AVR Freaks

Great, thank you. I'm starting to read all the tut's!

Before I begin seriously, I need to get my rig and order everything in one batch (shipping overseas from the US), to be able to test and work with the tut's.

Can you guys help me with the CPU/chip selection for STK-500, then? It must be compatible with the default STK-200 setup, so that I can plug it in STK-500 and code like this will work:

In the article, they mention it's for AT89/90 series, but I have no idea which kind. Probably one for the analog sockets, but I'm not sure about that either.

I'm beginning to worry if it's not too complex. :)

UPDATE: Yes, Forth would make sense, I guess, thanks. I'll check it out and see if there's a compiler available somewhere.

I was hoping to meet with people who would know this kind of stuff perhaps from personal experience.

Basically, all I need from you guys is help with the AVR selection for STK-500 to match the capabilities of a "stock" STK-200. The programming & circuit design is not a problem. AVR docs and available tut's will suffice. I have to order appropriate HW first, though, and that's the part for which I need your expertise.

OK, I'm a bit wiser now. :)

Arduino uses chips that have 28 pins. Of that there are 12 digital in/out pins (6 of which can drive PWM with independent timers) and 6 analog-in's (or 6 digital in/out's, if so desired).

Normally, PWM has only 256 step resolution, but PWM timer 1 can be set to a 16-bit resolution by direct register manipulation, which is awesome, because I need at least 10-bit resolution PWM.

The number of pins will be enough and the board is nicely compact. Plus, I can always pull the ATmega out, connect it to a 16MHz crystal and a voltage regulator and use it inside the scales without the development board. Arduino is just a programmer, right? If I supply 5V to the AVR and let it tick @ 16MHz, it will work just fine without the board, correct?

If this isn't the right place to discuss Arduino, just tell me, but I think these questions are relevant, because I'm really deciding on an AVR model here.

Are the pin numbers correct for ATmega328p?
- LCD = 7 pins (in 4-bit mode)
- 10-bit PWM = 2 pins (PB1,2)
- LED = 1 pin
- photodetector = 1 pin
- button = 1 pin

Max. current on these pins, however, is 40 mA, that's not much. What if I need more to drive the coil? What's the usual solution here?

Thank you.

Wow! What a beast. :) Thank you, very useful information.

From there I found there's also Arduino Mega2560, which has quite a lot of pins too, though not as many as yours. :) These non-Uno models have their chips hardwired, I see. I hope regular Uno will suffice.

I see. OK. Your're right. On the UNO, I'll need the analog-in's port for 1 analog-in, 2 pins from the 8-13 pins port for Timer 1 PWM, and that leaves only 0-7 port from which I'll need pins 0 & 1 for serial communication.

So I'd definitely need to split the LCD pins across different ports. Or use the MEGA version, which is what I'm gonna do, because the UNO pin count is tight as it is, without any extras.

Thank you all for these nuggets of gold! Yesterday morning, I knew absolutely nothing about this and now? I'm aware of my options, know what I need and why I need it, basic principles, requirements and constraints or this platform... Amazing. I knew you'd be of great help, guys.

I think I understand. I'm gonna read some theory about this and read your post again, to make sure I get the full implications.

EDIT: In the mean time, if you feel like it, would you watch this 3min video and tell me what kind of feedback they use? It's the same project, but the feedback looks much better. There's a rough circuit sketch, which I don't really understand:
http://www.national.com/en/video...

My device would keep the needle between a LED and a detector - kind of precarious. Hard to guess which way to move when it leaves the position and not very accurate, I'd say.

EDIT2: OK, now that you've taught me stuff, :) I recognize it's also a LED/detect method, so nothing new there, but it looks better - he uses some I/V converter and op-amp. Why? I don't know. Sensitivity, perhaps.

Damn! I should have paid more attention at school. What does integral mean in this context (I can do integration/derivation symbolically)? I'll have to research that when I'm in the final fine-tuning stage. :)

BTW: I'm looking at those PS tests and ground loops and I've noticed an oscilloscope there. I should probably get one, if it's not too pricey. Do you know these cheap ones? For a super-amateur like me, could be just the ticket:

http://www.hwkitchen.com/product...
http://www.hwkitchen.com/product...

The second one is more expensive, although the feature set looks great. I don't know many of them, but can see the difference. Being able to see Vmin/max, shape, etc. must be extremely useful.

