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- Shakedown analysis (6)
Coincident events in two scintillator crystals coupled to photomultipliers (PMT) are detected by processing just the digital data of the recorded pulses. For this purpose the signals from both PMTs are continuously sampled by free-running ADCs at a sampling rate of 40 MHz. For each sampled pulse the starting time is determined by processing the pulse data. Even a fairly simple interpolating algorithm results in a FWHM of about 2 ns.
We are developing an X-ray computed tomography (CT) system which will be combined with a high resolution animal PET system. This permits acquisition of both molecular and anatomical images in a single machine. In particular the CT will also be utilized for the quantification of the animal PET data by providing accurate data for attenuation correction. A first prototype has been built using a commercially available plane silicon diode detector. A cone-beam reconstruction provides the images using the Feldkamp algorithm. First measurements with this system have been performed on a mouse. It could be shown that the CT setup fulfils all demands for a high quality image of the skeleton of the mouse. It is also suited for soft tissue measurements. To improve contrast and resolution and to acquire the X-ray energy further development of the system, especially the use of semiconductor detectors and iterative reconstruction algorithms are planned.
Within the developments for the Crystal Clear small animal PET project (CLEARPET) a dual head PET system has been established. The basic principle is the early digitization of the detector pulses by free running ADCs. The determination of the γ-energy and also the coincidence detection is performed by data processing of the sampled pulses on the host computer. Therefore a time mark is attached to each pulse identifying the current cycle of the 40 MHz sampling clock. In order to refine the time resolution the pulse starting time is interpolated from the samples of the pulse rise. The detector heads consist of multichannel PMTs with a single LSO scintillator crystal coupled to each channel. For each PMT only one ADC is required. The position of an event is obtained separately from trigger signals generated for each single channel. An FPGA is utilized for pulse buffering, generation of the time mark and for the data transfer to the host via a fast I/O-interface.
Structural design analyses are conducted with the aim of verifying the exclusion of ratcheting. To this end it is important to make a clear distinction between the shakedown range and the ratcheting range. In cyclic plasticity more sophisticated hardening models have been suggested in order to model the strain evolution observed in ratcheting experiments. The hardening models used in shakedown analysis are comparatively simple. It is shown that shakedown analysis can make quite stable predictions of admissible load ranges despite the simplicity of the underlying hardening models. A linear and a nonlinear kinematic hardening model of two-surface plasticity are compared in material shakedown analysis. Both give identical or similar shakedown ranges. Structural shakedown analyses show that the loading may have a more pronounced effect than the hardening model.
Rezension zu: K. Zimmermann, Technische Mechanik – multimedial. Fachbuch Verlag Leipzig (2000)
(2002)
Thin film chemical sensors based on chalcogenide glasses for „electronic tongue“ applications
(2001)
Characterising an insect antenna as a receptor for a biosensor by means of impedance spectroscopy
(2001)
Pulses from a position-sensitive photomultiplier (PS-PMT) are recorded by free-running ADCs at a sampling rate of 40 MHz. A four-channel acquisition board has been developed which is equipped with four 12-bit ADCs connected to one field programmable gate array (FPGA). The FPGA manages data acquisition and the transfer to the host computer. It can also work as a digital trigger, so a separate hardware trigger can be omitted. The method of free-running sampling provides a maximum of information, besides the pulse charge and amplitude also pulse shape and starting time are contained in the sampled data. This information is crucial for many tasks such as distinguishing between different scintillator materials, determination of radiation type, pile-up recovery, coincidence detection or time-of-flight applications. The absence of an analog integrator allows very high count rates to be dealt with. Since this method is to be employed in positron emission tomography (PET), the position of an event is also important. The simultaneous readout of four channels allows localization by means of center-of-gravity weighting. First results from a test setup with LSO scintillators coupled to the PS-PMT are presented here
Weak Representation of the Cumulative Hazard Function under Semiparametric Random Censorship Models
(2001)
Fatigue analyses are conducted with the aim of verifying that thermal ratcheting is limited. To this end it is important to make a clear distintion between the shakedown range and the ratcheting range (continuing deformation). As part of an EU-supported research project, experiments were carried out using a 4-bar model. The experiment comprised a water-cooled internal tube, and three insulated heatable outer test bars. The system was subjected to alternating axial forces, superimposed with alternating temperatures at the outer bars. The test parameters were partly selected on the basis of previous shakedown analyses. During the test, temperatures and strains were measured as a function of time. The loads and the resulting stresses were confirmed on an ongoing basis during performance of the test, and after it. Different material models were applied for this incremental elasto-plastic analysis using the ANSYS program. The results of the simulation are used to verify the FEM-based shakedown analysis.
The load-carrying capacity or the safety against plastic limit states are the central questions in the design of structures and passive components in the apparatus engineering. A precise answer is most simply given by limit and shakedown analysis. These methods can be based on static and kinematic theorems for lower and upper bound analysis. Both may be formulated as optimization problems for finite element discretizations of structures. The problems of large-scale analysis and the extension towards realistic material modelling will be solved in a European research project. Limit and shakedown analyses are briefly demonstrated with illustrative examples.
An ISFET-based penicillin sensor with high sensitivity, low detection limit and long lifetime
(2001)
Novel concepts for flow-rate and flow-direction determination by means of pH-sensitive ISFETs
(2001)
Penicillin detection by means of field-effect based sensors: EnFET, capacitive EIS sensor or LAPS?
(2001)
Traglast- und Einspielanalysen sind vereinfachte doch exakte Verfahren der klassischen Plastizitätstheorie, die neben ausreichender Verformbarkeit keine einschränkenden Voraussetzungen beinhalten. Die Vereinfachungen betreffen die Beschaffung der Daten und Modelle für Details der Lastgeschichte und des Stoffverhaltens. Eine FEM-basierte Traglast- und Einspielanalyse für ideal plastisches Material wurde auf ein kinematisch verfestigendes Materialgesetz erweitert und in das Finite Element Programm PERMAS implementiert. In einem einfachen Zug-Torsionsexperiment wurde eine Hohlprobe mit konstanter Torsion und zyklischer Zugbelastung beansprucht, um die neue Implementierung zu verifizieren. Es konnte gezeigt werden, dass die Einspielanalyse gut mit den experimentellen Ergebnissen übereinstimmt. Bei Verfestigung lassen sich wesentlich größere Sicherheiten nachweisen. Dieses Potential bedarf weiterer experimenteller Absicherung. Parallel dazu ist die Eisnpieltheorie auf fortschrittliche Verfestigungsansätze zu erweitern.