1、 南京大學畢業設計(論文)外文資料翻譯系部： 機械系 專 業： 機械工程及自動化 姓 名： 學 號： 外文出處： servo systems 附 件： 1.外文資料翻譯譯文；2.外文原文。 指導教師評語：該同學翻譯的外文資料原稿，緊扣本次畢業設計主題。中文翻譯稿語言通順，與原文表達的意思基本相符，基本符合畢業設計的要求，基本達到了預期的目的。 成績評定為中。 簽名： 年 月 日注：請將該封面與附件裝訂成冊。附件1：外文資料翻譯譯文伺服系統由輸入軸隨意運動引起輸出軸運動的一種形式是同步發送接收系統。另一種形式是伺服機構或伺服系統同步系統可在相當大的距離內在兩個分開的軸之間工作，但不提供力矩放大傳

2、送到負載的力矩不能超過輸入力矩。由于這個原因，并且在傳送大力矩時，偏差角增大，同步系統只能用丁轉動刻度盤和指針，移動控制活門和驅動其他小力矩負載。另一方面，伺服系統可提供要求移動大負載的大力矩，只需給輸入軸加上很小的力矩。遙控運行不是伺服系統的固有特性，但可以通過數據傳送裝置實現，通常同步器作為系統的一部分。 一、伺服系統的基本要求 伺服系統是一種具有響應和執行指令的裝置，伺服系統必須能夠滿足五項基本要求，它們是： a.伺服系統要能夠接收規定期望結果的指令。 b伺服系統要能夠估計存在條件。 c伺服系統要能夠把存在條件與期望結果相比較得出差值或偏差信號。 d伺服系統要能夠依據偏差信號，發出校正指

3、令，正確地改變存在條件到期望結果。 e伺服系統要有執行校正指令的方法。 為使伺服系統滿足五項基本要求，它必須具有一個偏差檢測元件和一個操縱負載位移馬達的控制器。 二、伺服系統部件 伺服系統包括偏差指示器和按照圖1方式連接輸入和輸出軸的控制器。伺服系統的目標是驅功輸出軸通過保持偏差角（輸出軸離輸入軸的角位移偏差）盡可能接近于零而重復輸入軸運動。偏差指示器確定了偏差角的幅值和方向。在偏差指示器信號控制下，控制器在減小偏差的方向上給輸出軸施加力矩。伺服系統是個閉環或稱作反饋系統，因為加到控制器的信號引起輸出軸轉動，改變了偏差角，這樣又改變了加到控制器的信號。圖1偏差指示器和控制器可以采用很多種形式。

4、控制器必須含有伺服馬達或一些產生輸出為矩的裝量。伺服系統可按照所用伺服馬達的類型劃分為電動式、液壓式、氣動式或機械式。本文只討論電動伺服系統。電動伺服系統使用多種電動馬達。除了伺服馬達外，控制器一般還包括功率放大器，能使來自偏差指示器的弱信號轉換為較大功率供給馬達。這種功率放大器通常稱作伺服放大器。偏差指示器最常見的是同步裝量。電動伺服系統中，同步發電機和控制變換器機械聯接到輸入和輸出軸上，控制變換器即偏差指示器，其轉子電壓用作控制器的輸入信號。圖2大部分航空電子應用中的伺服系統是帶控制變換器偏差指示器的電動伺服系統。這種伺服系統的框圖如圖2所示。若這個系統用直流馬達，圖中放大器必須具有將同步

5、系統的交流電壓整流，同時進行功率放大的功能，若使用交流馬達，則需要交流放大器圖2所示的系統中，輸入軸與輸出軸之間的偏差角，確定了由控制變換器產生并送到伺服放大器的偏差電壓的相位和幅值。偏差信號依次控制由伺服馬達加到輸出軸上力矩的方向和幅值。電動伺服系統使用很多類型的伺服馬達，而在航空電子應用中，兩相感應電機是應用最廣的。因而本文只討論兩相感應伺服馬達。三、平衡電位計型偏差指示器伺服系統中作偏差指示器的另一種裝置是平衡電位計（見圖3）本系統中有兩個電位計用在電橋中，一個電位計用于指令控制，另一個電位計機械藕合在伺服機械的輸出軸上。兩個裝置的差值將產生偏差信號。依次引起伺服放大器或控制器轉動輸出軸

