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?