Chief Delphi - [FTC]: Drive Platform- Design Exercise

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Chief Delphi (http://www.chiefdelphi.com/forums/index.php)

- FIRST Tech Challenge (http://www.chiefdelphi.com/forums/forumdisplay.php?f=146)

- - [FTC]: Drive Platform- Design Exercise (http://www.chiefdelphi.com/forums/showthread.php?t=137636)

DavisDad

25-10-2015 11:20

Re: [FTC]: Drive Platform- Design Exercise

We added about 7 lbs to prototype and tested (with a fully charged battery) and the prototype had enough power to climb. See link to video below; sorry for wrong orientation.
https://www.youtube.com/watch?v=Kt9V27x_JaA
Butt.. now the modified cogs fold down and loose grip. We're working on a better profile for grinding the cogs that will be stronger in bending:

DavisDad

25-10-2015 19:23

Re: [FTC]: Drive Platform- Design Exercise

Quote:

Originally Posted by DavisDad on FTC Q&A forum

Hello,

The 10-08-2015 Q&A post (#39) in the "Robot Inspection and Build Rules" thread defines how tires can be tested for passing inspection. We have tested a cogged drive belt idea and the prototype design failed the test requirement: "run the wheels at full power for 15 seconds". The wheels spun and abraded the surface of the tile. See video of test: https://youtu.be/-63vg4V7UfM

Question: May we limit the power of the wheels in programming so that static friction force is not exceeded by the torque from the motor and the wheels do not spin?Is "full power" 100% of motor controller available power, or 100% of game controller command to robot.

a: If there is any possibility that the motor will run at 100% power during either autonomous or tele-operated mode, then the tread must be tested at 100% power. It is a violation of Gracious Professionalism for a team to run the motors at one power during inspection and at a higher power level during a match. Teams that are caught doing this will be disqualified from the event.

I guess they think we work for VW ;)

DavisDad

27-10-2015 05:39

Re: [FTC]: Drive Platform- Design Exercise

I've been thinking about the Q&A response about limiting the wheel spin and how we might design/code the bot to ensure that the field is not damaged. In my engineering job, there's a formal method of evaluating how systems can fail: "Failure Modes and Effects Analysis (FMEA)". I'm thinking we analyze, test and documented our work with this engineering tool. A team could present the work to the inspectors.

Here's the next prototype wheel profile we plan to test for gripping the churros:

DavisDad

01-11-2015 10:11

Re: [FTC]: Drive Platform- Design Exercise

We got the prototype to climb the 50 deg section: Test-5 YouTube Video

We did the following to modify cogged belt for optimize profile to engage with churros:

Made a grinding wheel with optimized profile. This was interesting as we stacked 4 1/16" grinding wheels and shaped the negative profile with diamond grinding blade.
Made a jig to hold grinder and wheel on mill. This allowed us to index the wheel on every other cog and raise/lower the grinder with pretty accurate x-axis location and good control of the z-axis (up/down) grinding travel.
Ground each profile. Removed every-other cog and small vertical cut on adjacent cogs.
Cleaned each cogged belt to remove gummy residue. Grinding makes a gummy, tarry residue (burned rubber) which needs to be removed from belt. We used mineral spirits and then TSP to remove the mineral spirits.

DavisDad

01-11-2015 10:37

Re: [FTC]: Drive Platform- Design Exercise

Review of prototype performance to date:

Successful climb of the 30 and 50 deg zones requires very precise design of wheels
Center of gravity (COG) will need to be controlled. At 50 deg zone, the COG needs to be at the front axle to have equal force on front and back wheels.
The wheels need to be controlled to a slow rotation to keep the wheels' contact with the churros in static friction; too fast causes slip (dynamic friction) and chatter (bounces on the cogs). We had issues with the motor controllers maintaining full power at the slow speeds and high torque; when the battery wasn't fully charged, the back motor would loose torque.
This prototype only models half the drive platform and does not account for potential problems with left/right wheels being out of alignment.
The cogged belt failed the sSoftTile damage test. It is possible to use current design if we could prove to the inspector we have controls to prevent wheel slippage on level ground. But... this would be difficult to do in software and we couldn't be sure the inspectors would pass the design.

I'm thinking this design is too risky to pursue for FTC 2016. We're going to continue developing the platform as part of this "design exercise", but do not recommend this design for competition build.

