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  #1  
Old 01-11-2012, 08:19 PM
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The VEX Raptors The VEX Raptors is offline
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Strange Wheel Base Issues

Our team has encountered a most unusual problem: during a match, only when plugged into a competition switch, one side of our wheel base stops working. It does not happen during all matches. As far as I can tell, it happens randomly- but ONLY when plugged into a field or competition switch. It isn't an overheating problem; our wheel base consists of four brand-new high-power motors with internal torque gearing and we have intentionally strained the robot when not plugged into a field and it had no such issue.

Our code is the standard Competition Template, and the software experts in Michigan State University's robotics program have told us they don't know of any code problem that could stop only two motors. Our lift motors have never done this.

Our wheel motors are wired through a power expander. We know there is no problem with the motors or their wires, because we had the problem at our first tournament with older motors and before our second tournament we replaced them with new ones. We think the power expander may be to blame.

Have any other teams had this problem? If so, what was the cause, and how did you fix it?
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Old 01-11-2012, 08:51 PM
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Re: Strange Wheel Base Issues

Quote:
Originally Posted by The VEX Raptors View Post
<snip> We have eliminated the following causes: Overheating, motor damage, and loose wires. <snip>
How have you eliminated overheating?
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Old 01-11-2012, 09:03 PM
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jpearman jpearman is offline
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Re: Strange Wheel Base Issues

It would help if you could post more comprehensive details as I outlined in this post yesterday.

It's almost impossible that the field control or competition switch can affect only two motors. Have you run the robot on the field tiles outside of a competition? On several occasions I have seen a design that when faced with the different friction of the field tiles, as compared to say carpet, had issues with the drive. This was usually due to lack of rigidity so that when the robot was turning the motors were working to hard. Do you have any pictures of your drive you could post?
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Old 01-12-2012, 06:38 AM
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Re: Strange Wheel Base Issues

After a match when it stops, we can unplug it from the switch and drive it off of the field. Also we have had numerous practice meetings where it preformed perfectly on the tiles without a competition switch.
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Old 01-12-2012, 07:28 AM
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mediumdave mediumdave is offline
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Re: Strange Wheel Base Issues

Perhaps the breakers in the extender are, ahem, breaking, but unplugging the cord gives your robot just enough time to cool down and begin working again. We had a similar problem in the finals matches...
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Old 01-12-2012, 01:16 PM
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Re: Strange Wheel Base Issues

I believe a power expander has 4A max capacity,
whereas a Cortex has 4A for ports 1-5 and 4A for ports 6-10.
So putting all 4 wheels on one power expander doesn't maximize this current bottleneck.
If you think the power expander is the problem, have you tried not using it?
You don't think the motors themselves are the problem, even though they are new (and untested)?

Correlation is not perfectly linked to Causation;
there may be other common factors that you don't recognize and list.

I would try using the scientific method, (form a testable hypothesis...) and see if that helps you debug the problem. There are lots of other email threads with similar problems, suggestions, and results of experimenting, if you know how to use the search features of the forum.
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Old 01-12-2012, 01:20 PM
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Re: Strange Wheel Base Issues

Quote:
Originally Posted by The VEX Raptors View Post
Also we have had numerous practice meetings where it preformed perfectly on the tiles without a competition switch.
Drive the robot around at your meeting for upwards of 15 minutes straight (or similar large number of minutes) without stopping once. This will help determine if the motors are burning out...

//Andrew
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Old 01-12-2012, 03:05 PM
Hen Janko Hen Janko is offline
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Re: Strange Wheel Base Issues

you can do what Andrew said or it might be just the extenders malfunctioning.
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Old 01-12-2012, 03:22 PM
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Re: Strange Wheel Base Issues

Thank you all for your suggestions. We will experiment with bypassing or maybe replacing the power expander.

This is just so infuriating because in the finals we couldn't do anything but drive in circles.

I have no pictures of the drive base to post, so I will try to explain it as best I can:
There are four high-powered motors geared for torque; two on each side.
On each side, there are three wheels driven with five gears (similar to the protobot, where two motors drive four wheels), with the motors driving the gears that are between the wheels. This puts more motor power to the ground.

I will post more updates when we do some experiments at our meeting tomorrow. Thanks again for your suggestions!
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Old 01-13-2012, 12:35 PM
jgraber jgraber is offline
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Re: Strange Wheel Base Issues

Quote:
Originally Posted by The VEX Raptors View Post
There are four high-powered motors geared for torque; two on each side.
On each side, there are three wheels driven with five gears (similar to the protobot, where two motors drive four wheels), with the motors driving the gears that are between the wheels.
Exactly what kind of wheels? 2.75"? 4"? Omni vs hi trac vs bare plastic?

Turning circles on vex foam,
at a competition where anti-static spray has been used on the foam,
requires much more torque from the motors than straight driving on hard-tile.
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