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More Myxa tuning questions

Hello,
Since last time, we’ve been able to integrate Myxa onto our multicopter successfully, but I had a few more questions regarding optimal performance:

  1. With the default voltage ramp settings, the motor was not very responsive at all, so we increased the voltage ramp to 50 V/s (a value i found in one of the Myxa forum threads) and the stability of the drone improved quite a bit while we were flying as a quadcopter. However, the forum recommends increasing current_ctl_bw as necessary to maintain stability. I haven’t done this yet, because I found that the motor does not “fail”/ESC does not report any error. However, my bench testing was not very thorough, and since switching to an octocopter, we’ve been seeing some instability (may or may not be related to the PX4 rate controller). I wanted to know if you had any tips on a more robust way of checking/stress testing for motor stability.
  2. Continuing from the previous note, what is the maximum recommended voltage ramp? FYI we are running the XOAR TA6012 130KV motor at 12S/50V, 24 inch propeller. The motor is an 18N24P configuration with a max continuous current of 19.9A. I am wondering if the high motor voltage of 50V requires the use of a higher voltage ramp to maintain the same responsiveness.
  3. We’re still hearing some annoying whistling sounds sometimes as well as a lot of vibration from the motors (and in general higher acoustic output than would be ideal). Granted that this might be a completely unrelated mechanical problem, but I wanted to check if there’s anything you’d recommend adjusting or testing. I did motor identification without the propeller.
  4. Wondering if we should stick with voltage control mode or switch to RPM. What are the benefits of RPM control mode, both in stability and efficiency? We’ve been reluctant to switch so far since we’re worried about motor responsiveness and ensuring our RPM controller tunes are the best they can be.

Thanks!
Kalyan

Hello, just wanted to add that we tested with a much higher voltage ramp (150V/s) and a current_ctl_bw of 0.08. The drone performed much better, which makes me wonder if an even higher value will be of benefit or if the remaining tuning should be done on PX4’s PID control loops? I wanted to check before increasing the value even further to make sure there are no adverse effects.

Please let me know as soon as you can!

Thanks,
Kalyan

Hi Kalyan,

As described in this post, increasing of the current control bandwidth can lead to instability of static operation, while an increase in the voltage ramp can. Before switching to an octocopter together with the autopilot, you should first test at least one axis on the test bench in the entire range of speeds and setpoint steps. I want to emphasize that the maximum voltage ramp and bandwidth values are best checked on the test bench. These values may vary depending on the size of the propeller, motor, etc.

The annoying sounds at some speed values in static operation may be due to the big current control bandwidth value.

The RPM control mode has two main advantages. First, it can improve responsiveness by increasing the order of the system. Secondly, in this mode, the system is less susceptible to the influence of load disturbances caused, for example, by gusts of wind.
But there are also disadvantages. In particular, the controller is more difficult to be tuned in this mode.
This mode has no effect on energy efficiency.