2 motor aren't synchronized

I have built a robot car with rectangular base and 4 wheels. 2 big wheels at the back of of car are operated by 2 different motors. And 2 small wheels in the front aren’t operated by motors, they are free to move when 2 big wheels are turned.

The problem is that when the robot car is ready to move, the big left wheel always moves and responds a little bit faster than the right one. As a result, it moves in curve-like path instead of straight line.

I have replaced wheels, controller and set one of wheels to less powerful that another. It still moves in a curve-like path, either turn to left or right side.
I have already spend 4-5 hours on this.

So, how can i make it move in nearly or exactly straight line?
Help me please !!!

I moved this thread into the general forum so you can solicit the opinions of the rest of the VEX community. Maybe some of them have some feedback for you.

Some suggestions of things to look at:

*]Frictional drag on one side of the drivetrain will cause it to arc slightly. Try to minimize “drag” on both sides of the robot, or at least try to get both sides to have equal drag.
*]Most DC motors spin at a slightly different speed in FORWARD then REVERSE. If synchronization is critical, make sure both motors are spinning in the same direction.
*]Fix it in software? (Not my forte).

i have re-built 2 sides, it still moves in curve-like path.
How to fix it in software? or should i do something on 2 front wheels (add friction in one side) ?
Also, i find that if friction on 2 front wheels is is very small, it moves more circular.

Give me some suggestions.

Have you tried using a differential and one motor to power the back wheels and making a steering assembly to steer? Or are you making it so it can go one direction and then turn 90 degrees without planning the trun ahead?
Hope this helps some:)

Do you have Optical Shaft Encoders? You can eliminate the drifting with these, found here: http://www.vexrobotics.com/vex-robotics-optical-shaft-encoder-kit.shtml

Due to the fact that parts can’t be machined to perfection, there will always be some difference when using two separate, unlinked motors to drive a vehicle. You’ll never get the motors, drive train, etc to match up and be the same. The “solution” is to live with it, or engineer around it.

If you are set on a “perfect” skid steer solution, you should search the google for “dual differential drive” and have a look. It is a little hard for me to explain, but instead of using 1 motor to drive each side (2 motors total), you use two motors connected by sets of gears (in a specific way) such that 1 motor drives the outputs and the other motor creates a difference between them.

So when motor 1 is going forward and motor 2 is stopped, the vehicle goes in a perfectly straight line. When motor 1 is going forward and motor 2 is going in one direction or another, the bot might skid left or right.

What I don’t recall is… does this limit you to a skid left/right turning only? Or can you still pirouette?

Check whether or not your drivetrain is rubbing anywhere, which can cause substantial friction for these tiny motors. Also, do you have the motors feeding into a Y splitter cable? You could be experiencing signal lag.

hope this helps

Have you tried driving it in tank and arcade styles?

Could you adjust the trim on one side to even it out?:confused:

Some one has suggested adjust the trim on the remote control. This may work. If you look in the manual Appendix E " Transmitter menus" you will find info on how to scale the output from a joystick axis.

You would want to use a linear scale for your application.

You just keep adjusting the scale for the axis that controls the faster motor, in your case it’s the left wheel.
Start at 100% scaling and keep stepping down the value until it drives fairly straight. The manual mentions you can stay in the scale menu while trying out the effect of your changes have so it should be fairly quick. Just make a change, press the joystick (s) hard forward to see what happens if it’s wrong decrease the scale % and try again.

It will probably still have a slight tendency to one side because you can’t get an exact match but it will be a lot better.