Fast Drives Getting Tired Quickly

VEX Robotics Competition Discussion VRC
#1

So I’m going to talk about drive speeds in terms of m/s cause it’s way easier to compare drive speeds with different wheels and setups (x-drive)

Here’s a good calculator where you can enter wheel radius and rpm to get linear velocity
http://www.endmemo.com/physics/rpmlinear.php

So generally speaking I’ve made relatively slower drives before change up:
1.05 m/s
1.32 m/s
1.50 m/s (only for a short time)

and for change up i’ve tried speeding stuff up:
1.44 m/s
1.55 m/s

It drives very well but after like 10 minutes or less. It just dies, it fairly suddenly slows down considerably. It stops driving straight etc. etc.

I’ve tried fixing it mechanically fixing friction etc. etc. but It free spins pretty well. But it is still a possibility that it is a mechanical problem,

I don’t know about you guys but this has never been a problem for me personally. even during cortex days, it has never been THIS bad. For turning point, our puncher would get tired. For tower takeover, our intakes would get tired but never our drives. We’d always be able to get 30 minutes or more of at least driving around at a time on these lower rpms. So in change up I’m on a new team with different parts and stuff so that’s why I might suspect it might be a motor/electronics thing. This team has probably been using the same motors since turning point likely being fixed in house, while on my old team our coach basically reserved a constant supply of new motors for us which was pretty poggers.

How much drive time do you guys usually get ?

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#2

We have a very special way of building our bases, that has its flaws but never in my years has the drive base gotten exhausted or even warm for that matter. If anything, systems on the robot like a lift, intake, so on will slow down and die before the drive base.

Edit: I’m just trying to answer the question at the bottom, but I am happy to give some clarification if needed.

#3

this season robots are so light, I don’t think your drive should ever stall during practice. it should be able to run as long as it has enough battery, and you don’t like push other robots around or anything like that.

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#4

I guess I should add to mine remembering that robots are light. Our robot this year is pretty heavy, although we haven’t been able to test yet as we don’t have a field. But last year we were about the same weight, shoved a lot of rubber wheel robots wall to wall, and never even had warm motors. I don’t really know how this contributes but I feel it’s reasonable to mention.

#5

i cannot express my appreciation for this statement with words

10 Likes
#6

Does the age of a v5 motor actually affect how fast it burns out?

#7

It shouldn’t. Add some code to monitor motor temperature and perhaps other motor related parameters such as power so you can determine if the issue is in fact with the motors.

8 Likes
#9

Also how light was your robot to get 30 minutes of driving without burning out lol

#10

only robots that are either really quite heavy or that have to push a lot should burn out in 30 minutes of driving. if your robot does, especially this year, you probably have some friction problems in your drive.

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#11

Interesting thing to note, X drives will overheat faster than an equivalent speed tank drive.

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#12

How heavy would you say is “quite heavy”

#13

I’m not super good at quantifying things, but this season I would say like 20 lbs or more is quite heavy. For context, my robot is 15 lbs, and it definitely could weigh less if I optimized it for weight. But I run a 5:3 ratio with 3.25" omnis and I never stall so I don’t think that’s necessary.

it also depends greatly on the gear ratio your drive is at. if you have a 1:1 drive then you really shouldn’t be stalling ever. But if you’ve got a much faster drive, I can see it stalling with a 20 lb robot in under 30 minutes.

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#14

huh thats funny, our robot is 17 lbs holding 3 balls and running 257 rpm on 4" omnis… and yet we still burn out in under 10 minutes. We define “burning out” when the motors hit 50 C. We’ve already taken apart and analyzed our drive for friction several times already. We’ve tried lubing the gears (to no avail). If we freespin the wheels by hand it spins smoothly. If we lift up the robot and let the drive motors just spin normally at max speed, they draw between 0-3 W (to be more specific, theres 2 motors on each side, one would be drawing like 0.1-0.2W while the other would draw 2-3W, I’m assuming its like this because theyre not perfectly in sync when they move and one is playing a slight catchup with the other because if you start and stop it, the motor that’s drawing 2-3W changes randomly). If turning with only one side, both draw 10-11W, and when moving forward and backward its like 4-5W on all of them. At this point I don’t know what else to try.

2 Likes
#15

That’s an interesting data point.

Is it because X-Drives are harder to build sturdy and well aligned (versus tank drive) and they flex more under load resulting in higher friction due to misaligned axles and bearings?

Or, is friction between the omni-wheels and their rollers as well as friction between sides of the omni wheels and chassis’ bearings plays more significant role? (because omnis need to push on chassis with their sides when moving forward)

3 Likes
#16

This that you have stated is all correct.

#17

It is more of a note on how X drives work.
Examples
If we had a 4 motor drive and could magically gear it 1.41:1 for speed. We would have the torque of 2.84 motors accelerating us in the X axis. Given robot with weight W we could calculate the amount of torque required to accelerate (for ease) 1.41 m/s^2 at the wheel and 1.0 m/s^2 at the motor.

Now if we had a 4 motor X drive and rotate our perspective so its a 4 motor + drive. Now we can clearly see that only 2 motors are pointing in each axis. And for accelerating the robot to 1.41m/s^2 each axis must accelerate at 1 m/s^2. We can calculate the amount of torque required for robot with weight W in each axis individually.
Analysis
In terms of the weight of the robot, each of the axis of the + drive are separate. Which means there are 2 motors trying to accelerate weight W to full speed with an X drive and 2.84 trying to accelerate the same weight W to full speed on a tank drive. Imagine if the Y axis motors didnt spin and the X axis motors ran full speed, its a 2 motor tank drive right? How long would you trust a 2 motor tank drive to drive a competitive robot?

In terms of total velocity AND pushing force we can combine the effort of both axis together and do the classic 1.41* but when we are worrying about the robot driving its own weight around, they dont get any help from each other.

18 Likes
#18

that’s very insightful, thanks for sharing tabor. always wondered why they had to be so light (aside from the obvious 1.41)

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#19

our tp and tt bots were very very light

#20

how ever i haven’t been able to weigh my cu bot yet

#21

How?

We have the same drive as you (it weighs 14 lbs) yet it burns out after 15-20 minutes of drive calm drive (no pushing or anything). How are you able to keep the drive motors so cool?