This chapter has provided information basic to understanding servo systems and their components. The following is a summary of specific points in the chapter.
The OPEN-LOOP CONTROL SYSTEM is controlled directly, and only by an input signal. It has no feedback and is therefore less accurate than the closed-loop control system. The open-loop system usually requires an operator to control the speed and direction of movement of the output.
The CLOSED-LOOP CONTROL SYSTEM is the most common type used in the Navy. It can respond and move loads quickly and with greater accuracy than the open-loop system. The closed-loop system has an automatic feedback system that informs the input that the desired movement has taken place.
The SERVO SYSTEM is classified as a closed- loop system when it is capable of:
Accepting an order and defining the desired result, Evaluating present conditions, Comparing the desired result with present conditions and obtaining a difference or an error signal, Issuing a correcting order, and changing the existing conditions to the desired result, and Obeying the correcting order.
The BASIC SERVO SYSTEM is normally made up of electromechanical parts, and consists of a synchro-control system, servo amplifier, servo motor, and some form of feedback.
The POSITION SERVO has the goal of controlling the position of the load. In the ac position servo system, the amplitude and phase of the ac error signal determine the amount and direction the load will be driven.
In the dc position servo system, the amplitude and polarity of the dc error signal are used to determine the amount and direction the load will be driven.
The VELOCITY SERVO is based on the same principle of error-signal generation as the position servo, except that the VELOCITY of the output is sensed rather than position of the load. When the velocity loop is at correspondence, an error signal is still present, and the load is moving at the desired velocity.
The ACCELERATION SERVO is similar to the velocity and position servos except that the acceleration of the load is being sensed rather than the position or velocity. In this loop, the tachometer of the velocity loop is replaced with an accelerometer.
TIME LAG is a servo characteristic defined as the time between the input of the signal and the actual movement of the load. Time lag is undesirable and is reduced through the use of high-gain amplifiers. Damping systems are then added to attain smooth, efficient operation.
An OVERDAMPED system will not be subject to oscillations but takes an excessive amount of time to reach synchronization. An UNDERDAMPED system provides instant response to an error signal but results in the load oscillating about the point of synchronism. Somewhere between overdamped and underdamped, designers achieve adequate accuracy, smoothness, and a moderately short synchronizing time.
DAMPING is used to stabilize a system-to minimize or eliminate the problem of overshoot. The simplest form of damping is FRICTION CLUTCH damping. MAGNETIC CLUTCH damping is similar to friction clutch damping. The difference is in how the flywheel is coupled to the shaft of the servo motor. Magnetic coupling uses a magnetic field to draw two friction plates together to produce damping. Another method uses the magnetic field set up by a pair of coils or one coil in conjunction with a conducting surface (flywheel) to produce damping.
ERROR-RATE DAMPING is defined as a method of damping that "anticipates" the amount of overshoot. This form of damping corrects the overshoot by introducing a voltage in the error detector that is proportional to the rate of change of the error signal. The stabilization network used for error-rate damping consists of either an RC differentiating network or an integrating network. The components of the RC network are chosen to tailor the stabilization network to the requirements of the servo system.
Frequency response of a servo is the range of frequencies to which the system is able to respond in moving the load. The ideal system can respond to whatever frequencies are present in the input signal. Frequency response is a good way of judging servo performance. In a given servo system, good Frequency response provides maximum stability and minimum time lag.
The BANDWIDTH of a servo amplifier, ideally, must be able to accept only the range of frequencies that represent valid servo signals.
Amplifier bandwidth is another compromise in achieving optimum servo operation.
A POTENTIOMETER is one of the simplest position sensor devices and is generally used because of its small size, high accuracy, and output, which can be either ac or dc. Its primary disadvantages are limited motion, limited life due to wear, and high torque required to rotate the wiper contact.
SUMMING NETWORKS can be used as error detectors in servo systems to add algebraically two or more inputs and a feedback error voltage.
The E-TRANSFORMER is a magnetic error detector that can be used in systems limited by large angular movements. Output signals are either in phase, 180° out of phase, or zero, depending on the direction of the E-transformer's armature motion. The amplitude of the signal is determined by the amount of armature motion. The basic E-transformer can only detect motion in one axis.
A CROSSED-E TRANSFORMER (or pickoff) is two E-transformers placed at right angles to each other. This type of error detector is capable of detecting error in both horizontal and vertical directions.
A CONTROL TRANSFORMER (CT), when used as a magnetic error detector, can rotate through unlimited angles. The output of this type of CT is always an ac servo error signal that must be demodulated if it is used with a dc servo motor.
A RATE GENERATOR (tachometer), when used in the velocity servo loop, is the heart of the feedback loop. The tach senses velocity (speed) of the load. A tachometer can be either an ac or dc rate generator. The output frequency of the ac tach is the same as the reference frequency, varying only in phase depending on the direction of rotation.
DEMODULATORS convert ac error signals to dc error signals. The dc signal is required to drive a dc servo amplifier.
A SERVO AMPLIFIER used in an ac or dc servo system must have a flat gain, minimum phase shift, low output impedance, and low noise level.
DC SERVO MOTORS can control heavy loads, and are widely used in servo systems. The speed and direction of the dc servo motor can be varied easily by varying the armature current.
The MULTI-LOOP SERVO SYSTEM combines several closed and/or open servo loops together to control a common load.