[Show abstract][Hide abstract] ABSTRACT: Abstract
Camera based systems have been a preferred choice in many motion tracking applications due to the ease of installation and the ability to work in unprepared environments. The concept of these systems is based on extracting image information (colour and shape properties) to detect the object location. However, the resolution of the image and the camera field-of- view (FOV) are two main factors that can restrict the tracking applications for which these systems can be used. Resolution can be addressed partially by using higher resolution cameras but this may not always be possible or cost effective.
This research paper investigates a new method utilising averaging of offset images to improve the effective resolution using a standard camera. The initial results show that the minimum detectable position change of a tracked object could be improved by up to 4 times.
[Show abstract][Hide abstract] ABSTRACT: The reduction of the lead time in measurement and reverse engineering, and the increased requirements in terms of accuracy and flexibility, have resulted in a great deal of research effort aimed at developing and implementing multi-sensor systems. This paper describes an effective competitive approach for using a tactile probe to compensate the data from a laser line scanner to perform accurate reverse engineering of geometric features. With the data acquired using laser scanning, intelligent feature recognition and segmentation algorithms can be exploited to extract the global surface information of the object. The tactile probe is used to re-measure the geometric features with a small number of sampling points and the obtained information can be subsequently used to compensate the point data patches which are measured by laser scanning system. Then, the compensated point data can be exploited for accurate reverse engineering of a CAD model. The limitations of each measurement system are compensated by the other. Experimental results on three parts validate the rapidity and accuracy of this multi-sensor data fusion approach.
Preview · Article · Sep 2014 · International Journal of Advanced Manufacturing Technology
[Show abstract][Hide abstract] ABSTRACT: There is a requirement for improved 3D surface characterisation and reduced tool wear, when modern computer numerical-controlled (CNC) machine tools are operating at high cutting velocities, spindle speeds and feed-rates. This research project investigates vibration-induced errors on a CNC vertical machining centre under dynamic conditions. A model of the machine structural dynamics is constructed using the Finite Element Method (FEM) for the comprehensive analytical investigation of the machine vibration behaviour. The analytical model is then validated against the measured results obtained from an experimental modal analysis (EMA) investigation. A correlation analysis of the simulated and experimental modal analysis results is undertaken in order to improve the accuracy of the model and minimise modelling practice errors. The resulting optimised model will need further sensitivity analysis utilising parametric structural analysis and characterisation techniques in order to identify a potential for vibration reduction using passive methods.
No preview · Article · Aug 2014 · ARCHIVE Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science 1989-1996 (vols 203-210)
[Show abstract][Hide abstract] ABSTRACT: This paper describes evaluation of a method of measuring the straightness of motion of machine tool axes using a taut wire and an optical sensor head mounted at the tool point location. In contrast to commonly used taut wire instruments, straightedges or laser-based methods, this solution combines low cost, simplicity of setup and automated data capture while achieving state of the art accuracy suitable for application on precision machine tools. A series of tests are discussed which examine the performance of the new sensing head and different wires which highlight the suitability of the taut wire properties as a straightness reference. Experimental results obtained on a production machine tool are provided with respect to the accuracy and repeatability of both the proposed taut wire system and a laser interferometer operated under the same conditions. The reference errors of wires made of different materials are compared and the wire catenary is separated from the measurement results. The uncertainty budget for taut wire and laser systems is presented and expanded uncertainty of 4 μm obtained for both. During the experiment, the method showed excellent repeatability with two standard deviations of 1.5 μm over a measuring range of 1.5 m; this performance matches that of a commercial laser interferometer-based straightness reference to within 0.1 μm.
[Show abstract][Hide abstract] ABSTRACT: To meet the requirement of both high speed and high accuracy 3D measurements for dimensional metrology, multi-sensor measuring systems have been developed to measure, analyse and reverse engineer the geometry of objects. This paper presents a new development in coordinate unification called the “centroid of spherical centres” method, which can be used instead of the traditional method which uses three datum-points to perform the geometric transformation and unification of tactile and optical sensors. The benefits of the proposed method are improved accuracy in coordinate unification and the method is used to integrate a coordinate measuring machine (CMM) and optical sensors (structured light scanning system and FaroArm laser line probe). A sphere-plate artefact is developed for data fusion of the multi-sensor system and experimental results validate the accuracy and effectiveness of this method.
