PDF 1.938kB

4.6. CONSTRUCTING TF SYSTEMS 39
validity of TF Data was introduced. Three values, namely the point in time t at which the TF Data was
captured, the period of time t valid how long the TF Data is valid and the type of the transformation
have to be added. Since the timestamp needed for the quality measurement is equal to t, the only
values that have to be added are t valid and the type of the transformation.
All the values described until now define the Communication Object which in pseudo code could
look like shown in Listing 4.1.
float64 x, y ,z
float64 q0, q1, q2, q3
Listing 4.1: Values for the Communication Object
# 3D vector
# Quaternion
string parent_frame
string current_frame
uint32 seq_id
# name of the parent frame
# name of the current frame
# optional sequence number
time timestamp
float64[] state_vector
float64[][] covariance_matrix
enum joint_type
float64 jx, jy, jz
# time when TF Data was captured
# state vector of size n
# covariance matrix of size n x n
# type of the joint
# 3D vector specifying joint axis
time time_valid
enum frame_type
# period of time, the data is valid
# dynamic (fixed/moving) or static frame
4.6 Constructing TF Systems
4.6.1 System Modelling
In this section some real world examples with the elements described in Section 4.3 will be presented.
These examples show that the introduced elements allow to model any kind of system, since they
provide a representation for any relevant element of the system.
For the next examples a setup where a laser ranger is mounted on an arm is considered. The arm
is statically mounted on a mobile robot base. The components which are involved are base, arm, laser
and another component named xy. The latter component is interested in the transformation chain of
the laser ranger and the laser scan published by the laser component. The corresponding tf tree can
be seen in Figure 4.20. First the modelling of this setup with the ROS tf system is shown.
dynamic
static
Figure 4.20: TF tree for the example setup.