Development of a force amplitude- and location-sensing device designed to improve the ligament balancing procedure in TKA

A force-sensing device to intraoperatively measure knee joint forces and moments to improve the ligament balancing procedure during total knee arthroplasty has been developed. It consists of two sensitive plates, one for each condyle, a tibial base plate and a set of wedges to vary the tibio-femoral gap. Each sensitive plate contains three deformable bridges instrumented with thick-film piezoresistive sensors, which allow the measurement of the amplitude and location of the tibio-femoral contact forces. Their net varus-valgus moment is then computed and regarded as the parameter characterizing the ligamentous balance. An accuracy study demonstrated that the maximum force amplitude and location error are 1.4 N and 0.6 mm respectively, whereas the mean absolute errors are 0.5 N and 0.2 mm. A control experiment with a plastic knee joint model equipped with spring-ligaments attested the device’s suitability for the purpose of ligament balancing. Furthermore, an in-situ trial on a cadaver demonstrated the appropriateness of the measurement scale as well as the consistency between the acquired data and the surgeon’s perception. The obtained results indicate thereby that the developed force-sensing device has a strong potential to offer useful quantitative information and effective assistance during the ligament balancing procedure in total knee arthroplasty.

Approaches on the Thermal Management and Parameter Estimation of Solid Oxide Fuel Cells and their Systems

Leonidas Tsikonis

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices that convert chemical energy from fuel directly into electrical and thermal energy. They present high electrical efficiency and because of this feature they are considered to be an important power source alternative. However, due to their high temperature of operation, SOFCs are subject to degradation, that is, their ability to produce electricity deteriorates in time. The reasons and the conditions that contribute to degradation are multiple. Among the studied factors, operating temperature is an important one. It has been found that a stable and controllable thermal environment can mitigate this undesirable phenomenon. Therefore, the pursuit of appropriate thermal conditions during their operation is of paramount importance for their lifetime and consequently for their commercialisation. Studying and obtaining such conditions is also known as thermal management. Another important point regarding the study of SOFCs is the development and use of adequate mathematical models that the engineer and generally the researcher will use as tools to analyse their behaviour and will determine conditions for safe and efficient operation. For practical applications, these models must be validated against experimental data. The result of such validation is the determination, or calibration, of the model parameters so that the output of the model fits measurements taken in the laboratory to the best possible extent. This process is called parameter estimation or system identification. This thesis tackles both aspects. In its first part it works on the thermal management of a specific product, an SOFC autonomous unit fed with methane, producing electrical power and liberating hot off-gases that can be used as a source of thermal power. A dynamic model of the physical system is developed and validated against experimental data. During this process, it was found that measurement of gas temperatures using thermocouples may be severely biased due to radiation effects of surrounding solids to the thermocouples. In order to overcome this hurdle, the phenomena that take place around the thermocouple were incorporated into the system's mathematical model. Such a systematic error may have important consequences on the thermal management and generally on the control of the SOFC system. Indeed, an investigation effectuated in this thesis revealed how incorrect gas temperature measurements affect the system's control to such an extent that, depending on the conditions, may lead to system failure. The validation of the system model proved to be particularly challenging. This fact incited the author to investigate the factors that contribute to reliable parameter estimations. This is the subject of the second part of this thesis. A first study was performed on model-based Design of Experiments (DoE) for SOFC button cells, i.e. on how measurements on small cells may be optimised. To our knowledge the study treats for the first time the issue of repetitive measurements and their impact on the quality of parameter estimation systematically. It distinguishes the notion of measurements, which can be repetitive, from that of measurement points (or design points), which are defined once for an experiment. Introducing a new type of graph that provide information on quality criteria as functions of the number of measurement points for constant number of measurements, it stresses the importance of repetitions of measurements during experiments, which up to now has been neglected. A mathematical formula is also given that helps the experimenter define the necessary number of repetitions for a specific degree of precision of the information matrix. The theoretical findings on Design of Experiments are validated experimentally with measurements on button cells. The thesis introduces a novel method with which repetitive parameter estimations are obtained using data from polarisation curves. This method allows the calculation of histograms for the model parameters approximating this way their stochastic behaviour. Standard deviations, covariance and correlation matrices are calculated directly from the available data, avoiding the approximate calculation based on sensitivity (Jacobian) matrices. The experimental results showed again the importance of repetitions on the quality of parameter estimation. A rule-of-thumb is introduced suggesting that the number of repetitions of measurements needs to be at least equal to the number of calculated parameters plus three in order to avoid high correlations. However, repetitions do not influence the values of parameters or the fit to the experimental data, but their precision. These values are influenced by the number of measurement points. Another point of importance shown in this thesis is that there is a counterbalance between the parameters' variances and covariances and their correlations, that is, if the one improves, the other deteriorates. This is demonstrated to be in accordance with the Cramèr-Rao theorem in statistics. A consequence of this is the conclusion that parameter estimation cannot be obtained to an arbitrarily high precision. Another consequence is that assessment of system identification based on only one of the widely known criteria may not be adequate, a result that concurs with the findings of other authors in the field of DoE. Finally the available results from the afore-mentioned method revealed a potential relation between correlations of parameters and the non-uniformity of their histograms. Among the investigated examples, a case of fast degradation exhibits how the introduced parameter estimation method may be used as a diagnostics tool. However such a use requires the employment of models that describe the phenomena adequately, especially after degradation. The thesis ends with the calculation of the histograms of the parameters of the SOFC unit's heat exchanger providing a stochastic perspective of parameter estimation at an SOFC system level.

