Transmissions

[Veselin Kolev]

Team 1072 (Harker Robotics Team) used a TechnoKat style dog shifter this year. Two speeds, two CIMS, geared to go 6 feet and 11 feet per second. Man.. what a beauty. Of course we totally ignored the white papers and designed everything ourselves... just to improve even more on an already very good design. We first of all did not use a flex coupler, instead we put a chain sprocket directly onto the output shaft of the transmission. We also made the whole thing with .25" and .125" plates screwed together, you could literally take the shifter OUT by unscrewing 5 bolts. And the thing was sturdy... even thought the transmissions were tiny they could pack a punch. For the record, or robot in low can push TWO other robots with the kit transmission. And we did this during competition. That is how we know..

As for shifting, it was very smooth and quiet, no jolt or sudden lurch of the robot. I was thinking about putting sycros on if shifting was too rough, but we tested it and they shifted fine. I highly reccomend any teams with access to a mill and lathe with the proper tooling and the urge to make a 2 speed transmission to consider a dog-style shifter like this.

[bradleym1559]

Originally Posted by Tristan Lall

Back in 2000, Woburn's Blizzard 1 had just such a control. But it wasn't included to surprise people--there was a bit more pragmatism involved! As it happened, that robot had front-wheel-drive (old Johnson drills) and rear casters, and because the weight distribution was very biased toward the rear, the robot had an annoying tendency to over-rotate when trying to manoeuvre. That button was in there so that full speed could be achieved if necessary (and usually with a fixed rear wheel deployed, raising the robot off of its casters), but under normal conditions, the robot wouldn't be so difficult to control. I'm not so sure it was of much actual use, though, since that robot was by far the worst-driving of any of the Woburn family (of those that I've taken for a spin, at least--that's the last six, plus one of the ancient Canada FIRST robots). It certainly wasn't an efficient or elegant solution, as sanddrag pointed out.

This sort of solution is only useful if you don't care about the power output (in 2000, we knew we had enough power to drive around at half-throttle), or conversely, don't care about the current and efficiency (we should have cared--we threw 30 A breakers on some occasions, that year, and were limited to a 60 A main fuse, not a 120 A main breaker). You're effectively decreasing the voltage of the motors--for instance, by half--which just doubles the current needed to sustain a certain power level. This is why it is fundamentally crazy to control something like a (DC permanent magnet brush) motor at low throttle settings (i.e. the Victor's output voltage is a fraction of the motor's rated quantity); you're just going to force the current way up, and the efficiency way down. Low efficiency just means heat losses, and to compound the problem, the internal fans don't work well at low speed.

So what's the right way? Shift gears. You'll have two speed ranges that allow the driver to avoid overstressing the motors by running them too slowly.

Of course, sanddrag already knows all of this, so I suppose that this is for everyone else out there....

That was exactly our problem (casters)...we had to limit our power because the robot was uncontrollable with the program we were provided with...so we had to change the gearing...but to save money we did it in the program. During the competition we increased the speed just to see what would happen..Divide by 2 instead of 3, and i couldnt control it (I was the driver)...all our mods to the program seemed to become more ineffective the faster the robot got. But next year we will need a more concrete solution to our gearing...because that was really our only issue--our robot was too slow. First year mistakes...we learn. This is what CD is for! Thanks for all the input!