TriTrack power issue

Lynxmotion Rovers, Sumo, and Tracked
1

Hi All,

I purchased a tri-track, assembled it naked, and found the performance to be beyond sluggish - about 3"/sec and impossible to rotate @ 7.2v.

I swapped in a much beefier set of gear-motors (Banebots 28020-385) making almost 200oz-in of torque @ 7.2v with gobs of RPM.

So that ran the naked base pretty well, but they were overloading a Sabertooth 2x10, so I subbed an RS80 (80A/channel continuous).

So, all is said and built, weight totals out at 6lb 1.3oz.

At 7.2v on the test bench, it’s nearly impossible to stall the motors by hand from the sprocket, the sprockets will suck my hand in and proceed to shred it. That should be a good sign. :wink: At 7.2v on the test bench, the tracks spin well enough, forward/back/mix-turn. Put it on the back of Melamine and it will move forward at about 1"/sec, and won’t turn at all. Motors are relatively hot, I can still wrap my hand around them and hold them, but they’re certainly stalling. Smell pretty stalled after a few minutes too.

At 9.6v, the 'bot will operate, very speedy forward and back, but after a few minutes of turning left/right, the motors get very hot and definitely have that “I’m gonna let my magic smoke out if you keep it up” smell.

Of course the 80A/chan motor controller is cool as a cucumber and couldn’t care less.

So I’m kinda stymied. The tracks seem to be… tracking … straight. They do ride on the plastic inner and outer shield instead of the bushings which seems a bit odd - and that leads me to suspect a friction problem.

Is this normal positioning or is that out-of-spec? It rides up at the peak of the triangle and the front bend on the plastic instead of the bushing… It’s like that under load or zero load, and was like that from the moment it was assembled with the original motors.

Any other idea? Yes, it’s a pound heavy, but with that size motors surely that’s not an issue?

[edit: I should probably note: Wire gauge to the motors is more than adequate - 10G stranded, 11" max run length. Batteries are: 4500mAh Tenergy Propel @ 7.2v or 9.6v, dedicated to drive - they’re fresh and take a full charge. I don’t have a ammeter handy that’s large enough to throw in-line, although I’m considering acquiring one if I don’t solve this soon]

TIA for any advice!

http://www.jlrdesigns.com/bruce-lm-small.jpg

2

Here’s a few shots to illustrate:

This is the fit on the top of the triangle - clearly we can see it rides on the plastic, not on the bushing:

http://www.jlrdesigns.com/4lynx/track-fit-top.jpg

Here’s the back of the triangle:

http://www.jlrdesigns.com/4lynx/track-fit-bot-back.jpg

I think this is excessive friction :wink::

http://www.jlrdesigns.com/4lynx/track-fit-wear.jpg

Thoughts? Suggestions?

Tia!

3

Hey adrenalynn,

Nice looking bot! :smiley:

Not surprising really considering the motors are 12vdc, and only getting a little more than half the voltage. :wink:

I suspect the motor modification may have allowed the tracks to make contact at the top of the triangle. None of our in-house test bots show that kind of wear. With the power those motors have, they should easily overcome a little friction from the tracks riding along a 1/8" piece of lexan. I would remove the tracks completely just to rule out any gearbox friction that may exist. I’m a bit perplexed as well…

4

Hi Jim,

Thank you for the quick reply and thoughts! And the kind words! You haven’t seen the super-cool part - the body-worn wireless arm controller with flex sensor on the fingers to control the doors of the box. :slight_smile:

I didn’t mean to suggest that the original motors were performing under-spec. Alas, given the competition event that this is designed for, I’m limited as to battery voltages permitted. (9.6max)

I always am concerned modifying any track system - they can be really touchy in my experience…

I measured the original motor shaft placement in respect to the hole itself using digital laser/pin calipers (+/- 2/10000ths) and then carefully placed mine. The new measurements are within 1/1000th" all the way around the shaft (maximum deviation is just a smidge over 1/2300th actually) - maybe “paranoid” is a better word… :wink: The shaft is the same length to 1/100th", cut on the scroll saw with slow speed and lots of foaming cutting lube/oil, so there’s no heat discoloration that would lead to suspect the gear lube is cooked. Given the tolerance on the sprocket and hub (not to mention the bearings riding in lexan) - I believe it’s excessively within tolerance.

