servomechanism refers to a device or combination of devices that automatically

controls a mechanism or a source of power or energy. Servomechanisms

automatically compare the controlled output of a mechanism to the controlling

input. The difference between the settings or positions of the output and the input

is called the error signal, which regulates the output to a desired value.

Servomechanisms may be mechanical, electrical, hydraulic, or optical. The process

of sending the error signal back for comparison with the input is called feedback,

and the whole process of the input, output, error signal, and feedback is called a

closed loop. The closed-loop system, also known as a servomechanism, has some

means of incorporating mechanical feedback from the output to the input. A sensor

at the output end generates a signal that is sent back to the input to regulate the

machine behavior. The term servomechanism correctly applies only to systems

where feedback or error-correction signals help control mechanical position or

other parameters. For example, an automotive power window control is not a

servomechanism, because 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.

Purpose Of Servomechanism:

1. Accurate control of motion without the need for human attendants (automatic

control),

2. Maintenance of accuracy with mechanical load variations, changes in the

environment, power supply fluctuations, and aging and                      deterioration

of components (regulation and self- calibration),

3. Control of a high-power load from a low-power command signal (power

amplification) and,

4. Control of an output from a remotely located input, without the use of

mechanical linkages.

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. A simple example is the driver of a car. His eyes tell

him where he is on the road, and compare it to where he should be, and this

information makes its way to his brain. The brain decides what action should be

taken in order to move the car from where it is to where it should be, and sends a

signal to the muscles in the arm, turning the steering wheel to realign the car.

All servomechanisms have the following parts:

1. A way to measure what is desired and what is being accomplished,

2. A way to transport this information,

3. A way to determine the difference between the actual condition and the desired

condition,

4. A means to amplify this difference (which is often small) and use it to move the

actual condition towards the desired condition

In the example of the car, (1) are eyes, (2) is the optic nerve and pathways to the

brain,(3) is the brain, and (4) are the arms and steering wheel. A small turn of the

wheel translates into a major turn for the car.

Servo loop elements and their interconnections. Cause-and-effect action takes

place in the directions of arrows. (After American National Standards Institute,

Terminology for Automatic Control, ANSI C85.1)

The illustration shows the basic elements of a servomechanism and their

interconnections; in this type of block diagram the connection between elements is

such that only a unidirectional cause-and-effect action takes place in the direction

shown by the arrows. The arrows form a closed path or loop; hence this is a single-

loop servomechanism or, simply, a servo loop. More complex servomechanisms

may have two or more loops (multiloop servo), and a complete control system may

contain many servomechanisms.

All servomechanisms have at least these basic components: a controlled device, a

command device, an error detector, an error-signal amplifier, and a device to

perform any necessary error corrections (the servomotor). In the controlled device,

that which is being regulated is usually position. This device must, therefore, have

some means of generating a signal (such as a voltage), called the feedback signal,

that represents its current position. This signal is sent to an error-detecting device.

The command device receives information, usually from outside the system, that

represents the desired position of the controlled device. This information is

converted to a form usable by the system (such as a voltage) and is fed to the same

error detector as is the signal from the controlled device. The error detector

compares the feedback signal (representing actual position) with the command

signal (representing desired position). Any discrepancy results in an error signal

that represents the correction necessary to bring the controlled device to its desired

position. The error-correction signal is sent to an amplifier, and the amplified

voltage is used to drive the servomotor, which repositions the controlled device.

(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).

In many applications, servomechanisms allow high-powered devices to be

controlled by signals from devices of much lower power. The operation of the

high-powered device results from a signal (called the error, or difference, signal)

generated from a comparison of the desired position of the high-powered device

with its actual position. The ratio between the power of the control signal and that

of the device controlled can be on the order of billions to one.

History

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 make use of 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.

Applications

Servomechanisms are useful to control motion without human attendants, or to

maintain the accuracy of an environment like a power plant, and to control action

from a remote isolated station. The controller typically uses (and has) much less

power than that of what is being controlled. Almost always it is the position or

velocity which is being controlled.

Servomechanisms are used to control mechanical things such as motors, steering

mechanisms, and robots. Servomechanisms are used extensively in robotics. A

robot controller can tell a servomechanism to move in certain ways that depend on

the inputs from sensors. Multiple servomechanisms, when interconnected and

controlled by a sophisticated computer, can do complex tasks such as cook a meal.

A set of servomechanisms, including associated circuits and hardware, and

intended for a specific task, constitutes a servo system.Servo systems do precise,

often repetitive, mechanical chores. A computer can control a servo system made

up of many servomechanisms. For example, an unmanned robotic warplane (also

known as a drone) can be programmed to take off, fly a mission, return, and land.

