Tuesday, August 21, 2012

TOBL2: Press Pause

This past weekend my hard drive kicked the bucket. It's kinda my own damn fault. My external hard drive died a while ago, my computer started wheezing a bit in the Spring, and I didn't do anything about it. But I guess that's what you get for moving around all the time. Hopefully it can be recovered at some point, though it will probably be expensive.

Progress on TOBL2 is at a bit of a stand-still while I resolve computer issues (currently booting Ubuntu off a flash drive w/o a hard drive), finish up my summer internship, and transition from Colorado back to Maryland. But don't fret! It's really quite excruciatingly close to being done. Check it out:

Essentially needs code (old code is now gone :/)and some slight mechanical tweaking to get the tensioning spot on. Next time I'll take it apart and show off some of the cool machining I did last week.

It also turns out Maker Faire extended the Maker application deadline (shock) to this Friday, but it doesn't look like I'll be able to take advantage of that. More likely TOBL1 will be used as a placeholder get to get in the door. Can't do much about a hard drive dying so I'm proud of how far I got. I'll press play again after relocation.    

In the meantime debating updating computers a year earlier than anticipated...

Saturday, August 11, 2012

TOBL2: Build Week 2

Sitting down feels nice right now. This week was quite a push to keep TOBL2 on World Maker Faire schedule. If I'm honest, I don't think I'll be able to pull off a fully functioning bot for the August 17th deadline,  because I need to focus on my actually work work. A TOBL2 running the old iPhone controller or maybe a short autonomous program and no servo clutching is definitely still possible. So that's what I'll focus on. But I'm getting ahead of myself, here's where the bot's at after this week.

I tend to stay in the machine shop until the time I need to leave to make up for it coincides with the start of the afternoon mountain rain. Cold, but totally worth it. This week I machined all the mounting blocks which resolve the 90 degree angle between the drivetrain and electronics shelf. Remember the ridiculous custom stainless steel standoffs I made for TOBL1? I suppose these mounting blocks would be the machining analogue to those standoffs. Except machining really small things beats machining stainless any day. By a lot.

Here they are. The two on the left are for the battery tray and the four on the right support the electronics.

The only issue I can spot so far is the taper on the electronics plates from the water jet. The sides of these plates butt up against the motor plates and create a little gap. That's easy to fix.

The motor controller diodes also appear to be impeding one of the servos. That's another easy fix.

Oh yeah, and slightly more troubling issue: the belt appears to be undersized. I need to cut the motor shafts before attaching the outer plates and knowing for sure, but it's looking a bit tight when I just hack it together. The next size up belt has three more teeth so I'll probably invest in a set of those. Don't really need to worry about the slack thanks to the torsion spring tensioners.  

Still left to do (mechanically):

1. Add springs (coming in the mail Monday)
2. Shorten mounting blocks to make way for servo
3. Grind down electronics plates
4. Cut motor shafts, attach outer plates and see if belts too tight (if not skip 5)
5. Replace belts
6. Assemble 
7. Add servo linkage (probably won't worry about that 'til post-deadline)     

It's a bit ridiculous to go through every little mechanical task that needs doing, so ignore that if you like, it's really just a reference for me. This is definitely doable in 5 hour blocks of machining. I'd like to be done with this list by Tuesday, leaving the last three days to wire it back up, get decent code running, and capture the required media for the Maker application. This week we should finally see this thing move! Build week 3 engage.

P.S. Last weekend I moved for legitimately the 5th time this summer. So that's why there's a new backdrop for my pictures every week. I think this is my favorite so far. 

Saturday, August 4, 2012

TOBL2: Build Week 1

Enough waiting around. At the beginning of this week I received some of the last bits and pieces needed to finish TOBL2. Here's what I got:

The parts from the water jet turned out really nicely! Seeing these parts in isolation makes me realize what a bizarre hobby/design sense I have. But, I suppose the endless hours of SolidWorks dimensioning has paid off. The only slight hitch is that Big Blue Saw messed with the dwg file and forgot to include all of the outer plates - those should come in Monday.

Whilst I wait I've had the privilege of full access to an impressively well-equipped machine shop at work. They used to do production in-house so now there's just an overkill shop full of mills, lathes, a wire EDM, a water jet, and so forth. A MechE candy store, if you will. And the guys there are great. Direct quote: "Don't buy any more hardware or material, okay? We have anything you need."  This past week I've been scarfing down lunch and spending the rest of the break in the shop machining.

The first order of business was to clean up those inner plates.

With all those countersinks I tested the motor mounting assembly.

No interference! While playing with this something occurred to me: why the four letter words was I going to use tension springs to tension the belt? The thrust bearings are a perfect place to wrap torsion springs; which is a much cleaner solution. I guess you can't do all your design work in front of a screen.

But before getting too excited a little bit of math is in order. The Hitec HS-45HB feather servos are rated to provide maximum 1N of force for a 1cm arm (i.e. stall torque 1kg-cm). There is an arm on the motor mount too...about 1inch or 2.5cm where the servo will catch it. The motor is about 1.25cm from the point of rotation. On McMaster, the weakest spring that fits around a 5-40 screw is 0.64 in-lbf or 0.072 N-m. Then we do a simple moment balance:

ΣM = 0 = kθ + mgl - FL

θ = (FL-mgl)/k

If we make the safety factor 2, and assume the servo can only output 0.5 N, the spring will overcome the servo at about 1.2°. For safety factor 1 we can get up to 5.2° travel. Somewhere in that range should be enough lift to disengage the belt. If not, coils can be removed to decrease k. Here's a clearer picture of the plan (I realize this is already like plan C):

The next things that need doing are:

1. Clean up the outer plates when they come in
2. Press all the bearings
3. Machine the interfaces between the PCB shelf and inner plates
4. Servo linkage
5. Torsion springs
6. Assemble!

This is the plan for next week. In the background I'll be working on the new controller. I've been learning some python specially for the occasion and am hoping to be competent enough to write a controller that will use keyboard inputs (for now). Another new goal is to have TOBL2 operable enough to get video to submit for World Maker Faire New York. The submission deadline is in two weeks, on August 17th. Presumably I could always enter TOBL1 like last year and then bring TOBL2, but let's see if we can't do this honestly.