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1、基于以太网的高精度时间同步实时数控系统摘要：本文提出了一种时间同步的方法来实现高精度的分布式时钟，基于实时以太网的电脑数控(CNC)系统的环型拓扑。建议同步方法采用频率补偿和对等透明时钟机制，利用一个实时的同步协议成实时嵌入的时间同步通信周期。一个最佳的比例积分控制器，然后设计的频率补偿基于模型和分析机制时间同步的过程。此外，硬件这段时间同步策略的实施进行说明。最后，一个实时的基于以太网的数控系统具有环形拓扑的建立和实验进行验证的性能提出同步方案。实验结果表明，提出的时间同步方法实现同步精度高。峰 - 峰抖动测量，只有一个和四个波动，分别为42.42和95.5纳秒。关键词：网络化数控系统；时间
2、同步实时以太网；频率补偿；透明时钟1、引言随着工业的不断提高自动化和现代化的制造系统，有应用共享数据的不断增长的需求网络控制系统1，2。例如，在应用程序中与大量的传感器，致动器或控制器，如计算机数值控制（CNC）加工中心，柔性制造系统，印刷机器和机器人3-5，实时控制网络可以被用于执行信息分布式节点之间的交流。而不是传统的集中制造系统直接配线设备连接在一起，这些基于网络的系统提供了多种优势，如可扩展性，可靠性，和远程控制能力6。另外，它们减少了配线连接的问题，并传输长度的限制和减少安装，重新配置，维护时间和成本5。这些类型的计算机集成制造系统被称为网络化数控系统，其中的功能模块，如传感器，伺服
3、驱动器，以及数字控制（NC）电脑分布通信网络互连7。在现场级网络化数控系统，大量的命令和反馈信息定期在分布式节点之间传输高频率。因此，重要和具有挑战性在这些应用中的问题是如何控制反馈信息是准确和即时传输，以及对每个单独的传感器或执行器实现同步采样或同步致动1。要制定出这些问题，许多不同的实时控制网络，如控制区网络，Profibus和串行实时通信规范，已在网络数控系统在过去的十年8-10。然而，随着网络数控系统的快速发展，带宽将成为这些领域的瓶颈巴士提供高速通信highaccuracy多轴协调控制。因此，实时以太网（RTE）作为替代品已经出现网络数控系统的信息交流由于其低廉的价格，确定性通信，和
4、高传输速度7，11，12。在实时以太网系统，必须在每个节点的本地时钟一些方法来同步管理通信周期和消息调度策略13。主要有以下三个方法同步是艺术的研究：网络时间的状态协议（NTP），全球定位系统（GPS），IEEE1588标准14。NTP是最流行的协议，用于NTP是最流行的协议，用于广域网应用工作或局域网络，以确保同步精度在订单上的毫秒15，16。然而，这是在RTE的数控系统作为网络不足其发送周期为几毫秒。对全球定位系统，工作微秒14的顺序，供资由美国能源部和控制防御。但是，一个GPS接收器，需要在每个节点在广泛和明确外，应安装环境，因为GPS信号很容易受阻建筑和金属12。第一个IEEE 158
5、8是在2002年存在了第一个IEEE 1588标准化是标准化在2002年和指定一个精确时间协议，具有高同步精度在次微秒的范围内，其最新的版本是IEEE 1588-2008 17，它定义了一个透明的时钟，来解决这个问题的指数积累的时间同步误差在级联网络。基本思想IEEE 1588是使用时间戳里面包交换时节点之间的信息。最近，一些研究人员已经提出并论证IEEE1588的时间同步解决方案，为网络数控系统12。提出了一种时间为分层的真实timenetworked的数控系统基于以太网/互联网的同步策略。采用IEEE 1588协议，以满足硬实时任务的要求，同时结合GPS和NTP解决同步之间的的NC服务器和
6、数控核心问题的计算机。然后，基于IEEE 1588时间同步和被认为是实时通信在18。考虑到要求在联网的数控高精度同步系统，一个集总的基于帧的实时通信用硬件实现的协议IEEE 1588协议中引入19。上述研究结果给了我们一个明确的说法在网络的不同的时间同步方法数控系统。然而，频率调整从时钟和时间同步级联网络不被认为在这些同步方法，这两个方面也很重要。网络数控系统的同步。究其原因是采样周期内的位置，速度，由从属时钟驱动电流回路，和频率它们之间的差异，最终会影响精度the motion 20。此外，线型或环型拓扑外地一级的网络是常用的数控系统由于其布线成本低。因此，本级联交换机引入的误差积累这些网络
7、的拓扑结构，应考虑在同步的方法。在本文中，我们提出了一种基于IEEE 1588的时间同步计划，以实现高精度分布式在现场级网络数控的时钟系统，这是用来定期安排所有任务并进行同步采样或驱动之间的分布式轴。不像我们以前的工作21异步采样或驱动工作时间同步方案，而不是由在一个控制器中的补偿算法。本文作出以下贡献：首先，建议同步方法采用频率补偿和对等透明时钟机制，从而实现高精度在网络数控系统与分布式时钟环形拓扑结构。此外，实际的时间同步协议被设计为将同步到实时通信周期的方法。此外，一个最佳的比例 - 积分（PI）的时钟伺服要取得同步精度高基于该模型和分析的时间同步的过程。事实证明，最优PI时钟伺服can
8、 give的最小积分平方误差（ISE）的性能指标。该同步算法是很好地适应所有基于IEEE 1588的实现，特别是对基于硬件的实现.第二，我们以前的工作22的基础上，我们给出了硬件实现的时间同步策略，从而实现实时同步协议和主要任务数控系统，通过数字信号处理器（DSP），现场可编程门阵列（FPGA），和实时以太网技术。在发达的实施，一种改进的媒体访问控制器（MAC）协议结合的运动的一些功能控制在一个FPGA芯片。此外，分布式节点在修改后的MAC协议，通过同步“飞”的时间戳，以节省网络带宽和处理时间。三，实验平台使用的实时以太网环形拓扑的建立和实验进行验证的有效性所提出的同步方法。实验结果表明，提出
9、的同步计划实现同步精度高现场级网络化数控系统。峰峰值抖动测量只有42.42和95.5纳秒一和四啤酒花，分别。本文的其余部分组织如下：第2节是专门配方的问题。在第3节，我们提出了一个同步方法解决了问题，网络的时间同步数控系统。然后，在硬件实现提出的时间同步策略第4节中描述。几个实验conducted conducted的第5节。在最后一节，结论和未来研究的角度。2、时间同步的问题基于RTE-数控系统如上文7中，网络结构在分层实时网络数控系统有三个不同的级别，即非实时的网路，软实时以太网，以及硬实时以太网。这分类的功能，以及基于在网络化数控系统的信号特征工业以太网的透明模型。该这项工作的主要目的集
10、中的时间同步在现场级联网的数控系统，这表明硬实时以太网。该网络用于通信的物理信号，如位置，速度，电流，由装置网络编码的信息，其特点包括线型或环型拓扑结构网络是常用的架构。命令和反馈信息定期分布式节点之间数据传输的大小是小的，但可能必佳的发送频率23。现场级的系统架构联网的数控系统示于图中。 一个主从系统架构具有环形拓扑在数字通信系统。 CNC system controls整个数字通信，由插补，位置控制，PLC，NC代码解释，刀具补偿，运动控制等7。伺服和I / O现场设备。在网络化数控系统，设备分布，并有自己的处理单元和定时功能。因此，同步所有设备是非常重要的。高精度的主要有三个问题,同步，
11、它可以被描述如下：1在数控系统中的抖动：数控系统是基于在标准PC上使用一个网络接口卡（NIC）来实现信息交流所述现场设备。当数控系统的选择的主节点中的时间同步，同步精度主要是确定其可靠性低的时间戳。即使一个实时操作系统是使用的时间戳的精度是微秒的顺序12。2时间同步算法分析与设计：这个问题，包括如何构建从时钟的频率调整如何建模和算法驱动设计这些从属时钟。3同步误差积累在RTE网络，环型拓扑：1588 - 2008已经定义了一个透明时钟来解决同步问题的指数积累级联网络错误。然而，离开的方法整合透明打开实时通信时钟机制。如果NIC有足够的缓冲区的微控制器计算能力，因此，它可以接收从数控系统的命令和
12、完成一些任务，如精细的内插时，位置环路，与现场设备和信息交流。然后，第一个问题很容易通过设置处理作为主节点的网卡，因为基于硬件的时间戳，可实现在所有的网络节点。将要描述的网卡和硬件实现的后来。因此，时间同步方法在下一节中提出处理余下的两个问题，实现高同步基于RTE-数控系统的精度。High-precision time synchronization in real-timeEthernet-based CNC systemsXing Xu Zhenjiang Xing Xinhua Wu Xiang yang ZhuReceived: 13 November 2011 / Accepted:
