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dc motor
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Motor TutorialKevin M. Lynch Laboratory for Intelligent Mechanical SystemsNorthwestern UniversityEvanston, IL USA
Types of Motors DC (brushed)
Stepper
RC Servo
Solenoid
Types of Motors DC (brushed)
Stepper
RC Servo
SolenoidWorkhorse, high powerSimple to use, two wiresTorque proportional to current, steady state constant-load speed proportional to voltageRequires gearingRequires feedback
Types of Motors DC (brushed)
Stepper
RC Servo
Solenoid
Useful for low-torque applications with no surprisesNo feedback requiredOne step per pulseMore involved driving circuit
Types of Motors DC (brushed)
Stepper
RC Servo
Solenoid
High torque, useful for positioning applications Feedback and gearing built inPosition commanded by persistent pulse trainLimited motion (less than 1 revolution)
Types of Motors DC (brushed)
Stepper
RC Servo
Solenoid
For on-off applicationsSimple to useShort strokePowered in only one direction; requires external spring for return
DC MotorsLorentz 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
Inside a DC Motor
DC MotorsV = IR + L (dI/dt) + kewt = kt I
VvoltageI currentR resistanceLinductancewspeedttorquekeelectrical constantkttorque constantwtspeed-torque curvesfor two voltages
DC Motor Specs
Driving a DC MotorSwitches and relaysTransistorsLinear push-pull stage with op ampIdeally: H-bridge and PWM
H-bridge and PWMPWM: 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 Issuesflyback diodes for inductive kickheat sinks for transistorscapacitors to smooth voltage spikesother noise issues, isolation
GearsGear ratio G wout= win / G tout = h G tin (h = efficiency)
Many typesspur, planetary, worm, lead/ball screw, bevel, harmonic...
Encoder FeedbackAnother option:potentiometerUS Digital
Feedback ControlProportional (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 MotorBipolar: 4 wires
Unipolar: 5 or 6 wires+Valternately ground oneend of coil or other1423ABA BR RR R L LL LL L R RRL
Animation of UnipolarTaken from http://www.cs.uiowa.edu/~jones/step/
Driving a StepperUse logic on/off signals at 2, 7, 10, 15.
RC Servo Motor3 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.
SolenoidPlunger attracted or repelled by current through a coil.
May be driven by a relay or transistor.
Questions?