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  #11  
Old 05-05-2012, 11:08 AM
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Re: Transmission VS Comprimised single speed?

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Originally Posted by mbrunn View Post
the difference that blowing air on one motor even if you released 100psi from 3 tanks onto a single motor is very insignificant because it is NOT up against the edge as you stated before. there is a pocket of air and it would be difficult to do this without making a hole in the motors for airflow's sake.
Really? Oh when I looked inside it looked like it was on the edge...

This wasn't a realistic suggestion by the way.
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  #12  
Old 05-05-2012, 11:47 AM
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Re: Transmission VS Comprimised single speed?

If we were allowed to drill holes in the motors (nothing electrical, only the casing, and I remember someone asking if they could do this to pass an axle through), you could make a pneumatic chamber out of lexan that would be able to push a good bit of air though the motor casing with pnumatic tubing. Again, I don't know if this is legal or practical, but just an idea. Also, the PTCs trip for a reason, to prevent the motor from drawing too much current. I don't know if messing with that is a great idea.
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  #13  
Old 05-05-2012, 11:58 AM
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Re: Transmission VS Comprimised single speed?

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Originally Posted by magicode View Post
If we were allowed to drill holes in the motors (nothing electrical, only the casing, and I remember someone asking if they could do this to pass an axle through), you could make a pneumatic chamber out of lexan that would be able to push a good bit of air though the motor casing with pnumatic tubing. Again, I don't know if this is legal or practical, but just an idea. Also, the PTCs trip for a reason, to prevent the motor from drawing too much current. I don't know if messing with that is a great idea.
Motors "may NOT be altered from their original state in ANY way" <R15>
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Old 05-05-2012, 02:36 PM
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Re: Transmission VS Comprimised single speed?

Quote:
Originally Posted by magicode View Post
If we were allowed to drill holes in the motors (nothing electrical, only the casing, and I remember someone asking if they could do this to pass an axle through), you could make a pneumatic chamber out of lexan that would be able to push a good bit of air though the motor casing with pnumatic tubing. Again, I don't know if this is legal or practical, but just an idea. Also, the PTCs trip for a reason, to prevent the motor from drawing too much current. I don't know if messing with that is a great idea.
If you don't mind, could you source this? All I have ever seen about modifying motors is <R15> which says that you can't.
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  #15  
Old 05-05-2012, 05:40 PM
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Re: Transmission VS Comprimised single speed?

That's correct, you can't modify the motors. Targeting the cooling at the PTC would be especially silly since it doesn't actually solve the problem of the motor overheating.

Also, on the matter of a transmission giving the motors "time off" to cool down - if your robot is pushing another robot while both robots are stationary then it will be drawing stall current regardless of how low it is geared. If it's pushing another robot at all then it's likely to be drawing high current even when geared low. If you have a high torque setting specifically for pushing other robots then I would expect you to trip breakers more often rather than less - but crucially, you would trip breakers much less often than your opponents. If you don't mind playing a little bit rough and potentially reducing the lifetime of your parts then a normal speed / pushing speed transmission might be very competitive.
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  #16  
Old 05-05-2012, 06:58 PM
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Re: Transmission VS Comprimised single speed?

My differential drive robot that I was driving around at Anaheim had one speed at about 1:5 for speed and another speed at about 1:1 (on 4" wheels). It could drive for about 7 seconds on high-speed before it croaked and we had to wait another 7 seconds until we could do that again.
But if we switched to high-torque after about 5 seconds of high-speed, we were fine forever, and could switch back to speed every once and a while.

If you are pushing and aren't moving, your motors will brown out. And if you're trying to push a robot and you aren't moving, you should give up (unless their time is more valuable than yours). However, the advantage to having a high-torque mode really comes when you can push a robot, that is, when you're facing resistive forces but can push through them.

JVN is right is saying that it can give the motors some time off. Running at a speed that is pushing it in itself for the entire match will surely not be kind to your motors, so some time in low-speed mode could be crucial if you want your robot driving for 2 minutes.
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  #17  
Old 05-05-2012, 07:30 PM
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Re: Transmission VS Comprimised single speed?

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Originally Posted by Ephemeral_Being View Post
If you don't mind, could you source this? All I have ever seen about modifying motors is <R15> which says that you can't.
I never claimed that it was legal to modify the motors. I proposed something that could be done If it was legal. It looks like it is not legal.
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  #18  
Old 05-05-2012, 07:55 PM
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Re: Transmission VS Comprimised single speed?

just remember that simplicity is key. Having a transmission would be cool and all but having something simple that is easy to fix if it breaks.
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  #19  
Old 05-06-2012, 01:36 PM
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Re: Transmission VS Comprimised single speed?

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Originally Posted by Vex Mundi View Post
That's correct, you can't modify the motors. Targeting the cooling at the PTC would be especially silly since it doesn't actually solve the problem of the motor overheating.

Also, on the matter of a transmission giving the motors "time off" to cool down - if your robot is pushing another robot while both robots are stationary then it will be drawing stall current regardless of how low it is geared. If it's pushing another robot at all then it's likely to be drawing high current even when geared low. If you have a high torque setting specifically for pushing other robots then I would expect you to trip breakers more often rather than less - but crucially, you would trip breakers much less often than your opponents.
This is not true.
If your gear ratio is low enough, you'll eventually slip your wheels.

Think of it this way.
If the robot is pushing against a "stationary" object...
The frictional force between the wheels and the floor acts as a brake on the robot's geartrain. You can easily calculate this max frictional force (robot weight * coefficient of friction / number of wheels).

You can back up the gear-train and calculate the load torque this frictional force will apply on the motor.

The motor will apply as much torque as possible (up to it's rated stall torque) to overcome the load -- based on how much load the motor sees determines how fast it spins, and how much current it draws.

So if your robot is geared very slowly, by the time the frictional load is applied on the motor it has been greatly reduced by mechanical advantage, and the motor won't see much load at all, which means it won't draw a lot of current, and can push "all day long" without popping thermal breakers.

If you're tripping breakers too much --- SLOW DOWN YOUR ROBOT.

-John
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  #20  
Old 05-06-2012, 09:31 PM
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Re: Transmission VS Comprimised single speed?

Well, what you can do is try to design a transmission involving the least amount of gears, due to friction. That will lower heat.

As for our team's robot, I think we will try to come up with an effective gear ration and make our bot single speed. We might add in a torque gear with a simple transmission.
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