Problem with KitBot: Motors move at different speeds
 

Hi y'all,
So we just loaded up an arcade drive program onto the kitbot, the programming is correct and right now when the bot moves forward, the right side spins faster, when it goes backwards the left side moves faster. We thought it might have been the tension between the two plates and we loosened the bolts, but it is still doing this.
The motor controllers are paired up with y cables so we only have a left and right output.
Could it be possible that somehow the cims move faster in one direction comparing to the other?

Have you tried calibrating the speed controllers?

Re: Problem with KitBot: Motors move at different speeds
 

We are having the same problem. We found that one of our motors had a bad connection. Check the voltage on both sides of your motor controllers while the motors are running.

Re: Problem with KitBot: Motors move at different speeds
 

We had a problem with speed controllers malfunctioning last year. Drive the robot forward at full speed (and assuming your using the kit Talon SR Motor Controllers) the light should be solid green when moving forward at full speed. Do this on blocks so you can observe the lights easier. If you have one of the lights on the motor controllers for the left side are different from the rest, your problem is either a connection, or the motor controller itself.

Re: Problem with KitBot: Motors move at different speeds
 

You can do some troubleshooting by only powering one of the motors on each side at a time. Remove the circuit breaker from the power going to the other one. See if one of the motors is turning slower with a given throttle command.

The advice to calibrate the speed controllers is good.

Re: Problem with KitBot: Motors move at different speeds
 

If none of the other things mentioned help (and I expect that they will; all seem more likely than any of these), I have to wonder if somehow there's a drag or misalignment that makes things want to rotate to the left.

The KOP shouldn't have this issue, but the easiest way I can imagine this sort of behavior is if the four wheels that are supposed to make contact with the carpet at any given time aren't in the same plane. Go forward: it'll rock back, put weight right, and you'll get traction on the right. Go back, it'll rock forward, and you'll get traction on the left. I suppose this effect could also be caused by a saddle-shaped floor.


It probably won't make a difference if you're using the KOP wheels that are exactly in the same line, but as we've swapped out for omni dualies, we were careful to make sure that both outboard belts were on one side (rear) and both inboard belts on the other (front) to avoid anything peculiar. Also check that the opposite faces of the chassis are the same length and that the frame corners are square. An off-center COG shouldn't cause behavior like this, but it's worth checking if you're still trying things at this point.

The problem is more obvious when it's propped up on blocks, when its throttle forward the right motor starts moving first and moves faster, throttle backwards the left one moves first and faster.... All motor controllers are with good connection and in sync, will try changing them up one by one.

Re: Problem with KitBot: Motors move at different speeds
 

What kind of speed controllers are you using? With factory calibration, Talons and Jaguars don't have the same neutral point. If your code isn't set up for the proper one you will get the offset effect you are seeing.

Have you calibrated your speed controllers yet? Do that before you try anything else.

Re: Problem with KitBot: Motors move at different speeds
 

And are you using all the same speed controllers? Last year someone wired all of one side with Victor 884s and the other with 888s. They respond differently to inputs (the 888 is more linear), so the robot veered until the electrical people were enlightened.

They are all brand new talons in the KOP, however we haven't calibrated them...... Or learned how to.......

Re: Problem with KitBot: Motors move at different speeds
 

From the Talon manual:

Found that here. Hope that helps.

Re: Problem with KitBot: Motors move at different speeds
 

In that case, ignore my previous post this thread; it was based on possibly uneven loading.

Thanks!!!!! We'll do that today!!

Re: Problem with KitBot: Motors move at different speeds
 

Before you calibrate your Talons, make sure your program is telling the roboRIO to send Talon-compatible signals. What programming language are you using? How are you defining your drive motors?

Re: Problem with KitBot: Motors move at different speeds
 

We're using Java and are importing the WPILib's Talon package, we've paired up the talons for each side with Y cable so ultimately we only have two PWM outputs.

Re: Problem with KitBot: Motors move at different speeds
 

One other item to add. Sorry if I missed it being posted earlier.

Electric motors are designed to be more efficient when operating in a certain direction. The armatures are timed with the magnetic field via the commutator brushes to give more torque/power in the desired rotational direction of operation. When that same motor is operated in reverse it will have less power. Depending on the motor maybe not significantly less but definitely less

When your robot is going forward one set of motors is spinning in their "forward" direction and the other side is going "backwards". When you go in the opposite direction the other side will be less powerful.

This is a common reason to see the issue you're having. Some teams have done a very simple fix to this in software by only allowing one set of motors to go to 85% throttle in one direction and the other set to do the same when in reverse. While not overly scientific it usually does the trick.

Re: Problem with KitBot: Motors move at different speeds
 

PaulEleven - we are having this exact same issue. Talons from KOP, talons selected in labview, code is right.

Did you ever figure it out? If so, how did you fix it?

We did the calibration, but apparently motors do run slightly faster in one direction or the other. We're gonna make do, and our actual competition chassis minimizes the stall tension from the two plates squeezing the wheels

Re: Problem with KitBot: Motors move at different speeds
 

We've seen the same issue every year so far with our kit - pretty much the same thing. Motor spinning in the forward direction will behave a certain way and slightly different when they are spinning in the backward direction. We've seen some gearboxes that would allow both sides to be driven in the same direction and have been considering them... The software solution is still cheaper though.

Re: Problem with KitBot: Motors move at different speeds
 

Still beats the drill motors that used to be included in the kit. Those motors had a much more significant directional bias.