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autopilot

Posted By:
tim uszuko
Homebuilder or Craftsman
6
Posts
1
#1 Posted: 1/6/2011 10:30:28

Has anyone ever built or does anyone know how to adapt radio controlled(rc) auto stabilizers that use mems gyros to build a simple autopilot in experimental aircraft?



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Eric Page
4
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0
#2 Posted: 1/13/2011 22:13:53

Tim,

Given your question, I'm assuming you're handy with electronics.  If so, here's a system that might be adapted to your purpose.  It's intended for home-built drones, but I see no reason it couldn't fly an experimental aircraft just as well.

http://diydrones.com/notes/ArduPilot

Cheers, and good luck!

Eric



Peter Pengilly
Homebuilder or Craftsman
2
Posts
1
#3 Posted: 1/15/2011 03:35:46

Tim,

You may find that the electonics is the easy part of making an autopilot! Assuming that the RC servos have sufficient power to drive your control surfaces, and that you are able to over-ride them when needed, figuring out how much control surface to use in any given situation, and for how long, is the tricky part. I don't know of any on-line resource that gives a good description of the problems involved.

For a heading or track hold device, you may like to consider how much bank angle will you use to correct a given heading deviation? How much aileron will you use to achieve that bank angle at 70kt, at 100kt and 130kt?

For a pitch device, are you holding speed or altitude? How much elevator do you use for small and large height deviationsat various speeds? If the power is reduced are you going to maintain altitude at all costs (until the airplane stalls)? If you slow down/speed up a lot how will the autopilot tell the pilot is significantly out of trim when the autopilot disengages?

Capturing your aircraft's flight control characteristics may be one of the most challenging aspects. Best of luck with the project.

Peter



Ray Toews
Homebuilder or Craftsman
16
Posts
4
#4 Posted: 1/16/2011 17:11:21

I have a True Trak which I am fairly happy with but the thought occured to me while on a long cross country, why not use light weight radio control servos and run them on trim tabs instead of using heavy high power servos on the controls.

There seems to be very little movement during normal flight, qa little more in turbulence but still very small movements.

Build a board which will respond to the MAC trim servos?

Ray



tim uszuko
Homebuilder or Craftsman
6
Posts
1
#5 Posted: 1/21/2011 09:46:55

Thanks for the imput guys. I looked at diy drones but decided to try the feiyu fy-21 stabilization autopilot. If anyone is interested in looking it is under nem hobby. I found their sevos can be found to produce  torque up to 10.6kg/cm. This should be enough to drive the controls in roll but am not sure in pitch. I am anticipating allot of growing pains in getting this system up to speed but it should work. In the world of homebuilding it is no different than anything else. Allot of study...,then work,... then anticipation....and finally accomplishment. Tim



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Brian Pease
3
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0
#6 Posted: 1/27/2011 09:50:00
I am thinking along the same lines.  I have a True-Trak pilot I am installing for heading control.  For altitude hold, Iam going the RC/UAV route with an RC servo controlling the existing trim tab.  I purchased a used "monster truck" servothat is probably overkill.   Perihelion Design (periheliondesign.com) sells a simple manual servo controlthat also provides the 6VDC.  Turning the knob moves the tab.  THese 2 items are all one needs for manual trim.  I purchased a PDC20 Altitude Hold control off Ebaythat is designed for RC/UAVs.  Unav no longer sells it.  It has a pressure sensor and microprocessor and will integrate with the manual trim control.  THe only issue is switching the unit from manual to hold mode, which requires the construction of a simple pulse generator to mimic the RC PWM signal.  THere are gain adjustments, and the servo is a programmable digital type with adjustable rop rotation limits, speed, fail-safe position, etc.  I have not spent much, and it might even work!

tim uszuko
Homebuilder or Craftsman
6
Posts
1
#7 Posted: 2/1/2011 19:20:40

brian.....I am going to use the fy21 for a reasonable heading hold. I found servos from pololu that supply 33lb*inch for 55 bucks and they tie into the fy21 and tey dont need a servo interface circuit...At least that is what the engineer said. The fy21 uses a barometric/gps system for altitude hold so it should be accurate. I plan to not use external trim tabs on my longez. I am going to mount everything internally. I think the hardest part is going to be coming up with a clutch system for the servos as they will be directly connected to the control belhorns. I dont know....maybe the friction will be low and I may not even need a clutch...we will see.

The entire system is around 400 bucks including the radio interface that communicates with the fy21. Tim



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Brian Pease
3
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0
#8 Posted: 2/1/2011 22:00:10

Tim, It will be interesting to see if you can make the autopilot work.  As an Electronics Engineer I have some idea of the limitations of the servos, which is why I chose to try to just drive a trim tab for altitude hold, which only requires ounces of force.  The RC servos are not designed to hold a high force for a long time.  They can overheat.  Current  flows through the motor continuously to prevent rotation, and increases with the holding force.  You will want to trim your plane for zero stick force before engaging the autopilot.

Another thing to consider is the DC power for the servos, which could peak at >1Amp each when in motion.  You might want to run heavy wires directly to the servos instead of passing the DC through the small wires of the autopilot.  See what the designers say.

The slip clutch is another issue.  My True Track roll servo can be moved with one finger, although it has no clutch.  The heavy duty RC servo I have is quite stiff and has a high gear ratio.  I doubt that it could take the abuse of being forced to move when not powered for very long.  For safety you would want a slip clutch that would allow you to take control without taking time to shut off the autopilot.  Hopefully, the drag can be set low enough to not be annoying.   Without a clutch, your controls will be locked by the servos when the autopilot is on.  You would definitely want a handy switch to cut servo power. The True Track has adjustable servo force which is set just high enough to maintain roll control in turbulence.  The altitude hold is just trim and can be easily overridden.



Brian Pease
3
Posts
0
#9 Posted: 2/2/2011 09:57:46

Vantec.com has a giant servo with 27 ft-lb torque that runs on 12VDC, model SSPS105.  It would require a current limiter, limit switches, or a resettable poly fuse.

The crude way to couple the servo is with a spring or bungee cord.

Search on "slip clutch" or "friction clutch" for devices to allow manual override, with extra drag.

Search on "electromagnetic clutch" for devices to completely release the servo.



tim uszuko
Homebuilder or Craftsman
6
Posts
1
#10 Posted: 2/2/2011 14:51:30

thanks for the input Brian. How much friction does that monster servo have when you turn it manually? Are those gears not made to be turned when the power is off? Looks like I will need a clutch system if the drag is too high. The rest of the fy21 system looks promising. If experimental autopilots wern't so expensive I would just buy off the shelve.

I looked into making a simple wing leveler using a turn coord. with the gyro that sends commands to a servo but anything that is certified is just overpriced for what you get.     if I could find a good used navaid device at a decent price I would consider that. But those systems are hard to find. Tim



TCU
tim uszuko
Homebuilder or Craftsman
6
Posts
1
#11 Posted: 2/2/2011 22:14:02

Brian...I am not sure if I can find a slip clutch that will fit in line with the servo. The long ez uses internal springs for trim. There isnt anything hanging out in the wind. perhaps with the correct spring tension ...that migt work if it is connected properly with the servo. what do you think? Tim



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Rap McBurney
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0
#12 Posted: 5/2/2011 16:47:36

I realize this post is a couple months old, but I have a question:  I have a navaid AP1 head with no servo.  What type, brand, model number RC servo can be driven by the AP1?  Has anybody done it?  I don't want to drive a control surface like the S2 servo does, I want to drive a roll trim tab for a simple wing leveler.  Thanks,  Rap