The board osc. w/o display is cheap, but the GUI looks too simple, like it doesn't have as many functions as the DSO Quad. Plus, PC required.

I'm asking because I would shell out €174 for a useless junk without batting an eye. :)

Do you think it would be useful for this kind of project? I only have a meter like this:
http://www.northerntool.com/imag...

EDIT: Thanks! My email is foceni at gmail. It's not secret.

Yep. Entirely thanks to you guys. I'm gonna order the HW and start hacking away.

In the mean time, I have some physics classes to remember, learn about PIM, oscilloscope use, Mega2560 chip, get Arduino toolchain, research battery/charger options, etc.

I'm gonna drop back here whenever I have a related question, if that's OK.

Before that, do you know how this is used?
http://www.hwkitchen.com/product...
http://www.hwkitchen.com/product...

If you hook them up to a LiPo battery which is powering the board - is the battery usable at the same time? Does the current then come "from the battery" or "from the charger". In reference to the voltage stability we spoke about for this project. I can't find how this is connected and how it works... :-/

David

OK.

And last, but not least - do you know of a good EU e-shop for IC's, LCD's, breadboards, etc? Czech Republic has a great tradition in hobby electronics, but I can't find anything remotely as comprehensive as e.g. Digi-Key. Regular specialized shops are a dime a dozen here, but their supply is too restricted compared to on-line catalogs. They're very cheap, however (good for resistors, caps, diodes, etc).

OK, reporting progress.

- Read up on PID generals, have needle-stabilization algo more or less prepared.
- HW order prepared for shipment, most stuff is 3x. :)
- Learned Forth basics and fully decoded the source code from the PDF. Rewriting in C is going to be trivial.
- In the process learned about DDR, PORTx/PINx, serial com, LCD setup & use, PWM setup for hi-res
- Now learning how op amps work in case I need them

Here's a redrawn circuit of the newer design. I'm not really getting what is hooked where:

Personally, I would hook up the galvo/resistor on the PWM pin, IR-LED on next pin, IR-detector (with optional op-amp) on next pin. That way, I have control of all three parts.

In his design, I don't see what should be hooked to the board. There's Vmeasure for sensing, Vbias (probably) feeding the whole circuit (that looks completely wrong to me). Why have everything on one circuit?

1) What do you think?
2) All op amps of this kind have 5 pins: +, -, Vout and V+/V- powering the positive/negative output amplification. In his design, he hooks photo detector current to the "-" input, puts Vbias on the "+". I guess he also hooks up the +5V supply to the V+. No V- (GND connected there?), because the signal is all positive. Correct? I suspect not. :) I really don't get it yet. What's the purpose of Vbias.

Thanks, David

Excellent. Thing are coming together... :) Thanks guys!

I'm commited to "doing it my way". :) Different HW, different approach. Whatever will work the best during testing, I'll use that.

Based on what I've learned and can comprehend, this is my intention:

1) I'll drive the galvo with PWM, nothing else in that circuit (with a pot for fine tuning galvo range)

2) I'll use IR-LED with phototransistor and a pennon to get analog reading. Definitely. Let's say at 50% of the LED-OFF/LED-ON current range, the pennon is exactly in the middle of them. That'll be the fixed (zero) position for the scale and I can tell (by a rise/drop in the photo current) whether I need to push or pull the galvo.

3) Scale will calibrate itself by:
- reading (A) PWM % required to get "zero" with no load
- reading (B) PWM % required to get zero with 100mg load
- calculate a linear function approximating these two points

After boot, the scale balances itself to zero, acquiring value A (LCD says "starting"). It keeps balancing (LCD says "calibrate"), until user places 100mg reference load and presses "CALIB" button. That will set value B. When the scale has value A & B, it will start displaying the weight on LCD.

This will be optimized by storing A & B in EEPROM (AE, BE), so as not to need calibration every time. After boot, A is calculated and if AE & BE are available, it is offset by the same amount A is offset to AE). Not very precise, but hey, what do you want w/o calibration. :)

"TARE" button will save current weight to "T" and subtract it from the live weight measurement (until user presses TARE with no load). By default T=0.

4) The actual weight measurement: PWM is giving 0-5V in at least 1024 increments. Optical feedback is used to change the duty cycle +/- to get to the "zero" value, which I know means "pennon is in middle" (and for now is hard-coded). A is PWM% to get empty needle to zero position, B is PWM% to get 100mg to zero position. Weight (mg) = (PWM% - A) * (100 / (B - A)) (linear interpolation).

