Position Of Workpiece Research Articles

A design of an automated compact positioning system for workpiece positioning in machine tool workspace

The paper constitutes a study into the design of a novel type of a compact positioning system based on the constrained parallel mechanism. In this study, a design of low-height rotary table is presented for automated leveling, which can be mounted directly on the machine tool table without a significant decrease of the workspace. The authors propose a modification of parallel mechanism by introducing four extensible leg design and a table block which motion is constrained around a sphere in order to achieve higher rigidness. The table is driven by four hydraulic linear actuators which are connected with the linkages. Three different designs are discussed focusing on its capabilities of load transmission and compactness. The best solution was verified using FEM in order to test its potential rigidness and maximal contact stress occurring between interacting blocks. The designed model allows rotating the table about the sphere center using three independent rotations. The development of an automated compact positioning system can contribute to the reduction of manual time-consuming leveling operations without significant reduction of machine tool workspace.

Determining stable equilibria of spatial objects and validating the results with drop simulation

Robotic pick and place processes require the workpieces to be prepositioned and aligned to a fixed pose. This ensures the accurate grasping, but lacks flexibility and efficiency. Camera based solutions are available for simple workpieces, but no simple solution exists for complex ones. We offer a universal software, that determines the stable equilibrium poses of the workpiece from 3D model, and renders the top view image of it. This image can be compared to one captured by a single camera setup to determine the workpiece position and orientation in realtime for robotic application. The results are validated applying drop test simulation.

Point Cloud Registration Based on MCMC-SA ICP Algorithm

Point cloud registration is very important for workpiece positioning and error evaluation. Generally, the Iterative Closest Points (ICP) algorithm is always adopted as the first choice in fine registration, but requires a more appropriate initial condition to avoid falling into the local minimum. As a result, the ICP shows poor robustness and adaptability when dealing with the point cloud with measuring noise and outliers, complex surface, the far distance between the measured and the design. In this research, an improved ICP method is developed based on the simulated annealing (SA) and the Markov chain Monte Carlo (MCMC) to achieve the global minimum under given any initial conditions. The SA based on Pincus theorem is applied to calculate the rotation angles and translation vectors of the correspondences. To improve the convenience of SA, the MCMC method is applied to improve its sampling ability to satisfy the Pincus distribution. Furthermore, the MCMC-SA ICP algorithm for point cloud registration was designed. Three different kinds of complex curved surfaces were employed to verify the visibility of the developed MCMC-SA ICP approach. By comparing with the traditional ICP, it is indicated that the improved ICP based on MCMC-SA showed better global optimization ability.

Trajectory planning method of robot sorting system based on S-shaped acceleration/deceleration algorithm

To improve the sorting accuracy and efficiency of sorting system with large inertia robot, this article proposes a novel trajectory planning method based on S-shaped acceleration/deceleration algorithm. Firstly, a novel displacement segmentation method based on assumed maximum velocity is proposed to reduce the computational load of velocity planning. The sorting area can be divided into four parts by no more than three steps. Secondly, since the positions of workpieces are dynamically changing, a dynamic prediction method of workpiece picking position has been presented to consider all the possible positions of the robot and the workpiece, so as to realize the picking position prediction of the workpiece at any positions. Each situation in this method can constitute an equation with only one solution, and the existence of the solution can be verified by the proposed graphical method. The simulations of the motion time of the sorting process show that the proposed method can significantly shorten the sorting time and improve the sorting efficiency compared with the previous method. Finally, this method was applied to the Selective Compliance Assembly Robot Arm (SCARA) robot for experiments. In the physical picking experiment, the missing-pick rate was less than 1%, which demonstrates the efficiency and effectiveness of this method.

