毕业设计（论文）基于单片机的微电机闭环控制系统设计

天津职业技术师范学院毕业设计论文目 录摘 要2一、题目分析3二、设计论证3（一）微电机的选择3（二）现在普遍采用单片机作为电动机的控制器3（三）直流电动机电枢的PWM的调压调速原理3（四）转速检测的选择4三、控制系统的硬件6（一）单片机6（二）PWM调压原理6（三）L298N的直流控制8（四）增量式光电编码盘的工作原理9（五）光电码盘与单片机的接口10四、系统的软件设计11（一）应用软件设计的基本要求11（二）系统软件设计11（三）流程图11五、测试方法及结果12（一）测试过程及方法12（二）测试结果12六、误差分析及处理12附图（原理图）13程序14结 束 语17致 谢 词18外文资料翻译19参 考 文 献24微电机闭环控制系统 作者：XXX摘 要本文介绍了用8051单片机实现的微电机闭环控制系统。该系统可实现微电机的正反转、无级调速等。速度调节采用PWM，速度检测采用增量式编码盘作为反馈检测元件，将转速转换成脉冲信号。在工业控制中，需经常对转速进行控制。由于直流电动机具有良好的转速特性，常用于宽范围内平滑调速、起制动性能好的自动控制系统中。本文论述的是用单片机实现的微电机闭环控制系统。电动机调速系统采用单片微型处理机实现数字化控制，是电气传动发展的主要方向之一。数字量的运算不会出现模拟电路中所遇到的零点漂移问题，被控量可以很大，也可以很小，都较易保证足够的控制精度。文中将介绍单片机用作直流电动机速度调节器的硬件和软件。关键词8051单片机 微电机 闭环控制系统 The Miniature Motor Closed-loop control systemAutor:pan jian fengAbstract This text introduction close with 8051pair that one-chip computer realize the miniature motor control systematic.The systerm that this text introduced to realize with 8051 machines the miniature motor forward and reback、 have no class regulate speed ect .Regulate speed dopt the PWM , and the flat-out examination adopt to increase the deal type light telegraphic code the dish to be used as the flat-out and versa to examine the dollar the piece, and will turn to soon convert pulse signal. Motor transfer speed adopt single slice of microprocessors realize that digitalization is controlled systematically, It is one of the mains direction of electric transmission development. Digital operation of quantity can appear simulation circuit zero point run into drift about the question, It is can very small too easier to guarantee enough precision of control if person who is accuse of is can very heavy .Single slice of introduction of text lieutenant general the pulse signal proceeding for hardware for machine used toing the direct current motive controling with software, that system passing the versa of code dish regulate to realize the wreath control of shutting of right tiny electrical engineering. Key words 8051 One-chip computer Miniature Motor Closed-loop control system 一、题目分析微电机的闭环控制系统就是用微机实现微电机的正反转、速度调节，能实现无级调速，具有闭环控制的系统。该系统具有技术先进、经济合理、维护方便、应用范围广等特点。主要应用于家用电器、机器人、军事等领域。二、设计论证微电机，就是指容量和尺寸都比较微小的电机。（一）微电机的选择直流电动机是依靠直流电源运行的电动机。与交流电动机比较，其特点：（1）使用直流电源，尤其适用于干电池、蓄电池一类独立直流电源；（2）电动机的转速依赖于电动机参数及电压，原则上可设计成任一转速运行，而且转速易于调节与控制。（3）除了无刷直流电动机外，具有接触式换向器，易磨损，可靠性比异步电动机差，换向过程产生火花与无线电干扰。（4）在相同功率和转速时，直流电动机价格比异步电动机高。 因此，直流电动机宜用于要求启动转矩高且容易允许转速变化，或要求调节转速及配上稳速装置后，在电源电压和负载变化情况下转速恒定，或要求使用电池以便携带等场合的一些机械的驱动，如生产自动化装置、自动化仪表、录音机、携便式电动类日用电器及电动玩具等。（二）现在普遍采用单片机作为电动机的控制器实际上可作为电动机控制器的元件还有多种，例如工业控制计算机、可编程控制器（PLC）、数字信号处理器（DSP）。 工业控制计算机可谓功能最强大，它有极高的速度、强大的运算能力和接口功能、方便的软件环境；但由于成本高、体积大、所以只用于大型控制系统。 