motor tutorial kevin m. lynch laboratory for intelligent mechanical systems northwestern university...
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Motor Tutorial
Kevin M. Lynch
Laboratory for Intelligent Mechanical Systems
Northwestern University
Evanston, IL USA
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Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
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Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
Workhorse, high power
Simple to use, two wires
Torque proportional to current, steady state constant-load speed proportional to voltage
Requires gearing
Requires feedback
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Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
Useful for low-torque applications with no surprises
No feedback required
One step per pulse
More involved driving circuit
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Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
High torque, useful for positioning applications
Feedback and gearing built in
Position commanded by persistent pulse train
Limited motion (less than 1 revolution)
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Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
For on-off applications
Simple to use
Short stroke
Powered in only one direction; requires external spring for return
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DC Motors
Lorentz Force Law: F = I x B
F = force on wireI = currentB = magnetic field
Right hand rule:index finger along I, middle finger along B, thumb along F
N S
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Inside a DC Motor
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DC Motors
V = IR + L (dI/dt) + ke
= kt I
V voltage
I current
R resistance
L inductance
speed
torque
ke electrical constant
kt torque constant
2 - permanent magnet3 - housing (magnetic return)4 - shaft5 - winding
speed-torque curvesfor two voltages
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DC Motor Specs
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Driving a DC Motor
Switches and relays Transistors Linear push-pull stage with op amp Ideally: H-bridge and PWM
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H-bridge and PWM
PWM: Rapidly switch between S1-S4 closed and S2-S3 closed
Averages to effective voltage across motor between -V and +V depending on time spent in S1-S4 and S2-S3 states
Switch control signals are simply digital signals
Use an H-bridge chip or build out of transistors
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L293 H-bridge chip
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Practical Issues
flyback diodes for “inductive kick” heat sinks for transistors capacitors to smooth voltage spikes other noise issues, isolation
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Gears
Gear ratio G
out = in / G
out = G in ( = efficiency)
Many types
spur, planetary, worm, lead/ball screw, bevel, harmonic...
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Encoder Feedback
Another option:potentiometer
US Digital
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Feedback Control
Proportional (Integral-Derivative) ControlMultiply position/velocity error by a gain to get control
signal (and perhaps add integral and derivative of that error multiplied by other gains)
Usually implemented on computer
Can be implemented with op amps
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Stepper Motor
Bipolar: 4 wires
Unipolar: 5 or 6 wires+V alternately ground one
end of coil or other
1 42 3A B
A BR RR R L LL LL L R R
RL
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Animation of Unipolar
Taken from http://www.cs.uiowa.edu/~jones/step/
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Driving a Stepper
Use logic on/off signals at 2, 7, 10, 15.
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RC Servo Motor
3 wires: power, ground, control
Control signal sets the position.
High pulse every ~20 ms determines set angle; pulse width between ~0.5 ms and ~2 ms, indicating the two ends of angle range
Internal gearing, potentiometer, and feedback control.
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Solenoid
Plunger attracted or repelled by current through a coil.
May be driven by a relay or transistor.
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Questions?