Pulling the tracks and running the sprocket alone, I measure 148oz-in @ 7.2v @ Stall using my not-so-super-hyper-trick, hand-held hanging digital fish scale. :wink: Obviously my fingers will give a little to that kind of torque (elbow on the bench), so it’s always going to read a little low. It also deformed my test rig, which probably lowered the max reading a bit too. It read over 230oz-in @ 9.6v and broke my scale. Max RPM reading at 7.2v was 816RPM and 1062RPM @ 9.6v no load (laser digital tach with reflectors placed on the outer-most tips of the sprocket teeth).

I really need to suck it up and spring for a big ammeter, my bench Fluke will only read to 10A and my 20A handheld costs more for a new fuse than the meter is worth. :wink:

I’m still at a loss to explain this. The only thing I can think of is that the motors are breaking down under high load. The tracks don’t seem to deform with the 6lb total finished load, and there is no detectable hot-spots on the sprockets under even 12x magnification. I hung the motor controller on some 750watt @ 24v motors, and it loads them up happily and warms up a bunch more - I can’t measure their torque with my fish scale - it’d break the scale and/or my hand… Motor controller is set for unlimited current up to its thermal cutoff and isn’t lighting the over-current or over-temp lights, and the power transistors are reading at room-temp.

I just pulled the batteries off and ran remote cable @ 12G stranded, bringing the load under 5lb total, and it behaves the same. Electronics are on a separate 7.2v battery and circuit entirely.

Sorry for the lengthy posts - I figured more diagnostics info is better than less. :wink:

Thanks again!

[edit] RPM photo - this wasn’t peak, trying to do everything at once and shooting photos is a challenge - but close enough.

http://www.jlrdesigns.com/4lynx/good-rpm.jpg

5

I think I can call this one.

I noticed high-spots in the lexan wall when I was looking at the motor mount. They were burned-in low after the arc and then seemed to bulge out (I’ll go to last nights pics and see if they’re visible there - my camera batteries were dead after the last image I posted)

I got out my favorite Warranty Voider and sanded down the spots where it was making contact, and now it runs around and turns and such at 7.2v.

I’m not overly happy with those BaneBots motors though. I may go looking for a replacement…

6

adrenalynn,

You might try Servo City.

Big selection of 3-12vdc gear head motors.

Some people complain about Servo City service, but I have had no problems with them and have place four orders in the last month.

Just a thought.

Regards,
TCIII

7

Thanks, TCIII!

I’ve had pretty good luck with them historically as well. Some crummy shipping, but…

I think it was simply binding force from friction. No guarantee I didn’t overheat the Banebots that way too. I’ve since sanded down all the high spots and polished the surfaces, and lubricated the link friction points with white lithium/teflon and it goes like, well, greased lightning. :slight_smile: Motors are still too warm, but I think that’s a left-over from the bad-ol’-stalls.

If another set don’t work out, I’ll look to ServoCity for replacements.

Thanks again!

8

If you run motors significantly under voltage under a load, they may tend to act more like big wire wound power resistors instead of motors.

9

Yup, thanks! These motors are rated at 7.2v

Stalling motors causes them to become like not-so-big resistors too. :wink:

10

Hi again,

Have you made any other modifications to the track system? They look awfully tight. On our in-house bot you can lift the tracks off the front of the assembly enough to easily stick a pencil between the tracks and the lexan components. Did you do anything that would affect this? Something is not adding up here…

11

Hi Jim,

That’s good info, thanks!

No, no other modification (other than sanding down the Lexan with a fine sanding block on a Dremel yesterday, which got it rolling). The motor shafts ride in the original holes, and the tracks read (on the fish scale again) nearly identical tension - below a tenth of an ounce - to the original motors/tracks. I first built everything exactly to the instructions so I could take exact measurements.

The sprockets were a couple tenths of a mm out of alignment with the idlers, not enough to matter, but I lined them up to the best of my measurement equipment yesterday. They’re within a hundredth of true, but the Lexan flexes more than that.

The only change I made originally was to add new motor mount holes keeping the shaft in the identical position to the original. And I selected my motors to have the same 6mm shafts.

