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Proceedingsofthe 2009 IEEE International Conference on Mechatronics and Automation August 9 12 Changchun China DesignofSoftware Architecture for NC Engraving Machine Based on Embedded Linux Junqi Zhao InstituteofAstronauticsandAeronautics UniversityofElectronic ScienceandTechnologyofChina Chengdu Sichuan Province 610054 China zhao junqi Abstract Embeddedsystem hasalready beenapplied widely inindustrycontrolandmanyotherfields As the sizeand complexity of embedded applicationsystemincreasing the embeddedsoftwaresystemisbecomingmoreandmore complexity Itisespeciallyimportanttodesignsuitable embedded softwarearchitecturefor a special control object At thesametime variousfunctionalandnon functional requirements of NC numerical control engravingaredemanded continually At present most of the software architectures for NC engraving machinearewithout operating systemsupportand notbeabletomeetthoserequirements Althoughafew engraving control system with commercial operating system support it is expensiveandnot good for enterprise developing software with independent intellectualpropertyrights Inorder to solve the above problems the new softwarearchitecturefor NC engraving machine was designed based on embedded Linux andDARTS method in thispaper The systemdataflow was analyzed bydataflow diagram andthen the system was divided into various tasks by the HGommaprinciple Furthermore the softwarearchitecturewasputforward Theinter task synchronizationandcommunication was also analyzed Finally thereal timeanalysiswasmade Byusingthissoftware architecture the software system become simpleandclear and different control component based on different time granularities can beintegratedseamlessly Throughpractical application it was proventhatvarious requirements of NC engraving system can be realized with low cost The flexibilityandadaptability of the systemarealso improved greatly Index Terms ARM Embedded Linux Engraving FPGA Software Architecture 1 INTRODUCTION As the size and complexityofembedded software system increasing the importanceofsoftwarearchitecturegoes beyond the algorithms and data structuresofthe computation Designing and specifying the overall system structure emerges as a new kind of problem 1 A good architecture can help ensure that a system will satisfy key requirements in such areas as performance reliability portability scalability and interoperability Abadarchitecturecanbedisastrous Architecture design is high level design for overall system from the system pointofview Different experts will define software architecture on their own After all it is a tool to solve practical problems in various domains and contexts thus various architectural models or definitions have been released Although various definitions are somewhat different we can Hui Li and Longfei Peng InstituteofAstronauticsandAeronautics UniversityofElectronic ScienceandTechnologyofChina Chengdu Sichuan Province 610054 China kelly li see a large degreeofcommonality at the coreofallofthem witch is the notion that the architecture of a system describes its gross structure This structure illuminates the top level design decisions including things such as how the system is composedofinteracting parts where are the main pathwaysof interaction what are the key properties of the parts The key stepofarchitecture design is to divide system into some manageable subsystems and design interfaces between the subsystems Software architecture design should not only consider how to meet the system s functional and performance requirements but also the non function such as reliability scalability portability and availability There is not a unified approach for software architecture especially in the embedded system Embedded Linux is referred to tailor standard Linux through the miniaturization treatment and can be stored on non volatile storage or microcontroller which capacity is only a few hundred KB or a few MB Itis a special Linux operating system that is applied to special embedded system 2 Linux has emerged as a mature high performance stable and reliable alternative to traditional proprietary embedded operatingsystem 3 Linuxnotonlypossessesthe adaptability flexibility and stability but also possesses strong network functions Moreover Linux is a sortoffree software with open source codes 4 At present most of the software architectures for NC engraving machine are without operating system support and not be able to meet various requirements Those architectures have a simple structure but the software development cycle is long the development cost is expensive the software quality is not guaranteed as well as the portability and scalability is poor Although a few with commercial embedded operating system such as Windows CE in NC engraving system the cost ofproduction is high due to expensive royalties So it is not good for enterprise developing software with independent intellectualpropertyrights ButifanembeddedLinux operating system with open source is used cost of production can be reduced because no licensing fee is needed and the cross development tool chain is also free In addition there are many other advantages such as reduced development time becauseofopensourcedevicedriversandreusable applications convenientdevelopmentenvironment configuration using module function file system management andeasyresolutionofaproblemfromopensource communities and so on 5 On the basisofabove enterprises 978 1 4244 2693 5 09 25 00 2009 IEEE2894 can developNCsoftwarewithindependentintellectual property rights and other various requirements can also be satisfied Embedded Linux has recently become a current research focus in embedded system II MODELOFHARDWARE Model is the abstractionofreality which can make clear descriptionofthe reality In order to get the engraving control system model the main hardware partsofthe system are abstracted as shown in Fig I Fig Modelofhardware The hardware of the NC engraving controller is