No preview · Article · Apr 2014 · Optics and Lasers in Engineering
[Show abstract][Hide abstract] ABSTRACT: This paper proposes and discusses an active dual-sensor autofocusing method for measuring the positioning errors of arrays of small holes on complex curved surfaces. The dual-sensor unit combines an optical vision sensor and a tactile probe and is designed to achieve rapid automated measurements in a way that can be adapted to be suitable for deployment on a manufacturing machine tool. Mathematical analysis is performed to establish the magnitude of the deviation from the optimal focal length that is induced by the autofocussing method. This evaluation is based on the geometrical relationship and interaction between the radius of the tactile probe with both the measured holes and the complex-curved surface. A description is provided of a laboratory-based standalone dual-sensor autofocusing unit and test rig that was built to perform experimental validation of the method. This system is estimated to have a focusing uncertainty of 11 μm deriving mainly from the inaccuracy of the X-Z translation stage and the maximum permissible error of the tactile probe.
A case study is presented which evaluates the accuracy of a pattern of Ø 0.5 mm small holes on an elliptic cylinder. A mathematical analysis of that problem and practical results from both the tactile and optical sensors are provided and discussed. It is estimated that the deviation in optimal focusing induced by this automated method is between -23 μm and +95 μm. This is sufficiently accurate to ensure that the optical device can capture the entire space outline of each of the small holes on the complex curve surface clearly and can therefore identify its centroid from the image to provide a measurement of the position.
No preview · Article · Apr 2014 · Sensors and Actuators A Physical
[Show abstract][Hide abstract] ABSTRACT: This paper presents and evaluates an active dual-sensor autofocusing system that combines an optical vision sensor and a tactile probe for autofocusing on arrays of small holes on freeform surfaces. The system has been tested on a two-axis test rig and then integrated onto a three-axis computer numerical control (CNC) milling machine, where the aim is to rapidly and controllably measure the hole position errors while the part is still on the machine. The principle of operation is for the tactile probe to locate the nominal positions of holes, and the optical vision sensor follows to focus and capture the images of the holes. The images are then processed to provide hole position measurement. In this paper, the autofocusing deviations are analyzed. First, the deviations caused by the geometric errors of the axes on which the dual-sensor unit is deployed are estimated to be 11 μm when deployed on a test rig and 7 μm on the CNC machine tool. Subsequently, the autofocusing deviations caused by the interaction of the tactile probe, surface, and small hole are mathematically analyzed and evaluated. The deviations are a result of the tactile probe radius, the curvatures at the positions where small holes are drilled on the freeform surface, and the effect of the position error of the hole on focusing. An example case study is provided for the measurement of a pattern of small holes on an elliptical cylinder on the two machines. The absolute sum of the autofocusing deviations is 118 μm on the test rig and 144 μm on the machine tool. This is much less than the 500 μm depth of field of the optical microscope. Therefore, the method is capable of capturing a group of clear images of the small holes on this workpiece for either implementation.
[Show abstract][Hide abstract] ABSTRACT: This paper presents an evaluation method for the rapid and automatic detection of position errors of arrays of small holes on complex-curved and freeform surfaces that can satisfy the special demands of the aviation and automobile industries. The evaluation is based on the dual-sensor autofocusing method. The dual-sensor unit is the combination of a tactile probe and an optical vision sensor. The tactile probe detects the focal position for the optical vision sensor by probing the distance between the objective lens of the microscope and the location of each small hole. The optical vision sensor focuses to this position for capturing the image of the artifact under inspection. As a case study, a pattern of φ 0.5 mm small holes centripetally drilled with equal-angular distribution on the circumference of an elliptical cylinder shell is investigated. The autofocusing errors caused by the radius of the tactile probe and the position errors of the small holes are evaluated mathematically. Subsequently, a standalone dual-sensor autofocusing unit is built and integrated into a user-controllable 3D coordinate test rig. It is used to autofocus and capture the images of small holes. The centroid positions and deviations of the holes are automatically and rapidly detected from the captured images.