13、 17 May 2012 / Published online: 1 June 2012 Springer-Versa London Limited 2012Abstract: This paper presents a time synchronization method to achieve high-precision distributed clocks in real-time Ethernet-based computer numerical control (CNC) systems with a ring topology. The proposed synchronizat
14、ion method adopts the frequency compensation and peer-to-peer transparent clock mechanisms, and utilizes a real-time synchronization protocol to embed the time synchronization into the real-time communication cycle. An optimal proportional-integral controller is then designed for the frequency compe
15、nsation mechanism based on the model and analysis of the time synchronization process. Besides, the hardware implementation of this time synchronization strategy is described. Finally, a real time Ethernet-based CNC system with a ring topology is built, and experiments are conducted to verify the pe
16、rformance of the presented synchronization scheme. The experimental results demonstrate that the proposed time synchronization method achieves high synchronization precision. The peak-to-peak jitters are measured to be only 42.42 and 95.5 ns for one and four hops, respectively.Keywords: Networked CN
17、C system Real-time Ethernet Time synchronization Frequency compensation Transparent clock1 IntroductionAlong with the continuous improvement in industrial automation and modern manufacturing systems, there are increasing demands for applying a shared data network in control systems 1, 2. For instanc
18、e, in applications with a large number of sensors, actuators or controllers, such as computer numerical control (CNC) machining centers, flexible manufacturing systems, printing machines and robots 35, real-time control networks can be used to perform information exchanges among distributed nodes. R
19、ather than traditional centralized manufacturing systems with directly wiring devices together, these network-based systems provide several advantages such as scalability, reliability, and remote-control capability 6. In addition, they reduce the problems of wiring connection and transmit length lim
20、itation and decrease installation, reconfiguration, and maintenance time and costs 5.These types of computer-integrated manufacturing systems are called networked CNC systems, where functional modules such as sensors, servo drivers, and the numerical control (NC) computers are distributed and interc
21、onnected by communication networks 7. In the field level of networked CNC systems, a large amount of command and feedback messages are periodically transmitted among distributed nodes at a high frequency. Hence, the important and challenging issues in these applications are how the control and feedb
22、ack information is accurately and instantly transmitted and how each individual sensor or actuator realizes the synchronous sampling or the synchronous actuation 1. To work out these problems, many different real-time control networks, like control area network, Provirus, and serial real-time commun
23、ication specification, have been presented in networked CNC systems during the past decade 3, 810. However, with the rapid development of networked CNC systems, the bandwidth becomes the bottleneck of these field buses to provide high-speed communication for high accuracy militaries coordinated cont
24、rol. Thus, the real time Ethernet (RTE) has emerged as an alternative for information exchanges in networked CNC systems because of its low price, deterministic communication, and high transmission speed 7, 11, and 12. In real-time Ethernet systems, the local clock in each node must be synchronized
25、by some methods to manage both the communication cycle and the message scheduling 13. The following three main methods of synchronization are the state of the art of the research: Network Time Protocol (NTP), Global Positioning System (GPS), and IEEE Standard 1588 14. NTP is one of the most popular