EPFL2011

High sensitivity field asymmetric ion mobility spectrometer

Mario Andres Chavarria Varon

Field Asymmetric IonMobility Spectrometry (FAIMS) is a very promising technique for chemical detection. FAIMS systems are portable, highly sensitive, relatively inexpensive and can work at atmospheric conditions. These attributes make it a very desirable technique to replace current bulky and expensive detection systems, such as Mass Spectrometry devices, especially for on-site experiments. Therefore, this work focuses on improving the FAIMS limits of detection (LoD) in order to expand the application fields for FAIMS technology. With this aim, the different factors affecting the FAIMS sensitivity and resolution were analyzed by theoretical and experimental studies. An ion mobility simulation software was developed based on the physical models for FAIMS proposed in literature. This software is able to simulate the ion trajectories inside the ion filter and the expected ion spectra, making it an important tool for theoretically determining the effect of different parameters in FAIMS measurements. It also helps the user to predict and understand the experimental results. The possibility of improving the fabrication process was evaluated by implementing two different techniques: PCB manufacturing and screen printing. The PCB plates were fabricated on FR4 substrates with copper electrodes while the screen printed plates used alumina substrates and silver-palladium electrodes. The prototypes developed with PCB techniques showed a very similar behavior to the standard alumina systems. Therefore, the PCB fabrication approach is a good option to reduce the fabrication complexity and costs of the FAIMS devices. Two atmospheric pressure ionization sources, corona discharge and UV, were tested for possible use in the FAIMS setup. Both sources showed similar ionization efficiency levels. Between the two sources, the UV is a simpler system that requires very low optimization and has very stable ionization rates. Consequently, it was selected as the ionization method for this work. All the required electronics for the ions filtering and detection were also developed. The designed separation voltage circuit, based on the power nMOS inverter configuration, is able to produce rectangular waveforms of up to 1000 Vpp in the MHz range. The compensation voltage (CV) circuit can scan the ion spectra by generating a voltage ramp from -20 to 20 V. The fabricated low current amplifier has a gain of 10^11 with a noise of about 4 fA/Hz^0.5 limited by the thermal noise of the feedback resistor. The experimental parameters can be controlled and monitored by a custom user interface software developed in LabVIEW®. This software also digitalizes, displays and save the measured data. Finally, several measurements were performed to determine the influence of the different experimental parameters in the FAIMS sensitivity and resolution. In order to have a better understanding of the phenomena occurring within the FAIMS, the parameters¿ effects on each of the FAIMS section were studied separately. Also, for the first time, the effects of a synchronous modulation technique (lock-in) in the FAIMS ion current detection were analyzed. Based on the results of these studies, the LoD of the FAIMS system was significantly improved. The extrapolated acetone LoD levels, from the experimental signal amplitude and base line noise, of the developed device are 900ppt and 40ppt at carrier gas flows of 3.05 and 9.5 l/min respectively.

EPFL2016

Quantum optomechanics at room temperature

Hendrik Schütz

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017