Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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Servos and the Arduino-BOT
Board of Education
Shield
Stop
Continuous-rotation
servo
Typical signal used
to turn the servo
1.5 ms
1.5 ms
20 ms
References:
1) Arduino-BOT Lectures #1-5 - http://faculty.tcc.edu/PGordy/Egr120/
2) Robotics with the Board of Education Shield for Arduino web tutorials http://learn.parallax.com/ShieldRobot
3) Board of Education Shield for Arduino documentation http://www.parallax.com/Portals/0/Downloads/docs/prod/robo/35000-BOEShield-v1.2.pdf
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4) Arduino web site (software, microcontrollers, examples, and more) - http://www.arduino.cc/
Arduino-BOT Lecture #2 EGR 120 – Introduction to Engineering
Servo - A servo is a single device that contains:
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• Motor
• Gearbox that gears down the motor to provide slower speeds (than most
motors) and higher torque (power to turn).
• Built-in electronics such that the motor position or speed can be controlled by
a series of pulses. The servo is powered using 5V (4.8 – 6.0 V)
Arduino-BOT Lecture #2 EGR 120 – Introduction to Engineering
Connecting Servos to the BOE Shield for Arduino
We could use digital outputs to control the servos, but servo ports have been
conveniently provided for plugging in the servos.
Servo ports
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Arduino-BOT Lecture #2
Servo port jumper
EGR 120 – Introduction to Engineering
The servos can be powered either by 5V or by Vin, which is the unregulated
battery voltage (perhaps 7.5V for 5 AA batteries). Our servos are rated to use
4.8 – 6.0V, so the jumper should be set to 5V as indicated below.
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Arduino-BOT Lecture #2
Plugging in the servos
EGR 120 – Introduction to Engineering
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The servos should be plugged in as shown below. If they are already plugged, in check
them to be sure that they are correct.
1) Be sure that the black wire is in the appropriate position.
2) Be sure that:
• The LEFT servo is plugged into Port 13
• The RIGHT servo is plugged into Port 12 (the ball is at the rear ).
To LEFT Servo
To RIGHT Servo
Arduino-BOT Lecture #2
Types of servos
EGR 120 – Introduction to Engineering
Pass around servos of each type in class
There are two types of servos:
• Unmodified servos – the servo can turn over a certain range, such as 180º
– Used commonly on RC (radio control) cars and airplanes for steering, moving
control flaps, etc.
– Available in hobby stores, such as Hobbytown USA or Debbie’s RC World
– The servo position is controlled by varying the pulse width of a control signal
(see below)
0 degrees
90 degrees
180 degrees
0º
180º
90º
• Modified servos
– Servo has been modified internally so that it will turn continuously.
– Commonly used as a drive motor for a robot, such as the Arduino-BOT.
– The servo speed is controlled by varying the pulse width of a control signal
(see below)
Full speed CW
Stop
Full speed CCW
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CW
Stop
CCW
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
Servo Signals
Servos are typically operated using
pulse-width modulation, where they
receive control signals to make them
turn.
The pulse width varies from about
1.3ms to about 1.7ms (or 1300 us to
1700 us) and the pulses are separated
by 20ms. These values are set by the
manufacturer.
Varying the pulse as described can be
used to make a continuous servo turn
at any speed between full speed CW
to full speed CCW (typically about 50
rpm max).
The pulses must continue to keep the
servo moving. If the pulses stop, the
servo stops.
1.3 ms
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1.3 ms
Full speed CW
20 ms
1.5 ms
1.5 ms
Stop
20 ms
1.7 ms
1.7 ms
Full speed CCW
20 ms
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
Controlling servos with the Arduino
Sending a series of pulses to a servo is
similar to turning ON and OFF and
LED. Recall our earlier example
below where an LED on digital pin 13
turns ON for 0.5 s and OFF for 0.5 s.
The LED will blink indefinitely.
0.5 s
LED
ON
0.5 s
LED
OFF
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Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
Controlling servos with the Arduino
The code on the previous page can be
adjusted to make a servo on servo port
13 move full speed CW.
1.3 ms
1.3 ms
Full speed CW
20 ms
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Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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A better way to control servos
The Arduino programming language includes a better way to control servos. It
involves using the Arduino servo library.
The library is actually a class. Using a class allows you to declare objects and
use member functions that work with those objects. Programming with classes
is called object-oriented programming. You will learn more about objectoriented programming using C++ in EGR 125.
Useful Arduino servo library instructions:
• #include <servo.h>
• Servo servoLeft, servoRight;
• servoLeft.attach(13);
• servoLeft.writeMicroseconds(1300)
• servoLeft.detach();
// include the servo library (class)
// declare two new objects in class Servo. Any
// name is OK, but servoLeft is descriptive
// associate the left servo with digital
// pin 13 (Servo Port 13)
// send a pulse train continuously to
// servo port 13 with a high time of
// 1.3ms & a low time of 20 ms (default)
// disassociate the left servo from current port
// this will stop the servo
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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A better way to control servos
So, the previous example used to turn a servo full speed CW on port 13 can be
re-written as:
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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Starting and stopping a servo
The previous example was used to turn a servo continuously.
Suppose that we wanted it to only turn for 10 seconds?
One way to accomplish this is to turn it on, wait for 10 s, and then turn it off.
Notes:
1. The argument for the delay( ) function is an unsigned long int which can have a value from 0
to 4,294,967,295 so the max delay is 4,294,967,295 ms = 4,294,967.295 s  8.2 years
2. We will use the delay( ) function for delays of several seconds in our programs, but this is
typically discouraged as most Arduino functions will be halted during the delay (such as
reading sensors).
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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Servo speed data
In Team Assignment #2 we will determine exactly how fast each servo on the
Arduino-BOT will turn for each pulse width. This will allow us to:
1) Select the proper pulse width for different speeds
2) Match the two servos so that they turn at the same rate
Recall that in order to make the servo turn at any speed between full-speed CW
and full-speed CCW, the pulse width should vary from
1.3ms to 1.7ms or from 1300 us to 1700 us.
So the corresponding commands are:
servoRight.writeMicroseconds(1300); // full-speed CW
servoRight.writeMicroseconds(1700); // full-speed CCW
Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
Example: Left wheel servo test
Each servo is a little different from the next, so in
Team Assignment #2, each group will test their
two servos by varying the pulse width gradually in
order to determine the servos’ speed and direction
for each value of pulse width. A table somewhat
like the one on the right will be determined for
each servo. Teams will count the number of wheel
revolutions for 15 s and multiply by 4 to get
revolutions per minute (rpm).
Vary the pulse
width from 1300 to
1700 and count the
wheel revolutions
Pulse Width (us)
Servo speed (CW rpm)
1300
40
1320
40
1340
39
1360
37
1380
34
1400
30
1420
25
1440
19
1460
12
1480
3
1500
0
1520
-3
1540
-11
1560
-18
1580
-27
1600
-31
1620
-35
1640
-39
1660
-42
1680
-42
1700
-43
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Arduino-BOT Lecture #2
EGR 120 – Introduction to Engineering
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Team Assignment #2
In Team Assignment #3 you will record the number of RPM’s for each pulse
width for each servo. You will also graph the information (similar to the graphs
shown below). Your data will be used in later labs to determine which pulse
widths are needed to make your Arduino-BOT move around a track.