26、protocols used for the applications working on wide area networks or local area networks, ensuring synchronization accuracy on the order of milliseconds 15, 16. However, it is insufficient in the RTE networks of CNC systems as its transmission cycle is several milliseconds. The GPS, working on the o
27、rder of microseconds 14, is funded and controlled by the United States Department of Defense. But a GPS receiver is needed at each node and should be installed in the wide and clear-outside environment because GPS signals are easily obstructed by building and metal 12.The first IEEE 1588 is standard
28、ized in 2002 and specifies a precision time protocol, with high synchronization accuracy in the sub microsecond range, and its latest version is IEEE 15882008 17, which defines a transparent clock to solve the problem of the exponential accumulation of time synchronization error in cascaded networks
29、. The fundamental idea of the IEEE 1588 is to use timestamps inside packets when exchanging information between nodes. Recently, some researchers have proposed and demonstrated IEEE1588-based time synchronization solutions for the networked CNC systems. Zhang et al. 12 proposed a time synchronizatio
30、n strategy for the hierarchical real-time networked CNC system based on Ethernet/Internet. The IEEE 1588 protocol was adopted to meet the requirement of the hard real-time tasks while it combined the GPS and the NTP to solve the synchronization issue between the NC server and the NC core computer. T
31、hen, the IEEE 1588-based time synchronization and the real-time communication were considered together in 18. Considering the requirement of high-precision synchronization in the networked CNCsystem, a lumped frame-based real-time communication protocol with a hardware implementation of the IEEE 158
32、8 protocol was introduced in 19. The abovementioned research results gave us a clear view of various time synchronization methods in the networked CNC systems. However, the frequency adjustment of slave clocks and time synchronization in cascaded networks were not considered in these synchronization
33、 methods, and these two aspects are also important for synchronization in networked CNC systems. The reason is that the sampling periods of the position, velocity, and current loops are driven by the slave clocks, and the frequency differences among them will ultimately affect the motion accuracy 20
34、. Besides, the line or ring topology is commonly used in the field level of networkedCNC systems due to its low cabling cost. Hence, the error accumulation introduced by cascaded switches of these network topologies should be considered in the synchronization method.In this paper, we present an IEEE
35、 1588-based time synchronization scheme to realize high-precision distributed clocks in the field level of networked CNC systems, which are used to schedule all periodic tasks and perform the synchronous sampling or actuation among distributed axes. Unlike our previous work 21, the asynchronous samp
36、ling or actuation is worked out by a time synchronization scheme rather than by a compensation algorithm in a controller. This paper has made the following contributions:First, the proposed synchronization method adopts the frequency compensation and peer-to-peer transparent clock mechanisms, thus a
37、chieving high-precision distributed clocks in networked CNC systems with a ring topology. Also, a real-time synchronization protocol is designed to incorporate the synchronization method into the real-time communication cycle. Moreover, an optimal proportional-integral (PI) clock servo is presented
