Hans-Jürgen Roscher

Control of sheet-metal forming processes with piezoactuators in smart structures

The most important project in sheet metal forming is streamlining the material flow since each rejects increases production costs. Using the multipoint cushion device together with an elastic blankholder makes it possible to actively manipulate the material flow in the flange range. This allows major enhancements in the deformation ratio, especially with the novel high strength materials in car body production. State-of-the-art is multiple draw pins to initiate the force on selected points on the blankholder. Admittedly, the cushion plate does not allow optimum force allocation because it is situated between hydraulic pressure rollers and draw pins. Replacing selected draw pins with piezoactuators for generating high forces allows systematic control of the force progression at critical forming areas during sheet draw-in. The system, consisting of the piezostack actuator, dynamometer and components for force initiation, was built as a compact unit with low resilience with the intension of using the inherent sensory properties of the piezostack actuator to measure force. Applying this principle throughout allows a reduction of hydraulic components which eventually lead to a less expensive one- point cushion device. Initial finding have already been arrived at in the context of a research project at the Fraunhofer Institute for Machine Tools and Forming Technology in Chemnitz, Germany in cooperation with a partner from the automobile industry. A draw pin was replaced ad hoc with a highly durable piezoactuator integrated in a force control cycle. The force progression during the sheet draw-in could be accurately adjusted according to a predetermined master curve. The master curve was taken up in the unregulated process and represents the quality criteria of a formed useable part. The real-time MATLAB Simulink XPC- Target simulation tool was used to develop an adjustment strategy that connects the specific signals of the press control (such as the tappet path, the die cushion position and the die cushion force) with the reference force (i.e., the master curve) and the actual force of the piezoactuator.

Hybrid cutting of granite by use of ultrasonic assistance

Hybrid technologies offer an important approach to enhance existing limits of conventional cutting manufacturing processes. Superposition of the infeed with adapted ultrasonic vibrations enables reductions of machining forces. This results in diminished tool wear and longer tool life. Furthermore, an increase of removal rates can be achieved. Successful machining of recalcitrant metal-based materials by ultrasonic assisted systems creates a high potential to gain similar effects in machining of mineral-based materials. This will be studied in this article. The state of the art for the machining of stone uses geometrically undefined cutting edges. This paper focuses on the geometrically defined cutting of granite with additional ultrasonic assistance. Cutting tests at a test station with linear cutting motion are being performed. The ultrasonic frequency is maintained at 20 kHz. Different oscillation amplitudes are applied to influence process forces and the wear of the used carbide metal and polycrystalline diamond cutting segments (PCD). A method to observe the wear is developed by use of a stereomicroscope and a 3D measurement system. This will enable conclusions about the applicability of the geometry of the cutting segments and the process parameters. Due to the significant different wear rates of both cutting materials, the cutting force progression by using PCD tools shows completely different characteristics compared to the machining with carbide metal tools.

Adaptronic tools for superfinishing of cylinder bores

Today in the production of internal combustion engines it is possible to make pistons as well as cylinders, for all practical purposes, perfectly round. The negative consequences of the subsequent assembly processes and operation of the engine is that the cylinders and pistons are deformed, resulting in a loss of power and an increase in fuel consumption. This problem can be solved by using an adaptronic tool, which can machine the cylinder to a predetermined nonround geometry, which will deform to the required geometry during assembly and operation of the engine. The article describes the actuatory effect of the tool in conjunction with its measuring and controlling algorithms. The adaptronic tool consists out the basic tool body and three axially-staggered floating cutter groups, these cutter groups consist out of guides, actuators and honing stones. The selective expansion of the tool is realised by 3 piezoelectric multilayer-actuators deployed in a series - parallel arrangement. It is also possible to superimpose actuator expansion on the conventional expansion. A process matrix is created during the processing of the required and actual contour data in a technology module. This is then transferred over an interface to the machine controller where it is finally processed and the setting values for the piezoelectric actuators are derived, after which an amplifier generates the appropriate actuator voltages. A slip ring system on the driveshaft is used to transfer the electricity to the actuators in the machining head. The functioning of the adaptronic form-honing tool and process were demonstrated with numerous experiments. The tool provides the required degrees of freedom to generate a contour that correspond to the inverse compound contour of assembled and operational engines.

New modular piezo actuator with built-in stress-strain transformation

As known, the electrical induced strain of conventional piezoceramic materials is limited by 0.12 % (2 kV/mm), which often requires strain transformation designs, like levers, in order to meet application needs. High fabrication accuracy and low tolerances are crucial points in mechanical manufacturing causing high device costs. Therefore, we developed a piezoelectric composite actuator with inherent stress - strain transformation. Basically, piezoceramic sheets are laminated with spring steel of a certain curvature, which can be realised by a comparatively simple fabrication technique. The working diagram of these composite bow actuators showed a high level of performance adaptable to a wide range of applications. The authors established the value chain covering the piezoceramic formulation, the processing technology and the design in view of optimum system performance. The paper presents an overview of the design principles, simulation and various aspect of fabrication technology including lamination, sintering and polarization. The new devices are useable in different sectors, for example in automotive industry as solid state transducer or as the active part in injectors. Moreover, the composite bow actuators may find application in microsystems technology, micro optics and micro fluidics as well as vibration dampers. The composite bow actuators can be used as single component transducer, as well as multi-bow actuator in series or parallel combination on demand.

Vibration damping with active carbon fiber structures

This paper presents a mechatronic strategy for active reduction of vibrations on machine tool struts or car shafts. The active structure is built from a carbon fiber composite with embedded piezofiber actuators that are composed of piezopatches based on the Macro Fiber Composite (MFC) technology, licensed by NASA and produced by Smart Material GmbH in Dresden, Germany. The structure of these actuators allows separate or selectively combined bending and torsion, meaning that both bending and torsion vibrations can be actively absorbed. Initial simulation work was done with a finite element model (ANSYS). This paper describes how state space models are generated out of a structure based on the finite element model and how controller codes are integrated into finite element models for transient analysis and the model-based control design. Finally, it showcases initial experimental findings and provides an outlook for damping multi-mode resonances with a parallel combination of resonant controllers.