Re: Problems in Timing Belt drive system in 6WD robot
--- In SeattleRobotics@yahoogroups.com, twcarroll@... wrote:
> In a message dated 1/25/2013 2:56:58 P.M. Pacific Standard Time,
> davida@... writes:
> I have not observed this behavior with jBot. Basically the timing
> belts/pulleys/motors are strong enough to twist up the original plastic
> driveshafts without the timing belts jumping teeth in the pulleys:
> I eventually replaced the drive shafts with steel CV joints. I don't
> think I have ever seen the timing belts jump teeth on the pulleys. Perhaps
> your pulleys are too small?
> Relatedly, my experience is that lowering the center wheels is not needed
> with this platform design, and actually will decrease the performance of
> the platform. YMMV.
> If you check out the "first movement" video of jBot spinning in place you
> can see that the wheels on the front and rear are sliding sideways quite a
> bit. But the center wheels are tracking nicely around a circle, no
> sideways movement, like you might expect from a traditional 3-wheel platform. I
> believe this is opposite from what Tom was saying:
> The center two wheels will track smoothly as they have the greatest
> traction as they are virtually going straight forward or backward with very
> little side slip. The front and back pairs are sliding more sideways as
> they are further away from the center of the turns. However, they (the front
> and back 2 pairs) have the advantage of double the number of wheels
> turning and thus, twice as much ground friction as they would have if only 2
> wheels. Hmmm. Go figure.
> Can you grab the wheels and rotate them by hand and force the timing belts
> to slip while holding the motors fixed? I cannot on jBot. The tires
> begin to slip on the wheels before that. (BTW, standard practice is to
> super-glue the tires to the wheels to prevent them from slipping, if you haven't
> already done that. I found it was necessary on jBot)
> The only other difference that I see in your setup is that jBot uses a
> reduction between the motor and the first pulley, while you are using a
> gear-up. Not sure what difference that might make. The belts on jBot are taut
> but not tight (they don't "sing" when plucked). Again, perhaps the pulleys
> you are using on the axles are too small? (i.e., too tight of a radius
> for the torque required). I don't have jBot here with me but as memory
> served I used 2 inch pulleys on the axles.
> looking good!
> --- In SeattleRobotics@yahoogroups.com, twcarroll@ wrote:
> > Just as Robert mentioned, looking at the photograph, it looks as if
> > using three solid shafts with the toothed pulley / wheels freely
> > on each, and no differential. 6-wheeled robots have a problem with this
> > arrangement. During turns, the robots basically tries to turn around a
> > in the center. The two center wheels turn the slowest during turns,
> > whereas the four end wheels are turning faster as they are further away
> from the
> > center of the turn. If they are digging in the grass or pavement, the
> > force on the shaft and pulley is enough of a difference between front
> > middle to cause the belt to slip off the pulley a few notches or more.
> > Quite frankly, if you're 'under the gun' to get the robot completed in
> > time for the RoboMagellan contest, I'd remove the center wheels
> > You're really not going to gain enough in traction with 6 wheels over 4
> > to make any difference at all in the contest.
> > Ackermann steering is the best for this type of robot but that requires
> > whole new set-up in your steering and control. Six wheels look 'cool'
> > are best used with differentials in the sets of wheels. Differential
> > steering, or as some say- "skid steering," with more than 2 wheels eats
> up a lot
> > of power when making turns. That's why it is called skid steering.
> > You'll never see a car with this type.
> > Good Luck,
> > Tom C.
> > Maybe it's not the belts, but somewhere else in your drive train, say
> > between the pulleys and the tires? I'm not familiar with your setup. Do
> > have differentials in the axles that turn? Are you sure the "popping"
> > caused by the loading of the drive train due to the difference in path
> > between the inner and out tires when turning? Have you tried it on a
> > slippery surface such as ice? That would at least let each tire spin as
> > wanted to when turning.
> > Robert
> > ____________________________________
> > From: "KM6VV"
> > Sent: Friday, January 25, 2013 12:51 PM
> > I'm having problems with the timing belt drive system in a 6WD robot I'm
> > building for Robomagellan competition in the upcoming Robogames.
> > (Mechanical design courtesy DPA).
> > Pictures in LM forum:
> > _http://www.lynxmotion.net/viewtopic.php?f=9&t=7839&hilit=6WD&start=45_
> > (http://www.lynxmotion.net/viewtopic.php?f=9&t=7839&hilit=6WD&start=45)
> > The belt system appears to run fine in straight runs, grass, concrete,
> > etc., even when moving quite fast. The problem arises when the 'bot
> > starts to make a turn (more torque required). I start hearing loud
> > "popping" sounds, which I take to be the belt(s) jumping teeth.
> Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
Thanks. I'm still not finding the exact motor,
but I get the idea. 45 oz-in 142 RPM ??
my 10 kgf-cm = 138.873866 oz-in. So maybe I've just got too much
torque? I like my ground speed so far, should be 6-7 MPH with the
1:1.533 gears I have. 1:1 gears would give me about 4.5 MPH.
On 1/26/2013 4:13 PM, dpa_io wrote:
> Alan, yes motors are 24 volt Pittman GM9413J820 (or something like that) with 20:1 gear reduction and 500 CPR encoder.
Drive pulley is 15 tooth and main pulley 30 tooth.