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2.2. COMPONENT-BASED ROBOTIC MIDDLEWARES 7
easy definition of the robots kinematic. To publish the frames a robot state publisher is provided
which determines the tf frames from the raw joint data.
The provided tools in ROS greatly support the developer with the use of coordinate frames, because
the framework takes care of how the transformations are done and how the information is
exchanged between components. The developer just has to specify the transformation his interested
in. However, ROS handles all transformations in the same way and does not divide between static and
dynamic frames. There is also no way in ROS to determine the quality of a transformation chain to
find out if it satisfies the requirements of the application. Further it always publishes all TF Data to all
components using tf, also if the TF Data is not needed to perform the transformation the component
is interested in.
The ROS framework inspired this work through its powerful build in support for transformations
and the use of URDF to describe the kinematics of a robot. Further the disadvantages of ROS showed
topics where improvements are needed.
2.2.2 OROCOS
The Open Robot Control Software (OROCOS) project [6] is a framework which allows components
to run in hard real time and on multiple computers. It comes with a special Kinematics and Dynamics
library called KDL. This library supports the modelling and computation of kinematic chains out of
primitives like segments and joints. After the construction of the model, transformations between
frames can be easily calculated by different solvers. Right now only forward kinematic solvers are
implemented but own inverse kinematic solvers can be added. Further the use of the KDL library is
not only limited to robotics, it is also useful for computer animated figures or human models.
The KDL library in OROCOS is a good tool to solve kinematic chains but OROCOS has no special
build in support to distribute information of the tf tree between different components. Thus the
developer has to decide on his own how this information should be exchanged and how components
deal with this information. Further like in ROS there is also no method the determine the quality of a
transformation chain.
2.2.3 Robot Technology Middleware (RT-Middleware)
OpenRTM-aist [1] is an open source implementation of an RT-Middleware and fully compatible with
the Robot Technology Component Specification from OMG [25]. The framework provides a component
model for distributed RT systems and also defines standard interfaces for the components. It
supports components at three different levels. Components for fine granularity level, such as a motor
or sensor. Components of middle-fine granularity level such as a mobile robot with some sensors and
rough granularity level such as a humanoid robot.
Further each component has an activity which can be in different states like INITIALIZE, READY,
ACTIVE, ERROR. The states allows entry, do and exit actions which make it possible to control the
action of many components similarly. In the paper where RT-Middleware [1] was proposed it is said
that coordinate system handling is indispensable for RT-Middleware and it should be either realized
as a service or imported into the RTM specification. Until now its not available in the official RTM
release neither as a service nor as an import into the specification.
Nevertheless there exist implementations in form of components in projects where RTM is used.
One example is [26] where an intelligent space called iSpace is developed on the base of OpenRTMaist.
For the iSpace several utility components were implemented where one is responsible for coordinate
frame transformation. The input for this component is the data that should be transformed and