可编程控制器则正好相反，它只能完成逻辑判断、定时、计数和简单的运算。由于功能太弱，所以只能用于简单的电动机控制。单片机介于工业控制计算机和可编程控制器之间，它有较强的控制功能、低廉的成本。人们在选择电动机的控制器时，常常是在先满足功能的需要的同时，优先选择成本低的控制器。因此，单片机往往成为优先选择的目标。（三）直流电动机电枢的PWM的调压调速原理 众所周知，直流电动机转速n的表达式为n= （1-3）式中U电枢端电压；I电枢电流；R电枢电路总电阻；每极磁通量；K电动机结构参数由此式可得，直流电动机的转速控制方法可分为两种，对励磁磁通进行控制的励磁控制法和对电枢电压进行控制的电枢控制法。其中励磁控制法在低速时受磁极饱和的限制，在高速时受换相火花和换相器结构强度的限制，并且励磁线圈电感较大，动态响应较差，所以这种控制方法用的很少。现在大多数应场合都使用电枢控制法。线性放大驱动方式是使半导体功率器件工作在线性区。这种方式的优点是：控制原理简单、输出波动小、线性好、对临近电路的干扰小；但功率器件在线性区工作时会将大部分电功率用于产生热量。效率和散热问题严重，因此这种方式只用于数瓦以下的微小功率直流电动机的驱动。绝大多数电动机采用开关驱动方式。开关驱动方式是使半导体功率器件工作在开关状态，通过脉宽调制PWM来控制电动机电枢电压，实现调速。1、在PWM调速时，占空比是一个重要参数。以下3种方法都可以改变占空比的值。（1）定宽调制法 这种方法是保持t1不变，只改变t2，这样使周期T也随之改变。（2）调宽调频法这种方法是保持t2不变，只改变t1，这样使周期T也随之改变。（3）定频调宽法这种方法是使周期T保持不变，而同时改变t1和t2。前2种方法由于在调速时改变了控制脉冲的周期，当控制脉冲的频率与系统的固有的频率接近时，将会引起震荡，因此这2种方法用的很少。目前在直流电动机的控制中，主要使用定频调宽法。2、PWM控制信号的产生方法有4种。（1）分立电子元件组成的PWM信号发生器这种方法是用分立的逻辑电子元件组成PWM信号电路。它是最早期的方式，现在已被淘汰了。（2）软件模拟法利用单片机的一个I/O引脚，通过软件对该引脚不断地输出高低电平来实现PWM波输出。这种方法要占用CPU大量时间，同时单片机无法进行其他工作，因此也逐渐被淘汰。（3）专用PWM集成电路从PWM控制技术出现之日起，就有芯片制造商生产专用的PWM集成电路芯片，现在市场上已有很多种。这些芯片除了有PWM信号发生功能外，还有“死区”调节功能、保护功能等，在用单片机控制直流电动机中，使用专用PWM集成电路可以减轻单片机的负担，工作更可靠。3、单片机的PWM口新一代的单片机增加了许多功能，其中包括PWM功能。单片机通过初始化设置，使其能自动地发出PWM脉冲波，只有在改变占空比时CPU才进行干预。（四）转速检测的选择1、 用测速发电机测速 用直流测速发电机测速是比较简单的。只要经过适当的滤波电路滤去杂波，再经过模数转换，就可输入计算机。考虑到转速有正有负，测速发电机的输出电压也有正有负，设计模数转换电路时，应当考虑负电压的转换问题。2、用微机中的定时/计数器测速除了用测速发电机测速外，也可以用微机中的定时/计数器配合增量式光电码盘的输出信号而测出转速。具体的方法有M法、T法和M/T法三种。（1）M法测速M法测速是在规定的检测时间T秒内，对光电码盘输出的脉冲个数m1进行计数。转速为： n= （1-4-1）式中P光电码盘转一周发出的脉冲数。 实际上在T内的脉冲数m1一般不是整数，而用微机中的定时/计数器测得的脉冲个数只能是整数部分，因而存在着量化误差。例如要求误差小于百分之一，则m1应大于100。在一定的转速下要增大m1以减小误差，可增大检测时间T；但考虑到光电码盘测速的主要目的是在数字伺服系统中测取速度反馈量供速度闭环使用，故检测时间T也不能太长，一般在0.01s以下。由此可见，减小量化误差的方法最好是增加光电码盘每周输出脉冲数P。 M法测速适合于测量高转速，因为在P及T相同的条件下，高转速时m1较大，量化误差较小。（2）T法测速T法测速是在码盘输出的一个脉冲周期内对高频时钟脉冲的个数m2进行计数。转速为：n= （1-4-2）式中f0高频时钟脉冲的频率。为了减小量化误差，m2不能太小，所以T法在测低转速时精度较高。当然，转速也不宜太低，以免码盘发出的一个脉冲周期太长，影响测量的快速性。为了提高测速的快速性，应当选用P值较大的光电码盘。（3）M/T法测速M/T法测速是在稍大于规定时间T的某一时间T1内，分别对光电码盘输出的脉冲个数m1和高频时钟脉冲个数m2进行计数。于是可求出转速n= （1-4-3）T1的开始和结束都应当正好是光电码盘脉冲的上跳沿，这样就可保证检测的精度。通过比较论证，此系统的设计采用单片机控制直流微电机，用专门的PWM集成电路产生的脉冲，定频调宽法进行调压调速，用微机中的定时/计数器配合增量式光电码盘M/T法的输出信号而测出转速反馈到输入端进行调节完成闭环控制。此系统的方框图如图所示。 图（2-1）三、控制系统的硬件 微电机控制系统的组成如 图（3-1） 直流电动机单片机 PWMM控制键码盘图（3-1）（一）单片机89C52芯片是MCS-51系列的单片机，管脚与8031芯片是完全兼容的，并且89C52芯片内部带有8K在系统可编程的闪速存储器，具有256*8位的内部存储器RAM，32根可编程的I/O线，5个中断源，不需要扩展外存储器，节省了芯片，降低了成本，也节约了接口资源。89C52还有功耗低的优点，采用单CPU系统是为了不间断地输出PWM控制信号。（二）PWM调压原理图3-2-1是利用开关管对直流电动机进行PWM调速控制的原理图和输入输出电压波形。在图3-2-1（a）中，当开关管MOSFET的栅极输入高电平时，开关管导通，直流电动机电枢绕组两端有电压Us。t1秒后，栅极输入变为低电平，开关管截止，电动机电枢两端电压为0。t2秒后，栅极输入重新变为高电平，开关管的动作重复前面的过程。这样，对应着输入电平的高低。直流电动机电枢绕组两端的电压波形如图3-2-1（b）所示。 电动机的电枢绕组两端的电压平均值U0为U0=Us (3-2-1)式中占空比，=占空比表示了在一个周期T里，开关管的导通时间与周期的比值。的变化范围为01。