Game Development Reference
In-Depth Information
carried inertia and does not allow the use of identical motors on all the axes. The
motors on the first generation actuators are located in the base and the movements are
transmitted to the different articulations by cable and pulley systems. This increases
complexity and reduces reliability.
On the other hand, a fully parallel structure (with as many sub-structures between
the fixed base and the robot gripper as the degrees of freedom, with each sub-structure
carrying a motor) presents the advantage of identical motors in the base, which reduces
the loaded mass and complexity. In addition, the segments of the robot only work in
traction and compression, which makes it possible to lighten them. On the other hand,
the work space is limited (mainly in rotation) with respect to the size of the robot.
In rotation, it depends on the place where the robot gripper is located.
The tight rope structures operate on the same principle but the segments are
replaced with cables that help to directly actuate the robot gripper. These structures are
very simple and light as they require no structural component. In addition, it is very
easy to obtain a significant work space by simply extending the cables. However, as the
cables act as unidirectional robot actuators, it is necessary to have a redundant acti-
vation (minimum m
1 motors for m degrees of freedom). The control must take this
redundancy into account tomanage the voltage in the cables and ensure that it is always
positive. In addition, if this solution is adapted for force feedback interfaces in trans-
lation, it becomes complex for interfaces with more than 3 DOF with force feedback.
Thus, an interface with 6 degrees of freedom requires 7 cables, which limits the
orientation of the work space if we want to prevent them from getting tangled. The
access to this type of interface is also difficult because the cables must be uniformly
distributed all around the robot gripper.
The two-stage mixed structures make a compromise between the serial and parallel
structures. They have a significant work space. In particular, the travel in orientation is
independent of the place where the handle is located, as the translations and rotations
are carried out by two separate stages. Moreover, the translation stage motors are
located in the base, which reduces the loaded mass. On the other hand, the rotation
stage presents the same problems as a serial arm: Either the motors are loaded, thus
increasing the mass, or the rotation stage is actuated from the base, which involves
complex transmissions.
Finally, the parallel mixed structures (with fewer sub-structures between the fixed
base and the robot gripper than the degrees of freedom, with each sub-structure carry-
ing several motors) provide the advantage of a relatively significant work space even
though the rotation field depends on the place where the mobile platform is located.
Moreover, the motors are close to the base and the elementary branches are identical.
Irrespective of the architecture used, the structure of the robot must use light and
rigid materials (carbon fibre, aluminium, etc.).
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8.3.3.2 Motorisation
The selection of motorisation is very important during designing of an actuator because
the quality of the force feedback directly depends on the performance of the robot
actuators. The selection criteria of the motors are:
Force capacity: A robot actuator must be able to generate a sizeable torque (with
respect to its mass), whether continuously or at peak. For safety reasons, it is
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