Enabling Advanced CNC Programming with opennc Controllers for HSM Machines Tools

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1 High Speed Mach. 2016; 2:1 14 Research Article Open Access Jean-Yves Hascoet and Matthieu Rauch Enabling Advanced CNC Programming with opennc Controllers for HSM Machines Tools DOI /hsm Received Mar 4, 2016; accepted Mar 9, 2016 Abstract: The manufacturing area has benefited from various progresses over the last decades, such as High Speed Machining (HSM), but CNC programming is still based on dated practices and habits with a segmented unidirectional CAD/CAM/CNC data chain. Decision power is limited at the NC controller stage and online process optimization is difficult. In contrast, advanced programming approaches, such as STEP-NC [9], aim to unify the product/process data within a consistent environment from design to machining. Hence, opennc controller solutions which allow access to their internal algorithms, have emerged. It is consequently possible to implement new tool path control algorithms that respond directly to the actual machining condition. This paper focuses on the interest of opennc controllers to develop advanced programming approaches for HSM processes. The major drawbacks of legacy controllers in the implementation of advanced tool path generation methods are discussed and the most significant opennc projects are reviewed. An integrated test platform has been developed. The advanced HSM programming methods enabled by this opennc controller are discussed. 1 Introduction The manufacturing area has benefited from various progresses over the last decades: the machines have become faster, smarter and safer to realize the potential of High Speed Machining (HSM) and to meet today s global challenges. However, CNC programming is somehow still based on dated practices and habits with a segmented uni- Jean-Yves Hascoet: Institut de Recherche en Génie Civil et Mécanique (GeM), UMR CNRS 6583, 1 rue de la Noe, BP92101, Nantes Cedex 03, France; jean-yves.hascoet@ec-nantes.fr Matthieu Rauch: Institut de Recherche en Génie Civil et Mécanique (GeM), UMR CNRS 6583, 1 rue de la Noe, BP92101, Nantes Cedex 03, France; matthieu.rauch@ec-nantes.fr directional CAD/CAM/CNC data chain. NC controllers have hardly any decisional power in the manufacturing process; as a result, online process optimization is highly difficult. In contrast, advanced programming approaches, such as STEP-NC [9], aim to unify the product/process data within a consistent environment from design to machining. Hence, a central role is given to machines tools because they feed the data chain with online process data and are capable of adapting to the real process situation in order to propose flexible and adaptive product manufacturing solutions. Within this context, opennc controller solutions have emerged in the middle of the nineties. Their principle is to allow access to the internal algorithms for tool path interpolation and control, and to enable the addition of specific modules for advanced programming, monitoring or interpolation purposes. It is consequently possible to implement new tool path control algorithms that respond directly to the actual machining situation of the machine. Within this context, this paper focuses on the interest of opennc controllers to develop advanced programming approaches for HSM processes. First, the drawbacks of legacy controllers in the implementation of advanced tool path generation methods are discussed. Then, the main principle of opennc controllers is discussed, followed by a thorough review of the most significant opennc projects developed over the last few years. The last section of the paper focuses on an integrated test platform in our lab and the advanced HSM programming methods enabled by its opennc controller. 2 Drawbacks of legacy controllers Modern CNC controllers for HSM are high value added products that often cost one third of the price of an industrial machine tool. Usually, CNC machines are built according to the architecture described in Figure 1. Three major components are included in the control part: 2016 J.-Y. Hascoet and M. Rauch, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

2 2 J.-Y. Hascoet and M. Rauch Figure 1: Typical components of a CNC machine. Numerical Control Kernel (NCK), which reads and interprets G-code, and prepares interpolation functions. Human Machine Interface (HMI), which enables visualization and allows parameters and programs modification. Program Logic Control (PLC), which runs sequential control of the machine (tool change, I/O, etc.) For maintenance and industrial property protection reasons, most legacy CNC controllers from venders, such as Siemens, Fanuc, Heidenhain, etc., have a closed architecture [5]. Access is only allowed to some portions of the control system, in particular lower level and non-real-time segments. This closed architecture entails several drawbacks [27]: 1) Only specifically specified suppliers can provide compatible products. 2) Controllers cannot cope with frequently needed updates to meet the machining requirements. 3) Consequently, service, maintenance and repair costs are very high. In addition to the above drawbacks, online control is very difficult to implement because there is no direct feedback of the process data. The actual behavior of the machine is difficult to identify. In order to overcome these difficulties, envisioned solutions aim to integrate opencontrollers on the machines tools. Although some CNC control developers offer the option to adapt machine products and functions to specific needs, this option requires direct intervention by the developer. Access is only allowed to some portions of the control system, particularly lower level and non-real-time segments, while the rest of the system are closed. Moving towards the vision of a fully functional, portable, interoperable, scalable and extendible control system needs to follow the model highlighted in Figure 2. The black-box system in Figure 2 represents almost all current controllers that do not allow or only grant partial access to the backbone of the controller. Breaking down such systems into modular pieces that can ensure interoperability is a necessity. Figure 2: Decomposition of control functionality [18].

