Exert-O'Meter
Since Sumo is all about
pushing, it seams logical that some method was needed to measure "push".
That method had to be reliable and repeatable. The most direct
method of measurement would be a robot pushing on a vertical plate.
Load cell type postal scales should function normally in a vertical
position, so a search was begun for an accurate, inexpensive scale.
A Sunbeam digital postal scale was on sale at a local retail outlet and
was purchased.
The scale's capacity was 10 pounds in 0.2 ounce increments, or 5 Kg in 5
gram increments. This range and increment seemed reasonable for
the application.
A simple fixture was made out
of scrap 1/4 and 1/8 inch plywood for holding the scale vertically.
The 10" by 16" test area was made from melamine shelving material to provide
a non-wearing surface. Unfortunately, the melamine was white which
has a negative effect on line sensors. Some reprogramming is
required to use the Exert-O'Meter. The center of the scale is 5/8
inch above the melamine surface.
Alignment Initial tests on the Exert-O'Meter were conducted with Runt. At the start of the test Runt weighed 3 pounds 2.8 ounces and was equipped with foam rubber "yellow dot" tires. Runt could push 2 pounds 9 ounces. After the tires were cleaned with alcohol, Runt pushed 3 pounds 4 ounces. Thin fiber washers were added under the servo mounts so that all four tires were parallel to the ground. Runt pushed 4 pounds 12 ounces.
There were three conclusions from the initial test: There is a definite advantage to wheel alignment (duh!). Clean tires work much better than dirty tires. Exert-O'Meter is capable of useful measurements.
Toe-in and Silicone Coating With Runt weighing in at 3 pounds 3.0 ounces, it could push 4 pounds 2 ounces. A little toe-in was added with fiber washers and push increased slightly to 4 pounds 4 ounces with much more consistent results. The toe-in was removed and Runt was back up to 4 pounds 12 ounces. So, toe-in does not have a major impact on pushing.
A set of tires was coated with clear silicone sealant and push went up to 7 pounds 2 ounces. Runt was re-weighed at 3 pounds 4.0 ounces. The one ounce addition of silicone improved push by 2 pounds 6 ounces. This demonstrates that it is possible for a Sumo robot to push twice its weight without magnets or suction! Never again run a Sumo robot with bare foam rubber tires.
February, 2002 NEIRG Lead was added to Runt so that it weighed 6 pounds 9.6 ounces. With dirty silicone coated tires, Runt could push 8 to 9 pounds. With clean tires, Runt recorded between 10+ pounds to "off scale" readings. Already, Exert-O'Meter is obsolete.
Urethane Tires A form was constructed to mold four 2 1/4" diameter tires. The mold was capable of handling up to 2 1/2 " wide tires. Using the urethane mixture from the Portland Area Robotics Society web page (Tap Plastics part A and B Urethane RTV and Polytech part C) four tires were made for Runt and eight tires were made for Sticky.
Sticky, without the extra
lead, weighed in at 3 pounds 10 ounces. Clean, old silicone coated
tires could push 3 pounds 7.2 ounces. New, molded urethane tires
pushed 7 pounds 10 ounces, but one tire was spinning off of the ground
and the other three motors were stalled.
Smaller, more powerful 7.2 volt motors were obtained from Lynxmotion for Sticky which dropped the weight to 3 pounds 5.6 ounces. With old clean silicone coated tires, Sticky could push 3 pounds 12 ounces. Molded urethane tires increased Sticky's weight to 3 pounds 10.2 ounces and achieved a push of 9 pounds 12 ounces, or 268% ORW (Of Robots Weight). Since I like silicone, a set of tires was molded using Polytech Platsil 71-20. With these tires, Sticky pushed 8 pounds 12 ounces, or 238% ORW.
Exhume weighed in at 2 pounds 5.2 ounces and could push 4 pounds 8 ounces with clean, molded urethane tires. There was much jumping around indicating that more work must be done with controlling the power.
Stabilize Chassis The previous test with silicone tires was disappointing. A new set was molded with the Platsil 71-20 softened with Polysil Silicone fluid. A 3 pound 10.2 ounce Sticky now pushed 10 pounds 12.4 ounces to "off scale", or 296% ORW, but the tires were wobbling on the axles. They didn't run true because the hubs were made to fit a 1/4" shaft and the motor shafts were a little smaller at 6 mm. The hubs were replaced with correctly sized hubs and Sticky easily pushed off the scale at over 11 pounds 2 ounces, or 307% ORW. Overall, the pushing was much more stable. The motors didn't quite stall, but were turning very, very slowly.
February, 2003 NEIRG The first test of molded urethane tires was the February, 2003 NEIRG. Silicone molded tires were not used because they appeared to be less predictable than the urethane, and the urethane tires have already experienced several months of testing without any failures. Both Sticky and Exhume won their classes with the original urethane tires.
February, 2004 NEIRG Weighing in at 2 pounds 15.4 ounces with a set of the original molded urethane tires, Excuse first pushed 5 pounds 0.2 ounces. Once the tires and surface were cleaned, the push went up to 8 pounds 12 ounces, or 295% ORW. In order to stay out of the 3 pounds class with Exhume, larger batteries were added to Excuse to produce a 3 pound 4.2 ounce Sumo robot. At that weight, Excuse could push 11 pounds, or 337% ORW. Clearly a higher range is required for the Exert-O'Meter.
Son of
Exert-O'Meter More to come with a Pelouze 40 pound scale.