Full-text · Article · Feb 2014 · International Journal of Precision Engineering and Manufacturing
[Show abstract][Hide abstract] ABSTRACT: Manufacturing of high-quality components and assemblies is clearly recognised by industrialised nations as an important means of wealth generation. A "right first time" paradigm to producing finished components is the desirable goal to maximise economic benefits and reduce environmental impact. Such an ambition is only achievable through an accurate model of the machinery used to shape the finished article. In the first analysis, computer aided design (CAD) and computer aided manufacturing (CAM) can be used to produce an instruction list of three-dimensional coordinates and intervening tool paths to translate the intent of a design engineer into an unambiguous set of commands for a manufacturing machine. However, in order for the resultant manufacturing program to produce the desired output within the specified tolerance, the model of the machine has to be sufficiently accurate. In this paper, the spatial and temporal sources of error and various contemporary means of modelling are discussed. Limitations and assumptions in the models are highlighted and an estimate of their impact is made. Measurement of machine tools plays a vital role in establishing the accuracy of a particular machine and calibrating its unique model, but is an often misunderstood and misapplied discipline. Typically, the individual errors of the machine will be quantified at a given moment in time, but without sufficient consideration either for the uncertainty of individual measurements or a full appreciation of the complex interaction between each independently measured error. This paper draws on the concept of a "conformance zone", as specified in the ISO 230:1-2012, to emphasise the need for a fuller understanding of the complex uncertainty of measurement model for a machine tool. Work towards closing the gap in this understanding is described and limitations are noted.
No preview · Article · Jan 2014 · International Journal of Metrology and Quality Engineering
[Show abstract][Hide abstract] ABSTRACT: This study presents a novel application of the Transmission Line Matrix Method (TLM) for the modelling of the dynamic behaviour of non-linear hybrid systems for computer numerical control (CNC) machine tool drives. The application of the TLM technique implies the dividing of the ball-screw shaft into a number of identical elements in order to achieve the synchronisation of events in the simulation, and to provide an acceptable resolution according to the maximum frequency of interest. This entails the use of a high performance computing system with due consideration to the small time steps being applied in the simulation. Generally, the analysis of torsion and axial dynamic effects on a shaft implies the development of independent simulated models. This study presents a new procedure for the modelling of a ball-screw shaft by the synchronisation of the axial and torsion dynamics into the same model. The model parameters were obtained with equipments such as laser interferometer, ball bar, electronic levels, signal acquisition systems, etc. The MTLM models for single and two-axis configurations have been simulated and matches well with the measured responses of machines. The new modelling approach designated the Modified Transmission Line Method (MTLM) extends the TLM approach retaining all its inherent qualities but gives improved convergence and processing speeds. Further work since, not the subject of this paper, have identified its potential for real-time application.
Preview · Article · Jan 2014 · ARCHIVE Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science 1989-1996 (vols 203-210)
[Show abstract][Hide abstract] ABSTRACT: Abstract
The application of industrial robots for machining of complex parts is a very promising field that refers to the ability of the robot to produce parts with complex geometries that typically requires multi-axes machine tools. However, there are many restrictions that limit the use of robot in advanced manufacturing industry such as positioning errors and low stiffness. This work investigates the positioning accuracy and the path repeatability of a Mitsubishi RV-1A robot with six degree of freedom during machining movements by using a Faro laser tracker. The results obtained show a significant motion error caused by the robot, even with the absence of cutting force. A comparison has been made between the positions of the commanded path measurements taken by the robot controller and the actual position of the end effector of the robot measured by the tracker.
[Show abstract][Hide abstract] ABSTRACT: Reducing downtime and assuring a high degree of accuracy of production machines, especially machine tools, have become increasingly important as the demand for higher production rates and closer tolerance continues to grow. The growing understanding of the importance of both calibration and maintenance in the evolving industrial scenario and the technological advancements of recent years has yielded the development of advanced metrology equipment and predictive maintenance programs. Predictive maintenance and similar programmes are tools that have been designed to reduce downtime by avoiding unpredictable machine failures. These programmes have been adopted in some industries to improve operational efficiency and reduce machine breakdown. However, extensive diagnostic procedures can take machines out of service for longer periods than are acceptable for some manufacturers.
