stepper motors name: mr.r.anandaraj designation: associate. professor department: electrical and...
TRANSCRIPT
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STEPPER MOTORSName : Mr.R.AnandarajDesignation : Associate . Professor Department : Electrical and Electronics EngineeringSubject code :EC 6252
Year : II ECE A & BUnit : IV
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Stepper Motor Basics
S
N
Stator: made out of coils of wire called “winding”
Rotor: magnet rotates on bearings inside the stator
• Direct control of rotor position (no sensing needed)
• May oscillate around a desired orientation (resonance at low speeds)
• Low resolution
printerscomputer 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 leadm otor
Red
B lue
A+
A-
B+ B-
Yellow W hite
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
grating
light emitter
light sensor
decode circuitry
Real
A diffuser tends to smooth these signals
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Optical Encoders
• Relative position - direction
- resolution
grating
light emitter
light sensor
decode circuitry
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Optical Encoders
• Relative position - direction
- resolution
grating
light emitter
light sensor
decode circuitry
A
B
A
B
A lags B
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Optical Encoders
• Relative position - direction
- resolution
grating
light 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
V e back emf
R
windings’ resistance
e is a voltage generated by the rotor windings 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 motor’s torque depends only on the current, I.
e = ke
= k I
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kke
Controlling speed with voltage
DC motor model
V e
R
• The back emf depends only on the motor speed.
• The motor’s torque depends only on the current, I.
e = ke
= k I
• Consider this circuit’s V: V = IR + eIstall = V/Rcurrent when
motor is stalledspeed = 0
torque = max
How is V related to
V = + ke R k
- or -
= - + R ke V
Speed is proportional to voltage.
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speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV
max torque when stalled
no torque at max speed
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speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV stall torque
no torque at max speed
Linear mechanical power Pm = F v
Rotational version of Pm =
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speed vs. torque
torque
speed
ke V
at a fixed voltage
R kV stall torque
max speed
Linear mechanical power Pm = F v
Rotational version of Pm =
power output
speed vs. torque
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speed vs. torque
torque
speed
ke V
R kV
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 .
We can control the voltage applied V.
V = + ke R k
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