Development of an integrated robotic polishing system

This thesis presents research carried out as part of a project undertaken in fulfilment of the requirements of Loughborough University for the award of Philosophical Doctorate. The main focus of this research is to investigate and develop an appropriate level of automation to the existing manual finishing operations of small metallic components to achieve required surface quality and to remove superficial defects. In the manufacturing industries, polishing processes play a vital role in the development of high precision products, to give a desired surface finish, remove defects, break sharp edges, extend the working life cycle, and meet mechanical specification. The polishing operation is generally done at the final stage of the manufacturing process and can represent up to a third of the production time. Despite the growth automated technology in industry, polishing processes are still mainly carried out manually, due to the complexity and constraints of the process. Manual polishing involves a highly qualified worker polishing the workpiece by hand. These processes are very labour intensive, highly skill dependent, costly, error-prone, environmentally hazardous due to abrasive dust, and - in some cases - inefficient with long process times. In addition, the quality of the finishing is dependent on the training, experience, fatigue, physical ability, and expertise of the operator. Therefore, industries are seeking alternative solutions to be implemented within their current processes. These solutions are mainly aimed at replacing the human operator to improve the health and safety of their workforce and improve their competitiveness. Some automated solutions have already been proposed to assist or replace manual polishing processes. These solutions provide limited capabilities for specific processes or components, and a lack of flexibility and dexterity. One of the reasons for their lack of success is identified as neglecting the study and implementing the manual operations. This research initially hypothesised that for an effective development, an automated polishing system should be designed based on the manual polishing operations. Therefore, a successful implementation of an automated polishing system requires a thorough understanding of the polishing process and their operational parameters. This study began by collaborating with an industrial polishing company. The research was focused on polishing complex small components, similar to the parts typically used in the aerospace industry. The high level business processes of the polishing company were capture through several visits to the site. The low level operational parameters and the understanding of the manual operations were also captured through development of a devices that was used by the expert operators. A number of sensors were embedded to the device to facilitate recording the manual operations. For instance, the device captured the force applied by the operator (avg. 10 N) and the cycle time (e.g. 1 pass every 5 sec.). The capture data was then interpreted to manual techniques and polishing approaches that were used in developing a proof-of-concept Integrated Robotic Polishing System (IRPS). The IRPS was tested successfully through several laboratory based experiments by expert operators. The experiment results proved the capability of the proposed system in polishing a variety of part profiles, without pre-existing geometrical information about the parts. One of the main contributions made by this research is to propose a novel approach for automated polishing operations. The development of an integrated robotic polishing system, based on the research findings, uses a set of smart sensors and a force-position-by-increment control algorithm, and transpose the way that skilled workers carry out polishing processes

Geometrical Error Analysis and Correction in Robotic Grinding

The use of robots in industrial applications has been widespread in the manufacturing tasks such as welding, finishing, polishing and grinding. Most robotic grinding focus on the surface finish rather than accuracy and precision. Therefore, it is important to advance the technology of robotic machining so that more practical and competitive systems can be developed for components that have accuracy and precision requirement. This thesis focuses on improving the level of accuracy in robotic grinding which is a significant challenge in robotic applications because of the kinematic accuracy of the robot movement which is much more complex than normal CNC machine tools. Therefore, aiming to improve the robot accuracy, this work provides a novel method to define the geometrical error by using the cutting tool as a probe whilst using Acoustic Emission monitoring to modify robot commands and to detect surfaces of the workpiece. The work also includes an applicable mathematical model for compensating machining errors in relation to its geometrical position as well as applying an optimum grinding method to motivate the need of eliminating the residual error when performing abrasive grinding using the robot. The work has demonstrated an improved machining precision level from 50µm to 30µm which is controlled by considering the process influential variables, such as depth of cut, wheel speed, feed speed, dressing condition and system time constant. The recorded data and associated error reduction provide a significant evidence to support the viability of implementing a robotic system for various grinding applications, combining more quality and critical surface finishing practices, and an increased focus on the size and form of generated components. This method could provide more flexibility to help designers and manufacturers to control the final accuracy for machining a product using a robot system

