外文翻译伺服系统

南京大学毕业设计(论文)外文资料翻译系部： 机械系 专 业： 机械工程及自动化 姓 名： 学 号： 外文出处： Servo Systems 附 件： 1.外文资料翻译译文；2.外文原文。 指导教师评语：该同学翻译的外文资料原稿，紧扣本次毕业设计主题。中文翻译稿语言通顺，与原文表达的意思基本相符，基本符合毕业设计的要求，基本达到了预期的目的。 成绩评定为中。 签名： 年 月 日注：请将该封面与附件装订成册。附件1：外文资料翻译译文伺服系统由输入轴随意运动引起输出轴运动的一种形式是同步发送接收系统。另一种形式是伺服机构或伺服系统同步系统可在相当大的距离内在两个分开的轴之间工作，但不提供力矩放大传送到负载的力矩不能超过输入力矩。由于这个原因，并且在传送大力矩时，偏差角增大，同步系统只能用丁转动刻度盘和指针，移动控制活门和驱动其他小力矩负载。另一方面，伺服系统可提供要求移动大负载的大力矩，只需给输入轴加上很小的力矩。遥控运行不是伺服系统的固有特性，但可以通过数据传送装置实现，通常同步器作为系统的一部分。 一、伺服系统的基本要求 伺服系统是一种具有响应和执行指令的装置，伺服系统必须能够满足五项基本要求，它们是： a.伺服系统要能够接收规定期望结果的指令。 b伺服系统要能够估计存在条件。 c伺服系统要能够把存在条件与期望结果相比较得出差值或偏差信号。 d伺服系统要能够依据偏差信号，发出校正指令，正确地改变存在条件到期望结果。 e伺服系统要有执行校正指令的方法。 为使伺服系统满足五项基本要求，它必须具有一个偏差检测元件和一个操纵负载位移马达的控制器。 二、伺服系统部件 伺服系统包括偏差指示器和按照图1方式连接输入和输出轴的控制器。伺服系统的目标是驱功输出轴通过保持偏差角（输出轴离输入轴的角位移偏差）尽可能接近于零而重复输入轴运动。偏差指示器确定了偏差角的幅值和方向。在偏差指示器信号控制下，控制器在减小偏差的方向上给输出轴施加力矩。伺服系统是个闭环或称作反馈系统，因为加到控制器的信号引起输出轴转动，改变了偏差角，这样又改变了加到控制器的信号。图1偏差指示器和控制器可以采用很多种形式。控制器必须含有伺服马达或一些产生输出为矩的装量。伺服系统可按照所用伺服马达的类型划分为电动式、液压式、气动式或机械式。本文只讨论电动伺服系统。电动伺服系统使用多种电动马达。除了伺服马达外，控制器一般还包括功率放大器，能使来自偏差指示器的弱信号转换为较大功率供给马达。这种功率放大器通常称作伺服放大器。偏差指示器最常见的是同步装量。电动伺服系统中，同步发电机和控制变换器机械联接到输入和输出轴上，控制变换器即偏差指示器，其转子电压用作控制器的输入信号。图2大部分航空电子应用中的伺服系统是带控制变换器偏差指示器的电动伺服系统。这种伺服系统的框图如图2所示。若这个系统用直流马达，图中放大器必须具有将同步系统的交流电压整流，同时进行功率放大的功能，若使用交流马达，则需要交流放大器图2所示的系统中，输入轴与输出轴之间的偏差角，确定了由控制变换器产生并送到伺服放大器的偏差电压的相位和幅值。偏差信号依次控制由伺服马达加到输出轴上力矩的方向和幅值。电动伺服系统使用很多类型的伺服马达，而在航空电子应用中，两相感应电机是应用最广的。因而本文只讨论两相感应伺服马达。三、平衡电位计型偏差指示器伺服系统中作偏差指示器的另一种装置是平衡电位计（见图3）本系统中有两个电位计用在电桥中，一个电位计用于指令控制，另一个电位计机械藕合在伺服机械的输出轴上。两个装置的差值将产生偏差信号。依次引起伺服放大器或控制器转动输出轴，直到电桥平衡为止。平衡电位计产生的偏差信号与控制变换器产生的偏差电压完全是用于同样方式。图3CT（控制变换器）的指令一般来自离CT一段距离的同步发送器转轴的运动，而平衡电位计转轴指令则可加到一个电位计滑动触点的转子上。这种系统的实例正如ARN-21塔康收发机的频道选择器。平衡电位计的输入部分位于频道选择器的控制端，电位计的另一部分位于收发器组件。电位计的滑动触点机械联接到晶体状六角转塔上。四、两相感应电动机 经常用于驱动伺服机构输出轴的各类交流马达中，最重要的是两相感应电动机。这种电机在小容量伺服系统中有着广泛的应用，例如用于机载塔康的距离和方位指示器驱动系统和用于驱动雷达平面位量指示器的偏转线圈，还用于大多数航空电于设备上。为了了解采用感应电动机的伺服机构，首先要知道这些电机的特性。图4图4给出了表示双极、双相感应电动机的转于、剖面和电路图。定于和转子都是由薄钢片叠加构成的。定子有两个相同的线圈：A线圈和B线圈。如此排列可使两个线圈的磁场相互垂直转子可采用线绕式短路绕组或鼠笼式绕组。鼠笼绕组由转于槽内的导电条组成。这些导电条由转子两端的导电环短接。定子线圈通常供给幅值相等、相位相差90度的交流电，这种交流电可直接从两相电源系统得到，或从单相电源利用移相电路的方式获得如图4 (d)所示。正如图4 (e）的相量图所示，B线圈电流IB因线圈的感抗而滞后于外加电压E.A线圈电流IA因电容C的容抗大于A线圈的感抗而使电流超前于外加电压E。适当选择电容值的大小，可改变IA相对E的相位，使lA和IB的相位差接近90度。由于电流流过两个线圈，通过转子铁芯的总磁通的幅值是常量，并由定子电流的频率确定磁场沿电动机轴转动的速度。图5表明了旋转磁场是如何产生的。图中标明了在电流的一个周期内各段区间磁通的方向。图中顶行描述了一个电流周期内每隔90度时，定子合成磁场的状态。图5图中所示的任何时刻的瞬时磁通，是流过两个线圈电流产生的合成磁通。相位角为零度时仅A线圈有电流，磁通方向沿A线圈轴线内上。90度时，仅B线圈有电流，磁通方向向右。18度时，由于A线圈电流在负值方向上，所以磁通方向向下。顶行图示说明了供电电流每个周期内，磁通旋转一周的变化。图5中第二行给出了两个线圈都有电流时，相位角为中间值的磁通状态。由于定子线圈的每匝都分布在槽内，这种方式使气隙磁通密度随转子的角度呈正弦规律变化，磁通在整个旋转过程中保持恒定幅值。例如。在45度相位角，流过A线圈的电流幅值是最大值的0.707倍这个电流产生向上的磁通也是最大值的0.707倍。B线圈也流过幅值为最大值0.707倍的电流，产生最大值的0.707倍的向右的磁通。两个成直角的磁通线圈，其合成相量转到45度方向，幅值等于每个单个线圈的最大幅值。 旋转磁通的速度叫做同步转速，它由线圈电流的频率决定。对于工作在400周秒电源的双极电动机来说，其同步转速为2400转分。 两相感应电机中，若把任何一个定子线圈的接线端子互换，则旋转磁通的方向将反向。换句话说，每个定子电流移相18度时，将使电动机反转。当A线圈电流IA极性反向时，磁通的旋转方向也由顺时针变为反时针。 旋转磁通穿过转子导体。在转子导体中产生电势，因此有电流流过转子短路绕组，图6表示磁通旋转的某一瞬间，转子电流的方向可用圆点和叉号表示。（图中假定转子转速近似等于同步转速，且转子电流与转子感应电势同相）字母N和S代表定子旋转磁场的北极和南极。N和S代表转子电流产生的转子磁场北极和南级。因此转子成为一个磁体，试图朝定子磁场方向调节自己，由此产生转矩，方向是使转子沿定子旋转磁场方向转动。