Servo systems can be programmed to do assembly-line work and other tasks that

involve repetitive movement, precision, and endurance.

A servo robot is a robot whose movement is programmed into a computer. The

robot follows the instructions given by the program, and carries out precise

motions on that basis. Servo robots can be categorized according to the way they

move. In continuous-path motion, the robot mechanism can stop anywhere along

its path. In point-to-point motion, it can stop only at specific points in its path.

Servo robots can be easily programmed and reprogrammed. This might be done by

exchanging diskettes, by manual data entry, or by more exotic methods such as a

teach box. When a robot arm must perform repetitive, precise, complex motions,

the movements can be entered into the robot controller’s memory. Then,when the

memory is accessed, the robot arm goes through all the appropriate movements. A

teach box is a device that detects and memorizes motions or processes for later

recall.

The constant speed control system of a DC motor is a servomechanism that

monitors any variations in the motor’s speed so that it can quickly and

automatically return the speed to its correct value. Servomechanisms are also used

for the control systems of guided missiles, aircraft, and manufacturing.

The power steering system in an automobile is an example of a servomechanism.

The direction of the front wheels is controlled by the angle of the steering wheel.

Should the motion of the car turn the front wheels away from the desired direction,

the servomechanism, consisting of a mechanical and hydraulic system,

automatically brings the wheels back to the desired direction. Another example of

a servomechanism is the automatic control system by which a THERMOSTAT,

(q.v.) in one of the rooms of a house controls the heat output of the heating

furnace. Other examples include automatic pilots used on ships, aircraft, and space

vehicles, in which the direction of motion of the vehicle is controlled by a compass

setting. Unmanned spacecraft are automatically turned to point their cameras, radio

antennae, and solar panels in the desired directions by servomechanisms. The input

in that case is the sensing of the direction of the sun and stars, and the output is the

control of small jets that turn and orient the spacecraft.

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.

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

equipment. They were also used in military applications, such as an antiaircraft

gun that tracks a plane via radar. As the plane moves the radar gives the plane’s

position information to the gun, which computes the new position of the plane and

realigns. This process can go indefinitely. Some other applications are satellite

tracking and satellite antenna alignment systems, automatic machine tools, star-

tracking systems on telescopes (since the stars’ position changes as the earth

rotates), and navigation systems.

RC servos are hobbyist remote control devices servos typically employed in radio-

controlled models, where they are used to provide actuation for various mechanical

systems such as the steering of a car, the flaps on a plane, or the rudder of a

boat.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. RC servos are

composed of an electric motor mechanically linked to a potentiometer. Pulse-width

modulation (PWM) signals sent to the servo are translated into position commands

by electronics inside the servo. When the servo is commanded to rotate, the motor

is powered until the potentiometer reaches the value corresponding to the

commanded position. Due to their affordability, reliability, and simplicity of

control by microprocessors, RC servos are often used in small-scale

robotics applications.The servo is controlled by three wires: ground (usually

black/orange), power (red) and control (brown/other colour). This wiring sequence

is not true for all servos, for example the S03NXF Std. Servo is wired as

brown(negative), red (positive) and orange (signal). The servo will move based on

the pulses sent over the control wire, which set the angle of the actuator arm. The

servo expects a pulse every 20 ms in order to gain correct information about the

angle. The width of the servo pulse dictates the range of the servo’s angular

motion. A servo pulse of 1.5 ms width will set the servo to its “neutral” position, or

90°. For example a servo pulse of 1.25 ms could set the servo to 0° and a pulse of

1.75 ms could set the servo to 180°. The physical limits and timings of the servo

hardware varies between brands and models, but a general servo’s angular motion

will travel somewhere in the range of 180° – 210° and the neutral position is

almost always at 1.5 ms.RC Servos are usually powered from either NiCd or

NiMH packs common to most RC devices. More recently these systems are

powered by Lithium Polymer (LiPo) packs. Voltage ratings vary from product to

product, but most servos are operated at 4.8 V or 6 V DC from a 4 or 5 cell NiCd

or NiMH battery, or a regulated LiPo pack.

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.

Today servomechanisms are used in automatic machine tools, satellite-tracking

antennas, remote control airplanes, automatic navigation systems on boats and

planes, and antiaircraft-gun control systems. Other examples are fly-by-wire

systems in aircraft which use servos to actuate the aircraft’s control surfaces, and

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

autofocus cameras also use a servomechanism to accurately move the lens, and

thus adjust the focus. A modern hard disk drive has a magnetic servo system with

sub-micrometre positioning accuracy.

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