Hi Tim,
you could use a switching board i.e. Pololu and a FBL-unit. The board switches the Pixhawk in or out on your command to either use Pixhawk+FBL-unit or pure FBL-unit.
Tim, I did not have a problem getting stable flight with the Pixhawk with FBL. But I’ve only ever done one heli. The rest are flybar and those are simple.
The problem I am scared of is EKF blowup, and like Marcus suggested, you will only have manual control of the helicopter if that happens by using an external FBL unit with some way to bypass Pixhawk. The bypass is done internally with flybar because Acro is straight thru with flybar mode 1. With FBL it’s not, so you may want to consider Markus’ suggestion for ultimate failsafe reliability in the event you lose EKF. And I don’t think EKF can be turned off in Copter. There is a AHRS_EKF_TYPE param that has 0 value to disable it, but I think that reverts to regular EKF like in Copter 3.3. The other two options are EKF2 and EKF3.
On your GPS cable, does it have connectors on both ends? And if so, what type of connector?
JST? Can’t you just take the connector apart, unsolder the terminals, and make up a longer cable out of a piece of Cat5e 8-wire twisted pair and put the ends back on it? That’s what I’ve done with other uBlox M8N GPS/compass units. Except the Chinese uBlox units come with the wire going into the unit with no connector. So have to take the unit apart and de-solder the wires from the GPS board and compass module, and solder on the longer cable and put it all back together. Has always worked fine here.
Chris,
Regarding EKF, i have read a few accounts of things not going well with it. Hopefully i dont have issues, im only using sensors in the Pixhawk and the Here GNS, so im hoping with reduced load…
As for the cable, i just used cat5e and soldered. The connectors are the new ones used by the Pixhawk 2.1.
Last night i deceided to boot it up and have a look through MP. Sone issues there, horizon was pooched as i calibrated on the bench and then side mounted it. I attempted to compensate for that by adjusting the parameter for orientation, but never got it to calibrate the accels after i got the horizon displaying correctly.
I reloaded firmware and started from scratch, lemme tell ya its alot easier calibrating everything with the unit off the frame as its an 800mm helicopter…got it done though
Rc calibration went as expected, only issue i have now is a “motor interlock failure” when i attempt to arm the unit.
Also, i get no movement from the servos at all, im assuming they only operate after arming?
Ill have to look into it further tonight.
Tim
Update… Servos are working, swash is moving, took a bit of time to get everything set up properly and moving as it should… Set to 60,60,180 with a phase offset of 60. Seemes to behave correctly.
Now, as to how the gyros behave in stabilize…? Im used to a standard fbl, and to verify correct compensation you tip the heli and the swash corrects in the opposite direction, is this true of the pixhawk as well?
Im looking for a way to verify correct operation on the ground before each flight as i do with other systems.
----Specificity regarding Pixhawk 2.1… How hot is the unit supposed to get during use? I bumped it after i was done setup this evening and was suprised how hot it was. I dont have a quantifiable number, was too lazy to go get the infra red, but ill be sure to have it nearby tomorrow for a reading.
----also pixhawk 2.1 related, i keep getting “bad gyro health” messages in MP. I did calibration and all seemed good?
Tim
Yes, that is correct Tim. You’ll have to adjust the rate PID’s and feedforward to get the swash movement you want. Frankly I kinda go by “looks right”. If the swash seems to compensate too much, I turn down the feedforward. If it looks sluggish, I turn it up. When I fly it if I get rockin’ and rollin’ I turn down the P gains and turn up the FF. If it’s steady I turn up the P gains until it’s not, and then turn 'em back down to where it is.
Once you get it tuned to your liking I recommend doing quite a bit of flying at different speeds, etc in Stabilize before moving on to the altitude controlled modes. A heli that’s stable at hover can have problems in high speed flight. Better to find those problems where you got a little more manual control of the heli so you can tune them out with less drama.
This is normal it has heated IMU.
Sensors run optimally at 60°C.
