An adaptronic strut for machine tools with parallel kinematics for compensation of the influence of geometric errors is introduced. Implemented within the strut is a piezoelectric sensor-actuator unit separated in function. In the first part of this contribution, the functional principle of the strut is presented. For use of one piezoelectric transducer as both, sensor and actuator as so-called self-sensing actuator, the acquisition of the sensing signal while actuating simultaneously using electrical bridge circuits as well as filter properties are examined. In the second part the control concept developed for the adaptronic strut is presented. A co-simulation model of the strut for simulating the controlled multi-body behavior of the strut is set-up. The control design for the strut as a stand-alone system is tested under various external loads. Finally, the strut is implemented into a model of the complete machine tool and the influence of the controlled strut onto the behavior of the machine tool is examined.

In machine tools of parallel structure with two or three translatory degrees of freedom the rotatory degree of freedom is kinematically locked. Yet due to geometric faults, for example assembly errors or different geometries due to production tolerances, such machine tools exhibit an additional rotational behavior. Stresses within the structure occur leading to deflections of the tool center point, and thus, reducing the quality of the workpiece. For compensating these errors an adaptronic strut which can be implemented within such a machine tool has been developed. The strut comprises a piezoceramic sensor-actuator unit for controlled correction of those static and quasi-static deflections. Piezoceramic elements were chosen due to their high positioning accuracy and the small installation space required. The functional principle of a scale with a vibrating string is used for measuring the external load. A control concept for the adaptronic strut is introduced. Finally, after implementing the strut in a model of an exemplary machine tool the compensation of influences due to specified geometric errors is examined.

AbstractPhase–locked loops are widely used within communication technology for synchronizing signals by tracking their phases. Their functional principle can be applied for maintaining the resonant excitation of a beam whose eigenfrequency changes over time and, thus, representing an alternative excitation method for the adaptronic strut for machine tools shown in [1]. In this contribution an experimental set–up for resonant excitation of a composite beam is introduced. A particle is mounted on the beam, its position and mass are are adjustable to modify the eigenfrequency of the beam. The simulation results of the analytical examination of the test–rig are compared with the experimental results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In Kombination mit dem Funktionsprinzip einer Schwingsaitenwaage konnen piezoelektrische Wandler im Bereich kleiner Frequenzen gleichzeitig sensorisch und aktorisch betrieben werden. In der vorliegenden Arbeit werden zunachst die hierfur erforderlichen Grundlagen untersucht. Darauf aufbauend werden anhand einer adaptronischen Strebe fur Werkzeugmaschinen eine mogliche Umsetzung des Konzepts und ein Einsatzbeispiel vorgestellt. Umfang: IV, 179 S. Preis: €27.90 | £26.00 | $49.00

Due to leakage resistances measuring slowly changing loads using piezoelectric sensors is rather difficult while measuring static loads is impossible. Using the functional principle of a scale with a vibrating string a work-around to this problem is found. By evaluating the relation between the load onto the string and the eigenfrequencies of bending vibration of the string the external load onto the string can be determined. Instead of a string of circular cross-sectional area a thin beam can be used which leads to a preferential vibration direction in direction of least bending resistance. To achieve large signal amplitudes the beam should be excited and vibrating close to its eigenfrequency. To remain in a state of resonance at all times the frequency of excitation must follow frequency alterations due to external influences. Such a frequency locking can be achieved by applying phase-locked loops (PILL) which are widely used within communication technology. In this contribution an experimental set-up for resonant excitation of a composite beam is presented. A small mass is mounted onto the beam. Its position is adjustable to modify the eigenfrequency of the beam. Using piezoelectric macro-fiber composites for sensing and actuating as well as an integrated PLL circuit the resonant excitation of the beam can easily be achieved even for changing eigenfrequencies.

