DCS PLC Difference


In the field of thermal automation of thermal power plants,DCS andPLC It's two completely different and inextricably linked concepts.DCS andPLC It's a combination of computer technology and industrial control technology, The main control system of thermal power plant usesDCS, andPLC Mainly used in auxiliary workshop of power plant.DCS andPLC There are operator stations to provide means of human-computer interaction, All rely on the controller based on computer technology to complete the control operation, All passI/O The card completes data exchange with primary element and actuator, All have communication systems called networks.DCS andPLC So similar, Why there are totally different concepts, How to choose in engineering practice? This article from the historical evolution, Technical characteristics, A summary of development direction, I hope it can be used for reference for thermal engineering professionals. Among themDCS The situation isNETWORK6000+ take as an example, Try to give examples, elaborate and clear.

1,DCS andPLC Historical evolution and core concepts of



DCS English for DCS(TOTAL DISTRIBUTED CONTROL
SYSTEM) Abbreviation. The dispersion of danger, Data set.70 Entering the market in the mid-s, Complete analog quantity control, Instead ofPID Operation based analog control instrument. First put forwardDCS The idea is that instrument manufacturers, At that time, it was mainly used in the chemical industry. andPLC to60 Developed in the late S, A programmable controller called logic operation(Programmable
Logic Controller), AbbreviationPLC. Mainly used in automobile manufacturing industry.

DCS andPLC The design principle of,
PLC It is developed from imitating the original relay control principle,70 AgePLC Only switch logic control. It performs logical operations with storage, Sequential control, timing, Instructions for operations such as counting and operation; And operate through digital input and output, To control all kinds of machinery or production process. The control program compiled by the user expresses the process requirements of the production process. Deposit it inPLC User program memory of, Run according to the content of stored program one by one, To complete the operation required by the process flow.

DCS It was developed on the basis of op amp. Put all the functions, The relationship between process variables is designed successfully.70 Mid-sDCS Only analog quantity control.


DCS andPLC The main difference of the controller is in the operation of switching value and analog value, Even though the two later infiltrated each other, But there are still differences.80 After the age,PLC Except logical operation, Some control loop algorithms are also added, But it is difficult to complete some complex operations,PLC Programming with ladder, The operation of analog quantity is not very intuitive in programming, Programming is cumbersome. But in terms of solving logic, Show fast benefits. andDCS Encapsulate analog and logical operations with function blocks, The expression of logical operation and complex analog operation is very clear, But relativePLC The expression efficiency of logical operation is low.


DCS andPLC The differences in history are obvious, Has a significant impact on their subsequent development. however, The most influential, It's not a technical difference of origin, It's the difference in the concept of its origin.DCS The core concept of risk dispersion, Computer control system of data set, thereforeDCS Development process of, It's the constant use of computer technology, Latest achievements in communication technology and control technology, To build a complete distributed control system,DCS It provides users with a complete, safe, reliable, efficient and flexible solution for industrial control. andPLC The core concept of PLC, The purpose is to replace the relay, Execution logic, Time, Sequential control functions such as counting, Establishment of flexible program control device. therefore,PLC The main line of continuous development is to continuously improve various capability indexes, Provide users with a perfect and flexible control device.

DCS It's a system,PLC It's a device, This is the fundamental difference between the two concepts. The impact of this difference is profound, It permeates every aspect of technology economy.

2,DCS andPLC Technical characteristics and mutual penetration

Different conceptual basis, Different development paths makeDCS andPLC They have different technical characteristics, And the development of technology is not closed, Mutual learning and mutual penetration also run through the development process.

2.1, Control processing capacity




We know, OnePLC Controller, Can handle thousands ofI/O spot( Up to up to8000 MultipleI/O). andDCS Controller, Generally, only a few hundred can be processedI/O spot( No more than500 individualI/O). Is itDCS Is the technical level of developers too poor? I'm afraid not.. In terms of the requirements of distribution system, Control concentration is not allowed, The controller with too many points is useless in practical application,DCS Developers don't bring a lot of development at allI/O Demand driving of point controller, Their main focus is to provide the reliability and flexibility of the system. andPLC Dissimilarity, As an independent flexible control device, The stronger the ability to bring points, of course, means the higher the technical level, As for the application level of the whole control system, It's mainly about the engineers and users, Instead ofPLC Manufacturer's core objectives.


