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【文章內(nèi)容簡(jiǎn)介】 in a direction to reduce the error. The servo is a closedloop, 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 al tered 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 twophase induction motor is most monly 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 CATOR Another type device used as an error detector in servo systems is the Balanced Potentiometer (see Fig. 3). In this Figure 3 Typical Balanced potentiometer System. System, two potentiometers are used in a bridge circuit, where one potentiometer is a mand 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 t
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