由式（3-2-1）可知，当电源电压Us不变的情况下，电枢的端电压的平均值U0取决于占空比的大小，改变值就可以改变端电压的平均值，从而达到调速的目的，这就是PWM调速原理。.m1m2TcTdabctPLG图（3-2-2）在速度闭环控制系统中，测速装置属于反馈环节，转速检测的精度和快速性直接影响系统的静动态性能。MT法测速（如图3-2-2）是在对光电脉冲发生器输出脉冲个数m1计数的同时，对高频脉冲的个数m2也进行计数。m1反映转角，m2反映测速时间，通过计算可得转速值n。该法在高速及低速时都具有较高的精度。测速时间Td由脉冲发生器脉冲来同步，即Td等于m1个脉冲周期。由图3-2-2可见，从a点开始，计数器对m1和m2计数，到达b点，预定的测速时间Tc到，微机发出停止计数指令，因为Tc不一定恰好等于整数个脉冲发生器脉冲周期，所以计数器仍对高频脉冲继续计数，到达c点时，脉冲发生器脉冲的上升沿使计数器停止，这样m2就代表了m1个脉冲周期的时间。设高频脉冲频率为f，脉冲发生器每转发出P个脉冲，则电动机转速为：n=(rpm) (3-2-2)我们将PWM通过298（H桥）控制电机的转速。298的工作频率在1KHz10KHz之间，即在此频率范围内才能保证通过的电流连续。为此我们才用了2KHz的PWM波，其周期为T=500us,并将其均分成十份（通过高低电平之间的延时得到同一频率不同占空比的PWM波）。波形图如下：图（3-2-3）（三）L298N的直流控制如图（3-1）图（3-1）功能表如表（3-1）：表（3-1） 输入功能 Von=HC=H；D=L正转C=L；D=H反转C=D高速制动Von=LC=X；D=X任一转速停止 L=低电平 H=高电平 X=任意（四）增量式光电编码盘的工作原理增量式光电编码盘不象绝对式光电编码盘那样测量转动体的位置，它是专门测量转动体角位移的累计量。1、 增量式光电编码盘的结构与工作原理增量式光电编码盘是在一个码盘上只开出3条码道，由内向外分别为A、B、C如图4-1（a）所示。在A、B码道的码盘上，等距离的开有透光的缝隙，2条码道上相临的缝隙互相错开半个缝宽，其展开图如4-1（b）所示。第3条码道C只开出一个缝隙，用来表示码盘的零位。在码盘的两侧分别安装光源和光敏元件，当码盘转动时，光源经过透光和不透光区域，相应的，每条码道将有一系列脉冲从光敏元件输出。码道上有多少缝隙，就会有多少个脉冲输出，将这些脉冲整形后，输出的脉冲信号如图4-1（c）所示。例如，国产SZGH-01型增量式光电编码盘采用封闭式结构，内装发光二极管（光源）、光电接收器和编码盘，通过连轴节与被测轴连接，将角位移转换成A、B两路脉冲信号，供可逆计数器计数，同时还输出一路零位脉冲信号作为零位标记。它每圈能输出600个A相或B相脉冲和一个零位脉冲，A、B相脉冲信号的相位相差90。2、 编码盘方向的判别编码盘方向的判别可以采用，4-2所示电路实现。（a） (b)经过放大整形后的A、B两相脉冲分别输入到D触发器的D端和CP端，如图4-2（a）所示，因此，D触发器的CP端在A脉冲的上升沿触发。由于A、B脉冲相位相差90，当正转时，B脉冲超前A脉冲90，触发器总是在B脉冲处于高电平时触发，如图4-2（b）所示，这时Q=1，表示正转；当反转时，A脉冲超前B脉冲90，触发器总是在B处于低电平时触发，这时Q=0表示反转。A、B脉冲的另一路经与门后，输出计数脉冲。这样，用Q或Q反控制可逆计数器是加计数还是减计数，就可以使可逆计数器计数脉冲进行计数。C项脉冲接到计数器的复位端，实现每转动一圈复位计数一次计数器。这样，无论是正转还是反转，计数值每次反映的都是相对于上次角度的增量，形成增量编码。（五）光电码盘与单片机的接口单片机与增量式光电编码盘的硬件接口非常简单，只要把编码盘经过整形的输出信号直接接到单片机的I/O口即可。四、系统的软件设计为了使微机控制系统各种硬件设备能够正常运行，有效的实现电机各个控制环节的实时控制和管理，除了要设计合理的硬件电路，还必须要有高质量的软件支持。（一）应用软件设计的基本要求（1） 实时性（2） 可靠性（3） 易修改性（二）系统软件设计此系统采用C语言编程，其单片机的P00口为电动机的驱动，P02口为电动机正转，P03为电动机反转，外部中断0是编码器的输入。（三）流程图图（4-1）图（4-2） 图（4-3）五、测试方法及结果（一）测试过程及方法1、 检查硬件接线电路是否完善。2、 仿真器与硬件电路相连，在仿真软件里调入原程序，进行编译后对程序进行单步调试，检查程序是否与设计思路一致，调试完毕将原程序的二进制或十六进制文件烧入单片机中，与硬件电路相连接，检查系统工作是否正常。（二）测试结果 电机能实现正反转、无级调速，当电机没达到额定转速时，能进行自动调节，实现预期的效果。六、误差分析及处理1、 测量误差当编码盘测得的速度反馈到输入端时，由于测得的是速度平均值n,此值越小系统越精确。例如：电机启动时速度为n0,十秒后速度为n1，此时n1并非所要的速度，编码盘应测得n1作为反馈，但实际反馈过来的是n。这样造成测量误差。2、 系统误差此误差有电源干扰、线路干扰、程序的误操作、算法的不完善等。附图（原理图）程序#include void delay ( unsigned char x);void zz(void); /电机正转函数void fz(void); /电机反转函数sbit zl=P00; /电动机驱动sbit zz=P02; /电机正转按键sbit fz=P03; /电机反转按键unsigned char rr2=0x0a,rr3=0x06,rr4=0x00,rr5=0,rr6=0;unsigned char count=0x00; count1=0x00;count2=0x00;main() unsigned char start1=0x00,start2=0x00; IT0=1; TMOD=0x01; TH0=0x00; TL0=0x00; EA=1; ET0=1; EX0=1; while (1) if (P0_2=0 & rr5=0) start1=0xff; rr5+; delay(3); TR0=1; if (start1=0xff) zz(); if (P0_3=0 & rr6=0) start2=0xff; rr6+; delay(3); TR0=1; if (start2=0xff) fz(); if (P0_2=0) if (count1count2)rr3+;zz();EX0=1;ET0=1; if (P0_3=0) if (count1count2)rr3+;fz();EX0=1;ET0=1; void zz (void)unsigned char rr1;for (rr1=0;rr1rr2;rr1+) zl=1; for (rr1=0;rr1rr3;rr1+) zl=0;void fz (void)unsigned char rr1;for (rr1=0;rr1rr2;rr1+) zl=0; for (rr1=0;rr1rr3;rr1+) zl=1;void delay ( unsigned char x)unsigned char i,j;for (i=0;ix;i+) for(j=0;j=50;j+) ;void intt0_sever() interrupt 0 using 0 EX0=0; count=+count; EX0=1; void intt1_sever() interrupt 1 using 1 ET0=0; EX0=0; count1=count; count=0x00; if (rr4=0) count2=count; rr4=rr4+1; TH0=0x00; TL0=0x00; TF0=0; 结 束 语 本次毕业设计共历时十二周，但留给我思考的东西却很多。使我更加深刻地理解了“学无止尽”，“温故而知新”这些古语。毕业设计不但是一个对过去所学知识进行综合运用的过程，更是一个学习新知识的过程。通过这十二周的毕业设计，我对大学五年所学过的许多知识进行了实际应用；同时，学到了许多新的知识，并将其应用到实际中，使理论与实际真正地挂上钩。并对实际工业控制理论有了更深一步的理解，这些对于我以后的工作及生活有着很大的帮助。我在整个设计过程中完成了资料查找、方案确定和论文的撰写。在毕业设计这段时间中，从理论知识的学习到软件的设计都得到了指导老师郑桐老师的精心指导和支持，克服了种种困难，终于圆满完成了此次毕业设计。我由最初的懵懂，通过翻阅大量的与设计有关的资料，查阅和研究大量相关的书籍，详细了解了单片机控制理论，到最后逐渐掌握了整个设计的技术和经验。但是，由于我对这一课题的经验相对缺乏，以致于我刚开始动手设计时有所茫然，在郑老师的耐心指导下，及其他几位同学的齐心协力的工作下，最后，我圆满完成了该设计。经过这次毕业设计，不仅巩固了自己的专业知识，学到并掌握单片机设计思路，通过理论与实际相结合，提高了解决实际问题的能力，也更进一步提高了我的动脑、动手能力和自己的专业技术水平。培养了吃苦和协作精神，为我即将走向工作岗位打下了良好的基础。 致 谢 词短暂几个月的毕业设计即将结束，在此，我首先感谢我们的母校，在我们即将步入社会，走向工作岗位之际为我们提供了这样一个良好的学习与动手锻炼的机会，并为我们提供了良好的毕业设计场所和实验设备。同时，也感谢自动化系领导老师在我设计过程中给予了极大的支持和帮助。 在本次设计过程中，我的指导老师郑桐老师不管是在学习还是在工作上都给予我悉心的指导和帮助，在此，向郑老师表示我诚挚的谢意，并感谢我的同学在毕业设计期间给予我的帮助、支持和指导。外文资料翻译Principles of Variable-frequency A. C. DrivesIn many modern variable-speed drives the demand is for a precise and continuous control of speed with long-term stability and good transient performance. The d. c. motor has satisfied these requirements satisfactorily in many applications, but the mechanical commutator is often undesirable because of the regular maintenance required. This causes difficulty when service interruptions cannot be tolerlated or when the motor is used at inaccessible locations. A. C. motor such as the squirrel-cage induction motor and the synchronous reluctance motor have a robust rotor construction which permits reliable maintenance-free operation at high speed. The simple rotor construction also results in a cheaper motor and a higher power/weight ratio. Unfortunately, the induction motor and reluctance motor are both inflexible in sped when operated on a standard constant-frequency a. c. supply. The reluctance motor operates synchronously at a speed which is determined by the supply frequency and the number of poles for which the stator is wound. The induction motor runs slightly below synchronous speed. For intermittent operation at reduced speeds, stator voltage control of the induction