provided with the architectureofARM and FPGA Field Programmable Gate Array The master control chipofthe controller adopts 32 bit RISC ReducedInstructionSet Computer ARM microprocessor chip S3C2440 of Samsung Company which based on ARM920T structure It has high work efficiency up to 400MHz and many kindsofgeneral interfaces such as integrated UART for serial port integrated USB CEMERA LCD VGA and Ethernet controllers etc Furthermore it provides on chip memory manage unit MMU which is used to realize virtual memory management so as Linux can be easily ported to ARM The Cyclone FPGAEPlC6of Altera Company was adopted as the slave chip to lighten the burdenofthe master control chip NAND Flash memory is used for non volatile program and data storage Main memory is synchronous dynamic random access memory SDRAM and might contain anywhere from a few megabytes to hundredsofmegabytes depending on the application The main processing flowofthe system is described as follow keying informationofcontrol panel is sent to ARM through serial port ARM then process and send the corresponding control data to FPGA or display panel FPGA is mainly doing front end processing work of sensor and control information 6 The signals sent by FPGA are transformed and isolated by the adapter panel and then are sent to the driver which will drive the stepping motorofthe engraving machine so as to control the cutting tool path To realize the functionsofhomingof engravingand tool changing the sensor signal of engraving machine will be feed back to FPGA through adapter panel Information is displayed on display panel which is LCD or VGA In all the controller built on embedded technology can reducethe systemhardwarescale facilitate application 2895 development cut down the cost and enhance the system s reliability and real time performance III TASKPARTITION Data flow diagram DFD is an important design aid tool in system development which allows the developerofa system to graphically show the flow of data in the system DFD describe and analyze system more precisely than any other artifact specifically in modelingofreal time embedded system 7 The ultimate data flow diagramofthe engraving machine system is shown as Fig 2 A task represents the executionofa sequential program or a sequential component in a concurrent program Each task deals with one sequential thread of execution hence no concurrency is allowed within a task However overall system concurrency is obtained by having multiple tasks that execute in parallel The system decomposition by DFD has already been preliminarily analyzed The dataflowofthe system is clearly revealed thus functionsofthe system and dataflow between them can be able to recognize conveniently The next step is how to identify the concurrent tasks in the system by applying the task structuring criteria and the DFD Here the DARTS Design Approach for Real Time System method was used to decompose the system into smaller and more manageable units with well defined interfaces between them Its central theme is to address the key aspectsofstructuring the system into concurrent tasks and define the interfaces between them DARTS uses a setoftask structuring criteria which be called H Gomma principles for identifying the concurrent tasks in the system as well as a setofguidelines for defining the communicationandsynchronizationinterfacesbetween various tasks I I I I Fig 2 The DFDofengraving machine The task structuring criteria are a setofheuristics derived from experience that obtained in the designofconcurrent system The main consideration in identifying tasks is the asynchronous natureofthe functions within the system The HGommaprinciplesaregroupedintothreeseparate categories based on how they are used to assist in the task structuring activity The first is event dependency criteria The event dependency criteria address how and when a task is activated including device I O dependency periodic event control function and user interface dependency et al The second is task cohesion criteria The cohesion criteria provide a means of assessing the asynchronous natureofthe functions and hence a basis for determining whether and or how functions should be combined into concurrent tasks including sequentialcohesion temporalcohesionandfunctional cohesion The third is task priority criteria including time critical and computationally intensive 8 Toachievethegoalofstructuringasysteminto concurrent tasks the task structuring criteriaofH Gamma principle arc applied to the functions shown in the data flow diagram Based on these criteria the engraving software system can be divided into seven parts which arc shown as follows i e the dashed line frame in Fig 2 a UserInterfaceTask todisplaytheinteraction information with the user such as coordinate part origin and process progress as well as other state information etc b Keyboard Processing Task to receive the control command from the control panel such as selecting file system setting or parameter setting manual movement etc then make the corresponding treatment c File Processing Task to read file from U disk then interpret the G code or HPGL code processing file The result will be stored in buffer d Data Processing Task to read data from the buffer and process it through the control algorithm such as treatment ofsmalllineblocks JudgebigS accelerationand deceleration control interpolation processing e Interrupt Service Task to send control pulsesofthe stepping motors to FPGAwhen ARM received interrupt signal from FPGA f Extended Functions Task to realize some extended functions Thesefunctions includingupdatethecontrol program remote control network management and simulation ofthe movementofthe tool etc can be added according to different requirement g FPGAProcessing Task to implement real time functions such as send control pulses to the stepping motors real timely receive and process sensor signal etc Tasks a f are implemented in the ARM becauseof its powerful computing ability Task g is