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Measurement of nonrigid freeform surfaces by coordinate measuring machine
Abstract
Nonrigid part could be subjected to significant distortion after the removal of manufacturing forces. This condition, known
as free-state variation, is principally due to weight and flexibility of the part and the release of internal stresses resulting
from fabrication. The present work deals with the inspection of freeform surfaces belonging to nonrigid parts. A manufactured
aeronautic component, named Mid Cowling, is considered as case study. The design of an appropriate fixture equipment will
be firstly presented: it enables both to simulate the mating part interface and to locate the part in coordinate measuring
machines working volume. Then, a method for evaluation of a freeform surface with respect to the nominal one will be presented.
This evaluation is based on Euclidean distance between actual and nominal surfaces. Finally, an analysis of the part deformation
presented in order to evaluate the measurement process in terms of interaction of the measurement system with the inspected
part will be proposed. The adopted method, based on a finite element analysis, was proposed in order to evaluate the interaction,
due to the measuring force, between the touch probe and the inspected surface and thus its effect on the measurement result.
KeywordsFixturing equipment-Freeform surface-Nonrigid part-Inspection-Coordinate measuring machine
... The first works about the measurement of flexible components by a CMM with a touch probe are described in Refs. [12,13]. Once defined and simulated the assembly process of a freeform component, the distortions due to the nominal contact force of touch probe on that component were evaluated by a finite element analysis (FEA) and a method of inspection was proposed [12]. ...
... [12,13]. Once defined and simulated the assembly process of a freeform component, the distortions due to the nominal contact force of touch probe on that component were evaluated by a finite element analysis (FEA) and a method of inspection was proposed [12]. The deviations from nominal measured on a flexible component, constrained as in the actual assembly stage, were used as input in a FEA software to evaluate the stress field due to the assembly constraints [13]. ...
... [14,15] with a further reduction of constraints to clamp a flexible component and the use of a FEA software to correct the coordinates of the measurement points by using as input the actual probing and clamping forces. A schematic representation of the relationship between this paper and the previous works on the measurement of flexible components [12][13][14][15] is shown in Table 1. In the rows of the Table 1 are shown the factors used to compare the different works that are reported on the five columns. ...
Thin components in composite material are of great interest in many fields, but the component deformation, known as spring-in and warpage, arise during the manufacturing process; so, it is really important to verify if deformations are within the indicated tolerances. However, the inspection of thin components can be difficult if it is carried out by a coordinate measuring machine because of their high flexibility. The aim of the present work is to present a method to inspect flexible components in composite material as rigid ones by means of a coordinate measuring machine with a touch probe. The proposed method foresees to acquire the probing forces and the clamping forces, due to the fixturing equipment, by force sensing resistor sensors and uses them in a finite element analysis to evaluate the corrections to be applied to the coordinates of points acquired through a coordinate measuring machine. The proposed method was applied to a L-shaped component and all results were compared with those due to a laser measurement system.
... The output geometry of one stage is the input of the next stage, and thus, the assembly geometry is modified from station to station [3,4] due to the succession of assembly operations [5]. In addition, the flexibility of aeronautical components makes their inspection arduous because of the deformations induced by the gravity load and by fixturing conditions [6]. The inspection of such components is generally performed when components are in the assembly configuration, at different stages of the assembly process. ...
... However, the assembly conditions are not necessarily the same as the conditions of use. R. Ascione and W. Polini [6] propose a method to control component geometry using coordinate measuring machines (CMM) in their conditions of use. These conditions are reproduced thanks to modular equipment. ...
This paper deals with a method to extract the free-state shape of aeronautical assembly components from part measurements independently of the assembly configuration of use. Knowledge of the free-state shapes enables to assess the geometrical conformity of an assembly through the assembly simulation using Finite Element Method (FEM). The component is measured, using optical means in a given configuration for which the setup is well-known. A coarse cleaning is thus applied on the measured data to obtain manipulative data in the CAD model frame. Meanwhile, displacements due to the measuring setup and to gravity are evaluated from the nominal geometry using FEM. Finally, the free-state shape, as a finite element mesh, is achieved by moving the nominal mesh nodes by a distance equal to the measured defect minus the evaluated displacement. The approach is applied to an aeronautical component.
... Ascione and Polini [10] propose to evaluate the geometry of the component using a tool simulating its position in its use configuration. In this way, the deviations between the geometry of the assembled component and the nominal geometry are directly identified. ...
... Geometrical specifications on the specimen Specifications 1,2,3,3,4,6 in Figure 11 are those defined by Anselmetti's CLIC method [15,16] in the models for the tolerance of positioning characteristics. Specifications 8,9,10,11 are examples of functional requirements transcription. The same goes for specification 7, which fulfils a requirement limiting the translation defects x and rotation around z of part 3 with respect to part 1. ...
When talking about ISO tolerancing of a mechanical component it generally comes along with the hypothesis that the component is rigid. Our study deals with a turbine blade on which significant constraints are exerted during operation, causing deformation of its aerodynamic part. For that reason, ISO specification of rigid parts do not apply on the entire component. Therefore, this contribution is about the proposition of a tolerancing method that permits to perform ISO tolerancing of different parts of the component while taking into account the possible deformations. On the example of a turbine blade the developed approach based on existing methodologies is explained in detail containing tolerance writing and deformation analysis.
... The inspection is accomplished by virtually comparing this scan mesh with the associated nominal CAD model to evaluate geometric deviation of the manufactured part with respect to assigned tolerances. Conventional inspection methods apply a hard inspection fixture [4] to keep non-rigid parts in its functional state, but ongoing studies on fixtureless inspection methods intend to eliminate the need of these complex and expensive fixtures. Fixtureless non-rigid inspection methods [1,[5][6][7][8][9][10][11][12][13] are developed as CAI methods in which different approaches are applied to compensate for the flexible deformation of measured manufactured part in a free-state. ...