You can turn it off if you are doing long bench test by setting IMU_TARGET_TEMP to 0.
About bad gyro health, try waiting for it to warm up then reboot.
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Just remember that in a flybarless rotor the P gain is much more necessary than VFF. Always add more P gain and if it gets too snappy then back off of the VFF. If you can get away with zero VFF and a high enough P gain to satisfy desired response then that is the best way to go. If you start seeing an instability then you need to back off on P gain.
Tim’s 700 heli has the same servo linkages mine has, with push/pull and bellcranks. Mine is flybar, Tim’s is FBL so they use different, more powerful and very fast servos in the FBL ones. These are very snappy handling helicopters made and set up for extreme 3D flight, including the flybar ones. But they are also quite easy to tune compared to a little one because of their size. And Tim’s is a stretch mod so he’ll be running quite low headspeed. Should be a really stable machine.
Can’t tell from the photos if you put the bigger 800 blades on the tail when you stretched it, Tim? If you didn’t just be aware that the default yaw rate settings are a little “soft” on rate P so as to prevent a surprise on your first takeoff in the event you’re not right on the anti-torque to counter it. On a bigger heli I would kind of recommend turning up the yaw rate feedward to about .05 if you use the default rate P to make sure you got plenty of yaw authority with the radio to counter it if it doesn’t want to hold its heading. You can tune the rest later once you find out how it acts.
Just for reference, this is the settings I run on rate yaw on a 700 with the servo linkage in the 16mm throw position. You could likely just stick my numbers in there and not have any surprises on takeoff other than maybe a tail wiggler if the D gain is a little sharp for your setup.
Loosing yaw control on a big heli is not fun.
ATC_RAT_YAW_D , 0.007
ATC_RAT_YAW_FILT , 4
ATC_RAT_YAW_I , 0.17
ATC_RAT_YAW_ILMI , 0
ATC_RAT_YAW_IMAX , 0.25
ATC_RAT_YAW_P , 0.35
ATC_RAT_YAW_FF , 0
Chris,
Regarding the servos and linkages, yeah the cyclics are very fast compared to most, probably overkill, but i got a great deal from Bert K at OHB this past December in FL, so i grabbed a bunch while at the funfly.
Now i decided to leave the push-pull linkages on for my intial testing to see if that allows me to get the gain a little higher, i have the newer FL style setup on another heli and was shocked how low of a head gain i can run on my regular heli fbl’s, so i assumed Pixhawk would be no different. Thise newer head styles with a direct link to the swash are very efficent and fast with todays crazy fast servos. My cyclics are faster than the tail servo that came on the heli was eapecially as its HV at 8 volts.
Now the tail, i spent many hours getting it buttery smooth. Sanding, shaping, polishing and lubricating with some Dow Corning 111 so its leagues better than it was when first built. Also, i upgraded the tail box, auto gear and jackshaft with helical gears that run at a higher ratio than stock. I also run Rail 115mm blades vs the 105mm stock aligns. Needless to say, the heli is purpose built for LHS running. Oh boy did the helicals really smooth out the drive train, and it has good holding power at some pretty low rotor speeds.
I appreciate the setup advice and the numbers, to that effect i was noticing some very sluggish tail behavior when in active mode under heli setup. Very multirotor-ish if that makes sence… I definatly have alot of setup still ahead of me.
Oddly enough, calibrating the esc went seeming well. Usually calibration with the Castle edge/ice2 esc’s can be a chore with other FBL’s ive used. Eventually i plan to swap out the Castle with a Kontronik with AFW, but for now it will do.
Hopefully being methodical and not rushing this will lead to a successful first flight. Having an 800mm heli spinning wildly in front of you, or darting off in a random direction is not a fun experience. Been there, and since its swinging the 800mm wide chord blades, they look like spinning canoe paddles, or a flying wood chipper.
Tim
Thanks for the info. 60c is quite warm, 140f, so id say thats about where it was then. Dident realize it warmed up that much.
Will definatly try a reboot if i get “bad gyro heath” again.