3 Enabling CNC Programming with opennc Controllers for HSM Tools 3 3 Emergence of open CNC controllers 3.1 Definition According to the IEEE technical committee, an open system can be defined as a system which enables wellimplemented applications to be integrated and runs on various platforms from multiple vendors. These systems must also interoperate with the other pre-existing software and hardware applications and provide consistent user interface styles [8]. To define an open system formally, the following four requirements are defined [14]: Portability, which refers to the ability of an Application Module to be transferred and used on different platforms without modification while ensuring the capabilities of the Application Module are maintained. Application Modules are building blocks that contain basic information for execution of a specific task. They are used as the foundation for other higher functionality complex programs. Extendibility, which refers to the capability to add any Application Module to a system without decreasing their capabilities or creating conflicts between them. Interoperability, which refers to the Application Modules interactions in a system. They must be able to work consistently together and exchange information in a defined standard way. Scalability, which refers to the ability to meet user requirements and system implementation. The functionality and performance of the Application Module as well as size of the hardware can be changed and adapted to fit various system sizes. This definition and the requirements are illustrated in Figure 3. To attain these relationships, several precursors are necessary. Inputs into the systems must be: Vendor neutral, such that both scalability and extendibility criteria are attained. It will also maintain the neutrality of the system with no link to proprietary interests. Consensus driven, such that user and vendor groups can control the evolution of the system. This will ensure that the immediate needs are met while visions of the future are driven by perceived future needs of the aforementioned groups. Standard based, such that inputs into the system can be governed by consensus-driven national and international standards, guaranteeing a wide network of participants and interests. Freely and openly available, such that any 3 rd party interested in the system can attain it. Moving towards the vision of a fully functional, portable, interoperable, scalable and extendible control system needs to follow the model highlighted above. The system platform is based on different modules, accepted standards and interfaces. The smaller the size of the units, the more options will be available and the more open the system will become [10]. 3.2 Open CNC Architectures Efforts The use of open architecture CNC is gaining importance as a promising industrial automation technology. It allows integration of equipment, pre-defined interfaces for configuration, and improved machine tool communication. Various benefits arising from using an open architecture when developing a new CNC include lower-cost electronics, increased functionality, easier customization, and higherperformance computers. Several types of open architectures are being developed in the USA, Europe, and Asia. All of them use a standard personal computer for control. The following is a summary of some key developments and functionalities of the worldwide effort to define standard system architecture for an open CNC controller. In this paper, the focus is on three projects, which provide the most comprehensive environments OSEC Figure 3: Control system openness criteria [18]. Started in 1994, the Open System Environment for Manufacturing Consortium (OSE) was tasked to create an open