... Therefore, the CAI methods that apply rigid registration only, such as iterative closest point (ICP) algorithm [17], do not fit the inspection of non-rigid parts in a free-state. To this end, classical inspection methods are used by restraining non-rigid parts in physical fixtures [4] during the inspection process. However, significant drawbacks of these complex fixtures, where the setup and repeatability of the fixtures are costly, lead to developing inspection methods by eliminating the need of fixtures. ...
Recent developments in the fixtureless inspection of non-rigid parts based on computer-aided inspection (CAI) methods significantly contribute to diminishing the time and cost of geometrical dimensioning and inspection. Generally, CAI methods aim to compare scan meshes, which are acquired using scanners as point clouds from non-rigid manufactured parts in a free-state, with associated nominal computer-aided design (CAD) models. However, non-rigid parts are deformed in a free-state due to their compliance behavior. Industrial inspection approaches apply costly and complex physical inspection fixtures to retrieve the functional shape of non-rigid parts in assembly-state. Therefore, fixtureless inspection methods are developed to eliminate the need for these complex fixtures and to replace them with simple inspection supports. Fixtureless inspection methods intend to virtually (numerically) compensate for flexible deformation of non-rigid parts in a free-state. Inspired by industrial inspection techniques wherein weights (e.g., sandbags) are applied as restraining loads on non-rigid parts, we present a new fixtureless inspection method in this article. Our proposed virtual mounting assembly-state inspection (VMASI) method aims at predicting the functional shape (in assembly-state) of a deviated non-rigid part (including defects such as plastic deformation). This method is capable of virtually mounting the scan mesh of a deviated non-rigid part (acquired in a free-state) into the designed assembly-state. This is fulfilled by applying permissible restraining forces (loads) that are introduced as pressures on surfaces of a deviated part. The functional shape is then predicted via a linear FE-based transformation where the value and position of required restraining pressures are assessed by our developed restraining pressures optimization (RPO) approach. In fact, RPO minimizes the orientation difference and distance between assembly mounting holes on the predicted shape of a non-rigid part with respect to nominal ones on the CAD model. Eventually, the inspection is accomplished by examining the mounting holes offset on the predicted shape of the scan model concerning the nominal CAD model. This ensures that the mounting holes on the predicted shape of a scan model in assembly-state remain in the dedicated tolerance range. This method is evaluated on two non-rigid parts to predict the required restraining pressures limited to the permissible forces during the inspection process and to predict the eventual functional shape of the scan model. We applied numerical validations for each part, for which different types of synthetic (numerically simulated) defects are included into scan meshes, to determine whether the functional shape of a geometrically deviated part can be virtually retrieved under assembly constrains.
... The inspection of flexible parts is not straightforward as gravity loads as well as part fixturing induce part deformations [3,4]. Flexible part shape is generally assessed when components are in assembly configuration. ...
... In this direction, Ascione and Polini [4] propose to assess part geometry using a fixturing equipment designed in order to reproduce the configuration (location and orientation) of the part to be inspected in its use configuration after assembly. The measurement is performed using a contact probe, which involves a displacement which is negligible compared to the part geometry deviations. ...
This paper deals with an approach to identify geometrical deviations of flexible parts from optical measurements. Each step of the approach defines a specific issue which we try to respond to. The problem of measurement uncertainties is solved using an original filtering method, which permits to only consider a few number of points. These points are registered on a mesh of the CAD model of the constrained geometry. The shape resulting from deflection can be identified through the finite-element simulation of the part’s deformation due to its own weight and the measuring set-up. Finally, geometrical deviations are obtained by subtracting geometrical deflections to measured geometrical deviations. The method is illustrated in an experimental test case.
... This latter requirement is an endeavor that often involves the use of an expensive and dedicated fixture. This issue is best illustrated in the case of a freeform aerospace component in Ascione and Polini's [3] paper (see Fig. 4). ...
... Fixturing equipment assembled on the base plate of the coordinate measuring machines [3] Furthermore, the cost of using dedicated fixtures, such as conformation jigs, are amplified since two fixtures are usually required-one for the supplier and the other for the client as a guarantee that they can independently asses the part's quality. ...
In a free state, nonrigid parts can take on different shapes compared to their design model. Such behavior of nonrigid parts introduces particular challenges to engineers during the geometric and dimensional requirements specification and inspection steps. Given that the choice of inspection method is guided by the requirement specification type, this paper presents two sections dealing, respectively, with the specification and the inspection method used for nonrigid parts. Accordingly, this paper proposes a categorization of the particular specification methods used for the geometric dimensioning and tolerancing of nonrigid parts under the American Society of Mechanical Engineers and International Organization for Standardization standards, as well as a review of the available approaches for the fixtureless inspection method of these parts. Typical applications of each specification method, as well as the advantages and drawbacks of their use, are proposed as general guidance. Finally, the current research trends into the fixtureless inspection methods of nonrigid parts are underscored.
... Thestateoftheartinmachinevisioninspection researchandtechnologyhasbeenpresentedrecently byMalamasetal.[21].Theyclassifiedthe contemporaryapplicationsintheindustryaccording totheirmeasuredparameters(i.e.dimensions, surface,assemblyandoperation)andtotheir degreesoffreedom.Aftertheremovalof manufacturingforces,flexiblepartcouldbe subjectedtosignificantdistortion.Thisfree-state variationisprincipallyduetoweightandflexibility ofthepartandthereleaseofinternalstresses resultingfromfabrication.Theinspectionof freeformsurfacesbelongingtonon-rigidpartshas beenpresentedbyAscioneandPolini[4].Intheir work,theyproposedafixtureassembly methodologythatenablesbothtosimulatethe matingpartinterfaceandtolocatethepartin coordinatemeasuringmachinesworkingvolume. Then,theyusedamethodfortheevaluationofthe actualsurfacewithrespecttoitsnominalmodel basedontheirEuclideandistance.Finally,amethod basedonafiniteelementanalysiswasproposedto evaluatetheeffectsofthemeasuringforce,induced bythetouchprobeontheinspectedsurface,onthe measurementresults.Forthealignmentof deformablepartsthatdonotrequireanyfixtures, Weckenmannetal.[25]aswellasJaramilloetal. ...
