontally or
vertically, but, when it turns, the point clouds are not transformed
properly, it seems that the rotation is greater than desired, but when
the robot reachs again an horizontal or vertical position the clouds
appear in a more correct position again.

I read in the xsens manual that the quaternions returned are unit
vectors, so I try to normalize the resultant quaternion in this way:

double a,b,c,d;
a = stransform.getRotation()[0];
b = stransform.getRotation()[1];
c = stransform.getRotation()[2]-0.41;
d = sqrt(1 - a*a - b*b - c*c);

transform_rotation2 = tf::Transform(tf::Quaternion(a,b,c,d),
            tf::Vector3(bswisstransform.getOrigin()));

But this is worst. So, the d value seems of great importance. Which is
the mistake in the values of the gyroscopes I am using?

Thanks in advance.