6、，直到電橋平衡為止。平衡電位計產生的偏差信號與控制變換器產生的偏差電壓完全是用于同樣方式。圖3ct（控制變換器）的指令一般來自離ct一段距離的同步發送器轉軸的運動，而平衡電位計轉軸指令則可加到一個電位計滑動觸點的轉子上。這種系統的實例正如arn-21塔康收發機的頻道選擇器。平衡電位計的輸入部分位于頻道選擇器的控制端，電位計的另一部分位于收發器組件。電位計的滑動觸點機械聯接到晶體狀六角轉塔上。四、兩相感應電動機 經常用于驅動伺服機構輸出軸的各類交流馬達中，最重要的是兩相感應電動機。這種電機在小容量伺服系統中有著廣泛的應用，例如用于機載塔康的距離和方位指示器驅動系統和用于驅動雷達平面位量指示器的偏

7、轉線圈，還用于大多數航空電于設備上。為了了解采用感應電動機的伺服機構，首先要知道這些電機的特性。圖4圖4給出了表示雙極、雙相感應電動機的轉于、剖面和電路圖。定于和轉子都是由薄鋼片疊加構成的。定子有兩個相同的線圈：a線圈和b線圈。如此排列可使兩個線圈的磁場相互垂直轉子可采用線繞式短路繞組或鼠籠式繞組。鼠籠繞組由轉于槽內的導電條組成。這些導電條由轉子兩端的導電環短接。定子線圈通常供給幅值相等、相位相差90度的交流電，這種交流電可直接從兩相電源系統得到，或從單相電源利用移相電路的方式獲得如圖4 (d)所示。正如圖4 (e）的相量圖所示，b線圈電流ib因線圈的感抗而滯后于外加電壓e.a線圈電流ia因電

8、容c的容抗大于a線圈的感抗而使電流超前于外加電壓e。適當選擇電容值的大小，可改變ia相對e的相位，使la和ib的相位差接近90度。由于電流流過兩個線圈，通過轉子鐵芯的總磁通的幅值是常量，并由定子電流的頻率確定磁場沿電動機軸轉動的速度。圖5表明了旋轉磁場是如何產生的。圖中標明了在電流的一個周期內各段區間磁通的方向。圖中頂行描述了一個電流周期內每隔90度時，定子合成磁場的狀態。圖5圖中所示的任何時刻的瞬時磁通，是流過兩個線圈電流產生的合成磁通。相位角為零度時僅a線圈有電流，磁通方向沿a線圈軸線內上。90度時，僅b線圈有電流，磁通方向向右。18度時，由于a線圈電流在負值方向上，所以磁通方向向下。頂行

9、圖示說明了供電電流每個周期內，磁通旋轉一周的變化。圖5中第二行給出了兩個線圈都有電流時，相位角為中間值的磁通狀態。由于定子線圈的每匝都分布在槽內，這種方式使氣隙磁通密度隨轉子的角度呈正弦規律變化，磁通在整個旋轉過程中保持恒定幅值。例如。在45度相位角，流過a線圈的電流幅值是最大值的0.707倍這個電流產生向上的磁通也是最大值的0.707倍。b線圈也流過幅值為最大值0.707倍的電流，產生最大值的0.707倍的向右的磁通。兩個成直角的磁通線圈，其合成相量轉到45度方向，幅值等于每個單個線圈的最大幅值。 旋轉磁通的速度叫做同步轉速，它由線圈電流的頻率決定。對于工作在400周秒電源的雙極電動機來說，

