Found at: http://www.he.net/~imagesco/index.html
SOLAR1.txt
Solar Ball Robot
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The inspiration for this robot originally came from Richard Weait of
North York, Toronto. He create a light seeking robot in a ransparent
globe (ball). More recently Dave Hrynkiw from Calgary Canada, picked
up the ball so to speak and developed a series of light seeking mobile
solar ball robots.
There are two functions to this mobile robot that are interesting.
First is the method of locomotion. Inside the globe is a gear box.
Each end of the gear boxes shaft are secured and locked to opposite
sides on the inner surface of the transparent globe. The shafts are
locked to the inside of the sphere so that they can not rotate,
forcing the gear box itself to rotate. When at rest, the weight of the
gear box keeps it at bottom dead center ( the gear box facing down)
and the ball resists rolling. When the gear box is activated, the gear
box begins to rotate inside the globe. (the gear box is forced to
rotate, because the shafts are locked to the inside of the sphere).
This moves the center of gravity of the ball forward, causing the ball
to roll forward.
The second function relates to the power supply for the gear box. The
original solar robots has an on-board power supply that provided
intermittent power to the gear box. The on-board power supply consists
of a solar cell, main capacitor and a slow oscillating or trigger
circuit. When exposed to sunlight, the solar cell begins charging the
circuit's main capacitor. When the capacitor reaches a certain voltage
a trigger circuit dumps the store electricity through a high
efficiency motor connected to the gear box causing the robot to move
forward a little.
The solar ball robot described here uses a similar gear box assembly,
but for power uses two standard AA batteries. The disadvantage to
batteries is that they must be replaced when worn out. The advantage
however is that they supply continuous power to the robot allowing one
to easily study its behavior, locomotion and mobility.
With the original solar ball robot one needs to use time elapse
photography to study these effects. The charging of the capacitor
takes a few minutes, depending upon the intensity of sunlight. When
the electricity is discharge into the motor the robot lurches forward
a short distance. For example, 10 hours of motion with the original
solar ball can be compressed into a few minutes of study with this
robot.
While this particular robot doesn't incorporate the electronics for an
on board power supply, it still uses a light trigger. The circuit
shown in figure 1 controls the power from the batteries to the gear
box motor. The circuit reads the level of illumination that the robot
sees. If the light level is high enough it turns on the motor to the
gear box. The trip level of the circuit is user adjustable using
potentiometer V1.
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SOLAR2.txt
While this particular robot doesn't incorporate the electronics for an
on board power supply, it still uses a light trigger. The circuit
shown in figure 1 controls the power from the batteries to the gear
box motor. The circuit reads the level of illumination that the robot
sees. If the light level is high enough it turns on the motor to the
gear box. The trip level of the circuit is user adjustable using
potentiometer V1.
Gear Box:
Before we get into the construction of the robot, lets first look at
the gear box. There are two gear boxes available, see figure 2.
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Figure 2
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The gearbox labeled SGB-01 must be assembled. This gear box has a
slide switch that one can use to easily change the gear ratio. The
gear ratios of this box range from 6.8:1 to 808:1. The height of the
gear box makes it a tight fit in the transparent sphere. If you decide
to use this gear box when assembling the unit set the gears to the
highest ratio available (808:1). This is the gear box used in the
prototype.
The gearbox label SGB-02 comes fully assembled. Physically this
gearbox is shorter than the SGB-01 and is easier to fit inside the
sphere. It has a 4000:1 gear ratio. The gear ratio may be change but
that requires you disassemble the unit. If I were building this robot
again I would choose this gear box. First for its physically smaller
size and second for the higher gear ratio. The higher gear ratio will
make the robot move slower.
In the prototype even though I have the SGB-01 gear box set to the
808:1 ratio, the robot still travels just a little too fast. I would
prefer it to go slower.
Robot Construction:
The shell is the first component for consideration. It must be
transparent and large enough to hold the gear box and electronics. The
shell used in my prototype has a diameter of 5 1/2 inches. If you can
not find a suitable shell locally, you can purchase one from Images
Company, see suppliers list. The plastic shell is fragile. Do not have
your robot try to climb or fall down stairs, it is sure to crack and
break.
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Separate the two halves of the shell. The first job is to locate the
center of the half sphere. This is where we will connect the shafts of
the gear box. Locating the center at first appears much easier than it
actually is. To find the center I was forced to trace the diameter of
the shell on white paper. Then draw a box around the drawn circle that
touched the circle on four sides, see figure 3. Drawing diagnal lines
from the corners of the box I was able to locate the circle center.
