Mechatronics Project 1

I decided to take a mechatronics class at FIT because it would be different and very useful. The class is split into two portions: learning to program an Arduino pro-mini using C and Atmel AVR-Studio, and then learning Matlab/Simulink XPC. We have to use C because "the arduino IDE is too easy".

Project 1 was to make a stopwatch using a 4 digit 7-segment display and the arduino pro mini 328 5V/16MHz. It must count up in increments of 0.01s using interrupts/timing, and have a button for start/stop and reset. I used the FTDI/USB device to program the arduino (with bootloader) through AVR-Studio.

The breadboard circuit shown below was wired. The display has pins for each segment and a common cathode for each digit. There are 330Ohm resistors in series between the arduino and each anode. There is a NPN transistor for each cathode that act as a switch. When an arduino pin drives the base of the transistor high (through a current limiting 10kOhm resistor), they turn on, grounding the cathode, allowing individual digit control. The digits are cycled through rapidly to take advantage of persistence of vision. The button is hooked up to pin 13 on the arduino, which is the one with the onboard LED. This made using it as an input somewhat difficult because the onboard pullup resistor is 20kOhms, which results in a pin voltage of around 1.7V instead of 5V because the onboard LED is drawing current. So I had to drive the voltage on the pin high as the trigger instead of grounding the pin as I would have normally done.

As for code, there is a main loop that contains the button polling routines. There is also an interrupt service routine (ISR) that handles the digit counting and timing via a bunch of volatile variables and if trees. There is a "on" routine that takes care of turning on and off the correct segments for each number 0-9. I can't make the code available since the class is still running. Apparently the more common way to do it is to have the ISR act as a 0.01s timer that calls a separate function that handles the digit counting, which would probably avoid the use of volatile variables.

In addition to the component datasheets, the resources I used are below:

• http://www.jayconsystems.com/tutorial/atmerpt1/
• http://www.jayconsystems.com/tutorial/s7sd/
• http://elecrom.wordpress.com/2008/02/12/avr-tutorial-2-avr-input-output/
• http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=40348&highlight=programming+101
• http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=37871
• http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=50106

L3 Rocket

Purpose: document the build and flight of my Tripoli Level 3 certification rocket

I've always liked the Phoenix AIM-54A missile. I had an Estes kit of it when I was little that I put a 24mm motor mount in it. I flew it probably close to 50 times before it died. I've seen a few larger scale Phoenix missiles, but I've never seen a 1/2 scale kit, so I decided to custom build one.

I got the specs from here. Using stock 7.5" ID, 7.75" OD phenolic tubing results in a 0.517 scale, which is close enough. Given that, the body tube length should be about 63", and the nose cone should be slightly less than 3 to 1. Though it's not ogive in real life, 3:1 7.5" ogive nosecones are commercially available, so I decided to use one and make the body tube slightly less than 63" to compensate for the slightly too long nose cone. I decided on a 3ft long, 98mm motor mount and 1/4" thick 12ply wood fins. I'm building it super tough, so it should be able to go supersonic (large M or small N motors).

The rocket was designed in OpenRocket and Solidworks.

Solidworks model of stretch 1/2 scale Phoenix

OpenRocket model of Phoenix

OpenRocket model of Stretched Phoenix

OpenRocket is predicting ~7000ft for the stretched (dual deploy) Phoenix on a medium M. It predicts ~2000ft on a medium K for the non-stretched Phoenix. Both configurations will use about 4lbs of nose weight. This is due to the stubby and spread-out nature of the fin area, causing the CP to be further forward than usual for a rocket this size.

My dad gave me two 48" long, 7.5" phenolic tubes, two 12" couplers, and 48" of 98mm phenolic motor mount tube. He also has (had) tons of fiberglass, so he let me use it. This wasn't my first time making composite rocket tubes, but I didn't have all the supplies I was used to (MIT Rocket Team, DBF had vacuum bagging supplies we used to make tubes with...I was spoiled) at dad's house, so I had to do a basic wet wrap. I measured and cut two wraps of ~10oz 0-90 E-glass and two wraps of ~4oz 0-90 S-glass for each tube. I used medium cure time West Systems epoxy and wet out the FG as I was laying/stretching it around the tubes.

Setup for layup. Katie's L3 kit is in the background

Lots of fiberglass

Finished layup

Heated cure

Each tube went from 3lb pre-glass to ~5lb 12oz post-glass. Final OD was ~7.75". The surface finish was terrible (no peel ply or release, no vac bag, etc. ), so I had to do a ton of sanding and filling (two coats of automotive spray primer and filler). You can see the result in some of the pics below. Total time spent on the tubes was probably 20-30 hours. 