10、其同步轉速為2400轉分。 兩相感應電機中，若把任何一個定子線圈的接線端子互換，則旋轉磁通的方向將反向。換句話說，每個定子電流移相18度時，將使電動機反轉。當a線圈電流ia極性反向時，磁通的旋轉方向也由順時針變為反時針。 旋轉磁通穿過轉子導體。在轉子導體中產生電勢，因此有電流流過轉子短路繞組，圖6表示磁通旋轉的某一瞬間，轉子電流的方向可用圓點和叉號表示。（圖中假定轉子轉速近似等于同步轉速，且轉子電流與轉子感應電勢同相）字母n和s代表定子旋轉磁場的北極和南極。n和s代表轉子電流產生的轉子磁場北極和南級。因此轉子成為一個磁體，試圖朝定子磁場方向調節自己，由此產生轉矩，方向是使轉子沿定子旋轉磁場方向

11、轉動。附件2：外文原文（復印件）servo systemsone means of causing an output shaft to follow the arbitrary motion of an input shaft is the synchro transmitter-receiver system. another means is the servomechanism or servo system. synchro systems can operate between shafts separated by a considerable distance but cann

12、ot supply torque amplification-the torque delivered to the load can never exceed the input torque. for this reason, and because the error angle increases when large torques are transmitted, synchro systems are employed only to turn dials and pointers, move control values, and actuate other low torqu

13、e loads. servo systems on the other hand, can supply the large torques required to move heavy loads, and only a very small torque need be applied at the input shaft. remote operation is not inherent in a servo system but may be obtained if data transmission devices, usually synchros, are made part o

14、f the system.1. basic requirements of servosa servo system is a device that has the ability to respond to and carry out an order. a servo system must be able to fulfill five basic requirements. they are:a. a servo must be able to accept an order which defines the result that is desired.b. a servo mu

15、st be able to evaluate the existing conditions.c. a servo must be able to compare the desired result with the existing conditions, obtaining a difference, or error, signal.d. a servo must be able to issue a correcting order, based on the error signal, which will properly change the existing conditio

16、ns to the desired result.e.a servo must have the means of carrying out the correcting order.in order for a servo system to meet the five basic requirements, it must possess an error detecting device and a controller that operates the load positioning motor.2. components of servo systemsa servo syste

17、m comprises an error indicator and a controller connected to the input and output shafts in the manner shown in fig. 1. the object of the servo system is to cause the output shaft to repeat the motion of the input shaft by maintaining the error angle (deviation in angular position of output shaft fr

18、om that of input shaft) as near to zero as possible. the error indicator determines the magnitude and direction of the error angle. under control of the signal from the error indicator, the controller exerts a torque on the output shaft in a direction to reduce the error. the servo is a closed-loop,

19、 or feedback system, because a signal applied to the controller causes rotation of the output shaft and thus changes the error angle with the result that an altered signal is applied to the controller.figure 1 basic servo system.the error indicator and controller may take a wide variety of forms. th

20、e controller must include a servo motor or some device for developing the output torque. servos are classified as electrical, hydraulic, pneumatic, or mechanical in accordance with the type of servo motor used. only electrical servo systems will be discussed in this course. electrical servos use ele

21、ctric motors of some sort. the controller often contains, in addition to the servo motor, a power amplifier to enable the weak signal from the error indicator lo control the large amounts of power supplied to the motor. this power amplifier is generally referred to as the servo amplifier. the error

22、indicator is most frequently a synchro device. in electrical servo systems a synchro generator and control transformer are connected mechanically to the input and output shafts. the control transformer is then the error indicator, its rotor voltage serving as input signal for the controller.the most

23、 used servo in avionics applications is the electrical servo with control transformer error indicator. a block diagram of this servo system is drawn is fig. 2. if a dc motor is used in this system, the amplifier in the figure must include a means of rectifying the alternating voltage from the synchr

24、o together with a means of increasing the power level. if an ac motor is used, an ac amplifier is required. in the system shown in fig. 2 the error angle between the input and output shafts determines the phase and magnitude of the error voltage developed by the control transformer and fed to the se

25、rvo amplifier. this in turn controls the direction and magnitude of the torque applied to the output shaft by the servo motor. many types of servo motors are used in electrical servo systems, but for avionics applications t the two-phase induction motor is most commonly used. it will be the only ser

26、vo motor discussed in this course.figure 2 block diagram of an electrical servo using a control transformer as an error detector. 3- balanced potentiometer error indi catoranother type device used as an error detector in servo systems is the balanced potentiometer (see fig. 3). in thisfigure 3 typic