Determination of the optimal imaging parameters in industrial computed tomography for dimensional measurements on monomaterial workpieces

Industrial computed tomography (CT) plays a key role in 3D coordinate metrology as an alternative to conventional coordinate measuring machines. CT measurement uncertainty depends on the setup parameters with which CT scans are performed. Currently, there is no established model that can describe the relationship between CT setup parameters and measurement uncertainty, and thus determine the optimal settings for a given measurement task. In practice, CT users choose setup parameters intuitively causing high variability in the measurement results. In this study, we enhance and validate an analytical method for optimizing imaging parameters. The proposed method is based on the assumption that minimizing the contribution of imaging parameters to measurement uncertainty corresponds to (1) maximizing image contrast and signal, (2) minimizing geometric blurring, and (3) minimizing image noise and CT artifacts. It also requires information about the workpiece nominal geometry and material composition. The proposed method calculates the optimal photon energy for precise surface determination, given the workpiece position and orientation. Tube voltage and prefilter are determined so that the resulting x-ray spectrum has an effective energy equal to the optimal energy and beam hardening artifacts are minimized. Tube current is chosen to maximize image signal, while avoiding blurring and excessive generated tube power. Exposure time is chosen as the shortest time that fully exploits the detector dynamics. The proposed method was validated by analyzing the standard deviations of CT measurements on a test phantom. An analysis of variance on the uncertainty components showed that the predicted parameters were globally optimal.

Predicting the Effect of Field Shaper in Electromagnetic Welding Using FEM

The research work presents a 3D numerical simulation to predict the distribution of magnetic field and pressure in the desired region of the tubular jobs using coils with and without field shapers in electromagnetic welding (EMW). For finite element (FE) simulation, single-turn (ST) and multi-turn (MT) (disk type) coils are considered to analyze the effects of field shapers with varying material combinations in ANSYS MAXWELL environment. The change in magnetic field distribution on the outer peripheral region of the flyer tube along axial direction with and without field shaper is graphically shown and compared. Also, variation of magnetic pressure on the flyer tube with respect to increasing voltage and time is compared individually for various material combinations of ST/MT coil and field shaper. Results show that in welding simulation (FE) using single-turn Cu and SS coils with field shapers of Be, Nb and Ta, the magnetic pressure increases by 34, 12 and 11% and 35, 12 and 11%, respectively, as compared to the coil with no field shaper. In case of multi-turn SS coil with Cu and Be field shapers, the magnetic pressure increases by 16 and 10%, respectively. The novelty of the work is the proper guideline to select the material combinations for ST/MT coils and field shapers and positioning of work pieces inside the coil in order to achieve maximum utilization of input energy in EMW system.

Use of dual coolant displacing media for in-process optical measurement of form profiles

In-process measurement supports feedback control to reduce workpiece surface form error. Without it, the workpiece surface must be measured offline causing significant errors in workpiece positioning and reduced productivity. To offer better performance, a new in-process optical measurement method based on the use of dual coolant displacing media is proposed and studied, which uses an air and liquid phase together to resist coolant and to achieve in-process measurement. In the proposed new design, coolant is used to replace the previously used clean water to avoid coolant dilution. Compared with the previous methods, the distance between the applicator and the workpiece surface can be relaxed to 1 mm. The result is 4 times larger than before, thus permitting measurement of curved surfaces. The use of air is up to 1.5 times less than the best method previously available. For a sample workpiece with curved surfaces, the relative error of profile measurement under coolant conditions can be as small as 0.1% compared with the one under no coolant conditions. Problems in comparing measured 3D surfaces are discussed. A comparative study between a Bruker Npflex optical profiler and the developed new in-process optical profiler was conducted. For a surface area of 5.5 mm × 5.5 mm, the average measurement error under coolant conditions is only 0.693 µm. In addition, the error due to the new method is only 0.10 µm when compared between coolant and no coolant conditions. The effect of a thin liquid film on workpiece surface is discussed. The experimental results show that the new method can successfully solve the coolant dilution problem and is able to accurately measure the workpiece surface whilst fully submerged in the opaque coolant. The proposed new method is advantageous and should be very useful for in-process optical form profile measurement in precision machining.