If I had left the couplers stock, I ran the risk of coupler failure (breaking in half), which is common in these large rockets during high accelerations. So I fiberglassed the insides with 2-3 layers of 10oz glass. Balloons were used to hold the glass against the insides, then popped after curing.

 

I wanted to do 1/8" G10 fins, but I do not have access to a CNC router or large enough laser cutter here. Custom fins would have been ~$400, which is crazy, so I bought a $50 sheet of 12ply plywood from aircraftspruce. I penciled the lines using a cardboard stencil and checked them all with a ruler. 

I then cut them out with an oscillating saw. I tried to sand them with a palm sander and sanding block, but quickly realized it really needs to be done with a belt sander, particularly the beveling. I'll finish them later. I'm going to build a jig for the FIT shops's belt sander and do the beveling there.

I slotted the tube next using a Dremel and reinforced cut-off wheel. I made two slits lengthwise per slot, then broke the material in the slot out using needle nose pliers. Fiberglass is brittle, and the thin/small amount on the ends of each slot made breaking it out easy. I then finished the ends of the slots with a 1/4" sanding drum attachment.

All tubes done

All tube ends were sanded smooth, treated with thin CA, then sanded smooth again to prevent phenolic wear.

Left to do:

• Buy plywood centering rings, bulk head, nose cone, altimeters
• Cut motor mount tube
• Finish sanding fins
• Make motor mount
• Install fins
• Filleting 
• Build altimeter bay
• Minor stuff
• Final primer coat, sanding, then painting

The fins will have internal and external fillets. The rear two centering rings will not be installed until the set of fins they contact are completely done so that the volume they enclose can be filled with expanding foam. The shock-cord mount will be a pair of U-bolts in the forward centering ring. Altimeter bay will be classic rods + sled construction. I haven't picked the altimeters yet. The goal is to be finished by December.

Next time I build a big rocket, I will probably order pre-glassed tubes unless I can figure out a way to get a much nicer surface finish. I'll get them pre-slotted, too, unless I build a nice router tube slotting machine/jig. I want a CNC router and laser cutter in my home shop eventually, so that will make fins significantly easier. 

Random Verizon Rant

I hope someone out there finds this remotely useful. I never imagined how big of a pain it would be to switch from a post-paid account to a pre-paid account on verizon.

Let's say you have a postpaid account with multiple lines and the contract end date has come. Now let's say you want to get a new phone for one of those lines, but you want to get that phone on a prepaid account, but you also want to keep your phone number. That's what I wanted. I got on verizon.com to shop for some prepaid smartphones and ended up buying two, one for each of the lines on my account, and 1 month of prepaid "allset" accounts with new numbers/sim cards to go with them...all under the guidance of a online verizon sales rep. She said that I would just call and they could activate the phones with my old number. Simple right? WRONG.

I was on the phone with verizon for no less than 4 hours being passed around like a hot potato. Almost no one knew how to handle this.

Tips and lessons learned to avoid this headache:
1. Do not buy more than one prepaid phones with prepaid accounts at the same time online. Make sure they are in separate orders. Shipping is free, so not a big deal.
2. Make sure your billing address is your shipping address. Their online ordering system can't handle separate billing and shipping addresses. Otherwise, you will have to call customer service for an hour to get them to re-enter the order with your billing zip code to get it to go through.
3. If you want to use your new prepaid accounts that you paid for, do NOT follow any online instructions on activating your new phones, i.e. do NOT switch the SIM cards. The only way to activate a prepaid phone with your old number is to call them.
4. Do NOT call late at night. They will go home and leave you on hold until the line dies.
5. Start with verizon wireless customer support and say "agent" in the menu. That will transfer you to a person. Explain to that person that you want to activate a new prepaid account and phone you purchased from verizon.com with a number you currently have in a postpaid account.
6. WARNING: IF you ported a number from another carrier, do not let them deactivate your old line, or the number will revert back to the original carrier. There is another process they have to follow that involves higher-ups if your number was originally from another carrier.
7. Verizon stores (the legit ones) can't help you with this problem.
8. Ask lots of questions and be skeptical

The process is as follows: First to have customer service deactivate your old line and reserve the number. Then they have to toss you to the prepaid department. I think if you follow the above tips and have an original verizon number, the prepaid department can simply activate your new phone and prepaid account with your old number.

