Transcript Radio Frequency - Robotics Academy

CURRENT DRAW Overview
In this lesson you will learn:
• Current flow
• The causes of excessive current
flow
• How current is affected by work
• How to measure DC motor current
with a multimeter
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Powering a vehicle is a basic concern of any robot designer. In this
lesson you will investigate how the amount of power needed for a vehicle
varies according to how much work it has to perform. Since it is not
convenient to measure power directly, you will measure current, which
varies directly with power. In order to do this, you will modify the battery
powering your robot so that you can put it in series with the multimeter.
Then you will observe and record the amount of current the robot needs
as it performs increasing amounts of work. You will increase the amount
of work required for the robot by increasing the angle the robot climbs by
regular increments. After plotting your data on a graph, you will analyze
the graph and demonstrate the relation between work and current, and,
by extension, between work and power.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Materials needed:
• Digital multimeter
• Vex Squarebot
• Radio control transmitter
• Board or plywood (8 ft. by 12 in.)
• White foam board (8 ft. by 4 ft.) or
other board suitable for drawing.
• Protractor
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
What is current?
• Current is the movement of
electrons through a circuit.
I
I
I
I
I
• In the Ohm’s Law formula, I=E/R,
the letter I is used to represent
current. E represents volts
(Electromotive Force) and R is
resistance.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Why is it important to understand
the concept of electrical current
flow?
• In any circuit, the amount of
current flow will determine how
much work is being performed.
• Proper control of current will allow
efficient operation of DC motors
and servo devices.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
In robotic applications, the
following items will determine how
much current is flowing through
the DC motor circuits:
• Wheel size
• Gear ratios
• Weight (how heavy is the moving
robot?)
• Mechanics (Friction)
• Terrain (Are there hills or
obstructions in the running
surface?
• Battery voltage and condition
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Keep in mind:
• In general, a small motor does
less work than a larger one
• Motor size does not necessarily
determine a required battery
voltage
• Current and voltage ratings are
determining factors in how much
work can be performed by a
particular DC motor
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Gear Ratios:
• There is an inverse relationship
between gear ratio and current
draw, if all other factors are held
constant.
• The higher the gear ratio, the less
current will be required for the
same driving gear speed.
• The lower the gear ratio, the
more current will be required for
the same driving gear speed.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Wheel size:
• There is a direct relationship
between current draw and wheel
size, if all other factors are held
constant.
• The smaller the wheel diameter,
the less current will be required
for the same driving gear speed.
• The greater the wheel diameter,
the more current will be required
for the same driving gear speed.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Weight:
• There is a direct
relationship between
weight and current
draw, if all other factors
are held constant.
• Increasing the weight
the motors move will
increase the current
draw, if the driving axle
speed remains the
same.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Gears, alignment and leverage:
• Improper alignment of axles,
shafts and gears will have a
braking effect, which will increase
the motor current, compared to
proper alignment.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Battery voltage:
• If a 6 volt battery is used on a 12
volt motor, it will run more slowly
(or may not run at all) and will not
produce the proper power.
• If a 12 volt battery is used on a 6
volt motor, the motor will draw
excessive current and be
destroyed.
• Weak or dead batteries will not
provide adequate voltages for
correct motor operation.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Introduction
Terrain:
• Irregular surfaces and steep
grades will also affect current
draw, if all other factors are held
constant
• In this lesson, we will investigate
what happens to current draw
when we require a robot to
navigate progressively steeper
grades.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Set up
Build the incline indicator. You
will need:
• A protractor
Starting
point
• A straight edge
• A flat 8’ x 12” board
• Any 8’ x 4” white board that
can easily be drawn on. A
whiteboard or white foam
board will work well.
• Set angles in 5° increments
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Set up
Use the
protractor and
Starting point
the straight edge
to draw lines on
the white board:
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Set up
Watch the set up video. It covers:
• Wiring the robot battery in series
with the multimeter
• Modifying Squarebot so you can
mount the multimeter on it
securely.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Lesson
Watch the lesson video. It covers:
• The experimental procedure. You
will find the current that the robot
uses when traveling on different
inclines. You will then use that
data to draw conclusions about
current and work.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Sample Data
Angle of
Incline
Voltage
0
0.63
5
0.75
10
0.90
15
1.07
20
1.14
25
1.24
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
CURRENT DRAW Extension Activity
Compare dragging weight to
rolling weight and see if you can
find a proportional relationship:
• How do current readings from
dragging weight compare to
current readings from rolling
weight?
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.