Interchangeable parts | EPFL Graph Search

Interchangeable parts are parts (components) that are identical for practical purposes. They are made to specifications that ensure that they are so nearly identical that they will fit into any assembly of the same type. One such part can freely replace another, without any custom fitting, such as . This interchangeability allows easy assembly of new devices, and easier repair of existing devices, while minimizing both the time and skill required of the person doing the assembly or repair. The concept of interchangeability was crucial to the introduction of the assembly line at the beginning of the 20th century, and has become an important element of some modern manufacturing but is missing from other important industries. Interchangeability of parts was achieved by combining a number of innovations and improvements in machining operations and the invention of several machine tools, such as the slide rest lathe, screw-cutting lathe, turret lathe, milling machine and metal planer. A

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

The modelling of natural gas combustion with detailed as well as reduced kinetic mechanisms has been subject to many publications already. Most of the mechanisms are validated at higher temperatures and ambient pressures. Only recently - in parallel to a gain of interest in the homogeneous charge compression ignition (HCCI) engine concept - there are more efforts trying to reproduce the low temperature chemistry, and in particular autoignition of natural gas under engine-like conditions. This work is part of a project on stationary natural gas engines equipped with unscavanged prechambers. It has been demonstrated that it is possible to reduce the emissions below the Swiss limits both with natural gas and biogas with the spark ignition prechamber, while keeping high efficiencies of around 38 %. We are now aiming at converting the prechamber from spark ignition to autoignition leading to longer maintenance intervals in the case of stationary natural gas engines and to a probable cleaner combustion - HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front - compared to prechamber spark ignition. In order to be able to model the fluid dynamics and the chemistry inside the cylinder, a good chemical model is one of the most important aspects. To obtain an appropriate mechanism - by validating and/or optimising existing mechanisms - experiments have been conducted in the rapid compression machine facility at the University of Lille with different mixtures of methane, ethane and propane. The relative air-to-fuel ratio λ varied from 1 to 1.6, the temperature and pressure at the end of compression varied from 850 to 925 K and 13 to 21 bar respectively. The compression ratio was constantly kept at 9.2 for all experiments and the final temperature and pressure were defined by varying the charge and the composition of the inert gases (Ar/N2). The compression time was between 50 and 60 ms for all experiments. For modelling the ignition delay a zero-dimensional model assuming homogeneity was used, based on the measured pressure and calculated core temperature at the end of compression of each experiment. The chemical solver package used for the simulations is CHEMKIN. Different mechanisms (PRF, GRI3.0, Konnov, …) have been tested. Most of the mechanisms are mainly validated at higher temperature ranges and therefore their applicability to ignition delay prediction in our conditions is questionable. Several attempts to adjust the GRI mechanism (version 1.2) to the lower temperature regime exist, whereof we chose the UBCMech and Petersen mechanism (GRI+RAMEC) to test on our simulations. Both mechanisms initially were developed based on pure methane mixtures while the UBCMech recently was extended to model natural gas as well. The experimentally obtained ignition delays vary from 40 ms at high pressures and temperatures to more than 300 ms for the lower regime of the experimental space. For the ignition delays up to 60 ms the best fit was obtained by a reduced version of PRF accounting only for the C1 to C3 compounds. Konnov’s mechanism and UBCMech are under predicting the experimental ignition delays while the GRI3.0 mechanism (with and without RAMEC sub module) over predicts the ignition delay for the natural gas mixtures. It has to be mentioned that the GRI3.0+RAMEC mechanism as well as the initial mechanism of Petersen (GRI1.2+RAMEC) on the other hand under predict the experimental ignition delay for pure methane mixtures. This indicates that the low temperature chemistry for methane that has been added in the RAMEC sub module is enhancing ignition too strong. Replacing part of methane in pure methane mixtures by equivalent amounts of ethane and propane - know as ignition promoters - lead to longer ignition delays in the simulations. But as the GRI3.0+RAMEC mechanism (without nitrogen chemistry) is the smallest of all tested mechanisms considering number of species and reactions, it still is an interesting option for coupling it to a computational fluid dynamics code as we are planning to do. We though performed a sensitivity analysis to obtain a set of the most dominating reactions and used a multi-objective optimisation algorithm to optimize the reaction rates within their limits of uncertainty. The results show a good fit for the short ignition delays of natural gas mixtures but still an under prediction of the methane delays. For the longer ignition delays the assumption of a constant core temperature during the post compression phase becomes questionable and the under prediction of all mechanisms of the experimental delays is an obvious consequence. A heat loss model predicting the temperature evolution has to be implemented in the simulations to properly simulate the ignition delays.

