Post on 13-Nov-2014
description
SERVOMOTORS
Submitted to:
Mr. Abdul Salam SaitAssociate Professor
•A Servo is a small device that has an output shaft.
• This shaft can be positioned to specific angular positions
by sending the servo a coded signal.
• As long as the coded signal exists on the input line, the
servo will maintain the angular position of the shaft.
• As the coded signal changes, the angular position of the
shaft changes.
Definition
• Servo motors are used for angular positioning, such as
in radio control airplanes. They typically have a
movement range of 60 degree but can go up to 90
deg.
• The output shaft of a servo does not rotate freely, but
rather is made to seek a particular angular position
under electronic control.
• They are typically rated by torque and speed. A servo
rated 40 ounce-in/.21 means that at 1 inch from the
hub, the servo can exert 40 ounces of force and move
60 degree in 0.21 sec.
What makes a Servo?
Servo motors and are constructed out of basic DC
motors, by adding:
• some gear reduction
• a position sensor for the motor shaft
• an electronic circuit that controls the motor's
operation
• The basic hobby servo has a 180:1 gear ratio.
The motor is typically small.
• Typically, a potentiometer (variable resistor)
measures the position of the output shaft at all
times so the controller can accurately place
and maintain it’s setting.
Inside a Servo
Types of servo
1) D.C. Servomotor:-
• Shunt wound d.c. motor
• Series wound d.c. motor
• Compound wound d.c. motor
2) A.C. Servomotors:-
Induction motor a.c. motorSynchronous a.c. motor
DC Servo Motors
• DC servo motors are controlled by DC command signals applied directly to coils
• The magnetic fields that are formed interact with permanent magnets and cause the rotating member to turn
• One type of PM uses a wound armature and brushes like a conventional DC motor, but uses magnets as pole pieces
• Another type uses wound field coils and a permanent magnet rotor
Wound Armature PM Motor
• Armature contains wound coils• Current is supplied by brushes• Pole pieces are made of permanent magnets• Typically 2 or 4-pole structure• Similar characteristics to a DC shunt motor
Moving Coil Motor
• The stator field is provided with 8 pairs of permanent magnets
• Arranged to provide alternating magnetic fields
• The armature is made of thin disc of fiberglass laminated with copper conductors
Moving Coil Operation
Brushless DC Motor
• Contains a permanent magnet rotor and fixed stator winding
• Stator windings are typically three-phase
• Contains a converter that converts DC into pulsating DC
• DC pulses are applied to stator windings to produce a rotating field
• Also contains a sensor to detect the position to switch the current stator coils
Stepper Motors
• Conventional servo motors are classified as continuous rotation motors
• Stepper motors rotate through a specific number of degrees, or steps, then stop
• Each incoming pulse results in the shaft turning a specific angular distance
• Stepper motors can control velocity, distance, and direction of mechanical load
Permanent Magnet Stepper Motor
• PM stepper motors have rotor teeth made of permanent magnets
• Reaction of the rotor teeth to stator fields provides torque for the motor
• Signals are applied to the stator to determine direction and step rate of the rotor
Variable ReluctanceStepper Motor
• The variable reluctance stepper motor uses electromagnetic stator poles
• The soft iron core is un-magnetized
• The rotor is toothed, alternating N-S– The more teeth, the greater
the resolution
Stepper Motor Terminology
• Stepping Rate - maximum number of steps the motor can make in one second
• Step Angle - number of degrees per arc the motor moves per step– Step angle is determined by the number of rotor
teeth and stator poles used
Stepper Motor Speed
• Stepper motor speed depends upon the step angle and stepping rate
n = Y x S
6
n = Speed in RPM
Y = Step angles in degrees
S = Steps per second
6 = Formula constant
Microstepping
• Stepper motors tend to jerk at low speeds
• Stepper motors have limited resolution
• Microstepping overcomes these problems– Uses simulated sine waves that increment or
decrement in small steps called microsteps
20
MicrostepperOperation
AC Servo Motors
• Controlled by AC command signals applied to the coils
• AC Brushless Servo Motor– Operates on the same principle as single-phase
induction motor
AC Brushless Servo Motor
• Two windings– Main winding
– Auxiliary winding
• Electronic drive provides the necessary phase shift for motor operation
AC Servomotors
•It is a two phase a.c. induction motor. There are two
winding, one fixed or reference winding is supplied
with a fixed voltage and frequency from a constant
voltage source. Second winding is called control
winding, with variable supply voltage of same
frequency.
•The stator has two distributed windings displaced 90
electrical degree apart.
•One winding is the reference phase and is connected
to a constant voltage source.
•The other winding is the control phase and is
supplied with a variable voltage of the same
frequency as the reference phase but is phase-
displaced by 90 electrical degree.
•For balanced two-phase voltages (Va=Vb), the motor torque-
speed characteristic is similar to that of a three-phase
induction motor.
• For low rotor resistance this characteristic is non-linear.
• Such characteristic is unacceptable in control systems,
however if the rotor resistance is high the characteristic will
be linear
•over a wide range of speed, as shown in the figure.
Control
• An external controller (such as the Basic Stamp) tells
the servo where to go with a signal know as pulse
proportional modulation (PPM) or pulse code
modulation
• PPM uses 1 to 2ms out of a 20ms time period to
encode its information.
• The servo expects to see a pulse every 20 milliseconds (.02
seconds).
• The length of the pulse will determine how far the motor
turns.
•A 1.5 millisecond pulse will make the motor turn to the
90 degree position (often called the neutral position).
Modified Servos
• Servo motors can also be retrofitted to provide continuous rotation:– remove mechanical limit (revert back to DC
motor shaft)– remove pot position sensor (no need to tell
position)– apply 2 resistors to make the servo to “think” it
is fully turning.
•The idea is to make the servo think that the output shaft is always at the 90 degree mark.
• This is done by removing the feedback sensor, and replacing it with
an equivalent circuit that creates the same readings as the sensor
being at 90 degrees.
Then, giving it the signal for 0 degrees will cause the motor to
turn on full speed in one direction. The signal for 180 degrees
will cause the motor to go the other direction.
Since the feedback from the output shaft is disconnected, the
servo will continue in the appropriate direction as long as the
signal remains.
IN GARMENT INDUSTRY
Industrial Sewing machines are equipped with one of the two motors
generally- The clutch motor or the SERVO MOTOR
A sewing machine is much easier to control with the help of a servo
motor, due to speed control.
This is especially used by the beginners, who don’t require high speed
stitching.
ADVANTAGES OF SERVO MOTOR
• The two major advantages of using a SERVO
MOTOR in sewing machines and other machines in
the sewing operation are that, one can always control
the speed of the motor and that they are completely
silent when the pedal is not engaged.
• The other advantages include:
• They consume up to 90% less energy than clutch motors.
• 1/3 times lighter than clutch motors• Reverse motor rotation with the flick of a switch• Adjusted speed remains the same no matter
how hard the pedal is pressed.
Uses• Position control
• Speed control