DEVELOPMENT OF A LOW COST PRECISION POLISHING MACHINE BASED ON PARALLEL KINEMATIC SYSTEM

The increasing demand on mass production of high precision parts, has pushed the precision manufacturing industry to develop reliable precision finishing processes such as Bonnet polishing to address market requirements. Indeed, the nature of the surface to be polished plays an important role in the design of a possible polishing machine. A gap within the research in polishing for precision industry needs has been identified. Small parts with <50mm x 50mm and possible freeform curvature containing small slopes cannot be polished with available bonnet polishing (BP) processes on market. This is caused by the tool head size and the tool holder being bigger than part curvature or the part itself. Although, the BP process has a huge potential for surface roughness improvement and form accuracy, it is generally seen in industry as an expensive solution for a non-deterministic finishing process. Therefore, this project has sought to develop a BP machine to cover the gap with an innovative and inexpensive design. In order to develop a machine which responded to the market expectations all possible requirements were listed from a customer point of view. Based on the requirement, a machine concept was produced. Market analysis helped to identify sub-systems of the machine. FEA analysis of the design was performed to check for stress distribution and displacement due to its own mass. Additional assembly parts are designed and a prototype of the machine was produced. The designed machine is tested for its ability as precision polishing machine. Flat surfaces of P20 tool steel were targets for polishing to nanometric surface finishes. Empirical experiments helped to identify parameters which influenced the surface roughness. Taguchi method were then used to optimise the parameters for better surface roughness. Optimum parameters conditions helped to reach less than 10 nm Ra systematically and repeatedly. The samples were also polished using re-circulating slurry techniques, and the obtained results were discussed. Further, pre polishing, Grolishing processes capable of improving surface roughness from ground finish to mirror like finish were developed for cost effective manufacturing procedures. The material removal was analysed to identify parameters capable of improving surface roughness over a step grolishing process. Two grolishing procedures were developed. Both processes produced nanometric range surface finishes. Other variations in results were compared and discussed. Although, machine axis has the ability to produce freeform movement, tool holders need to be improved to facilitate the identification of the distance between tool origin and workpiece origin. Therefore, a new spindle holder assembly is produced to hold the tool and an optical measurement device DRI used to evaluate accurately the distance separating the tool-workpiece origin and further align the workpiece inclination with respect to the machine axis. A CAD-CAM package is also developed to generate programme capable of performing freeform curvature

The Application of Zeeko Polishing Technology to Freeform Femoral Knee Replacement Component Manufacture

The purpose of this study was to develop an advanced 7-axis Computer Numerical Controlled (CNC) Polishing Machine from its successful original application of industrial optics manufacture into a process for the manufacture of femoral knee components to improve wear characteristics and prolong component lifetimes. It was indentified that the successful manufacture of optical components using a corrective polishing procedure to enhance their performance could be applied to femoral knee implant components. Current femoral knee implants mimic the natural shape of the joint and are freeform (no axis of symmetry) in nature hence an advanced CNC polishing machine that can follow the contours associated with such shapes could improve surface finish and conformity of replacement femoral knee bearing surfaces, leading to improved performance. The process involved generating machine parameters that would optimize the polishing procedure to minimize wear of materials used in femoral knee implant manufacture. Secondly a design of a Non-Uniform Refind B-Spline (NURBS) model for control of the Polishing Machine over the freeform contours of the femoral component. Completing the process involved development of a corrective polishing process that would improve form control of the components. Such developments would improve surface finish and conformity which are well documented contributors to wear and hence the lifeline of orthopaedic implants. By the means of comparison of this technique to that of a conventional finishing technique using pin-on-plate disc testing it was concluded that performance of the CNC polished components was an improvement on that of the conventional technique. In the case of form control their were slight indications through small decreases in peak to valley (PV) error that the process helped reduce form error and could increase the lifetime of femoral knee replacement components. The overall study provided results that indicate the the Zeeko process could be used in the application of polishing of hard-on-hard material combinations to improve form control without compromising surface finish hence improving lifetimes of the implant. The results have their limitations in the fact that the wear test performance was only carried out on orthopaedic implant materials using a pin-on-plate wear test rig. Due to the time limitations on the thesis it can be said that further analysis of correcting form without compromising surface finish on entire implant systems under full joint simulator testing which would provide mre realistic contitions would a more definitive answer be achieved

Design and Optimization of a Robot for Abrasive Waterjet Polishing of Hydraulic Turbine Blades

