This is another Open Source project brought to you by IF.
You know how we all always wonder about which motor is best for our planes, and then which prop works better. Along the way we get offered lots of useful data, and anecdotal advise - which is useful, but sometimes conflicting. Plus it is hard to choose because there are so many options available and combinations. Well, as you can guess, I'm one of those guys who can't live with too much ambiguity and guesswork. So, this project was born, out of a conversation I had with Mark late one night about how cool it would be to be able to settle this issue once and for all. For people who are into this kind of thing, who need actual numbers, I thought of calling this the Automated Normalizing Algorithm project (or ANAL for short). Okay, maybe not.
The device is an MCU controlled thrust test rig. It will be able to control the motor power, and at the same time, read out voltage, current, power, thrust, throttle, and perhaps some other data. (provision has been made for temp and RPM but not currently used). With the MCU controlling the test, hopefully there will be good repeatability.
One of the key components for this project is to have a digitally readable thrust sensor. If you look around in the market, you will find that this usually means some kind of load cell or strain gauge. These things are not cheap, however, and can be quite involved to set up and calibrate. Maybe a bit too involved for my liking. So, I went out to the shops and found myself a 5kg range digital kitchen scale (CAMRY EK5350) with the hope of hacking it so that it can be used for my purposes. This cost me some USD28 or so and was a gamble. In fact, you can probably find cheaper ones on e-bay by my impatience was getting the better of me.
Okay, so now the whole project hinged upon my successfully getting a digitally readable thrust sensor. I got home, put in the 9V battery and fired it up to test it. Sure enough - 5kg limit before giving the ERR message. Time to whip out the screw drivers and after a little examination, I found out how the case comes apart. Opening the case reveals a standard strain gauge used as part of a Wheatstone bridge implemented on one single PCB. The strain gauge feeds the bridge which is in turn processed by a quad op-amp. The output then goes to a MCU under a blob of resin so there's no way to tell what the part is. Measuring the voltage from the op-amp proved difficult since the output range was in the order of less than 100mV for full-scale, which will require a high-resolution ADC or a scaling-amp or both. I wasn't really intending to do so much work for what seemed like a relatively simple project.
So I figured, there must be more than one way to skin a cat, and no hacker worth his salt will get stumped so easily, or give up and start asking others! So, out comes the oscilloscope and after some probing, I found an unusual square wave around a CD4066 switch array. This was interesting because it looked like some kind of PWM. Following the signal through the switch got me a nice clean PWM signal inversely proportional to the weight! Bingo! Got it. This will work perfectly for me. Simple tests reveal that this will do the trick so my 28 dollar gamble paid off and we're in business.
<to be continued>
Edited by Daniel Wee, 18 November 2007 - 08:48 AM.