Another indicator to control processing capacity, Computation speed, In people's impressionPLC Also ratioDCS Much faster. From a certain point of view, It's true,PLC High efficiency in performing logical operations, implement1K Logic program does not arrive1 Millisecond, Its control period( withDI Input direct deliveryDO Output as an example) Can be controlled in50ms within; andDCS In the same way when dealing with logical operation and analog operation, The control period is usually100ms Above. We usePID Algorithm to compare, Can be foundPLC Execute onePID In milliseconds, andNETWORK6000+DCS OfT2550 Controller solves onePID Also need1 Millisecond, This explanationPLC andDCS It's equivalent to the actual computing power, Of this modelDCS Controller even stronger. The difference in control period is mainly related to the scheduling design of the controller. largePLC Often use a pairCPU To complete the operation of analog quantity, mainCPU High speed switch operation, So even if the speed of analog operation is average, The speed performance in switching control is excellent. andDCS At the same speed to deal with switching and analog operations, The index of control period is not ideal. New-typeDCS Controller learning largePLC Design, Significant improvement in control cycle performance. withNETWORK6000+DCS OfT2550 Controller as an example. The controller can set four tasks with different priorities, The minimum operation period can be set to10ms, Cooperate with high speedI/O Card parts, The control cycle can reach15~20ms. While the analog operation is set in other tasks with long cycle.

2.2 Data communication exchange


Data communication exchange mainly refers to the control system network and its data exchange form. In this respectDCS Has a natural advantage. Distributed system“ Dispersed” Mainly reflected in the independent controller,“ focus” Mainly reflected in the human-computer interaction device with complete data, And it's the network that connects the distributed and centralized systems. therefore, fromDCS Early stage of development, The Internet has becomeDCS Core technology direction of manufacturers, Redundancy technology, Narrowband transmission technology isDCS The first successful R & D or application.PLC Mainly designed according to independent devices, his
“ network” It's actually serial communication.


Development and wide application of industrial Ethernet Technology, Flattened in formDCS andPLC Network gap. On the surface, a lotDCS andPLC Industrial Ethernet is applied, But in fact, the gap still exists. With manyPLC AdoptedMODBUS-TCP Take example.MODBUS Is serial communication protocol, Not network, There's no doubt;MODBUS-TCP Is it a network?? A lot of people have questions. Careful analysis,MODBUS-TCP Will beMODBUS Communication protocol loaded to EthernetTCP A way of communication over protocol, It has the shape of a network, But it is still a management mode of one master and many slaves, Transmission structure of data table. andDCS What about, Network6000+DCS OfELIN Net as an example, Although it is also based on Industrial Ethernet, But its application layer protocol is that European companies have accumulated nearly30 Ownerless token ofLIN Network protocol, stay1M OfOLIN,2.5M and20M OfARCNET There are long-term successful applications in Shangdu.ELIN online, Equality of stations, No main management station. And data communication is structured data with modules as units, Data management capability is comparable in non data table way.


withPID Module as an example, The basic data arePV,SP,OP, Data table transmission, You have to definePV,SP,OP The data address of is01,02,03, Other stations also receive data in the form of data tables, however01 What data is it?02 What data is it? Must pass data definition table to restore. The management of data table is cumbersome and error prone, Tens of thousands of data points in a large system adopt this method, Tile in datasheet for management, It's terrible. andNETWORK6000+DCS Module based structured management, Will onePID Process as a module, To visit itPV value, First access its module, withPID.PV In the form of. This will tile all the data, Classification and attribution are concentrated in small boxes, By module. Manage by component, The efficiency of management is greatly improved.


PLC Problems of data communication and exchange, Mainly fromPLC It has been developing as an independent device for a long time, No system concept; And it is mainly used in small control system, The problem is not obvious, So the development is slow. There are also some largePLC We have improved in this respect, But to achieveDCS It will take a long time to reach the level of.

