22-12-2012, 05:11 PM
Mechanical System and Control System of a Dexterous Robot Hand
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Abstract:
In recent years numerous robot systems
with multifingered grippers or hands have been
developed all around the world. Many different
approaches have been taken, anthropomorphic and
non-anthropomorphic ones. Not only the
mechanical structure of such systems was
investigated, but also the necessary control system.
With the human hand as an exemplar, such robot
systems use their hands to grasp diverse objects
without the need to change the gripper. The special
kinematic abilities of such a robot hand, like small
masses and inertia, make even complex
manipulations and very fine manipulations of a
grasped object within the own workspace of the
hand possible. Such complex manipulations are for
example regrasping operations needed for the
rotation of a grasped object around arbitrary
angles and axis without depositing the object and
picking it up again. In this paper an overview on
the design of such robot hands in general is given,
as well as a presentation of an example of such a
robot hand, the Karlsruhe Dexterous Hand II. The
paper then ends with the presentation of some new
ideas which will be used to build an entire new
robot hand for a humanoid robot using fluidic
actuators.
Introduction
The special research area 'Humanoid Robots'
founded in Karlsruhe, Germany in July 2001 is
aimed at the development of a robot system which
cooperates and interacts physically with human
beings in 'normal' environments like kitchen or
living rooms. Such a robot system which is
designed to support humans in non-specialized,
non-industrial surroundings like these must, among
many other things, be able to grasp objects of
different size, shape and weight. And it must also
be able to fine-manipulate a grasped object. Such
great flexibility can only be reached with an
adaptable robot gripper system, a so called
multifingered gripper or robot hand.
Mechanical system
The mechanical system describes how the hand
looks like and what kind of components it is made
of. It defines the mechanical design, e.g. the
number of fingers and the kind of materials used.
Additionally actuators, e.g. electric motors, and
sensors, e.g. position encoders, are settled.
Mechanical design
The mechanical design determines the fundamental
'dexterousness' of the hand, i.e. what kind of
objects can be grasped and what kind of
manipulations can be performed with a grasped
object. Three basic aspects have to be settled when
designing a robot hand:
• The number of fingers
• The number of joints per finger
• The size and placement of the fingers
To be able to grasp and manipulate an object safely
within the workspace of the hand at least 3 fingers
are required. To achieve the full 6 degrees of
freedom (3 translatory and 3 rotatory DOF) for the
manipulation of a grasped object at least 3
independent joints are needed for each finger. This
approach was taken for the first Karlsruhe
Dexterous Hand [1,2]. However, to be able to
regrasp an object without having to release it and
then pick it up again, at least 4 fingers are
necessary.
Sensor system
The sensor system of a robot hand provides the
feedback information from the hardware back to
the control software. This is necessary to perform a
closed loop control of the fingers or a grasped
object. Three types of sensors are used in robot
hands [7,8]:
• Gripper state sensors determine the position of
the finger joints, and hence the finger tip, and
the forces which act upon the finger. Knowing
the exact position of the fingertip makes precise
position control possible, which is necessary for
dexterous fine manipulations. With the
knowledge of the forces applied to a grasped
object by the fingers it is possible to grasp a
fragile object without breaking it.
Control system
The control system of a robot hand determines
which of the potential dexterous skills provided by
the mechanical system can actually be exploited.
As mentioned before the control system can be
subdivided in the control computer or hardware and
the control algorithms or software.
Control hardware
To cope with the requirements the control hardware
is usually distributed among several specialized
processors. For example the input/output on the
lowest level (motors and sensors) can be handled
by a simple microcontroller, which is also of small
size and thus can be integrated more easily into the
manipulation system. But the higher levels of
control need more computing power and the
support of a flexible real time capable operating
system. This can be achieved most easily with PClike
components.
Therefore the control hardware often consist of a
non-uniform, distributed computing system with
microcontrollers on the one end and more powerful
processors on the other. The different computing
units then have to be connected with a
communication system, like for example a bus
system.
Conclusion
To be able to perform dexterous fine manipulations
with a robot hand a suitable mechanical system and
control system is necessary. The introduced
criterions for these systems must be considered as
shown in this paper. This was done successfully for
the Karlsruhe Dexterous Hand II. This robot hand
is capable of grasping a wide variety of objects of
different shape, size and weight. The pose of a
grasped object can be controlled reliably, even
under external disturbances. Additionally complex
fine manipulations, like regrasping, are possible
with this system.
The novel hand to be built in the context of the
'humanoid robots' special research area, will be
anthropomorphic and mechanically based on a very
different concept called fluidic actors (see
figure 17) developed at the IAI in the Karlsruhe
research center [12]. However the principal
structure of the control software will be adapted
and used for the novel hand.