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