2.3, Configuration maintenance function



Configuration maintenance function includes logic configuration, Download modification, Operation and debugging, Remote diagnosis, etc.


early stage,PLC Mainly ladder,DCS Mainly based on module function diagram. After years of development, Adopted by International Electrotechnical CommissionIEC1131-3 The standard specifies five programming languages, Current mainstreamDCS andPLC All indicate that they meet this standard, Support several or all of the programming languages. Considering development efficiency and program readability, Module function diagram and sequence function diagram are becoming the main programming methods, Ladder logic and structured text become the development tool of custom module. largePLC More and more similar in configuration modeDCS, The gap is narrowing, And smallPLC Still dominated by ladder.


DCS After years of development, Accumulated a large number of advanced algorithm modules. for exampleNETWORK6000+ Device level module with, In one module, the basic control and fault alarm functions for equipment are completed, This module has also been passed as a unit in network communication, Greatly improve the efficiency of software development. A device pole module is equivalent to0.5K Ladder logic quantity of,PLC To do the same, It's going to be a lot more complicated.


Modifying in download, Operation and debugging, Remote diagnosis,PLC Lack of Solutions. andDCS From the beginning of design, it is from the perspective of system needs, With years of accumulation of perfect solutions. withNETWORK6000+DCS take as an example, The system can modify the control strategy online, You can also download control policies Online, During modification and download, No impact on the normal operation of the system.NETWORK6000+DCS Perfect virtualDCS function, It can not only be used for verification of configuration logic, And can be built into a complete virtualDCS Connect to model, Complete the simulation and debugging of the system.NETWORK6000+DCS Perfect safety measures, Provide remote debugging scheme based on WAN.

2.4, Hardware package structure



PLC Generally large bottom frame, ClosedI/O Module, Advantages and improvement of closed structureI/O Reliability of modules, Anti RF, Antistatic, Anti injury.PLC ModularI/O Number of points8 spot,16 spot,32 spot.

DCS Most of them are19 Inch standard case with plug-inI/O Module,I/O The module is of exposed structure. For each moduleI/O Number of points8 Point and16 spot, Rarely used32 Point module.


DCS This kind of structure originates from its application field mainly in large control objects,19 Inch standard case for dense layout, FewerI/O The number of points is due to the requirement of dispersion.PLC Bottom layout of机架,封闭式模件结构在管理和配置上更加灵活,单个设备的可靠性更高.因此,不少DCS也吸收了PLC在结构上的优点,采用了和PLC相似的封装结构,如I/A采用金属外壳,
NETWORK-6000+采用导电塑料外壳.

2.5,人机交互装置


在早期,DCS作为一个系统,其人机交互装置是DCS厂家提供的专用装置.而PLC厂家一般不提供人机交互装置,往往由工程商自主采用通用的监控软件来完成(如ifix,intouch,组态王).DCS集成的人机交互装置往往有着功能较专业,稳定性较好的特点,但是其价格也很高.随着PC技术的快速发展,一些通用监控软件发展很快,功能和性能逐渐超过了DCS厂家提供的专用装置.因此不少DCS厂家逐步放弃了专用的人机交互装置,转而和PLC一样也使用了通用的监控软件.DCS厂家使用通用监控软件并不是简单地拼装,而是在通用监控软件的基础上,通过合作开发,将自已多年积累的网络通讯技术,系统自诊断技术以专用软件包的形式保留和继承下来了.


例如,NETWORK-6000+早期曾经使用过基于专用操作系统的T1000人机交互系统,而目前主要使用基于FIX/IFIX或INTOUCH的T3500人机交互系统.其中的LINPOLL网络通讯包是由欧陆公司开发集成的.

3,DCS和PLC的市场情况和发展方向


在热工自动化领域,主厂房控制系统基本上毫无例外地使用DCS.而在辅助车间才使用PLC.其主要原因是早期的DCS系统非常昂贵,人们认为辅助车间的运行可以间断,可靠性要求不是很高,且模拟量控制要求较少,从降低成本的角度出发,往往选择PLC来构建控制系统.而锅炉,汽机和发电机的控制系统,要求长期稳定可靠地运行,信号中含有相当比例的模拟量,从系统的性能出发,人们不得不选择了昂贵的DCS.


