Limiting error between attitude controller target (ghost) and aircraft attitude in Trad Heli

Yes, so this is a way of exciting the aircraft with multiple step functions that would then allowing us to combine all those responses into a single representative step response. It ensures that a single resonance would not mess up the result or cause a resonance in the aircraft.

Provided I add the signal to the output of the rate controller (input of the output mixer). The aircraft should be able to be excited up to frequencies of half the output rate to the servo or esc’s. To make maximum use of the characterisation signal it should be injected as late as possible into the system and measured as early as possible. The problem with passing it through the attitude or rate loops is the signal will be attenuated/distorted significantly by the PID loops. This would significantly reduce the bandwidth that we can characterise.

The advantage of the BPSK signal is it will let us characterise the actuator/airframe/IMU system to half the IMU sample rate even if the servo output rate is only 50 Hz.

I’m fairly familiar with PSK, use it in ham radio applications for years and designed and built a few hardware modulators/demodulators for it. I don’t know what you’re using for a modulator/demodulator, or what the data rate is. But normal BPSK is two-phase, with the phases 180 degrees out of phase, and rather limited data rate since it’s modulated one bit per symbol. Or are you really using a quadrature 4-PSK scheme for higher data thru-put? And if so, modulating/demodulating 4-PSK involves a fair amount of processor overhead.

I guess the question is more along the lines of whether you are after just rigid body modes of the aircraft response or do you want to capture elastic modes as well. I know for bigger models the elastic modes may be closer to the rigid body modes. I suspect that most of the response you are interested will be less than 5 hz. With the X3 heli, I had FF_ENABLED off and a P gain of .05 and the aircraft was getting out of phase with the stick at less than a half hz. The gain was crossing zero db around 2 hz. Point being that even though the excitation frequency can be up to 25 hz for heli’s, I think our freq sweep should only go to 5 or 10 hz at most. For Helis I don’t think there will be useful control response above 5 hz. From what I have read, you need a sample rate of 25 times that to conduct a good spectral analysis.
Let me know what frequency range you want the chirp to sweep and I will look at getting some data.

I think that is 2.5 times. For most of my DSP work I like to have 3 or 4 times but when you are collecting data it doesn’t really matter because in the last bit you get what ever you get.

So regarding the frequency range. I want to support as much research as possible. So the maximum frequency range will be specified by the user. That way we can get really clean data at say 0 to 40Hz for control optimisation or 0 to 500Hz for mechanical analysis. In both cases the data is being recorded by the IMU at up to full rate of something like 2KHz.

That would be very interesting. At this stage I would be happy to get what every you thought was safe for you to collect.

Well then I just want to warn you and others that may do this on a helicopter. There is a rotor mode called the lead lag regressive that is lightly damped. You want to be very careful exciting this mode. Personally I may just choose not to sweep through this frequency. I don’t know for sure but I think it will be around 15 to 20 hz for our model helicopters. But you can just do a FFT on the lateral acceleration signal and look at what frequencies pop out. It will show up at about half of the rotor speed depending on how tight your blade mounting bolt is

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Good to know! Thanks @bnsgeyer

This is where the BPSK excitation is useful as it shows the full frequency response of the system with a bunch of small step inputs. If we use 1000 steps we can use 1/1000 of the magnitude to get the same accuracy of the response.