In the past years parallel robots demonstrated their capability in applications with high-dynamic trajectories.
Smart-structures offer the potential to further increase the productivity of parallel robots by reducing disturbing
vibrations caused by high dynamic loads effectively. To investigate parallel robots and their applications,
including suitable control concepts for smart-structures, the Collaborative Research Center 562 was founded by
the German Research Council (DFG).
The latest prototype within this research center is called Triglide. It is a four degree of freedom (DOF)
robot with three translational and one rotational DOF. It realizes an acceleration of 10 g* at the effector. In the
structure of the robot six active rods and a tri-axial accelerometer are integrated to control effector vibrations
in three translational DOF. The main challenge of this control application is the position dependent vibration
behavior. A single robust controller is not able to gain satisfying performance within the entire workspace.
Therefore a strategy for describing the vibration behavior by linearization at several operating points is developed.
Behavior in-between is approximated by a linear approach. On a trajectory robust controllers in all
operating points are smoothly switched by robust gain-scheduling. The scheduling parameters are fast varying
and though a suitable stability proof is defined, based on Small-Gain approach. Several transformations enhance
the results from Small-Gain Theorem and reduce the usual conservatism. Experimental data is used to show the
Parallel kinematics offer a high potential for increasing performance of machines for handling and assembly. Due
to greater stiffness and reduced moving masses compared to typical serial kinematics, higher accelerations and
thus lower cycle times can be achieved, which is an essential benchmark for high performance in handling and
However, there are some challenges left to be able to fully exploit the potential of such machines. Some of
these challenges are inherent to parallel kinematics, like a low ratio between work and installation space or a
considerably changing structural elasticity as a function of the position in work space. Other difficulties arise
from high accelerations, which lead to high dynamic loads inducing significant vibrations.
While it is essential to cope with the challenges of parallel kinematics in the design-process, smart structures
technologies lend themselves as means to face some of these challenges. In this paper a 4-degree of freedom
parallel mechanism based on a triglide structure is presented. This machine was designed in a way to overcome the
problem of low ratio between work and installation space, by allowing for a change of the structure's configuration
with the purpose of increasing the work space. Furthermore, an active vibration suppression was designed and
incorporated using rods with embedded piezoceramic actuators. The design of these smart structural parts is
discussed and experimental results regarding the vibration suppression are shown.
Adaptive joints are another smart structures technology, which can be used to increase the performance of
parallel kinematics. The adaptiveness of such joints is reflected in their ability to change their friction attributes,
whereas they can be used on one hand to suppress vibrations and on the other hand to change the degrees of
freedom (DOF). The vibration suppression is achieved by increasing structural damping at the end of a trajectory
and by maintaining low friction conditions otherwise. The additional feature to alter the DOF is realized by
increasing friction to the point where clamping happens. This can be used to support the change in the machines
configuration of parallel kinematics. Two kinds of adaptive joints are presented, both utilizing piezoceramic
actuators. The first kind features an adjustable clearance of the slide bearing that provides low friction for high
clearance conditions and great friction for reduced clearance. The second kind offers the possibility to reduce
the friction by moving the rubbing surfaces dynamically. For both joints experimental results are shown.
The paper closes with an outlook on ongoing research in the field of parallel robots for handling and assembly
with an emphasis on smart structures technologies.
Automation of handling and assembly, which are complex technological processes, requires qualified solutions. The longterm development goals are decreasing cycle-times and increasing quality of processing. These goals can be achieved by means of innovative concepts based on parallel kinematics which enable higher velocity and acceleration while maintaining at least the same accuracy as compared to conventional systems.
Principally, parallel kinematics are better suited for high accelerations than serial structures because the drive units can be mounted on the frame without the need to move their high masses. Additionally, parallel structures are stiffer than their serial counterparts. Two key features of the innovative concepts introduced in the paper are lightweight structural components which allow to reach even higher accelerations and integrated smart actuators and sensors to control the vibrations induced by the high accelerations.
This paper discusses modelling of parallel kinematics, control-strategies for the vibration suppression, and design-criteria for active rods. These active rods have built-in piezoceramic stacks serving as both sensors and actuators that provide the means to supresss the vibrations. A two-degree-of-freedom parallel structure with active rods is used as test-case and experimental results confirming the potential of smart parallel kinematics are shown. An outlook to the ongoing research in the field of parallel robots is given.