Hey Guys,
So I’ve had a question lingering in my mind the whole day. Which one is better to do for the Sack Attack competition?
A fully able-bodied transmission with 2 or more speeds or a single gear setup comprimising speed and torque through testing?
The thing is, that a multi-speed transmission takes up quite a bit of weight, slowing the robot down. Also, a transmission can lead to issues and also reduce driver efficiency, since time is crucial, and the time and thinking of changing gears can waste some time (IMO, you should make it easy for the driver to operate the robot to improve your score. Yes, it also does depend on the controller mapping). Also, every time another gear is added to the drivetrain, it reduces the power output to the wheels and creates more friction (This is why cars usually produce less power on the Dyno than their rated HP. Their rated HP is usually at the flywheel. ie, 2010 Ford Mustang GT is rated for 315 HP, but produces 280HP on the dyno.)
On the other hand, a single speed drivetrain saves weight, but leaves you at a comprimise, meaning that if your opponent is a good driver, and can effectively use his multi-speed transmission to his advantage (Assuming he has one), you are done for. Also, if you get stuck due to too much load, you are kinda screwed.
So, I have left myself with 3 questions:
Is it better to have a transmission or a single speed setup with a ratio satisfying both speed and torque?
Also, is there a method of creating a multi-speed drivetrain that reduces weight and driver/maintenance/power hassle?
Do you need a multi-speed drivetrain for the sack attack?
To answer your last question first:
No, of course not.
Robots without transmissions have been winning VEX games for years, and I don’t think that is likely to change.
You will certainly do perfectly well and will encounter minimal difficulties if you go with a single-speed drive.
In 95% of matches in VEX Sack Attack, having a transmission will not have a decisive advantage over single-speed drives. However, there are cases where you are being blocked and cannot push past the other robot where a two-speed drive can save the match. But for the most part, transmissions are just to show off (because they’re cool).
If I were going to make a transmission to put on a competition robot, it would have a “normal” speed that you would use 95% of the time. Then there would be a high-torque speed in case you were getting blocked or had to push your way through some sacks. The speed switcher would be a single differential per side of the drive base that locks one output or the other using pneumatics. It is very light and only adds a few gears to the drive train (albeit bevel gears).
Personally, going with one speed that we’ll get through seeing what works best. We are going to have a hard enough time getting the deisgn we’ve come up with this year to work without trying to throw in a transmission/differential as well.
To me, I would say that a single speed setup would be the best. The transmission system would probably need more than 6 motors for it. You only have 10 motors to spare for the Sack Attack game. A single speed setup is simple, and you would most likely need at most 4 motors. Even though the transmission system looks cool, you would need to risk a lot of the motors for it.
Thanks guys! I had a feeling at the beggining that a single setup would do just fine, but had a doubt due to the possible accumulating weight of sacks being collected! Thanks!
I agree with most of your statements, but want to add one other note…
These days VEX teams are pushing closer and closer to the razor’s edge. Faster and faster drivetrains. Higher and higher current draws on their motors. These high current draws result in thermal breakers tripping.
At least if a team has a low-gear to compliment their “razor’s edge” gear, they can give the motors (and thermal breakers) some time off.
Oh yes this is very true. My team had some rather large issues with the PTCs tripping if we tried to push too much stuff at once. So I guess if you want the top speed of your robot (not pushing anything) at the max gear ratio, you have to have a cool-down option for pushing stuff.
Or instead of a low-gear, you could always blow air onto the PTCs with pneumatics to cool them down
Wait just a minute. Can you explain to me how you would use a Pneumatic to act as a cooling system for the Cortex? I was under the impression it was a closed system that I couldn’t get to.
I’m referring to the PTCs in the 269 and 393 motors. The PTC comes right up against the plastic case of the motor so some gentle air on the case will help to cool it off (not by a lot, but every little bit helps).
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.
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.
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.
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.
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.
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.
