Ralf Keimer

Robust Gain Scheduling for Smart-Structures in Parallel Robots

Smart-structures offer the potential to increase the productivity of parallel robots by reducing disturbing vibrations caused by high dynamic loads. In parallel robots the vibration behavior of the structure is position dependent. A single robust controller is not able to gain satisfying control performance within the entire workspace. Hence, vibration behavior is linearized at several operating points and robust controllers are designed. Controllers can be smoothly switched by gainscheduling. A stability proof for fast varying scheduling parameters based on the Small-Gain Theorem is developed. Experimental data from TRIGLIDE, a four degree of freedom (DOF) parallel robot of the Collaborative Research Center 562, validate the presented concepts.

Smart-Structures Technology for Parallel Robots

Industrial robots play an important role in automation technology. A further increase of productivity is desired, especially in the field of handling and assembly, the domain of industrial robots. Parallel robots demonstrated their potential in applications with needs for high-dynamic trajectories in the recent years. Within the scope of the Collaborative Research Center (SFB 562)—‘Robotic Systems for Handling and Assembly’ the German Aerospace Center (DLR) and the Technical University of Braunschweig investigate smart-structures technology for parallel robots. In this paper the results of the main topics new active components, Finite-Element based elastic position dependent modeling and vibration control are summarized. The latest parallel robot called is equipped with new active rods. The design as well as the dimensioning of the rod with surface mounted piezo patch actuators is described. For trajectory based robot control, rigid body models are required. In parallel robots with vibration reduction a coupled approach is necessary in which elastic and rigid body equations are combined. The derivation of the equations for parallel robot is presented. Finally, the implemented vibration control is explained. In a position-dependent approach several robust controllers are switched to gain optimal control performance. A stability proof for the switching process is derived.

High-speed parallel robots with integrated vibration suppression for handling and assembly

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.

Design and Implementation of Adaptronic Robot Components

Parallel robots demonstrated their potential in applications with the needs for high dynamic trajectories in recent years [1]. In contrast to serial robots they meet the demands for higher accelerations at constant precision due to their fixed drives and large structural stiffness. On the other hand higher accelerations increase the risk of severe vibration of the structural parts. Therefore smart structures technology is introduced in parallel robotics for a further enhancement of the features of parallel robots. In consequence, fundamental investigations regarding adaptive mechanical components and adaptive systems are carried out in order to extend the application range of parallel robots. The development of specific active components being able to reduce the vibrations are investigated within the field of smart structures for parallel robots and are elucidated in the article. Manufacturing aspects and design principles are addressed.

Adaptronic Revolute Joints for Parallel Robots Based on Simultaneous Quasi-Statical Axial and Radial Clearance Adjustment

Robots based on parallel kinematics feature low moved masses, allowing for better dynamic performance compared to serial mechanisms. Otherwise, the known drawbacks, like occurrence of singularities or bad radio of work space to installation area, hinder their fully industrial establishment. In order to overcome some of these drawbacks, development of specific and optimized robot components, like rods or joints, becomes necessary. Development of joints for parallel robots is determined by numerous contradictory requirements which cause different goal conflicts. In this work, possibilities for dissolving of such goal conflicts by means of adaptronic joints (joints with integrated piezo-actuators) are discussed. To deal with these complex issues this paper focuses on three major areas: firstly, conventional joint concepts, including their main flaws; secondly, new, adaptronic joint concepts based on quasi-statical clearance adjustment with two laboratory prototypes and their improvements over the old solutions; thirdly and finally, some of possible consequences of the new joint concepts for the overall performance of parallel robots. By drawing on experimental results derived from laboratory tests, it is possible to show how implementation of the developed joint prototypes could influence friction characteristic of the whole robot system.Copyright

Smart structures technologies for parallel kinematics in handling and assembly

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 assembly. 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 structures 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.

Automated Synthesis of Robust Controllers for Smart-Structure Applications in Parallel Robots

Industrial robots occupy a central position in automation technology. Especially in the field of handling and assembly, the main domain of industrial robots, an increase of productivity is desired. Serial robots, where drives and links are located in a single chain, are widely used in those applications. A further enhancement of their productivity by shortening of cycle times is limited. In the past years, parallel robots demonstrated their potential in applications with needs for high-dynamic trajectories. Due to their fixed drives and large structural stiffness, they meet the demands for higher accelerations at constant precision, in contrast to serial robots. The direct consequence of large accelerations and decelerations during pick-and-place operations are high inertial forces that lead to vibrations and accordingly large decay times of the effector. To extend the possibilities of parallel robots under this operational conditions and in relation to cycle times and precision, the vibrations have to be suppressed effectively. A key technology for the control of such unwanted effects is smart-structures technology. Smart-structures technology uses structure integrated sensors and actuators to observe and control the vibrations of structures or parts of it. A general problem in smart-structures technologies is the sensitivity of the structures vibration behavior to structural changes. To guarantee the stability of the control loop beyond these changes, a redesign of the controller is necessary. To address this in the majority of cases unattended problem, a new and rapid procedure for a controller development chain including system identification is presented. This paper shows that an automated synthesis of a Robust Controller for a smart-structure system in a complex industrial robot with 4 DOF can be realized and verified. The algorithms are implemented on a real system and proven with measurements.

Parallel Robot Calibration Utilizing Adaptronic Joints

Model based geometric calibration is well known to be an efficient way to enhance absolute accuracy of robotic systems. Generally its application requires redundant measurements, which are achieved by external metrology equipment in most traditional calibration techniques. However, these methods are usually time-consuming, expensive and inconvenient. Thus, so-called self-calibration methods have achieved attention from researchers, which either use internal sensors or rely on mechanical constraints instead. In this paper a new self-calibration technique is presented for parallel robots which is motivated by the idea of constrained calibration. The new approach utilizes a special machine component called the adaptronic swivel joint in order to achieve the required redundant information. Compared to similar approaches it offers several advantages. The new calibration scheme is described and verified in simulation studies using a R RRRR -structure as an example.Copyright

Design and manufacturing of morphing fan blades for experimental investigations in a cascaded wind tunnel

Within this paper shape variable compressor blades for jet engines using piezoelectric composite actuators attached to the blade’s suction and pressure sides are investigated. By applying a voltage to these actuators it is possible to increase and to decrease the blade stagger angle by changing the camber angle. This study is about the engineering process and about the rst results of a cascaded wind tunnel experiment.

Design and Test of Adaptronic Joints With Quasi-Statical Clearance Adjustment in a Parallel Robot

Robots based on parallel kinematic structures feature low moved masses, allowing for better dynamic performance compared to serial mechanisms, eventually resulting in shorter cycle-times in handling and assembly. A side effect of this high dynamics is a higher level of vibration which has to be addressed in order to reach the full potential for short cycle-times. Some active measures for vibration-suppression were already presented previously. Another problem of parallel robots is a small ratio between working-space to installation-space. In order to overcome the problem (e.g. by changing configurations) specific components (e.g. joints) are needed. In this paper the use of an adaptronic joint with quasi-statical clearance adjustment integrated in the five-bar parallel structure is presented, addressing both vibration-suppression and configuration changes. Dimensioning of the adaptronic joint covers two aspects. Firstly increasing of friction up to blocking the joint in order to change configuration of the five-bar-structure. Secondly friction variation is used to optimize vibration damping and thus indirectly shorten cycle-times. The design and the test-setup for use of adaptronic joints in a parallel robot is presented.Copyright