Problem with KitBot: Motors move at different speeds

[PaulEleven]

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?

[dradel]

Have you tried calibrating the speed controllers?

[mastachyra]

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.

[Sohaib]

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.

[Alan Anderson]

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.

[GeeTwo]

Originally Posted by PaulEleven

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?

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.

[PaulEleven]

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.

[Alan Anderson]

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.

[Billfred]

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.

[PaulEleven]

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

[BobbyVanNess]

Originally Posted by PaulEleven

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

From the Talon manual:

Calibration
The calibration procedure takes the minimum, maximum and center
values of the PWM input signal and scales the output based on these
values. Calibrating the Talon will allow full range of control with PWM
signals that are not within the default range. Calibrating will also
correct any non-center issues with input devices such as gamepads
or joysticks.
To calibrate the Talon:
1. Press and hold the button labeled “CAL” with a paper clip. The
LED should begin to blink red/green.
2. Continue to keep the button pressed while moving the joystick
full forward and full reverse. You may do this as many times as
you like.
3. Center the joystick and then release the CAL button.
4. If calibration was successful, the LED will blink green several
times. If the LED blinks red several times, the calibration was
not valid. If this happens, the Talon will use the last valid
calibration values.
All calibration values are retained after power cycle or reset.

Found that here. Hope that helps.

[GeeTwo]

Originally Posted by PaulEleven

The problem is more obvious when it's propped up on blocks

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

[PaulEleven]

Originally Posted by bobby5150

From the Talon manual:







Found that here. Hope that helps.

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

[Alan Anderson]

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?

[PaulEleven]

Originally Posted by Alan Anderson

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?

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.