PS: Rewritten and in places better formatted Forth source easier to understand:
http://pastebin.com/ZEMn3DWX

Their PWM drive is based on bit flipping the 10-bit PWM duty cycle register bits to HIGH. Goes from MSB, when it overshoots, sets bit LOW and moves to the next one. This makes the needle as high as possible, but not so high as to leave the detector slot. Apparently, it too works fine (has clear boundaries too).

So there's no ADC in use at all. They go by PWM% and the slot, which is the same I'm going to do. Just in a slightly different way.

Hi, I have a question about op-amps. What's the deal with V+/V-? Does every amplifying configuration (with or without negative feedback) require both supplies? AFAIK, there's no V- available on AVR chips or Arduino-based boards. And I understand it's not a trivial task to "get" one. Everybody suggests the "two batteries" solution, but that's not possible when the supply is USB, wall-wart, etc. How do you make it work?

If I want to use an op-amp as a non-inverting amplifier, do I still have to supply both V+/V- on the two supply pins?

David

The opamp question was just out of curiosity. I'm reading about them a lot to learn something new and just in case.

I'll use just a transistor in the balance.

Your suggestion regarding the cap is to make an RC filter, right? I may have to add more caps to filter as much noise as possible. Like this guy:

I'm leaving for abroad (3 weeks) on business, but I may work on it at the hotel. Evenings are usually pretty boring there. I'll be online and keep researching.

I was thinking the level detector is going to be crucial (even more than stable supply). Consider how little the needle bends when you put an eyelash on it! The detector has to be able to recognize it. I need to "multiply" the tiny movement somehow. E.g. like placing the pennon at the farther end, not by the coil. Angular distance is longer there. Or come up with a different detector. Perhaps something based on magnetic fields.

Check the NatSemi version, his photo detector is just below the needle's center of rotation, where it's fixed. If he's measuring the tiny distance the circular plate turns, it must less sensitive than with a pennon at the other end of the needle, being lowered (dipped) into the detector...

What do you think?

I know, it only was attached as an illustration to my comment about maybe needing some caps to do filtering of imperfect DC.

You're the man! :) Thanks for helping so much. This is awesome. I'd never be able to design something like this.

Some questions:

1) I get the "5V reference with min. temp. coeff" part with LM336 (I found the data sheet). Even though I don't know why LM336 works like that in this setup (I know Zener diode function, but this has 3 terminals and is "configurable"), I trust the docs. But can you explain what R1, R3 and C1 are for? I suspect R1 is to limit the current or create a voltage drop, but that's all.

2) The second circuit I don't understand at all. Could you describe how it works?

Sorry for the dumb questions, but it really helps me in understanding & remembering how current behaves in circuits. I used to solve differential equations for R/C/L loops and compute R/U/I values anywhere in the cicruit (at school), but when I started this project I could hardly remember Ohm's law! Unbelievable.

David

OK, HW is here and I've been hacking on it the whole weekend. :)

Already, I have some results that I'm gonna present here. I've tried two configurations of the galvo connection. One with 200R (10mA) in a series and one with 2R0 (1mA).

For now, it's not mounted on anything and there's no photo-interrupter feedback. I use the serial connection to Arduino to set PWM duty cycle to any value I want and then read the results off the galvo.

I don't have any calibration weights yet, so I use 0.4mm stripped copper wire as a reference. Like this:
PI * (d/2)^2 * 0.00894 * length [units = mm, density = 8.94g/cm^3]
With that I made 3 weights:
3mm = 3.368592mg
10mm = 11.22864mg
20mm = 22.457280mg

The equation checks out, so I was surprised that such a tiny 3mm piece of thin wire would weigh as much as 3mg! But it looks like it's correct, wire is plain Cu and math looks OK to me.

I used super-precise measurements of the wire length and surprisingly enough, the measured values look perfectly linear on the graph. The only issue I have is with 0 (no weight). Check out the calibration graphs:

As you can see, measuring is perfectly linear with my weights, but in both configurations, the "zero" point (empty needle powered so it reads 0) is out. Not by much, but it's there and I don't have small enough weights so as to be able to see how it behaves down there. Perhaps you'll think of something, why it does this.

And finally, some stats. Very precise, but doesn't have much range in the sub-mg area (2k ohm). The galvo is super sensitive, the slightest air flow moves it, but I guess sub-mg weights are so tiny, that even these results are great.