My experience: I did the opposite of those tips (at the suggestion of various verizon representatives who were all wrong). Thus, my experience was painful. Once I finally got to people who could help me...First they deactivated the line I wanted to switch. Then I was tossed to another department. They tried to activate my new phone but it wouldn't work. Then the line I was on hold on died around midnight. The I called again in the morning.  They tried activating the phones but found that the problem might be that I had purchased two prepaid phones on same order and the system got confused. Then they realized the old number was from another carrier and had to reactivate it in the original postpaid account to save it. Then they tried splitting the order so that the new phone could be activated. This took me to hour 4. Eventually they tossed the problem to "systems", which I'm guessing means tech guys. I got a few calls back, but after another 5 or so hours, it was finally resolved.

If the mistakes under lessons learned had been my doing, ok, I wouldn't be upset. But everything I did was under the guidance of verizon representatives!! At least all of the verizon reps were nice and friendly, and the higher-ups I talked to were helpful.

Over 2 years...

Wow time flies. It's been 2 years since my last post. Life... Graduate work is sucking most of my time currently. I need to start my thesis (in the field of cyrogenic fluid slosh dynamics) soon, which won't make doing side projects any easier.

Projects update:
-EHB and LITE are suspended until further notice. I'm currently in a small apartment with very little access to machine tools, which makes making things difficult. They will eventually be built and will be followed by some other similar projects I have in mind, probably a scooter.
-I'm working on my Level 3 certification rocket. It's going to be a half scale Phoenix missile.
-Also designing a huge rocket with some MIT Rocket Team cruft. More details on that closer to launch day (BALLS 2015)

I'll post a build log of my L3 rocket similar to my L2 rocket when it's complete.

3 months....more ouch

Yeah...I haven't done anything on my boards. I got swept away by MIT's work load again. All my free time was spent leading the team that built this:

MIT DBF 2012 Competition Plane

It's our plane from the DBF competition this year. I am an aerospace engineer after all...gotta build some things that fly :P .

About DBF:
DBF (Design/Build/Fly) is an international RC aircraft competition sponsored by Raytheon, Cessna, and the AIAA. Every year in September, a new set of rules/missions are released, and 60-80 teams design, build, and fly a plane for those rules. The contest site location alternates between Tuscon, AZ and Wichita, KS. 1 plane is allowed, and the missions are usually very different, making it a complicated design/optimization problem. Also, weight is usually a HUGE scoring factor (in some years it's been the only one that really matters). Also, lithium batteries are not allowed...It's like Flintstones meet the Jetsons in that regard (NiMH battery tech meets top-of-the-line composite technology).
The missions this year were to fly as many laps as you can in 4 minutes, carry 3.75lbs of aluminum block for 3 laps, and carry 2L of water up to 100m as fast as possible and then dump it.

About MIT DBF:
We're a small team (~12) undergraduates, usually with one or two graduate advisers who help out here and there. No profs at MIT have the time to devote to helping the team out, so we're really undergraduate run.

About our plane:
Unfortunately, I can't tell you very much about our plane because DBF is SUPER competitive, and a lot of how/what MIT DBF does is secret. I can tell you what you see in the pictures though. This plane is the product of a semesters worth of design/analysis, and a semesters worth of building. (for a reference length: it's wing span is about 1.4m). It has composite reinforced (various amounts of carbon fiber/fiberglass/Kevlar...I won't reveal weights or locations) foam wing and tails. The airfoils are very precise thanks to the CNC foam cutters the Aero/Astro department has. The fuselage is made out of a honeycomb/fiberglass composite; we CNC milled our molds in house for this fuselage, and developed our own manufacturing processes. The propulsion system is: NiMH battery pack, Castle Creations ESCs with Axi Gold brushless motors, and APC propellers (different props for different missions).

More pics:

Filling with water.

The team.