... This free-state variation is principally due to weight and flexibility of the part and the release of internal stresses resulting from fabrication. The inspection of freeform surfaces belonging to non-rigid parts has been presented by Ascione and Polini [4]. In their work, they proposed a fixture assembly methodology that enables both to simulate the mating part interface and to locate the part in coordinate measuring machines working volume. ...
Non-rigid parts, in free-state, may have a considerable different shape than their nominal model due to dimensional and geometric variations of manufacturing process, gravity loads and residual stress induced distortion. Therefore, sorting profile deviation from a part's deformation by comparing the part's nominal shape to its scanned free-state shape is a challenging task. This task is a key step in the Iterative Displacement Inspection (IDI) algorithm used for the inspection of non-rigid parts without the use of costly specialized fixtures. This paper proposes the use of the statistical maximum normed residual test to improve the aforementioned identification task. Thirty two simulated manufactured parts are studied to show that the proposed method reduces the type I and II identification error of the IDI method.
... There are multiple downsides to using fixtures, including the timeconsuming set-up process, considerable purchase and operating expenses, and errors in CAI analysis if the setup on the fixture has not been performed adequately. These disadvantages have recently led researchers to attempt to circumvent the use of fixtures with flexible registration with complementary defect identification [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. A flexible registration is a deformation of the acquired point set that respects intrinsic (dimensional and shape) properties of the compliant part in order to avoid the misidentification of any existing defects or create new artificial ones. ...
Because of the effects of gravity and/or residual stress, some manufactured mechanical parts, such as sheet metals and skins, often have a significantly different shape in a free-state position as compared to their state-of-use position. These parts are described as compliant, flexible or nonrigid. Expensive specialized fixtures are currently used prior to performing geometrical inspection operations in order to maintain compliant parts in the state-of-use position. This paper introduces an automatic bi-criterion flexible registration method for the dimensional and geometric inspection of such parts. The proposed method deforms the data acquired via a non-contact scanner of a compliant part in a free-state position until it reaches the nominal CAD shape for inspection with conventional Computer-Aided Inspection (CAI) tools. In other words, the method neutralizes the deviations induced in a compliant part by the effects of gravity and residual stress, allowing the acquired data to be treated as if it were obtained from a rigid part, using already available conventional (rigid) CAI tools. A proposed algorithm based on the BOFR-2 (the 2nd version of a Bi-Objective Flexible Registration algorithm) method is validated against both virtual simulated and experimental real industrial case studies from the aerospace sector. The resulting cost reduction and agility increasing make this fixtureless method well adapted to the requirements of unit-batch production in the context of Industry 4.0.
... In a previous work, a method to inspect a flexible freeform component was suggested, once the configuration of the component during the assembly process through a special fixture equipment was simulated, and a finite element analysis was used to evaluate the distortions due to the probe nominal contact force on the part [15]. In a later work, the obtained deviations from nominal, measured through a CMM on a flexible part constrained how it was to be assembled, were used as input in a finite element model (FEM) analysis to estimate the stress and strain fields due to the constraint of the assembly [16]. ...
High flexibility components made by composite material are of great interest in many fields, from aeronautic and automotive industries to sport and design goods. During curing process, component deformation arise and are acceptable if the component can be brought within the indicated tolerance, once the component is assembled to final product mating components, by applying reasonable forces. In order to do so, the component needs to be inspected.
This work presents a high flexible component inspection method in composite material by means of a coordinate measuring machine (CMM) with a touch probe and some force sensing resistor sensors (FSR). The proposed method aims to acquire the probing forces by FSRs and uses them in a FEM tool in order to evaluate the correction to be applied to the coordinates of points acquired by CMM on high flexibility components.
... One approach to measure nonrigid part geometry, is to use a complex fixturing, as shown in [7]. The fixtures do reduce the deformation caused by measuring force, but the fixturing assembly can be extremely complex and time-consuming. ...
This research implements the Finite Element Analysis of the contact between the flexible part and the CMM touch-probe in order to support the Cyber-Physical Manufacturing Metrology Model and its metrology integration into coordinate measuring machine (CMM) inspection of flexible parts. Although optical metrology offers a solution for flexible part measurements, because there is no contact between the measurand and the probing system, not all geometrical features are accessible by the optical CMM probe. Therefore, it is important to quantify and to validate the deformation introduced by the contact measuring process. The Cyber-Physical Systems connect the virtual and physical worlds in a way that intelligent objects communicate and interact with each other. This research will connect the physical measurement system consisting of CMM and flexible plastic part with their digital representation, where contact is simulated by means of Finite Element Method.
... The manufactured part is clamped to the fixture, measurements of the parts surface are captured using a 3D sensor, and the measurements are compared to the computer aided design (CAD) model to evaluate the profile deviation. A well-documented example in the literature is found in Ascione and Polini [3]. ...
Sheet metal parts and large structures both distort under gravitational and fixturing loads. When it is physically or economically impractical to restrain the product during measurement, measurement data must be manipulated to determine whether the product conforms to geometric specifications. The proposed evaluation method relies on the super-position of the effects of gravity and fixturing restraints to deform the nominal model to the measurement data. Because these effects are calculated a priori, the deformation of the nominal model during measurement evaluation is computationally efficient. Comparing the distortion-compensated model directly to the inspection point cloud reveals profile deviations and locating forces.
... In a previous work, a method to inspect a flexible part during its working was proposed, once simulated the configuration of the part during the assembly process [16] through a special equipment described in ref. [17]. ...
