EFFICIENCY OF FLEXIBLE FIXTURES: DESIGN AND CONTROL

The manufacturing industries have been using flexible production technologies to meet the demand for customisation. As a part of production, fixtures have remained limited to dedicated technologies, even though numerous flexible fixtures have been studied and proposed by both academia and industry. The integration of flexible fixtures has shown that such efforts did not yield the anticipated performance and resulted in inefficiencies of cost and time. The fundamental formulation of this thesis addresses this issue and aims to increase the efficiency of flexible fixtures.To realise this aim, the research in this thesis poses three research questions. The first research question investigates the efficiency description of flexible fixtures in terms of the criteria. Relative to this, the second research question investigates the use of efficiency metrics to integrate efficiency criteria into a design procedure. Once the efficiency and design aspects have been established, the third research question investigates the active control of flexible fixtures to increase their efficiency. The results of this thesis derive from the outcome of seven studies investigating the automotive and aerospace industries. The results that answer the first research question use five criteria to establish the efficiency of flexible fixtures. These are: fundamental, flexibility, cost, time and quality. By incorporating design characteristics in respect of production system paradigms, each criterion is elaborated upon using relevant sub-criteria and metrics. Moreover, a comparative design procedure is presented for the second research question and comprising four stages (including mechanical, control and software aspects). Initially, the design procedure proposes conceptual design and verification stages to determine the most promising flexible fixture for a target production system. By executing detailed design and verification, the design procedure enables a fixture designer to finalise the flexible fixture and determine its efficiency. Furthermore, a novel parallel kinematics machine is presented to demonstrate the applicability of the design procedure’s analytical steps and illustrate how appropriate kinematic structures can facilitate the efficiency-orientated design of flexible fixtures.Based on the correlation established by the controller software’s design procedure, the active control of flexible fixtures directly affects the quality criterion of flexible fixture efficiency. This provides the answer to the third research question, on general control strategies for active control of flexible fixtures. The introduction of a system model and manipulator dynamics proposes force and position control strategies. It is shown that any flexible fixture using a kinematic class can be controlled, to regulate the force and position of a workpiece and ensure that process nominals are preserved. Moreover, using both direct and indirect force control strategies, a flexible fixture’s role in active control can be expanded into a system of actively controlled fixtures that are useful in various processes. Finally, a position controller is presented which has the capacity to regulate both periodic and non-periodic signals. This controller uses an additional feedforward scheme (based on the Hilbert transform) in parallel with a feedback mechanism. Thus, the position controller enables flexible fixtures to regulate the position of a workpiece in respect of any kind of disturbance

Integrated Assessment of Assembly fixtures Re-configurability

The needs of consumers are changing over time. As a result, the manufacturers are looking for new methods to adapt effectively and efficiently to market changes. These involve supplying customers with a variety of products in a reasonable time with decreasing the cost. Reconfigurable fixtures are an important means for dealing with increased product variety and shorter life cycles, as they help change between the product variants effectively and decrease the time and resources required to introduce new product variants. In this thesis, an integrated method to assess the reconfigurability of assembly fixtures is developed. This assessment is based on four core reconfigurability characteristics: scalability, modularity, convertibility, and customized flexibility. A clear definition of the scalability of the reconfigurable assembly fixtures was developed. A mathematical model for each characteristic of reconfigurable assembly fixtures was developed. Their indices were determined then combined using a radar plot to assess the reconfigurability of the reconfigurable assembly fixture. Welding tack fixture is chosen as a case study in this thesis. Two redesign recommendations were proposed. The results showed the most appropriate design with highest reconfigurability index because it was designed to produce the same number of product variants with less reconfiguration time, cost, effort, and complexity. The significance of research in this thesis is to help in the design stage of the assembly fixture by comparing different configurations for the assembly fixture to choose the best one and suggesting some changes for the assembly fixture design and configuration. This is essential to minimize the number of fixtures to be produced when the new part component/ variant is introduced

Recent research on flexible fixtures for manufacturing processes

Fixtures, are used to fixate, position and support workpieces, and form a crucial tool in manufacturing. Their performance influences the manufacturing (and assembly) process of a product. Furthermore, fixturing can form a significant portion of the needed investment and total process planning time for the manufacturing system. Many fixturing concepts, as contribution to increase the flexibility of the manufacturing system, are reported in the literature. The flexible fixturing designs can be classified into the following seven categories: modular fixtures, flexible pallet systems, sensor-based fixture design, phase-change based concepts, chuck-based concepts, pin-type array fixtures and automatically reconfigurable fixtures. It is observed that the more intelligent and automated fixturing systems are designed with the demands for automation in certain industries in mind. Furthermore, different fixturing solutions suit the engineering demands for different manufacturing areas, this means that for the foreseeable future all technologies will remain current. From the self-reconfigurable fixturing techniques a new fixturing capability is emerging: in process reconfigurability for the optimal placement of clamps and supports during the whole process time. These several concepts together with some recent patents are studied here. The paper concludes with some prospective research directions in the field of flexible fixturing