Allgemein zeigen Werkzeugmaschinen Verlagerungen im statisch/quasistatischen und im dynamischen Lastbereich auf. Ziel des Projekts ist die adaptronische Kompensation dieser Verlagerungen mit paralleler Bewältigung von Sensorund Aktorfunktion. In den ersten zwei Projektphasen wurde dazu bis heute auf den statisch/quasistatischen Bereich fokussiert, um in einer weiteren dritten Phase die Ausweitung des Konzepts auf den dynamischen Bereich anzugehen. Unter Ausnutzung des Prinzips einer Schwingsaitenwaage werden statische Verformungszustände zur Messung in ein dynamisches Eingangssignal für piezoelektrische Wandler gewandelt. Bild 1 verdeutlicht hierzu den prinzipiellen Aufbau des Systems. Dr.-Ing. Christian Munzinger, Dipl.-Ing. Stefan Herder, Dipl.-Ing. Martin Weis Institut für Produktionstechnik (wbk) Universität Karlsruhe (TH) Kaiserstr. 12, D-76128 Karlsruhe Tel. +49 (0)721 / 608-2449, Fax +49 721 / 699 503 E-Mail: weis@wbk.uka.de Internet: www.wbk-ka.de

An adaptronic strut, developed for compensation of the influence of geometric faults in machine tools with parallel kinematic structure, is examined. A simple oscillator model of the strut is built. First, the equations of motions for this simplified model are derived analytically. These information are used for designing a single variable state control based on the principles of the optimal least quadratic regulator (LQR). Afterwards, the controller concept is extended applying an additional PI-controller. Secondly, the strut is modeled using the commercial multi-body system simulation software Msc.Adams. The required system state which is not explicitly given within Msc.Adams primarily has to be estimated. For this task a Luenberger observer is implemented. A similar single variable state control is developed and both designs are compared among themselves when the adaptronic strut is examined under external loads. Finally, the strut is implemented into the model of the complete machine tool and its influence on the behavior of the machine tool is treated.

AbstractBei Werkzeugmaschinen mit Parallelkinematiken mit zwei bzw. drei translatorischen Freiheitsgraden sind die rotatorischen Freiheitsgrade kinematisch gesperrt. Aufgrund von Fertigungs‐ und Montageungenauigkeiten kommt es zu Verspannungen in der Kinematikstruktur. Daraus resultieren Verlagerungen des Werkzeug‐ bzw. des Werkst ückhalters und die Fertigungsgenauigkeit der Maschine und damit die Werkstückqualität werden reduziert. Der translatorische Anteil dieser Maschinenfehler kann durch die genannte Kinematikklasse korrigiert werden, der rotatorische Anteil nur sehr begrenzt.Durch den Einsatz kraftgeregelter Piezoaktoren in den Streben sollen die rotatorischen Maschinenfehler im laufenden Betrieb korrigiert werden. Ein erster Schritt hierzu ist die Modellbildung einer beliebigen Strebe, in die ein Modell eines Piezoaktors integriert ist. Die Strebe wird am Kopfende geführt und soll am Fußende eine Kraft erfahren. Durch Analyse und Simulation können die Einflüsse verschiedener Maschinen‐ und Prozessparameter untersucht werden. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Geometric faults in parts of machine tools with parallel kinematics lead to stresses in the structure and deflections of the tool center point, reducing the quality of the workpiece. Improving the design of machine tools can reduce these influences. In this paper an approach to compensate the influence of geometric faults in parallel kinematics based on the design of an adaptronic strut is introduced. The strut is divided in two halves and two piezoelectric transducers are implemented in between them, used as sensor and actuator respectively. A preliminary design of the adaptronic strut is presented. The problems of measuring low-frequency signals using piezoelectric transducers are considered in the design.Finally, a primary analytical model of the dynamical behavior of the adaptronic compensation unit is presented. The strut and its connection to the surroundings are regarded as a flexible multibody system, the equations of motion are derived using linear graph theory. Some simulation results are presented.