Studies in the field of predictive maintenance have resulted in cost calculations for the downtime associated with machine failure. Models have been presented to determine optimal intervals between repairs by minimising global maintenance costs. However, very little work has concentrated on optimising the frequency of machine tool calibration by assessing the downtime cost considering the contribution of both technical and commercial factors.
This paper give an introduction to causes of machine tools failures with respect to production of non-conforming parts and the importance of calibration and then it addresses the key factors to a cost function parameters that forms the basis of a strategy for scheduling machine tool calibration which takes into account these influences on part tolerance.
[Show abstract][Hide abstract] ABSTRACT: This research work describes an intelligent and modular architecture for controlling the milling process. For this purpose, it is taking into account the admissible input cutting parameters given by the stability lobes and restrictions of the system, calculating the quasi-optimal cutting parameters. The quasi-optimal cutting parameters are obtained considering a cost function with multi-objective purpose. Furthermore, parallel multi-estimation adaptive control architecture is proposed in order to allow adaptation of the system when the cutting parameters are changed, for example, due to production requirements. It incorporates an intelligent supervisory system to address the problem of choosing the most adequate control signal at each required time. The fundamental idea of the control system is to work automatically, with a simple interface with the operator, based around the admissible cutting parameter space given by the well-known stability lobes.
[Show abstract][Hide abstract] ABSTRACT: Thermal errors are often quoted as being the largest contributor to inaccuracy of CNC machine tools, but they can be effectively reduced using error compensation. Success in obtaining a reliable and robust model depends heavily on the choice of system variables involved as well as the available input-output data pairs and the domain used for training purposes. In this paper, a new prediction model “Grey Neural Network model with Convolution Integral (GNNMCI(1, N))” is proposed, which makes full use of the similarities and complementarity between Grey system models and Artificial Neural Networks (ANNs) to overcome the disadvantage of applying a Grey model and an artificial neural network individually. A Particle Swarm Optimization (PSO) algorithm is also employed to optimize the Grey neural network. The proposed model is able to extract realistic governing laws of the system using only limited data pairs, in order to design the thermal compensation model, thereby reducing the complexity and duration of the testing regime. This makes the proposed method more practical, cost-effective and so more attractive to CNC machine tool manufacturers and especially end-users.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a novel control architecture system which is composed of a multi-objective cost function which Pareto optimizes the programming of cutting parameters while controlling the milling process to new cutting conditions if new constraints appear. The paper combines a self-optimized module which looks for and finds Pareto optimal cutting parameters according to multi-objective purposes and, a multi-model control module which keeps the cutting forces under prescribed upper safety limits independently of programmed cutting conditions and material properties while maintaining the performance of the process. An intelligent algorithm acts as decision supportsoftware to automatically switch to the best performance tracking controller among those available at each required time.
[Show abstract][Hide abstract] ABSTRACT: Machine tool failures in industrial organisations disturb production operations and cause production loss. Predictive maintenance is one approach which has been successfully applied in some circumstances to allow scheduled production stoppages. It is an approach that reduces the need for reactive maintenance. Predictive maintenance is a tool that has been adopted in some industries to improve operational efficiency and reduce maintenance cost. As a result, monitoring equipment providing information about the systems conditions have evolved rapidly over the last years.
Machine tools can change or drift over time and usage in both their mechanical and electrical performance and so reduce in accuracy. This paper proposes a new method for maintaining machine tool accuracy that is complimentary to the predictive maintenance paradigm. This strategy, called predictive calibration, is a methodology that depends on the prediction of the degradation in machine tool accuracy based upon regular data capture.
Although introducing such a strategy will introduce a new cost, the aim is to offset this investment by optimising the operational efficiency and reduce the downtime cost. The main objective is achieved by monitoring the condition of the machine tool by collecting data using quick check measurement techniques or post-process quality data. Calibration should, therefore, be driven by the data measured from either the machine or the part. Building a database of inspection history by measuring the machine on a regular basis with relatively non-invasive methods will make the decision of scheduling extensive calibration accurate better informed process.
The project presents a new method of identifying new boundaries of machine tool working tolerance. These boundaries of tolerance reflect the degradation level corresponding to production capacities and the quality of the part produced.