Active fixturing: literature review and future research directions

Fixtures are used to fixate, position and support workpieces and represent a crucial tool in manufacturing. Their performance determines the result of the whole manufacturing process of a product. There is a vast amount of research done on automatic fixture layout synthesis and optimisation and fixture design verification. Most of this work considers fixture mechanics to be static and the fixture elements to be passive. However, a new generation of fixtures has emerged that has actuated fixture elements for active control of the part–fixture system during manufacturing operations to increase the end product quality. This paper analyses the latest studies in the field of active fixture design and its relationship with flexible and reconfigurable fixturing systems. First, a brief introduction is given on the importance of research of fixturing systems. Secondly, the basics of workholding and fixture design are visited, after which the state-of-the-art in active fixturing and related concepts is presented. Fourthly, part–fixture dynamics and design strategies which take these into account are discussed. Fifthly, the control strategies used in active fixturing systems are examined. Finally, some final conclusions and prospective future research directions are presented

Deriving a systematic approach to changeable manufacturing system design

It has long been argued that Factories are long life and complex products. The complexity of designing factories, and their underlying manufacturing systems, is further amplified when dealing with continuously changing customer demands. At the same time, due to research fragmentation, little if any scientific explanations are available supporting and exploiting the paradigm that "factories are products". In order to address this weakness, this paper presents research results arising from a comparative analysis of systematic "product design" and "manufacturing system design" approaches. The contribution emerging from this research is an integrated systematic design approach to changeable manufacturing systems, based on scientific concepts founded upon product design theories, and is explained through a case study in the paper. This research is part of collaboration between the CERU University of Malta and IAO Fraunhofer aimed at developing a digital decision support tool for planning changeable manufacturing systems.peer-reviewe

Reconfigurable Manufacturing Potential in Small and Medium Enterprises with Low Volume and High Variety:Pre-design Evaluation of RMS

AbstractGlobal competition forces companies to respond to fast changing market conditions by introducing new and innovative products rapidly and more often at competitive prices. Meeting these challenges sets strict requirements in order to cope with the variety of products during both ramp-up and production. The Reconfigurable Manufacturing System meets these challenges of high variety by adapting to the capacity and functionality needed when needed. Thus, implementing reconfigurable manufacturing affects the performance during both ramp-up and production. Still, the evaluation of the reconfigurable manufacturing potential has only received limited attention even though it is highly related to justifying its investment. One practical case based example of investigating the potential in reconfigurable manufacturing has though been carried out in high volume manufacturing. However, quantifying and investigating the potential in reconfigurable manufacturing for low volume manufacturing has to our knowledge not been carried out. Thus, the purpose of this paper is to measure the potential of reconfigurable manufacturing in low volume industry, carried out by use of a case-study in Danish industry. Thus, this paper presents an approach for decision support that can be applied by low volume manufacturing companies

Design, simulation and experimental investigation of a novel reconfigurable assembly fixture for press brakes

A reconfigurable assembly fixture is a major and important component of a reconfigurable assembly system. It isrequired for the assembly of a variety of press brake models inorder to reduce the assembly time and overall production time.The stages and requirements for the design of an assembly fixtureand understanding of the assembly process for press brakemodels were used to design a reconfigurable assembly fixture.A detailed design analysis of parts of the fixture and the hydraulicsystem is considered and presented in this article. The stress anddisplacement analysis of the parts is executed using Solidworksexpress simulation. The parameters of the hydraulic componentswere determined from force requirements, and the hydraulic system was modelled physically using Matlab Simscape hydraulics.The response of the hydraulic system was obtained for eachactuator in the system in order to depict the output of the actuators from the spool displacement of the valves. Stress analysisconducted on parts of the fixture showed that it can withstandmaximum stresses that are lesser than the yield strength of thematerial used for the part. It was also established that synchronization of hydraulic actuators can best be achieved by the use of asine input to the electrohydraulic valve. An experimental investigation was done using FESTO hydraulic test bench in order toobserve the synchronized extension and retraction of the hydraulic actuators. The simulation of the hydraulic system, electricsystem and the programmable logic controller was prepared using automation studio. The design is envisaged to provide the industries with relevant information on accurate location and gripping of press brake frames rather than turning and repositioning of the frame in order to fit other parts during assembly. The article provides relevant information on the design analysis of a reconfigurable assembly fixture for press brakes which is novel because articles on reconfigurable assembly fixtures have not considered its application to press brake assembly