High-flexibility components in composite material are of great interest in many fields, from aeronautic and automotive industries to sport and design goods. Their deformation during cure process, known as spring-in and warpage, is acceptable if it is within the indicated tolerance. The research aim of the present work is to present a way to inspect a high flexible part in composite material by means of a coordinate measuring machine with a touch probe. This means to define the fixturing equipment and the measurement strategy. The developed method was applied to an L-shaped part with a very small thickness and the obtained measurements were compared with those due to a laser system. The results show a good agreement between the two measurement techniques. Moreover, the further numerical simulations validate the developed contact measurement method.
... So far, many efforts on the shape prediction of flexible part in different configurations have been done. For example, Ascione and Polini gave a solution to the problem of nonrigid freeform inspection by a coordinate measuring machine [7]. But it is expensive and time-consuming to manufacture such equipment. ...
Due to the compliance and geometrical defects of composite parts, gaps exist between assembly components after the preassembly. Assembly requirements impose to fill these gaps to eliminate any unexpected internal stresses. Although it is identified as a problematic and expensive nonadded value stage, a gap measurement is still needed. This paper develops a numerical process for gap prediction before the assembly step. After the assembly components are scanned in a specified configuration, finite element meshes are created using the scanned data and the shape variations of each component caused by constraints and forces in different configurations are evaluated by finite element analysis. Assembly gaps are finally assessed by assembling the simulated preassembly shapes of all components. The feasibility of the proposed method is proved by an experiment.
... Ascione and Polini [14] discussed the dimensional inspection of non-rigid parts with freeform surface using inspection fixtures combined with Coordinate Measuring Machine (CMM). Abenhaim et al. [11] presented a review of previous research on the fixtureless inspection of non-rigid parts and proposed a classification of the specification methods used for the GD&T of non-rigid parts under the ASME and ISO standards. ...
Dimensional inspection is an important element in the quality control of mechanical parts that have deviations from their nominal (CAD) model resulting from the manufacturing process. The focus of this research is on the profile inspection of non-rigid parts which are broadly used in the aeronautic and automotive industries. In a free-state condition, due to residual stress and gravity loads, a non-rigid part can have a different shape compared with its assembled condition. To overcome this issue, specific inspection fixtures are usually allocated in industry to compensate for the displacement of such parts in order to simulate the use state and accomplish dimensional inspections. These dedicated fixtures, their installation, and the inspection process consume a large amount of time and cost. Therefore, our principal objective has been to develop an inspection plan for eliminating the need for specialized fixtures by digitizing the displaced part’s surface using a contactless (optical) measuring device and comparing the acquired point cloud with the CAD model to identify deviations. In our previous work, we developed an approach to numerically inspect the profile of a non-rigid part using a non-rigid registration method and finite element analysis. To do so, a simulated displacement was performed using an improved definition of boundary conditions for simulating unfixed parts. In this paper, we will improve on the method and save time by increasing the accuracy of displacement boundary conditions and using automatic node insertion and finite element analysis. The repeatability and robustness of the approach will be also studied and its metrological performance will be analyzed. We will apply the improved method on two industrial non-rigid parts with free-form surfaces simulated with different types of displacement, defect, and measurement noise (for evaluation of robustness).
... Consequently, in common inspection methods, as described in [3], dedicated holding fixtures are designed and used to compensate for the flexible deformation of non-rigid parts during inspection. However, these dedicated fixtures are very sophisticated and very expensive to set up in most cases. ...
Computer-aided inspection (CAI) of non-rigid parts significantly contributes to improving performance of products, reducing assembly time and decreasing production costs. CAI methods use scanners to measure point clouds on parts and compare them with the nominal computer-aided design (CAD) model. Due to the compliance of non-rigid parts and for inspection in supplier and client facilities, two sets of sophisticated and expensive dedicated fixtures are usually required to compensate for the deformation of these parts during inspection. CAI methods for fixtureless inspection of non-rigid parts aim at scanning these parts in a free-state for which one of the main challenges is to distinguish between possible geometric deviation (defects) and flexible deformation associated with free-state. In this work, the generalized inspection fixture (GNIF) method is applied to generate a prior set of corresponding sample points between CAD and scanned models. These points are used to deform the CAD model to the scanned model via finite element non-rigid registration. Then, defects are identified by comparing the deformed CAD model with the scanned model. The fact that some sample points can be located close to defects results in an inaccurate estimation of these defects. In this paper, a method is introduced to automatically filter out sample points that are close to defects. This method is based on curvature and von Mises stress. Once filtered, the remaining sample points are used in a new registration, which allows identifying and quantifying defects more accurately. The proposed method is validated on aerospace parts.
... Ascione and Polini [13] dealt with the free-form surface inspection of non-rigid parts using inspection fixtures combined with CMM. Abenhaim et al. [11] presented a review of the previous researches for the fixtureless inspection of non-rigid parts and proposed a classification of the specification methods used for the GD&T of non-rigid parts under the ASME and ISO standards. ...
Quality control is an important factor for manufacturing companies looking to prosper in an era of globalization, market pressures, and technological advance. The functionality and product quality cannot be guaranteed without this important aspect. Manufactured parts have deviations from their nominal (CAD) shape caused by the manufacturing process. Thus, geometric inspection is a very important element in the quality control of mechanical parts. We have focused here on the profile inspection of non-rigid parts which are widely used in the aeronautic and automotive industries. Non-rigid parts can have different forms in a free-state condition compared with their nominal models due to residual stress and gravity loads. To solve this problem, dedicated inspection fixtures are generally used in industry to compensate for the displacement of such parts for simulating the use state in order to perform geometric inspections. These fixtures and the inspection process are expensive and time-consuming. Our aim is therefore to develop an inspection method which eliminates the need for specialized fixtures by acquiring a point cloud from the displaced part using a contactless measuring system such as optical scanning and comparing it with the CAD model for the identification of deviations. Using a non-rigid registration method and finite element analysis, we will numerically inspect the profile of a non-rigid part. To do so, a simulated displacement is performed using an improved definition of boundary conditions for simulating unfixed parts. In this paper, we will apply an improved method on two industrial non-rigid parts with free-form surfaces simulated with different types of displacement, defect, and measurement noise.