另外,分析一下主厂房DCS和辅助车间控制系统的市场竞争情况,我们会发现一个有趣的现象.主厂房DCS的竞争往往在不同品牌的供应商或代理商之间展开,竞争激烈,DCS的价格不断下调.而辅助车间控制系统的竞争往往在同一品牌PLC的各个工程商之间进行,门槛较低,竞争更加激烈,但是PLC的价格下调幅度却并不如DCS明显.主要原因是DCS的生产商直接参与竞争,在巨大的市场压力下,不断下调设备制造费用和工程实施费用.而PLC的生产商不直接参与竞争,各个工程商只能下调自身有限的工程费用,空间有限.从现在情况看来,DCS与高档PLC的价格差距已不明显,辅助车间仍然较多地采用PLC,是市场的惯性使然.


随着国内电厂装机容量的不断扩大及电力系统改革的推进,对辅助车间控制的要求也不断提高,在这个大环境,DCS系统进入辅助车间控制已成为趋势.NETWORK6000+DCS因其综合的技术经济优势,已经并将继续在辅助车间控制方面发挥越来越大的作用.在辅助车间应用广泛的PLC也并不会就此退出热工自动化的历史舞台,前所未有的竞争压力,将会促使PLC厂商在技术上向DCS标准靠拢,在价格上作出更大的努力.
市场竞争的结果,将使用户获获得更大的利益.

4,结论


DCS和PLC作为计算机技术和控制技术结合的产物,为火电厂热工自动化水平的提高都作出了各自的贡献.由于两者在应用上有较大的相通性,在不同的时期,其各自的技术或价格优势,都会直接影响到其市场地位.而市场的反应也会或快或慢地反映到各自的技术发展和价格调整上.从总的趋势来看,DCS和PLC在技术上的融合和促进将会是竞争的主流,而在性价比方面,你来我往地不断攀升,也将是发展的主旋律.






PLC常见的六大应用


可乐的工控技术

摘要: 似乎无所不能啊,都来看看PLC在控制系统中都扮演了什么!!! 这是一张典型的PLC控制系统的框图
1,用于开关量控制,PLC控制开关量的能力是很强的.所控制的入出点数,少的十几点,几十点,多的可到几百,几千,甚至几万点,由于它能联网,点数几乎不受限制,不管多少点都能控制,所控制的逻辑问题可以是多种多样的:组合的,时序的,即
...



1,用于开关量控制


PLC控制开关量的能力是很强的.所控制的入出点数,少的十几点,几十点,多的可到几百,几千,甚至几万点,由于它能联网,点数几乎不受限制,不管多少点都能控制,所控制的逻辑问题可以是多种多样的:组合的,时序的,即时的,延时的,不需计数的,需要计数的,固定顺序的,随机工作的等等,都可进行.

PLC的硬件结构是可变的,软件程序是可编的,用于控制时,非常灵活.必要时可编写多套或多组程序,依需要调用.它很适应于工业现场多工况,多状态变换的需要.


用PLC进行开关量控制实例是很多的,冶金,机械,轻工,化工,纺织等等,几乎所有工业行业都需要用到它.目前,PLC首用的目标,也是别的控制器无法与其比拟的,就是它能方便并可靠地用于开关量的控制.

2,用于模拟量控制

模拟量,如电流,电压,温度,压力等等,它的大小是连续变化的.工业生产,特别是连续型生产过程,常要对这些物理量进行控制.


作为一种工业控制电子装置,PLC若不能对这些量进行控制,那是一大不足,为此各PLC厂家都在这方面进行大量的开发.目前,不仅大型,中型机可以进行模拟量控制,就是小型机,也能进行这样的控制.PLC进行模拟量控制,要配置有模拟量与数字量相互转换的A/D,D/A单元.它也是I/O单元,不过是特殊的I/O单元.


