Twitching motors


For some odd reason, the motors on our drive base twitch whenever I drive forward, back, turning. They go in short spasms. But whenever we pick the robot off the ground and drive, it works fine, the motors run smoothly. It is only when the robot is on the ground that it twitches.

We also have a lift mechanism attached to the claw on top of the robot.

We’re not sure if its a programming issue or a mechanical issue. Maybe the lift system is too heavy, but I’m not sure if that will affect the drive.

Attached is our code.

operator control.jpg

We are having similar issues with RobotC code:

Still no real resolution or cause.

However, we have found that if we download straight to the PIC (not via VexNet), it seems to have fixed the problem most of the time.

We currently have easyC V4 for cortex so we can’t download our code to the PIC.

We changed out our motors twice and even tried the high-torque motors thinking it was a torque problem, but it still twitches.

The lift seems to be fine, the motors run smoothly, it’s just the drive motors.

it does sound like a torque problem considering they work fine when the robot is lifted. How heavy is your robot and what ratio do you have? because it may be too much even for the high strength motors, or something maybe short circuiting?

Our gear ratio is 60:36. Do you recommend that I try the 84:60 gear ratio w/ the high-torque motors or have 1 standard motor connecting to each of the 4 wheels?

Currently, I do not know the exact weight of the robot, but I estimate it to be around 10 pounds-ish.

Sorry, i dont understand what you are saying. But i would recommend trying direct drive and if the twitching is still there, it wont be torque. How many motors do you have driving it at 60:36?

We only have 2 motors driving at that gear ratio.

Sorry, I meant having 4 motors total, connecting to the 4 wheels w/o any gears. (If that makes any more sense)

I will try direct drive, hopefully it will work. Our competition is only in 2 days.

ok, good luck! let us know if it works or not. We had a 84:60 ratio earlier in the season with 2 HS motors on speed and it burnt them out, so i think that 60:36 is a bit much anyway. Oh, and your code looks fine to me

Thank you so much for your help! And yes, I will let you know if everything works out.

You said you have Y cables on the wheel motors, but not on the arms, right?
You could try converting from tank2 to tank4, with no Y cables on the wheel motors, and put the Y cables on the arm motors instead.

What size wheels are you using?

With a 10 pound robot and 4" wheels, 60:36 with only two motors could be pushing it in terms of torque. Do your motors get hot?

jgraber: we’re not using any Y-cables

Chris is me: yes, we are using 4" wheels. We’re planning on keeping the 4" wheels but going with direct drive, one motor per wheel. The motors get warm but not extremely hot.

FYI: we tried the direct drive and it works now! :smiley:
And our robot is 10.6 lbs
thanks you for your help!

In general, if your motors are “twitching” (stalling) they’re overloaded. You can fix this by adding more motors, reducing the gear ratio (what you are doing now), or by lightening your robot. The configuration you’re describing is gearing your robot for about 2.8 feet per second, or a very fast speed for an average Vex robot. It can be done but it requires careful engineering and the use of more than one motor per side of the drivetrain.

good to know you got it working! Good luck for the competition also!