motor is satisfactory. Sub-synchronous speed control of a wound-rotor induction motor is obtained by means of a converter cascade for slip-energy recovery. However, efficient wide-range speed control of the reluctance motor or cage-rotor induction motor is only possible when a variable-frequency a. c. supply is available. Consequently, in this text, attention is largely concentrated on the variable-frequency method of obtaining variable-speed a. c. motor operation.Hitherto, the various techniques for the speed control of a. c. motors often required auxiliary rotating machines. Nowadays, static a. c. drives, using thyristors (silicon controlled rectifiers) as solid-state switching devices, have been developed and marketed in Europe and in the United States of America. Falling thyristor prices and an appreciation of the performance possibilities have created a growing interest in solid-state a. c. drives, and consequently the variable-frequency drive, in particular, is being increasingly applied in industry.In a static frequency converter, the fixed-frequency a. c. supply voltage is transformed to a variable-frequency output which is used to power conventional a. c. motors. In this text, attention is concentrated on static frequency converters using thyristors, but much of the text is also applicable to transistorized converters. Power transistor of tens of amperes and several hundred volts are now available and, for an inverter rating of less than 5kVA, a transistorized inverter may be cheaper than a thyristor inverter. Consequently, transistor inverters are economically competitive for a. c. motor drives of less 5 h. p. , but thrusters are essential for higher powers.The problem in selecting a variable-speed drive for a particular application is to choose the system which can most economically provide the required range of speed control with the desired accuracy and speed or response. The a. c. commutator motor has been widely used, since it can be supplied directly form the a. c. mains, but for a reversible drive with continuous speed control over a very wide range the d. c. motor has been the most popular solution. The separately excited d. c. motor is rapidly and efficiently controlled by variation of the armature voltage and field current. In recent years the d. c. supply has been obtained form the a. c. network by means of static converters which permit the controlled rectification of the alternating voltage so that a variable direct voltage is provided for the armature. Precise speed control is achieved by adopting closed-loop feedback methods.However, the d. c. motor is not the ideal