4 4 J.-Y. Hascoet and M. Rauch architecture for Japanese industrial automation in many sectors. They created the Open System Environment for Controllers (OSEC). The basic goals were to control manufacturing equipment while improving performance and facilitating maintenance [6]. The architecture was meant to provide the eventual users, machine builders, control and software vendors with a standard industrial platform on which common industrial controllers could be based. These controllers would then be modified to suit the different end-users, thus promoting technical and commercial developments of industrial machines. Development for this architecture, focused on five levels [3]: 1. Workstation planning: operation and process control. 2. Workstation managing: tool movement and control sequencing. 3. Plan interpretation: trajectory control and discrete event control. 4. Trajectory generation/ discrete input and output (I/O): axis motion and triggering. 5. Servo controls: axis and device controls. To aid in the execution of this architecture, the consortium developed the OSEC API based on an interface protocol that is used to exchange messages between controller software components. Each component represents functional and real-time blocks within the system that are interconnected through the API as shown in Figure 4. The blocks can be defined as objects so that their background operations are hidden. The benefit of this architectural approach is that, although the OSEC architecture can be used to define the structure of each block, the system as a whole is not defined and not limited during the implementation phase. This guarantees that numerous options are available for implementation and these options can include de- vice drivers, inter-process communication, static libraries and DLL, hardware such as controller cards, etc. Other options such as software modules could be added to monitor or control various software executed within the system. The system architecture is designed such that it is not limited to a single set model that can be used for implementation. This allows multiple possibilities that can be tailored and defined to suit the size and performance requirements of end users HOAM-CNC The Hierarchical Open Architecture Multi-processor system (HOAM-CNC) was proposed by Altintas et al. as an open platform for implementing machining algorithms [1, 2]. It focused mostly on hardware integration in a modular design but also defined the software architecture that has been implemented. The real-time hardware architecture is shown in Figure 5a. In this iteration, the hardware architecture proposes two separate buses. The main bus contains the operating system and deals with the direct monitoring of activities, data processing, HMI and the adaptive control. A secondary CNC bus manages the real-time position and speed controls of the servo drives and axes. The introduction of two separate control buses allows modular expansions of the system. Additional processing modules can be added to the main bus while axis control can be added to the CNC bus in order to extend the operational capabilities of this system. The HOAM software architecture is shown in Figure 5b. The function of this architecture depends again on the modular nature of its design. This architecture is separated into two classes of tasks: toolpath control task and machine process monitoring/control task. The toolpath control tasks are broken down into several functional jobs that communicate to and are scheduled by the CNC interface module within the system master control. Within the system master control lies a custom developed real-time micro-kernel that takes care of scheduling and orchestration activities. The machine process control tasks are imbedded within the CNC master control and they communicate and send data/requests to the system master control via the CNC interface module. Figure 4: OSEC Open Architecture with API [18].

5 Enabling CNC Programming with opennc Controllers for HSM Tools 5 (a) (b) Figure 5: HOAM-CNC Architecture [2]. (a) Hardware; (b) Software LinuxCNC (formerly EMC-EMC2) The Enhanced Machine Controller, EMC, stems from an initiative by NIST to study and develop a comprehensive machine controller. LinuxCNC is the current extension of the original controller with updated functions. The system is stable and performing. The basic architecture of this controller is shown in Figure 6 and it features four components: a motion controller (EMCMOT), a discrete I/O controller (EMCIO), a task executor which coordinates them (EMCTASK) and Graphical User Interfaces (GUI s). One of the most significant advances in the LinuxCNC framework is the Hardware Abstraction Layer (HAL). It allows simple hardware modules to be assembled and interconnected to form a complex system. One major benefit of the HAL is its ability to control the modules as black-boxes so that can be interchanged, modified or replaced without the need for a complete system change. It also allows partial or full system testing that are similar to what would be done in the integration of physical hardware to troubleshoot or optimize a piece of hardware. The flexibility provided by the HAL greatly simplifies the interface with the control hardware. It also includes significant source code optimizations from previous versions while extending support for implementation on a large variety of machines, even those with complex kinematic structures. The HAL allows real-time data transfer from the controller to requested control hardware or other lower-level software modules. It is the framework for hardware drivers and software module development for realtime execution [7]. Performed cyclically in real-time, motion control activities such as tool path planning, direct and inverse kinematic computation and output to motor control subsystems are all performed within the EMCMOT. The EMC API integrates hardware into the controller by creating HAL modules without modifying any of the control codes. This creates the modularity within the control of this architecture. The discrete I/O module EMCIO is not in the real-time environment and it controls all functions that are not directly related to the actual motion of the machine axes. It is implemented as a single I/O controller with a hierarchical structure for each of its functions as sub-controllers and is directly connected to the HAL. Hence, EMCIO can be envisioned as adaptable and portable. A critical module within the EMC framework is EMC- TASK. It is a task level command handler and provides program interpretation for the RS-274 NGC machine tool programming language (G code) [13]. This module is hierarchically superior to both EMCIO and EMCMOT because it monitors the activities and status of those subordinate modules. The EMCTASK receives and analyses commands from local or remote Graphical User Interface (GUI) or from other processes above. It then translates those commands into NML that are either dispatched within itself or sent to the subordinate modules. The GUI gives the user access to needed parameters and options on the system. In LinuxCNC, several interfaces are currently have been developed. They can be run either natively or remotely (Figure 7). There also is the option to modify and expand GUI-based programs to match specific application requirements through the use of the Virtual Control Panel (VCP) [23]. Monitoring and data acquisition functions are available too. The implementation of EMC2 has been successfully tested and used in a variety of machine tools. Since each hardware module added to the HAL is visible as a black box, integration into the system is made simple because