How the competition went:
Not well. Fucking tornadoes. I'll start from the beginning though: The plane made it to Wichita via freight no problem. It was SUPER windy every single day there. First day, we successfully completed Mission 1 (the speed mission) in the morning with 6 laps, at the end of which, an ESC fried (it was a bad one...we weren't anywhere near the amp limit). Then we decided to call it a day and go do Mission 2 flight testing at a small RC airfield nearby. The next morning we successfully completed Mission 2 (aluminum block passenger mission) in 20+mph winds. We were sitting in the top 5 at that point (top 3 if you didn't count the heavy planes that had already finished all 3 missions, but were way too heavy to place in the final top 5). Then we were waiting around for our next turn (they cycle through everyone for flight attempts) to do Mission 3 when they closed the flight line due to some rain and high (30ish) winds. It's unfortunate...we designed our plane to be very fast, so it could have handled the wind. Oh well, we decided to go back to the hotel and wait for tomorrow, the last competition day. That night, 90 tornadoes ripped through Kansas, out of those, 10 hit Wichita, and 1 happened to go right through the competition site, wrecking everything. They had to cancel the competition; no reschedule, nothing. We were super bummed: 1000's of man-hours and 1000's of dollars went into that plane. But we went back to the small airfield and completed Mission 3 (2L water dump mission) in ~40s in 35mph+ winds. We did the math...if we had completed Mission 3 in anywhere close to that time at the competition, we would have won. It sucks...a lot. The judges decided to get rid of the 3rd cycle of flights in the competition (the one we did M2 on), so we ended up in like 12th (because we only had one mission score...), or something. The places are total bullshit this year; 1/2 the teams never even flew, and the ones who placed high would have placed much lower had half the competition not been canceled.
I'm just glad Tuscon doesn't have tornadoes.


I'd like to thank the MIT Edgerton Center, the MIT AIAA Chapter, the MIT GEL Program, Lincoln Laboratories, the Department of Aeronautics and Astronautics, Aurora Flight Sciences, and Lockheed Martin for their support this year, and hopefully in many years to come!

-Jed Storey
President, MIT DBF

2 months...ouch

Needless to say (but I will say it anyways), I've been busy. Leading MIT's DBF team has taken up every bit of my spare time since november. Well, almost...

I got the hubcaps and spacers for LiteBoard's hub motors machined:

I did the spacers on the lathe, but the hubcaps I did entirely in the mill. I really like the mill method, rather than going from the lathe to the mill. It took a lot less time. I didn't check one of my drill bits though, and being a student shop, it was damaged, so the tip broke off in my part. I was lucky that it had gone most of the way through before breaking off, though, so I was able to get the tip out. I also decided to try loctite bearing compound this time, so I made the bearing holes slightly oversized.

Even though I haven't been building much, I've been gathering supplies: 2 hobbyking 80A car esc's, 2 more 6S 5000mAh packs (in addition to the 2 I have left over from ELB), all of the metal stock I'll need, 4 1.25"x4" colsons, a massive lipo charger and power supply, a $15 hobbyking 2.4GHz 2 channel radio, etc.

The next machining I'll do is probably coring out the colsons.

Left to do:

• core out colsons
• machine axle adapters and hubs
• wind stators
• order rotor plates
• find/buy hall effect sensors
• assemble motors
• test motors/esc's
• assemble massive lipo pack (6S4P)
• laser cut grip tape design
• assemble board
• add lights
• +more

Wow that's a long list :/ . I hope I have time this semester to work on it. I'll only be taking 3.5 classes to give me time for DBF and this, so we'll see how it goes.

Here we go!

I’ve been busy, but I’ve somehow found time to work on LiteBoard. Oh right…what is LiteBoard?

Now Presenting LiteBoard:

But first, some history: I want a light*, fast way to get around campus, which is the same premise and idea that evolved into ELB. Unfortunately, ELB turned out to be more of a prototype and a learning experience. Now I want a light*, fast, and polished product to get around campus. Sure, there are commercially (and cheaper) available options, such as a bicycle (can’t pick it up and take it to class…also not original and would get stolen), roller blades (no way…I’d kill myself on those), kick scooters (lammeee), electric scooters (lammeerrr), hub motor scooters (so not original)…but I wanted a longboard. So I came up with the EHB concept, and started acquiring parts for it. But once I did a weight build-up, I realized that EHB wouldn’t actually be much lighter than ELB, so I suspended work on it. I’ve been riding around on the mountainboard I bought for EHB for awhile now…I have to say, it is a great way to get around. It’s just slow: I can only maintain about 10mph.

LiteBoard is the V3 concept that I came up with after completing ELB. V2 is/was EHB, but even though it would address most of the problems ELB has, it didn’t address one very important one: weight. ELB weighs 40lbs. EHB is predicted to weigh 35lbs. Liteboard: 25 lbs.

Not enough time to do a rendering=screen shot.

Lite will be a 2WD hub motor powered electric longboard (actually a longboard this time, and not a mountainboard). Each motor will be ~1100W, producing ~3.9Nm of torque each. Lite’s estimated top speed will be ~25mph. The basis of Lite is this longboard:

Pending cool grip-tape design

Randal II 50deg 180mm trucks, ABEC11 97mm Flywheels, and a bamboo/maple deck I got off of ebay (it was incredibly cheap, yet very high quality. ebay=good source for longboard parts). The deck needed some modifications to use the massive 97mm Flywheels:

Initial cutout lines. 

