AHRS

That's "Attitude and Heading Reference System", and the next big part of the helicopter project (possibly the biggest part, if this off-the-shelf solution doesn't work out).

The plan is to use this board from SparkFun: it contains a 3-axis accelerometer, 3-axis gyro, 3-axis magnetometer, and a ATmega328 processor with bootloader setup for in-circuit serial programming. And at least one group has built an AHRS out of it. I haven't looked through their code, but if they're at all competent it should give some indication of the board's capabilities.

What I'll need to evaluate is its susceptibility to drift (this is the big one; I think this'll be what ultimately limits the system's performance) and the level of noise (also problematic, but at least I have ways of dealing with it). When I get to the stage of implementing the control system, I can add the plant dynamics into the estimator model (I wonder if the microprocessor is up for calculating Kalman gain matrices) and that should only make it better, right?

Next steps:

1. Get the thing running with the pre-flashed firmware. Check that output values are reasonable and look for drift. Also try running it in the refrigerator to check temperature compensation.
2. Install the AHRS code and evaluate its performance. If necessary, write some client software to display its output (maybe MATLAB or Java, for easy graphics).

And on the Arduino side:

1. Get Arduino talking to servos (will require 3.3V to 5V hardware)
2. Get Arduino talking to receiver (will probably require 3.3V to 5V hardware). An oscilloscope would be nice.
3. Get Arduino talking to both at the same time (try powering everything off helicopter 5V regulated supply)

In terms of hardware, will need something with enough connectors for the servos and receiver, as well as 5V to 3.3V level shift. And for integration:

1. Get Arduino talking to AHRS (over software SCI at first, then try SPI)

And that should be it. Arduino mini has 18 I/O pins available, need 3 for SPI, 6 inputs (from receiver), 6 outputs (to servo, ESC). That leaves 2...which I might use 2 for I²C to a camera module. Hmm...I guess I should take a look at serial interface servo signal drivers...I might even be able to find a 5V driver with a 3.3V serial interface.

update: No magical chip to read/output servo signals. My search mostly turned up tutorials on how to implement it on a PIC or AVR, which is probably a bad sign. A 12-bit PWM generator might work: at 60 Hz, I'd get 8 bits of resolution for pulses between 1 ms and 2 ms, which is...okay. Well, I can control the output timing to have no overlaps, so I guess I could deal with the output latency of a shift register or I/O expander. That saves 3 or 4 pins.

Ultimately, anything involving measuring or generating a precise servo pulse would require a crystal or else a clock line from the Arduino. And programming a PIC (and therefore designing ICSP into the servo I/O board) would be more trouble than it's worth. Right?

update 2: ATmega328 doesn't have a bajillion input capture/output compare channels like the HC12. Also, it's an 8-bit processor! How quaint. It does allow any I/O pin to trigger a pin change interrupt, though, so I can use that ISR with a free-running timer to measure pulse lengths. And I'll stagger the output pulses.

Also, I don't really need the extra pins at all. This count just shows that I have exactly as many as I need. And if I really need to, I can cannibalize the SCI Tx/Rx pins.

Insulation

The day before yesterday it finally got warm in our apartment, recalling the summer days when we'd open the door and all the windows at dusk to exchange in the cool air. It seems so long ago...

But then today it rained and there was no sun and it was cold. I'd turn on the heat, but it just seems so ineffectual...you hear it burning gas, and the room warms up briefly but cools back down to steady-state in an hour or so. As a result, we've spent most of the winter being cold, bundled up in sweaters and/or blankets.

So up on the list of apartment search criteria (for next time): double-pane windows and doors that close properly.

More helicopter options

Looked around some more, people on the Internet seem to bash on the E-flite CP series quite a lot. The options now seem to be:

E-flite CP+
$200 at D&J Hobby, $180 online
528mm main rotor, 298 g
3-in-1 (mixer/ESC/gyro) module
FM receiver
11.1V 800 mAh Li-Po
370 brushed main motor, N30 tail motor

E-flite CP Pro 2
$230 at D&J Hobby and online
515mm main rotor, 325 g
2-in-1 (mixer/ESC) module, G110 heading lock gyro (11 gram)
Spektrum AR6100 2.4 GHz receiver (DSM2)
11.1V 800 mAh Li-Po
370 brushed main, direct-drive N60 tail

E-sky Honey Bee CP 3
$100 online only
525mm main rotor, 410 g
3-in-1 module
2.4 GHz receiver
11.1V 1000 mAh Li-Po
370 brushed main, direct-drive tail

E-sky King 3
$160 online only
600mm main rotor, 390 g
All discrete components
2.4 GHz receiver
11.1V 1500 mAh Li-Po
400 brushless main, shaft-driven tail

E-sky Belt CP V2
$170 online only
680mm main rotor, 670 g
All discrete components
2.4 GHz receiver
11.1V 1800 mAh Li-Po
450 brushless main, belt-driven tail

The advantage of the E-flite models is that they're available (and so are their replacement parts) at a nearby retail store. Its disadvantage is that it is rather expensive.