6 6 J.-Y. Hascoet and M. Rauch Figure 6: EMC2 Systems architecture [23] and example of HAL configuration. the modules are connected to the abstraction layer using HAL pins defined as either inputs or outputs OpenNC controller projects comparison and selection For a better look at each of the architecture available, the EU funded FP7 FoFdation has provided a comparative synthesis of each of them (Figure 8) [5]. The table in Figure 8 provides comparisons between the main segments of each major architecture. It also includes comparison of their respective API. From Figure 8, it appears that there are too many areas to evaluate for better comparisons. Moreover, the starting points to develop these opennc projects were often different; thus, incompatibilities arise between them. The main architectural structure, method of communication,

7 Enabling CNC Programming with opennc Controllers for HSM Tools 7 Figure 7: Example of a LinuxCNC GUI. API structure and mandates about the definition of application modules are all important consideration points, and they seem to create debilitating problems as far as a unifying architecture is concerned. Most of these projects focus on modules architecture and information flow [16]. From the data of Figure 8, a feature comparison analysis for the three major architectures reviewed in the previous section is presented in Figure 9. This analysis shows that OSEC is difficult to implement because the description of the modules is limited; HOAM project is very conceptual and difficult to integrate in a machine although advanced functions such as monitoring and collision avoidance have been developed. Finally, LinuxCNC benefits from existing implementations on laboratory scale machines and has a very active development community. It has been considered as the best candidate to be implemented on the high speed industrial machine tool in the research lab. In the next section, the integration of LinuxCNC and other controllers on a single industrial machine at Ecole Centrale Nantes is explained. 4 Integrated Test Platform (ITP) at Centrale Nantes, France 4.1 Implementation of opennc controllers To experiment the potential of open NC controllers, an industrial functional machine tool of the laboratory, a Cincinnati Milacron Sabre, has been updated to operate with the LinuxCNC open controller [21]. It has been the first industrial machine of this scale to implement this opennc controller to the authors best knowledge. At the same time, the NC controller has been extended with the integration of SPAIM (STEP-NC Platform for Advanced and Intelligent Manufacturing [20]) so that the advantages of existing advanced CNC programming methods developed in the lab can be explored. During these modifications, the Cincinnati machine was further extended to become an all-encompassing Integrated Test Platform (ITP). The primary goal was to safe-

8 8 J.-Y. Hascoet and M. Rauch Figure 8: Comparison of opennc controller architectures [5]. Figure 9: Simplified illustration of the different criteria to select an opennc controller for the ITP guard the functionalities of the original NUM controller, while enabling two different guest controllers LinuxCNC and Fidia CNPC 143 to all cohabit within the same machine tool (Figure 10). This controller cohabitation setup allows for the influence of each controller on the milling process to be distinguished and also to demonstrate the potential of each specific programming technology, such as smoothing curves, NURBS, optimizations, etc. The physical implementation with LinuxCNC consists of two on-board computers. On the first computer, PC1 with Linux OS, the LinuxCNC software and hardware (motion boards, encoders and I/O boards) are installed. The connections between the PC and the machine axes are handled by this station. It is responsible for the real-time motion control based on a RT-Linux kernel. The second computer, PC2 with Windows OS, contains accompanying soft-

