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奥迪A6L悬架系统原理与检修【汽车类】【1张CAD图纸】

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关键词：: 奥迪 a6l 悬架系统原理检修汽车 cad图纸

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奥迪A6L悬架系统原理与检修

24页 12000字数+说明书+外文翻译+1张CAD图纸

A0 奥迪A6悬架系统.dwg

外文翻译--温度控制简介和PID控制器.doc

奥迪A6L悬架系统原理与检修说明书.doc

第一章引言4

1.1 课题的意义4

1.2 国内外研究现状4

第二章汽车悬架系统7

2.1汽车悬架系统简介7

2.2 汽车空气悬架结构组成10

2.3 汽车空气悬架系统的特性13

2.4 汽车空气悬架的优缺点14

（一）汽车空气悬架的优点15

（二）汽车空气悬架的缺点15

第三章奥迪A6L悬架系统16

3.1 奥迪A6简介16

3.2 空气悬架系统18

3.2.1空气悬架系统的结构18

3.2.2 空气悬架系统的原理19

3.2.3 空气悬架系统的主要特点19

3.3 奥迪A6悬架系统原理19

第四章奥迪A6L悬架系统检修21

4.1奥迪A6空气悬架系统布局21

4.2 显示检修22

4.3 系统部件23

参考文献24

附录一：英文专业文摘及翻译25

附录二：外文文献原文32

摘要：汽车悬架系统是传递车身与轮胎之间各种力和力矩的连接装置。汽车悬架系统基本上是由弹性元件、减振器和导向机构三大部分组成。这三部分分别起缓冲、减振和导向作用，共同承担传递轮胎与车身之间的各种力和力矩的任务。悬架作为上述各种力和力矩的传动装置，其性能的好坏是影响汽车行驶平顺性和操纵稳定性最重要、最直接的因素。

ABSTRUCT:：The vehicle suspension system is a connecting device, which transmit various force and moment between body and tire. The vehicle suspension system is mainly consisted of spring components, damper components and orientation machine. The three parts respectively performs the functions of cushion damping and orientation, and cooperate with each other to finish the task that transfers all kinds of forces and torques from tires to vehicle body. As this kind of equipment the transferring characteristics of the suspension system is the most important and direct factors that influence the vehicle ride performance and handling performance. 随着空气悬架应用的推广，对空气弹簧、导向机构及控制机构的研究也得到了重视。J. R. EVANS等人在1970年做了空气弹簧垂直特性实验，建立空气弹簧垂直动态特性模型；1994年做了空气弹簧的侧向特性实验，在大频率和大幅值情况下，测量了空气弹簧在不同载荷下的侧向力和变形。Katsuya Yoyofuku等通过研究振动频率和弹簧反应之间的关系，分析管道和气室对弹簧特性变化的影响。交通部重庆公路科学研究所的丁良旭对空气悬架的一些性能进行了探讨，拟合了空气弹簧的特性曲线。 Jon Bunne和Roger Jable研究了空气悬架对传动系统振动的影响。John Woodrooffe通过试验分别评价了重型货车空气弹簧悬架和钢板弹簧悬架的路面附着性和行驶平顺性。

空气悬架系统的特性：

1、空气弹簧的特点

（1）空气弹簧具有非线性特性，可将其特性曲线设计成理想形状。如图1所示空气弹簧特性曲线，静、动刚度随着载荷的增加而增大。

（2）空气弹簧质量轻，内摩擦极小，对高频振动有很好的隔振、消声能力。

（3）空气弹簧的刚度和承载能力可以通过调节橡胶气囊内的压力来调整。

（4）空气弹簧制造工艺复杂，费用高。

2、空气悬架对整车性能的影响

（1）空气悬架为刚度可变的非线性悬架。当簧载质量变化时，刚度随之变化，以保持空载和满载时车身高度相同，悬架固有频率基本不变。根据需要，可以选择不同的气囊工作高度，获得理想的固有频率，从而得到良好的行驶平顺性。

