Executioner
Late in 2004 work began on a suitable replacement for Excuse. Since Excuse was originally designed for the 3 pounds sumo class, a purpose built 3 Kg sumo would be far more competitive. The primary design goal would be substantially higher speeds and more weight built in to the structure.
There are many, very powerful, small electric motors available for R/C cars and airplanes. The major problem with these motors is that there were designed to turn most efficiently in only one direction, counter clockwise. That is, they have advanced timing built into the motor brushes. The motors in my robots will need to run in both directions, counter-clockwise on the left side of the robot and clockwise on the right side of the robot. So neutral timing or, preferably, advanced counter clockwise on the left side and advanced clockwise on the right side of the robot.
The first
chassis was powered by four Speed 380 motors from
The Robot Marketplace
and four PGHM-09 gearboxes (no longer available) from
Lynxmotion. Also,
a gear puller and a timing adjustment tool were acquired. The major
problem with this arrangement was that the axles had too much play.
High precision bearings reduced the play, but the axles could not hold
the tires square to the ground for maximum traction.
In March, 2005, a major change in robot construction techniques arrived in the form of a Smithy Midas 1220 Ltd with a three axis digital readout. Now, much more advanced work is possible. The chassis with the Speed 380 motors was boxed up and forgotten.
R/C car motors
and Lynxmotion 27:1 gearboxes (PGHM-14) were the next step. Stock wind
(27 turn) R/C car motors do not come with adjustable timing and
adjustable timing motor cans only come with much higher current
armatures, requiring much more powerful motor controls. The logical
solution is a stock armature in a modified motor can.
Tower Hobbies
Speed Stocker 27 Turn Racing Motors provided the armatures and
Tower
Hobbies Speed Secret 14T Quad Modified Motors provided the cases. The
two fit together like there were made for each other. The speed control
that I have been using was capable of 18 amps continuous per side. With
double the MOSFETs, 18 amps per motor should be ample.
New wheels
were machined out of Delrin. The wheels were about 1.5” diameter and
1.65” long. They extended 1.25” over the gearboxes. Aluminum hubs were
placed outside the wheels for easy access to the axle set screws.
A chassis was built similar to Excuse II and ten GP3700 NIMH batteries were installed. Initial runs were awesome! 130 inches per second speed with almost instantaneous acceleration and deceleration must be seen to be appreciated. Accidentally, I programmed in a two second run. The robot careened off of two dining room chairs, the dining room table, and two walls. Once it stopped, the gearboxes did not sound very good. A disassembly of the boxes revealed many stripped nylon planet gears.
It was obvious
in the photos on the web site that the current Lynxmotion PGHM-14
gearboxes were different from the ones that I just stripped. A few days
later, four new gearboxes arrived. Upon disassembly, it was discovered
that they had four planet gears per stage and the planet gears were
metal. That is how they achieved the 5.9 foot/pound instantaneous
torque rating.
To achieve 20 cm width, the snap ring and washer were removed from the axles, .100” was removed from the back plate, and .050” was removed from the front plate. Also, the OD of the gearbox was reduced from 1.42” to 1.34” to better fit the wheels. The modified gearboxes were assembled onto the motor using new M3.0 bolts and lock washers with Loctite 290 between the pinion and motor shaft, and between the back plate and the motor case. The robot was now 19.8 cm wide.
Four of the batteries were removed to slow the thing down to a manageable speed. The remaining six GP3700 MINH cells should be more than enough. The new gearboxes were briefly broken in and a new speed test produced 85 inches per second. The chassis, motors, gearboxes, and batteries weighed 4 pounds 8.8 ounces. A push test resulted in over 13 pounds of force. This is more in line with my original design goals.
The front
bumper is machined out of a hunk of ½” x 6” 1018 steel. Since this was
my first serious milling project, a partial bumper was machined out of
aluminum to get practice in the order of cuts and hold down techniques.
Once work on the steel bumper began, it was surprising how much longer
it took to mill steel than aluminum. Also, the original High Speed
Steel end mills soon gave way to carbide end mills. It took several
broken end wills to work out the proper techniques.
As in
Extrasensory, precise locations for the IR LEDs and IR receivers were
machined into the bumper. For the first time in any of my sumo robots,
line sensors were included in the bumper. The line sensors work very
well, but, so far, they are not included in the software. I still
haven’t figured out how to use them to benefit the robot.
