APS复习过控英文版.doc

Process control engineeringWhat have you learned in this course?1.An overview of process control system2.Process modeling3.Single-loop control system designplex process control systemDetail1- An overview of process control systemA process control system is usually refer to industrial process control, it aim to maintain the output of a specific process within a desired range. It r s extensively used in industry z such as in oil refining, paper manufacturing, chemicals and so on. Following is a basic and general block diagram of process control system:……(fig.)A regulator^ontroller) is usually a PLC(programmable logic controller) or a single-chip. Actuator can be a valve or a motor. Detector and transmitter usually contain sensors and corresponding signal processing circuit； such as filter and amplifier.A simple example is a single-tank level control system (pic•课本173 页)bock diagram ……(fig.).The liquid level is constantly detected by sensor and compared with the setpoint value, in order to gen erate an error signal. The n con troller receives the error signal and produces a con trol signal, acti ng on the valve to adjust the in flow to reduce the error and to keep the level as close as possible to expected value・In this system, liquid level is con trolled variable, and inflow is con trol variable.2.Process modelingThere are roughly two method to obtain the T.F model of a process: mechanisnrvanalysis method and testing method.Mechanism-analysis method:By analyzing the mechanism of process, we can obtain the ordinary differential equation(ODE)f then we can get the F.T. The method applies only to those process that we already accurately un dersta nd its in ter nal function.For example, a single capacity process, (fig.)(注意推导过程)Suppose the process is a sei仁regulating process. (A self-regulating process can reach to a new steady state by itself without human or other devices' help, when interferenee appears.)Multi-capacity process modeling:……Non self-regulating process modeling: such as a single-capacity process with constant flow pump.Testi ng method:Use step or pulse signal as in put, and record output signal to get resp onse curve. By analyzing the response curve, we can obtain its T.F. And many times, this procedure requires an extensive experienee.For example, if we get the response curve of a process, like this (fig.), there's no inflection point(turning point), so we know it is a first-order process；so the T.F must be likethis(equation.). And the parameter T z K can be determined according to some certain points (equation.). To get more accuracy T.F, we can repeat it several times and use the average value as final result・If the response curve is kind of S-shape (fig.), then it is a second or a higher order process. To obtain its T.F need to refer to some professional book, because many of them require rich experienee. And if the curve is like this (fig) it shows that the process contains a pure lag, which can be expressed as e'ts.Notice: if we want to get a discrete model we can use least square method, which usually complete by computer software, like MATLAB.3.Single-loop control system designSingle loop control system: with only one feedback loop z which mea ns only one con trolley one valve and one detector.The requireme nts of in dustrial process control lie in three major aspects: security, system performa nee and economy. System performa nee in eludes stability, rapidity and accurac y. The major procedure to design a process control system: process modeling choose control scheme, choose suitable control instruments and setting parameters of controller.3.1To choose control scheme5.Performanee indexes:(fig.) (equation.)Steady-state error C, decay rate 屮，overshoot Mp%, sett I i ng time ts f and some overall indexes: IAE(integral absolute error), ITAE(integral time absolute error), ISE(integral square error), ITSE(integral time square error).6.Choose con trolled variable:Direct choose: choose variable that can directly reflect the quality of products. E.g in a level con trol system we choose level as con trolled variable.In direct choose: whe n the direct variable is difficult to observer or to determi ne, we should choose a variable that has a single-value function with the direct variable. E.g in a fractio nating tower system, the concen trati on of product should be the direct con trolled variable, but it difficult to directly detect the concentration, so we choose temperature as con trolled variable, because it has a sin gle-value function with concen trati on of product.7.Choose control variable, basic principles:L Gain of forward path should be as big as possible, and gain of disturbanee should be as small as possible・8.Time constant of process should be small, but of disturbanee should be big, and time lag of process should be small・3.2To choose suitable control instrumentsChoose detector and transmitter:should have quick resp on se(ge nerally T<0.1T0)；eno ugh accuracy, and some esse ntial signal process circuit (see Signal Detection and Conversion).Choose valve:we choose valve mainly according to its discharge characteristic (fig.) and its switch type. Discharge characteristic in elude logarithmic characteristic and lin ear characteristic, when process is linear, choose linear characteristic, when it is nonlinear, choose logarithmiccharacteristic. Switch type in elude gas-ope n type and gas-close type, we choose it according to safety and economy. E.g in a boiler system, valve of fuel use gas-open type, valve of water use gas-close type； to ensure the safety.Choose controller (choose control strategy):The most widely used control method in in dustrial process is PID(proportio n in teg rati on differentiation) control.PID equation: G(S)=K P+T I(1/S)+T D S/ or time domain: u(t)=……P: G(s)=Kp z Apply the proportional control can reduce the steady state error, but usually it is not used alone. And if gain K P is too big. it may cause the system in stability ・ E.g.