（2）空气悬架质量轻，弹簧刚度低,高速行驶时，轮胎与地面的附着能力强，制动距离短；转向时，过多转向和不足转向倾向减小，转向稳定性强，提高了整车的操纵稳定性。

（3）空气弹簧内的空气压力直接反映了簧载质量，可取空气压力作为信号，控制制动缸内的气压，来控制制动时的制动力，更好地保证了行驶安全性。

（4）可通过给空气弹簧气囊充气或放气来调节车身高度。在平坦的路面上，降低车身高度，保持空气阻力系数为最佳值，可以减小油耗或在功率不变的情况下获得最大车速。在崎岖不平的道路上，为了通过障碍物，可以提高车身高度。

（5）减少整车的振动噪声,提高汽车零部件使用寿命。

（6）由于空气悬架刚度低，轮胎动载荷小，能够降低载重汽车对高速公路的破坏。汽车空气悬架系统主要由空气弹簧、导向机构、高度控制阀、减振器、横向稳定器和缓冲限位块等组成。以空气弹簧为弹性元件,利用空气的可压缩性实现其弹性作用的。通过压缩空气的压力能够随着载荷和道路条件变化进行自动调节,不论满载还是空载,整车高度几乎没有变化,可以大大提高乘坐的舒适性。

（一）空气弹簧

空气弹簧是一橡胶/帘布结构的气囊，以空气为介质，利用空气具有的压缩弹性的性质所制成的弹簧。根据橡胶气囊工作时的变形方式，空气弹簧一般分囊式空气弹簧和膜式空气弹簧（如图1）。囊式空气弹簧由夹有帘线的橡胶制成的气囊和密闭在其中的压缩空气构成。气囊外层由耐油橡胶制成单节或多节，节数越多弹簧越软，节与节之间围有钢质腰环，防止两节之间摩擦。气囊上下盖板将空气封于囊内，其主要靠橡胶气囊的挠曲获得弹性变形。膜式空气弹簧由橡胶片和金属压制件组成，在盖板和底座之间放置一圆柱形橡胶气囊。其主要靠橡胶气囊的卷曲获得弹性变形。囊式空气弹簧寿命较长、制造方便、刚度较大，常用于载货汽车上；膜式空气弹簧尺寸较小、弹性特性曲线更理想、刚度较小，常用于轿车上。

图2-1 空气弹簧

（二）导向机构

由于空气弹簧只能承受垂直载荷，要传递作用在车轮和车架〔或车身〕之间的一切力（纵向力和侧向力）和其力矩，必须在汽车空气悬架中设计导向机构。导向机构的形式很多，目前典型的导向机构有如下几种。

1、板簧式导向机构

主要用于复合式空气弹簧悬架中，钢板弹簧主要作用为导向元件，同时也承担一部分载荷，兼起一部分弹性元件的作用。日野、日产及韩国的部分大中型客车都采用这种悬架结构型式。这种导向机构对于具有纵梁或类似纵梁的汽车底盘结构布置比较方便，对于传统的工字型锻造前梁和铸造后桥壳，不需进行改造，可直接装置空气悬架。常见有纵置半椭圆钢板弹簧导向机构和纵置四分之一椭圆钢板弹簧导向机构。

2、纵向单臂式导向机构

纵向单臂式导向机构类似于纵置四分之一椭圆钢板弹簧导向机构，所不同的是纵臂采用刚性臂，而纵置四分之一椭圆钢板弹簧导向机构采用弹性臂，这种导向机构必须设置横向推力杆，用来承担侧向力。该导向机构可降低汽车纵向倾覆力矩中心位置，增加了车身抗纵倾能力。前悬架导向臂一般较长。可保持主销后倾角不变。通常导向臂与车桥及车架弹性连接，消除了刚性连接应力集中的影响，也可减少噪声的传递。

3、A形导向机构

A形导向机构可以看成是纵向单臂式导向机构的特殊型式，它将两根纵置刚性臂通过与车架上一点的连接构成A形架，在传递纵向力的同时还传递侧向力。A形架可避免导向机构内的附加载荷，克服了纵向单臂式导向机构的缺点。A形架的另一优点是可使左右空气弹簧中心距较大，大大提高了悬架的侧向角刚度。但是该结构为了减少轮胎磨损，避免空气弹簧有过大的垂直位移，常把A形架做得很大以增加摆臂长度，使得导向机构尺寸和重量变大。

