next topic »

[on:]

I'm trying to figure out a way to make a simple and low cost device to help balance RC brushless motors.

While I'd be open to other suggestions, this is the idea that I had in mind that seems the simplest.

I could attach the motor to a loosely mounted motor mount that has an accelerometer (or some other sensor) attached.  It could monitor the values of that sensor, and each time one particular axis is at its peak value, it could strobe an LED light.  Should work like a timing light, each time the heavy part of the motor comes around, the light strobes so it basically freezes motion, showing exactly where the heavy side is.  Allowing you to make adjustments until the motor is in balance.

My biggest holdback is trying to figure out what sort of accelerometer, or other low cost sensor will function quickly and accurately enough to work. 

Anyone have any suggestions what sensor to use, or some other general pieces of advice towards making this project?

[Reply #1 on:]

I don't know if it would be a problem in practice, but I'd be concerned about how high a frequency you need to spin the motor at for the imbalance to be detectable, and the resulting frequency at which you need to read the accelerometer.

I wonder whether you could use some sort of piezo mounting as an AC signal generator and then use a peak detection circuit to trigger a strobe when the peak occurs. What that doesn't do is give you any idea of the magnitude of the imbalance, which of course you need to know to make any correction and also to know when the balance is good enough. Maybe you could detect the signal level as well? Quite a lot of electrical hardware to design though, to get it working.

[Reply #2 on:]

Could be interesting project. SFE has a huge list of devices:
https://www.sparkfun.com/search/results?term=acce&what=products I just check one  for speed of conversion, and it's good up to 400 Hz.
https://www.sparkfun.com/products/9652   400 Hz translates to 24000 RPM, which is quite high, and should be fine for vast majority of motors.

[Reply #3 on:]

400 Hz translates to 24000 RPM, which is quite high, and should be fine for vast majority of motors.

Remember that this is your sampling frequency, not the motor speed. You would probably want to sample at least a hundred times per revolution in order to get a reasonable angular resolution. Given that the effects of imbalance are proportional to speed squared you really need to be spinning the motor somewhere near its normal working speed to balance it. If that's 10,000 rpm, it would need a sampling frequency of 16600 Hz to give 100 samples per revolution. Maybe you don't need that many samples per revolution - I don't know how many you need in practice, but I'm sure it's a lot more than 1.

[Reply #4 on:]

Well, I wonder if it would be good idea to sample with 399 Hz (1 Hz less than the revolutions per second). Then the beat frequency would be 1Hz. Very easy to detect This is bascially the same approach as I use here: http://blog.blinkenlight.net/experiments/measurements/power-grid-monitor/. The tricky part would be to infer the angle of the imbalance by analyzing the phase of the beat frequency. I never tried this but I would expect that this is feasible.

[Reply #5 on:]

As an afterthought: somehow it would be necessary to find the "0" position of the axle --> need also some kind of sensor to detect the current axle position.

[Reply #6 on:]

If anyone thinks a piezo would work better than an accelerometer, I'd be open to hearing that idea.  My main criteria for this project is low price and simplicity.

Considering right now the only way I can attempt to balance motors is basically, see how much it vibrates, start taking random guesses as to where to add weight, and just keep using trial and error to try and make it feel like it vibrates less.  Not efficient at all, in fact I usually waste a bunch of time doing that and give up. 

Obviously I'd like this motor balancer to work as good as possible, but considering I am a bit of an arduino newbie, if I go trying to make something too overly complex, it will never get done.

[Reply #7 on:]

For newbie, you better to start with "off-the-shelf" sensor, simply because you must be an expert in both: technology/science and electronics to be able to design sensor. Depends how much time you have it would take a years.  I'd start from research on "off-the-shelf" complete balancing system, than move one step at a time from output to input, trying to improve algorithm/math, interface / ADC, only last stage - sensor.