A/D单元是把外电路的模拟量,转换成数字量,然后送入PLC;D/A单元,是把PLC的数字量转换成模拟量,再送给外电路.作为一种特殊的I/O单元,它仍具有I/O电路抗干扰,内外电路隔离,与输入输出继电器(或内部继电器,它也是PLC工作内存的一个区,可读写)交换信息等等特点.


这里的A/D中的A,多为电流,或电压,也有温度.D/A中的A,多为电压,或电流.电压,电流变化范围多为0~5V,0~10V,4~20mA,有的还可处理正负值的.这里的D,小型机多为8位二进制数,中,大型多为12位二进制数.A/D,D/A有单路,也有多路.多路占的输入输出继电器多.有了A/D,D/A单元,余下的处理都是数字量,这对有信息处理能力的PLC并不难.中,大型PLC处理能力更强,不仅可进行数字的加,减,乘,除,还可开方,插值,还可进行浮点运算,有的还有PID指令,可对偏差制量进行比例,微分,积分运算,进而产生相应的输出,计算机能算的它几乎都能算.

这样,用PLC实现模拟量控制是完全可能的.


PLC进行模拟量控制,还有A/D,D/A组合在一起的单元,并可用PID或模糊控制算法实现控制,可得到很高的控制质量.用PLC进行模拟量控制的好处是,在进行模拟量控制的同时,开关量也可控制.这个优点是别的控制器所不具备的,或控制的实现不如PLC方便.当然,若纯为模拟量的系统,用PLC可能在性能价格比上不如用调节器.

3,用于运动控制


实际的物理量,除了开关量,模拟量,还有运动控制.如机床部件的位移,常以数字量表示.运动控制,有效的办法是NC,即数字控制技术.这是50年代诞生于美国的基于计算机的控制技术.当今已很普及,并也很完善.目前,先进国家的金属切削机床,数控化的比率已超过40%~80%,有的甚至更高.
PLC也是基于计算机的技术,并日益完善.PLC可接收计数脉冲,频率可高达几k到几十k赫兹,可用多种方式接收这脉冲,还可多路接收.有的PLC还有脉冲输出功能,脉冲频率也可达几十k,有了这两种功能,加上PLC有数据处理及运算能力,若再配备相应的传感器(如旋转编码器)或脉冲伺服装置,则完全可以依NC的原理实现种种控制.高,中档的PLC,还开发有NC单元,或运动单元,可实现点位控制.运动单元还可实现曲线插补,可控制曲线运动.所以,若PLC配置了这种单元,则完全可以用NC的办法,进行数字量的控制.新开发的运动单元,甚至还发行了NC技术的编程语言,为更好地用PLC进行数字控制提供了方便.



4,用于数据采集


随着PLC技术的发展,其数据存储区越来越大.如德维森公司的PLC,其数据存储区(DM区)可达到9999个字.这样庞大的数据存储区,可以存储大量数据.数据采集可以用计数器,累计记录采集到的脉冲数,并定时地转存到DM区中去.数据采集也可用A/D单元,当模拟量转换成数字量后,再定时地转存到DM区中去.PLC还可配置上小型打印机,定期把DM区的数据打出来.

PLC也可与计算机通讯,由计算机把DM区的数据读出,并由计算机再对这些数据作处理.这时,PLC即成为计算机的数据终端.

电力用户曾使用PLC,用以实时记录用户用电情况,以实现不同用电时间,不同计价的收费办法,鼓励用户在用电低谷时多用电,达到合理用电与节约用电的目的.

5,用于信号监控


PLC自检信号很多,内部器件也很多,多数使用者未充分发挥其作用.其实,完全可利用它进行PLC自身工作的监控,或对控制对象进行监控.对一个复杂的控制系统,特别是自动控制系统,监控以至进一步能自诊断是非常必要的,它可减少系统的故障,出了故障也好查找,可提高累计平均无故障运行时间,降低故障修复时间,提高系统的可靠性.

6,用于联网,通讯

PLC联网,通讯能力很强,不断有新的联网的结构推出.

PLC可与个人计算机相连接进行通讯,可用计算机参与编程及对PLC进行控制的管理,使PLC用起来更方便.