A model-based residual approach for human-robot collaboration during manual polishing operations

A fully robotized polishing of metallic surfaces may be insufficient in case of parts with complex geometric shapes, where a manual intervention is still preferable. Within the EU SYMPLEXITY project, we are considering tasks where manual polishing operations are performed in strict physical Human-Robot Collaboration (HRC) between a robot holding the part and a human operator equipped with an abrasive tool. During the polishing task, the robot should firmly keep the workpiece in a prescribed sequence of poses, by monitoring and resisting to the external forces applied by the operator. However, the user may also wish to change the orientation of the part mounted on the robot, simply by pushing or pulling the robot body and changing thus its configuration. We propose a control algorithm that is able to distinguish the external torques acting at the robot joints in two components, one due to the polishing forces being applied at the end-effector level, the other due to the intentional physical interaction engaged by the human. The latter component is used to reconfigure the manipulator arm and, accordingly, its end-effector orientation. The workpiece position is kept instead fixed, by exploiting the intrinsic redundancy of this subtask. The controller uses a F/T sensor mounted at the robot wrist, together with our recently developed model-based technique (the residual method) that is able to estimate online the joint torques due to contact forces/torques applied at any place along the robot structure. In order to obtain a reliable residual, which is necessary to implement the control algorithm, an accurate robot dynamic model (including also friction effects at the joints and drive gains) needs to be identified first. The complete dynamic identification and the proposed control method for the human-robot collaborative polishing task are illustrated on a 6R UR10 lightweight manipulator mounting an ATI 6D sensor.

Workpiece Position in the Machining of Multiple Holes

The radius of the circle tangential to the axes of all the holes being machined is established. A method is proposed for selecting the workpiece position in the machining of several holes.

Investigation on Enhancement in Photochemical Machining

Micro-machining techniques are generating considerable interest in manufacturing industries. Photochemical machining (PCM) is one of such techniques in which the material is etched with the help of etchant in the controlled corrosion process. Machining area is defined by application of photoresist layer over the material, which ensures the removal of desired area. This process is regarded as stress free machining in which no mechanical stress is imposed on the workpiece while machining. Achieving accuracy in the PCM process is one of the prime challenges to overcome. Material removal and accuracy of PCM process can be enhanced by selecting proper positioning of workpiece inside the etching tank and preparing accurate photo-tool. This paper investigates various modes of workpiece positioning and their effect on material removal mechanism and determines most suitable positioning of workpiece to enhance performance of PCM process. An attempt to attain accurate photo-tool has also been presented in this paper.

Automatic Close-optimal Workpiece Positioning for Robotic Manufacturing.

Robot programming is still an expert dependent and not automatically optimized task. In order to make this process more automatic and intuitive for the end-user, this paper presents a novel approach to determine a close-optimal workpiece pose for different robotic manufacturing processes like welding and milling. The approach is based on a model-based interpretation of the Product, Process, and Resource (PPR) components defined in an internally developed Computer-Aided Manufacturing (CAM) software. After the interpretation addressed to simplify the path planning, an algorithm uses sample-based motion planning techniques and optimization algorithms, in order to find optimal motions in reaction to infeasible states of the robot (i.e. maximum joint limits and reachability) and a close-optimal workpiece pose. The optimized path planning is achieved by exploring an interpreted Configuration Space (C-space) using a Degree of Freedom (DoF) of the Robot Manufacturing Processes (RMP) and by interpreting its constraints. Simulation results are presented for robotic welding and milling task by optimizing welding orientations and robot stiffness respectively in the path planning and the joint movements and gravity cost criteria in the workpiece positioning. Optimization of these criteria could be used in RMP to address improvement of the process quality.

Analysis Method of Degrees of Freedom for Integrated Positioning

In order to ensure the machining requirements, the very first issue to be solved during machining is the positioning of workpieces on lathes or fixtures. Due to different shapes of workpieces, integrated positioning is adopted in most cases. While, the analysis of degrees of freedom for integrated positioning is quite difficult, therefore, the author developed a certain analysis method of degrees of freedom for integrated positioning—individual part analysis. It has been used to solve many workpiece positioning problems, which can provide much reference for design and analysis of positioning scheme.