9 Enabling CNC Programming with opennc Controllers for HSM Tools 9 Figure 10: opencnc Manufacturing System Implemented. Figure 11: Overview of CNC cohabitation on the ITP. ware packages such as CAD and CAM. It is also where the SPAIM platform inhabits and it will be responsible for all the advanced programming and control. On the ITP, a hard-wired switch permits switching from one CNC controller to the next and each controller has complete control over the Sabre machine. The switch represents a physical diversion of the signal sources for the input signals that are used to control the machine axes (spin- dle, linear machine axes and hand-wheel). It also changes the signal sinks of the output signals that are sent out by encoders and limit switches (Figure 11). A primary benefit of this ITP is that because the core of the tool-path interpolator is open on the LinuxCNC side, it can be updated to be able to directly control Nurbs or Bsplines, to work with the process planner for updating the work plan, or proposing new tool-path parameterizations in order to optimize the machining process according to the machine behavior. With this multiple-controller setup, direct comparison between CNCs and their functionalities is now made possible on the same machine structure. 4.2 ITP performances evaluation Before accessing the capabilities of the above setup, it is important to verify that the developed ITP meets the requirements associated with open systems. Portability is ensured by the Linux exploitation system itself. Interoperability is made possible by the use of Profibus and Linux-

10 10 J.-Y. Hascoet and M. Rauch Figure 12: X and Y positions for a 200 mm square tool path. Figure 13: Examples of tool paths screenshots with LinuxCNC (Lock, Square and Line). CNC HAL. The system can be extended with new NC functions (NURBS tool paths generator, link to SPAIM, etc.). The scalability is guaranteed by the ability to implement and use LinuxCNC for other CNC processes such as incremental sheet forming and additive manufacturing, for example. LinuxCNC enables the simulation of the actual running of a tool path by including a dynamic configuration of the machine tool, where parameters, such as speeds, accelerations, and jerks, can be easily modified. These modifications have a software impact on the tool path interpolation profiles. Servo control parameters, such as PID gains, can also be modified. Several experimental tests were conducted based on direct measurement of tool motions and positions. They attest that the ITP is fully capable to meet the requirements associated with the tolerance of the parts. Tool positions are accurate with a maximal deviation of 0.05 mm. (Figure 12). Addition tests (Figure 13) based on real feedrate recordings and machining time measurements for several sets of parameters provide the same conclusions regarding this capability. In parallel, ballbar acquisitions were conducted for both LinuxCNC and Fidia NC ITP configurations. The results obtained on the Cincinnati Machine are shown in Figure 14. It is clear that LinuxCNC drives the machine tool with a higher accuracy than the Fidia controller in terms of squareness, circularity, geometric deviations, etc. As a result, the LinuxCNC can be considered as capable as legacy NC controllers. With the capabilities of LinuxCNC proven to be comparable to proprietary modern NC controllers, the next session will focus on advanced tool path generation methods enabled by the open nature of the implemented NC controller. 4.3 Advanced CNC programming methods Addition of a NURBS tool path interpolator The ITP implemented in the laboratory can be enhanced with advanced functionalities. In order to illustrate the scalability criterion, a NURBS interpolator was integrated to the ITP of the laboratory. NURBS curves, which are usually not accessible in legacy controllers [4, 12, 22], are often used to describe CAD models. As a result, the numerical data chain has been homogenized, as shown in Figure 15,

11 Enabling CNC Programming with opennc Controllers for HSM Tools 11 Figure 14: Ballbar tests results with a 1000 mm/min feedrate and a 150 mm radius. Figure 15: ECN 4 waves test part, geometry of the 4 th wave, CAD modeling with Rhino and computed tool path in LinuxCNC. Figure 16: Feedrate simulation for linear and NURBS interpolation on a wave profile. which represents those tool paths difficult for using linear and circular interpolations. With NURBS interpolation, manufacturing tool paths are not only more accurate and lead to lighter program files [26], but also are carried out more smoothly on the machine tool because of their dynamic parameters are smoothly continuous. Machined surfaces using NURBS show very few cutting marks because feedrates are more stable than those with linear interpolation. To illustrate this capability, Figure 16 compares the feedrate simulation results for a wave geometry machined with either linear interpolation or NURBS format. It appears that the NURBS format enables the real feedrate to be very close to the programmed values along the waveform. In contrast, with linear interpolation, the real feedrate is often quite low with many undesirable variations.