The half sphere is then position
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SOLAR3.txt
sphere you may be able to eyeball the center and mark it on the sphere
with a magic marker. I tried once or twice with less than ideal
results. Finally I taped the paper on a 1/2" piece of wood and drilled
a small hole at dead center. Then I placed a small dowel, about 2.5"
long in the hole, making sure it was perpendicular to the wood. Place
the half sphere over the fixture, lining up its diameter with the
drawn circle, the dowel locates the center of the sphere fairly
accurately. Mark the center of one half sphere then the other.
The next step is to make a drive locking fixture in the sphere that
prevents the gear box shaft from turning. This forces the gearbox
itself to rotate inside the sphere, changing the center of gravity and
moving the robot along. The drive fixture must at the same time allow
the globe to be assembled or unassembled at will. The system I devised
is illustrated in figure 4. Although I built all the drive components
out of transparent plastic, you can fabricate the parts out of other
materials like brass and wood.
The first component is a small length of tubing 5/8" OD, 1/2" ID about
3/8" long. This tubing is glued to the center of the half sphere.
Using the marks as a guide. Inside the tubing, glue a 1/2" diameter
half round about 3/8" long. This piece may be glued inside the tubing
before the tubing itself is glued into the sphere.
Next machine a small length of 1/2" diameter solid rod. On one end of
the rod a 3/8" half section is removed. This is accomplished using a
hack saw or coping saw. First make a cut directly down the center of
the rod about 3/8" deep. Then make a horizontal cut to remove the half
section. Check to make sure this shaft fits easily, into the 3/8" tube
and half round assembly inside the half sphere. If not, file the cut
end it until it does.
At the opposite end of this rod, drill a hole down the center that
will fit the shaft from the gear box.
Note: on the prototype robot I made the second shaft a drive
connection also. Only when the robot was finished did I realize that
this was unnecessary. A single drive connection works just as well as
a double.
The second half sphere is easier to make. Glue a small length of 5/8"
OD, 1/2" ID tubing to the center of the half sphere, using the mark as
a guide. Cut a small length of 1/2" diameter solid rod. Check to make
sure the shaft fits easily into the 5/8" tubing. If not, obtain a
small piece of 100 grit sandpaper. Wrap the sandpaper around 1/2"
length on the end of the shaft. Twist the sandpaper around on the
shaft to sand the end. Continue sanding until the end of the shaft
fits easily into and out of the tubing. Next, drill a hole down the
center that will fit the shaft from the gear box.
We want the gear box to be positioned in the center of the
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SOLAR4.txt
sphere. Place the shaft of the gear box in the plastic rod. Place the
rod in on half sphere of the globe. Position the gear box so that it
will lie in the center. Mark the depth the gear box shaft must go into
the plastic rod on the gear box shaft. Remove the gear box shaft. Mix
a small amount of 2 part epoxy glue. Coat the gear box shaft with the
epoxy glue and insert it into the plastic rod. Let the glue set before
proceeding.
Once the glue has dried on the first shaft we must glue the other
plastic rod on the opposite side of the shaft. Position the glued rod
into the half sphere. Place the other plastic rod on the opposite
shaft. Place the other half sphere together with the first. Gauge the
depth the gear box shaft must be inserted in the plastic rod, then add
another 1/8" of depth for error. Glue and let set.
Check your work while the glue is setting on the second shaft to
insure that you can close the sphere properly.
Electronics
The electronic circuit is a light activated on-off switch. When the
light level is too low the circuit shuts off power to the gear box.
The user adjusts the sensitivity of the circuit using potentiometer
V1.
How it Works:
The circuit configures a CMOS op-amp as a voltage comparator. A
comparator monitors two input voltages. One voltage is set up as a
reference voltage called "Vref". The other voltage is the input
voltage called "Vin", which is the voltage to be compared. When the
Vin voltage falls above or below the Vref, the output of the
comparator (pin 6) changes states.
The two input voltages are applied to pins 2 and 3. Pin 2 (inverting
input) is connected a reference voltage of approximately 1.5V, using a
simple voltage divider made of resistors R1 and R2. Photosensitive
resistor R3 makes up another voltage divider in conjunction with
potentiometer V1, that is connected to the non-inverting input (Pin 3)
of the op-amp.
