Reliability Assessment of a spacecraft mechanism using Axiomatic Design

Transcription

1 Reliability Assessment of a spacecraft mechanism using Axiomatic Design Jose Thomas #, Tina Raju 2, SujithKumar.N 3, Y. S. Shankar Narayan 4 PG Scholar, 2 Associate Professor, Department of Mechanical Engineering, MA College of Engineering, Kothamangalam, Kerala, India 3,4 Scientist/Engineer, Reliability Assurance Mechanical Division, Systems Reliability Group, ISRO Satellite Centre, Bangalore, Karnataka, India Abstract a system is the ability of a system to perform its required functions under stated conditions for a specified period of time. This paper discusses the reliability assessment of payload door mechanism using axiomatic design. Evaluation of the payload door mechanism was done by using axiomatic design and the critical components were identified. The reliability of these components is assessed by using stress strength interference method. Keywords Design evaluation, Axiomatic design, Design reliability, Stress-Strength interference model. I. INTRODUCTION System reliability is the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is associated with unexpected failures of products or services and understanding why these failures occur is key to improving reliability. Reliability is a critical design attribute for space systems and an important metric in spacecraft design and optimization. It is the aggregate result of the reliability of spacecraft subsystems, and as such, analysing the reliability of these subsystems and their relative contribution to spacecraft failures is important for reliability growth plans and targeted subsystem improvements. This paper deals with the reliability assessment of pay load door mechanism. Figure.Pay load door mechanism Payload is the part of a vehicle s load, especially an aircraft s, from which revenue is derived; cameras, cargo etc. payloads are placed inside a stationary frame and the stationary frame is covered by using pay load door. Payload door is used as a cover for the detector during ground testing and during launch. Deployment mechanism has to open door on orbit and will be retained with spacecraft. Payload will be fixed with spacecraft. Payload door will be with hinges at one end and hold down mechanism with the other end. Mechanism requirement arises since the door, in closed position, protects the detectors from radiation damage during transit to moon and then needs to be opened upon reaching the lunar orbit to enables the detectors to view the lunar surface. Before opening the door, it is planned to calibrate the detectors using calibrating sources (FE-55). These calibrating sources are mounted on inner surface of the door. The door opening mechanism is a single shot operation and the door is deployed and latched permanently. Pay load door is under development and reliability should be assessed functionally for meeting requirements. However measuring reliability does not make a product reliable, only by designing in reliability can a product achieve its reliability targets. The objective of design for reliability is to design a given product that meets its requirements under the specified environmental conditions. The process of ensuring that the final product conforms to user is called design validation. Design validation is done to review the effectiveness and efficiency of a particular design to meet its functional requirements. Validation of payload door design can be done by ensuring that each customer requirement is converted to functional requirements to design parameters. These design parameters are met by components realized by capable processes. Validation of payload door design is done by confirming that the design parameters are traced to customer requirements and none of the customer requirements/functional requirements is left out in the design. Design validation helps to finds out the critical factors which can be used to ensure reliability of the system. Assessment of the reliability of the payload cover mechanism has to be carried out to ensure functioning of the payload since it is a single point failure. So identifying all functional requirements and design parameters and addressing the failure modes which prevents meeting the above requirements are essential in the product assurance. As part of product assurance of the payload cover mechanism, validation of the design is carried out using axiomatic design and design reliability has to be estimated. ISSN: Page 28

2 II. DESIGN VALIDATION AND RELIABILITY In this study, two works are carried out. The first work is the design validation of pay load door mechanism using axiomatic design. After evaluating the design, the critical components should be identified. The second work is to calculate the reliability of the critical components. In order to avoid failure the critical components should have high reliability. A. Design validation using Axiomatic Design Axiomatic Design methodology is used to find the critical components which causes total failure of the system Axiomatic Design is a design methodology using matrix methods to systematically analyse the transformation of customer needs into functional requirements, design parameters and process variables. It was created and popularized by Professor Suh of the Massachusetts Institute of Technology (Suh 990, 2000). First the objective of the problem is defined and then how the axiomatic tool can be applied to find the critical components. The function of the AD tool starts from customer requirements and mapping it through FRs and DPs. This process helps in the creation of a design matrix for visualizing the interaction and to find out the critical components in which failures may occur within the design. The result of axiomatic design is obtained as coupling and how each FRs and DPs are physically integrated to sub-assemblies and components. The listed couplings helps to find out the effect of change in DPs and FRs. Till this step the first axiom in axiomatic design is followed. The second axiom is not followed because there is only one design to validate. The information axiom is useful when there is more than one design that satisfies the independence axiom equally and the best design is the one with the least information. In order to built quality into the design, customer requirements acts as a bench mark for developing any new product or updating an existing design. First step of the Axiomatic design methodology is to define the objectives. The objectives of the problem were defined and then how the Axiomatic design tool can be applied to find out the critical components is explained. Customer Requirements The customer requirements hold the information obtained from the customer. The customer requirements are To protect the detectors during launch and transit to lunar orbit Uncover the detector to map lunar surface in the lunar orbit. Functional Requirements From the above mentioned customer requirements the information is transferred in to a minimum set of Functional requirements. The functional requirements are Protect the swept charge devices from electron radiation (80MeV) during transit orbit. Door should be in closed position during launch and transit to lunar orbit. Release the cover after reaching the orbit Locking the cover after deployment. Status monitoring Door should house the calibrating sources. Design Parameter Design parameters are the parameters which satisfies the functional requirements. The design parameters in the physical domain are listed below. Cover Hold down mechanism Hinge mechanism Latching mechanism Switches Mounting interface After the domains are defined appropriately the next step in axiomatic design is to perform a functionally based decomposition of the obtained top level functional requirements. This type of decomposition is called Zigzagging decomposition. Decomposition helps to convert the elements of the design into hierarchy until a complete detailed design is obtained or till the design is complete. Figure 2.Decomposition of first level functional requirements into sub levels Figure 3.Decomposition of second level functional requirements into sub levels ISSN: Page 29