附件2：外文原文（复印件）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 cannot 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 torque 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 of 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 must 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 conditions 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 system 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 from 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, 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. The 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 electric 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 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 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 synchro 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 servo 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 servo 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 Typical 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 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 comes 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. The 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 of 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 on 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 induction 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 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-phase 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 coil 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 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 of 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 produced 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 in 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 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 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 rotates is called the synchronous speed and depends upon the frequency of the coil currents. For a two-pole motor operated from 40U cps power, the synchronous speed is 24.000 rpm.Trip direction of rotation of the flux in a two-phase induction motor is reversed if the terminals of either stator coil are interchanged. In other words, a 180 phase shift in either stator current will reverse the direction of the rotation of the motor. With the current Ia reversed in polarity it can be seen that the direction of rotation of the magnetic flux is counterclockwise rather than clockwise as before.Rotation of the magnetic flux past the conductors of the rotor induces voltages in these conductors, and currents therefore flow in the short-circuited rotor winding. The direction of rotor currents are indicated by dots and crosses in Fig. 6 for a particular instant during the rotation of the flux. (In the figure, rotor currents are assumed to be in phase with the rotor induced voltage. This assumption is correct only when the rotor speed nearly equals the synchronous speed. ) The letters N and S indicating the rotating north and south poles of the stator, and N and S represent the north and south poles of the rotor produced by the rotor currents. The rotor is thus a magnet that tends to align itself -with the stator field. A torque is produced in a direction to make the rotor follow the rotating magnetic field of the stator.