Replies to This Discussion

Permalink Reply by Ritchie on November 17, 2010 at 11:20am

Can you confirm whether you have an airspeed sensor?
FBW_A needs one so if you don't have it something will be up.

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Permalink Reply by DaveyWaveyBunsenBurner on November 17, 2010 at 11:25am

Thanks ritchie,

Yes i do, fbw b is the problem though!

Appreciate your input.

D

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Permalink Reply by Ritchie on November 17, 2010 at 12:56pm

Oh poop thats my theory gone then. I was thinking you didn't have one and the without it the maths was going weird.
Have you updated everything (including the libraries)?

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DeveloperPermalink Reply by jasonshort on November 17, 2010 at 9:47pm

There is no elevator control in FBW B. Sorry, It's just for simulation of the autopilot. You are giving it altitude error commands which will cause the plane to loose altitude by slowing the motor. The slow speed will trigger the elevator and make the plane come down. If your throttle min is too high, you won't be able to come down very quickly or at all.

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Permalink Reply by DaveyWaveyBunsenBurner on November 18, 2010 at 1:41am

Jason,

Thanks for the information. I read the description for FBW B and thought it was identical to FBW A but with automated throttle control to maintain a cruising airspeed. I didn't realise airspeed was being used to gain/loose altitude in this mode instead of the elevator! Strange! FBW B doesn't seem to suite itself well to planes such as the Easy star with a high thrust line I guess.....

Anyway, that tells me not to worry about FBW B and just move on to AUTO!

Thanks for your time,

D

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Permalink Reply by DaveyWaveyBunsenBurner on November 18, 2010 at 7:28am

Jason,

As you clearly understand Fly By Wire Mode B wayyyy better than me, could I be cheeky and ask for a small explanation of the PID tuning for this mode? IT seems to be by far the least understood AP mode.

I took an early lunch to have another go at Fly By Wire B and I now kinda get it, but looking at the PID tunings/options for FBW B I'm a little lost. For example:

1. AIRSPEED_CRUISE: Does this get used in FBW B? It's not in the FBW B section. I presume it is still applicable.

2. THROTTLE_CRUISE: If we tell Ardupilot that we want to fly at X meters a second, why do we need this value? Surely AP just ramps up the throttle till it gets to AIRSPEED_CRUISE and that's it? Don't see why this is neccesary?

3. AIRSPEED_FBW_MAX: How does this relate to the climbing etc of the aircraft? I'm flying along at AIRSPEED_CRUISE, I want to climb, so I pull back on the stick, this increases the motor RPM, which in turn increases the speed. If I make THROTTLE_MAX something big like 100%, will that be limited by AIRSPEED_FBW_MAX or will the need to increase altitude over ride AIRSPEDD_FBW_MAX?

Sorry if these questions are silly or I am not understanding the mode properly, it seems to be a bit of a mystery this one!

Many thanks

Dave

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DeveloperPermalink Reply by jasonshort on November 20, 2010 at 11:38am

Hi,

1- IN FBW B, Airspeed Cruise is controlled by your throttle using a range between the AIRSPEED_FBW_MIN and MAX
2 - THROTTLE_CRUISE you'r right we could use the I term to get there but this is how it's done today. I don't have a better explanation other than we're open to new ideas. BTW, many people don't use an airspeed sensor.
3 - The plane converts excess speed into altitude. If you are flying faster than airspeed_fbw_max you will climb, slowing the plane. Climbing will also tell the AP that you need to go down to go back to the desired altitude. This doesn't happen in FBW B because your elevator controls the altitude_error.

In most planes, these control laws work remarkably well. In an Easystar pointed to the wind, they can be problematic. Doug has since bought a similar slow flyer, so we will be taking suggestions and investigating new laws for such slow planes.

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Permalink Reply by DaveyWaveyBunsenBurner on November 22, 2010 at 4:02pm

Thanks a lot Jason,

Its a tricky problem, my easy star noses right down on throttle up and takes a long time (and height!) to recover. I dont know if the ability to hold a little up elevator when throttling up/or when throttle is above x percent is feasible (not to gain high merely hold level flight) or if that throws the maths out of the window!

Anyway, appreciate your Continued effort on the "legacy" AP!

Will be testing your latest build this week weather permitting.

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