440521$76955208$基于PLC的音乐喷泉系统设计 - 图文

北方民族大学学士学位论文 基于PLC的音乐喷泉控制系统--软件设计

registers and can fetch data on an absolute address basis. This can lead to data processing errors (e.g., from the wrong input) you won't encounter with the central database of a DCS. Some PLCs use proprietary networks, and others can use LANs. Either way, the

communication functions are the same-fetch and put registers. This can result in bottlenecking and timing problems if too many PCs try communicating with too many PLCs over a network. A PLC may have a third-party package for operator interfaces, LAN interface to PCs and peripherals, PLC data highway or bus, redundant controllers with local and distributed I/O, local MMI and local programming capability. The PLC would have redundant media support, but not the redundant communication hardware or I/O bus hardware you'd find in a DCS. A PLC would have preprogrammed I/O cards for specific signal types and ranges.

In the beginning, there was the microprocessor. This small chip, with its integrated functionality, laid the technological foundation for the birth of PLC and DCS. The first PLC application was in the automobile industry in 19'70. Necessity to satisfy application requirements, as requested by General Motors, became the mother of invention for the PLC in discrete manufacturing. The PLC was first used commercially in machine control applications for metal cutting, hole drilling, material handling, assembly and testing for GM's Hydramatic Model 400 automatic transmission .

The PLC replaced hard-wired electromechanical relays and provided greater flexibility by eliminating the necessity of reworking hard-wired panels to accommodate process and application changes. The PLC allowed engineers to make faster and easier production changes that translated into dramatic reductions in both cost and cycle time. PLCs have dramatically evolved since their inception and have greater functionality and application. Advances in operator interface software have provided engineers with a user friendly environment and a configurable window into all process control applications.

The DCS was introduced in the mid 1970s and revolutionized the process control industry. The DCS was the first practical and comprehensive replacement by large and bulky hard-wired custom control panels. At that time, DCS control philosophy was centralized and liability was minimized through distribution over the entire control system. It offered for the first time, the ability for all engineering disciplines to program a process control computer through configurable software without necessarily requiring high level programming skills. DCS provided the user with larger boundaries on flexibility, speed of control, security, manufacturing consistencies and reliability than ever before.

Generally in-process control, the procurement of a control system is the largest single

expenditure automation engineers are faced with. The control system, in most cases, is the brain and nervous system of a production facility. The purpose of the control system is to supervise, control, monitor, schedule, document and record process parameters that are vital to production. Since automation is critical to manufacturing, selection of a control system, by definition, is equally important. Trying to decide between control systems with similar functionality can be very subjective, confusing and time consuming.

Once the Want objectives have been defined, a weight is assigned to each with respect to its relative importance. The most important objective is given a weight of 10, all other objectives are weighted in comparison with the first, from 10 down to 1. All Want objectives are evaluated and scored for degree of compliance, on a scale from 1 to 10. The weighted Want objectives are then multiplied by the score to result in a weighted score. Selection is awarded to the control system

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北方民族大学学士学位论文 基于PLC的音乐喷泉控制系统--软件设计

with the highest total weighted score. This structured analytical process for control system selection, is efficient, less subjective and quantifiable. Engineers can use these tools easily to quantify and justify both technical and commercial selection of PLC/DCS control systems. Comparing the differences between the functions of PLC and DCS for the purposes of

selection has become more difficult for engineers today. There no longer exists a clear black and white difference in application and functionality that was so evident in the 1970s. PLCs over the past two decades have entered applications that were traditionally in the DCS domain. This trend continued into the 1980s and 1990s and has, to some degree, polarized control engineers to voice strong preferences in favor of application solutions using PLCs, DCSs or a hybrid of both in certain applications. The continued encroachment of PLCs into the DCS domain has consistently increased from sharing limited common functionality in the 1980s to major overlapping of functions in the 1990s. The DCS has also taken advantage of new technology to improve its performance to handle discrete signals. Some DCSs have incorporated functionality such as relay ladder logic, function block and structured text programming that were traditionally found in PLCs. By the year 2000, expect to see a functional convergence of the two systems into one unified PLC/DCS control system for large applications, costing the same as today's PLC-based control systems.

Comparing and choosing between PLC and DCS based control systems can sometimes be quite difficult, tedious and somewhat subjective, depending on the application. The user's technical background as well as work experience. The debate between which system is a better choice when it comes to application, cost and performance, will be decided only by the user in his final selection. The intention of the following common system issues, is to highlight differences most often encountered in the selection process.

Cost. Initial hardware and software costs, in almost all cases, are less for PLCs than DCSs. However, in a large system that requires heavy integration and custom coding, PLC software cost may be higher and will cancel any cost savings that may have been realized initially. It is essential for this reason to define accurately and completely all system functions and needs in the form of a user's requirement document and carefully evaluate compliance of control system proposals accordingly.

Batch sequencing. In large processes that require production of multiple products and/or batches, including recipe management, the DCS outperforms the PLC. However, in small,

dedicated batch production with limited recipes, where batch management is not critical, PLCs are a cost-effective and practical solution.

Security.DCS networks are designed to offer high availability and full redundancy in all system components, with no single point of failure. Tight coupling between operator interface, controllers and system software allows greater security and assurance that all components will work well together. Not all levels of PLCs can offer the same system-wide redundancy, and similar DCS features must be designed in to the PLCs by the user.

Reliability . Historically, reliability for both PLCs and DCSs has a solid track record and is rated equal.

Diagnostics. Certain PLCs and DCSs can be considered almost equal in diagnostics. However, DCSs can handle system diagnostics more easily. Both can go down to reading

diagnostics at a point level as well as health of hi way, 1/O cards and CPU loading. Since system reliability is very high for both PLC and DCS, this may not be a major consideration.

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