stepper motor construction and its functions
TRANSCRIPT
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STEPPER MOTORS
Name : Mrs.A.Sangari & Mrs.P.T.Subasini
Designation : Sr. Lecturer & Lecturer
Department : Electrical and Electronics Engineering
Subject code : 131307
Year : II ECE A & B
Unit : IV
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Stepper Motor Basics
S
N
Stator: made out of coils ofwire called winding
Rotor: magnet rotates onbearings inside the stator
Direct control of rotor position (no sensing needed)
May oscillate around a desired orientation (resonance at low speeds)
Low resolution
printers
computer drives
SN
Electromagnet
stator
rotor
N S
Current switch in winding
==>Magnetic force
==>hold the rotor in a position
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Increased Resolution
Half stepping
S
S
N
N
angle
torque
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Increased Resolution
More teeth on rotor or stator
Half stepping
S
S
N
N
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Increased Resolution
More teeth on rotor or stator
Half stepping
S
S
N
N
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How to Control?
Step TableStep Red Blue Yellow White
0 + - + -
1 - + + -
2 - + - +
3 + - - +
4 + - + -
4 lead
motor
Red
Blue
A+
A-
B+ B-
Yellow White
4 Lead Wire Configuration
Clockwise Facing Mounting End
Increase the frequency of the steps => continuous motion
Each step, like the second hand of a clock => tick, tick
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Motoring along...
direct control of position
precise positioning (The amount of
rotational movement per step depends
on the construction of the motor)
Easy to Control
under-damping leads to oscillation at low speeds
torque is lower at high speeds than the primary alternative
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DC motors -- exposed !
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Position Sensors
Optical Encoders
Relative position
Absolute position Other Sensors
Resolver
Potentiometer
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Optical Encoders
Relative position- direction- resolution
grating
light emitter
light sensor
decode
circuitry
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Ideal
Optical Encoders
Relative position mask/diffuser
gratinglight emitter
light sensor
decode
circuitry
Real
A diffuser tends to
smooth these signals
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Optical Encoders
Relative position- direction- resolution
gratinglight emitter
light sensor
decode
circuitry
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Optical Encoders
Relative position- direction- resolution
gratinglight emitter
light sensor
decode
circuitry
A
B
A
B
A lags B
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Optical Encoders
Relative position- direction- resolution
gratinglight emitter
light sensor
decode
circuitry
A
B A leads B
Phase lag between A and B is 90 degree
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Optical Encoders
Detecting absolute position
wires ?
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Other Sensors
Resolver
= driving a
stepper motor
Potentiometer
= varying
resistance
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Control
What you want to control = what you can control
For DC
motors:speed voltage
N
S
N SV
wV e back
emf
R
windingsresistance
e is a voltage generated by the rotorwindings cutting the magnetic field
emf: electromagnetic force
Control: getting motors to do what you want them to
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Controlling speed with voltage
DC motor model
V e
R
The back emf depends only on the motor speed.
The motors torque depends only on the current, I.
e = kewt = k
tI
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ktke
Controlling speed with voltage
DC motor model
V e
R
The back emf depends only on the motor speed.
The motors torque depends only on the current, I.
e = kewt = k
tI
Consider this circuits V: V = IR + eIstall = V/R
current when motor
is stalled
speed = 0
torque = max
How is V related to w ?
V = + kewt R
kt
- or -
w = - t +R
ke
V
Speed is proportional to voltage.
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speed vs. torque
torquet
speed w
ke
V
at a fixed voltage
R
ktV
max torque when stalled
no torque at max speed
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speed vs. torque
torquet
speed w
ke
V
at a fixed voltage
R
ktV
stall torque
no torque at max speed
Linear mechanical power Pm = F v
Rotational version of Pm = t w
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speed vs. torque
torquet
speed w
ke
V
at a fixed voltage
R
ktV
stall torque
max speed
Linear mechanical power Pm = F v
Rotational version of Pm = t w
power output
speed vs.
torque
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speed vs. torque
torquet
speed w
ke
V
R
ktV
power output
speed vs.
torque
gasoline enginemax speed
stall torque
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Back to control
Basic input / output relationship:
How to change the voltage?
We want a particular
motor speed w .
We can control thevoltage applied V.
V = + kewt R
kt
V is usually controlled via PWM -- pulse width modulation
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PWM
PWM -- pulse width modulation
Duty cycle:
The ratio of the On time and the Off time in one cycle
Determines the fractional amount of full power delivered to the
motor