Stephan Algermissen

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.

Antenna element design for a conformal antenna array demonstrator

Conformal and structurally integrated antennas will play an increasing role in future airborne applications in various fields of interest such as Communication, Navigation, Electronic Warfare or RADAR. However, these antennas are subject to static deformations and vibrations caused by aerodynamic loads which significantly diminish the antennas performance. These effects are investigated in the scope of the interdisciplinary NATO Research Task Group SET-131. Within this group a demonstrator for a conformal antenna with active vibration compensation was initiated. In this paper the single element design for the planned demonstrator is discussed. 12

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.

Experiments on Active Control of Counter-Rotating Open Rotor Interior Noise

The efficiency of future aircraft has to be increased because of the CO2 restrictions layed down by the European Union. Two key technologies to reach this ambitious goal are a consequent light-weight design of future aircraft and new engine concepts like the Counter-Rotating open rotors (CROR). However, the combination of lightweight materials like carbon-fibre-reinforced plastics (CFRP) with CROR is acoustically demanding because of the very high sound pressures emitted by this type of engine and the poor transmission loss of CFRP structures in the lower frequency range. Therefore, this work conducts a preliminary study to improve the transmission loss of a CFRP panel excited by a synthesised CROR pressure field in lower frequency range. As a first step, a typical aircraft fuselage panel mounted in a sound transmission loss facility is equipped with actuators and sensors to implement multiple-input multiple-output (MIMO) feedforward control of flexural vibration. The CFRP panel is excited via a CROR pressure field synthesised by a 112-channel loud speaker array. The active vibration control (AVC) system is realised by accelerometers and inertial exciters. A considerable vibration reduction of the flexural vibration on the accelerometers is achieved. The local attenuation around the accelerometers leads to a new controlled vibration pattern that radiates sound in a different way than the uncontrolled one. The difficulties in reducing the radiated sound power through the AVC system are due to low observability, the “pinning” effect, and the restructured vibration patterns. All of these effects are studied in detail through surface vibration scans and sound intensity measurements. Additionally, the radiation resistance matrix is used to analyse the controlled vibration patterns.

Closed-loop subspace identification for vibration control of structure integrated antenna arrays

Structure integrated or conformal antenna arrays consist of several spatially distributed patches. Integrated in aircraft fuselages the array is excited by inherent engine and turbulent boundary layer induced vibrations. These disturbances lead to distortions of single patch orientation and thus to unknown phase differences in the patch signals. Smart-structures technology is the key to overcome this problem. Structure integrated sensors and actuators measure structural vibrations and induce annihilating vibrations which are calculated by a controller. This paper presents a closed-loop sub-space identification combined with a self-tuning controller approach. The objective is to identify the controlled plant operating in closed-loop in flight. Thereby, changes in the vibration behavior of the supporting structure due to temperature gradients e.g. are monitored. A subsequent re-synthesis of the controller by self-tuning guarantees optimal performance during the entire flight. For identification a two stage LQ decomposition is combined with a sequential LQ decomposition in order to handle large data sets. In experiments on an active plate demonstrator a parameter study leads to good identification results without degrading controller performance.

Experimental Synthesis of Sound Pressure Fields for Active Structural Acoustic Control Testing

For next generation aircraft, contra rotating open rotor propulsion systems are currently discussed. Their economic advantages are in conflict with their high noise emission, which concentrates in distinct frequency bands. To bring contra rotating open rotor engines into operation at commercial aircraft and to maintain the passenger comfort level, active systems for noise reduction are considered. In this article, a contra rotating open rotor noise simulator for the future testing of active systems is presented. The simulator consists of a 14 × 8 loudspeaker array, which is placed close to a test fuselage. Driving each loudspeaker by a particular signal, complex sound pressure fields can be synthesized on the fuselage. The algorithm presented here calculates loudspeaker signals to synthesize the target sound pressure spectra on the fuselage. These target spectra were derived from numerical contra rotating open rotor engine simulations, coupled with a Ffowcs Williams–Hawkings solver to propagate the calculated sound pressure field toward virtual surface microphone positions on the test fuselage. Requirement for such synthesis is the knowledge of the transfer paths from all loudspeakers to all surface microphones. In experiments, these paths are measured, and the loudspeaker signals are synthesized by the algorithm. Finally, measurements with the loudspeaker array and a microphone array are shown which prove the concept.

Research on vibration control and structure integration of antennas in NATO/RTO/SET-131

Integration of antennas into ground, air or water vehicles is a critical task due to the increasing number of RF and microwave systems used for communication, RADAR or ESM. Structural aspects such as mechanical or thermal stability, aerodynamics or outer appearance are of great importance. Methods and technology for the integration of single antennas or antenna arrays into structures of different type (e.g. glass or carbon fibre composite materials) are currently being developed at different companies and research institutes and are scope of research work in the NATO task group SET-131. The performance of a system should not be affected by the influence of the carrier platform or by electromagnetic interaction between different systems. Another important aspect are variations of the shape of a vehicle or its parts which may also degrade the overall system performance. There are different reasons for these changes, e.g. vehicle motion, moving parts such as rudders or turbines, or the impact of a projectile or collision.