implemented in the FPGA due to its real time processing ability IV ARCHITECTURE D ESIGN A Software Architecture Software architecture typically plays a key role as a bridgebetweenrequirementsandimplementation The software architecture for NC engraving machine controller based on embedded Linux is shown in Fig 3 and each partof the architecture is also described below SoftwareArchitecture unction Modules I User lru erfaceIIKeyboard ProcessingIIrileProcessing I I Interrupt ServiceIIData Proce ingIIblendedFunctionsI IRoot FileSystemI IGCII ISystem ManagementI YAFFS Qt EmbeddedjInterface I EmbeddedLinuxKernel I I BSP Flash SDRA M t Sll l CD CS 900 Serial Camera etc I Hardware Fig 3 Software architecture BSP Board Support Packet is a software developing package between the hardware layer and the software layer It is usedto cover the differencesofhardware bootthe operating systemand provide device driver It is a key interface between hardware and embedded operating system The version of embedded Linux kernel is 2 6 which many new features have been added to support the wide applicationofembedded system It can be customized and ported based on ARM to meet our special requirements of the NC engraving system At the same time using embedded Linuxcan simplifysystem design Complex application program can be divided into some simple tasks which are managed by the embedded Linux Based on the embedded Linux kernel root file system and embedded graphical user interface GUI were selected with the requirementsofthe system System management interface is also supported by embedded Linux The YAFFS yet another flash file system is adopted for root file system which is also a kindofLog structured file system like JFFS JFFS2 It is specially designed for NAND Flash and suitable for mass storage devices YAFFS is thus designed not only to adapt to the features of NAND Flash but also to better use the advantages of NAND Flash to reach best performance It uses log structure error checking correction and technique to improve NAND Flash s robustness The adventofYAFFS makes the low cost NAND chip effective and robust YAFFS is highly portable and can run on Linux Qt Embedded is a famous commercial embedded GUI tool has great cross platform ability can be conveniently applied in present popular OS such as embedded Linux QUE can directly communicate with I O devices Furthermore its systemarchitectureorientedforobjectmakesitscode structuralized reusable and run fast QUEsupports frame buffer driver and can directly write frame buffer without X server or X lib support In this way it saves memory cost and improvesapplication running efficiency 9 Considering advantages mentioned above this paper adopts it as embedded lightweightGUIcomponentlibrary basicconstruct tool oriented to industry monitoring 2896 ReceiveMessage r 1 Output data selector unit Message Queue message mess3ge Fig 6 Message queue Input data selector unit 2 IUX I Send Message r 1 and rather than a binary semaphore a mutex lock will be used to represent mutex The producer and consumer running as separate threads will move items to and from a buffer that is synchronized with these empty full and mutex structures FIFOallowsthemoduleswithinadesigntobe decoupled The FIFO interface is simple reducing design time FIFO is seldom bottlenecks since the width and depth can be adjusted and is well suited for connecting custom hardware directly to the embedded processors and is simple to implement with a short design cycle and small code size 12 Theping pangoperationisintroducedintothe asynchronous FIFO design to make FPGA and ARM to realize seamless buffering and processing for high speed data streams as well as to ensure the pipeliningofFPGA processing The typical ping pang operation method is shown as Fig 5 13 Fig 5 Ping pang operation The processing flowofping pang operationofFPGA is followed as the input data from ARM is firstly allocated to Data buffer module I through Input data selector unit and then at next switching time the input data will be buffered in Data buffer module 2 At the same time the data buffered in Data buffer module I last time is sent to Data Processing Module for processing through Output data sciector unit At the third time the input and output data will be exchanged to the two data buffer modules again The steps mentioned above will be repeated and repeated The data buffer module is FIFO in our system The switching signal for data selector unit is the interrupt signal which sent by FPGA when it needs data That means when the data buffered in oneofthe two data buffer modules has been processed then FPGA will send an external interrupt signal to ARM So ARM will send data to FPGA in interrupt service routine FPGA will buffer the data in this data buffer module when it has received Meanwhile the Data Processing Module can still process the data in the other data buffer module Ping pang operation promotes the concurrency of the system Another way used to achieve the same effect for other tasks is via a message queue which is for the operating system to provide the meansofcooperating processes to communicate with each other as shown in Fig 6 Consumer Producer Fig 4 Producer Consumer problem To allowproducerand consumerprocessesto run concurrently we must have an available bounded buffer of items that can be filled by the producer and emptied by the consumer This buffer will reside in a regionofmemory that is shared by the producer and consumer processes A producer can produce one item while the consumer is consuming another item Concurrent access to shared data may result in data inconsistency The producer and consumer must be synchronized so that the consumer does not try to consume an item that has not yet been produced II In order to solve synchronization semaphores will be used For this project standard counting semaphores will be used for empty and full On the basis of services provided by the parts mentioned above the application tasksofNC engraving except for FPGA p