Control Technology
1.Introduction to Control Engineering
Whenever energy is to be used purposefully, some form of control is necessary, in recent times there has been a considerable advance made in the art of automatic control. The art is, however, quite old, stemming back to about 1790 when James Watt invented the centrifugal governor to control the speed of his steam engines. He found that while in many applications an engine speed independent of load was removed the speed increased.
In a simple centrifugal governor system, variations in engine speed are detected and used to control the pressure of the steam entering the engine. Under steady conditions the moment of the weight of the metal spheres balances that due to the centrifugal force and the steam valve opening is just sufficient to maintain the engine speed at the required level. When an extra load torque is applied to the engine, its speed will tend to fall, the centrifugal force will decrease and the metal spheres will tend to fall slightly. Their height controls the opening of the steam valve which now opens further to allow a greater steam pressure on the engine. The speed thus tends to rise, counteracting the original tendency for the speed to fall. If the extra load is removed, the reverse process takes place, the metal spheres tend to rise slightly, so tending to close the steam valve and counteracting any tendency for the speed to rise.
It is obviously that without the governor the speed would fall considerably on land. However, in a correctly designed system with a governor the fall in speed would be very much less. An undesirable feature which accompanies a system which has been designed to be very sensitive to speed changes is the tendency to “hunt” or oscillate about the final speed. The real problem in the synthesis of all systems of this type is to prevent excessive oscillation but at the same time produce good “regulation”. Regulation is defined as the percentage change in controlled quantity on load relative to the value of the controlled under condition of zero load. Regulators form an important class of control system, their object generally being to keep some physical quantity constant (e.g. speed, voltage, liquid level, humidity, etc.) regardless of load variation. A good regulator has only very small regulation.
The automatic control of various large-scale industrial processes, as encountered in the manufacture and treatment of chemicals, food and metals, has emerge during
the last thirty years as an extremely important part of the general field of control engineering. In the initial stages of development it was scarcely realized that the theory of process control was intimately related to the theory of servomechanisms and regulators. Even nowadays complete academic design of process control systems is virtually impossible owing to our poor understanding of the dynamics of processes. In much of the theory introduced in this book, servomechanisms and regulators are used as example to illustrate the methods of analysis. These methods are, however, often applicable to process control systems, which will be themselves introduced separately.
2. Programmable Controllers
In the 1960s, electromechanical devices were the order of the day as far as far as control was concerned. These devices, commonly known 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 panels called for 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became a very great challenge. Cost became another issue, for in spite of the low cost of the relay itself, the installed cost of the panel could be quite high. The total cost including purchased parts, 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 a control panel. With relays, this was a 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 light of this, it was not uncommon to discard an entire control panel in favor of 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 (1)could be
used on the factory floor, (2)could have there “logic” change without extensive rewiring or component changes, 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 t o 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?
Figure 10.1
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 va riety 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, photo sensors, thermocouples, position sensing devices, and bar code reader as input; and pilot light, display devices, motor starters, DC and AC drivers, solenoids, and printers as outputs.
控制技术
1.控制工程绪论
只要有目的地利用能量，都有必要采取某种控制形式。
近年来，在自动控制领域取得了相当大的进步，其实这种技术历史悠久，可以追溯到1790年，当时瓦特就发明了离心式调速器来控制蒸汽发动机的转速。
他发现在一些应用中有必要保持发动机转速不随负荷扭距的变化而变化。
但实际上，当加一个负载时速度就会下降或者去除负载后速度就会正加。
在一个简单的离心调速系统中，发动机转速的变化被探测并用来控制进入发动机的蒸汽压力。
在稳定条件下，由于离心力的作用和蒸汽阀的开度足够维持发动机转速所要求的水平，瞬间与金属摆球的重量平衡。
当额外的负荷扭距加到发动机上，发动机的转速会下降，离心力减少，金属球将轻微下降，开口度控制器蒸汽阀的开启，当开口度变大时将更多的蒸汽压力加到发动机上，这样转速就要上升，抵消了最初转速下降的倾向。
如果额外的负载去除后，将发生相反的过程，金属球要轻微上升，蒸汽阀倾向于关闭，抵消了一些转速整加的倾向。
显然，没有这种调节器，速度就会将到底，然而，经过一个合适的调节系统，转速下降会很少。
伴随高灵敏度的转速控制系统的出现，也会产生一些人们不希望出现的新问题，即控制量紧随被控制量（转速），从而在稳定的转速附近发生不规则的振荡和摆动。
所有这种系统的真正的问题是预防过度振荡的同时产生良好的调节作用。
调节作用被定义为负载条件下被控制量的数值相对空栽条件下被控制数值的变化百分比。
各种调节器构成了一个重要的、完整的控制系统，他们通常能够保证各自所控对象（对应）的物理量（如速度、电液面高度、湿度等）在负载变化时保持恒定。
一个好的调节器有很少的调节量。
在过去的30年里，控制工程通用领域中的一个极其重要的部分“自动控制”已经出现，并且被广泛应用于诸如化工、食品加工、金属加工等各种各样的大规模的工业控制过程控制当中。
在发展的最初阶段，很难想象过程控制理论与随动系统和调节器理论密切相关。
甚至在现在，过程控制系统的完善设计实际上不可能归功于我们对过程动力学的那些有限的理解。
本书介绍的大部分理论中，一随动系统和调节器为例阐述分析方法，然而这种方法通常适用于过程控制系统，那些都是他们自己分批提出的。
2.可编程控制器
20世纪30年代，在控制器受到关注之前，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，更明确地说，应用于汽车工业。
这些继电器使用成百上千的金属导线，他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，继电器的性能是基本稳定的。
但是，继电器盒通常需要安装三百到五百甚至更多的继电器，于是可靠性及维修和保养问题就不可避免地摆到了我们的面前。
成本问题成为另一问题，尽管继电器本身成本很低；但是继电器盒的安装成本很高，每个继电器总的成本，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美元之间不等。
更糟糕的事情是，控制面板需要经常不段地更改。
对于继电器来说，这是一个昂贵的事实。
因为这一更改过程需要大量的劳动在控制面板上重新接线。
以外，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修成为很头疼的事。
按照这样的考虑，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接线元件的控制面板，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。
正如在昂贵的电动车辆的生产线上一样。
越来越清楚地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。
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。
这些早期的控制器，即可编程逻辑控制器（PLC）是能应用于工厂车间的最早系统。
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，它能够很轻易地诊断和修补。
观察可编程控制器在最近15年取得的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？改装过的计算机替代这些设备，正如电子器件替代继电器，甚至设计出了工具来作为继电器的替代品。
我们现在有机会来审视一下前景，回顾过去，可编程控制器带给制造业是什么？
可编程控制器都包含了基本的功能模块，参见图10.1。
为了理解控制系统
图10.1
的关系，我们将检查一下每一个模块。
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、存储变量数据的变量存储器，中心部分具有控制程序执行和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提供控制水平的信号、模拟信号或是应用于某一特定应用的“智能”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，其中包括按钮、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、发动机、DC和AD驱动器，螺线管和作为输出的打印机。