2006

Miniature Mobile Systems for Inspection of Ferromagnetic Structures

Patrick Schoeneich

Power plants require periodical inspections to control their state. To ensure a safe operation, parts that could fail before the next inspection are repaired or replaced, since a forced outage due to a failure can cost up to millions of dollars per day. Non-Destructive Testing (NDT) methods are used to detect different defects that could occur, such as cracks, thinning, corrosion or pitting. Some parts are inspected directly in situ, but may be difficult to access; these can require opening access holes or building scaffoldings. Other parts are disassembled and inspected in workshops, when the required inspection tools cannot be moved. In this thesis, we developed innovative miniature mobile systems able to move within these small and complex installations and inspect them. Bringing sensors to difficult-to-access places using climbing robots can reduce the inspection time and costs, because some dismantling or scaffolding can be eliminated. New miniature sensors can help to inspect complex parts without disassembling them, and reduce the inspection costs, as well. To perform such inspections, miniature mobile systems require a high mobility and keen sensing capabilities. The following approach was used to develop these systems. First, different innovative climbing robots are developed. They use magnetic adhesion, as most structures are made of ferromagnetic steel. Then, vision is embedded in some of the robots. Performing visual inspections becomes thus possible, as well as controlling the robots remotely, without viewing them. Finally, non-visual NDT sensors are developed and embedded in some of the robots, allowing them to detect defects that simple vision cannot detect. Achieving the miniaturization of the developed systems requires strong system integration during these three steps. A set of examples for the different steps has been designed, implemented and tested to illustrate this approach. The Tripillars robots, for instance, use caterpillars, and are able to climb on surfaces of any inclination and to pass inner angles. The Cy-mag3Ds robots use an innovative magnetic wheel concept, and are able to climb on surfaces of any inclination and to pass inner angles, outer angles and surface flips. The Tubulos robots move in tubes of 25 mm diameter at any inclination. All robots embed the required electronics, actuators, sensors and energy to be controlled remotely by the user. Wireless transmission of the commands signals allows the systems to maintain their full mobility without disturbing cables. Integrating Hall sensors near the magnetic systems allows them to measure the adhesion force. This information improves the security of the robots, since when the adhesion force becomes low, the robots can be stopped before they fall. The Tubulo II uses Magnetic Switchable Devices (MSDs) for adhesion. An MSD is composed of a ferromagnetic stator and one or more moving magnets; it has the advantage of requiring only a low force to switch on or off a high adhesion force. MSDs have the advantage of being easy to clean of the magnetic dust that is present in most real environments and that sticks strongly to magnetic systems. As an additional step toward inspection, a camera is embedded on the Cy-mag3D II and the Tubulos. It allows these robots to inspect visually the structures the robots move in, and to control them remotely. The perspective of a climbing robot in an unknown environment is often not enough to give the user a sense of its scale, and to move efficiently in it. A distance sensor is designed and embedded on the Cy-mag3D II, which increases the user's perception of the environment substantially; Finally, an innovative miniature Magnetic Particle Inspection (MPI) system was developed to inspect turbine blades without disassembling them. An MSD is used to perform the required magnetization. The system can automatically inspect a flat surface, performing all the required steps of MPI: magnetize, spray magnetic particles, record images under UV light and demagnetize. Thanks to the strong integration and miniaturization, the system can potentially inspect complex parts such as steam turbines.

EPFL2012

Interchangeability with thresholds and degradation factors for Soft CSPs

Boi Faltings, Nicoleta Neagu

Substitutability and interchangeability in constraint satisfaction problems (CSPs) have been used as a basis for search heuristics, solution adaptation and abstraction techniques. In this paper, we consider how the same concepts can be extended to soft constraint satisfaction problems (SCSPs). We introduce two notions: threshold alpha and degradation factor delta for substitutability and interchangeability, ( (alpha) substitutability/interchangeability and (delta) substitutability/interchangeabi-lity respectively). We show that they satisfy analogous theorems to the ones already known for hard constraints. In (alpha) interchangeability, values are interchangeable in any solution that is better than a threshold alpha, thus allowing to disregard differences among solutions that are not sufficiently good anyway. In (delta) interchangeability, values are interchangeable if their exchange could not degrade the solution by more than a factor of delta. We give efficient algorithms to compute ( (delta) / (alpha) )interchangeable sets of values for a large class of SCSPs, and show an example of their application. Through experimental evaluation based on random generated problem we measure first, how often neighborhood interchangeable values are occurring, second, how well they can approximate fully interchangeable ones, and third, how efficient they are when used as preprocessing techniques for branch and bound search.