1、Jimei university,LESSON 27,CONTROL THEORY,Jimei university,To control a device or system is to be able to adjust or vary the parameters which affect it.,Jimei university,This can be achieved manually or automatically depending upon the arrangements made in the system.,Jimei university,All forms of c
2、ontrol can be considered to act in a loop.,Jimei university,The basic elements present in the loop are a detector(检测器), a comparator(比较器) and a regulator(调节器).,Jimei university,All of which surround the process and form the loop (Fig. 6 (a).,Jimei university,Comparator,Controller,Process Of System,D
3、etector,Desired value,(a) Basic elements in a control loop,Jimei university,This arrangement is an automatic closed loop if,Jimei university,if the elements are directly connected to one another and the control action takes place without human involvement(卷入、牵连).,Jimei university,A manual closed loo
4、p would exist if one element were replaced by a human operator.,Jimei university,It can be seen therefore that in a closed loop control system the control action is dependent on the output.,Jimei university,A detecting or measuring element will obtain a signal related to this output which is fed bac
5、k to the transmitter(传送器).,Jimei university,From the transmitter the signal is then passed to a comparator.,Jimei university,The comparator will contain some set or desired value of the controlled condition which is compared to the measured value signal.,Jimei university,Any deviation(偏差) or differe
6、nce between the two values will result in an output signal to the controller.,Jimei university,The controller will then take action in a manner related to the deviation and provide a signal to a regulating unit.,Jimei university,The regulating unit will then increase or decrease its effect on the sy
7、stem to achieve the desired value of the system variable (Fig. 6(b).,Jimei university,Controller,Comparator,Transmitter,Detector,Process Of System,Regulating unit,Desired value,(b) Actual elements in a control loop,Jimei university,The transmitter, controller and regulating unit are supplied with an
8、 operating medium in order to function.,Jimei university,The operating medium may be compressed air, hydraulic oil or electricity.,Jimei university,For each medium various types of transmitting devices, controllers and regulating units are used.,Jimei university,Some of the terms in common use in co
9、ntrol system will now be defined.,Jimei university,OPEN LOOP CONTROL SYSTEM,It is one in which the control action is independent of the output, that is, the output in no way controls the system.,Jimei university,This form of control can be used, and has been used, on some accommodation heating syste
10、ms.,Jimei university,The quantity of steam flowing through the heaters is controlled by a valve, the setting of which is determined by the deck temperature.,Jimei university,As long as this temperature remains constant, so will the heat to the air and to the accommodation.,Jimei university,However,
11、if the air flow (or the steam pressure) changes the air could become cooler or hotter,Jimei university,but the steam valve would remain in the same position as it has no feed back of information on the air temperature.,Jimei university,FEEDBACK,This is the transmission of a signal representing the o
12、utput i.e. the controlled condition, for comparison with the input to the system.,Jimei university,Feedback will increase accuracy.,Jimei university,CLOSED LOOP CONTROL SYSTEM,The system is one in which the control action is dependent on the output.,Jimei university,The marine examples are steering
13、gear, boiler control, cargo control, engine control, etc.,Jimei university,The measured value of the output is being fed back to the controller which compares this value with the desired value for the controlled condition and,Jimei university,and produces an output to alter(改变) the controlled condit
14、ion if there is any deviation between the values.,Jimei university,MEASURED VALUE,Actual value of the controlled condition.,Jimei university,DESIRED VALUE,The value of the controlled condition that the operator desires to obtain.,Jimei university,SET VALUE,The value of the controlled condition to wh
15、ich the comparing element is set. It is usually the desired value.,Jimei university,DEVIATION,It is the difference between measured and desired values.,Jimei university,This signal, probably converted into some suitable form such as voltage to hydraulic output or voltage to pneumatic output etc.,Jim