... Ascione and Polini [10] deals with the free-form surface inspection of non-rigid parts with inspection fixtures combined with CMM. Abenhaim et al. [9] presents a review of the previous approaches for the fixtureless inspection of non-rigid parts and proposes a classification of the specification methods used for the GD&T of non-rigid parts under the ASME and ISO standards. ...
In order for manufacturing companies to thrive in an era of globalization, market pressures and technological developments, quality control is key. Without this aspect, it is not possible to ensure the functionality and quality of products. Due to errors that occur during the manufacturing process, manufactured parts have deviations from their nominal geometry. Therefore, one of the important aspects of the quality control of mechanical parts is geometric inspection. With the help of automated inspection, costs can be reduced during process. In our research project, we have focused on the profile inspection of non-rigid (flexible) parts. In fact, several mechanical parts used in the aeronautic and automotive industries can be considered non-rigid. This category of parts may have significantly different shapes in a free-state condition than the design (nominal) model due to gravity loads and residual stress. Generally, to solve this problem, special inspection fixtures are used in industry to compensate for the deformations of such parts to simulate the use state to perform geometric inspection. These dedicated fixtures are very expensive and the process is very time-consuming which reduces competitiveness. We aim to develop a non-rigid inspection technique to eliminate the need for specialized inspection fixtures by using a non-contact measuring system such as optical scanning and comparing the obtained point cloud from the distorted part with the nominal model to identify deviations. Using a non-rigid registration method and finite element analysis, we will apply a virtual (numerical) inspection fixture instead of a physical fixture. The simulated displacement will be performed with improved boundary conditions for simulating unfixed parts.
... In 2010, Ascione and Polini [7] proposed a fixture assembly methodology that enables both to simulate the mating part interface and to locate the part in a coordinate measuring machine's working volume. Then, they used a method for the evaluation of the actual surface with respect to its nominal model based on their Euclidean ...
Dimensioning and tolerancing standards assume inspection operation, unless otherwise specified, must be done in Free State. This can be problematic when dealing with compliant parts. The inspection of compliant parts needs specialized fixtures because in Free State they may have a significantly different form than their nominal model (CAD) due to inherent variations in the manufacturing process, gravity loads, and residual strains. These specialized fixtures pose difficulties, bear significant costs to industry, and the process is very time-consuming. To address these challenges, this paper proposes a new method for quantifying flexibility/rigidity of the mechanical parts. Subsequently, a novel approach named IDB-CTB is proposed to fixtureless inspection of deformable bodies by curvature estimation and Thompson-Biweight test. This approach combines the Gaussian curvature properties of manufactured compliant parts, one of the intrinsic properties of the geometry, with the Thompson-Biweight statistical test based on the extreme value notion as an identification method. The aim is to distinguish profile deviation due to the manufacturing process from a part’s deformation due to its flexibility in order to determine whether the tolerance fits the CAD model or not. The IDB-CTB approach is tested on two sets of case studies. Three simulated, typical industrial sheet metal case studies were performed in the first set, and an experimental case study in the second one. The low percentage of errors in defect areas and in the profile deviations estimated compared with their reference ones in most cases reflects the effectiveness of the proposed approach.
... Malamas et al. [1] presented the state of the art in machine vision inspection research and technology by classifying the contemporary applications in the industry according to their measured parameters and their degrees of freedom. Ascione and Polini [2] used a method for the evaluation of the actual surface with respect to its nominal model based on their Euclidean distance. They proposed a method based on a finite element analysis (FE) to evaluate the effects of the measuring force, induced by the touch probe on the inspected surface, on the measurement results. ...
Nowadays, a complicated and expensive conformation jig is needed to inspect the nonrigid parts. In a free-state condition, these parts may have a significant different shape than their nominal model (CAD) due to gravity loads and residual stress. In this paper, we present a new method for automatic fixtureless inspection of nonrigid parts. The inspection in our case is limited to the profile deviation as required by ASME Y14.5 standard and the defects are dent shapes. Our method combines the curvature estimation, one of the intrinsic properties of the geometry, with the Thomson statistical test in order to identify the defects due to the inherent variations of the manufacturing process from the deformations due to the flexibility of the part. The method is tested and validated on a simulated flexible part representing a typical sheet metal from the transport industry.
... This free-state variation is principally due to weight and flexibility of the part and the release of internal stresses resulting from fabrication. The inspection of freeform surfaces belonging to non-rigid parts has been presented by Ascione and Polini [2]. In their work, they proposed a fixture assembly methodology that enables both to simulate the mating part interface and to locate the part in coordinate measuring machines working volume. ...
Non-rigid parts in free state condition may have a different form than their nominal CAD model due to dimensional variation, gravity loads, residual stress induced distortion and/or assembly force. The Iterative Displacement Inspection (IDI) algorithm has been developed in order to resolve this problematic. In this paper we propose an improving of the identification techniques used to distinguish between the defects due to the manufacturing process and the deformations due to the positioning of the part and its flexibility. The maximum normed residual test has been implemented in the identification module of the IDI algorithm. A quasi-constant surface, an omega shape and a freeform surface representing typical structure parts in the transport industry are tested, giving satisfying results in most cases.
Due to requirements of high-precision geometric tolerances for some high precision products, forming errors and clamping errors are unavoidable in the machining process. In this paper, an on-position measurement method is proposed for position error compensation based on the combination datum theory. Firstly, the mathematical representation model of the coaxiality of parts with the shallow holes is constructed based on the combination datum theory. Secondly, an on-position measurement method is proposed with constructing the mathematical model of the profiles of the surfaces based on laser displacement sensors (LDSs) to calculate the coaxiality error of parts on-position based on the combination datum theory. Based on it, the position of the spindle of the machine are adjusted to present the clutter again to compensate for position errors of the holes. Finally, the experimental results demonstrate the effectiveness and correctness of the proposed position error compensation method based on the on-position measurement device.
