1,909 research outputs found

    Hiukkasilmaisimen tukirakenteen toleranssianalyysi ja laadunvarmistus

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    Particle detectors are some of the largest and most complicated devices ever built, that are used to study the smallest building blocks of our universe. Design and construction can take years and engineers and physicists have to guarantee that the detectors finally operate as required. The Large Hadron Collider (LHC) at CERN is being updated to operate at higher luminosities. Consequently, many of the detectors that study the collisions produced by the LHC are being upgraded. In this thesis, an abstracted feature based model is presented for allocating geometric tolerances for a novel support structure of the future tracking detector of the Compact Muon Solenoid (CMS) experiment. This new tracking detector will have a unique section where silicon sensors are mounted in different angular orientations, providing unparalleled track reconstruction efficiency while lowering costs tremendously. The tolerance analysis is based on the functional requirement of hermeticity in the CMS tracker: silicon sensors within each of the tracker’s layers must overlap slightly so that particles cannot traverse a layer without hitting at least one active sensor surface. Imprecisions in the tracker’s real construction geometry cause the sensors to deviate from their nominal positions. This may generate gaps through which particles can traverse without hitting a sensor. Because the CMS tracker is a first-time-right production, with limited or even no possibilities for corrections in the mechanical construction, the composed model of the tolerances is based on worst-case analysis. The second part of this thesis is dedicated to quality assurance; the simulated geometric tolerances are utilized to develop accuracy requirements for dimensional measurements. Dimensional measurement plans are presented for the various assembly stages of the studied sub-detector. The chosen measurement methods utilize either a coordinate measurement machine (CMM) when the geometry and dimensions allow it, or photogrammetry. In case of photogrammetry, detailed descriptions are provided as where to position photogrammetric targets to achieve the required measurement accuracies. As a result, it is shown that an abstracted feature based model is suitable for synthesizing tolerances of the CMS trackers support structures. Such model can also be integrated into existing software at CERN, to allocate tolerances for other support structures. Additionally, comprehensive dimensional measurement plans have been developed for the quality assurance of the sub-detector.Hiukkasilmaisimet ovat yksiä suurimmista ja monimutkaisimmista laitteista joita on koskaan rakennettu. Niitä käytetään tutkimaan universumimme pienimpiä ja perustavanlaatuisimpia palasia. Hiukkasilmaisimien rakentaminen vie usein vuosia, ja niinpä niitä rakentavien insinöörien ja fyysikoiden tulee olla varmoja, että ne toimivat ennalta määritetyllä tavalla valmistuttuaan. CERN:issä sijaitsevaa suurta hadronitörmäytintä valmistellaan uudistukseen, jonka jälkeen se toimii korkeammalla luminositeetillä. Tästä johtuen useat ilmaisimet jotka tutkivat törmäyttimen tuottamia törmäyksiä uudistetaan myös. Tässä työssä esitellään abstrakteihin piirteisiin perustuva malli, jonka avulla asetetaan toleransseja kompaktin myonisolenoidin uuden jäljittimen tukirakenteisiin. Mallin toleranssianalyysi pohjautuu jäljittimen hermeettisyyteen liittyvään toiminnalliseen vaatimukseen: jäljittimen kerroksissa olevien piisensoreiden tulee limittyä hieman päällekkäin, jotta partikkelit eivät pääse kulkeutumaan kerroksen läpi osumatta ainakin yhden piisensorin aktiiviseen pintaan. Tukirakenteen toleranssit aiheuttavat näiden sensoreiden paikaan poikkeavuutta niiden nominaalisesta sijainnista, jolloin kerroksiin voi syntyä rakoja joista partikkelit pääsevät läpi. Koska uusi jäljitin rakennetaan vain kerran, toleranssianalyysimalli ratkaisee toleranssit pahimman mahdollisen skenaarion menetelmällä. Työn toisessa osassa hyödynnetään laskettuja toleransseja määrittelemällä niiden perusteella tarkkuusvaatimukset tukirakenteen dimensionaaliselle mittaukselle. Dimensionaalisen mittauksen suunnitelmat esitellään jäljittimelle sen kokoamisen eri vaiheissa. Valitut mittaustavat käyttävät hyödyksi joko koordinaattimittauskonetta, tai fotogrammetriaa. Fotogrammetrian tapauksessa keskitytään fotogrammetristen maalien paikkoihin, jotta mittauksen tarkkuusvaatimukset toteutuisivat. Työn tuloksena näytetään, että esitelty abstrakteihin piirteisiin perustuva malli soveltuu kompaktin myonisolenoidin jäljittimen tukirakenteiden toleranssisynteesiin. Malli voidaan myös integroida jo CERN:issä käytössä oleviin ohjelmiin, jolloin toleranssit voi asetta muillekin tukirakenteille kuin tässä työssä tutkitulle. Tukirakenteelle lasketut toleranssit esitellään taulukkomuodossa. Lisäksi, työssä suunniteltiin kattavat dimensionaalisen mittauksen menetelmät tutkittavan tukirakenteen laadun varmistamiseksi

    A statistical tolerance analysis approach for over-constrained mechanism based on optimization and Monte Carlo simulation

