SERVOS

LEARNING OBJECTIVES

Upon completion of this chapter you will be able to:

• Define the term "servo system" and the terms associated with servo systems, including open-loop and closed-loop control systems.
• Identify from schematics and block diagrams the various servo circuits; give short explanations of the components and their characteristics; and of each circuit and its characteristics.
• Trace the flow of data through the components of typical servo systems from input(s) to outputs(s) (cause to effect).

SERVOS

Servo mechanisms, also called SERVO SYSTEMS or SERVOS for short, have countless applications in the operation of electrical and electronic equipment. In working with radar and antennas, directors, computing devices, ship's communications, aircraft control, and many other equipments, it is often necessary to operate a mechanical load that is remote from its source of control. To obtain smooth, continuous, and accurate operation, these loads are normally best controlled by synchros. As you already know, the big problem here is that synchros are not powerful enough to do any great amount of work. This is where servos come into use.

A servo system uses a weak control signal to move large loads to a desired position with great accuracy. The key words in this definition are move and great accuracy. Servos may be found in such varied applications as moving the rudder and elevators of a model airplane in radio-controlled flight, to controlling the diving planes and rudders of nuclear submarines.

Servos are powerful. They can move heavy loads and be remotely controlled with great precision by synchro devices.

They take many forms. Servo systems are either electromechanical, electrohydraulic, hydraulic, or pneumatic.

Whatever the form, a relatively weak signal that represents a desired movement of the load is generated, controlled, amplified, and fed to a servo motor that does the work of moving the heavy load.

Q.1 What is a servo?

CATEGORIES OF CONTROL SYSTEMS

A control system is a group of components that are linked together to perform a specific purpose. Generally, a control system has a large power gain between input and output. The components used in the system and the complexity of the system are directly related to the requirements of the system's application.

Control systems are broadly classified as either CLOSED-LOOP or OPEN-LOOP.

Closed-loop control systems are the type most commonly used in the Navy because they respond and move the loads they are controlling quicker and with greater accuracy than open-loop systems.

The reason for quicker response and greater accuracy is that an automatic feedback system informs the input that the desired movement has taken place. Upon receipt of this feedback information, the system stops the motor, and motion of the load ceases until another movement is ordered by the input. This is similar to the system that controls heat in many homes. The thermostat (input) calls for heat. The furnace (output) produces heat and distributes it. Some of the heat is "fed back" to the thermostat. When this "feedback" raises the temperature of the room to that of the thermostat setting, the thermostat responds by shutting the system down until heat is again required. In such a system, the feedback path, input to output and back to input, forms what is called a "closed loop." This is a term you will hear and use often in discussions of control systems. Because closed-loop control systems are automatic in nature, they are further classified by the function they serve (e.g., controlling the position, the velocity, or the acceleration of the load being driven).

An open-loop control system is controlled directly, and only, by an input signal, without the benefit of feedback. The basic units of this system are an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job. Open-loop control systems are not as commonly used as closed-loop control systems because they are less accurate.

OPEN-LOOP CONTROL SYSTEM

As we stated previously, an open-loop control system is controlled directly, and only, by an input signal. The basic units of this type consist only of an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.

The open-loop control system is shown in basic block diagram form in figure 2-1. With this system, the input is a signal that is fed to the amplifier. The output of the amplifier is proportional to the amplitude of the input signal. The phase (ac system) and polarity (dc system) of the input signal determines the direction that the motor shaft will turn. After amplification, the input signal is fed to the motor, which moves the output shaft (load) in the direction that corresponds with the input signal. The motor will not stop driving the output shaft until the input signal is reduced to zero or removed. This system usually requires an operator who controls speed and direction of movement of the output by varying the input. The operator could be controlling the input by either a mechanical or an electrical linkage.

Figure 2-1. - Open-loop control system basic block diagram.

THE CLOSED-LOOP CONTROL SYSTEM

A closed-loop control system is another name for a servo system. To be classified as a servo, a control system must be capable of the following:

• Accepting an order that defines the desired result
• Determining the present conditions by some method of feedback
• Comparing the desired result with the present conditions and obtaining a difference or an error signal
• Issuing a correcting order (the error signal) that will properly change the existing conditions to the desired result
• Obeying the correcting order

We have discussed the open- and closed-loop control systems and defined a servo system as a closed-loop control system. Although not technically accurate by definition, open-loop control systems are also often referred to in the Navy and many publications as servo systems even though they lack one of the five basic requirements, that of feedback.

Q.2 In an open-loop control system, what action reduces the input to zero so the load is stopped at the desired position?
Q.3 What basic requirement of a closed-loop system (not present in open-loops) enables present load position to be sensed?

OPERATION OF A BASIC SERVO SYSTEM

For the following discussion of a servo system, refer to figure 2-2, view (A), view (B), view (C) and view (D).This closed-loop servo system is the most common type in the Navy today. It is normally made up of electromechanical parts and consists basically of a synchro-control system, servo amplifier, servo motor, and some form of feedback (response).

Figure 2-2A. - A basic servo system (closed-loop).

Figure 2-2B. - A basic servo system (closed-loop).

Figure 2-2C. - A basic servo system (closed-loop).

Figure 2-2D. - A basic servo system (closed-loop).

The synchro-control system provides a means of controlling the movement of the load, which may be located in a remote space. The servo amplifier and servo motor are the parts of the system in which power is actually developed (to move the load).

As you remember, the controlling signal from a CT is relatively weak, too weak to drive an electric motor directly. In views A through D of figure 2-2, assume that the control signal will be initiated by a handcrank input connected to the synchro transmitter (CX). The dials located on the CX and the CT indicate the positions of the synchro's rotors, while the dial on the load indicates the position of the load.

In view A, the dials of both the CX and the load indicate that the load is in the desired position. Because the load is where it should be, there will be no error signal present at the servo amplifier and no power to the servo motor.

In view B, the rotor of the CX has been moved by the handcrank to 90. (This indicates that it is ordered to move the load by 90.) Notice that the rotor of the CT is still at 0. The CT now develops a signal, called the ERROR SIGNAL, which is proportional in amplitude to the amount the CT rotor is out of correspondence with the CX rotor. The phase of the error signal indicates the direction the CT rotor must move to reduce the error signal to zero or to "null out." The error signal is sent to the servo amplifier. In view C, the error signal has been amplified by the servo amplifier and sent on to the servo motor. The motor starts to drive in the direction that will reduce the error signal and bring the CT rotor back to the point of correspondence. In this case the motor is turning clockwise.

The mechanical linkage attached to the servo motor also moves the rotor of the CT. This feedback causes the amplitude of the error signal to decrease, slowing the speed at which the load is moving.

In view D, the servo motor has driven both the load and the rotor of the CT, so that the CT is now in correspondence with the CX rotor. As a result, the error signal is reduced to zero (nulled). The load stops at its new position. Note that in this servo system, we moved a heavy load to a predetermined position through the simple turning of a handcrank. In responding to the handcrank, the servo system performed a basic positioning function .

Two key points for you to remember, thus far, about the operation of the closed-loop servo system are:

The original error (movement of the CX rotor) was "detected" by the CT. For this reason the CT is called an ERROR DETECTOR. The servo motor, in addition to moving the load, also provides mechanical feedback to the CT to reduce the error signal. For this reason the servo motor is also called an ERROR REDUCER.

Q.4 An error signal is the difference between what two quantities fed to the CT (error detector)?
Q.5 What are the two functions of the servo motor in the system shown in figure 2-2?