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Motor Tutorial Kevin M. Lynch Laboratory for Intelligent Mechanical Systems Northwestern University...
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Transcript of Motor Tutorial Kevin M. Lynch Laboratory for Intelligent Mechanical Systems Northwestern University...
Motor Tutorial
Kevin M. Lynch
Laboratory for Intelligent Mechanical Systems
Northwestern University
Evanston, IL USA
Types of Motors
• DC (brushed)
• Stepper
• RC Servo
• Solenoid
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
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
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)
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
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
Inside a DC Motor
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
DC Motor Specs
Driving a DC Motor
Switches and relays Transistors Linear push-pull stage with op amp Ideally: H-bridge and PWM
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
L293 H-bridge chip
Practical Issues
flyback diodes for “inductive kick” heat sinks for transistors capacitors to smooth voltage spikes other noise issues, isolation
Gears
Gear ratio G
out = in / G
out = G in ( = efficiency)
Many types
spur, planetary, worm, lead/ball screw, bevel, harmonic...
Encoder Feedback
Another option:potentiometer
US Digital
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
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
Animation of Unipolar
Taken from http://www.cs.uiowa.edu/~jones/step/
Driving a Stepper
Use logic on/off signals at 2, 7, 10, 15.
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.
Solenoid
Plunger attracted or repelled by current through a coil.
May be driven by a relay or transistor.
Questions?