为了充分发挥计算机的作用,可实行一台计算机控制与管理多台PLC,多的可达32台.也可一台PLC与两台或更多的计算机通讯,交换信息,以实现多的对PLC控制系统的监控.PLC与PLC也可通讯,可一对一PLC通讯,可几个PLC通讯,可多到几十,几百.


PLC与智能仪表,智能执行装置(如变频器),也可联网通讯,交换数据,相互操作.可联接成远程控制系统,系统范围面可大到10公里或更大.可组成局部网,不仅PLC,而且高档计算机,各种智能装置也都可进网.可用总线网,也可用环形网.网还可套网.网与网还可桥接.联网可把成千上万的PLC,计算机,智能装置组织在一个网中.网间的结点可直接或间接地通讯,交换信息.


联网,通讯,正适应了当今计算机集成制造系统(CIMS)及智能化工厂发展的需要.它可使工业控制从点(Point),到线((Line)再到面(Aero),使设备级的控制,生产线的控制,工厂管理层的控制连成一个整体,进而可创造更高的效益.这个无限美好的前景,已越来越清楚地展现在我们这一代人的面前.




以上几点应用是着重从质上讲的.从量上讲,PLC有大,有小.所以它的控制范围也可大,可小.小的只控制一个设备,甚至一个部件,一个站点;大的可控制多台设备,一条生产线,以至于整个工厂.可以说工业控制的大小场合,都离不开PLC.






做好一个PLC项目的全部过程

可乐的工控技术



我们学PLC有时候是为了做项目,做项目需要做些什么步骤呢,学习什么知识点,看看老工程师怎么告诉你……

一,做一个PLC项目的大体流程如下:

1,熟悉好现场环境和工艺流程;

2,设计出安全可靠的控制方案;

3,画出电气控制原理图;

4,确定好材料,制作材料物资明细表;

5,编写PLC程序,组态监控画面,设计PLC 机柜接线图,并同时制作PLC机柜;

6,沟通甲方,现场施工;

7,现场调试,并完善工艺控制方案;

8,组织甲方验收项目.

二,PLC 设计原则


1,最大限度的满足被控对象提出的各项性能指标


为明确控制任务和控制系统应有的功能,设计人员在进行设计前,就应深入现场进行调查研究,搜集资料,与机械部分的设计人员和实际操作人员密切配合,共同拟定电气控制方案,以便协同解决在设计过程中出现的各种问题.

2,确保控制系统的安全可靠


电气控制系统的可靠性就是生命线,不能安全可靠工作的电气控制系统,是不可能长期投入生产运行的.尤其是在以提高产品数量和质量,保证生产安全为目标的应用场合,必须将可靠性放在首位,甚至构成冗余控制系统.

3,力求控制系统简单

在能够满足控制要求和保证可靠工作的前提下,应力求控制系统构成简单.只有构成简单的控制系统才具有经济性,实用性的特点,才能做到使用方便和维护容易.

4,留有适当的裕量


考虑到生产规模的扩大,生产工艺的改进,控制任务的增加,以及维护方便的需要,要充分利用PLC易于扩充的特点,在选择PLC的容量(包括存储器的容量,机架插槽数,I/O点的数量等)时,应留有适当的裕量.

三,PLC设计的基本步骤

在进行PLC控制系统设计,尽管有着不同的被控对象和设计任务,设计内容可能涉及诸多方面,又需要和大量的现场输入,输出设备相连接,但是基本内容应包括以下几个方面:

1,明确设计任务和技术条件

设计任务和技术条件一般以设计任务书的方式给出,在设计任务书中,应明确各项设计要求,约束条件及控制方式.因此,设计任务书是整个系统设计的依据.

2,确定用户输入设备和输出设备


用户的输入,输出设备是构成PLC控制系统中,除了作为控制器的PLC本身以外的硬件设备,是进行机型选择和软件设计的依据.因此,要明确输入设备的类型(如控制按钮,行程开关,操作开关,检测元件,保护器件,传感器等)和数量,输出设备的类型(如信号灯,接触器,继电器等执行元件)和数量,以及由输出设备驱动的负载(如电动机,电磁阀等).并进行分类,汇总.

