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Unread 10-26-2009, 09:03 AM
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[OCCRA]: Servo issue at competition

At the competition this weekend we had an issue with the servo that helps us score. The servo is attached to a rotating arm that pushes against the ball to push it out of our "claw" and into the basket.

When we practice at home and when we were practicing for most of the competition the servo had no issues as long as we didn't run it to the point where it overheated. In our last match, the servo moved once back, and then stoped responding, preventing us from scoring a point which lead to us losing the match. Afterwards when we tested the VEX in the pits, the servo would sometimes only move once in each direction and then stop responding. Other times, it would keep working.

Does anyone have any idea of what may have caused this problem? If the servo can't work consistantly, then we have to swap a motor in and make a lot of changes.
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Unread 10-26-2009, 12:49 PM
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Re: [OCCRA]: Servo issue at competition

How hot was the servo? The thing that comes instantly to my mind is that it overheated - we had a similar problem, but used rubber bands to solve it (we had the entire ball supported on an arm by the servo). Also, check the wiring. We *had* position control on our elevator, but the wire on the potentiometer keapt falling out. We disabled it in code for that competition, and have fixed it for the next one. If you decide to switch to a motor, you can use a potentiometer to get position control, basically a software servo (more on that if you want). Also, how are you controlling the servo? Buttons? Joystick axis? How much load is on the servo (how big is the arm?) And are you able to replicate the problem? Try running it repetitively and see how long it takes to fail. also, check for mechanical problems like the shaft not in all the way. Also, VEX motors and servos are known to act erratically when they only receive power, maybe the connector is only partially in or a wire is loose? Try coding a joystick axis directly to a servo or pot [pwm05 = PWM_in1; //right X] or [pwm05 = Get_Analog_Value(rc_ana_in01) /4; //First Pot to Servo] and see if it still acts strangely, possible because of a programming problem. How old is the servo, VEX servos and motors have little thin plastic gears that are known to break after a few years of competition. If you have spares (they come with the motors and servos; little white gears in a little plastic bag) you can try that if nothing else works. That's all I have.
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Unread 10-26-2009, 04:53 PM
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Re: [OCCRA]: Servo issue at competition

The servo overheating was our first idea. However, the match where the servo failed was right after lunch, so it had been sitting for at least 25 minutes without being on.

We have been running it a lot to see what went wrong and we can't recreate it. We think it may have been a one time bug. I also have no clue how old the servo is, but its working fine now and I haven't changed a thing. Even the battery was the same as I worked on it this morning, so I don't think it is programming, I think either something somewhere got stuck or the microcontroller decided to mess with us...

We are going to try again this Thursday with minor program and build tweaks and see if it works. Thanks for trying to help.
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