2 Motor Drivetrain Help

Hi forum,
My team has a drivetrain with 2 motors, and an intake on top. We’re having trouble with getting the base to turn. It can move forwards and backwards just fine, and it can even turn when moving, but if you try to turn from a standstill, it wont move. Any suggestions on what to do?image image image
Side note, the intake isn’t the issue as it doesn’t turn without anything on top either.

Try increasing the width of the base, the larger the width the easier it is for the base to do point turns.

Another trick you can try is to move the center of mass further away from the geometric center of the robot.

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put your robot as picture 1, wheel not touching ground, make turn and see if the wheels are moving in the right directions

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We have tried it where the wheels are not touching the ground, and they both move in opposite directions just fine.

I feel like your robot is too narrow for point turning. If your robot isn’t wide enough, it won’t do point turns. Try using longer c-channels as a test brace between the two sides of your chassis to see if it fixes your problem. It seems from your design that you can just unscrew the 5 sections bracing the sides and lightly screw some 7’s to see if it helps.

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Our robot is also 25 wide, same as your bot, however our wheels are closer together (front and back wheels). Having the wheels in a square formation helps turning as the outer wheels do not have to travel a farther distance.

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I suspect there is imbalanced friction on the four wheels. add force to test each wheel forward and backward.

Adding a wheel in the middle so that you have a 6 wheel drive will take 50% of the load off the front and back wheels which have to be dragged when turning.

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Apart from what everyone else said, check to see if the chain is catching on anything. Even a few millimeters of screw head sticking out can jam the chain

Edit: try this last when all other options have failed, as it is very hard to detect and test for.

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Which gearboxes do you have in those motors? With the motors disengaged from their shafts (slide motor shaft out of motor, but not out of the rest of the mechanism) do the wheels spin freely for a second or two if you spin them by hand?

We are using 200RPM motors at the moment. Yes, the wheels do spin freely when the motor shafts are taken out of the motors.

Surprised no one has mentioned this yet…
You have a 2:1 ratio from your motors to your wheels… meaning your wheels are being run at 400rpm. With 2 motors, this is not enough torque. Change your sprocket ratio.
In addition, what do you mean when you say robot does not turn?

  • Does it just stall when you try to turn?
  • Do the gears skip?

Try to be more descriptive when you post something like this next time.

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good observation. but not sure if the sprocket ratio explain it all. The robot can turn while moving forward but not can turn while standing still. It seems to me there is another factor plays too.

Try (2) non-Omni wheesl on the frony or back (whichever is heaviest). Four Omni wheels work best if mounter at 45 degrees in each corner. Having all (4) mounted parrallel yields less lateral traction.

why is so? love to learn the mechanism. thanks

An xdrive… just to improve turning… that’s a bad idea.

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Nope. Turning while moving forward is easier than turning while standing still. That was the exact same behavior of the robot I built back in 2007, and it lacked enough torque to turn itself. The cause was different, as my robot suffered from excessive friction from having no omni wheels, but the behavior is the same. Increase your torque and your turning will be fine.

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But if the wheels are chained together. Wouldn’t the rear wheel cut off the front drive wheel. Causing the motor to drive against itself?th

your design should work when turning. By “not working” do you mean the motors are struggling to drive the wheels, the gears skiping etc? did you test turning the robot with the wheels midair?

Ratio of width (wheel track) to length (wheel base) affects ability to turn, even with omnis. Either more torque (so slower but stronger) or change ratio, wider vs. shorter length front-to-back. (Or both.)

Try it, see for yourself. You can take the motors off, try to move motor gears with your fingers, you will get feedback…

It will “drive” forwards relatively easily, turn not-so-much. The torque vs speed ratio works back from the wheels (driven) to the motor (driving) in reverse.