200 ohm (10 mA):
- PWM @ zero = 54 (5%)
- max = 156mg
- resolution = 0.160mg (156 / (1024 - 54))

2k ohm (1 mA):
- PWM @ zero = 347 (33%)
- max = 26mg
- resolution = 0.038mg (26 / (1024 - 347))

To get near 1µg sensitivity, one would have to use a 20k (or 200k) resistor, which would (by extrapolation) give max lift of about 4mg (0.66mg), but you need more than that just to move the needle, so it would be impossible with this particular galvo.

Nevertheless, despite these facts, I'm quite satisfied. Plus, it may be possible my calibration weights are off by order(s) of magnitude :) and in that case all these numbers would shift down and make for a pretty neat 1µg scale. :)

BTW, I use the small galvo from a generic 0-15V panel of about 10x10cm. I removed the fitted resistor to get my own range.

So, to sum up:

1) Can you see why the zero point doesn't fit the linear progression?

2) How do you think the NatSemi guy got so much precision with such a huge galvo? I mean my needle is really a needle, his needle is massive in comparison. The movements on his scale must be invisible by the human eye.

Thanks.

I'll reply in depth later tomorrow, I'm driving back to Germany on a project in the morning. I'll be a busy week, but maybe It'll take the Seeeduino with me and experiment after work. If so, I'll take some pictures for you. If not, they'll be available next weekend.

I also connected the detector, but on its own, not via the board. It looks good, I've experimented with different resistors on the transistor emitter and had the most success with 300R. On it, I read 2.9V when the slot is free and 1.2V when the tiny needle passes through in the center-most part.

I've tried larger resistances and with them I got full 5V when free, but the voltage drop was much smaller when the needle passes through. I'm not sure why, guess it has something to do with transistor saturation etc., but the 1.7V drop with 300R should be sufficient to recognize some pretty tiny movements.

The galvanometer is pretty cheap - there is no special bearing, just a coil rotating around its axis, the axis being a needle (sharp on both sides). It's kept in place by two spiral springs (one on each side) that twist in the opposite direction. Inertia doesn't seem to be a problem, but the clearance is. I had to fiddle with how tight the axis needle fits between the bearings and rotate the springs, otherwise when you moved the needle up a lot, it returned slightly higher than it started and the same when moved down. It doesn't return to the exactly same position.

I unscrewed the bearing as much as possible and now it moves freely and the clearance is minimal, but still visible by the naked eye. I guess something much pricier would be required to solve this.

Calibration - I also thought about that. :) But I don't have a sub-g scales, so it wouldn't help me. I found a rather cheap calibration weight set: 1mg - 100g for $47 (class M2, probably). In the E1/E2 class, you pay this much for one piece! :)

You could use these - cheapest I've ever seen, by much! For me, with intl. shipping, they'd cost more than the ones I ordered:
http://www.americanweigh.com/pro...

You must tell me more about the DDS, sounds interesting.

No pennon at this time. It would be better with, but I need to hang a scoop at the end of the neeedle (like a reverse parachute) and even with the lightest of materials, it's gonna drag it down and lower the max capacity somewhat.

On the other hand, with the pennon, I'd have a clear idea where it is in the slot - voltage ranging from full on to full off. Without it, I need to hunt for the lowest voltage possible, but not really knowing if I'm moving too far down or up. That's rather inconvenient and would have to be solved in SW.

The noise - good idea. The problem is I'm not sure that's gonna help with the "zero/empty" position. In that position, even the slightest PWM +/- moves the correct amount up/down. It doesn't stick. It moves smoothly with or without any load.

I think the non-linearity of this point may be caused purely by physics. I mean when I say that a needle in the horizontal position is "0", I'm actually already pushing some weight - the needle itself (not 0mg).

Then, when I place extra weight at the tip, I'm pushing the needle + the weight, but I also shifted the center of gravity of the combined weight and its weight isn't proportional to the empty needle, because of how lever/fulcrum works.

What do you think? I want to fix this one first and them move on to the interrupter & pennon.

What's weird is that if my theory is correct, the deviation of the empty needle should be consistent, but if you look at those graphs, it's not. First was lighter than the expected linear progression, the second was heavier.

DDS: I see, it's a signal generator, nice. For testing purposes? That's your page you linked here? If so, you've been quite busy I see. :)