Next steps:

Increase gear ratio from 2:1 to 3:1 by using a larger wheel pulley.
Build other half of the drive platform to see how the whole design performs.
With whole platform, test different controls set-ups for tele-op driving and autonomous climbs.

wgardner

01-11-2015 10:57

Re: [FTC]: Drive Platform- Design Exercise

I'm looking forward to seeing how it works with both sides built! Thanks for continuing to share!

DavisDad

12-11-2015 18:10

Re: [FTC]: Drive Platform- Design Exercise

Modern Robotics is selling a new version of the Matrix 12V motor: 12v 6mm Motor Kit

This fixes the only thing I don't like about the Matrix motor; 4mm shaft requires pretty high precision machining of couplings and we've had issues with the set-screws loosening. 6mm (just under 1/4") shaft has more meat and is now compatible w/ Tetrix and AM gear. Now adapting to 1/8"square shafts will be a lot easier: drill a rod half way 6mm and other half with No. 30 drill size, then broach through No. 30 hole.

I prefer the Matrix planetary gears to the others' spur gear designs.

DavisDad

15-11-2015 12:43

Re: [FTC]: Drive Platform- Design Exercise

We've completed the first whole prototype chassis build. We increased the pulley" gear ratios from 2:2 to 3:1. Now ready to mount electronics.

Greg Needel

15-11-2015 16:22

Re: [FTC]: Drive Platform- Design Exercise

Quote:

Originally Posted by DavisDad (Post 1504840)

Modern Robotics is selling a new version of the Matrix 12V motor: 12v 6mm Motor Kit

Is this a legal motor for use in FTC? It does not seem to be listed in the game manual on the approved motor list.

DavisDad

15-11-2015 17:48

Re: [FTC]: Drive Platform- Design Exercise

Probably not legal. Maybe next year...

DavisDad

15-11-2015 17:52

Re: [FTC]: Drive Platform- Design Exercise

19 lbs

DavisDad

17-11-2015 18:29

Re: [FTC]: Drive Platform- Design Exercise

We won't be able to test on mountain for a while; don't want to distract the team with this "design exercise". Here's a video running it around the house. Need to glue the fan belts on AM wheels; they work their way off when pivoting.

It has better turning than I expected; not much chatter.

https://youtu.be/h68eNyzMy44

RRLedford

20-11-2015 15:37

Re: [FTC]: Drive Platform- Design Exercise

Quote:

Originally Posted by DavisDad (Post 1505935)

I highly recommend that you use black Shoe Goo urethane adhesive to glue the treads to the wheels. It has extremely high peel strength, and bonds exceptional well to most materials. We use it in FRC for the attachment of tread to hubs and have now ceased using any rivets at all, since it bonds so aggressively and durably. Multiple years and no failures

Beware though that it is solvent based and needs at a lot of time to dry and cure. I suggest at least 48 hours, or even more if the path for solvent to escape from is long.

DavisDad

21-11-2015 12:35

Re: [FTC]: Drive Platform- Design Exercise

Quote:

Originally Posted by RRLedford (Post 1506761)

I highly recommend that you use black Shoe Goo urethane adhesive ...

Thanks RRLedford- I've ordered some. I haven't used it, but it sounds like just the right stuff. Thanks for the advice!

DavisDad

22-11-2015 09:51

Re: [FTC]: Drive Platform- Design Exercise

The next step is to develop a controls design strategy. Since were're only designing the drive platform, with little consideration of other game functions, we're looking at these requirements:

We must address the wheel's damage to the SoftTiles as required by FTC test: "run the wheels at full power for 15 seconds". We'll need to have controls that prevent the wheels from spinning on the tiles.
In order to score the maximum autonomous climbing points, we'll need to navigate accurately to the high zone.
We'll need to be able to drive through debris (balls & blocks) in autonomous while maintaining our course to the high zone.

For effective navigation, we've purchased a sensor that I've been interested in for a while: navX-MXP Robotics Navigation Sensor. This board uses the Invensense MPU-9250. I got interested in this when I saw David Sachs' video: Sensor Fusion on Android Devices: A Revolution in Motion Processing. I'm hoping that with the FTC specific Android software support by Kauai Labs and my son finds the time to help me with JAVA, we can achieve the navigation requirements. The sensor may also be useful for anti-slip control.

I've done the simple programming so far using App Inventor (AI), but will need to use Android Studio with the navX-Micro.

NOTE: I don't think that driving into the high zone is the best approach for the Res-Q game; the most successful teams I've seen (YouTube) are reaching from the low or mid zones and avoiding the difficulties of the high zone. But... my son and I want to bring this prototype design as far as possible to achieve the original design intent.

All times are GMT -5. The time now is 17:58.

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