Of the E-sky models, I appreciate how cheap the Honey Bee is, but I think the King's discrete components may be easier to work with. It's also a bit bigger, but what does that mean? The Belt is bigger still, but that's starting to get into uncomfortably-big territory. I still need to figure out where to fly this thing.

So, E-sky King it is? Will have to sleep on it, but that looks like the winner for now.

update: xheli.com and eskyhelicopters.com have the King 3 for $186 and $195, respectively, with tax and shipping. helidirect.com has it for $160 (no tax, free shipping), but they only have it in red. These seem to be the only U.S. distributors. Oh noes!

Also, saw a video of the E-flite Blade mSR. It looks really fun...

Cowfish, back from the hospital

We put the littlest cowfish back into the main tank. We did a full treatment of erythromycin (4 days) then kept him in quarantine another day and a half, feeding him flake and small pellet food, both of which he seemed to like  very much (at least much better than the shrimp pellets).

Once back in the tank, he found the other cowfishes and they seemed to get along all right. I'm still concerned about his missing pectoral fin, but there's nothing we can really do about that.

Helicopter update

Looked around some more; I think the E-flite Blade CP Pro 2 is the one I want. I saw an older model (CP+) at D&J Hobby, and it looks pretty good. I'd originally thought a fixed-pitch heli would be better, but it looks like there aren't any good ones on the market.
It's got a 6-channel receiver that feeds into a motor speed controller (for the main motor and the tail motor), which has an integrated gyro (which might be okay; I'll just account for it when closing the control loops). But it's not one of those integrated receiver/motor controllers, so that's good. Brushed motor, so no tach signal, but that shouldn't be too hard to add. It's got electronically mixed CCPM, which is good, and better yet, the transmitter can be switched out of that mode (into unmixed elevator/aileron/collective signals).
I'll have to decide, though, whether to go for the one at the hobby shop or to try to get a CP Pro 2. Will have to call around to see who has that in stock. I hope it'll be okay with the weight I plan to add. I guess one advantage to a modular gyro is that I can remove it to save weight.

Talked to Prof. Rock about the project. He thinks the helicopter dynamics won't be too bad, but sensing will be difficult. First, there is the issue with accelerometer drift when trying to get a good attitude reading. We'll see how that goes. I'm curious, too, about how the magnemometer pans out.
Next there is the issue of velocity sensing. Ideally, the user is dealing with a velocity command. He thinks it'd be pretty interesting to run optical flow on a simple webcam. Checked Sparkfun, and it looks like it'd be doable in terms of price and size. Will need good attitude and altitude information to decouple pitch/roll from x/y translation, though. Hey, maybe with good attitude and perceived x/y translation, I can get altitude... And as an added bonus, it becomes an aerial photography platform.
Finally is the issue of the ideal user interface. Can we get by without velocity sensing? My argument is that humans have trouble dealing with 1/s² plants (i.e., issuing a force command) because the system is underdamped, so we just need damping. Or, recasting the human interface question in a different light, what if we take the human to be a simple proportional controller? The user has a desired position, and he issues a command based on the error he sees. If it's a velocity command, that's great, because it's a first order system. If it's an acceleration command, then with insufficient damping and human time delay, it'll be unstable. But we should be able to deal with that, right? What I like about this approach is that it takes advantage of the human's sensors and puts that into the loop.
Oh, one final thing that I thought would be cool: if the magnemometer works well for heading, then we could interpret all cyclic commands in a fixed (w.r.t. yaw) reference frame, thereby freeing the user from orientation considerations...

Helicopter buying considerations

Backordered the Sparkfun parts. Now to look at a helicopter.

