My team is probably going to yell at me for this, because I designed it with the input of only one other team member. Having been enthralled by AM products in the past, I designed one into my gearbox. Having never used the fisherprice, I was hoping I could get some feedback on them. Is this use sane? I’ve heard of the heat issues, so I’m designing a heat-sink at the same time.
Output speed for a 4 wheel is 4fps and 12fps. Bolts to frame, directly driving the middle wheel. Questions and comments welcome, especially if you’ve used an FP this way.
I know that both 968, and 254 used the Fisher Price motor with the AM planetary gearbox in their 06 drive. They had to replace quite a few FP motors in this setup, but my understanding is that this was due to a problem with the AM gearbox which has been fixed in the newer version.
I imagine it would work fine. Recently, I was working on a plan to mod AndyMark 2-speeds to add FP w/ AM planetary, in addition to the 2 CIMS; when I “ran the numbers” (ie used JVN’s old calculator to model the combiner), the load was perfectly acceptable. My gearing plan slightly different (faster low gear, slower high), but the predicted FP current draw to slip the wheels was 13 amps in low gear and 32 in high. I wouldn’t want to get into an extended pushing match in high, but the motor should last fine if you try to only push in low.
We usually put a FP in addition to the two small CIMs. No problems with overheating, but be very careful to wire it correctly. We accidentally wired the practice ones backwards, and as the FP isn’t powerful enough to overcome the two CIMs we didn’t notice until they were both burned out in a blaze of magic smoke and smell.
Where you might have problems with the FP motor is under stall conditions, my understanding is that they smoke if they have power applied and aren’t able to turn because of too much of a load being applied (ie. good traction, and an immovable object in the way). The armature has low thermal mass, and heats up real quickly…no way to put a heat sink on it either, that I know of.
I’m curious as to why you chose to use 3 motors per side.
You’re going to be traction limited in low gear at 4 FPS, whether you use 2 motors per side, or 3.
You aren’t going to push anyone in high gear anyways, so the only benefit from an extra motor I see is slightly lower current draw (I assume you’re dropping the center wheel, in which case it doesn’t even matter).
Seems like unnecessary effort, and extra weight, for not much of a tangible benefit.
Actually, this is designed for a tank track. We’re not really traction limited with around 60 inches square of red linatex on the ground. (I haven’t really taken physics yet, so correct me if that’s flawed. A discussion with a mentor yielded that answer). There’s no middle wheel to be lowered. As for not pushing someone in high gear, you never really know when you’re going to get defended against, and not be practiced enough to downshift before the pushing begins.
To my understanding, that shouldn’t matter. Friction (which determines your traction) does not really change based on surface area. There was a discussion about this in the Technical Discussion forum a few weeks ago. I think the linatex has a higher coefficient of friction (compared to the rubber gum or blue nitrile), which would result in a slightly higher available friction force, which would result in being able to push somewhat stronger. However, I don’t think that the added torque from the third motor would really make a difference, as with the gear ratio already in place, your maximal torque would already be well above the maximal traction you can get.
Your conjecture is, in fact, flawed. On an ideal surface, friction and surface area are unrelated. FIRST carpet is not an ideal surface; however, I have never seen any conclusive evidence to support the idea that surface area and friction on it are related. (Unless you have real rigid ‘spiky’ traction materials, like 71’s 2002 file cards or the old Technokat tread designs, but those have been illegal since 2003?, so that isn’t really relevant)
Since red linatex has a mu of 1.6 instead of the mu of 1.3 that I did my earlier calcs with, I need to revise what I said. Most definitely, do not use this design if you intend to push in high gear; the FP will be drawing a bit over 41A, which is 65% of a FP’s stall current. I do not think that this will last too well. You may not get a sudden smoking of the FP (well, if you push a lot in high you will, but not if it is just a tad here and there), but I think you will probably slowly loose power as you melt insulation on the windings, which causes windings to short out, reducing power. In low gear, you are still fine; the FPs only draw 15A. If I intended to use a gear for pushing, I would feel comfortable with FPs drawing 16A at the traction limit, but not much more than that.
On a sidenote: when trying to do drivetrain calculations, has anyone else been getting 403 forbidden errors when trying to use Google calculator due to their query looking like something malware would produce.
It’s been proven time and time again that surface area does not affect traction.
One case in which it would is like someone said with Beatty’s file cards (or velcro, etc). If you have something actually interlocking with the fibers of the carpet, it would take more force to displace you.
Since those treads are totally smooth, with no raised tread profile, you can’t even mimic that interlocking action between carpet fibers and your tread material.
P.S. Without a dropped idler on your treads, I think you may find turning in high gear to be pretty poor. Practically every successful treaded robot I’ve seen lately drops the center idler to shorten the effective wheelbase.
Coefficent of friction is not static value with rubber. As the weight per square inch decreases the coefficent of friction actually goes up. This was proven by several different experiments that are posted in the thread that you mentioned. So assumeing 1.6mu is fine for baseline calculations but will vary depending on how you implement it. The mu vlaue could increase by 20%or more depending on implementation.
So the euqation is:
mu*Normal force=“Tractive Force”
If you normal force stays the same but your mu goes up by 20% you will see a 20-30% greater tracitve force thus
1.2mu*Normal Force = 1.2Tractive Force
(I use the term tractive force to mean the force that you must apply to break staic friction and start relative motion between the ineracting materials.)
Just so you know manual labor the outback tracks have a pivot wheel so you don’t get the full length of engagement on the tracks. So change 60 inches of engagement to about 30 inches but it is still 5 times the engagment of a 6wd robot.
I hope some day to do an exhaustive experiment to settle this question once and for all but I don’t have the time or the money right now.
I think it’s an important distinction here…If you design a mechanism based on an assumption, you may end up finding out later that you didn’t need 6 motors, and could have saved time, weight, and effort by just using 4, for example.
It’s my personal opinion that 6 motor drives have a very small niche, and are almost never a better choice than 4 motors. The only reason I would ever use 6 motors is if I was making a single speed drivetrain and wanted to keep my speed high, while still having some torque (and I probably wouldn’t, even then).
Oh, no worries. With motors, I recall something around 10 (I don’t actually have the CAD right now). Plus, if you’re building tank tracks, your primary focus is probably defense, so you might as well put as much power to the ground as you can get.
I’m hoping people will comment here, as this thread has the more updated version of this on it’s intended mechanism.
Last year we used the track system that is modeled, our tracks NEVER slipped against the carpet, we have even pulled a van while I sat down in front of the robot, video is on google video. I never saw our track slip so the extra power might actually be helpful.