Measuring Motor RPM

[backtrack9]

Ok everyone - has anyone found a good repeatable method for measuring the speed of these little motors? I am not talking about running it on a treadmill (very inacurate) to get the speed of the vehicle and then backing out the motor speed through calculations. The Tamiya speed checker is similar to this, but I am still leery about the results. I have been looking at using a technique outlined on the website by Roman Black - www.romanblack.com - showing how to measure the back EMF pulses from the motor over a given time period to indicate motor RPM. Also there is the method of measuring the current draw at various loads to calculate the motor's characteristics and then from these calculations use this motor as a generator when connected to a drive motor. This simply makes a dyno system with the test motor coupled to the generator motor. To test the speed, you would then connect the test motor to generator motor and only power the test motor and then measure the voltage created by the generator motor. This voltage is directly proportional to the speed of the generator motor shaft, which is the same speed as the test motor. The only problem is that this proportionality constant can only be found by first knowing the response of the generator motor at various RPMs. This means that you need to know an RPM to calculate an RPM. I'm sorry for such a long posting, but I am very frustrated at this point. As far as explanations are concerned, I can only explain things to a small degree - not much time - so please do some research before asking too many questions. I might be able to offer some links to motor theory and calculations. Thanks for your input.

[neurokinetik]

I could design and build a tachometer, I just haven't felt like doing it yet. You could do it with an optical wheel, a slot-type optical sensor, and a couple of counter chips. Alternatively, they sell tachometers for model airplanes that read off of the propellors. Something like that could be made to work as well.

[TypeZer0]

this could work....for almsot the price of an xmod though http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=TCH-4&type=store

[neurokinetik]

Nice find. I may end up picking one of those up.

[backtrack9]

Thanks for the ideas everyone. Just one small word of caution about the laser tachometer method. These units require a reflective target be placed on the rotating shaft to be measured. These targets are sometimes as small as 1/8"X1/8" and sometimes as large as 3"x3". The higher quality tachometers (>$500) are the ones that can be used for smaller targets. I am not sure if the unit mentioned above requires such a large target or not. I think that for $50 it might be worth a test. One method to get around this would be to place a larger diameter adapter on the shaft of the motor so that you can use a larger target. Possibly this target adapter could be made out of plastic, wood or even rubber. Just something to put on the end of the shaft so that a larger target could be used. Thanks again for all of your help.

[neurokinetik]

Well, I would just hook it up to a wheel that I swiped out of an arcade trackball. That's about 2" in diameter.

[backtrack9]

Ok, so I picked up one of these laser digital tachometers to see if they could indeed be used to measure the speed of these little motors. After placing a small target adapter on the end of the shaft of a Plasma Dash motor, I turn on the juice (2 AA batteries) and tested it out. The thing works like a champ. I just have to align it properly and I get a real-time RPM reading. AWESOME!!!!. Here is just a quick taste of some results for the Plasma Dash under no-load conditions: 20,580 rpm @ 2.56V (old/weak AA's). I measured the no-load current at 1.18A. This is exciting. Now I can quantify any gains made on motor modifications. I will try to measure some other motors and give results. I think Neuro will soon be able to provide the same info with his new o-scope. I just want to say thank you to TypeZero for the great heads-up on this useful little tool.

[nobodyhere962]

off the topic but
when was xmod sci put up?

[suprafreak33]

with in the last week

[TypeZer0]

Originally posted by backtrack9
I just want to say thank you to TypeZero for the great heads-up on this useful little tool.

no prob. i was actually looking for this small gadget that i saw from Lego a year or two ago where it would measure the speed of the car you build by measuring the rotation of the shafts. coudln't find it so i started looking for small tachometers and came across that. glad it works well. maybe you coudl put up numbers on the motors you have and maybe we can compare it with neuro's measurements