3 Figure 4.Decomposition of third level functional requirements into sub levels Coupling and physical integration Coupling occurs in AD when a functional requirement cannot be easily controlled by changing its corresponding design parameters. The independence axiom helps in pointing out any coupling in the design. Unwanted coupling results in unintended consequences and makes a design difficult to control or adjust. When two things are coupled it means that they cannot be adjusted or changed independently. The coupling obtained from the design matrix is listed below. DP: Fork end bracket The change in parameters of the fork end bracket will affect the load path provided and deployment angle. So any changes in the parameters of the fork end bracket have to accommodate the effect in functional requirements. DP: Bearing The change in parameters of the bearing will affect the energy stored, load path provided and deployment angle. So any changes in the parameters of the have to accommodate the effect in functional requirements. DP: Eye end bracket The change in parameters of the eye end bracket will affect the energy stored, load path provided and deployment angle. So any changes in the parameters of the have to accommodate the effect in functional requirements. DP: Torsion The change in parameters of the will affect the energy stored and deployment angle. So any changes in the parameters of the have to accommodate the effect in functional requirements. Physical integration is the final step of the design process. This is the step where the functional requirements are integrated into components of the assembly. Designer should integrate functional requirements into minimum number of components keeping as much independence as possible (with minimum coupling). There are mainly two sub assemblies in payload door mechanism.. Deployment hinge and Latch mechanism The functional requirements related to deployment and latch mechanism are latching slot for the cam, to latch the door, energy for deployment, storage of energy, angle for deployment, angle of locking and angle of deployment. These functional requirements are physically integrated to deployment hinge and latch mechanism. 2. Hold down and release mechanism The functional requirements, included protect SCDs from electron radiation, door should be in closed position during launch and transit, release the cover after reaching the orbit, house the calibrating sources, ability to take launch load, provide load path, to measure the hold down load, to cut the Kevlar rope, load carrying capacity of the rope and provide pushing force against rope are physically integrated to hold down and release mechanism. The functional requirement cover stiffness is controlled by snubber location, number and location of hinges & hold down and snubber force. So the components related to the functional requirements of these two sub assemblies are critical. Any type of failure to these components leads to the total failure of the system. The critical components identified are, hinge shaft, shaft pin, compression, latch strip, splice holding bracket, washer, fork terminal, heater and load bracket. B. Reliability Assessment Reliability is the probability that an item will perform its specified mission satisfactorily for the stated time when used according to the specified conditions. Reliability must be designed into a product or service. Most important aspect of reliability is to identify cause of failure and eliminate in design if possible otherwise identify ways of accommodation. the system is the product of reliabilities of its subassemblies. Here Stress strength interference method is used to find the reliability of the syste2m. By using the Stress Strength interference method the reliabilities of each critical component can be calculated. It is found to be useful in situations where the reliability of a component or system is defined by the probability that a random variable S (representing strength) is greater than another random variable s (representing stress). While it makes intuitive sense that a component is deemed to have failed when its strength is lower than the applied stress, this model is not entirely restricted to stress and strength. It can be applied to any situation or problem where the random variable S represents any performance related characteristic of the system under question and s serves as a criterion that determines ISSN: Page 30