Tim
Tim, why are you calibrating the ESC? aren’t you going to use the governing function of the ESC? Just curious.
Bill,
Now VFF, is that feed forward in the traditional sence? Not that im going to try and apply conventional FBL tuning techniques here, but I know I run mid range, to upper mid range feed forward numbers usually. I like a more direct feel when flying 3D, but that being said i guess that has no merit as it applies here as this heli is definatly not flying 3D.
I know ive seen the term VFF referenced a few times in my reading, but am drawing a blank on the definition.
My goal is certainly going to be to get the P gain as high as possible. My experiences with every other system agrees with that sentiment, whether it be a multi or a 3D heli, the P gain was the key to good flight. I always go right to the point of ossicilation, then backed it off a cpl percent.
I havent gotten that far yet, but im wondering what filters i have access to with rc3.5 i could employ to get gains up without pesky feedback ossicilations?
Tim
Last time the esc was used, it was with Spektrum gear with Set RPM Gov mode. Now i am flying with FrSky and when i tried to arm the esc it wasent seeing low throttle so i had to put it back to Linear Throttle mode and calibrate. Now it arms as it should and i can set up my headspeeds in the Castle Gov and move on.
Tim
Tim,
VFF is just the parameter name. The full name is ATC_RAT_XXX_VFF where XXX is the axis. So this parameter (VFF) resides in the PID controller and takes the error between the requested rate and the actual rate and applies this gain and sends it to the mixer. It is as close as you will get to direct control[quote=“timbaconheli, post:22, topic:16800”]
I havent gotten that far yet, but im wondering what filters i have access to with rc3.5 i could employ to get gains up without pesky feedback ossicilations?
[/quote]
The only filter I know of in 3.5 is a first order low pass that is applied to the error signal prior to going to the PID controller. My notch implementation is pretty easy to incorporate but it also filters the error and not the gyro signal (that’s why it was easy). But as long as the freq you are trying to attenuate is greater than 5 hz, it works pretty well.
I’m not Bill, but VFF is Velocity Feedforward. It basically bypasses the rate PID loop. With a flybar where you have a built-in mechanical rate controller and you tune it by changing flybar weight or paddle pitch, you simply turn off the rate PID loops by setting everything to zero and turn up the feedforward until you get the response you want.
With FBL you will use the electronic rate controller built into Pixhawk and all the params start with ATC_RAT and apply to roll, pitch and yaw. It is very much like tuning some FBL controllers. It is just a matter of knowing where to find the settings. Make sure you use the Full Parameter List or Tree in MP or APM Planner2. Don’t use any of the Extended Tuning pages etc because those are specific to multi’s and will make settings for you in your helicopter you will be less than impressed with 
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That’s definitely good. Should have no problem with yaw authority.
Curious what headspeed you’re running with the 800 stretch? 1,000 - 1,100 rpm?
Historically i run 1000-1300 rpm with the 800. Had a good tail at those speeds after the blade and gear swap.
Tim
Tim, another comment on this. I have no problem with those push/pull linkages with the bellcrank. I’ve always felt they provide a nice straight link to the swash with more convenient and less packed-in arrangement of the servos. And the ratio on the bellcrank should be like 1:2.1. So you don’t need to use the crazy fast direct linked servos. I rather like that setup myself, it’s quite robust at the expense of a bit extra weight, but also really reliable. I wouldn’t even think of changing it.
The 3D rage has bred a lot of stuff in heli’s that lighter, faster, more responsive. But I don’t necessarily consider all that stuff to be good for a UAV heli.
Yeah, im really hoping the push pull setup helps reduce any feedback loops and ossicilation with the servos. When i fly 3D with my other 800 that has direct to swash links, i often think about how much punishment those poor servos take with no leverage in between.
Tim
The push/pull provides a redundant link to the bellcrank. And the bellcranks are hefty with good bearings in them where the bellcrank shaft goes thru. I think it’s a really heavy duty system. I think you’ll find it easy to tune with Ardu.