This paper deals with a method to extract the free-state shape of aeronautical assembly components from part measurements independently of the assembly configuration of use. Knowledge of the free-state shapes enables to assess the geometrical conformity of an assembly through the assembly simulation using Finite Element Method (FEM). The component is measured, using optical means in a given configuration for which the set-up is well-known. A coarse cleaning is thus applied on the measured data to obtain manipulative data in the CAD model frame. Meanwhile, displacements due to the measuring set-up and to gravity are evaluated from the nominal geometry using FEM. Finally, the free-state shape, as a finite element mesh, is obtained by moving the nominal mesh nodes by a distance equal to the measured defect minus the evaluated displacement. The approach is applied to an aeronautical component.
Due to the fact that large forgings are critical components in the large machinery equipments, the dimensions measurement of the key parts plays an important role in the forging process. A non-contact real-time dimension measurement system based on green laser scanning for the large thermal forgings is proposed in this paper. First of all, the mathematical measurement model is established. The mapping relationship between the two-dimensional (2-D) image and the three-dimensional (3-D) outside dimensions of forgings can be acquired. Secondly, a light plane translational scanning model is derived, and the optical centre motion trail is calculated by using linear regression analysis in this paper. Finally, the measurement of forging's overall outside dimensions is realized by a data fusion of each section's dimension information through continuous scanning the forging and the real-time capturing images. The experimental results demonstrate that the measurement method proposed in the paper is viable.
Reverse engineering (RE) has emerged as a crucial tool in product design for facilitating reuse of design and relevant knowledge, by which new design can be accelerated. Reconstruction of free-form surface is the main focus of RE. To meet the demand of rapidly and precisely digitizing unknown complex free-form surface, which is the first step of RE, a multi-sensor approach characterized by an integrated use of a contact scanning probe (SP) and a point laser probe (PLP) is proposed. Both of the two digitizers are mounted on the Z axis arm of a coordinate measuring machine (CMM), and the distance between the laser beam of the PLP and the stylus of the SP in feed motion direction is adjusted to be equal to the specified scanning line spacing, so that the PLP is capable of measuring SP’s next scanning path as the SP is performing digitization task. With the priori path information acquired using PLP, measurement path planning algorithms are applied to program the next scanning path for SP real-timely. The planned scanning path can be subsequently used to automatically guide the SP and the CMM for fast and precise digitization of complex surfaces. A prototype system has been developed to demonstrate the feasibility of the proposed new method through experiment. Experimental results indicate that the proposed approach enables rapid, flexible, and high-precision digitization of complex free-form surface in reverse engineering.
In this thesis, we focus on the automation of the fixtureless geometric inspection of non-rigid (or compliant) parts. The primary objective of this project is to handle virtually this type of component and their point cloud, which represents a scan taken in a Free State condition, by eliminating the use of very expensive and complicated specialized fixtures posing productivity problems for manufacturing companies. This topic is a very high interest in the transport sector and, more specifically, in the aerospace one in order to significantly improve its productivity and its degree of competitiveness. The thesis is organized by articles. The study is divided over four phases. The first three phases will be represented by three journal papers and the fourth phase is presented as an appendix. The first phase of this work is intended to improve the identification module of an existing inspection mathematical tool « IDI: The Iterative Displacement Inspection » which has been developed by the research team working under the supervision of professor Tahan at ÉTS. The identification module aims to distinguish between defects that are due to the manufacturing process and deformations that are due to the flexibility of the part (gravity and residual stress effects). We propose to replace the original module with a new one which is based on the extreme value statistical analysis. We demonstrate that the new module remarkably reduces the type I and type II errors. In addition, unlike the identification method of the IDI, the proposed one does not require a user-specified threshold based on a trial and error process. In the second phase of this study, we propose an original approach to measure the flexibility/rigidity of the mechanical components. We introduce a factor that represents the ratio between the maximum displacement resulting from the deformation of the part and its profile tolerance and we present the results in a logarithmic scale. Three different régions were defined as giving a clear idea to the manufacturing industry about the situation of the parts on the flexibility scale. Subsequently, we propose a new fixtureless inspection method for compliant parts: the IDB-CTB « Inspection of Deformable Bodies by Curvature and Thompson-Biweight » method. This approach combines the Gaussian curvature estimation, one of the intrinsic properties of the surface which is invariant under isometric transformations, with an identification method based on the extreme value statistics (Thompson-Biweight Test). The low percentage error in defect areas and in profile deviations estimated reflects the effectiveness of our approach. In the third phase of this thesis, we propose a novel method that can be considered as complementary to the IDB-CTB approach. In addition to the profile deviations, we aim to detect the localization defects. We introduce two criteria that correspond to the specification of compliant parts: the conservation of the curvilinear distance and the minimization between two objects (Hausdorff Distance). We adapt and automate the Coherent Point Drift; a powerful non-rigid registration algorithm widely used in medical imagery and animation, for satisfying these two criteria. We obtain satisfying results by applying the third approach on a typical aerospace sheet metal. The conclusion of this thesis summarizes the scientific contributions through our work on the fixtureless inspection of compliant parts and the perspective related with it. In the appendix, we introduce a graphical user interface (GUI) created to handle the proposed approaches as well as the case studies bank developed in the training at Bombardier Aerospace Inc.
Nowadays, optimization of manufacturing and assembly operations requires taking into account the inherent processes variations. Geometric and dimensional metrology of mechanical parts is very crucial for the aerospace industry and contributes greatly to its. In a free-state condition, non-rigid parts (or compliant parts) may have a significant different shape than their nominal geometry (CAD model) due to gravity loads and residual stress. Typically, the quality control of such parts requires a special approach where expensive and specialized fixtures are needed to constrain dedicated and follow the component during the inspection. Inspecting these parts without jig will have significant economic impacts for aerospace industries, reducing delays and the cost of product quality inspection. The Iterative Displacement Inspection (IDI) algorithm has been developed to deal with this problem. In this paper, we propose a statistical approach based on the extreme value analysis to improve the identification module of the IDI. We tested our robust IDI algorithm on a simulated aerospace sheet metal part. The experiments show that the proposed approach is more robust and effective and extends the original IDI identification module in its methodology and applications.
The measurement technology of the hot-state size for heavy shell ring forging is researched. A measurement technology is proposed and designed in this paper. The outer diameter is measured by laser scanning technology. The relationship between temperature and size is derived. Using this relationship, the inner diameter is measured by combining the outer diameter and temperature. The temperature is measured by infrared temperature measurement technology based on three-level interference filter. The measuring method is feasible according to the experimental result. Thereby, the measurement for hot-state size of heavy shell ring forging is achieved, and the requirement of the large forgings' green manufacture is fulfilled.
A general scheme to validate the shape of a deformable part consists in performing a non-rigid alignment between measurements on the part’s surface with its CAD model. In many algorithms, this process requires the acquisition of a complete model of the inspected part, including regions near its fixation points. This paper proposes a system to perform inspection without the need to digitize the entire part’s surface or regions near fixation points. This algorithm uses instead of standard fixation points, surface feature points to compute the non-rigid transformation. Various tests on real parts show that a reduction of up to 58% of the RMS deviation in less than 3 iterations can be obtained using a single view of the part’s surface.
The increasing use of advanced measurement tools and technology in industry over the past 30 years has ushered in a new set of challenging computational problems. These problems can be broadly classified as fitting and filtering of discrete geometric data collected by measurements made on manufactured products. Collectively, they define the field of computational metrology for the design specification, production, and verification of product geometry. The fitting problems can be posed and solved as optimization problems; they involve both continuous and combinatorial optimization problems. The filtering problems can be unified under convolution problems, which include convolutions of functions as well as convolutions of sets. This paper presents the status of research and standardization efforts in computational metrology, with an emphasis on its classification and synthesis.
Local and global evaluation methods for shape errors of free-form surfaces are presented. The local evaluation methods use the changes of principal curvatures between the original CAD data and actual surfaces. The global evaluation methods use the aggregate normal vectors for the characterization of portions of surfaces. Both methods are applied for the evaluation of sheet metal formed by using numerical simulation data and coordinate measurement data.
The authors describe a general-purpose, representation-independent method for the accurate and computationally efficient registration of 3-D shapes including free-form curves and surfaces. The method handles the full six degrees of freedom and is based on the iterative closest point (ICP) algorithm, which requires only a procedure to find the closest point on a geometric entity to a given point. The ICP algorithm always converges monotonically to the nearest local minimum of a mean-square distance metric, and the rate of convergence is rapid during the first few iterations. Therefore, given an adequate set of initial rotations and translations for a particular class of objects with a certain level of `shape complexity', one can globally minimize the mean-square distance metric over all six degrees of freedom by testing each initial registration. One important application of this method is to register sensed data from unfixtured rigid objects with an ideal geometric model, prior to shape inspection. Experimental results show the capabilities of the registration algorithm on point sets, curves, and surfaces
Inspection of machined objects is one of the most important quality control tasks in the manufacturing industry. Ideally, inspection processes should be able to work directly on scan point data. Scan data, however are typically very large scale (i.e., many points), unorganized, noisy, and incomplete. Therefore, direct processing of scanned points is problematic. Many of these problems may be reduced if reconstruction methods exploit diverse scan data, that is, information about the properties of the scanned object. This paper describes this concept and proposes new methods for extraction and processing of diverse scan data: (1) extraction (detection of a scanned object's sharp features by the sharp feature detection method) and (2) processing (scan data reduction by the geometric bilateral filter method). The proposed methods are applied directly on the scanned points and arc completely automatic, fast, and straightforward to implement. Finally, this paper demonstrates the integration of the proposed methods into the computational inspection process.
Engineering surfaces comprise form and waviness errors, which are separated from the measured surface by establishing a reference surface that represents these errors. An attempt is made for the first time to fit a reference surface for simultaneous separation of form and waviness errors. A second-degree polynomial and a set of sinusoidal functions are taken as basis functions to represent form and waviness, respectively, and fitting is done using a nonlinear least-squares method. Different examples of surfaces are considered and a comparison is also made with 3D Gaussian filter to bring out the nature of reference surfaces obtained by the present fitting approach.
The digitization of mathematically unknown free-form curves is frequently required. Current industrial practice relies on the experience of an operator to determine the direction of a CMM probe's approaching vector and the number of samples to represent a complex curve. An automatic procedure is proposed to generate recursively the approaching vector direction as well as the sampling rate. Several techniques including a linear prediction, a polynomial prediction, and a spline interpolation method have been presented and compared. Experimental results show that the spline interpolation method provides the best digitization accuracy and requires the least number of sampling points.
The present paper describes an artefact based approach to obtain traceability of freeform measurements on coordinate measuring machines. First, the requirements for the traceability of freeform measurements and a strategy for the development of a feasible solution are presented. A new concept of artefact, called the “Modular Freeform Gauge” (MFG) has been developed. It is based on physical modeling of a given freeform surface by a combination of items with regular geometry, well calibrated on their dimensions and form. The relative position is accounted for during the procedure; this information is used to generate a “calibrated” CAD model as reference for freeform measurements. The architecture of the artefact, its collocation in the traceability chain, and the calibration procedure are described.Finally, a procedure for the uncertainty assessment of actual freeform measurements is presented. The work here described has been focused on implementation of the uncertainty assessment procedure for freeform measurements on turbine blades. A task-specific Modular Freeform Gauge was developed for this application.