3,选择PLC的机型

PLC是整个控制系统的核心部件,正确,合理的选择机型对于保证整个系统的技术经济性能指标起着重要的作用.

PLC的选型应包括机型的选择,存储器容量的选择,I/O模板的选择等.

4,分配I/O地址,绘制I/O接线图

通过对用户输入,输出设备的分析,分类和整理,进行相应的I/O地址分配,并据此绘制I/O接线图.

至此,基本完成了PLC控制系统的硬件设计.

5,设计控制程序


根据控制任务和所选择的机型以及I/O接线图,一般采用梯形图语言设计系统的控制程序.设计控制程序就是设计应用软件,这对于保证整个系统安全可靠的运行至关重要,必须经过反复调试,使之满足控制要求.

6,必要时设计非标准设备

在进行设备选型时,应尽量选用标准设备.如无标准设备可选,还可能需要设计操作台,控制柜,模拟显示屏等非标准设备.

7,编制控制系统的技术文件

在设计任务完成后,要编制系统的技术文件.技术文件一般应包括设计说明书,使用说明书,I/O接线图和控制程序(如梯形图等).



PLC工程师

四,PLC的选型


选择适当型号的PLC机是设计中至关重要的一步.目前,国内外PLC生产厂家生产的PLC品种已达数百个,其性能各有特点,价格也不尽相同.所以,在设计时,首先要根据机型统一的原则来考虑,尽可能考虑采用与本企业正在使用的同系列的PLC机,以便于学习,掌握,维护的方便性,备品配件的通用性,且可减少编程的投资.在此基础上还要充分考虑下面因素,以便选择最佳型号的PLC机:

1,PLC的功能


在选择PLC时,首先应对系统要求的输入,输出有详细的了解,即输入量有多少,输出量有多少,哪些是开关(或数字)量,哪些是模拟量,对于数字型输出量还应了解负载的性质,以选择合适的输出形式(继电器型,晶体管型,双向可控硅型).


在确定了PLC机的控制规模后,还要考虑一定的余量,以适应工艺流程的变动及系统功能的扩充,一般可按10~15%的余量来考虑.另外,还要考虑PLC的结构,从I/O点数的搭配上加以分析,决定选择整体式还是模块式的PLC.


在确定了PLC的输入量和输出量的点数及性质后,就可以进一步确定各种I/O模板的型号和数量.开关量I/O模板的规格标准有4,8,16,32,64点,点数多的模板,每点平均价格相对较低.对开关量I/O模板的外部接线方式可分为隔离式和汇点式,隔离式的每点平均价格较高.如果信号之间不需要隔离,应选用汇点式的I/O模板.在整体式PLC机中,各个I/O端子也有隔离式和汇点式之分,以满足不同电压等级的输入/输出器件的需要.

2,PLC的功能


要根据该系统的控制过程和控制规律,确定PLC机应具有的功能.各个系列不同规格的PLC机所具有的功能并不完全相同.如有些小型PLC只有开关量的逻辑控制功能,而不具备数据处理和模拟量处理功能.


当某个系统还要求进行位置控制,温度控制,PID控制等闭环控制时,应考虑采用模板式PLC,并选择相应的特殊功能的I/O模块,否则这些算法都用PLC的梯形图设计,一方面编程困难,另一方面也占用了大量的程序空间.另外,还应考虑PLC的运算速度,特别是当使用模拟量控制和高速计数器等功能时,应弄清PLC机的最高工作频率是否满足要求.

3,用户程序存储器的容量


合理确定PLC的用户程序存储器的容量,是PLC应用设计及选型中不可缺少的环节.一般说来,用户程序存储器的内存容量与内存利用率,开关量I/O总数,模拟量I/O点数及设计者的编程水平有关.

简单的估算公式:

内存字数=(开关量I/O总点数+模拟量I/O点数 X 16) X 10


式中:每个模拟量通道(或I/O点)相当16个开关量I/O点.在此基础上,可考虑留有20~25%的裕量.对于工艺比较复杂的系统,应适当增加存储器的容量,否则,当控制较复杂,数据处理量大时,可能出现存储器容量不够的问题.







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