solution to the problem of variable-speed motor operation. The commutator consists of a large number of copper segments separated by thin sheets of mica insulation. This elaborate construction increases the cost of the d. c. motor and reduces the power/weight ratio. Brush and commutrtor wear are accentuated by sparking, and the mica insulation limits the voltage between segments. The total armature voltage is therefore limited to a maximum of about 1500V. The magnitude of the armature current and its rate of change are restricted by commutation difficulties, and the speed of rotation of the d. c. machine is limited. The squirrel-cage induction motor, on the other hand, has a rotor circuit consisting of a short-circuited winding which can often be made from a single casting. There is no necessity to insulate the rotor bars from the surrounding laminations, and the cage rotor has a low inertia and can operate at high temperature and high speed for prolonged periods without maintenance. In addition, the cost of the cage-rotor induction motor is only about one-sixth of that for a d. c. motor of the same speed and horsepower rating. The power/weight ratio of the squirrel cage motor is about twice that of the d. c. machine, and induction motors are manufactured in much larger power ratings since voltage can be 15kV or more. It is obvious, therefore, why numerous attempts have been made to obtain economic and efficient speed control of the cage-rotor induction motor.Microprocessors Generation Control Technology The first attempts at powerplant automation date back to about 1985. They were truly “hybrid” in the sense that they were part electric, part electrohydraulic, and part pneumatic. TVAs 900 MW Bull Run station had electronic controls around 1962. In 1960, utilities installed their first large digital computers. The configuration of these early direct digital control (DDC) systems combined analog and digital controls in a true hybrid function as we know that term today. Despite the back-up computer, however, if elements or subsystem components failed, the entire control scheme went down, and manual controls had to override in an attempt to keep the unit or station from tripping out. Control equipment in generating stations can range from relatively simple, discrete devices that control or monitor a single function, to complex multifunction automated systems and subsystems. The functions controlled may include boilerpressure and temperature trends, ramping power outputs up or down, switching fans, pumps, coal pulverizes, entire FGD systems, scrubbers, and a host of other plant auxiliaries turning