This paper presents a method of reconstructing a triangular surface patch from dexel data generated by ray casting to represent solid models for applications, such as virtual sculpting and numerically controlled (NC) machining simulation. A novel contour generation algorithm is developed to convert dexel data into a series of planar contours on parallel slices. The algorithm categorizes the dexels on two adjacent rays into different groups by using a "grouping" criterion. The dexel points in the same group are connected using a set of rules to form subboundaries. After checking the connections among all the dexel points on one slice, a connection table is created and used to obtain the points of connection in a counterclockwise sequence for every contour Finally, the contours on all the parallel slices are tiled to obtain triangular facets of the boundary surface of the 3D object. Computational costs and memory requirements are analyzed, and the computational complexity analysis is verified by numerical experiments. Example applications are given to demonstrate the described method.
The problems of the substitute geometry for features of size are considered and an algorithm for synthesis of the substitute features (SF) is developed. Three and only three classes of surfaces are proved to have an incomplete set of position and orientation deviations within the SF equation: cylinders with any directrix, surfaces of revolution with any meridian, and helical surfaces with any profile. The form accuracy of multidimensional features relating to these classes is considered: ellipsoid of revolution, epitrochoidal cylinder, and Archimedean screw. The deterministic consideration is accompanied by evaluation of the uncertainty of the standard assessments of the geometric accuracy and capacity of the computing procedure.
Volumetric models of 3D objects have recently been introduced into the reverse engineer-ing (RE) process. Grid-based methods are considered as the major technique for recon-structing surfaces from these volumetric models. This is mainly due to the efficiency and simplicity of these methods. However, these grid-based methods suffer from a number of inherent drawbacks, resulting from the fact that the imposed Cartesian grid in general is not well adapted to the surface, neither in size nor in orientation. In order to overcome the above obstacles a new iso-surface extraction method is proposed for volumetric models. The main idea is first to construct a geometrical field that is induced by the object's shape. This geometrical field represents the natural directions and a grid cell size for each point in the domain. Then, the imposed volumetric grid is deformed by the produced geometrical field toward the object's shape. The iso-surface meshes can be extracted from the resulting adaptive grid by any conventional grid-based contouring technique. The proposed method provides better approximation of the unknown surface and exhibits anisotropy, which is present inherently in the surface. Moreover, since the produced meshes are quad-dominant, Catmull-Clark subdivision surfaces are directly constructed from these meshes.
In the automotive industry, steadily increasing demands on the accuracy of the size and form of drawn sheet metals and the rising complexity of measurement tasks have meant that more and more measurements have to be made on the shopfloor with CNC coordinate measuring machines for workpiece inspection and process control.It is usually assumed that the uncertainty of CMMs has a sufficiently small value for reliable inspection of tolerances within a common range of about 1 mm. The characteristic values like E3 for length measurement uncertainties or R for the probing uncertainty, as specified by the manufacturers, only describe which uncertainties can be expected for specific measurement tasks under ideal conditions. However, these measurement tasks are only performed very rarely during the normal use of the instrument. Statements about uncertainties of other measurements cannot be derived directly from these values.Using the example of measurements on car-body parts, it is shown what uncertainty contributions can really be expected from the measuring device, workpiece, environment, and operator. Not only the device-specific uncertainty but also other influences play a major role. The effects of environmental conditions, especially temperature and operator-specific actions can make at least the same contribution to the measurement uncertainty.
The scope of this keynote paper is to present the state of the art in the metrology of freeform shapes with focus on the freeform capabilities of the most important measuring techniques and on related metrological issues. Some examples of products are presented, for which the metrology of freeform shapes is important to guarantee the desired functional performance of the product. A classification of freeform measuring tasks and the corresponding metrological requirements are presented. A review of the most important measuring techniques is presented along with their capabilities for freeform measuring tasks. Specification and verification of freeform surfaces, including data evaluation and comparison to specifications are discussed, along with the measurement uncertainty and traceability of freeform measurements.
Precision inspection has been widely used in manufacturing to measure the dimensional accuracy of parts and products to meet the quality requirements. For regular geometric features, coordinate-measuring machines (CMM) can be used effectively to assess the accuracy and tolerances. For parts with free-form surfaces, the inspection becomes complex. Therefore, numerous researches have been carried out to tackle both fundamental and application issues concerning free-form surface inspection. In addition to academic research, some commercial packages have also been developed.This paper provides a comprehensive literature review of methodologies, techniques and various processes of inspections of parts with free-form surfaces. The specific topics cover: measurement data acquiring methods including contact and non-contact measurement approaches; inspection planning; geometric description methods of design models or measurement data; and, the free-form surface localization and comparison techniques, which are emphasized in this paper and mainly include the establishment of corresponding relationship, 3D transformation solving, measurement data to design model comparison or surface to surface distance calculations. Other issues, such as the influence factors to the localization/registration process, definition and inspection of free-form surface tolerance, and discussions on the functions of some commercial inspection packages available on market, are also discussed.
This paper describes a general purpose, representation independent
method for the accurate and computationally efficient registration of
3-D shapes including free-form curves and surfaces. The method handles
the full six-degrees of freedom and is based on the iterative closest
point (ICP) algorithm, which requires only a procedure to find the
closest point on a geometric entity to a given point. The ICP algorithm
always converges monotonically to the nearest local minimum of a
mean-square distance metric, and experience shows that the rate of
convergence is rapid during the first few iterations. Therefore, given
an adequate set of initial rotations and translations for a particular
class of objects with a certain level of 'shape complexity', one can
globally minimize the mean-square distance metric over all six degrees
of freedom by testing each initial registration. For examples, a given
'model' shape and a sensed 'data' shape that represents a major portion
of the model shape can be registered in minutes by testing one initial
translation and a relatively small set of rotations to allow for the
given level of model complexity. One important application of this
method is to register sensed data from unfixtured rigid objects with an
ideal geometric model prior to shape inspection. The described method is
also useful for deciding fundamental issues such as the congruence
(shape equivalence) of different geometric representations as well as
for estimating the motion between point sets where the correspondences
are not known. Experimental results show the capabilities of the
registration algorithm on point sets, curves, and surfaces.