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Omni-wheel accuracy
I am currently using Robot C to program with a tetrix kit. We have a base drive train with omniwheels and is setup like so:
/------\ | | | | \------/ I already have the code for telop written but in testing the motors have a big problem with rolling. When the bot goes diagonally, the motors slow to a stop at random (not every time) rather than just stopping. Also, occasionally, when the train is rotated, two motors stop and two motors roll to make it roll about an extra 45 degrees. Note that this occurs with different motors every time. Does anyone have any ideas on how to compensate for this? Message me if you want to see the code, I don't want it leaking out everywhere. Note: this is an FTC robot. |
Re: Omni-wheel accuracy
Lift the bot off the ground and tell us what the wheels do when you command forward, reverse, rotate clockwise, rotate counterclockwise, strafe left, and strafe right. |
Re: Omni-wheel accuracy
I would suggest ensuring that you have feedback on each of the wheels. On our 3 wheel omni platform we drove it with and without velocity feedback (RobotC has this built in) and the difference was night and day. http://www.youtube.com/watch?v=qobHqf45SLc is before we put feedback on... but here is the one with feedback. http://www.youtube.com/watch?v=Qwg2cgNTAZE
In both cases, disregard the audio tracks they don't really add anything... |
Re: Omni-wheel accuracy
Can you give me an example line of how to enable the feedback?(Too lazy to go dive around for it) I'm assuming encoders are not needed for this...
Thanks! |
Re: Omni-wheel accuracy
If you set the motors to brake instead of coast when your controller axes are within your dead band, that should solve your problem.
I agree that putting encoder feedback on the wheels is nice, but it's expensive ($56 per encoder), the encoders are touchy to get mounted properly, and it takes extra expertise to figure out how to program them once you have the instruments setup. |
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Re: Omni-wheel accuracy
Our team has built 4 wheel omni drives like OP is describing on several occasions, and our experience is that they drive pretty nicely without encoder feedback. Maybe the 3 wheel version is touchier.
Encoders could solve the problem described by the OP. Setting the motors to brake instead of coast can solve it faster and cheaper. Adding encoders has additional benefits such as making the robot drive in a straight line and enabling more accurate autonomous driving. I like instrument feedback. I just don't think it's worth $200 in this case. |
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Edit: my memory is poor. There is a separate VI provided in the FTC Toolkit called "Stop Motors". It's not an option in the run motors VI. |
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I would disagree that simply placing it in brake mode would fix the core problem of motors moving at different speeds. It may fix the majority of it but for our team it was worth the $150 it cost to actually address the issue and to give us accurate autonomous navigation. It was also worth it to teach the lesson of the importance of feedback systems to my students. I guess, my suggestion to OP would be to try the cheap and fast solution and if it meets your needs cool if not move on to using feedback. |
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In this specific situation, I don't think the core problem is best described as "motors are moving at different speeds." According to the quote below, sometimes the motors are continuing to run after the driver stops telling the robot to drive, while other times the motors stop right away. That problem as described would go away if the motor was told to stop instead of letting it coast. Quote:
No philosophical disagreements here... like I said, I agree that using encoders is better. |
Re: Omni-wheel accuracy
Try calibrating the joysticks or adding some joystick deadband. Sometimes, even when the joysticks appear to be centred, they may not be giving an output of 0, which causes the motors to slowly turn. We had this problem in Vex, after linearizing the motor speed controller output, and adding some deadband really helped.
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