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Unread 12-03-2006, 14:15
Andrew Schreiber Andrew Schreiber is offline
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Servos to control a door

On our robot this year we are using a door to control feeding to our shooter/elevator. Currently we are using a motor (don't ask me the type I'm just a programmer) with a wheel to control this and it works fine. This is just for my personal knowledge. Does anyone know if a servo would work for this function. It is just to stop the balls from moving through an opening and i know that servos can actually be decently powerful (we use them for shifting on our dewalt transmissions which i sometimes cant move by hand). Now take into account that I am not all that mechanically minded i just know that a servo is lighter than the motor and wheel assembly + relay. One of our mentors suggested it but i don't think the idea got any testing. Is anyone using an approach like this?
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Unread 12-03-2006, 14:24
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Al Skierkiewicz Al Skierkiewicz is offline
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Re: Servos to control a door

Damien,
It isn't that easy to predict if a servo would work where a motor is now the control. In general, the servo has a lot of torque for it's size but it is afterall, all plastic. The gears inside, the shafts, the coupling are all white plastic. They also have a limited range that is much less than 360 degrees.
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Unread 12-03-2006, 14:34
Andrew Schreiber Andrew Schreiber is offline
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Re: Servos to control a door

Thanks for the reply, id have to measure the pressure that a full mag of balls would exert on that point and the figure the torque on the servo to get an exact value. And since im just a programmer they dont let me do much outside of programming and electrical (kidding). We dont need 360 degrees of rotation and because we jus need to put something (a bar even) in the way to stop the balls falling through. From a purely theoretical standpoint how feasible would it be to stop the weight of one ball from rolling thru an opening using a servo which moved a simple bar into position? Ill work the physics out sometime later today to figure out about how much weight all the balls exert on the bottom ball and then figure out a good way of mouting it.
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Unread 12-03-2006, 16:26
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Re: Servos to control a door

we use a servo to drop balls from our hopper into our shooter, however there is a minor problem with the servo tweaking out at the beginning of auto mode, which has caused a ball to drop into the shooter before it spools up
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Unread 12-03-2006, 16:32
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Re: Servos to control a door

ok, we use a gravity fed hopper (see image below) and the door (not on in the pic) is at the base, we use our elevators to bring the ball to our shooter. so we wont have to worry about the wheels not ramping up in time because it takes long enough to get from our elevator base to the shooter. Thanks as this gives me hope that we may be able to lighten things up a bit there...



btw, the left robot shows the area im talking about. the ramp right above our elec box feeds to the elvator and the door goes at the bottom of that (look for the dewalt and its right there)
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Unread 12-03-2006, 18:20
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Re: Servos to control a door

A standard servo, like the one allowed to us in the rule book, is rated as 42 ounce inches of torque.

Honestly, I don't know how they measure it. I am assuming that it is a stall torque at a nominal 5 volts on the output shaft. Adding linkage can increase this, however as Al said, these guys are all plastic (either nylon or delrin). You can also gang two or more servos together on the same task. As some have stated, it can and is being done, but requires a little bit of thought. Easier methods, like using a full size motor or pneumatics come with a weight and complexity penalty, but this is one of those instances where overkill is good for a FIRST robot.

Just a caution- Servos are run off the backup battery. Not only will a low battery inhibit proper camera function, it will weaken servos to. I've found the effect to be pretty profound on R/C cars. When your voltage drops below a certain level, servos become just about worthless. If you depend on servos and the camera, consider opting for a larger backup battery. There is a weight penalty, but it will likely be worth the extra capacity to you.

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Unread 12-03-2006, 18:39
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Re: Servos to control a door

The plastic gears in the servo's can be damaged very easily by shock loads. There are servo's that are built very tough but they are not permitted. Using a large motor for this function probably is a good idea.
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Unread 12-03-2006, 18:54
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Re: Servos to control a door

ok. thanks for the feedback. now i know more about a servo as well as understanding why they did what they did.
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Unread 13-03-2006, 15:36
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Re: Servos to control a door

thinking it over, is it legal to modify the servos to use metal gears (difficult im betting) because i know it is legal to modify transmissions (or redesign entirely) so I would guess that it is the same basic concept. this would make them more shock resistant (or so i think). This is just me thinking and probably wont ever happen.
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Unread 14-03-2006, 07:38
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Re: Servos to control a door

You may not modify the servos.
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