4 compression Flat washer failure. Any state where S falls below s represents the component to be in a state where it is deemed unacceptable or to have failed. Once the distribution and parameters of S and s are determined, the reliability can be calculated by estimating the probability S>s, which is computed as shown. Where, R = / = mean value of strength = standard deviation of strength = mean value of stress = standard deviation of stress Stress should be different for different sections. So by applying proper cross section equations stress can be calculated. Strength of a material is always constant. If the material used for the manufacturing of component knows the strength of the material can be easily calculated from the design data book. Standard deviation of stress is calculated from the test results conducted by ISRO and the standard deviation of strength from design data book. Thus the reliability of each component can be easily calculated. Total reliability of the system is the product of reliabilities of its critical components. This is how the total reliability of the system is calculated. Since the system is series, total reliability of the system is the product of all the reliabilities of its critical components. Total reliability = reliability of x reliability of Hinge shaft x reliability of shaft pin x reliability of Fork terminal x reliability of Load bracket x reliability of splice holding bracket x reliability of Latch strip x reliability of compression x reliability of Flat washer x reliability of Heater. the system = x x x x x x x x x x = hinge shaft latch strip t = shaft pin splice holding bracket fork terminal load bracket Reliability of System III. RESULTS AND DISCUSSION The reliability of pay load door mechanism was assessed using axiomatic design. Critical components of pay load door mechanism were identified and the reliability of these components is calculated using stress strength interference theory. According to axiomatic design, the validation started from the needs of the customer. The obtained customer requirements are mapped to functional requirements and then in to design parameters. Parent level functional requirements were decomposed into its lower level functional requirements. Since the payload door design was large, decomposition was done with the help of free mind software which helped to visualize the decomposition process more clearly. The evaluation of the design was done with the help of independence axiom. The final design matrix showed four couplings. The effect of change in the coupled design parameters on other functional requirements has been found out design validation was performed. The final step of axiomatic design is the physical integration. Through this physical integration, the critical components are easily identified. The reliability of payload door mechanism was found to be Here the calculated reliability is almost equal to one which reveals that the payload door mechanism is high reliable. IV. CONCLUSION AND FUTURE WORK Axiomatic Design provides a systematic way of evaluating the design of payload door mechanism. The customer requirements are traced to physical integration through functional requirements and design parameters. The process ensured that each functional requirement is fulfilled by its corresponding design parameters and no requirement is left out unsatisfied. The results obtained from the analysis of payload door mechanism are used to check for incorporation of functional requirements into subassemblies and components. Checking for incorporations, were able to establish how physical integration is done and physical integration shows how components are put together to fulfil the customer needs. Design evaluation of payload door mechanism was done by using first axiom (independence axiom) in axiomatic design. The use of first axiom helps in listing all the coupling in the design. The information for listing the number of coupling present in the design is obtained from the design matrix. The obtained results will help in making design decisions in future modifications and scaling of design. In the present work, the use of process domain has not been employed. It is a reformulation of the design parameters in terms of the processes that can generate the physical realities in the previous domain. It describes how the elements of the physical domain will be created, acquired, or manufactured. Tests & ISSN: Page 3

5 evaluation and quality assessment strategies can then be devised for each and every FR-DP relationship. Introduction of new design ideas for payload door can be compared with the old design by using the second axiom i.e. information axiom, in axiomatic design. The present work deals with only the first axiom (independence axiom) for the evaluation of pay load mechanism. The second axiom will help to identify the strength and weakness of the design when decomposed functionally. It can be used in the process domain for establishing the best process available to fulfil the design parameters. Defining the process domain and the application of second axiom into the process domain can be done as future work. REFERENCES [] Bae S., Lee J. M., and Chu, C. N., Axiomatic Design of Automotive Suspension Systems CIRP Annals Manufacturing Technology, 5(), 2002, 5 8. [2] Bulent Gumus M.S, Axiomatic Product Development Lifecycle, Dissertation, Texas Tech University, December [3] Rausand, M. and Hoyland, A., System Reliability Theory, Wiley,2004. [4] Keller, A.Z., Reliability Aging and Growth Modeling, In Reliability Modeling and Applications, A.G. Colombo and A.Z.Keller Eds., Kluwer, 986. [5] Misra, K.B., Reliability Analysis and Prediction, Elsevier, 992. [6] Christopher A. Brown, Elements of Axiomatic Design, Cazenovia, NY, 29 March [7] M.F.Pellissetti, G.I.Schueller, H.J.Pradlwarter, A.Calvi, S.Fransen, M.Klein, Reliability analysis of spacecraft structures under static and dynamic loading, Computers & Structures 84(2006) [8] Jean-Francois Castet, Joseph H. Saleh, Beyond reliability, multi-state failure analysis of satellite subsystems: A statistical approach, Reliability Engineering And System Safety 95,(200) [9] Gregory F.Dubos, Jean-Francois Castet, Joseph H. Saleh, Statistical reliability analysis of satellites by mass category: Does spacecraft size matter?, Acta Astronautica 67 (200) [0] Saleh, J.H. and Castet, J., Spacecraft Reliability and Multi- State Failures, Wiley, 200. ISSN: Page 32