12 12 J.-Y. Hascoet and M. Rauch Figure 17: STEP-NC a unique formalism from CAD to NC controller [19]. In this area, ECN ITP has been one major concrete output of EU FP7 project FoFdation concerning the development of a smart NC controller. The integration of an opennc to a SPAIM controller enables to realize true benefits from STEP-NC, such as on line toolpath parameters modification according to the shop floor live situation (planned cutting tool not available) or machining strategy change by adapting to machine tool kinematic characteristics (Figure 18). These benefits come from the total control on the internal interpolation algorithms enabled by the opennc controller Realization of a fully STEP-NC machine tool controller With the same purpose of STEP (STandard for Exchange of Product data model) standard (ISO 10303) [17] for CAD, STEP-NC standard [9] has been developed to significantly enhance the communication between CAD/CAM systems and NC controllers (Figure 17). The manufacturing stage is fully incorporated in the product development process: modifications made on the shop-floor stage can be saved and fed back to the design stage, as ISO is fully STEPcompliant [25]. In addition, the STEP-NC approach is based on a unique NC formalism which is completely machine tool independent; thus, no post-processing operation is needed. In practice, the communication language used for STEP-NC is a high level one; the data exchanged are related to the machining features rather than low level axis coordinates control [27, 28]. In conclusion, STEP-NC enhances the manufacturing equipment effectiveness by developing intelligent programming approaches and making these equipment and software tools more interoperable by exchanging seamlessly high level data along the manufacturing data chain [19]. STEP-NC object-oriented programming shifts the toolpath generation to the machine NC controller. This opens a way to intelligent machines tools and essential interests of STEP-NC approach are dependent on the capabilities of the NC controller. Research efforts led to the emergence of various STEP-NC compliant platforms [15, 25, 28], such as SPAIM developed by the authors [20] which was enabled for industrial HSM machines tools. Up to know, SPAIM has been employed in combination with the existing legacy controllers of the tested machines. It has the advantage of making STEP-NC applications available with current manufacturing equipment, within a limited scope. To fully benefit from STEP-NC advanced approaches, access to the core of the NC controller is necessary. OpenNC are consequently ideal candidates Towards multi-process manufacturing This first successful implementation of multiple opennc controllers for machining opens a way to an extension of opennc projects for Additive Manufacturing (AM). The great ability to access, control and optimize each stage of the manufacturing process from CAD to NC process control will be of great use for the AM processes. Another important aspect is its ability to produce multi-process parts by benefiting from a unique integrated approach for material removal and additive manufacturing operations. STEP-NC multiprocess manufacturing has already been proposed by the authors in [11]. The same concepts and methods developed for advanced programming for machining are ideally implementable on machines tools with opennc controllers. For example, an experiment has been conducted to compare integrated multi-process AM/HSM approaches versus single HSM process in the case of a pocket with bosses manufacturing, from the STEP-NC part program to the actual physical part. The usual classical HSM manufacturing was implemented as a reference: HSM tool paths are generated by SPAIM by for both pocket and bosses features. Then the multi-process capabilities of SPAIM were test on a second similar part. HSM was used for the pocket and CLAD for the bosses. Bosses geometries were automatically removed to generate the milling tool paths, which were consequently simpler. Then, the bosses have been cladded with the AM process on the same machine, within the same numerical environment. Figure 19 illustrates the results of the study. The effectiveness of multi-process methods for these activities is evident. The use of opennc controllers to control both HSM and AM tool paths on the same machine will lead to optimize the parameterization of each process to their actual implementation. As the tool path generation and control are carried out by the NC controller, it will be possible to

13 Enabling CNC Programming with opennc Controllers for HSM Tools 13 Figure 18: Architecture of SPAIM with opennc controller implemented on the ITP. Figure 19: Single and multi-process scenario for the same part.

14 14 J.-Y. Hascoet and M. Rauch adapt them to the actual machining situation on the machine tool. 5 Conclusions The paper introduced the use of opennc controllers for intelligent machine tools with flexibility and adaptivity. Among the existing projects, LinuxCNC has been selected, implemented on an industrial HSM machine and augmented with advanced CNC programming approaches to become an intelligent integrated test platform. The methods proposed here covered most of the challenges associated with current manufacturing approaches, which include the use of high level tool path interpolation algorithms, the information consistency into the design-to-manufacturing data chain and the emergence of multiprocess approaches which integrate material removal and additive manufacturing operations. For all of these promising methods, the NC controller will have a central role. 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