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?