16、ei university,would be used to instigate(激起) corrective(改正、矫正) actionobject to reduce the error to zero.,Jimei university,OFFSET(偏移量、剩余偏差),It is sustained(持续的) deviation (or error).,Jimei university,TRANSMITTER (OR TRANSDUCER),It changes the signal received from the detector sensor(感受器) into some re
17、adily amplified output, usually electrical.,Jimei university,The term transmitter has now general application and is no longer confined(限制) to electrical terminology(专门名词, 术语) only,Jimei university,it could be mechanical movement to electrical output or to pneumatic output and so on.,Jimei universit
18、y,READING MATERIAL,A. CONTROLLER ACTION AND TYPES,Jimei university,TWO STEP(双位) (ON-OFF) CONTROLLER ACTION is the action of a controller whose output signal changes from one predetermined value to another when the deviation changes sign(符号).,Jimei university,This controller action is mainly on/off c
19、ontrol,Jimei university,for example, a refrigeration unit controlling room temperature,Jimei university,when the temperature rises to a predetermined value the refrigerator compressor motor automatically starts,Jimei university,then when the temperature falls to a predetermined value the motor is st
20、opped.,Jimei university,Proportional Controller Action is the basic action form of a continuous controller whose output signal is proportional to the deviation.,Jimei university,In the case of a boiler water level control, if the water level in the boiler, due to increased steam demand, drops by 20
21、mm,Jimei university,then the automatic controller output to the feed water inlet may change from 0.6 to 0.7 bar.,Jimei university,If the level drops by 40 mm, then the automatic controller output would change from 0.6 to 0.8 bar.,Jimei university,Offset or droop(固定偏差) is an inherent(固有的) characteris
22、tic of proportional controllers.,Jimei university,When a load change occurs in a controlled system,Jimei university,a proportional change occurs in the controller which results in an offset or steady state error(恒稳态误差) to the desired value.,Jimei university,INTEGRAL(积分) CONTROLLER ACTION is the acti
23、on of a controller whose output signal changes at a rate which is proportional to the deviation.,Jimei university,It is used in conjunction with proportional control in order to remove offset inherent in the proportional controller.,Jimei university,DERIVATIVE(微分) CONTROLLER ACTION is the action of
24、a controller whose output signal is proportional to the rate at which the deviation is changing.,Jimei university,The principal object of derivative control is to give quicker response to system changes.,Jimei university,Sophisticated(复杂的、完善的) controllers for rapid response, minimum offsets and rese
25、t action would be three term controllers,Jimei university,i.e. proportional plus integral plus derivative.,Jimei university,B. SUPPLY OF CONTROL SYSTEMS,Jimei university,Marine control systems are either pneumatic, hydraulic or electronic or a combination of these.,Jimei university,The electronics c
26、ould act as the nervous system(神经系统) with the pneumatics or hydraulics supplying the muscle.,Jimei university,The advantages of the systems will now be examined and it will be left as an exercise for the reader to list the disadvantages.,Jimei university,PNEUMATIC SYSTEM ADVANTAGES,1. Less expensive
27、 than electronic or hydraulic systems. 2. Leakages are not dangerous.,Jimei university,3. No heat generation, hence no ventilation required. 4. Reliable.,Jimei university,5. Not very susceptible(对敏感的) to variations in ships power supply. 6. Simple and safe.,Jimei university,ELECTRONIC SYSTEM ADVANTA
28、GES,1. Fewer moving parts hence less lubrication and wear. 2. Low power consumption.,Jimei university,3. System is either on or off, with pneumatic or hydraulic systems if they develop a leak, or dirt enters the system they become sluggish.,Jimei university,4. Small and adaptable(通用性强). 5. Very quic
29、k response.,Jimei university,HYDRAULIC SYSTEM ADVANTAGES,1. Nearly instant(立即的) response as fluid is virtually(实际质上的) incompressible(不可压缩的).,Jimei university,2. Can readily provide any type of motion such as reciprocating or rotary.,Jimei university,3. Accurate position control. 4. High amplification of power.,