The significance of this work is that machine tool accuracy is critical for high value manufacturing. Over-measuring the machine to ensure accuracy reduces productivity. This piece of work seeks to optimise the frequency of calibration to reduce unnecessary downtime while maintaining the machine at the required tolerance.
[Show abstract][Hide abstract] ABSTRACT: This paper describes a novel method to measure straightness error of an axis of motion with a system utilising taut wire, optical sensor and reference error cancellation technique. In contrast to commonly used taut wire, straightedge or laser-based methods it combines simplicity of setup and low cost with high levels of automation, accuracy and repeatability. An error cancellation technique based on two-point method is applied for the first time to a versatile reference object which can be mounted at any place of machine's working volume allowing direct measurement of motion straightness of a tool point. Experimental results on a typical machine tool validate performance of the proposed taut wire system with a commercial laser interferometer operating in the same conditions is used as a reference. The proposed method shows highly repeatable results of better than ±0.25 μm over the range of 0.48 m and measurement accuracy comparable to the interferometer of ±0.5 μm.
No preview · Article · Aug 2013 · Optics and Lasers in Engineering
[Show abstract][Hide abstract] ABSTRACT: In this paper, discrete time model reference control schemes for practical milling using different discretization of the continuous-time plant are presented. First, a basic controller scheme is addressed where a fractional order hold with pre-fixed value of the gain is used. Secondly, a multi-model scheme, which outputs different discretization in parallel with the continuous-time milling system transfer function under a fractional order hold (FROH) of correcting β ∈ [−1, 1], is dealt with. Then, an intelligent design framework is designed as a supervisory scheme with two hierarchical levels in order to find the most appropriate value for the gain β. For choosing the value of β, a tracking performance index is designed. It evaluates each pre-defined discretization of the continuous time milling transfer function and the scheme chooses the one with the smallest value of the index in order to generate the real control input to the plant. Two different methods of adjusting this value are presented and discussed. The first one selects a β-value among a fixed pre-defined set of possible values, while the second one the value of β is updated by adding or subtracting a small quantity.
Full-text · Article · May 2013 · International Journal of Precision Engineering and Manufacturing
[Show abstract][Hide abstract] ABSTRACT: Machine tools are susceptible to exogenous influences, which mainly derive from varying environmental conditions such as the day and night or seasonal transitions during which large temperature swings can occur. Thermal gradients cause heat to flow through the machine structure and results in non-linear structural deformation whether the machine is in operation or in a static mode. These environmentally stimulated deformations combine with the effects of any internally generated heat and can result in significant error increase if a machine tool is operated for long term regimes. In most engineering industries, environmental testing is often avoided due to the associated extensive machine downtime required to map empirically the thermal relationship and the associated cost to production. This paper presents a novel offline thermal error modelling methodology using finite element analysis (FEA) which significantly reduces the machine downtime required to establish the thermal response. It also describes the strategies required to calibrate the model using efficient on-machine measurement strategies. The technique is to create an FEA model of the machine followed by the application of the proposed methodology in which initial thermal states of the real machine and the simulated machine model are matched. An added benefit is that the method determines the minimum experimental testing time required on a machine; production management is then fully informed of the cost-to-production of establishing this important accuracy parameter. The most significant contribution of this work is presented in a typical case study; thermal model calibration is reduced from a fortnight to a few hours. The validation work has been carried out over a period of over a year to establish robustness to overall seasonal changes and the distinctly different daily changes at varying times of year. Samples of this data are presented that show that the FEA-based method correlated well with the experimental results resulting in the residual errors of less than 12 μm.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a novel control architecture system which is composed of a multi-objective cost function which Pareto optimises the programming of cutting parameters while adapting the milling process to new cutting conditions if new constraints appear.
The paper combines a self-optimised module which looks for and finds Pareto optimal cutting parameters according to multi-objective purposes and, a multi-estimation adaptive control module which keeps the cutting forces under prescribed upper safety limits independently of programmed cutting conditions and material properties while maintaining the performance of the process. A supervised controller acts as decision support-software to automatically switch to best performance tracking adaptive controller among those available at each required time.