Programmable Controllers
In 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 –type manufacturing processes and stand-alone machines. Many of these relays were in use in the transportation industry, more specifically, the automotive industry. These relays used hundreds of 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 called for 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became a very itself, the installed cost o f the panel could be quite high. The total cost including purchased pars, wiring and installation labor, could range from $30-$50 per relay. To make matters worse, the constantly changing needs of 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 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. 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 control needs.
That something, in the late 1960s,was the first programmable controller. This first ”revolutionary” system was developed as a specific response to the needs of the major automotive manufacturers in the United States. These early controllers, or Programmable Logic Controllers(PLC),represented the first systems that could be used on the factory floor, could have there ”logic” change without extensive rewiring 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 programmable controller area. The pioneer products of the late 1960s must have been confusing 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
“computers” disguised as electronics designed to replace relays. Even the programming tools were designed to appear as relay equivalent presentations. We have the opportunity now to examine the promise, in retrospect, what the programmable controller brought manufacturing？
All programmable controllers consist of the basic functional blocks shown in Figure 10.1.We will examine each block to understand the relationship to the control system. First we looked at the center, as it is the heart of the system .It consists of a microprocessor, logic memory for the storage of the actual control logic, storage or variable memory for use with data that will ordinarily change as a function of the control program execution, 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 signals for the CPU and converts them to voltage and current 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 “smart” I/O which are dedicated to a certain application task. The programmer is normally used only to initially configure and program a system and is not required for the system to operate. It is also used in troubleshooting a system, and can prove to be a valuable tool in pinpointing the exact cause of a problem. The field devices shown here represent the various sensors and actuators connected to the I/O. These are the arms, legs, eyes, and ears of the system, including pushbuttons, limit switches, proximity switches, photosensors, thermocouples, position sensing devices, and bar code reader as input; and pilot light, display devices, and bar code reader as input; and pilot light, display devices, motor starters, DC and AC drivers, solenoids, and printers as outputs.
图1
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本，大体在30美元到50美元不等。更糟糕的事情是，控制面板需要不断的更改。对于继电器来说这是个昂贵的事实。因为这一更改过程需要大量的劳动在控制面板上从新接线。此外，这些变化有时很少备有证明文件的。这就使得以后的再次维护成为一件很头疼的事情。按照这样考虑，丢弃整个旧的控制面板，同意使用一个适合的新的控制过程方式的相匹配的的接线元件控制面板，也是很常见的事情。这就给这些系统的维护成本增加了不可预知的、潜在的高成本。正如在按归的电动车辆生产线上一样。越来越清楚的认识到要使系统更可靠，更容易排除故障，更适合不断变化的控制过程需要，就必须改进控制过程。
documented翻译在20世纪60年代，出现了第一个可编程控制器。这是第一个“革命性”的系统，这个系统是按照美国汽车制造工业的特定要求开发研制出来的。这些早期的的控制器，即可编程逻辑控制器（PLC）是能应用于工厂车间的的最早系统。这些控制器在不需要大量重新接线或改变元件的情况下能够做“逻辑”变化，一旦出现问题，它能够很轻易的诊断和修补。观察可编程控制器在最近15年取得进步是很有趣的事情，在20世纪60年代末期早期的产品或许会使很多人感到惊恐和迷惑不解，例如，维修人员习惯了手动工具，那么对于电子仪器的部件和机电设备将会发生什么呢？改装过的计算机代替这些设备，正如电子器件代替继电器，甚至设计出了工具来作为继电器的替代品。我们现在来审视一下前景，回顾过去，可编程控制器带给制造业的是什么？
可编程控制器都包含了基本的功能模块，参见图1。为了理解控制系统的关系，我们检查以下每一个模
块。首先，我们看一下中心，它是系统的心脏。中心包括微处理器、存储当前控制逻辑的逻辑存储器、存储常变量数据的变量存储器，中心部分具有控制程序执行和为微处理器与存储器提供电力的功能。接着是I/O 模块，它的功能是为CPU提供控制水平信号，并把它们转化为适合连接工厂级别的传感器和调节器的标准电压和电流。I/O类型可以是数字信号、模拟信号或是应用于某一特定应用的“只能”I/O。程序员通常仅需编写程序，而不需要考虑
程序在系统中的运行。它也可以用来发现并修理系统故障，在检查系统故障的确切原因方面是个很有用的设备。在这里提到的设备代表了与I/O连接的各种传感器和调节器，他们是系统的手臂、腿、眼睛和耳朵，其中包括按扭、限制开关、行程开关、光敏元件、热电偶、位置传感器，作为输入的读卡机、标灯、显示设备、发电机、DC和AD驱动器、螺线管和作为输出的打印机。
图1

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