Little Flash is a 3d printed robot powered by ultracapacitors. To prevent getting stuck, she uses a bump switch and random path adjustments.
Making a couple of changes to the design allows me to reduce the part cost by forty percent. Here's Little Flash (left) and Little Flash 2 (right).
Daren Schwenke suggested a modification to the servo motors that allows me to remove the Arduino motor shield (a costly component). I do lose current detection, but that seems to be insignificant in the scheme of things. Changing the dc to dc converter to a "non metered basic unit" reduces costs further.
This year's competition is about moving toward a market solution for a given device--I thought I would at least try to move toward lower costs while maintaining functionality for Little Flash.
Little Flash is powered using three 350 farad capacitors (in series). Her drive motors are metal gear servos (with electronics and end stops removed).
Currently, she runs for over 25 minutes on a single charge and the charge is supplied using a constant current supply (10 amps for 40 seconds).
I expect to use three 3000 farad capacitors (in series) to quickly charge Little Flash--this could result in Big Flash; something we'll see in a few days.
Take the 3d printed wheels and add vacuum cleaner belts for tires.
Attach the Servo horn using 3mm screws.
Solder the capacitors in series and place them in the 3d printed capacitor holder (along with the on/off switch). Solder the (female) charging cable.
Attach the Arduino (with motor control shield) and dc-dc converter to the back of the blue capacitor holder. I used velcro for attachment.
Attach the lever switch and bracket to the robot body.
Add the "bump switch blade" to the lever switch bracket using 3mm screws. The blade should move very freely.
Secure the motors to the robot body (3mm screws). Add the wheels to the motor shaft (using the servo horn screw). Attach the capacitor holder to the robot body using screws. Attach the caster ball holder to the robot body using screws.
Insert the caster ball.
Set the output voltage for the converter to about 8 volts. Program the Arduino, charge the capacitors and she's ready to run.
Today is the big day--I prepared to (from a distance) connect 9000 charged farads (8.1 volts) to 1050 partially discharged farads (3.5 volts).
Nothing spectacular to report--probably got enough charge (in ten seconds) to run for 20 minutes.
Peak current during the transfer was around 45 amps and the hottest physical component (plug connector, #18 wire) reached 122 degrees F. The wiring and the connector represent about .1 ohm, so that resistance limits the transfer.
To charge Little Flash in one second, I would need to transfer about 400 amps. The connector and wire size to accomplish that (for a one pound robot) seem impractical--so I'll be happy with a ten second charge for now.
Using available materials, I'm preparing for the first "pretty quick charge" test.
Using painter's tape to secure the 3000 farad capacitors to scrap plywood, I wired in an automobile starter solenoid (rated for about 300 amps).
I plan to stand about 15 feet away (and operate the entire setup on concrete) when I connect "9000 charged farads to 1000 depleted farads." It should be interesting and will probably occur Monday, May 20.
Little Flash just runs around and stays out of trouble, but it's somewhat relaxing to let everything else go and just watch the movement. As John Lennon observed, "Time spent doing nothing is rarely wasted."
The schematic diagram for the robot is shown below:
The Arduino sketch for operation is available in the files on this site.
Solder the capacitors and on/off switch together. Place them in the capacitor holder.
Add the dc to dc converter to the switch side of the capacitor holder.
Solder leads to the lever switch.
Place the lever switch in the bumper bracket using 3mm screws and nuts.
Prepare the bumper to attach to the bracket--it will attach using 3mm screws that thread into the bracket.
Gather the wheels, servo horns and vacuum belts. Attach the servo horn using 3mm screws.
Place the "tire" (vacuum belt) on the wheel.
Drill holes and attach the bumper assembly to the frame.
Drill a hole and attach the caster bracket to the rear of the frame.
Insert the caster ball into the bracket.
Next, we modify the servo motors. Remove the screws from the backplate.
Remove the small "silver" screw (below the circuit board) that holds the pot in place.
Pull the pot straight out.
Cut the three wires and remove the pot.
Combine two 2k resistors. Solder the center wire to the connection of the two resistors.
Solder one wire to the end of a resistor and the other wire to the free end of the other resistor.
Tape the resistor assembly so that the resistor ends cannot make contact with anything else.
Insert the circuit board and resistors into the motor cavity. Screw the back on.
Using a file, remove the plastic nub on the servo mounting bracket.
Install the motors in the frame using 3mm screws and nuts.
Place the wheels on the servo shaft. Secure the wheels using the servo horn screw.
Route the wires between the motors toward the back of the frame.
Pull the wires through the hole in the capacitor holder and secure the capacitor holder using 3mm screws (holes will need to be drilled in the frame).
Solder two 1 Mohm resistors together. Add a wire from the center connecting point and insulate the assembly.
Solder one end of this assembly to "+ input" on the dc to dc converter. This device is used to monitor supply voltage and trigger the Arduino's onboard led when the supply voltage drops below about 4 volts.
Install the sketch for the Arduino (found in the files), then plug in all the wires according to the schematic and tie wires up for appearance.
Charge the capacitors, flip the switch and it will run just like Little Flash.