Encoders on Drive Motors?
 

Hi everybody,
So when we drive our team's prototype chassis, which has 6 wheels and a 2 motor drive, if we run both of the motors full speed straight forward, one side tends to be faster than the other, resulting in the robot trajectory actually curving instead of going straight. I was going to put encoders on both the motors in order to make sure that both sides were running at the same speed, but I don't know the labview code that I would use to do that. Could anybody show me an example of using encoders for that task?
Thanks,
Team #2367

Re: Encoders on Drive Motors?
 

I'm not a programmer, so I can't really help much there. However I do know mechanical pretty well, and a curving drivetrain means that there's something mechanically wrong with your drivetrain.

Get the mechanical guys to fix the problem. Otherwise you will be always reducing power to one side to compensate for the drivetrain problem. With the drive motors disconnected, you will find that one side of the drivetrain requires more effort to move than the other.

That being said, encoders on your drive motors are generally a good idea, especially when you're trying to move a specific distance in autonomous.

Re: Encoders on Drive Motors?
 

I pretty much am the mechanical guy, so what could be the root cause of the problem mechanically? The robot is just a chassis right now, so there isn't much on it except for the motors, wheels and the electronics board.

Re: Encoders on Drive Motors?
 

Chains too tight, gearboxes assembled improperly, bearings binding, wheels misaligned, motor manufacturing tolerances.

Try swapping parts side-to-side until the problem changes sides.

Motors, gearboxes, motor controllers, wheels, you get the idea.

Start with the easiest parts and work your way down the list.

Re: Encoders on Drive Motors?
 

Put it on blocks and see if you can determine if any of the wheels rotate less well then others.

Determine if the frame is warped.

With it on blocks, mark the wheels and set the speed controllers to the same value and do a visual encoder to see if it is the drive wheels or speed controllers that are doing it. Is it only at high speeds, or all the time?

To add encoders, there is at least one example that will get you started, and if nothing else, it will help with diagnosis and measurement of the difference between sides. To find the example, you can do it from the Getting Started window/Support, or you can go to the Help menu/Find Examples and look in the FRC/Robot and Motor section.

Greg McKaskle

Re: Encoders on Drive Motors?
 

A six-wheel drive base will tend to go in whatever direction the frame wants it to when the motors are driven with approximately the same power. I'd definitely look for warping or binding somewhere.

However, I'd also double-check the software to make sure the speed controllers being defined in the code match the ones physically installed. If you're using Talons, you must select Talon from the menu at the bottom of the Open 2 Motor Drive function. If you use (e.g.) Victors while telling the software you're using Jaguars, you'll get asymmetry in motor speeds.

Re: Encoders on Drive Motors?
 

It may also be useful to calibrate the speed controllers. New ones shouldn't need this, but older repurposed ones may have different calibrations.

Greg McKaskle

Re: Encoders on Drive Motors?
 

Thanks for the help! I'll try debugging it tomorrow.