dc motor tutorial
DESCRIPTION
dc motorTRANSCRIPT
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Motor TutorialKevin M. Lynch Laboratory for Intelligent Mechanical SystemsNorthwestern UniversityEvanston, 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
SolenoidWorkhorse, high powerSimple to use, two wiresTorque proportional to current, steady state constant-load speed proportional to voltageRequires gearingRequires feedback
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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
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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)
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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
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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
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Inside a DC Motor
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DC MotorsV = IR + L (dI/dt) + kewt = kt I
VvoltageI currentR resistanceLinductancewspeedttorquekeelectrical constantkttorque constantwtspeed-torque curvesfor two voltages
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DC Motor Specs
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Driving a DC MotorSwitches and relaysTransistorsLinear push-pull stage with op ampIdeally: H-bridge and PWM
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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
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L293 H-bridge chip
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Practical Issuesflyback diodes for inductive kickheat sinks for transistorscapacitors to smooth voltage spikesother noise issues, isolation
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GearsGear ratio G wout= win / G tout = h G tin (h = efficiency)
Many typesspur, planetary, worm, lead/ball screw, bevel, harmonic...
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Encoder FeedbackAnother option:potentiometerUS Digital
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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
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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
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Animation of UnipolarTaken from http://www.cs.uiowa.edu/~jones/step/
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Driving a StepperUse logic on/off signals at 2, 7, 10, 15.
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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.
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SolenoidPlunger attracted or repelled by current through a coil.
May be driven by a relay or transistor.
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Questions?