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SERVO CHARACTERISTICS
Friction Clutch Damping

Neets Module 15-Principles of Synchros, Servos, and Gyros
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2-12 (overshoot). When the load attempts to drive back to the desired position, the high gain of the amplifier may cause it to overshoot in the opposite direction. Therefore, the system must be stabilized to minimize or eliminate the problem of overshoot. This is done through DAMPING. Damping can be done by either introducing a voltage in opposition to the signal voltage or placing a physical restraint on the servo output. The actual function of this antihunting is to reduce the amplitude and duration of the oscillations that may exist in the system. Every system has one or more natural oscillating frequencies that depend on the weight of the load, designed speed, and other characteristics. The degree of damping is determined by the purpose and the use of the system. If the system is OVERDAMPED, it will not be bothered by oscillations. However, the large amount of restraint placed on the servo presents an additional problem. This is an excessive time requirement for the system to reach synchronization. Figure 2-8 is a graphic representation showing the time relationship with regard to degree of damping. Figure 2-8.—Degree of damping. An UNDERDAMPED servo system has other traits. The favorable one is its instantaneous response to an error signal. The unfavorable trait is an erratic operation around the point of synchronization because of the low amount of restraining force placed on the servo. Somewhere between overdamped and underdamped, there is a combination of desirable accuracy, smoothness, and moderately short synchronizing time. The simplest form of damping is FRICTION damping. Friction damping is the application of friction to the output shaft or load that is proportional to the output velocity. The amount of friction applied to the system is critical, and will materially affect the results of the system. Friction absorbs power from the motor and converts that power to heat. A pure friction damper would absorb an excessive amount of power from the system. However, two available systems have some of the characteristics of a friction damper, but with somewhat less power loss. These are the friction clutch and the magnetic clutch. Q-12.   If a position servo system tends to oscillate whenever a new position is selected, is the system overdamped or underdamped? Q-13.   If a position servo system does not respond to small changes of the input, is the system overdamped or underdamped?






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