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 control 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,Detector,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 loop 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 back 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 difference 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 system 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 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 control 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 systems.,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, 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 output 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 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 condition 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 which 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.,Jimei 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 readily 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,