View Full Version : Bump climbing issues
archer003
10-02-2010, 17:12
Hello all. Archer003 from 1982 here.
We have been working on the drive train recently, and we discovered a flaw in our design. When we attempted to climb over the bump, the bot couldn't make it over. We are currently working on attaching some new sprokets to try to gain more torque, but if it doesn't work we need to have our bases covered.
What I need, is some help trying to come up with ways to make our torque better, without using more CIM motors, since we are close to the max 5 as it is.
I included pictures of the front of our bot, the front wheels, and a profile of the current gear box. This is kind of a big portion of our robot, because it is too tall to go through the tunnel, so a quick response would be great. Thanks in advance.
~Archer003
Eugene Fang
10-02-2010, 17:18
You say that you want to change the sprockets to add more torque. Does the robot not climb over because the wheels stop spinning? or because the wheels start slipping? Keep in mind that if the wheels are slipping as you try to climb over, changing sprockets, adding CIMs, or whatever else you do to increase torque won't help.
DonRotolo
10-02-2010, 17:19
Good way to ask a question (rep given). A complete description of the problem, and some decent photos of the situation.
Anyway: It looks like those sprockets on the wheels are kinda small. I can't see exactly how many teeth they have, but if you double the number of teeth, you double the torque (but cut the speed in half).
Try something like that.
archer003
10-02-2010, 17:20
You say that you want to change the sprockets to add more torque. Does the robot not climb over because the wheels stop spinning? or because the wheels start slipping? Keep in mind that if the wheels are slipping as you try to climb over, changing sprockets, adding CIMs, or whatever else you do to increase torque won't help.
The motors stop spinning. Our driver says that it stalls when it hits the bumps, but when we pick up the bot, the wheels spin fine. So we think it is torque, because electrical, and code is all functional.
Eugene Fang
10-02-2010, 17:23
The motors stop spinning. Our driver says that it stalls when it hits the bumps, but when we pick up the bot, the wheels spin fine. So we think it is torque, because electrical, and code is all functional.
Thanks for the clarification. Do you know what your sprocket ratio is from the Toughbox to the wheels? As Don said, the sprockets on the wheels look a bit small.
archer003
10-02-2010, 17:24
Good way to ask a question (rep given). A complete description of the problem, and some decent photos of the situation.
Anyway: It looks like those sprockets on the wheels are kinda small. I can't see exactly how many teeth they have, but if you double the number of teeth, you double the torque (but cut the speed in half).
Try something like that.
Alright, but are there any certain stores/companies you know that could get us the sprockets? We want to avoid buying over the internet, a lot of our parts came in later than they told us, and we need to fix this soon, or we'll be stuck in 1/3 areas for the whole competion.
I agree with Don, nice job on asking the question and I also agree with his suggestion to try a larger gear on the wheel.
I have a question, there is a transmission in the picture, can you tell me which one it is? I think we are all looking to see what the gear ratios to the wheel is.
Eugene Fang
10-02-2010, 17:28
I have a question, there is a transmission in the picture, can you tell me which one it is? I think we are all looking to see what the gear ratios to the wheel is.
I'm pretty sure it's the Toughbox. It has a 12.75:1 ratio. I'd still like to know the sprocket ratio though.
http://www.andymark.biz/am-0145.html
Ice Berg
10-02-2010, 17:31
Another place you may be losing power is in the gearbox/motor setup. If your motors, or some gears are slightly misaligned, this could cause a lot of friction and hurt your power output. I would test the current going through your drive train motors when the wheels are free spinning and compare that the the cim motor cure. If the current is high, this would indicate some resistance in the drive train. We had this issue last year, and a little bit of tweaking made a big difference.
-Seth
archer003
10-02-2010, 17:34
Another place you may be losing power is in the gearbox/motor setup. If your motors, or some gears are slightly misaligned, this could cause a lot of friction and hurt your power output. I would test the current going through your drive train motors when the wheels are free spinning and compare that the the cim motor cure. If the current is high, this would indicate some resistance in the drive train. We had this issue last year, and a little bit of tweaking made a big difference.
-Seth
We considered this, but as I mentioned, we picked up the bot to test the wheels without the bump, and they were functioning fine. I think the motors and gears itself are properly attched.
Ice Berg
10-02-2010, 17:41
Another consideration would be to use fisher price motors along with the Cims. AndyMark sells a good gearbox that takes a fisher price motor and has the same output as a Cim motor (so you can mount it right into the tough box).
http://andymark.biz/am-0347.html
Although the fisher price motors are currently out of stock, you could order the gearbox by itself, or wait a few days for the fisher prices to come in (I called AM yesterday and they estimate having the FPs in by Friday).
Your concerns about shipping are valid, but my team has had good success with AM shipping, so I would say with fast shipping that would be a safe bet.
However, as other people have said, lowering your ratio is probably the best solution, unless there is some other issue associated with that.
Alright, but are there any certain stores/companies you know that could get us the sprockets? We want to avoid buying over the internet, a lot of our parts came in later than they told us, and we need to fix this soon, or we'll be stuck in 1/3 areas for the whole competion.
McMaster-Carr ships very quickly, unless you happen to have a McMaster distributor in the area, in which case you can send someone to pick the items up.
AndyMark might have shipping delays due to inventory shortages (something about 1000 teams all wanting the same item at once, or something like that), but once they ship, they ship quickly.
Also note that COTS items don't count against your withholding allowance (40lb). If they arrive at your shop before you leave for competition, bring them with.
