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gamibots, pager motors, soccerballcopters, things from ebay / china

been spending money lately
mixed feelings ’bout my consumerism of things i feel like i might only use once or twice but i tell myself it’s research 😛

gamibot

based on
http://www.howtoons.com/?p=3484
these are super cool because you use a business card instead of a toothbrush, i have twenty extra business cards but not twenty toothbrushes and it’d be a shame to buy new toothbrushes just to chop off their heads

all it needs is a business card, tape, a pager motor, and a battery 🙂 so cute.
pager motor placement affect movement a lot, e.g. I changed the placement a bit and this one moves faster and more forwards than sideways

anyway, to start this story off, i am co-teaching a few lessons in 4th grade for my d-lab education class. and last year when we went to China, +Cathy Wu bought a bunch of pager motors in preparation for making swarmbots. I then bought a couple dozen off of her this year in preparation for bristlebots but then realized I needed to solder leads to them. The type she bought is to the right in this picture:

I tried soldering wires onto the tiny motors (they are about 1cm long) and promptly desoldered the metal leads from motor. Well, crap.

Anyway, I ordered some pager motors off of ebay for 84 cents each and they arrived eight days from ordering from China. I expected it to take three weeks so it was a pleasant surprise. (Well, I bought from two sellers and the other packaged arrived in 10 days).

that’s a bag of 30 motors! i could fit them in wallet. they come with pre-formatted messages. I wonder if these two are actually the same seller.
Details
From two different ebay sellers:
“10pcs Pager and Cell Phone Vibrating Micro Motor 2.5V-4.0VDC With Two Leads s883”
x3 = 23.63 for 30, or 0.79 each. Arrived in 10 days
x1 = 8.36 for 10, or 0.84 each. Arrived in 8 days.
I emailed them to check there were two wires coming off of them and got a pretty prompt response.
and i get reimbursed for these, so yay ^__^
Note in particular that these are from Shanghai and not Shenzhen. According to Amy, who travels to China for work a lot, and Star, 

I found better “hard hardware” streets in Shanghai TBH, & don’t remember seeing tons of motors in SZ.

Yup, I think you are right. SEG looks like the place to go for electronics, but Shanghai had way more mechanical stuff. It was a fun visit, though!

soccerballcopeter

In other news I am playing poking at copters a bit for the lulz. (I have been spending more money ever since making decent money at fitbit last summer… no regrets! i think)

“Pro 6042 Flying Ball Scientific 3CH R/C UFO Remote Control Gyro LED Helicopter”
22.91 + 1.6 shipping
bought 3/26 arrived 4/4 (nine days)

all that it needed was 6 AA batteries and it even comes with two props. super cute. fits in palm.
and it flies!
uhm, I thought this would be totally harmless because it is so tiny. and it pretty much is, but i was surprised because it has a decent bite just crashing into you (i thought the soccerball shape would protect me from the props)

things from ebay/china

in another post some day, crazy cheap and awesome robots (even hexapods!) from taobao, the swarmbot entry for $10 robot competition, and cost reduction due to scaling analysis.

Also, can we just take a moment and reflect how amazing it is that I can get a functioning flying thing complete with wireless control shipping to be from China for a total of $25 USD? That’s like 1.5 arduinos without batteries or anything at all. Mass manufacturing is awesome.

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servo arm now drawing things with inverse kinematics

you can see some of the error modes i was encountering in the ^ shaped “straight line” where theta1 was clipping out because I was feeding it negative values based on my inverse kinematics calculated angles

i blame servo jitter for how shaky the lines are 🙂

this took forever and a heaping bowlful of confusion to get to where i am. actually i’m still confused. but basically i spent all day sunday working on this, emailed out to MITERS, and finally a hallmate, pranjal, helped me out (https://github.com/pranjalv123/servoarm) today and fixed where I was stuck at in an hour or two.

essentially he rewrote the code in python (I was actually starting to do this) to graph and understand what was going on. For instance, the bottom-most image is the working envelope of the robot arm;

Isn’t that fascinating? It’s like a  yin yang. If you play around with the servo arm this sort of working envelope makes sense.

So turns out my code was decently fine, the negative values just meant I was giving it bad inputs that it physically couldn’t reach given the arm link lengths I’d given it.

We initially tried to wrap the negative values around by doing mod180 ‘ing it,
 theta1 = ((int)theta1+180)%180
but this gives the sort of trajectory shown in the upper image
(I think… it may also be that elbowup needs to be false to generate that sort of trajectory)

setting elbowup to be true fixed a lot of issues to (which makes sense physically as more x,y coordinates can be reached if the elbow is up rather than down, it’s  easy to see if you play around with it … see the inverse kinematics chapter on http://www.eng.utah.edu/~cs5310/chapters.html if this elbowup/down stuff is coming out of nowhere — basically there are two combinations of theta 1 and theta 2 that will work for any given x,y coordinate and you just pick whether you want the elbow up or down solution)

and finally the working enveloped helped me pick correct x,y values to feed it. Initially I was just basing my x,y values off of the instructables www.instructables.com/id/Robotic-Arm-with-Servo-Motors/?ALLSTEPS but scaled down 1/3, which was just a guess of mine based on my servo joint limitations versus her motor joint limitations

it is also surprisingly close to the dimensions in mm I give it, eg.


double highY = 80; // line drawing targets in mm


double lowY = 20;


double staticX = 20;

will give me a line about 60 mm long (I put the link lengths in as mm), which is exciting.

I’m still not sure what’s going on with why the straight line up and down is at a 45degree angle, but that’s probably a constant offset problem. Fixable either in code or if I set the initial conditions on the angle the links are mounted on the servos better.

Next: faces?

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eyeglasses fixed with composites

earlier I posted about broken eyeglass frames (and was investigating TIG welding titanium).
they are fixed now, yay! courtesy of nick, some [edit 4/1/13 actually it is brown cotton thread], and some superglue (yay composites)
the secret is I actually have broken two pairs of glasses this term. @__@ these broke…well were the tiny screws are normally

the excellent thing about this is that it’s barely visible from the front.

i still intend to fix the other frame, but now it’s super low on the priority list so…

musings:
you know, I think I have a more hesitant attitude toward the word startup (at least, as applied to my own project) than even general MITERS. I can’t figure it out, except that for some reason I want to prove that I am being semi-realistic and not embellishing. It’s not so much an allergic reaction to the idea of people working on something they have a crazy vision about as a hesitancy, especially after my experience with the concept to company IAP accelerator by startlabs, to label myself as working on a startup until I’ve reached some degree of success.

projects blog (nouyang)