Audio Spectrum Analyser results

spectrum analyser output

Audio spectrum analyser results

I ran a quick test indoors playing with the audio spectrum analyser for my iPad.  Quite a cool piece of kit, though I did pay for a decent one.

The plot shows peaks at about ~65Hz, 126Hz and multiples of ~65Hz thereafter suggesting an RPM of about 3780.  Kv for my motors is 980 rpm / V which suggests 3.86 volts.

The PWM was set up with 1400us pulses – given the range is between 1000us and 2000us, this suggests 40% power from a freshly charged (12.2V) battery => 4.88V

To be honest, it doesn’t surprise me these two values don’t match.  In fact, I think Kv is as good as irrelevant for use with brushless motors, and in fact provides just another obfuscating factor to confuse the user:

Brushless DC motors work much like stepper motors. They are fed a 3 phase signal at a given frequency, and depending on the number of coils / magnets on the stator / rotor, the motor spins at a fraction of that 3 phase signal frequency. The amount of voltage driving the current they draw simply provides the power level required to drive the motors with a load (i.e. propellers) at the frequency defined by the 3-phase signal.

These days, the ESC uses a microcontroller to interpret the input PWM, and uses that simply as a factor to scale the motor rate from 0 to 100% motor spin rate.  The ESC drives the motors with 3-phase PWM-like digital signals. They use inductive feedback from the motors to track how much power (i.e. the width of the 3-phase PWM pulses) needs to be applied to the motors to make them spin at the desired rate, regardless of the load (i.e. propeller size / weight).

In the world of brushed motors where the magnets are stationary and the coils spin, I can see how Kv is relevant: the rate the motor spins is directly proportional to the voltage applied.  But brushed motors generate huge amounts of electrical noise, and wear out quickly, and so simply aren’t suitable for UAV’s (Unmanned Autonomous Vehicles – i.e. RC boats, planes, cars) of any kind.

IMHO, in the world of brushless motors, the mathematical value of Kv is frankly utterly meaningless, and only provides an arbitrary measure of what kind of battery can be used to provide suitable spin speeds for a given set of motors / propellers:

Kv of the order of 700 – 1000 are suitable for 3 cell / 11.1V LiPo’s with small propellers (8 – 10″)

As the Kv value drops into the say 300 – 700 range, then more cells are required to provide the same RPM – typically though, motors with these Kv values are intended for use with very big blades, where the required rpm to produce the same force is much less.

Obviously all the above is based upon my opinion, so may be utter bollox – I’m more than happy to be proven wrong.

P.S. On the plus side, the single sharp peaks at 65Hz intervals does suggest that all of my motors are rotating at the same speed, and none are obviously duff, which annoying means I still have no idea why yaw compensation is still plaguing Phoebe’s flights.

8 thoughts on “Audio Spectrum Analyser results

  1. silly question perhaps, but how do you link the frequency with RPMs ?

    “The plot shows peaks at about ~65Hz, 126Hz and multiples of ~65Hz thereafter suggesting an RPM of about 3780”

    • The peaks in the display are the noise created by the propellers as they rotate. Each prop blade passing will produce a peak, meaning the largest peak at 126 Hz is double the rotation speed in rotations per second. So half that and multiple by 60s per minute gives you 3780 rpm

      • Great, thanks!

        I was thinking that this could be used to measure the relationship between RPMs and PWM signal.

        As this might well be non-linear, meaning that potentially, one would need different PID settings for different parts of the curve?

        Or, if the non-linearity is significant, you may have to find out the best PID tuning for different regimes and then make a compromise when choosing a tuning that works OK spanning your operating range?

        • Hmm, interesting again. Assuming linearity for the moment, since motors come with a Kv gain specified where rpm = Kv * V, where V is essentially the ESC PWM ratio from the LiPo nominal 11.1v, you could know enough to remove the ESC from the system, and directly feed a MOSFET from a modified PWM signal to drive the motors directly.

          That really appeals to me as I really don’t like the presence of the ESCs – it’s microcontroller driver, and I have no idea what it’s doing.

          Certainly worth another play with the spectrum analyser tomorrow to at least see if there is linearity – if there is, I can feel a rebuild coming on to make my quad 100% Raspberry Pi controlled!

          Cheers – starting to feel I owe you a beer or two – are you anywhere near the Cotswolds?

  2. To make sure the 65Hz is from the props, you could also measure the background noise without the props turning… and substract that from your initial measurement…. Just a thought…

    • The analyser was running long before the blades were – as the blades span up, these spike rose up from the general background noise, so it was pretty clear. I guess you just has to be there 🙂

  3. Hi Andy,
    Thanks for your mention of my blog.

    I didn’t got time to perform the same test.
    What I would do is to see if there is a linear reletion betwen sound freq. and pwm setting. This will help to understand what is recorded in the app.
    For example in my hardware set up teh prop starts to turn for pwm>5% (1050 )

    • I agree, especially as the PIDs depend on this being roughly linear. I might give this a go next time I’m trapped indoors by wet English winter.

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