38、to obtain high synchronization accuracy based on the model and analysis of the time synchronization process. It is proven that the optimal PI clockservo can give the minimum integral square error (ISE) performance index. This synchronization algorithm is well adapted to all IEEE 1588-based implement
39、ations, especially for the hardware-based implementations. Second, on the basis of our previous work 22, we give a hardware implementation of the time synchronization strategy, which accomplishes the real-time synchronization protocol and the main tasks of the CNC system through digital signal proce
40、ssor (DSP), field-programmable gate array (FPGA), and real-time Ethernet technologies. In the developed implementation, a modified media access controller (MAC) protocol is combined with some functionalities of motion control in one FPGA chip. Also, the distributed nodes are synchronized by means of
41、 the modified MAC protocol with “on-the-fly” time stamping, to save network bandwidth and processing time.Third, an experimental platform using a real-time Ethernet network with a ring topology is built, and experiments are conducted to validate the effectiveness of the presented synchronization met
42、hod. The experimental results demonstrate that the proposed synchronization scheme achieves high synchronization precision in the field level of networked CNC systems. The peak-to peak jitters are measured to be only 42.42 and 95.5 ns for one and four hops, respectively. The remainder of this paper
43、is organized as follows: Section 2 is devoted to the problem formulation. In Section 3, we propose a synchronization method to solve the problems in the time synchronization of networked CNC systems. Then, the hardware implementation of the presented time synchronization strategy is described in Sec
44、tion 4. Several experiments are conducted in Section 5. In the last section, a conclusion and future research perspective are given.2 Problems in time synchronization of RTE-based CNC systems As mentioned in 7, the network architecture in a hierarchical real-time networked CNC system has three diffe
45、rent levels, namely, non-real-time Internet, soft real-time Ethernet, and hard real-time Ethernet. This classification is based on the functionality as well as signal characteristics in the networked CNC system with the transparent model of industrial Ethernet. The main purpose of this work focuses
46、on the time synchronization in the field level of the networked CNC system, which indicates the hard real-time Ethernet. This network is used to communicate physical signals, such as position, velocity, and current, by the means of network coding, and its characteristics including the line or ring t
47、opology is commonly used as the network architecture. Command and feedback messages are periodically transmitted among distributed nodes; data sizes are small, but the transmission frequency may be high 23.The system architecture of the field level of the networked CNC system is shown in Fig. 1. Mas
48、ter slave system architecture with a ring topology is used in the digital communication system. The CNC system controls the entire digital communication and consists of interpolation, position control, PLC, NC code interpretation, tool compensation, motion control, etc. 7. Servo and I/O are the fiel
49、d devices. In the networked CNC system, devices are distributed and have their own processing units and timing functions. Hence, synchronization of all devices is extremely important.There are three main problems to high-precision synchronization, which can be described as follows:1. Jitter in the CNC system: The CNC system is based on a standard PC and uses a network interfac