4、四连杆导向机构

四连杆导向机构是空气弹簧悬架系统广泛采用的一种结构型式。它常采用两种结构型式，一种主要用于前悬架；另一种主要用于后悬架。如依卡露斯256前悬架、三菱扶桑MP158前悬架等，这种导向机构采用一根上纵向推力杆，二根下纵向推力杆和一根横向推力杆组成。依卡露斯256后悬架、日野RE大客车后悬架等其四连杆导向机构采用两根上纵向推力杆在水平面内倾斜布置的方式，构成了一个三角形架,上推力杆不仅承受纵向力，也承受侧向力。

（三）高度控制阀组件

高度控制阀组件是用来控制空气弹簧内气体压力的执行机构，装配在车架和车桥之间，用来感知车身与悬架之间高度变化，即空气弹簧挠度变化，调整空气弹簧的刚度，使之维持在标准高度附近。高速时降低车身，保持汽车稳定性，减少空气阻力。在起伏不平的路面情况下，提高车身高度以提高汽车通过性。

高度控制阀根据阀门开闭对车身振动反应时间分为即时型和延时型。所谓即时型高度控制阀即当车身有相对位移时，高度控制阀就有充放气动作。这就要求控制设备精度高，气路密封性好，同时所有的控制设备每时都处于工作状态，工作负荷较大；延时型高度控制阀避免了这种频繁工作的现象。延时型高度控制阀通过延时装置产生阻尼，延缓阀门的动作，其延时时间一般为1—6s，通常使用时间为2—4s，即在两个振动周期左右不敏感，以节省压缩空气无益的消耗，减小了阀中各零部件的磨损，延长了高度控制阀的使用寿命，所以被普遍采用。

