A Servo Mechanism

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					                    A servomechanism

     A servomechanism, or servo, is an automatic device that
uses error-sensing negative 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, speed or other parameters. For example, an automotive
power window control is not a servomechanism, as there is no
automatic feedback that 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.
      A servomechanism may or may not use a servomotor. For
example, a household furnace controlled by a 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.
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. This procedure is one widely used
application of control theory.
       Speed control via a governor is another type of
servomechanism. The steam engine uses mechanical governors;
another early application was to govern the speed of water wheels.
Prior to World War II theconstant speed propeller was developed to
control engine speed for maneuvering aircraft. Fuel controls forgas
turbine engines employ either hydromechanical or electronic
governing.
      Positioning servomechanisms were first used in military fire-
control and marine navigation equipment. 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, andradio-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.
     Typical servos give a rotary (angular) output. Linear types are
common as well, using a leadscrew or alinear motor to give linear
motion.
      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.)
     In industrial machines, servos are used to perform complex
motion.
                      Servo Motors
      Servo motors are used in closed loop control systems in
which work is the control variable, Figure 9. The digital servo
motor controller directs operation of the servo motor by
sending velocity command signals to the amplifier, which
drives the servo motor. An integral feedback device
(resolver) or devices (encoder and tachometer) are either
incorporated within the servo motor or are remotely
mounted, often on the load itself. These provide the servo
motor's position and velocity feedback that the controller
compares to its programmed motion profile and uses to alter
its velocity signal. Servo motors feature a motion profile,
which is a set of instructions programmed into the controller
that defines the servo motor operation in terms of time,
position, and velocity. The ability of the servo motor to
adjust to differences between the motion profile and
feedback signals depends greatly upon the type of controls
and servo motors used. See the servo motors Control and
Sensors Product section.

     Three basic types of servo motors are used in modern
servosystems: ac servo motors, based on induction motor
designs; dc servo motors, based on dc motor designs; and
ac brushless servo motors, based on synchronous motor
designs.
        Figure 9 - Typical dc servo motor
system with either encoder or resolver
feedback. Some older servo motor systems
use a tachometer and encoder for
feedback.
       A servomotor (servo) is an electromechanical device in which an
electrical input determines the position of the armature of a motor. Servos
are used extensively in robotics and radio-controlled cars, airplanes, and
boats.

       You will be using an Airtronics 94102 Precision Heavy-Duty
Standard Servo. The position of the armature (in the Figure) is determined
by the duty cycle of a periodic rectangular pulse train. The duty cycle of a
rectangular pulse train is expressed in %: It is the ratio of the pulse duration
to the pulse period times 100% (See Figure for examples.)




Figure: Illustration of Servomotor Identifying the Armature




Figure: Examples of Duty Cycle Calculation

       The receiver sub-circuit of the lab project outputs a DC voltage.
Therefore it is necessary to convert the DC voltage produced by the receiver
into a rectangular pulse train whose duty cycle is determined by the DC
level. In this lab exercise you will investigate how the servo controller
circuit accomplishes this conversion from a DC voltage level to a
rectangular pulse train with a specific duty cycle.
  Components of the Servomotor Controller
                  Circuit
The servo controller circuit consists of several op-amps and Transistor-
Transistor Logic (TTL) components. Its purpose is to convert a DC voltage
into a rectangular pulse train whose duty cycle is determined by the DC
level. The servo responds to variations in the duty cycle of a 50 Hz
rectangular pulse train. The controller circuit is designed to produce a 50 Hz
rectangular pulse train with a duty cycle determined by the DC voltage level
at its input. The path of the signal through the circuit is shown in the block
diagram of Figure . A summary of each component is provided in the
following section.
            Control of standard servo motor
      Servos are controlled by sending them a pulse of variable
width. The control wire is used to send this pulse. The parameters for
this pulse are that it has a minimum pulse, a maximum pulse, and a
repetition rate. Given the rotation constraints of the servo, neutral is
defined to be the position where the servo has exactly the same
amount of potential rotation in the clockwise direction as it does in the
counter clockwise direction. It is important to note that different
servos will have different constraints on their rotation but they all have
a neutral position, and that position is always around 1.5 milliseconds
(ms).




      The angle is determined by the duration of a pulse that is
applied to the control wire. This is called Pulse width Modulation. The
servo expects to see a pulse every 20 ms. The length of the pulse will
determine how far the motor turns. For example, a 1.5 ms pulse will
make the motor turn to the 90 degree position (neutral position).
       When these servos are commanded to move they will move to
the position and hold that position. If an external force pushes against
the servo while the servo is holding a position, the servo will resist
from moving out of that position. The maximum amount of force the
servo can exert is the torque rating of the servo. Servos will not hold
their position forever though; the position pulse must be repeated to
instruct the servo to stay in position.
       When a pulse is sent to a servo that is less than 1.5 ms the
servo rotates to a position and holds its output shaft some number of
degrees counterclockwise from the neutral point. When the pulse is
wider than 1.5 ms the opposite occurs. The minimal width and the
maximum width of pulse that will command the servo to turn to a valid
position are functions of each servo. Different brands, and even
different servos of the same brand, will have different maximum and
minimums. Generally the minimum pulse will be about 1 ms wide and
the maximum pulse will be 2 ms wide.
From NEWNES ELECTRICAL POWER ENGINIEERS HANDBOOK. Page 291
               10.6 The brushless servomotor
       A synchronous machine with permanent magnets on the rotor is the
heart of the modern brushless servomotor drive. The motor stays in
synchronism with the frequency of supply, though there is a limit to the
maximum torque which can be developed before the rotor is forced out of
synchronism, pull-out torque being typically between 1.5 and 4 times the
continuously rated torque. The torque–speed curve is therefore simply a
vertical line.
       The industrial application of brushless servomotors has grown
significantly for the following reasons:
● reduction of price of power conversion products
● establishment of advanced control of PWM inverters
● development of new, more powerful and easier to use permanent magnet
materials
● the developing need for highly accurate position control
● the manufacture of all these components in a very compact form
       They are, in principle, easy to control because the torque is generated
in proportion to the current. In addition, they have high efficiency, and high
dynamic responses can be achieved.
       Brushless servomotors are often called brushless dc servomotors
because their struc-ture is different from that of dc servomotors. They rectify
current by means of transistor switching within the associated drive or
amplifier, instead of a commutator as used in dc servomotors. Confusingly,
they are also called ac servomotors because brushless servo-motors of the
synchronous type (with a permanent magnet rotor) detect the position of the
rotational magnetic field to control the three-phase current of the armature. It
is now widely recognized that brushless ac refers to a motor with a
sinusoidal stator winding distribution which is designed for use on a
sinusoidal or PWM inverter supply voltage. Brushless dc refers to a motor
with a trapezoidal stator winding distribution which is designed for use on a
square wave or block commutation inverter supply voltage.
       The brushless servomotor lacks the commutator of the dc motor, and
has a device (the drive, sometimes referred to as the amplifier) for making
the current flow accord-ing to the rotor position. In the dc motor, increasing
the number of commutator seg-ments reduces torque variation. In the
brushless motor, torque variation is reduced by making the coil three-phase
and, in the steady state, by controlling the current of each
phase into a sine wave.

				
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