driving straight

[ebarker]

This is a topic that comes up every year.

Assuming that you have taken care of calibrating the Jaguars I will move on to the issue of 'motor bias'.

DC motors have a tendency to run a little faster in one direction than in the reverse direction. It is a nature of the beast.

Since you have at least two motors, one on each side of the robot, and they are turning in opposite directions relative to each other, there is the motor bias problem.

So if one side is turning CW, the other side is CCW. And one of those modes is faster than the other.

And all of this is assuming you have calibrated the Jaguars, and that you didn't build in a drivetrain defect into a transmission, like a draggy failed bearing, etc.

So even with calibrated Jag's, perfectly matched transmission, it will not go perfectly straight.

[Jared Russell]

Drive 20 feet forward. Note which way the robot veered.

Drive 20 feet in reverse. Note which way the robot veered.

If the robot turned the opposite direction in reverse as forwards (e.g. it ended up near the original start point), one side of the drive is faster than the other in both directions. Check tension, motor free speeds, grease, calibration, etc.

If the robot turned the same way forwards and backwards (one side is faster forwards but the other side is faster in reverse), it may just be motor bias. You can (1) double check speed controller calibration, (2) just live with it, or (3) add sensors or other mechanisms to help correct the problem. Usually (1) and (2) will be good enough.

[Alan Anderson]

Originally Posted by ebarker

So even with calibrated Jag's, perfectly matched transmission, it will not go perfectly straight.

I have to disagree with that. We had a drivebase a number of years ago that could go perfectly straight even when only being driven by the motors on one side. If there is nothing causing binding in the drivetrain, the most important factor for how straight a robot will go is the alignment of the wheels.

[Bsteckler]

In addition to calibrating the Jags and checking the motors, you may also want to check your joystick calibration. One joystick could be set to put out a higher PWM value then the other, and thus one set of motors would be driving faster than the other set.

[basicxman]

This is quite common, it's near impossible for your robot to go perfectly straight on it's own. Even if your robot's weight was perfectly distributed the manufacturing process of any of your parts can have an effect.

If you have some encoders around, consider using them to make sure your wheels are traveling the correct distance and if not correcting their speed in code (see: closed-loop control systems).