Mecanum gearbox issues

[RichardNixon]

We have been having issues with our mecanum wheels turning at different speeds. We think it might be a gearbox issue but we couldn't find anything. It could be a sim motor issue. We have narrowed it down to the gearbox/sim area. Any thoughts on potential issues with mecanum drives?

[s_forbes]

Originally Posted by RichardNixon

We have been having issues with our mecanum wheels turning at different speeds. We think it might be a gearbox issue but we couldn't find anything. It could be a sim motor issue. We have narrowed it down to the gearbox/sim area. Any thoughts on potential issues with mecanum drives?

Can you clarify what gearboxes you are using? Also, if you turn the wheels by hand with the robot powered off, are certain wheels harder to turn than others?

[Chris_Ely]

Are your motor controllers calibrated? This is the most likely cause for motors turning different speeds. What motor controllers are you using?

*Also; CIM, not "sim".

[efoote868]

Originally Posted by RichardNixon

We have been having issues with our mecanum wheels turning at different speeds. We think it might be a gearbox issue but we couldn't find anything. It could be a sim motor issue. We have narrowed it down to the gearbox/sim area. Any thoughts on potential issues with mecanum drives?

This is a common issue, which is why teams will use encoder feedback to vary motor outputs in a control loop, to get each wheel turning at the desired speed set point.

[Nick Lawrence]

It could also be a center of gravity issue. If you don't have encoder feedback on each of the wheels, and the COG is not centered between the wheels in the X and Y direction, the robot may not drive straight or strafe straight as desired.

-Nick

[RichardNixon]

we are using vexpro single reduction gearboxes and there is a noticeable difference between the wheels when spun by hand.

[GeeTwo]

Originally Posted by RichardNixon

we are using vexpro single reduction gearboxes and there is a noticeable difference between the wheels when spun by hand.

The difference when spun by hand is the first place to look. There is apparently unequal drag (friction) among the wheels, which would definitely cause the unequal output speeds, especially at low speeds where the drag is a larger fraction of the total output of the motors. Check for misalignment in the various shafts, bearings, and gears. If you are unable to completely remove these differences and don't have enough time, energy, or expertise for encoders, you will probably want to find settings for each wheel which produces the same output speed when on blocks and driving slowly. Then add the differences in those amplitudes to the ones which require extra voltage. For example, if the four wheels require 10%, 8%, 7%, and 6% to make the same low speed, always add 4% to the amplitude of the first, 2% to the second, and 1% to the third. When driving the wheels backward, you will actually need to subtract these percentages to increase the amplitudes.

[efoote868]

Originally Posted by RichardNixon

we are using vexpro single reduction gearboxes and there is a noticeable difference between the wheels when spun by hand.

What motors / how many are you using per gearbox?

[Snowfire]

Originally Posted by RichardNixon

we are using vexpro single reduction gearboxes and there is a noticeable difference between the wheels when spun by hand.

We had that issue. It's probably is that you need to calibrate your motor controllers or the pinion gears/other gears are different teeth. VEXpro's 12 tooth and 13 tooth pinion gave the same pitch distance, so you might have a mix

Originally Posted by Snowfire

VEXpro's 12 tooth and 13 tooth pinion gave the same pitch distance, so you might have a mix

Correction: The 11T and 12T pinions have the same diameter (.7"OD), and the 13T and 14T pinions share the same diameter as well (.8" OD).

[Mark Sheridan]

Originally Posted by efoote868

This is a common issue, which is why teams will use encoder feedback to vary motor outputs in a control loop, to get each wheel turning at the desired speed set point.

Its actually better to get gyro feedback. You don't need encoder feedback unless it auto. A control loop on each wheel is possible but overkill. The WPI library code for mecanum has a gyro input. the library code is very good, I have lost a wheel before and the robot still drove reasonably well.

RichardNixon, is the speed difference something you noticed driving on a bench or is your robot having trouble driving on the ground. Is it having trouble going straight? Is it rotating too much when your translating?

[Snowfire]

Originally Posted by Andrew Lawrence

Correction: The 11T and 12T pinions have the same diameter (.7"OD), and the 13T and 14T pinions share the same diameter as well (.8" OD).

Thank you!

[Gregor]

Originally Posted by Mark Sheridan

Its actually better to get gyro feedback. You don't need encoder feedback unless it auto. A control loop on each wheel is possible but overkill. The WPI library code for mecanum has a gyro input. the library code is very good, I have lost a wheel before and the robot still drove reasonably well.

Encoders in teleop are still pretty useful if you can do it correctly. They make our movements much sharper.

[Mark Sheridan]

Originally Posted by Gregor

Encoders in teleop are still pretty useful if you can do it correctly. They make our movements much sharper.

In my experience, having tried PD control on mecanum, having the gyro was much more important. I agree that you could get a good control loop on each wheel but I never felt it was a must have feature. I actually got rid of it because I felt it was overkill because it consumed a large amount of resources for a small gain.

You and me probably seen our fair of mecanum drives with no gyros or encoders. To me the gyro is a must have feature for mecanums to operate effectively. I seen so many team struggle with mecanum because they did not realize the importance of using a gyro. Hopefully OP comments more but I am speculating its a control issue.