( root locus pic.)PD: G(s)=K P+T D s・ Only works when the error varies, and is not suitable for strongly changed process, such as pressure and flow process. Effect: improve the system's stability, transient performa nee and reduce sett I i ng time and overshoot Disadva ntage: difficult to achieve in practical system. In practicaljt is usually replaced by G(s)=K[ 1+(s+z)/(s+p)]f and let p>5乙And T D can not be too big z or may cause oscillation. It is useless for pure lag.PI: G(c)=K P+Ti(l/s). Effect: it can completely eliminate steady-state error, improve accuracy(i ncrease the type of system). Disadva ntage: resp onse time will in crease and stability become bad .If T【is too big, system will be un stable ・Gen erally speaki ng if we want to get a higher performa nee, we use PID control.3.3tuning parameters of controllerFor PID control, tuning parameters is to determine the coefficient K P/ T b T D.One of the most popular and basic method is called critical-gain method. The advantage od this method is that we can directly tune the parameter in closed二loop system, and do not need to know the characteristic of process, which means there's no need to know the model of process. The specific procedure is as following:Running the system with feedback loop, firstly we should adjust the Ti and K to smallest, and T D to largest. The n in crease K gradually until the output become oscillate, the n record gain K and oscillation frequency. Finally we can refer to the empirical formula to determine the parameter PID controller.Notice: this method dose not adapted to processes that do not allowed oscillation. E.g boiler-water supply system and burning system. For these processes, gain K should be adjust to let the output appears 4:1 decay or 10:1 decay, in stead of oscillatio n. For example (fig.).*3.4 Example to design a single-loop control system: milk powder drying system (a system to dry out milk to make milk powder).Pic. of system's instrument:……Control objective: to maintain a certain water content of milk, and require the error to be as small as possible・Choose con trolled variable: water content should be direct con trolled variable, but there's still no effective sensor to detect water content of milk, so we choose the temperature of dryer as in direct con trolled variable of system.Choose control variable: there are three can didate control variable: milk flow, air flow and steam flow. Milk flow decide the quantity of product, and according to economy principl巳we definitely want to maintain the quantity maximum, so milk powder can't be control variable. Then we see steam flow, the heat exchanger is an double-capacity process, with large timeconstant, if we choose it as control variable, the time constant of forward path may be to large, which can cause settling time increase, so steam flow is also not the best choice. As for air flow, this eleme nt has a smaller time constant, and the in flue nee of disturbanee is also the smallest, so we choose air flow as control variable.Choose valve: in order not to burn the milk powder and to protect the devices when the system suddenly breaks down, we choose gas-close type valve. And because the process is nonlinear z so we choose logarithmic characteristic valve・Regulator or controller: in order to reduce the settling time and overshoot, and improve the system's stability, we can choose PID control.Detector or sen sor: use thermal resista nee thermometer as sen sor f with three wire.plex process control systemComplex process control system refer to cascade control system, feed-forward control system and lag compensation control system and so on.4.1Cascade control systemThe architecture of cascade control system is as following:……(fig.)Primary con troller sec on dary con troller detector one detector twoInn er-loop process outer-loop process sec on dary disturba nee the first disturba neeCascade control system adapted to following situation:9.To overcome the capacity lag and pure lag, to achieve a higher transient performanee.10.To overcome some relative large disturbanee in inner loop.11.To overcome the nonlinearity of process・Design: The design of control scheme is similar to single loop control system・L Gen erally speaki ng inn er-loop process should contain more disturba nces, so that in flue nee of more disturba nces can be restrained. But in order to avoid the time constant of inner-loop to be too long, the process should be appropriately chosen.12.Primary con troller usually applies PID con trol, and sec on dary con troller usually use P con trol. The sec on dary con troller is kind of coarse tuning and it is a servoc on trol system. Primary controller is more likely a fine tuning it is an constant control system・13.when tuning the parameters of two controller, we usually use two-step tuning method. That is, we tune the sec on dary con troller firstly, the prin ciples are similar to sin gle-loop control system, then we tune the primary controller, treat the inner-loop as an element of a single-loop・Example: furnace temperature control systemHearth(炉膛) outlet intake cold materialControl objective: to maintain the temperature of output constant with small steady-state error. Major disturbanee: the variation of fuel flow, pressure and calorific value. And the variation of airflow. (fig.)Why cascade control system: when we use only single-loop control, the system performanee is not so satisfied because of the serious disturbances and the lag of process. Block diagram of system. (fig.)4.2feed-forward controlGen erally speaki ng, feed-forward con trol dose not applied alone in a system, usually with a single-loop system, or a cascade control system. (three fig.)Controller one controller two disturbanee pathApplied occasions: Want to overcome a major and especially serious disturbanee, and the disturba nee should be observable but uncon trollable ・To design a controller for disturbanee:The T.F of controller two is as following: W m(s)=……:W f(s) is the T.F of disturbanee path, W c(s) is the T.F of forward-path controller, W0(s) is the T.F of process.Disadvantage: form the T.F of controller two, we know that we need to get the T.F of process and disturbanee path r which is very difficult to obtain in a practical system・ And even if we can get the T.F, it is also too difficult to achieve in practical, because the control law can be very complicated and expensive.Example: furnace temperature control systemComparator set point value material outletThe perturbanee of fuel flow can be very serious, which seriously deteriorates system's performa nee.Why feed-forward con trol: the disturba nee is especially serious, and should be con trolled individually.Control objective: to overcome the perturbanee of fuel flow, and to achieve a higher performa nee.Block diagram of system: (fig.)4.3Smith-compensation controlGeneral architecture of smith-compensation control:……(fig.)T.F of process: W0(s)e'ts.T.F of Smith-compensation element: G(s)=……Applied occasions: process with large pure lag.Disadvantage: we need to know exactly the model of process, that is W o(s)e'ts, otherwise the performanee will be even worse・ As we know, usually it is difficult to know the accuracy T.F of process, which added the difficulty to system implementation.Example: various transfer system。