Heli System Identification / Logging Sample Time

Hello, I want to perform a system identification test on my helicopter. I’m planning to use matlab’s sysID tool for system identification. I’m thinking of using gyro_y data as system id input and rate_p_out output. But I’m having trouble getting the gyro_y output and the rate_p_out values ​​at the same time intervals. Can you suggest a solution? (Transfer function= gyro_y/rate_p_out)

Hi Burhan,
In order to get the data in a format suitable for analysis, you’ll have to parse it out of the RATE and SIDD messages according to the time listed with each message. I typically use the SIDD as the time for analysis and then use the last posted message for RATE and treat that as being the same time as the SIDD message. You could also interpolate the RATE message if you want it to be more time accurate but I have found my method to work fine especially if you are sampling data above 100hz.
In order to parse the data you will first need to convert the log file from a .bin which is binary to a .log which is ascii comma delimited. Or you can convert it to a .mat but I have never worked with the file in that format. It may be easier but I don’t know. If you are savvy with Matlab then you might want to try it.
Regarding identification using Matlab, are you planning to identify a bare airframe model? Be aware that due to the highly coupled nature of helicopters, the sysid mode will not allow you to sweep one axis without the other axes having highly correlated coupled inputs. This will affect the identification process.

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Thank you very much for your reply.
When analyzing the input-output signals on the log data, we get the output data very noisy. Do we need to do a filtering in Post-Processing?

Yes it is common to have noisy gyro signals. You could use the harmonic notch to help remove the higher frequency noise. Just remember that any filters you apply to the output without applying them to the input will show in the frequency responses and affect the identification. I normally don’t correct the input signals for the notch because it is up at 20hz for heli’s. The identifications that I have done only identified the model for frequencies up to 10 hz. Any other filtering, I would do post flight and apply it to both the input and the output signals.