Optimal workpiece positioning in flexible fixtures for thin-walled components

Flexible fixtures are finding widespread use in manufacturing processes of thin-walled components in the automotive and aerospace industries. Owing to its reconfigurability, a flexible fixture can take different layouts depending on the workpiece geometry and location. By finding the appropriate workpiece location, the deformation of workpieces under machining forces and their own weights can be reduced, hence increasing the quality of the final products. In this paper, we introduce a novel method for optimum workpiece positioning in rod type flexible fixtures for thin-walled components. First, the geometry of the workpiece is extracted from its CAD model. Second, a geometry-based method is proposed to find the optimal location of the workpiece relative to the fixture system. A composition of homogeneous transformations is performed to maximize the workpiece support capacity, initially place the workpiece in the fixture region, and maximize the number of active fixture elements (fixels). Third, the result is verified and visualized in a fixture verification module. An aircraft body panel is employed as an example to test the performance of the proposed method. Finally, conclusions and future studies are discussed.

A contribution for increasing workpiece location accuracy in a 3-2-1 fixture system

The 3-2-1 fixture design principle is the most used method to locate prismatic workpieces. To further increase location accuracy, the influence of friction and contact stiffness between workpiece and fixture parts should be minimized. This can be achieved by decreasing the structural stiffness of the locators in the tangential direction to the contact. Hence, based on flexure hinges, locators with two different stiffness values are investigated. To analyze the influence of locator stiffness, a special experimental setup is developed to study two locators positioned in the secondary plane of the 3-2-1 fixture. Experiments are carried out to compare the locators with flexure hinges to a rigid one. The displacement and rotation of the workpiece are investigated as a function of the clamping force. The experimental results show that the reduction in the tangential stiffness can improve the position and orientation accuracy of workpieces.

3D pose estimation of large and complicated workpieces based on binocular stereo vision.

A binocular stereo vision method is proposed for automatically locating the position and posture of workpieces, which is especially important when processing large, complicated structures, for example, the laser hardening and laser cladding of automotive die. First, a binocular stereo vision positioning system was designed and modeled, from which a method of background subtraction was proposed to extract the edge line of the foreground area. Furthermore, the intersection point of the workpiece contour line was taken as the characteristic point of the workpiece, and an algorithm that combines epipolar constraint with gray value similarity was proposed to quickly and accurately realize the feature points matching. Finally, experiments show that the workpiece can be positioned accurately, and that the precision of position recognition could be controlled within ±0.5 mm when the camera was 1m away from the workpiece, meeting the requirement of robot processing.

State-of-the-art in fixture systems for the manufacture and assembly of rigid components: A review

Abstract Basic work holding devices (e.g. vices), fixtures and jigs are used to construct a critical interface between a workpiece and an end-effector. This interface performs two main functions: location of the workpiece in the Euclidean space and preservation of the workpiece position against any loads. Despite the critical nature of the part-machine interface, limited attention has been given to work holding systems in the academic community. In this respect, the main objective of this paper is to systematically review the field of fixture design, thus allowing the classification of fixturing systems to identify research trends and niches. This review is broken into four sections: (i) basics of fixturing and work holding; (ii) fixtures for single components. The classification of these systems is based on an evolutionary trend that allows to see how the development of technologies, such as additive manufacture, sensing technologies and actuation systems, affects fixture design; (iii) fixtures for multi-parts (both for batch production and assembly operations), with an emphasis on the unique challenges that arise from the assembling process; and (iv) conclusions, denoting various research trends/opportunities in the areas of fixture design and fixture instrumentation. Examples of these prospects includes the integration of fixtures with sensing technology (incentivise by the growth of industry 4.0) and the construction of truly new multi-part fixturing systems, rather than just the expansion of single component fixtures.

An analytical model for predicting the machining deformation of a plate blank considers biaxial initial residual stresses

The initial residual stress in a blank is one of the main causes of machining deformation that has become a major concern in manufacturing aerospace monolithic structures. An analytical prediction model for machining deformation that considers biaxial residual stresses is proposed to investigate the quantitative relationship between deformation and initial residual stress. Finite element method (FEM) simulations and machining experiments are then conducted to validate the accuracy of the analytical model. Results calculated using the analytical model are consistent with the FEM and experiment results. Eventually, the quantitative relationships between deformation and initial residual stress under three typical machining strategies are determined. The effects of workpiece position in the blank on machining deformation are summarized based on these quantitative relationships.

