More electronics projects on mikekohn.net here.

August 7, 2010

Introduction

So while working on the Linksys Quadcopter project, I decided I needed to figure the speed the propellers were spinning so I could calculate thrust. I looked online for some cheap tachometers, but the ones I found required sticking a piece of reflective tape on the spinning object. Heh, that wouldn't mess up the calculation at all. So anyway, using the IR concept from my R/C car drag racing christmas tree project, I made this circuit that would count the number of pulses on the IR every second and output the result to the computer over RS232 which could then divide it by 2 (since there are 2 propellers) and multiply by 60 to get revolutions per minute.

Taking the RPM and propeller model and plugging it into this website: http://www.badcock.net/cgi-bin/powertrain/propconst.cgi thrust can be pretty accurately calculated. Measuring the weight of the Linksys Quadcopter on a scale with the motors off and with the motors running full speed showed 800 grams of thrust. Finding out the RPM of the propellers and plugging in this website showed the same result.

Pictures

	Here's a view of the output from the IR detector while running 10x3.8 propellers at almost full speed on the Linksys Copter. Originally I was calculating RPM just by the output of the IR into an oscilloscope, but after testing the IR part, I added the microcontroller which counts the IR pulses for me and outputs RPM on a computer screen.
	This is how the circuit board looks. At the very top right are the IR emitter and detector. I covered them with shrink tubing to stop leakage and some ambient light. The micro (on the far left is an Atmel ATtiny2313 and the chip in the middle is the unforutnately almost extinct DS275. I need to start a "SAVE THE DS275!" campaign.
	Schematic for the circuit.

Source code
tachometer.asm - ATtiny2313 firmware.
tachometer.c - Linux source to read from device.

Copyright 1997-2012 - Michael Kohn

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