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STEPPER MOTORS

Name : Mrs.A.Sangari & Mrs.P.T.Subasini

Designation : Sr. Lecturer & Lecturer

Department : Electrical and Electronics Engineering

Subject code : 131307

Year : II ECE A & B

Unit : IV

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Stepper Motor Basics

S

N

Stator: made out of coils ofwire called winding

Rotor: magnet rotates onbearings inside the stator

Direct control of rotor position (no sensing needed)

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

Low resolution

printers

computer drives

SN

Electromagnet

stator

rotor

N S

Current switch in winding

==>Magnetic force

==>hold the rotor in a position

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Increased Resolution

Half stepping

S

S

N

N

angle

torque

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Increased Resolution

More teeth on rotor or stator

Half stepping

S

S

N

N

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Increased Resolution

More teeth on rotor or stator

Half stepping

S

S

N

N

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How to Control?

Step TableStep Red Blue Yellow White

0 + - + -

1 - + + -

2 - + - +

3 + - - +

4 + - + -

4 lead

motor

Red

Blue

A+

A-

B+ B-

Yellow White

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

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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

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DC motors -- exposed !

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Position Sensors

Optical Encoders

Relative position

Absolute position Other Sensors

Resolver

Potentiometer

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Optical Encoders

Relative position- direction- resolution

grating

light emitter

light sensor

decode

circuitry

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Ideal

Optical Encoders

Relative position mask/diffuser

gratinglight emitter

light sensor

decode

circuitry

Real

A diffuser tends to

smooth these signals

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Optical Encoders

Relative position- direction- resolution

gratinglight emitter

light sensor

decode

circuitry

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Optical Encoders

Relative position- direction- resolution

gratinglight emitter

light sensor

decode

circuitry

A

B

A

B

A lags B

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Optical Encoders

Relative position- direction- resolution

gratinglight emitter

light sensor

decode

circuitry

A

B A leads B

Phase lag between A and B is 90 degree

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Optical Encoders

Detecting absolute position

wires ?

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Other Sensors

Resolver

= driving a

stepper motor

Potentiometer

= varying

resistance

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Control

What you want to control = what you can control

For DC

motors:speed voltage

N

S

N SV

wV e back

emf

R

windingsresistance

e is a voltage generated by the rotorwindings cutting the magnetic field

emf: electromagnetic force

Control: getting motors to do what you want them to

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Controlling speed with voltage

DC motor model

V e

R

The back emf depends only on the motor speed.

The motors torque depends only on the current, I.

e = kewt = k

tI

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ktke

Controlling speed with voltage

DC motor model

V e

R

The back emf depends only on the motor speed.

The motors torque depends only on the current, I.

e = kewt = k

tI

Consider this circuits V: V = IR + eIstall = V/R

current when motor

is stalled

speed = 0

torque = max

How is V related to w ?

V = + kewt R

kt

- or -

w = - t +R

ke

V

Speed is proportional to voltage.

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speed vs. torque

torquet

speed w

ke

V

at a fixed voltage

R

ktV

max torque when stalled

no torque at max speed

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speed vs. torque

torquet

speed w

ke

V

at a fixed voltage

R

ktV

stall torque

no torque at max speed

Linear mechanical power Pm = F v

Rotational version of Pm = t w

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speed vs. torque

torquet

speed w

ke

V

at a fixed voltage

R

ktV

stall torque

max speed

Linear mechanical power Pm = F v

Rotational version of Pm = t w

power output

speed vs.

torque

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speed vs. torque

torquet

speed w

ke

V

R

ktV

power output

speed vs.

torque

gasoline enginemax speed

stall torque

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Back to control

Basic input / output relationship:

How to change the voltage?

We want a particular

motor speed w .

We can control thevoltage applied V.

V = + kewt R

kt

V is usually controlled via PWM -- pulse width modulation

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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