RÉSUMÉ Dans l’industrie de fabrication de turbine hydraulique, toutes les surfaces de turbines qui sont en contact avec de l’eau devraient être polies afin d’obtenir la qualité et l’efficacité maximales. Pour cela, il est nécessaire d’utiliser une méthode de polissage qui peut avoir accès à toutes les surfaces des turbines incluant leurs bords, leurs zones restreintes et leurs courbures serrées. En raison des propriétés particulières qu’offre la technique de polissage par jet d’eau abrasif, celle-ci peut être utilisée pour accomplir cette tâche. Par conséquent, dans cette recherche, les propriétés de cette méthode non-conventionnelle sont examinées dans un premier temps et les principaux paramètres affectant ses performances sont alors déterminés. Ensuite, les conditions nécessaires de manipulations de la buse de pulvérisation vis-à-vis des surfaces courbes sont étudiées et les propriétés d’un bras robotisé pour manipuler celle-ci sont obtenues afin de réaliser cette tâche d’une manière appropriée. Par après, plusieurs mécanismes robotiques tels que des mécanismes sériels, parallèles à membrures, parallèles à câbles, et des robots hybrides sont considérés et leurs capacités à être utilisé dans ce processus sont analysées. Il est alors démontré qu’une l’architecture hybride est le meilleur candidat à retenir pour le design d’un robot de polissage par jet d’eau abrasif. Ensuite, l’architecture conceptuelle d’un robot hybride à 5 DDL est proposée. La structure du robot est constituée d’un mécanisme parallèle à câbles à 3 DDL et d’un poignet sériel à 2 DDL. Afin d’améliorer les propriétés cinématiques du mécanisme à câbles tout en minimisant le nombre d’actionneurs nécessaires, il est proposé d’utiliser des différentiels pour guider ce robot manipulateur. Aussi, la rigidité et la compacité du mécanisme sont améliorées en utilisant une liaison prismatique. Par la suite, les systèmes à câbles différentiels sont examinés et les différences entre leurs propriétés cinématiques et celles de systèmes actionnés indépendamment pour chaque câble sont décrites. Il est démontré que la force résultante de tous les câbles d’un différentiel à câbles doit être prise en compte dans son analyse cinématique. En effet, dans un système différentiel planaire, la direction de la force résultante n’est pas fixée vers un point particulier. Mais plutôt, elle se déplace dans le plan de ce système différentiel. Cette propriété peut être bénéfique pour les propriétés cinématiques des robots à câbles. En comparant deux types d’espace de travail de plusieurs robots planaires actionnés par des mécanismes différentiels par rapport à leurs équivalents pleinement actionnés, il est alors montré qu’en utilisant ces mécanismes, les espaces de travail des robots planaires à câbles peuvent être améliorés. Cependant, cette même propriété qui augmente la plage de variation de la direction de la force résultante dans un câble différentiel, diminue aussi son amplitude. Ainsi, le design optimal d’un différentiel à câble résulte d’un compromis entre ces deux propriétés.----------ABSTRACT In hydraulic turbine manufacturing, all surfaces of the turbines which are in contact with the water flow should be polished to obtain the desired quality and maximal efficiency. For this, it is needed to use an effective polishing method which can have access to all surfaces of the turbines including edges, narrow areas and tight bends. Because of the particular properties of the abrasive waterjet polishing technique, it can be used to accomplish this task. Therefore, in this research, the properties of this non-conventional method are first investigated and the main parameters affecting its performance are then determined. Next, the manipulation requirements of the jet nozzle over free-form surfaces are studied and the properties of a robotic arm to appropriately perform this task are obtained. Afterwards, several robotic mechanisms, e.g., serial, linkage-driven parallel, cabledriven parallel, and hybrid robots are considered and their abilities to be used in this process are investigated. It is then shown that a hybrid architecture is the best candidate for the design of an abrasive waterjet polishing robot. Next, the conceptual design of a 5-DOF hybrid robot is proposed. The structure of this robot is made of a 3-DOF cable-driven parallel mechanism and a 2-DOF serial wrist. To improve the kinematic properties of the cable-driven mechanism while the number of required actuators is kept at a minimum, it is proposed to use cable differentials to drive this manipulator. Also, the rigidity and compactness of the mechanism is improved through the use of a prismatic joint in its structure. Afterwards, differentially driven cable systems are investigated and the differences between their kinematic properties and these of independently actuated cables are described. It is shown that the resultant force of all cables of a cable differential should be taken into account in its kinematic analysis. Indeed, in a planar differential, the direction of the resultant force is not fixed toward a particular point. Instead, it moves within the plane of that differential. This property can be beneficial in the kinematic properties of differentially driven cable robots. By comparing two types of workspaces of several planar robots actuated by differentials with their fully actuated counterparts, it is then shown that using these mechanisms, these workspaces of planar cable robots can be improved. However, the same property that increases the range of variation of the resultant force direction in a cable differential, decreases its magnitude. Thus, the optimal design of a cable differential is a trade-off between these two properties. Next, a synthesis method is presented to find all possible arrangements of the cable differentials to generalize the idea of using such mechanisms in the design of planar cable robots. Additionally, the application of differentials in spatial robots is also investigated and it is shown that they have properties similar to the planar types

Behavioural Study of the Force Control Loop Used in a Collaborative Robot for Sanding Materials

[EN] The rise of collaborative robots urges the consideration of them for different industrial tasks such as sanding. In this context, the purpose of this article is to demonstrate the feasibility of using collaborative robots in processing operations, such as orbital sanding. For the demonstration, the tools and working conditions have been adjusted to the capacity of the robot. Materials with different characteristics have been selected, such as aluminium, steel, brass, wood, and plastic. An inner/outer control loop strategy has been used, complementing the robot¿s motion control with an outer force control loop. After carrying out an explanatory design of experiments, it was observed that it is possible to perform the operation in all materials, without destabilising the control, with a mean force error of 0.32%. Compared with industrial robots, collaborative ones can perform the same sanding task with similar results. An important outcome is that unlike what might be thought, an increase in the applied force does not guarantee a better finish. In fact, an increase in the feed rate does not produce significant variation in the finish¿less than 0.02 m; therefore, the process is in a ¿saturation state¿ and it is possible to increase the feed rate to increase productivity.Rodrigo Perez-Ubeda is grateful to the Ph.D. Grant CONICYT PFCHA/Doctorado Becas Chile/2017-72180157 and the University of Antofagasta, Chile.Pérez Ubeda, R.; Gutiérrez, SC.; Zotovic Stanisic, R.; Perles, A. (2020). Behavioural Study of the Force Control Loop Used in a Collaborative Robot for Sanding Materials. Materials. 14(1):1-19. https://doi.org/10.3390/ma14010067S11914