1st try: didn't remove enough.

Second try: Now the wheels don't bite the deck when I turn.

I think it looks awesome...

Why did I need 97mm Flywheels? …

Detailed Motor design:

I couldn't get a good angle of the internals. The axle extension is not round, but keyed to fit with the hub that is pressed into the stator.

Since ELB ended up with about twice as much torque as I’d ever need, I decided to shoot for the same amount of torque/motor for LiteBoard, but cut the number of motors in half. I followed the same design process I used to design ELB's and EHB's motors: Start with simple calculations, move on to SolidWorks CAD drawings, import DXF's into FEMM, modify CAD, repeat...

FEMM pre-diameter shrinking.

The motor underwent one large design change as a result of FEMM. Turns out that pretty much the whole stator was saturated, so I wasting a ton of energy. I shrunk the magnets from 3mm to 2mm thick, and shrank the whole OD of the motor by 4 mm, and only took a .1Nm hit in the process.

Estimated Motor Specs:

Specification	Value
# Phases	3
Power	1100W
Voltage	6S Lipo
Max current	50A+
Torque (FEMM)	3.9Nm
No-load speed	30mph
Stator dia	55mm
Stator length	35mm
Teeth	18
Turns/tooth	4
Rotor OD	70mm
Magnet thickness	2mm
Magnet grade	N40
# Poles	20
Motor width	60mm
Motor OD	100mm

The high current, low voltage method was picked in order to use HobbyKing’s 80A car ESC, which is about 1/3 of the cost of an equivalent Kelly Controller, and about ¼ the size. It also has a sensored option that I plan on using. I will likely get some pretty serious I^2R losses, but I’ll try to keep wires as short as possible and pack the stator with multiple parallel strands of magnet wire (thinking 22Ga). 8 parallel 22ga stands has same resistance/m as one 13ga strand…idk, I’ll see how many strands I can fit.

The stators are Scorpion Power Systems 5535 18T stators. They come in their S5535 motor series, which are about $400 a piece…which is why I bought the stators directly from them again ($140 for both, free DHL express shipping). Yet again, they are very high quality:

I officially love these guys' work.

I decided to bite the bullet and go with BigBlueSaw for waterjetting my rotor plates. Turns out that 11*.125” =35mm to within a fraction of a mm…I got lucky there. 22 plates will run my $250…ouch! But I did the math, and if I spent the amount of time working that it would take me to machine the rotors, I would come out ahead if I just have them cut for me (and waterjetting them here is a no-go…it’s freakin’ expensive).

Batteries: While I thought about trying A123 cylindrical cells for this one (specifically a 7S7P pack made of their 26650 cells), it turns out that it won’t fit under my deck. So I went with lipo again. Specifically, I’m going to buy 2 more 6S 5000mAh 25C packs from HobbyKing, and using the leftover packs from ELB, I’m going to make my own, slim 6S4P pack (22.2V, 20Ah).

Control: At first I’ll use a $15 hobbyking 2Ch. 2.4GHz system. But I’m planning on doing something like this eventually.

Tires: So my last post suggested a Colson 4”x7/8” rubber wheel was my best bet. But that was before I shrunk the OD of the motor from 74mm to 70mm, which puts it out of the range of the 7/8” one. So I’m going to use two 4”x1.25” ones.

From left to right: 4x7/8 turned out, 4x7/8, 4x1.25, 4x1.5,4x2

Note the huge void.

After digging a bit deeper: MORE huge voids.

 Next I tried the 1.5" wide one:

Only 1" wide of plastic inside (after turning out visible part of hub).  Guess I'll be using the 1.25" wheels (more plastic for press fit).

I was able to cut the rubber on these.

Again: another void...

Here's a table of 4" Colson rubber wheel data for your reference.

Safety: I’ll have the same setup as I did on ELB. A contactor sitting in the main battery line that is actuated by a magnetic reed-switch from a magnet strapped to my shoe. This will act as a fail-proof way to make sure that if I’m not on the board (anymore…), it doesn’t keep going. I’m also planning on wearing a helmet with it.

The ultimate goal is have a very sleek, finished product that achieves all of my criteria (safe, light, reliable, fast, etc…)

Anyways, that’s the plan so far. I should be able to get a lot of it done over IAP…we’ll see.

Oh, and to leave you with a cliffhanger: it’s called “Lite” for more than one reason ;) .