Replies to This Discussion

DeveloperPermalink Reply by Randy on April 17, 2012 at 5:05am

Wesley,

     A very clean looking helicopter.  Interesting to see where you mounted the APM2.  I've always used the mounting plates (although they're sold out at the moment) but it will be very interesting to see if your set-up works.  It would be good if it does because:

• it keeps the mag away from the motor
• no need for an external gps because it's out from under the heli (assuming the main blades don't cause any troubles)

     I can imagine that we would make that the standard if it works.

 

     I suspect your yaw problems are largely just down to PID tuning.  If the external gyro is enabled only the top PID contoller is used.  If no external gyro then both the upper angle rate controller and lower rate controllers are used.  It's important to get the rate controller right first...so you can enable PID logging in the logs and then use the channel 6 tuning function for CH6_YAW_RATE_KP (see right lower side of configuration screen of AP MissionPlanner).

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Permalink Reply by Wesley Chuen on April 17, 2012 at 7:06am

Thank you!  :D  I spent a lot of time to route the cable inside the frame to keep it clean looking.  

 

That mounting place may work for me, but I don't think it would be good as a standard.  The vibration there is affected by the tail quite a bit.  

 

With the SK720, big vibration would disable the self leveling function of the unit.  I also mounted it on the same spot on top of the anti rotation bracket.  I used the vibration log feature to find where the vibes came from and fix them one at a time.  Now my heli is super quiet and smooth.  I think it would work fine for me for the APM2.

 

On my friend's old 450XL, that mounting place did not work at all with the SK720.  He put his 450XL together years ago poorly and crashed many time.  I tried to help him to eliminate the vibes but ended up still have quite a bit.  When I used the self leveling on his, the vibration affected the accelerometers and made it quite tweachy and not easy to control.  After over a week of tuning, I finally gave up and moved his mounting location to the bottom under the main shaft since his old 450 has enough space there.  It then can self level and move very nicely.  The bottom under the main shaft suppose to have the least vibration in the entire heli, and it's the recommended mounting spot for the SK720.  Just that the 450 Pro has a narrow body, and the APM2 mounting plate is still out of stock.  Otherwise I would have gone that route as well.

 

Back to my original question.  I do not have an external gyro.  So I should keep the Gyro box unchecked?  Even though the HH seem to work well (in a way) when the Gyro box is checked?  My unit currently has the default PID values: Stablize Yaw P=0.7, I=0.02, IMAX=8 and Rate Yaw P=0.135, I=D=0, IMAX=50.  I suppose I'll try to adjust the Stabilize Yaw first...

 

I understand that the safety feature of the unit would disable the rudder servo when the throttle/pitch is at the bottom.  But even when I have the pitch up, the tail servo would only move a little bit.  Is it because it tried to work with the HH logic and lowered the movement?  

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DeveloperPermalink Reply by Randy on April 17, 2012 at 7:14am

Wesley,

      If you don't have an external gyro you should keep that external gyro box unchecked.  That will give you an angle controller with a nested rate controller.  For me, I've nearly always used an external gyro.  I think we need Robert to give the PID values that have worked for his yaw as a good starting point.

     It's easiest to find Robert and a couple of other people on the monster heli thread.

     I'll drop a note over there pointing back to this discussion.

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Permalink Reply by Wesley Chuen on April 17, 2012 at 7:20am

Great!  Thank you!  In the mean time, I'll play around those PID and try to see what their effects are.  At this point, I'll be happy just to get the rudder under "my" control.  lol.

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DeveloperPermalink Reply by R_Lefebvre on April 17, 2012 at 7:40am

Wesley, what are your tail PID?

You should not be using it in external gyro mode when you don't have an external gyro attached.  I think you just don't have the PID's correctly set up.  Most people have this difficulty, but usually get past it with the right PID settings.  I've posted them a few times, but I can't remember off the top of my head right now...

I think I have Stab_Yaw_P = 2, or something like that.  Stab_Yaw_I = 0.   Rate_Yaw_P = 0.350, and I is... I can't remember at all.  I also have a bit of D I've been playing with.  0.04 or something.

As for the returning to center when you feed a command, yeah, I've seen that as well and I can't figure it out.  In flight, I think it's not as bad as you would think.  I find the nose will just twitch left just a tiny bit before then following the command.  When you are holding it in your hand, the error looks bigger.  When really flying, it seems like the rate portion of the yaw control system catches it very fast.

Edit: Oh, I see your PID values now.  Stab_P of .7 is way too low.  You need at least 2, and I've even gone as high as 3 without issue.  The Rate_P of 0.135 is probably also too low.  Increase both of these similar to mine, and it should look a lot better.

You WILL need Rate_I , but Stab_I should probably be zero.

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Permalink Reply by Wesley Chuen on April 17, 2012 at 8:03am

Great!  Just entered your PID and the ridder finally moves!  :D  Thank you!

 

As I twist the heli, right now the rudder just stuck on one side.  I'll play with the PIDs and see what they do.

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DeveloperPermalink Reply by R_Lefebvre on April 17, 2012 at 7:45am

Oh, and about the mounting, very interesting, you did just what I was planning to do.  I've just ordered an HK450GT FBL yesterday.

I am going to machine an aluminum mount.  If it works, I think that it would be a great candidate for a 3D printed plastic part.

It sure makes for a nice clean installation.

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Permalink Reply by Wesley Chuen on April 17, 2012 at 8:14am

This is how I mounted mine.  

 

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DeveloperPermalink Reply by Randy on April 18, 2012 at 6:33am

I really wonder if that mounting mechanism is going to work.  I think it's going to have problems..I think it needs to be basically flat.  I'm pretty sure the compass at least isn't going to be happy like that.

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Permalink Reply by Wesley Chuen on April 17, 2012 at 9:07am

I kind of understand what the PID numbers do but what is the Stablize Yaw numbers versus the Rate Yaw numbers are for?  I find myself randomly change one or both area without understanding what these 2 area mean.

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DeveloperPermalink Reply by R_Lefebvre on April 17, 2012 at 10:11am

Rate Yaw P is *exactly* like the gain on an old non-HH gyro.  

Stab Yaw P is sort of like the HH that operates on top of the old non-HH gyro.  It controls how fast the the copter will try to turn to hit the target heading.  If you set it too high, it will overshoot the target and you can get some bouncing.

My suggestion to you is to set StabP to something low, like 1.  I to zero.  On the Rate, set I to zero and D to zero.  Then set the P to Ch6. tuning, and put it in the range of 0.200 to 0.800.  Start off with the knob close to the bottom so the active Rate P is something like 0.250.  Try to take off, but understand you will not have much yaw authority with the stick, but you just want to keep it nose in, and play with the knob until you get a decent hold without oscillation.  Just like a normal gyro.  There may be some requirement to hold a little bit of rudder stick to keep it straight, but that's fine for now.  Once you have a Rate P you are happy with, work on the Rate I.  You want to tune it up so that it just doesn't quite cause an oscillation.  Then work on the Stab P.

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Permalink Reply by Wesley Chuen on April 17, 2012 at 10:16am

Ah, I see.  Thanks again.

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