Background Information
Robotic devices all have one major feature in common. Whether they are robotic weather
surveillance satellites, orbiting astronomical observatories, or robotic arms - they all
require energy.
Efficient energy use and energy management systems are considered essential elements
of good robotic design.
Driven by the extremely high cost of importing energy into space, the development of
energy related technology has taken great strides forward.
Fortunately many of the components and subsystems developed for space-based robotic
technologies find their way into applications and products that we use everyday on Earth.
For example, the solar voltaic systems which power satellites and space stations, are
often used on Earth to convert solar energy into electricity in remote locations such as the
Australian Outback and the Canadian Arctic. Rechargeable metal-hydride voltaic cells
(hence batteries) with high energy-to-mass ratios are used in space for communications
satellites as well as on Earth to power cell phones and lap-top computers. Hydrogen
powered fuel cells similar to those used to provide electrical energy in the Space Shuttle
are being tested here on Earth as a power source for electric cars.
Almost all robotic devices used in space are programmed to operate at a minimum power
level (standby mode) until "awakened" to perform other tasks.
The Canadarm2, which can move masses larger than a school bus (even if filled with
students), uses no more power than that used by a few dozen light bulbs! The Canadarm's
are extremely energy efficient.
This efficient use of energy has found its way into most desk-top and lap-top computers
which are programmed to automatically enter onto "hibernation states" when userintervention has ceased for an extended period of time. The computer will "waken" when
keyboard (or mouse) activity alerts it to the fact that the user requires it to return to work.
As most house cats know, the ability to "snooze" is a great way to save energy. This
concept, known as software energy management is well known to "smart" robotic
devices.
Energy Transformations
Another way to save energy is to "store" it as potential energy for use at a later time.
Certain types of robotic devices have the ability to transform energy from one state to
another with very little energy being lost in the process.
Consider a robotic spacecraft which must rotate in order to look in another direction.
One way in which robotic spacecraft can change their orientation (i.e. the direction they
are facing) in space is to use the kinetic energy stored in (relatively) massive flywheels.
In this maneuver energy is transferred1 between the flywheel and the spacecraft. This
energy transfer can be made back and forth between the spacecraft and the flywheel with
surprisingly little loss of energy, allowing the spacecraft to "look" back and forth.
A more direct application of energy transformation applies to (prototype) electrical
automobiles. The energy usually wasted as heat by conventional braking systems, is
instead transferred to electrical generators that convert the vehicle's kinetic energy into
electricity which in turn is stored in the car's batteries. This energy can then be used to
accelerate the vehicle, turning the stored energy back into kinetic energy.
1
For simplicity engineers would generally consider the exchange of angular momentum in this
case, rather than energy.
An Autonomous Feed-back Device
In order to get a better understanding of energy and energy transformations a simple
robot-like toy is used as the basis for energy investigations.
Since this little device always returns to its starting position without user intervention it
"appears" to exhibit autonomous behaviour which gives it some "robot-like"
characteristics.
Since the device's motion is always in response to the user's input, (that is, it reacts to the
user's input) it provides mechanical "feed-back" through the process of energy
transformation.
Details for its construction are given in the Activity below.
Teaching Strategies
Prior to engaging students in a discussion about this device it is instructive to build one
for yourself. Experiment with it until you have it working well enough to operate it in a
predictable manner2.
1. Roll it. Allow students to speculate about what is happening.
2.
3.
4.
5.
Stop it at its furthest point. Hold it there . Release it.
Open it. Have student's re-assess their ideas.
Let them build one.
Have a contest. Furthest, fastest, longest duration.
2
For fun, work up an interesting scenario about the device to explain to your students (before you
begin).e.g. it is a highly sophisticated computer controlled IQ seeker. Always rolls towards the
person with the highest IQ ... hopefully it will roll back towards you after you have pushed it away.
The Amazing Robotic "Energy-Transformer"
Materials
1. one empty cylindrical tin (tea or peanut) with removable lid (approximately
500mL in size)
2. 4 - 6 elastic bands
3. 2 - 4 large metal nuts
4. 2 paperclips
5. tape
An extremely simple device to
illustrate the concept of energy
transformations is The Amazing
Robotic "EnergyTransformer".
It appears to be quite "friendly".
Whenever you try to roll it away
from you, it always "wants" to
come back.
Today's students are so
accustomed to high-tech toys that
they can be easily convinced that
this is a very smart, computer
controlled, high-tech robotic toy,
especially if you disguise it with
a few coats of high gloss enamel
paint and few Canadian Space
Agency stickers.
Of course the truth is that this is a
totally mechanical device. It's
seemingly mysterious behaviour
is a result of the energy
transformations which are hidden
inside the tin.
The kinetic energy imparted to the
tin to set it in motion is gradually
transformed into gravitational
potential energy and elastic
potential energy.
At its furthest position the
winding loop will have lifted the
suspended mass upwards against
the force of gravity.
When the tin stops rolling (at its
furthest position), the system
begins to convert the stored
potential energy back into kinetic
energy.
Of course no energy conversion
process can be 100% efficient.
Energy is lost and the tin
eventually stops moving.
The simplicity of the design is
shown in the accompanying
diagrams and photographs.
The tin is a rolling cylinder.
The essence of the design is a
loop of string with a mass
suspended on one side of the loop
and supported at the centre of the
circular ends of the cylinder.
As the cylinder rolls, the loop
winds up, lifting the suspended
masses thus adding gravitational
potential energy to the masses at
the expense of the kinetic energy
of the rolling cylinder.
Understanding energy
transformations is essential to
creating practical designs for
space applications.
Although string loops can be
used, elastic band loops work
much better.
In our design we used large nuts
with nylon sleeves inside
(commonly called lock-nuts). The
nylon sleeves reduce both friction
and chaffing with the elastic
bands. This makes the system
operate more smoothly and
lengthens the working life of the
elastic bands.
In our design the elastic bands
were simply "daisy-chained"
together.
Although not apparent in the
photo, four complete elastic bands
make up the loop. You should use
more or fewer elastic bands
depending upon the dimensions of
your cylinder and the size of your
elastic bands.
The elastic bands are held in place
at each end of the cylinder using
paperclips. A piece of tape may be
needed over the paperclip to
prevent the paperclip from turning.
Tea tins (or peanut tins) work well
for this since they have an almost
square aspect ratio ( i.e. their
diameter is almost the same as their
height). This shape gives the
suspended masses a lot of height to
gain as the tin rolls, improving the
energy-transformer's ability to
store gravitational potential energy.
Before testing your new energytransformer check that the elastic
loop is fully unwound (rotate the
lid to unwind it if necessary) and
that the masses do not touch the
sides (bottom) of the cylinder when
the lid is securely in place.
The design works best when the
mass ratio between the empty
cylinder (the tin) and the suspended
masses is close to one.
Also, elastic bands work better than
string.
Can you account for these facts?
The operation of this device relates to linear and rotational kinetic energy, elastic and
gravitational potential energy, and energy transformations.
The study of energy use and energy transformations is a critical element in the design of
robotic devices. For space based applications, energy considerations are as important as
mass limitations. Manipulating energy use through clever design is a key element in the
success of the space program.
When applied to terrestrial applications even modest improvements in the efficiency of
energy use can translate into huge savings in energy costs in the large scale robotic
operations of the earth-based production, assembly, and manufacturing industry.
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