There is no feedback resistor between the output (pin 6) and either of
the inputs (pins 2 and 3). This forces the op-amp to operate at its
open loop gain.
The CdS photoresistor is the light sensor. A photoresistor as it name
implies changes it resistance in relation to the intensity of the
light that falls on its surface. The CdS produces its greatest
resistance in total darkness. As the light intensity increases its
resistance decreases. In the circuit, the CdS cell is part of a
voltage divider. The changing resistance of the CdS cell changes the
voltage drop across the potentiometer V1, that is connected to pin 3.
As the light intensity increases the resistance
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SOLAR5.txt
of the CdS cell decreases which increase the voltage drop across the
potentiometer. This increased voltage drop is seen as a raising
voltage. The trigger voltage can be set for different light levels
using the potentiometer.
The electronic circuit is not critical. You can construct the circuit
using point to point soldering on a proto-typing bread board. Once the
circuit is complete you need to adjust the light level that will
activate the circuit using potentiometer V1. Make temporary
connections to the gear box motor using alligator clip wires. Power to
the circuit and gear box is obtain from two AA cells. The AA cell pack
is glued to the back of the gear box during final assembly. Make sure
the battery pak has a battery clip for easily disconnecting and
connecting power.
When making the light level adjustment use a low level of light to
activate the robot. When the robot is on the floor, if the light level
is set too high it will stop every time it passes under a shadow.
Putting it all Together:
Once the circuit is adjusted you are ready for the final assembly.
Glue the AA battery pack to the back of the gear box. Making sure that
no glue comes into contact with any of the gears. The electronic
circuit board is glued to the front of the gear box. Again making sure
none of the glue touches any of the gears. Connect the power supply.
At this point the gear box will probably start turning. To load the
mechanism inside the robot, bring all the parts into a dark room to
deactivate the circuit. Load the assembly inside the sphere.
Take the robot out into the light. The gear box should become active.
Place the robot on the floor. The robot should travel toward or in the
direction of light. If the robot does the pposite, stop the robot,
remove the gear box and electronics, and reverse the wires leading to
the motor.
Locomotion:
I was pleasantly surprised when I began observing this robot. I
originally though it would become trapped easily. Not so. When the
robot enters a corner and stops, the gear box inside begins swinging
all the way up and over radically shifting its weight over top dead
center and moving the robot out of the corner.
Advancing the design.
When I originally designed this robot I planned to use a steering
mechanism to track a light source. However the small steering
mechanism didn't have enough weight to turn the robot in any direction
quickly. In the long run other factors (terrain,
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SOLAR6.txt
obstacles, etc.) affect its direction. Hence I removed the steering.
But this is still a good research area for advancing the overall
design.
Adding Higher Behavior Module
As the robot stands, when a certain level of light is reached it
becomes active. We can add a higher behavior mode, feeding, by adding
a few more components (two solar cells and steering diodes) and
another comparator circuit. The second comparator circuit will
deactive the motor when the light illumination level becomes high
enough. Allowing the solar cells to charge the AA batteries which will
be changed to Nicads. Figure 5 illustrates the behavior. When the
light level is low the robot is off or we can say it is in a resting
mode. As illumination increases it reaches a point where the motor
turns on, the robot enters its searching mode. When the light level
increases significantly beyond this point (searching mode) the second
comparator turns off power to the gear box motor, allowing the two
solar cells to charge the AA Nicad batteries, or the feeding mode.
If anyone plans to add this feeding behavior circuit, keep track of
the current drain to the comparator circuits. It must not exceed the
current supplied by the solar cells or obviously no charging to the
Nicads will occur. If you would like to see this behavior added to the
robot contact me through this magazine.
Parts List
(1) 5 1/2" Transparent Plastic Globe
(1) Gear Box SGB-01 or SGB-02
(1) 6" length of 1/2" solid plastic rod
(1) 3" length 5/8" OD 1/2" ID Plastic Tubing
(1) 1" length 1/2" half round plastic rod
Electronics
(1) CMOS Op-Amp ALD 1702 or equiv.
(2) 33K ohm 1/4 watt resistor
(1) CdS Photoresistor
(1) 4.7K ohm potentiometer (PC mount)
(1) 15K ohm resistor
(1) .0047 uF capacitor
(1) Tip120 NPN darlington
(1) PC board
Available from
Images Company
PO Box 140742
Staten Island NY 10314
(718) 698-8305
All major credit cards accepted.
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