There are lots of sites with buying guides for beginner fliers. But I think what I'm looking for is slightly different because I'm looking for a helicopter that will be easy to close a control loop around, which may not necessarily be one that is easy to fly by hand. Or will I need to learn how to fly it by hand before handing it over to the control system? I hope not...

For one, bigger helicopters are supposed to be easier to fly because everything happens more slowly. That might not be as much of an issue with the control system. Not only is its reaction time much better than mine, it will have access to acceleration data (an idea: get another 2-axis accelerometer and mount it a distance away from the main IMU, to get angular acceleration) whereas a human operator only sees position.

Now, what about fixed vs. collective pitch? Cost and crashworthiness are both still extremely relevant, which suggests fixed pitch. What about the controllability? Fixed has one fewer control input, and the variable rotor speed is going to wreak havoc on the cyclic control transfer functions, which I suspect are strongly influenced by rotor speed (may need to install a rotor tachometer).

I guess we'll look more closely at the cost, then. It may also be important to get one with separate electronics so that I don't have to hack into some integrated receiver/motor controller module. And for this, the dumber the electronics, the better.

Soda is hypertonic!

So I was thinking about how I need sugar, or maybe acid, in my drink to really make me feel like my thirst is quenched (just drinking lots and lots of water doesn't seem to do it). And perhaps that's due to highly successful manipulation by soda companies, because soda (carbonation + sugar + acid) seems best at quenching thirst.

Anyways, I'd heard that drinking soda actually makes you more dehydrated. Is that true? The can of Sprite in front of me has 38 g of sugar in 355 mL of solution. At a molar mass of 180 g/mol (for glucose/fructose), this comes out to 0.59 M, or 590 mOsm/L. A saline solution has 9 g of NaCl (58.4 g/mol) per 1 L of solution, i.e. 0.154 M. And it dissociates completely, so it has an osmolarity of 310 mOsm/L.

So it's true! Well, maybe not. According to Wikipedia, urine has an average osmolarity of 500-800 mOsm/L without really trying (i.e. with a restricted fluid intake, healthy individuals produce higher concentrations). So maybe you're still hydrating when you drink 590 mOsm/L Sprite (I'm not counting the citric acid or carbonates, but my gut feel is that those are negligible compared to the sugar). The scary part is that it's just about half the concentration of 3.5% salinity sea water (35 g/L NaCl = 0.6 M = 1200 mOsm/L).

Update: SC points out a pretty fundamental flaw in my reasoning: sugar is special. There is active transport and all sorts of metabolism and other reactions that happen, and your kidneys try very hard not to even get any into the glomerular filtrate. So we can probably ignore the sugar, which most likely makes soda hypotonic. It'd be different if it were a 590 mOsm/L salt solution.

Helicopter project

I'm officially getting the helicopter project off the ground (so to speak). The goal is to build an auto-stabilizing (via active electronic control) R/C helicopter. Or rather, since I plan to buy a 5-channel ready-to-fly helicopter, the goal is to build an electronic control system that will interface with the helicopter's existing actuators and radio  receiver. Some thoughts on various parts of the project:
Interfacing with the helicopter: For this, I really need to buy a helicopter and get access to an oscilloscope, but if it's anything like the R/C plane I'd played with, then it's all PWM servo control signals, which will be easy to work with. Helicopter should also have a 4.8V battery back for the control electronics, and/or a 3.3V regulator on board?
Sensors: SparkFun has a pretty nifty sensor: 3 axis accelerometer, 3 axis gyro, 3 axis magnetometer, and an ATmega328 chip w/ bootloader, all on a small 3.3V board. The only downside is that it's $125, but I guess you have to bite the bullet somewhere. These sensors are expensive and tend to come in hard-to-solder packages (where all the pins are on the bottom). Board printing is not too cheap (AP Circuits $20 for a small board + $40 shipping; ugh) and doing the board design and programming an on-board microcontroller...it's just too much for now.
The other option, though, is to get an Arduino (which I'll need anyways) and some accelerometer or gyro breakout boards: they're not much more expensive than the sensors alone from DigiKey or Mouser, and it saves a lot of trouble. They have analog output, which is easy to read but prone to noise. (some might have PWM? Will have to look again to check for serial).


Next step: Well, unfortunately SparkFun is out of 3.3V Arduino Pro's and FTDI USB-Serial breakouts, so I'll just have to wait on that. Looks like buying the helicopter is the next step...

Cowfish!