（四）减振器奥迪A6L悬架系统原理与检修.gif

内容简介：: 附录一：英文专业文摘及翻译温度控制简介和PID控制器过程控制系统自动过程控制系统是指将被控量为温度、压力、流量、成份等类型的过程变量保持在理想的运行值的系统。过程实际上是动态的。变化总是会出现，此时如果不采取相应的措施，那些与安全、产品质量和生产率有关的重要变量就不能满足设计要求。为了说明问题，让我们来看一下热交换器。流体在这个过程中被过热蒸汽加热，如图1所示。 Fig. 1 Heat exchanger这一装置的主要目的是将流体由入口温度乃(f)加热到某一期望的出口温度T(f)。如前所述，加热介质是过热蒸汽。只要周围没有热损耗，过程流体获得的热量就等于蒸汽释放的热量，即热交换器和管道间的隔热性很好。很多变量在这个过程中会发生变化，继而导致出口温度偏离期望值。如果出现这种情况，就该采取一些措施来校正偏差，其目的是保持出口温度为期望值。实现该目的的一种方法是首先测量r(0，然后与期望值相比较，由比较结果决定如何校正偏差。蒸汽的流量可用于偏差的校正。就是说，如果温度高于期望值，就关小蒸汽阀来减小进入换热器的蒸汽流量；若温度低于期望值，就开大蒸汽阀，以增加进入换热器的蒸汽流量。所有这些操作都可由操作员手工实现，操作很简单，不会出现什么问题。但是，由于多数过程对象都有很多变量需要保持为某一期望值，就需要许多的操作员来进行校正。因此，我们想自动完成这种控制。就是说，我们想利用无需操作人员介入就可以控制变量的设备。这就是所谓自动化的过程控制。为达到上述目标，就需要设计并实现一个系统。图2所示为一个可行的控制系统及其基本构件。Fig. 2 Heat exchanger control loop首先要做的是测量过程流体的出口温度，这一任务由传感器(热电偶、热电阻等)完成。将传感器连接到变送器上，由变送器将传感器的输出信号转换为足够大的信号传送给控制器。控制器接收与温度相关的信号并与期望值比较。根据比较的结果，控制器确定保持温度为期望值的控制作用。基于这一结果，控制器再发一信号给执行机构来控制蒸汽流量。下面介绍控制系统中的4种基本元件，分别是：(1)传感器，也称为一次元件。(2)变送器，也称二次元件。(3)调节器，控制系统的“大脑”。(4)执行机构，通常是一个控制阀，但并不全是。其他常用的执行机构有变速泵、传送装置和电动机。这些元件的重要性在于它们执行每个控制系统中都必不可少的3个基本操作，即：(1)测量：被控量的测量通常由传感器和变送器共同完成。(2)决策：根据测量结果，为了维持输出为期望值，控制器必须决定如何操作。 (3)操作: 根据控器的处理,系统必须执行某种操作,这通常由执行机构来完成.如上所述,每个控制系统都有M,D和A这3种操作.有些系统的决策任务简单,而有些很复杂.设计控制系统的工程师必须确保所采取确保所采取的操作能影响被控变量,也就是说,该操作要影响测量值.否则,系统是不可控的,还会带来许多危害.PID控制器可以是独立控制器（也可以叫做单回路控制器），可编程控制器（PLCs）中的控制器，嵌入式控制器或者是用Vb或C#编写的计算机程序软件。PID控制器是过程控制器，它具有如下特征：连续过程控制；模拟输入（也被称为“测量量”或“过程变量”或“PV”）；模拟输出（简称为“输出”）；基准点（SP）；比例、积分以及/或者微分常数； “连续过程控制”的例子有温度、压力、流量及水位控制。例如：控制一个容器的热量。对于简单的控制，你使用两个具有温度限定功能的传感器(一个限定低温，一个限定高温)。当低温限定传感器接通时就会打开加热器，当温度升高到高温限定传感器时就会关加热器。这类似于大多数家庭使用的空调及供暖系统的温度自动调节器。反过来，PID控制器能够接受像实际温度这样的输入，控制阀门，这个阀门能够控制进入加热器的气体流量。PID控制器自动地找到加热器中气体的合适流量，这样就保持了温度在基准点稳定。温度稳定了，就不会在高低两点间上下跳动了。如果基准点降低，PID控制器就会自动降低加热器中气体的流量。如果基准点升高，PID控制器就会自动的增加加热器中气体的流量。同样地，对于高温，晴朗的天气(当外界温度高于加热器时)及阴冷，多云的天气，PID控制器都会自动调节。模拟输入(测量量)也叫做“过程变量”或“PV。你希望PV能够达到你所控制过程参数的高精确度。例如，如果我们想要保持温度为+1度或1度，我们至少要为此努力，使其精度保持在01度。如果是一个12位的模拟输入，传感器的温度范围是从0度到400度，我们计算的理论精确度就是4096除400度=0097656度。我们之所以说这是理论上因为我们假定温度传感器，电线及模拟转换器上没有噪音和误差。还有其他的假定。例如，线性等等。即使是有大量的噪音和其他问题，按理论精确度的110计算，1度精确度的数值应该很容易得到的。模拟输出经常被简称为“输出”。经常在0到100之间给出。在这个热量的例子中阀门完全关闭(0)，完全打开(100)。基准点(SP)很简单，即你想要什么样的过程量。在这个例子中一你想要过程处于怎样的温度。 PID控制器的任务是维持输出在一个程度上，这样在过程变量(PV)和基准点(SP)上就没有偏差(误差)。在图3中，阀门用来控制进入加热器的气体，冷却器的制冷，水管的压力，水管的流量，容器的水位或其他的过程控制系统。 PID控制器所观察的是PV和SP之间的偏差(或误差)。它观察绝对偏差和偏差变换率。绝对偏差就是一PV和SP之间偏差大还是小。偏差变换率就是PV和SP之间的偏差随着时间的变化是越来越小还是越来越大。如果存在过程扰动，即过程变量或基准点变化时一-PID控制器就要迅速改变输出，这样过程变量就返回到基准点。如果你有一个PID控制的可进入的冷冻装置，某个人打开门进入，温度(过程变量)将会迅速升高。因此，PID控制器不得不提高冷度(输出)来补偿这个温度的升高。一旦过程变量等同于基准点，一个好的PID控制器就不会改变输出。你所要的输出就会稳定(不改变)。如果阀门(发动机或其他控制元件)不断改变，而不是维持恒量，这将造成控制元件更多的磨损。这样就有了两个矛盾的目标。当有“过程扰动”时能够快速反应(快速改变输出)。当PV接近基准点时就缓慢反应(平稳输出)。我们注意到输出量经常超过稳定状态输出使过程变量回到基准点。比如，一个制冷器通常打开它的制冷阀门的34，就可以维持在零度(在制冷器关闭和温度降低后)。如果某人打开制冷器，走进去，四处走，找东西，然后再走出来，再关上制冷器的门- PID控制器会非常活跃，因为温度可能将上升20度。这样制冷阀门就可能打开50，75甚至100目的是赶快降低制冷器的温度然后慢慢关闭制冷阀门到它的34。让我们思考一下如何设计一个PID控制器。我们主要集中在过程变量(PV)和基准点(SP)之间的偏差(误差)上。有三种定义误差的方式。绝对偏差他说明的是PV和SP之间的偏差有多大。如果PV和SP之间偏差小那我们就在输出时作一个小的改变。如果PV和SP之间偏差大那我们就在输出时作一个大的改变。绝对偏差就是PID控制器的比例环节。累积误差给我们点儿时间，我们将会明白为什么仅仅简单地观察绝对偏差(比例环节)是一个问题。累积误差是很重要的，我们把它称为是PID控制器的积分环节。每次我们运行PID算法时，我们总会把最近的误差添加到误差总和中。换句话说，累积误差二误差1+误差2+误差3+误差4+。滞后时间滞后时间指的是PV引起的变化由发现到改变之间的延时。典型的例子就是调整你的烤炉在合适的温度。当你刚刚加热的时候，烤炉热起来需要一定时间。这就是滞后时间。如果你设置一个初始温度，等待烤炉达到这个初始温度，然后你认为你设定了错误的温度，烤炉达到这个新的温度基准点还需要一段时间。这也就被认为是PID控制器的微分环节。这就抑制了某些将来的变化因为输出值已经发生了改变，但并不是受过程变量的影响。绝对偏差比例环节有关设计自动过程控制器，人们最初想法之一是设计比例环节。意思就是，如果PV和SP之间的偏差很小那么我们就在输出处作一个小的修改；如果PV和SP之间的偏差很大-那么我们就在输出处作一个大的修改。当然这个想法是有意义的。我们在MicrosoftExcel仅对比例控制器进行仿真。图4是显示首次仿真结果的表格。(滞后时间二0，只含比例环节)比例、积分控制器 PID控制器中的积分环节是用来负责纯比例控制器中的补偿问题的。我们有另外一个Excel的扩展表格，表格上仿真的是一个具有比例积分功能的PID控制器。这里(Pig5)是比例积分控制器最初的仿真表格(滞后时间0，比例常数二04)。众所周知，比例积分控制器要比仅有比例功能的比例控制器好得多，但是等于0的滞后时间并不常见。 Fig .4 The simulation chart微分控制微分控制器考虑的是：如果你改变输出，那么要在输入(PV)处反映这个改变就需要些时间。比如，让我们拿烤炉的加热为例。 Fig.5The simulation chart如果我们增大气体的流量，那么从产生热量，热量分布烤炉的四周，到温度传感器检测升高的温度都将需要时间。