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    Tolerancing decisions can profoundly impact the quality and cost of the mechanism. To evaluate the impact of tolerance on mechanism quality, designers need to simulate the influences of tolerances with respect to the functional requirements. This paper proposes a mathematical formulation of tolerance analysis which integrates the notion of quantifier: ‘‘For all acceptable deviations (deviations which are inside tolerances), there exists a gap configuration such as the assembly requirements and the behavior constraints are verified’’ & ‘‘For all acceptable deviations (deviations which are inside tolerances), and for all admissible gap configurations, the assembly and functional requirements and the behavior constraints are verified’’. The quantifiers provide a univocal expression of the condition corresponding to a geometrical product requirement. This opens a wide area for research in tolerance analysis. To solve the mechanical problem, an approach based on optimization is proposed. Monte Carlo simulation is implemented for the statistical analysis. The proposed approach is tested on an over-constrained mechanism

    Recent Developments in Tolerancing Techniques

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    Mechanical tolerances are specified in conjunction with part geometry, material type andother technical specifications during the design of mechanical components and assemblies., These tolerances ensure the expected design function, and provide guidelines for the manufactureof designed parts. However, assigning proper tolerances for a design is a non-trivial task. Thispaper reviews the recent developments in the field of tolerancing techniques, particularly withreference to the use of computers, cost tolerance relationship and tolerance design for qualityimprovement of the product

    Automating Tolerance Synthesis: A Framework and Tools

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    This paper describes CASCADE-T—a new approach to tolerance synthesis that uses a complete representation of the conditional tolerance relations that exist between features of a part under design. Conditional tolerances are automatically determined from functional requirements and shape information. Tolerance primitives based on the virtual boundary requirements approach to tolerance representation are composed to form more complex tolerance relationships. Artificial intelligence techniques, including a constraint network, frame-based system, and dependency tracking are used to support flexible and detailed computation for tolerance analysis and synthesis

    Modeling of 2D and 3D Assemblies Taking Into Account Form Errors of Plane Surfaces

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    The tolerancing process links the virtual and the real worlds. From the former, tolerances define a variational geometrical language (geometric parameters). From the latter, there are values limiting those parameters. The beginning of a tolerancing process is in this duality. As high precision assemblies cannot be analyzed with the assumption that form errors are negligible, we propose to apply this process to assemblies with form errors through a new way of allowing to parameterize forms and solve their assemblies. The assembly process is calculated through a method of allowing to solve the 3D assemblies of pairs of surfaces having form errors using a static equilibrium. We have built a geometrical model based on the modal shapes of the ideal surface. We compute for the completely deterministic contact points between this pair of shapes according to a given assembly process. The solution gives an accurate evaluation of the assembly performance. Then we compare the results with or without taking into account the form errors. When we analyze a batch of assemblies, the problem is to compute for the nonconformity rate of a pilot production according to the functional requirements. We input probable errors of surfaces (position, orientation, and form) in our calculus and we evaluate the quality of the results compared with the functional requirements. The pilot production then can or cannot be validated

    Allocation of geometric tolerances in one-dimensional stackup problems

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    Many tolerancing problems on mechanical assemblies involve a functional requirement depending on a chain of parallel dimensions on individual parts. In these one-dimensional cases, simple methods are available for the analysis and the allocation of dimensional tolerances. However, they are difficult to extend to geometric tolerances, which must be translated into equivalent dimensional tolerances; this allows the analysis but makes the allocation generally impossible without Monte Carlo simulation and complex search strategies. To overcome this difficulty, the paper proposes a way of dealing directly with geometric tolerances in the allocation problem. This consists in expressing the functional requirement as a linear model of geometric tolerances rather than equivalent dimensional tolerances; the coefficients of the model (sensitivities) are calculated considering both the dimension chain and the standard definition of the geometric tolerances. The approach can be combined with any constrained optimization method based on sensitivities. The optimal scaling method, previously proposed for dimensional tolerances, is extended to geometric tolerances and used in two examples to demonstrate the simplicity of the overall workflow and the quality of the optimal solution

    Concurrent tolerance allocation and scheduling for complex assemblies

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    Traditionally, tolerance allocation and scheduling have been dealt with separately in the literature. The aim of tolerance allocation is to minimize the tolerance cost. When scheduling the sequence of product operations, the goal is to minimize the makespan, mean flow time, machine idle time, and machine idle time cost. Calculations of manufacturing costs derived separately using tolerance allocation and scheduling separately will not be accurate. Hence, in this work, component tolerance was allocated by minimizing both the manufacturing cost (sum of the tolerance and quality loss cost) and the machine idle time cost, considering the product sequence. A genetic algorithm (GA) was developed for allocating the tolerance of the components and determining the best product sequence of the scheduling. To illustrate the effectiveness of the proposed method, the results are compared with those obtained with existing wheel mounting assembly discussed in the literature

    Estimating the cost of functional requirements for tolerance allocation on mechanical assemblies

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    When allocating tolerances to geometric features of machine parts, a target variation must be specified for some functional requirements on the assembly. Such decision, however, is usually made from experience without consideration of its effect on manufacturing cost. To allow such an assessment, the paper describes a method for estimating the cost of a requirement as a function of its variation. The estimation can be done before solving a tolerance allocation problem, at the time the variation on the requirement is chosen as an optimization constraint. A simple expression for the cost of requirements of various types is obtained using the extended reciprocal-power function for the cost of part tolerances, and the optimal scaling method for tolerance allocation. As a result, the costs of both requirement variations and part tolerances can be treated in the same way; this allows a hierarchical approach to tolerance allocation, which can simplify the problem when dealing with complex dimension chains. Furthermore, simple calculations based on the proposed method suggest general cost reduction criteria in the design of assemblies
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