The littlest (what? The spellcheck isn't complaining? "littlest" is actually a word?!) cowfish (read: Panda Cory) is not doing so hot. We found him laying on his back yesterday, but a tap on the glass scared him back into a right-side-up orientation. A few minutes later, we found him sucked into the filter intake, but after turning the filter off he swam away.
Worried that he was infectiously sick, we quarantined him, and that's when we noticed (1) the water has a distinct blue-green tint and has an ammonia concentration of 0.25 ppm and (2) this fish's spine is a bit bent (though he seems to be able to straighten it) and he seems to be missing a pectoral fin.
So we did a 30% water change (the spare-tank setup makes this easy; dump the new water in the spare tank and it gets cycled into the populated tank over several hours) and hoped for the best.

This morning the cowfish still seems okay. I put in a small shrimp pellet, but I think that was a bad idea because I came back to see him doing barrel rolls in the quarantine bowl. The bowl (a yogurt container floating in the main tank) is pretty small (860 mL or 0.23 gallons). So I netted the fish out and replaced the quarantine water with main tank water. He looks okay now. I don't know how we're going to feed him, though. Maybe the algae wafers will be better. Or we could give him flake food, because there's nobody to compete with him.
He is showing some fraying at the ends of his fins. Fin rot? Or physical damage? If it's physical damage, is secondary infection an issue? So we decided to dose him with antibiotics (E.M. erythromycin).

Here's the plan: the package calls for 1 packet / 10 gallons (at 200 mg/packet, this makes a 5.3 ppm solution), once a day for 4 days. We're going to make a concentrated solution (21.2 ppm) and mix that into the quarantine water. What should be our target concentration? What concentration vs. time profile does the recommended dosage produce?
Wikipedia says erythromycin is metabolized through methylation in the liver and eliminated through bile, with a  half-life of 1.5 hours. But we have a very small fish in a lot of water, so is metabolism significant? Are there other things that may metabolize erythromycin? What about elimination? The instructions are very clear to remove any activated carbon filters during treatment. So without any metabolism or elimination, and with the specified water changes:
Day    ppm
  1    5.3
  2   10.6
  2*   8.0 (after 25% water change)
  3   13.3
  4   18.6
  4*  14.0 (after 25% water change)
This article gives 370 mg/kg as the LD50 for erythromycin injected into rainbow trout at 13°C. That's a pretty high upper limit. So even if "no metabolism/elimination" isn't an accurate assumption, it may still be a safe one...

update: SC pointed out that it actually wouldn't be too hard to split up the powder in a packet into 4 equal sized doses, so we'll just do that. We'll dose the water in a bucket at twice the specified concentration (so, 1/4 packet an a time into 1.25 gallons) and then dilute that by 1/2 when mixing into the quarantine bowl.

Tire adventures

Tires! Our front right tire was squeaking, and we had no idea why. But we decided to let it be because the squeaking would go away after a few minutes.

Gradually, though, it started to be accompanied by a vibration. You could feel it even at very low speeds, but it seemed to go away around 25 mph, and wouldn't come back until highway speeds.

Then yesterday I drove SC to the airport and it was pretty bad up at 65 mph. I hadn't driven the car in a while, so I couldn't tell if this was much worse than what it used to be. It'd also pull to the right (indicative of higher drag on the right wheel), which I didn't remember it doing before. On the way back, it was decidedly worse than it had ever been before: at a walking pace I heard and felt a periodic thumping, and at higher speeds it became a vibration. At medium speeds I guess the suspension took care of it.

When I parked, I took a look at the tire, and pretty luckily, the damage was on the part of the tire facing up, in plain view. It looked like a part of the tread was dented in. What?

To cut to the end of the story, it turns out these tires are 14 years old, the tread had separated (we're very lucky it didn't do so catastrophically), and I was looking at a bulge (from the side I was viewing it initially, it was hard to tell whether one part was abnormally high or whether another part was abnormally low) where it was hemorrhaged out (the steel belts had broken). Also spent 10 min. pumping up a flat 60 psi spare using a bike pump. I'm glad our spare is actually functional now...

Took it to America's Tire in Redwood City, got it replaced in 30 min. for $50 plus $22 labor+fees, and the dudes there were all right.

Oh, right: this time it was the front right tire. This had happened previously once before, with slightly different symptoms, to one of the front tires (don't remember which one...that's why this blog is necessary). They'd rotated the older new tire to the left rear wheel, and so this time they rotated the newer new tire to the right rear wheel.