PID控制器中微分环节具有抑制功能，因为有些温度增量会在以后不需要的情况下产生了。正确地设置微分系数有利于你对比例系数和积分系数的确定。文附录二：外文文献原文Introductions to temperature control and PID controllers Process control systemAutomatic process control is concerned with maintaining process variables temperatures pressures flows compositions, and the like at some desired operation value. Processes are dynamic in nature. Changes are always occurring, and if actions are not taken, the important process variables-those related to safety, product quality, and production rates-will not achieve design conditions. In order to fix ideas, let us consider a heat exchanger in which a process stream is heated by condensing steam. The process is sketched in Fig.1 Fig. 1 Heat exchanger The purpose of this unit is to heat the process fluid from some inlet temperature, Ti(t), up to a certain desired outlet temperature, T(t). As mentioned, the heating medium is condensing steam. The energy gained by the process fluid is equal to the heat released by the steam, provided there are no heat losses to surroundings, iii that is, the heat exchanger and piping are well insulated. In this process there are many variables that can change, causing the outlet temperature to deviate from its desired value. 21 If this happens, some action must be taken to correct for this deviation. That is, the objective is to control the outlet process temperature to maintain its desired value. One way to accomplish this objective is by first measuring the temperature T(t) , then comparing it to its desired value, and, based on this comparison, deciding what to do to correct for any deviation. The flow of steam can be used to correct for the deviation. This is, if the temperature is above its desired value, then the steam valve can be throttled back to cut the stearr flow (energy) to the heat exchanger. If the temperature is below its desired value, then the steam valve could be opened some more to increase the steam flow (energy) to the exchanger. All of these can be done manually by the operator, and since the procedure is fairly straightforward, it should present no problem. However, since in most process plants there are hundreds of variables that must be maintained at some desired value, this correction procedure would required a tremendous number of operators. Consequently, we would like to accomplish this control automatically. That is, we want to have instnnnents that control the variables wJtbom requ)ring intervention from the operator. (si This is what we mean by automatic process control. To accomplish his objective a control system must be designed and implemented. A possible control system and its basic components are shown in Fig.2.Fig. 2 Heat exchanger control loopThe first thing to do is to measure the outlet temperaVare of the process stream. A sensor (thermocouple, thermistors, etc) does this. This sensor is connected physically to a transmitter, which takes the output from the sensor and converts it to a signal strong enough to be transmitter to a controller. The controller then receives the signal, which is related to the temperature, and compares it with desired value. Depending on this comparison, the controller decides what to do to maintain the temperature at its desired value. Base on this decision, the controller then sends another signal to final control element, which in turn manipulates the steam flow.The preceding paragraph presents the four basic components of all control systems. They are (1) sensor, also often called the primary element. (2) transmitter, also called the secondary element. (3) controller, the brain of the control system. (4) final control system, often a control valve but not always. Other common final control elements are variable speed