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1、Programmable ControllersIn the 1960s, electromechanical devices were the order of the day as far as control was concerned. These devices, commonly know as relays, were being used by the thousands to control many sequential -ype manufacturing processes and stand-alone machines. Many of these relays w
2、ere in use in the transportation indust,ry more specifically, the automotive industry. Theserelays used hundredsof wires and their interconnections to affect a control solution. The performance of a relay was basically reliable-at least as a single device. But the common application for relay panels
3、 called for 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became avery itself, the installed cost of the panel could be quite high. The total cost including purchased pars, wiring and installation labor, could range from $30-$50 per rel
4、ay. To make matters worse, the constantly changing needsof a process called for recurring modifications of control panel. With relays, this was a costly prospect, as it was accomplished by a major rewiring effort on the panel. In costly prospect, as it was accomplished by a major rewiring effort on
5、the panel. In addition, these changes were sometimes poorly documented,causing a second-shift maintenance nightmare months later. In high of this, it was not uncommon to discard an entire control panel in favor lf a new one with the appropriate components wired in a manner suited for the new process
6、. Add to this the unpredictable, and potentially high, cost of maintaining these systems as on high-volume motor vehicle production lines, and it became clear that something was needed to improve the control process -to make it more reliable, easier to troubleshoot, and more adaptable to changing co
7、ntrol needs.That something, in the late 1960s,was the first programmable controller. This first ” revolutionarysys” tem was developed as a specific response tothe needs of the major automotive manufacturers in the United States. These early controllers, or Programmable Logic Controllers(PLC),represe
8、nted the first systems that could be used on thefactory floor, could have there ” logic ” change without exrtewnsiriivneg or component change, and(3)were easy to diagnose and repair when problems occurred. It is interesting to observe the progress that has been made in the past 15 years in the progr
9、ammable controller area. The pioneer products of the late 1960s must have beenconfusing and frightening to a great number of people. For example, what happened to the hardwired and electromechanical devices that maintenance personnel were used to repairing with hand tools? They were replaced with“ c
10、omputers disguised as electronics designed to replace relays. Even the program ming tools were desig ned to appear as relay equivale nt prese ntati ons. We have the opport unity now to exam ine the promise, in retrospect, what the programmable con troller brought manu facturi ng ?All programmable co
11、ntrollers consist of the basic functional blocks shown in Figure 10.1.We will exam ine each block to un dersta nd therelati on ship to the con trol system. First we looked at the cen ter, asit is the heart of the system .It con sists of a microprocessor, logic memory for the storage of the actual co
12、n trol logic, storage or variable memory for use with data that will ordinarily change as a function of the con trol program executi on, and a power supply to provide electrical power for the processor and memory .Next comes the I/O block. This function takes the control level sig nals for the CPU a
13、nd con verts them to voltage and curre nt levels suitable for connection with factory grade sensors and actuators. The I/O type can range from digital, analog, or a variety of special purpose “ smartI/O which are dedicated to a certa in applicati on task. The programmer is n ormally used only to in
14、itially con figure and program a system an dis not required for the system to operate. It is also used in troubleshooting a system, and can prove to be a valuable todh pinpointing the exact cause of a problem. The field devices show n here represe nt the various sen sors and actuators conn ected to
15、the I/O. These are the armslegs, eyes, and ears of the system, in clud ing pushbutt ons, limit switches, proximity switches, photose nsors, thermocouples, positi on sensing devices, and bar code reader anput; and pilot light, display devices, and bar code reader asin put; and pilot light, display de
16、vices, motor starters, DC and AC drivers, sole no ids, and prin ters as outputs.传感器行程开关模拟量输 入 土 LU 师中央处理器(CPU)逻辑控制器变量存储器动力供应电磁装置光电耦合电机接触器开关输入编程设备可编程控制器20世纪60年代，在控制器受到关注以前，机电装置一直是这个年代的流行产品。这些通常被称为继电器的装置被数以千计的系统应用来控制许多制造过程和单独的机 器。许多这样的继电器被应用于运输工业，更明确的说，应用于汽车工业。这些继 电器使用成百上千的金属导线，它们的相互联系将影响控制的解决方案.至少作为一个 单个的装置，继电器的性能基本是稳定的.但是，继电器盒装通常需要安装300到500 甚至更多的继电器，于是可靠性及维修和保养问题就不可避免的摆到我们面前.成本问 题成为另一个问题，尽管继电器本身成本很低，但是继电器盒子的安装成本很高，没 给继电器总的成本，其中包括购买零件，配线和安装工作的成
