Aircraft Avionics and the Prop. Avionics Interface.
TJHJ
Posts: 243
Hello all,
So short question does anyone know what the signal looks like coming from an S-Tec GPSS GPS auto pilot coupler to a century 2B?
The rest of the question.
I own a Piper Comanche 260B. Absolutely wonderful plane I fly as much as I can. The current autopilot has issues, it is almost 50 years old. It does what I call a walking thing. Really its a PID loop out of tune. So it flies back and forth across the centerline somedays faster somedays slower. The analog components are getting worn out. Aside from this it costs me a ton to even have it looked at. Any maintenance must be done by a certified Century repair shop, and I have had it repaired twice now at a cost of over $1000 each time, and it is better but it still does it. Really annoying to fly.
Right now buying a new S-Tec autopilot is well out of the question. At around 20 grand, there is no way in hell I can afford that. So I have been exploring alternatives. This is where I am at.
So I want to replace the autopilot with a Prop + HB25. I have spoken to a friend at the FAA and he said that if I can get it to work reliably. He can give me a single aircraft 337 install approval. So it will be legal but only in my plane.
Currently it is equipped with a Century 2B autopilot. I have the full schematics and breakdown on this module. ( I cant post it for 2 reasons, 1. Its like 80 pages long all goofy 8.5x16" pages. 2 Century is very protective of these and does not want them public on the internet. So if it would be helpful send me a PM, ill see what I can do) It was built in the 60's and is analog PID technology. This is a one axis autopilot. It only control roll motion. I have a stand alone S-Tec 30-AH altitude hold module which controls the vertical axis. It works wonderfully, it is also newer technology.
How the whole system works is GPS -> GPSS -> autopilot
So I have a Garmin 430 Nav/Com. This gives off the GPS course to the S-Tec GPSS which converts it to an analog heading signal(I think). This analog signal is then fed into the Century 2B module which thinks it is standard signal signal. Using this info it then flies the plane as it needs. I cant figure out what the hell this signal is supposed to look like. I tried going up and flying and measuring, but this is confusing as hell. Especially as I am trying to do this myself. (My only friend that understands electronics well enough is afraid of small aircraft. Figures).
With out the GPSS unit the Direction Gyro gives off the heading information. The GPSS unit intercepts and feeds a new heading signal into the unit.
The programming of the prop and HB-25 should not be to much of an issue as I have done quite a good amount of PID motor control work in the past. Now getting the signal into the prop is the real challenge. And figuring out what it is supposed to mean.
If I had to take a guess I am looking for varying positive/negative voltage that would be related to the course offset.
I have tried to just call S-Tec and Century but they tell me that they are unable to talk to me because I am not a certified dealer.
So anyone have any ideas? Anyone have any avionics experience they would be willing to share.
Thank you for your time,
TJ
PS. For more info on the S-tec GPSS see the attached PDF S-Tec.pdf page 10. Its not much but all that is available on their website.
For more info on the Century unit please see the attached PDF CENTIIB.PDF this is the pilots operating handbook.
So short question does anyone know what the signal looks like coming from an S-Tec GPSS GPS auto pilot coupler to a century 2B?
The rest of the question.
I own a Piper Comanche 260B. Absolutely wonderful plane I fly as much as I can. The current autopilot has issues, it is almost 50 years old. It does what I call a walking thing. Really its a PID loop out of tune. So it flies back and forth across the centerline somedays faster somedays slower. The analog components are getting worn out. Aside from this it costs me a ton to even have it looked at. Any maintenance must be done by a certified Century repair shop, and I have had it repaired twice now at a cost of over $1000 each time, and it is better but it still does it. Really annoying to fly.
Right now buying a new S-Tec autopilot is well out of the question. At around 20 grand, there is no way in hell I can afford that. So I have been exploring alternatives. This is where I am at.
So I want to replace the autopilot with a Prop + HB25. I have spoken to a friend at the FAA and he said that if I can get it to work reliably. He can give me a single aircraft 337 install approval. So it will be legal but only in my plane.
Currently it is equipped with a Century 2B autopilot. I have the full schematics and breakdown on this module. ( I cant post it for 2 reasons, 1. Its like 80 pages long all goofy 8.5x16" pages. 2 Century is very protective of these and does not want them public on the internet. So if it would be helpful send me a PM, ill see what I can do) It was built in the 60's and is analog PID technology. This is a one axis autopilot. It only control roll motion. I have a stand alone S-Tec 30-AH altitude hold module which controls the vertical axis. It works wonderfully, it is also newer technology.
How the whole system works is GPS -> GPSS -> autopilot
So I have a Garmin 430 Nav/Com. This gives off the GPS course to the S-Tec GPSS which converts it to an analog heading signal(I think). This analog signal is then fed into the Century 2B module which thinks it is standard signal signal. Using this info it then flies the plane as it needs. I cant figure out what the hell this signal is supposed to look like. I tried going up and flying and measuring, but this is confusing as hell. Especially as I am trying to do this myself. (My only friend that understands electronics well enough is afraid of small aircraft. Figures).
With out the GPSS unit the Direction Gyro gives off the heading information. The GPSS unit intercepts and feeds a new heading signal into the unit.
The programming of the prop and HB-25 should not be to much of an issue as I have done quite a good amount of PID motor control work in the past. Now getting the signal into the prop is the real challenge. And figuring out what it is supposed to mean.
If I had to take a guess I am looking for varying positive/negative voltage that would be related to the course offset.
I have tried to just call S-Tec and Century but they tell me that they are unable to talk to me because I am not a certified dealer.
So anyone have any ideas? Anyone have any avionics experience they would be willing to share.
Thank you for your time,
TJ
PS. For more info on the S-tec GPSS see the attached PDF S-Tec.pdf page 10. Its not much but all that is available on their website.
For more info on the Century unit please see the attached PDF CENTIIB.PDF this is the pilots operating handbook.
Comments
I have 14 years of Avionics experience, and could probably help if my name is not used anywhere in the fixing. Also you should have any agreements in writing from the FAA, friend or not.
I will take a look at the documents, and it will probably take a few days to get back to you. Feel free to PM me if you wish. I would rather keep the discussion off the message board. I do have friends which are still in the Avionics business and they may lend a hand, but also may be reluctant to do so (job security and all).
The signal could be a square wave which could in phase or 180 degrees out of phase depending on direction. There are a few different ways signaling was done with analog signals. Just because the signal is analog doesn't mean it will be a flat varying voltage. Autopilot systems were always way to complicated for what was needed, but the designers wanted them to be "unique".
The important part is to figure out the signal, which will require an o-scope. If you don't have one, find someone who does. You should be able to connect the probe temporarily to the input wire and read the signal. It should be pretty easy with the wire connected to be able to look over at the o-scope during flight.
James L
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