continuing my efforts to employ flight controllers in notoriously vibratory aircraft, i was reading the data sheet for the IMU in the speedybee f405 wing (invensense ICM-42688-P) and noticed that it has programmable sensitivities for the gyro and accel (data sheet pages 11 & 12).
i got to wondering if this could be (or is) used to ‘numb down’ the FC in shaky aircraft?
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ps…hopefully this is not a “support question”.
These “sensitivity” settings don’t really change sensitivity per se, they just change how the measurement is reported so you can choose between a wide measurement range or more decimal points.
I think the only reason you’d want to change this would be to increase the measurement range to prevent saturation of the accelerometer. I suspect that the accelerometer is already set at the least sensitive setting to give the largest measurable range of 16 g’s. Or maybe it’s set to report data in 20 bit mode, which has both the high precision and full range.
The only way to deal with vibration (besides making mechanical changes) is filtering, which offers the “numbing” you’re looking for, with the benefit of being able to target which frequencies are deafened. Nowadays Ardupilot has quite robust filtering features. Have you tried playing around with the excellent filter review tool?
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I think all IMU’s are configured this way. BMI088 is +/-24g. The big $$$ ADIS16470 is higher than that. Just to name a couple other IMU’s on FC’s supported by Ardupilot.
ok. thx guys.
i have only started to notice the filtering tools/params (haven’t attempted anything yet). i have to say, though, that reading through the notch filtering wiki pages gets me glassy-eyed for now. im just not familiar with the vocabulary and ideas, so it’s greek to me atm. i don’t know if it could be more beginner friendly or not - i’m guessing i’ll need to keep learning , and i will.
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i DO wonder about this, though …flying RC planes for a while, and in clubs with groups, there are some ppl that use the ‘gyros’ that just bolt right into these gas engine planes and don’t worry about vibes. some have a “gas engine” switch in the setup - obviously a vibration setting but, other than that, they just handle it. for instance …https://www.redwingrc.com/product.php?productid=1808
… so why is it so easy for them and AP with a multi-op FC needs such brain scratching and iso-mount mentality?
so why is it so easy for them and AP with a multi-op FC needs such brain scratching and iso-mount mentality?
This is the cost that AP pays for being so flexible. Because it can be used for any platform, additional configuration is needed to get it working well on any particular platform. The upside is that, thanks to having control over everything, you can probably get an AP-based controller tuned much better than the more plug-and-play ones.
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thx, rick. point taken.
i’m also considering that those application-specific ‘gyros’ are relatively expensive (aforementioned around $400). i’d be guessing that enables them to have plenty of flash, ect, so they could, if desired, employ dynamic in flight fft filters.
i’d be guessing that enables them to have plenty of flash, ect, so they could, if desired, employ dynamic in flight fft filters.
Then you might be happy to hear that AP can do that too!
The cost of those kinds of application-specific controllers might have been justifiable 10 years ago when they were the only tool for the job, but frankly I think they’ve just been coasting on that momentum ever since then.
Granted, the F405 is low performance/low cost and will be somewhat limited when it comes to how complex of an FFT filter it can handle, but in all likelihood it will do just fine, especially because the FFT filter is often inferior to a well-tuned throttle- or RPM-based notch filter, which it can handle easily.
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well now that’s music to my ears. now looking forward to playing with it. up to this point i’ve been engineering and making my own vibe iso mounts, but i’d like to scale down on that some.
i’ve got a few cuav v5/+, but just got a couple of the SB f405wings for lighter duty applications. the cuav’s have 2mb flash, the SB405 has 1mb. but, now, you’ve encouraged me with the rpm/throttle talk.
regarding the app-specific gyros, they still have a place. some of these RC events are full of giant scale, big money planes/copters and the owners use these gyros. not many guys are interested in flight controller setup/operation complexity.
Its a interesting example. I should add that I have never used such a device and I’m just speculating on how they work. I guess typically do pure rate stabilisation, no horizon levelling. This means the accelerometer is not really used. Clipping the accelerometer is what makes high vibrations very bad for AP. They also only have one thing to do, so they can run the rate stabilization loops very fast, this not only gives better stabilisation but also minimizes aliasing issues.
Ultimately, AP has more things to do, and is trying to solve for a full attitude/velocity/position solution.
The other side of it is also that AP tends to be used for things beyond just flying. If the flight controller is having a bad time due to vibrations your camera your mapping with will be too not to mention all the bolts coming loose and the cables fatiguing. If the vibrations can be fixed or mitigated at source then its a win - win.
I will also add the you do not have to use the fancy filters, typically they won’t help you anyway if the vibrations are very bad (you can’t filter out clipping). The filters are not used be the EKF, so again no filtering will help if the EKF is un-happy (fancy soft mounts help with both of these, but you need to know what your doing, or try several, fitting generic a soft mount is just as likely to make things worse). The filters simply allow the rate tune to be better so you can get a more responsive vehicle. We have only had them for a few years and planes flew fine before that. The are some cases where they can make a very noticeable improvement, but that is mostly copters (or the copter controller of VTOL planes).
TLDR: if you want the best performance possible then its worth getting into the filter stuff. If you just want to go flying (as I suspect most of the RC-gyro users do) then just go flying.
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the AR unit has gyro, accel, compass, baro. it also has a lot of function/features.
does it auto level? i don’t know, but some of the cheaper ones do (like the ultra cheap frsky sxr, spektrum as3x’s, etc).
thing is, they don’t have a gps, so RTH isn’t gonna happen (and i consider that a HUGE benefit). can they loiter for failsafe? i don’t know. i can set my channel failsafes for the sxr’s to kick on the autolevel with a little rudder offset and cruise throttle setting to get a rudimentary loiter (altitude ‘hold’ is a roll-of-the-dice, as it’s just based on an aircraft ‘level’ presetup).
these cheaper setups, i’ve found, still have problems with vibrations from the IC engines.
i can’t seem to find the imu part number on the ar to see if it is somehow made with more vibe immunity.
anyway, i cannot mount one of my FC’s on a gas engine airplane with anything close to rubber washers, foam tape, etc … the vibes always clip. requires a much more elaborate vibe mount. when i see these guys doing it with minimal mount precautions, i get all squinty-eyed and envious. to be fair, i haven’t investigated one of these in my planes, but they are a bit expensive to just try out, and i’d prefer a more capable FC.
Despite holding a masters in electronics/electro-mechanical engineering, I’d rather work on a mechanical solution to remove vibrations than dive into altering the programming/settings of the gyros and filters. First of all, I suspect that the mechanical approach will be faster and more simple, and secondly it will also solve other problems like video jello and vibration induced stress of the airframe and parts thereof.
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