An MX310
optical mouse was included. This time an
ezMOUSE from Multilabs
was used to convert the PS/2 from the mouse to serial for the Stamp.
The connectors were removed from the ezMOUSE and the printed circuit
board was cut down. Four wires from the MX310 to the ezMOUSE and four
more from the ezMOUSE to the Stamp board completed the mouse wiring. A
little heat shrink around the ezMOUSE provided electrical insulation,
and a little silicone glue attached the ezMOUSE directly to the MX310 board.
By the end of
January, 2006, all electrical and mechanical assembly was completed.
Testing and software setup revealed that the robot had 85 inches per
second speed along with awesome acceleration, braking, and turning
potential. At this stage, the name “Executioner” seemed to fit much
better than any others that had been suggested.
Software
testing under search conditions indicated that the main control loop
runs 10,000 times in 180 seconds. That is 18 milliseconds per loop.
Actually, about a third of that time is used up in mouse communications.
One changes were made as the result of the initial tests. The MX310 mouse was not as reliable as previous robots, so a 50 ohm resistor was added to increase current to the LED. The resistor approximately doubles the current in the MX310 IR LED. A test MX310 ran for 24 hours continuously at the doubled current without failure.
Executioner
was covered with 1/8” clear polycarbonate. Polycarbonate was used
because it is heavier than the carbon fiber, it absorbs IR and sonar
better than carbon fiber, and it looks good. Final weight was 6 pounds
1 ounce, or 2.750 Kg.
Pushing the
two buttons and counting red or green LED flashes was getting very
tiresome, particularly when there are eight or nine different start
routines. The first attempt to simplify this process was a
Comfile
CLCD-216 display. The advantage of this particular LCD was the 115.2
Kbps serial communications to the Stamp.
For initial testing, the LCD was placed in a box and a 4 wire USB male/female plug was used to connect the box to Executioner. The LCD displayed the sensor outputs, mouse outputs, and the present start routine. Two buttons on the box controlled the start routine selection and the actual start.
Executioner’s first outing was the May 6, 2006 event in Peoria. During the competition, the mouse lost contact with the surface too many times and ordered Executioner to spin to avoid a perceived threat. All of that spinning resulted in the rear tires coming off of the wheels.
In order to cure the tire problem, four new wheels were made. New polyurethane tires were molded with a hardness of 20 Shore A (the originals were 10 Shore A). After removing the tires from the mold, the tires were superglued to the wheels. The original tires were superglued, also. All future testing was done with the new tires and no more problems were encountered.
Further investigation revealed two major software problems with the optical mouse. One of the errors goes back to the original software that was used two years ago. Once those errors were corrected, the mouse response resulted in repeatable results. Presently, the thresholds for evasive action are being evaluated and updated.
The external
box for the LCD was too much of a hassle, so an attempt was made to
reduce the thickness of the LCD and mount it on Executioner’s top
polycarbonate plate. Two recessed push buttons were also installed.
Before that modification was completed, I received a new
Graphic Touch
Screen Display from PDA, Inc. Their web site is
simmetry.com and their
display is available through eBay from "dgodrik". The processor on the
Graphic Touch Screen Display can store up to 26 screens. The screens
are created on provided PC software and downloaded directly to the
display for saving. There are also several templates in the Graphic
Touch Screen Display for interpreting and reporting the touch screen
inputs. It takes only four ASCII characters (plus Carriage Return) to
load a saved screen and a template. Touch Screen inputs are saved until
the Stamp requests they be returned. It is very nice and easy to use!
Of course the
first screen has to be the name “Executioner” with the crossed axes.
The start screen displays “Executioner” at the top center. Immediately
below “Executioner” is the presently chosen start routine. In this case
the routine is “Search R/L”. Pushing the big arrow to the right of the
start routine initiates the five second countdown and the runs “Search
R/L” to the right. The left arrow runs “Search R/L” to the left. The
“00000” are the front sensors. In this case, they do not see anything.
Immediately below is “0 0” which is the two front line sensors. Next
to that are “000X” and “000Y” which display the mouse outputs. The up
and down arrows increment the start routines.
This photo
shows the Graphic Touch Screen Display for the “Angle” start routine.
The center, right center, and right sensors see an object, and both line
sensors see the surface. Once one of the start buttons is pushed, the
screen displays a countdown 5, 4, 3, 2, 1. If the screen is touched
during the countdown, then the program and display return to the
previous start screen.