pumps, conveyors, and electric motors. The importance of these components is that they perform the three basic operations that must be present in every control system. These operations are (1) Measurement (M) : Measuring the variable to be controlled is usually done by the combination of sensor and transmitter. (2) Decision (D): Based on the measurement, the controller must then decide what to do to maintain the variable at its desired value. (3) Action (A): As a result of the controllers decision, the system must then take an action. This is usually accomplished by the final control element. As mentioned, these three operations, M, D, and A, must be present in every control system. PID controllers can be stand-alone controllers (also called single loop controllers), controllers in PLCs, embedded controllers, or software in Visual Basic or C# computer programs. PID controllers are process controllers with the following characteristics: Continuous process control Analog input (also known as measuremem or Process Variable or PV) Analog output (referred to simply as output) Setpoint (SP) Proportional (P), Integral (I), and/or Derivative (D) constants Examples of continuous process control are temperature, pressure, flow, and level control. For example, controlling the heating of a tank. For simple control, you have two temperature limit sensors (one low and one high) and then switch the heater on when the low temperature limit sensor tums on and then mm the heater off when the temperature rises to the high temperature limit sensor. This is similar to most home air conditioning & heating thermostats. In contrast, the PID controller would receive input as the actual temperature and control a valve that regulates the flow of gas to the heater. The PID controller automatically finds the correct (constant) flow of gas to the heater that keeps the temperature steady at the setpoint. Instead of the temperature bouncing back and forth between two points, the temperature is held steady. If the setpoint is lowered, then the PID controller automatically reduces the amount of gas flowing to the heater. If the setpoint is raised, then the PID controller automatically increases the amount of gas flowing to the heater. Likewise the PID controller would automatically for hot, sunny days (when it is hotter outside the heater) and for cold, cloudy days. The analog input (measurement) is called the process variable or PV. You want the PV to be a highly accurate indication of the process parameter you are trying to control. For example, if you want to maintain a temperature of + or - one degree then we typically strive for at least ten times that or one-tenth of a degree. If the analog input is a 12 bit analog input and the temperature range for the sensor is 0 to 400 degrees then our theoretical accuracy is calculated to be 400 degrees divided by 4,096 (12 bits) =0.09765625 degrees. We say theoretical because it would assume there was no noise and error in our temperature sensor, wiring, and analog converter. There are other assumptions such as linearity, etc. The point being-with 1/10 of a degree theoretical accuracy-even with the usual amount of noise and other problems- one degree of accuracy should easily be attainable. The analog output is often simply referred to as output. Often this is given as 0100 percent. In this heating example, it would mean the valve is totally closed (0%) or totally open (100%). The setpoint (SP) is simply-what process value do you want. In this example-what temperature do you want the process at? The PID controllers job is to maintain the output at a level so that there is no difference (error) between the process variable (PV) and the setpoint (SP). In Fig. 3, the valve could be