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
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