27、al balanced potentiometer system.system, two potentiometers are used in a bridge circuit, where one potentiometer is a command control and the other potentiometer is mechanically coupled to the output shaft of the servomechanism. a difference between the two settings results in the production of an

28、error signal which, in turn, will cause the servo amplifier or controller to rotate the output shaft until the bridge is balanced. the error signal produced by the balanced potentiometers is used in exactly the same way as that error voltage from a control transformer.with the ct the order usually c

29、omes from a movement of a synchro transmitter shaft at some distance from the ct; but in the balanced potentiometer system, the order shaft may be attached to the rotor of the sliding contact of one potentiometer. an example of such a system is the channel selector in the arn-21 tacan transceiver. t

30、he input portion of the balanced potentiometer is located in the channel selector control head. the other part of the potentiometer is located in the transceiver unit. its sliding contact is mechanically connected to the turret containing the crystals.4. two-phase induction motorthe most important o

31、f the several types of ac motors used to drive the output shafts of servomechanisms is the two-phase induction motor. this motor has wide applications in low powered servo systems such as those used in the airborne taken range and azimuth indicator drive systems and in driving the deflection yokes o

32、n the radar ppi indicators, as well as most avionics applications. to understand servomechanisms employing induction motors it is first necessary to know the characteristics of these motors. figure 4 shows the rotor, cross section, and circuit diagram representation of a two-pole, two-phase inductio

33、n motor. stator and rotor are built of sheet steel laminations. the stator has two similar windings, coil a and coil b, arranged so their magnetic fields will be at right angles to each other. the rotor may carry either a short-circuited winding of wire or a squirrel-cage winding. the squirrel-cage

34、winding consists of conducting bars in the rotor slots the bars being short-circuited at each end of the rotor by conducting rings. the stator coils are usually supplied with alternating currents equal in magnitude but 90 degrees apart in phase. such currents may be obtained directly from a two-phas

35、e power system, or they may be obtained from a single-phase source by means of a phase shifting circuit such as that shown in fig. 4(d). as indicated in the vector diagram of fig. 4(e), the current ih in coil b lags the applied voltage e because of the inductance of the winding. the current ia in co

36、il a leads the applied voltage e because the capacitor c has a reactance greater than the inductive reactance of coil a. by proper choice of capacitor size the phase of ia with respect to e can be varied so that the phase relationship between ia and ib closely approximates 90 degrees. as a result of

37、 the currents flowing in both coils, the total flux through the rotor core is constant in magnitude and rotates about the axis of the motor at a speed determined by the frequency of the stator currents. the diagram of fig. 5 shows how the rotating flux is produced. this diagram shows the direction o

38、f the flux at various intervals in the current cycle. the top row of drawings in the figure shows the resultant flux field from the stators at 90 intervals in the current cycle.figure 4 two-pole two-phase induction motor.at any instant of time the flux shown in the diagrams is the resultant flux pro

39、duced by currents flowing in both coils. when the phase angle is 0, only coil a carries current, and the flux is directed upward along the axis of coil a. at 90, only coil b carries current, and the flux is directed to the right. at 180, the flux is directed downward because coil a carries current i

40、n the negative direction. the patterns in the top row show that the flux rotates one revolution for each cycle of the supply current. flux patterns obtained at intermediate values of phase angle, when current flows in both coils, are given in the second row of fig. 5. because the turns of the stator

41、 coils are distributed in the slots in such a way that the air gap flux density varies sinusoid ally with angle around the rotor, the flux remains constant in magnitude throughout its rotation, and the speed of rotation is constant. for example, at a phase angle of 45 coil a carries a current whose

42、magnitude is 0.707 of maximum producing a. flux in the upward direction which is 0.707 of maximum value. coil b also carries a current whose magnitude is 0.707 of maximum producing a flux to the right whose magnitude is 0.707 of maximum. the vector sum of these two flux components at right angles to each other falls at an angle of 45 and has a magnitude equal to the maximum magnitude of either individual components,figure 5 rotating field in two-phase induction motorfigure 6 relation of rotating flux to rotor currents and torque in a two-phase induction motorthe speed at which the flux