SERVOMECHANISM AND THEIR AREAS OF APPLICATION

INTRODUCTION

Servomechanism, or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.

James Watt's steam engine governor is generally considered the first powered feedback system. The windmill fantail is an earlier example of automatic control, but since it does not have an amplifier or gain, it is not usually considered a servomechanism.

The first feedback position control device was the ship steering engine, used to position the rudder of large ships based on the position of ship's wheel. This technology was first used on the SS Great Eastern in 1866. Steam steering engines had the characteristics of a modern servomechanism: an input, an output, an error signal, and a means for amplifying the error signal used for negative feedback to drive the error towards zero.

Electrical servomechanisms require a power amplifier. World War II saw the development of electrical fire-control servomechanisms, using an amplidyne as the power amplifier. Vacuum tube amplifiers were used in the UNISERVO tape drive for the UNIVAC I computer.

Modern servomechanisms use solid state power amplifiers, usually built from MOSFET or thyristor devices. Small servos may use power transistors.

The origin of the word is believed to come from the French “Le Servomoteur” or the slavemotor, first used by J. J. L. Farcot in 1868 to describe hydraulic and steam engines for use in ship steering

BLOCK DIAGRAM - OF A SINLE-LOOP SERVOMECHANISM (SERVO-LOOP)

A servomechanism is unique from other control systems because it controls a parameter by commanding the time-based derivative of that parameter. For example a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.

Servomechanism may or may not use a servomotor. For example a household furnace controlled by thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.

A common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.

OPEN LOOP AND CLOSED LOOP SERVOMECHANISMS

Servomechanisms are classified on the basis of whether they depend upon information sampled at the output of the system for comparison with the input instructions. The simplest servomechanisms are called open-loop servomechanisms and do not feed back the results of their output. Open-loop servomechanisms do not verify that input instructions have been satisfied and they do not automatically correct errors.

An example of an open-loop servomechanism is a simple motor used to rotate a television-antenna. The motor used to rotate the antenna in an open-loop configuration is energized for a measured time in the expectation that antenna will be repositioned correctly. There is no automatic check to verify that the desired action has been accomplished. An open-loop servomechanism design is very unsatisfactory as a basis for an antenna rotator, just as it is usually not the best choice for other applications.

When error feedback is included in the design the result is called a closed-loop servomechanism. The servo's output result is sampled continuously and this information is continuously compared with the input instructions. Any important difference between the feedback and the input signal is interpreted as an error that must corrected automatically. Closed-loop servo systems automatically null, or cancel, disagreements between input instructions and output results.

The key to understanding a closed-loop servomechanism is to recognize that it is designed to minimize disagreements between the input instructions and the output results by forcing an action that reduces the error.

AREAS OF APPLICATION.

v Servomechanisms were first used in military fire-control and marine navigation equipment.

v Today servomechanisms are used in- automatic machine tools,

v satellite-tracking antennas,

v remote control airplanes,

v automatic navigation systems on boats and planes,

v antiaircraft-gun control systems.

v fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces

v radio-controlled models which use RC servos for the same purpose.

v Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus.

v A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.

v Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.

v Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)

v In industrial machines, servos are used to perform complex motion.