them on or off, or providing intervention in crisis situations that might otherwise trip out a turbine/generator unit or even an entire plant. Typically, process-control computers have performed such monitoring and data-logging functions as in-plant alarm/warning, excursion and transient trending, post-trip review and analysis, and sequence-of-events recording of reactor-control and protection systems, with on-line data acquisition, alarming, logging, and data reduction. The early systems were based on analog computers, which extended the traditional use of analog instrumentation in stations. Years ago, however, control manufacturers realized the potential advantages of digital techniques. This led ultimately to direct digital control (DDC), still built around a central computer or central processing unit (CPU). DDCs advantages are related directly to the nature of the hardware. Calculations and nonlinear functions, such as square root, thermocouple conversions, etc, are easily done. Also, system tuning the setting of gains and time constants can be done directly in engineering units. The digital control system also offers better communications with the operator. Alarm messages and directional guides can be displayed on a CRT or printer, and process and control variables can be retrieved by the operator for display in engineering units. Tuning can be done on all control-system loops, in engineering units, from one central device.Finally, the digital-control system affords the infinitely greater flexibility that software has over hardware, parametric changes in a digital control system are made more quickly and more economically by rewriting the software.The on-going objective of the late 1970s was to fashion comprehensive system that incorporated the advantages of digital control. The solution has been achieved sequentially with the development of, first, miniprocessors, and then, microprocessors the “computer on a chip”. These major breakthroughs in electronic miniaturization have provided a cost-effective method of manufacturing systems in which control functions could be decentralized, or “distributed”. Each microprocessor comprises a CPU, an input-output unit, and appropriate logic circuits. Thus, a single microprocessor failure, like a single controller failure in an analog system, would force only those loops associated with that processor into the manual mode, while all other controls remain in automatic.In the early 1980s, some major US manufacturers introduced a new DDC system that used functionally distributed microprocessors and an input/output (I/O) subsystem especially designed for