controlling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system. What the PID controller is looking at is the difference (or error) between the PV and the SP. SETPOINT P，I，&D CONSTANTS Difference error PID control algorithm process variable output Fig .3 PID controlIt looks at the absolute error and the rate of change of error. Absolute error means-is there a big difference in the PV and SP or a little difference? Rate of change of error means-is the difference between the PV or SP getting smaller or larger as time goes on. When there is a process upset, meaning, when the process variable or the setpoint quickly changes-the PID controller has to quickly change the output to get the process variable back equal to the setpoint. If you have a walk-in cooler with a PID controller and someone opens the door and walks in, the temperature (process variable) could rise very quickly. Therefore the PID controller has to increase the cooling (output) to compensate for this rise in temperature. Once the PID controller has the process variable equal to the setpoint, a good PID controller will not vary the output. You want the output to be very steady (not changing) . If the valve (motor, or other control element) is constantly changing, instead of maintaining a constant value, this could cause more wear on the control element. So there are these two contradictory goals. Fast response (fast change in output) when there is a process upset, but slow response (steady output) when the PV is close to the setpoint. Note that the output often goes past (over shoots) the steady-state output to get the process back to the setpoint. For example, a cooler may normally have its cooling valve open 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler door-the PID controller is freaking out because the temperature may have raised 20 degrees! So it may crank the cooling valve open to 50, 75, or even 100 percent-to hurry up and cool the cooler back down-before slowly closing the cooling valve back down to 34 percent. Lets think about how to design a PID controller. We focus on the difference (error) between the process variable (PV) and the setpoint (SP). There are three ways we can view the error.The absolute error This means how big is the difference between the PV and SP. If there is a small difference between the PV and the SP-then lets make a small change in the output. If there is a large difference in the PV and SP-then lets make a large change in the output. Absolute error is the proportional (P) component of the PID controller.The sum of errors over time Give us a minute and we will show why simply looking at the absolute error (proportional) only is a problem. The sum of errors over time is important and is called the integral (I) component of the PID controller. Every time we run the PID algorithm we add the latest error to the sum of errors. In other words Sum of Errors = Error 1 q- Error2 + Error3 + Error4 + . The dead time Dead time refers to the delay between making a change in the output and seeing the change reflected in the PV. The classical example is getting your oven at the right temperature. When you first mm on the heat, it takes a while for the oven to heat up. This is the dead time. If you set an initial temperature, wait for the oven to reach the initial temperature, and then you determine that you set the wrong temperature-then it will take a while for the oven to reach the new temperature setpoint. This is also referred to as the derivative (D) component of the PID controller. This holds some future changes back because the changes in the output have been made but are not reflected in the process variable yet.Absolute Error/Proportional One of the first ideas people u

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