Geometrical product specifications. Datums and coordinate systems

The work is devoted to the relevant topic such as the technical products quality improvement due to the geometrical specifications accuracy. The research purpose is to ensure the quality indicators on the basis of the systematic approach to the values normalization and geometrical specifications accuracy in the workpiece coordinate systems in the process of design. To achieve the goal two tasks are completed such as the datum features classification according to the number of linear and angular freedom degrees constraints, called the datums informativeness, and the rectangular coordinate systems identification, materialized by workpiece datums sets. The datum features informativeness characterizes the datums functional purpose to limit product workpiece linear and angular degrees of freedom. The datum features informativeness numerically coincides with the kinematic pairs classes and couplings in mechanics. The datum features informativeness identifies the coordinate system without the location redundancy. Each coordinate plane of a rectangular coordinate system has different informativeness 3 + 2 + 1. Each coordinate axis also has different informativeness 4+2+Θ (zero). It is possible to establish the associated workpiece position with three linear and three angular coordinates relative to two axes with the informativeness 4 and 2. is higher, the more informativeness of the coordinate axis or a coordinate plane is, the higher is the linear and angular coordinates accuracy, the coordinate being plotted along the coordinate axis or plane. The systematic approach to the geometrical products specifications positioning in coordinate systems is the scientific basis for a natural transition to the functional dimensions of features position - coordinating dimensions and the size of the features form - feature dimensions of two measures: linear and angular ones. The products technical quality improving is possible due to the coordinate systems introduction materialized by datums sets in GPS standards and national standards. Standards on the datums and datums systems should cover all the geometrical specifications and all the processes of the products life cycle: designing, manufacturing, verification and exploitation.

A method for determining the degree of freedom of workpiece based on developed geometry theorem

The traditional 6-point positioning principle could not indicate the specific features of the workpiece constrained at the six locating points and could not provide the influence law of geometric relationship between each constraint and the DOF of the workpiece, which makes it difficult to adapt to various complex positioning and often mistakes DOF of workpiece to lead to fail in jig design. The normal line of the workpiece at the jig anchor point is defined as the positioning normal line in this paper. According to the number of the positioning normal lines and the geometrical relationships between them, a set of geometrical theorems on determining the DOF of the workpiece are deduced. The judgement criterion for the distribution of the normal line in over-positioning condition is also provided. The DOF of the workpiece and the solutions to the unreasonable positioning are provided by using the geometric theorems in several examples, which proves that the geometric theorems can quickly and accurately determine the DOF of the workpiece in the complex positioning. The geometrical theorems of the workpiece positioning develop and perfect the positioning theory of the workpiece and can make the method of determining DOF more precise, simple and intuitive; furthermore, the geometrical theorems are beneficial to accurately determine the DOF of the workpiece in computer-aided fixture design.

A Study of Variation in MRR Influenced by Work Piece Positioning on Copper and Stainless Steel During Wet Chemical Machining

Wet Chemical Machining is one of the nonconventional machining processes, which is based on the principle of control corrosion phenomenon referred as redox reaction. Moreover, the need to manufacture the micro featured components is increasing continuously because of process capabilities and advancement in the manufacturing techniques. The paper deals with the study of material removal behavior and comparative study on copper and stainless steel sheet having thickness of 100 μ-m with influence of work piece positioning. FeCl3 is the chemical etchant used in process and dry photoresist film for both side coating. For the comparative study two different geometrical configurations of regular hexagons with sides 3 mm and 5 mm were selected. The experiments were performed with the optimized results of input parameters like concentration of 600 gm/lit and temperature of 54°C. It is seen that material removal rate is non-uniform from each hexagon and varies from the top to bottom of sheet. This non-uniformity encountered is due to positioning of work piece while etching. As material removal is non-uniform, the intermediate measurements of thickness variation and irregular area variation is done with plunger dial gauge and USB optical camera respectively with least count of 1 μ-m. Thus, from the variation in thickness and area in each of the hexagon the volume of material removed is calculated and influence of positioning the work piece is analysed. It is observed that the stainless steel sheets require three times more time compared to copper for etching same sheet thickness.