It looks like that is a 10"-12" diameter wheel. To help we would need to know 3 things:
1) the diameter of the wheel
2) the number of teeth on the sprocket on the transmission
3) the number of teeth on the axle sprocket
If you are using the same wheels for all 4, you will probably not be able to turn either.
Something you can do right away is to swap in the KOP wheels and see if it is better.
O'Sancheski
10-02-2010, 19:29
The motors stop spinning. Our driver says that it stalls when it hits the bumps, but when we pick up the bot, the wheels spin fine. So we think it is torque, because electrical, and code is all functional.
you say the CIMs stop spinning... if you have any extra CIMs try swapping them out... they might be fried
archer003
10-02-2010, 20:46
you say the CIMs stop spinning... if you have any extra CIMs try swapping them out... they might be fried
As mentioned, it was fine in mid-air, but on the ground wasn't working, so I dont think its fried.
waialua359
10-02-2010, 21:14
I find it very hard to fry CIMs, without purposely doing so.
However, testing it in air (no load) is very different than with a load.
You should make sure it works perfectly fine.
On the other hand, if they were working fine prior to testing and you find its bad now, a design change may be in order.
Good luck.
DonRotolo
10-02-2010, 21:15
McMaster will get them to you tomorrow if you order by 6 pm today. You can also buy #35 sprockets in lost of other places, like Grainger. Use Google and see what's near, call them and drive there for same day service.
I know you lifted the robot into the air and it *seemed* fine, but we're trying to explain some of the finer points of drivetrains to you.
Measure the motor current with the wheels in the air. It should be only a few Amps. Let us know if you don't know how, happy to explain. (What this tells you is IF the gearboxes really are tight or not. They often are, and need an hour or 2 running time, unloaded, to loosen up. You can lose 25% or more of your power here!!!)
Please, please, tell us the sprocket sizes top & bottom! And how big those wheels are! We are problem-solvers by nature, we just can't help it, but a problem with insufficient data is our version of Hades. Have pity on us, tell us...
longrange
10-02-2010, 21:16
We anticipated a problem like this and changed the sprockets initially. We went with 11 and 32 from McMaster Carr and received them in 2 days. We discovered that putting two 11 tooth gears side by side we had to turn down half of the hub of one sprocket for the chain to clear the hub. Also we had a problem with the alignment of the cim motors as mentioned earlier. We ran the robot elevated and tested each motor in forward and reverse using a power supply to measure the current. After making slight position adjustments to the motors, no motor draws more than 5 amps @12.5 volts.
Also if you adjust your chain tensioners tooooooo tight you will load the system down.
archer003
10-02-2010, 21:27
McMaster will get them to you tomorrow if you order by 6 pm today. You can also buy #35 sprockets in lost of other places, like Grainger. Use Google and see what's near, call them and drive there for same day service.
I know you lifted the robot into the air and it *seemed* fine, but we're trying to explain some of the finer points of drivetrains to you.
Measure the motor current with the wheels in the air. It should be only a few Amps. Let us know if you don't know how, happy to explain. (What this tells you is IF the gearboxes really are tight or not. They often are, and need an hour or 2 running time, unloaded, to loosen up. You can lose 25% or more of your power here!!!)
Please, please, tell us the sprocket sizes top & bottom! And how big those wheels are! We are problem-solvers by nature, we just can't help it, but a problem with insufficient data is our version of Hades. Have pity on us, tell us...
We may have gotten it working today, but we are going to test it tomorrow. We have an assembly, so we were more focused on getting the kinks worked out to drive in the gym, so we will test the adjusted sprockets tomorrow, and I'll get back to you. All of you, thank you so much for the help!
Chris is me
10-02-2010, 21:37
I have a few questions.
Do you know the coefficient of friction of your tires on carpet? Can you calculate that? If your coefficient of friction is around 1, you should have no problems on the bump from that end.
What gear ratio is going through the box?
What happens when you drive your robot against a wall? Like, put your robot against a wall, and try to drive forward. Do the wheels spin in place?
You probably are short on torque. Ideally, when you're not shifting, you want your drive to be traction limited. If you're traction limited, your wheels will slip if you drive against a wall. This will probably require 2 more CIMs (you could use the FPs, but why wouldn't you use 2 more CIMs if you have them?) and an appropriate Toughbox gear ratio. It'll require you to move one or two around from the rest of your robot (you can probably replace them with FPs depending on the application), but it'll let you get over the bump or push a robot without moving at an unacceptably slow pace otherwise.
Valkyries_698
10-02-2010, 21:38
Alright, but are there any certain stores/companies you know that could get us the sprockets? We want to avoid buying over the internet, a lot of our parts came in later than they told us, and we need to fix this soon, or we'll be stuck in 1/3 areas for the whole competion.
http://www.grainger.com They have local stores
http://www.mcmaster.com they can deliver next day if you order by 5pm Mountain time
Ken Streeter
11-02-2010, 00:17
Alright, but are there any certain stores/companies you know that could get us the sprockets? We want to avoid buying over the internet, a lot of our parts came in later than they told us, and we need to fix this soon, or we'll be stuck in 1/3 areas for the whole competion.
Regarding sprockets, my personal favorite supplier for *inexpensive* sprockets is the "Burden Sales Surplus Center" at https://www.surpluscenter.com. With sprocket prices about 25% that of McMaster or MSC, they really can't be beat for teams on a tight budget. The problem I have with getting items from them is that UPS Ground shipping takes quite a few days from their location in Nebraska to our location in New Hampshire. However, with you being in Kansas, standard UPS Ground shipping may only be one day to you!
They usually ship items the day after the order is received, and are great folks to talk to on the phone.
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