EET 2261 Unit 14 INCOMPLETE

Floyd, Digital Fundamentals, 10
th

ed

EET 2261
Unit
14
INCOMPLETE

Servos and Stepper Motors



Read
Mazidi
,
Chapter
15.



Homework #14 and Lab #14 due next
week.



Quiz next week.

•
There are many kinds of electric
motors. (See
Wikipedia article
.)

•
In many motors, the rotor spins
continuously
,
with no way of precising controlling the
motor’s
output
(rotational position
and speed).

•
Servos

and
stepper motors

are two widely
used kinds of motors whose output
can

be
precisely controlled.

•
We’ll work with servos in Lab #5 and stepper
motors in Lab #6.

Servos and Stepper Motors

•
A
servo motor

(or
servo
) is a package
containing a DC motor connected via gears to
a shaft.

•
It also contains a feedback circuit that
precisely controls the shaft’s angle of rotation.

•
Unlike a motor whose rotor spins continuously,
a servo is usually used to set the shaft to a
specific angle of rotation and then hold it there
for a while.

Servos

•
Servos are commonly used in small robotics
and in
radio
-
controlled (RC) airplanes
, cars,
and boats.

•
In an RC airplane, for example,
servos may
control the
plane’s throttle, rudder, elevators
,
ailerons, landing gear, etc.

•
Next slide shows inner workings
of an RC
airplane.

Servos in RC Vehicles

Servos and Stepper Motors

Servo: Exploded View

Servo: Block Diagram

•
Diagram above demonstrates
feedback
, in which a
system’s output is fed back in as an input to provide
more accurate control over the output.


•
Feedback is a fundamental concept in most control
systems.

•
Servos have three wires:

•
Power (red)

•
Ground (black)

•
Control signal (yellow or white)

•
The LAB
-
X1 board has two connectors for
servos, labeled Servo1 and Servo2.

•
Look at the Port Order list to see which of the
PIC

chip’s ports these are connected to.

Wiring a Servo

•
The servo’s control signal is a 50 Hz pulse
train. Therefore, what is this signal’s period?

Controlling a Servo

Period = ?

•
The control signal’s
pulse width

determines
the shaft’s angle of rotation.

•
Typically this pulse width ranges from about
1.0 ms to about 2.0 ms, interpreted as follows:

•
1.5 ms pulse width: 0


rotation.

•
Pulse
width less than 1.5 ms: rotate
counterclockwise (
up to 45


or 90

,
depending on the servo).

•
Pulse
width greater than 1.5 ms : rotate
clockwise.

Controlling a Servo

•
PULSOUT

•
SELECT CASE

New PICBASIC
Commands in Lab
#5

•
Generates a pulse of specified width on an
output pin.

•
Example:
PULSOUT PORTD.0, 100
generates a 1
-
ms wide pulse on bit 0 of Port D.

•
Note that the pulse width = the number given
in the command times 10

s.

•
In Example, pulse width = 100


10

s

PULSOUT
(p
. 121
in PICBASIC manual)

•
Similar to IF...THEN, because it executes
different code depending on the value of a
variable.

•
Example
:

SELECT CASE a




CASE 1





PORTD = 70




CASE 2





PORTD = 85




CASE ELSE





PORTD = 0



END SELECT

SELECT CASE
(
p
. 131
in PICBASIC
manual)

•
Up to now we’ve been using the LAB
-
X1
board, which has a PIC16F877A chip
connected to peripheral devices (keypad, LCD
screen,…).

•
For Lab #6 we’ll use the RCG1 board, which
also has a PIC16F877A chip connected to
peripheral devices (six
-
digit 7
-
segment display,
temperature sensor,…).

RCG1 Board

•
Schematic diagram (
page 1

and
page 2
)

•
List of pins (organized by
port number

and by
pin number
)

Important Documents: RCG1
Board

•
A
stepper motor

is a digitally controlled motor
that allows precise control over the position of
the motor’s rotor.

•
Unlike a servo, there’s no limit to how far a
stepper motor can rotate.

•
Changes in the digital input rotate the motor’s
rotor by a precise amount, which is called a
step
.

•
Depending on the motor, this step may be as
small as 1


(or less) or as large as 45

.

•
On our motors, the step is 3.6

.

Stepper Motors

•
Recall that an
electromagnet

behaves like a
magnet only when current is passing through it.
It’s a magnet that you can “turn on” or “turn off.”

Electromagnets

•
Electromagnets are
constructed by wrapping wire
around an iron
-
alloy bar.
(Illustration from
Wikipedia
.)

•
The bar is called the
core
,
and the wrapped wire is called
the
winding
.

•
Stepper motors have permanent magnets
connected to the rotor and electromagnets
connected to the stator.




•
The digital signals controlling the motor turn
the electromagnets on and off in sequence,
which results in rotation of the rotor.

Magnets in a Stepper Motor

Simplified image from

John
Iovine’s

PIC


Microcontroller

Project Book
, 2
nd

ed.

Rotating a Stepper Motor

From John
Iovine’s

PIC


Microcontroller

Project Book
, 2
nd

ed.

In this example,
how big is each
step?

•
In the previous illustration, only one of the
electromagnets was on at a time.

•
We can get finer resolution by sometimes
turning on two electromagnets at a time. This
technique is called
half
-
stepping
.

Half
-
Stepping

•
Different designs result in several types of
stepper motors:

•
Unipolar

•
Bipolar

•
Universal (which combine characteristics of
unipolar

and bipolar motors)

•
Our motor is a
unipolar

stepper motor.

Types of Stepper Motors

•
The number of wires on a stepper motor
varies, depending on the motor’s type.

•
Bipolar Stepper Motor: 4 wires

•
Unipolar

Stepper Motor: 5 or 6 wires

•
Universal Stepper Motor: 8 wires

Wiring a Stepper Motor

•
The RCG1 board has a header designed for
use with a
unipolar

stepper motor.

•
The color labels on the board do not match the
wire colors on our stepper motor, but the Lab 6
handout tells how to resolve this problem.

Stepper Motors and the RCG1
Board