基于网络信息的自动化控制英文毕业论文Control Based on Network and InformationA Automation Networking Application AreasNetworking is used in all areas of automation. In factory automation, process automation and building automation networks perform diverse tasks. Likewise, there are distinct differences between tasks performed for applications in different industry sectors that all have unique characteristics and consequently varying requirements. The way devices are connected, configured, and exchange data also differ.There is no one-size-fits-all for industrial networks; rather, buses are optimized foter different characteristics. For example, factory automation and process automation are often used in harsh and hazardous environments where a production interruption is costly. These requirements contrast significantly with building automation, for example, where keeping costs low is a main driving force. Factory AutomationFactories with assembly-line manufacturing, as in the automotive, bottling, and machinery industries, are predominantly controlled using discrete logic and sensors that sense whether or not, for example, a process machine has a box standing in front of it. The network types ideal for simple discrete I/O focus on low overhead and small data packets, but they are unsuitable for larger, messages like configuration download and the like. Examples of this network type are Seriplex, Interbus-S, and AS-I (AS-Interface), which are sometimes called sensor buses or bit level buses. Other more advanced protocols oriented toward discrete logic include DeviceNet, ControlNet, and PROFIBUS (DP and FMS application profiles). These buses are sometimes referred to as device buses or byte-level buses. Factory automation involves fast-moving machinery and therefore requires quicker response than slower processes. Traditionally, these tasks have been handled by PLCs. Process AutomationProcess plants in industry segments like refining, pulp & paper, power, and chemicals are dominated by continuous regulatory control. Measurement is analog (here meaning scalar values transmitted digitally), and actuation is modulating. Of course, process industries also use some discrete control and the predominantly discrete manufacturing industries use some modulating. Fieldbus on/off valves are already available in the market, as are small remotely mounted I/O modules for discrete sensors. In the past, a DCS or single-loop controller did this. Process-related network include FOUNDATION Fieldbus, PROFIBUS (PA application profile), and HART. All these buses as a category are now typically referred to as fieldbus (without the capital F), though some would argue that one or the other does not belong. These three protocols were specifically designed for bus-powered field instruments with predefined parameters and commands for asset management information like identification, diagnostics, materials of construction, and functions for calibration and commissioning. In term of size, the networks used in industrial automation are considered to constitute local area networks (LAN) spanning areas no greater than a kilometer or two in diameter and typically confined to a single building or a group of buildings. Networks that extend only a few meters are insufficient, and networks that span cities or even the globe are overkill. Field and Host Tier NetworksEven within control systems for the process sector there is a need for different network characteristics at each tier of the control system hierarchy. At the field and there are instruments such as transmitters and valve posittioners that have their specific needs, and valve positioners that have their specific needs (Fig.5-1A-1). When fieldbus began to evolve, the process industry put a large number of requirements on the field-level network that were not met by other types of networks. Many new design considerations needed to be taken into account. On the upper tier, data from all the field-level networks have to be marshaled onto a single host-level network that also serves any tasks the plant may have that seem related to factory automation.Fig.5-1A-1Field LevelAt the field level, the dominant protocols for process instrument are HART, FOUNDATION Fieldbus H1, and PROFIBUS PA.HART is significantly different from the other two in that it is a so-called smart protocol, that is a combination of digital communication simultaneously superimposed on a conventional 4-20mA signal. As such, the HART protocol has been an ideal intermediate solution in the transition from analog. HART is compatible with existing analog recorders, controllers, and indicators while at the same time it makes possible remote configuration and diagnostics using digital communication. The HART protocol does allow several devices to be multidropped on a single pair of wires, but this is a capability infrequently explored because of the low update speed, typically half a second per device. For a