MC gyro cutoffs - Reduce IMU_GYRO_CUTOFF to 30Hz and disable MC_DTERM_CUTOFF #11950
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Test at 30Hz on a S500, default gains (except rate integrator a bit higher): At 20Hz I started to feel the delay on that vehicle with default gains, at 30Hz the feeling was quite good. I will increase the default to 30Hz in that PR and test on other vehicles. |
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This is already a very common change to make on typical hobby quality setups, so I'm in favor of changing the default for v1.9.0. The problem is the good setups where this default was appropriate are now going to get worse after an upgrade. In this case all we can really do is call attention to the change in the migration guide section of the release notes. |
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As most people who use drones are professionals, default parameters is most for beginners who uses F450 S500. Lots of PX4 users are in university, they good at algorithms but not quite good on making drones, they also do not have much time for adjust the parameters. I think maybe add a frame preference for large drones instead of change defaults is better, make students and testers can have an extremely flight at beginning, will also make them dig deeper. |
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The S500 needs 30 Hz - that is the whole point why changing the default makes sense. |
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https://twitter.com/bys_1123/status/1124762888147619840 |
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@PX4/testflights Can you test that PR on all you multicopters please? Make sure to have:
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@bys1123 Thanks for the feedback and the video. This PR is not just about the S500 and I understand and see that you vehicle flies properly with the current cutoff value. However, I can see in the video that you have an isolation mount under the autopilot, which certainly reduce the amount of vibrations transferred to the autopilot and makes the job of the digital filter; not everyone has that and we should make sure that even the drones with hard-mounted autopilots can fly with default values. This default value is also something needed for larger vehicles (e.g.: >15kg drones) that have low RPM rotors with a blade-pass frequency that can go down to 50Hz. Natural resonance modes of a frame are also often located in the 30-80Hz range on those vehicles and we should make sure that the sensitivity is low is that region. For you application, simply set the desired D-term cutoff and gyro cutoff in your dedicated config file or manually in you vehicle via QGC and you won't notice any difference. I still encourage users to tune the filters for their application as it can 1. improve attitude/rate tracking and 2. increase stability margins. Check the user guide for more details on how to tune the low-pass filters of the rate controller. |
| @@ -538,6 +538,8 @@ PARAM_DEFINE_FLOAT(MC_TPA_RATE_D, 0.0f); | |||
| * The D-term uses the derivative of the rate and thus is the most susceptible to noise. | |||
| * Therefore, using a D-term filter allows to decrease the driver-level filtering, which | |||
| * leads to reduced control latency and permits to increase the P gains. | |||
| * A good rule of thumb is to fix the time constant of this filter equal to one tenth of the | |||
| * derivative time constant: MC_DTERM_CUTOFF = 10 * MC_(ROLL|PITCH)RATE_P / (2 * pi * MC_(ROLL|PITCH)RATE_D) | |||
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This formula gives too high values for the setups I looked at (about 130). Also, setting this properly depends on the vibration frequencies, which are not in the formula, so I don't think it's very helpful advice.
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This rule means "If you choose that cutoff, you are sure that the filter will not add too much phase lag". I checked and in the config files we have more
MC_DTERM_CUTOFF ~= 5 * MC_(ROLL|PITCH)RATE_P / (2 * pi * MC_(ROLL|PITCH)RATE_D)
Maybe having that factor 5 is better for our setups given the amount of noise.
Should we remove completely the rule of thumb of the parameter description and add it to the documentation when we have more tests on that equation?
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If it means something different, why didn't you write that then? :)
Yes it sounds like it should be in the documentation, and more in detail as to what it means. And since we're lower by a factor of 2, and it still works fine, the formula doesn't seem to be a strict bound, so I question a bit its usefulness.
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@bresch Really thanks for your information. I'll transfer this info to Chinese community too. |
Tested on PixRacer V4:He made sure to have:MC_DTERM_CUTOFF = 0
Pre-flight Procedure: Flash firmware through QGC ✓ Flight Test:
No issues noted, good flight in general. Flight log: 1https://review.px4.io/plot_app?log=4278e8c1-fe6b-4ff9-a99f-e8eaede0c9bb Flight log: 2https://review.px4.io/plot_app?log=ca75be8c-45d4-4982-af4e-91deddc77fcb Flight log:Comparison to master.https://review.px4.io/plot_app?log=35de3fc0-fa47-4cc4-8558-109761e6fba4 Tested on Pixhawk 2 Cube V3:He made sure to have:MC_DTERM_CUTOFF = 0
Pre-flight Procedure: Flash firmware through QGC ✓ Flight Test:
No issues noted, good flight in general. Flight log: 1https://review.px4.io/plot_app?log=be819c09-6175-45e1-a771-52e134e03325 Flight log: 2https://review.px4.io/plot_app?log=06dbdf93-78d3-4b7b-96fd-3347f99b8e7f Flight log:Comparison to master.https://review.px4.io/plot_app?log=24027025-b9cd-4f23-97ed-67b1ec94061f |
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Pixhawk4 v5 Flash firmware through QGC ✓ Flight Test: Procedure logs: https://review.px4.io/plot_app?log=adac11f7-9500-4220-aba1-6a8a45288a6e master: https://review.px4.io/plot_app?log=04032970-bb7e-4aed-9f42-e9cdb1273029 |
Tested on Pixhawk 4mini v5Preflight Checks:
Modes Tested
Procedure Notes Logs:
Tested on Pixhawk Pro v4Preflight Checks:
Modes Tested
Procedure Notes Logs: |
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@jorge789 @dannyfpv @Junkim3DR Thank you guys! |
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Let's see if that helps with the reported osciallations. I would also take the rule of thumb out. There are too many influencing factors, it's probably better to just leave it untouched if you're not really tuning. We can give some real world examples as reference in the documentation.
…_CUTOFF. A low gyro cutoff is needed for most medium/large size drones as the structural natural and blade-pass frequencies are low. A higher value is still desirable for small platforms surch as racers or well isolated autopilots and should be tuned by the user. Specific values for config files are untouched. The cutoff filter for the D term is disabled here as the required cutoff frequency for the default D term of the rate controller is higher than the gyro cutoff. In that case, enabling the D term cutoff would just add some undesired phase lag to the derivative.
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@bkueng I removed the comment, please approve again |
CarlOlsson
changed the title
IMY_GYRO_CUTOFF decreased to
20Hz30HzA lot of people realized that their drone is more sensitive to noise since the last release - possibly due to the increased default cutoff frequency of the gyroscope. A low gyro cutoff is needed for most medium/large size drones as the structural natural and blade-pass frequencies are low and people are flying more and more large powerful drones.
MC_DTERM_CUTOFF deactivated by default
The cutoff filter for the D term is disabled here as the required cutoff frequency for the default D term of the rate controller is higher than the gyro cutoff. In that case, enabling the D term cutoff would just add some undesired phase lag to the derivative.
Proof:
is given by:
A common rule of thumb used in the industry is to set the D term filter cutoff frequency equal to one tenth of the derivative time constant (ref: controlguru.com (bottom of the page)):
The time constant of a 2nd order with a cutoff frequency of
Since the time constant of the derivative part of the controller is
We can express the desired filter cutoff as a function of the P and D gains:
Since the default gains of PX4 are:
MC_ROLLRATE_P = 0.15MC_ROLLRATE_D = 0.003The desired cutoff frequency would be
and is smaller than the gyro cutoff frequency of
20Hz30Hz.FYI @LorenzMeier
We will test it on a few vehicles ASAP