STEPPER MOTORSName : Mr.R.AnandarajDesignation : Associate . Professor Department : Electrical and Electronics EngineeringSubject code :EC 6252

Year : II ECE A & BUnit : IV

Stepper Motor Basics

S

N

Stator: made out of coils of wire called “winding”

Rotor: magnet rotates on bearings inside the stator

• Direct control of rotor position (no sensing needed)

• May oscillate around a desired orientation (resonance at low speeds)

• Low resolution

printerscomputer drives

SN

Electromagnet

stator

rotor

N S

Current switch in winding ==>Magnetic force

==>hold the rotor in a position

Increased Resolution

Half stepping

S

S

N

N

angle

torque

Increased Resolution

More teeth on rotor or stator

Half stepping

S

S

N

N

Increased Resolution

More teeth on rotor or stator

Half stepping

S

S

N

N

How to Control?

Step TableStep Red Blue Yellow White

0 + - + -1 - + + -2 - + - +3 + - - +4 + - + -

4 leadm otor

Red

B lue

A+

A-

B+ B-

Yellow W hite

4 Lead Wire Configuration

Clockwise Facing Mounting End

Increase the frequency of the steps => continuous motion

Each step, like the second hand of a clock => tick, tick

Motoring along...

• direct control of position

• precise positioning (The amount of rotational movement per step depends on the construction of the motor)

• Easy to Control

• under-damping leads to oscillation at low speeds

• torque is lower at high speeds than the primary alternative…

DC motors -- exposed !

Position Sensors

Optical Encoders Relative position Absolute position

Other Sensors Resolver Potentiometer

Optical Encoders

• Relative position - direction

- resolution

grating

light emitter

light sensor

decode circuitry

Ideal

Optical Encoders

• Relative position mask/diffuser

grating

light emitter

light sensor

decode circuitry

Real

A diffuser tends to smooth these signals

Optical Encoders

• Relative position - direction

- resolution

grating

light emitter

light sensor

decode circuitry

Optical Encoders

• Relative position - direction

- resolution

grating

light emitter

light sensor

decode circuitry

A

B

A

B

A lags B

Optical Encoders

• Relative position - direction

- resolution

grating

light emitter

light sensor

decode circuitry

A

B A leads B

Phase lag between A and B is 90 degree

Optical Encoders

• Detecting absolute position

wires ?

Other Sensors

• Resolver

= driving a stepper motor

• Potentiometer

= varying resistance

Control

What you want to control = what you can control

For DC motors:

speed voltage

N

S

N SV

V e back emf

R

windings’ resistance

e is a voltage generated by the rotor windings cutting the magnetic field

emf: electromagnetic force

Control: getting motors to do what you want them to

Controlling speed with voltage

DC motor model

V e

R

• The back emf depends only on the motor speed.

• The motor’s torque depends only on the current, I.

e = ke

= k I

kke

Controlling speed with voltage

DC motor model

V e

R

• The back emf depends only on the motor speed.

• The motor’s torque depends only on the current, I.

e = ke

= k I

• Consider this circuit’s V: V = IR + eIstall = V/Rcurrent when

motor is stalledspeed = 0

torque = max

How is V related to

V = + ke R k

- or -

= - + R ke V

Speed is proportional to voltage.

speed vs. torque

torque

speed

ke V

at a fixed voltage

R kV

max torque when stalled

no torque at max speed

speed vs. torque

torque

speed

ke V

at a fixed voltage

R kV stall torque

no torque at max speed

Linear mechanical power Pm = F v

Rotational version of Pm =

speed vs. torque

torque

speed

ke V

at a fixed voltage

R kV stall torque

max speed

Linear mechanical power Pm = F v

Rotational version of Pm =

power output

speed vs. torque

speed vs. torque

torque

speed

ke V

R kV

power output

speed vs. torque

gasoline enginemax speed

stall torque

Back to control

Basic input / output relationship:

How to change the voltage?

We want a particular motor speed .

We can control the voltage applied V.

V = + ke R k

V is usually controlled via PWM -- “pulse width modulation”

PWM PWM -- “pulse width modulation

Duty cycle: The ratio of the “On time” and the “Off time” in one cycle Determines the fractional amount of full power delivered to

the motor