The Lexan platform itself has only been modified with four new mounting holes for the arm support.

That’s the entirety of the modifications made.

Now that I have sanded down the side panels, I’ll run your pencil test and report back.

Thanks!

12

Ok - mine was too tight still to get a ball-point (couldn’t find a pencil :blush: ) in there. I pulled the sides off and put them on the mill and machined off exactly .0625" (1/16th) around each radius.

Now it looks like this:

http://www.jlrdesigns.com/4lynx/track-fit2-001.jpg
http://www.jlrdesigns.com/4lynx/track-fit2-002.jpg
http://www.jlrdesigns.com/4lynx/track-fit2-003.jpg
http://www.jlrdesigns.com/4lynx/track-fit2-004.jpg
http://www.jlrdesigns.com/4lynx/track-fit2-005.jpg

You know - I wonder if the laser-cutter just wasn’t having a bad calibration hair-day…

13

No the laser is pretty much dead on accurate. It’s like a stepper controlled plotter and there is no calibration. I grabbed a side panel from the shelf that was just made yesterday and compared it to the in-house bot which has panels that were cut when I designed the bot. They are exactly the same. I had a set of treads built and tested them to ensure they didn’t change. The sprockets are also injection molded so they didn’t change. I even tested the nylon bushings and standoffs… I’ve compared and tested everything I can. Your motor mod must have increased the tension on the tracks. I can find no other explanation. Bear in mind 20 thousandths off can make a BIG difference. Here is an image of our stock TriTrack Chassis to illustrate the clearance it has on the tracks…

14

Thank you for the effort and illustration!

Taking the material off the sidewall seems to have resolved the issue.

The max error for the new shaft placement was 1/2,300th, or 10/23,000th rather than 20/1,000th or 2/100th.

Regardless, mine aren’t quite that loose yet (nor were they with the stock motors), so I’m going to loosen 'em up a little more - but I do think it’s resolved, and the motors are behaving pretty happily. I’m encoding a video of it performing if you’d like to peek at it. No general publication until after I disassemble everything, turn it into a “kit”, let the kids build it, modify it a bit, practice, and then compete it in March. :slight_smile:

15

I agree it’s resolved somewhat, but I’m at a loss. At this point I just wanted to figure it out. Can you measure the distance from the front passive sprocket to the rear passive sprocket, closest edge to closest edge. It should be 6.935" minus the lasers kerf.

16

That’s sprocket to sprocket outside minus teeth? Let me take the micrometer up there, or I guess I should bring the 'bot down to the lab instead. :slight_smile:

Ok - I see what you’re looking for. Who stole my danged imperial hex wrenches?!?

17

Hmm, I get no where even in that neighborhood for inside edge to inside edge of sprocket. I must be misunderstanding - let me shoot a photo - sorry I don’t understand what measurement you’re looking for! :blush:

As you can see from this photo, nothing about the passive idler sprockets is in any way modified or changed from the stock build.

http://www.jlrdesigns.com/4lynx/sprocket-measure1.jpg

18

Um, did I write this? lol

What I meant was measure from the holes where the passive sprocket bearings go. :blush: Doh!.. I thought that would be easier than trying to measure from the center of the holes. Sorry for the confusing post.

19

Hi Jim,

No problem! I was wondering how the kerf played into what I thought you wanted measured. Now it makes more sense - that’s why I use photos. :wink:

Alas, it will need to wait a few days. Professionally I often work on very very precise components. I can measure to a fraction of a wavelength of light - but only up to 12cm. I can measure to the 10,000th inch with reasonable accuracy, but only up to 6". Anything bigger than that and I’m usin’ a ruler, which doesn’t really help us here. My laser calipers will do 8", but there’s not really anything good to put 'em against in that measurement.

I ordered a 16" micrometer but it won’t be here until next week. Apologies for the delay!

(Hmm. My Hilti laser will do 1/64th inch out to 600+ft. Wonder if I can get a measurement there…)

20

I’ve got the same problem. I should also mention that my motors are pulsing, they’re not spinning smoothly. I know this my sound stupid…So I just sand off the corners down a bit?.

(I’m using BS2 on BotBoard 2 with 2 Parallax HB-25 motor controllers and standard Tri-Track motors, running on 12V DC)

Please reply! :smiley: