Re: [Flightgear-devel] Faster responsiveness on the turn
On Thu, 05 Feb 2004 00:05:00 +0100, Roy Vegard Ovesen [EMAIL PROTECTED] wrote: I'm thinking that adding a second indicated-turn-rate property that is filtered with a higher bandwidth would be a good solution. I just tried this, and the control system performance boost was quite noticable. :-) This would be beneficial for all turn-rate-based autopilot implementations KAP140, S-TEC and probably many more. -- Roy Vegard Ovesen ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
On Wed, 04 Feb 2004 20:18:15 -0500, David Megginson [EMAIL PROTECTED] wrote: Roy Vegard Ovesen wrote: So I shouldn't touch the responsiveness then?!. But rather add a new property with better responsiveness. Out of curiosity, why do you think that the responsiveness should be better? It improves controller performance. But I still don't want to go beyond what is possible in the real world. I've flown briefly behind two small-plane autopilots (one newer, one older) and they were both extremely jerky things. Do you have any reason to believe that the AP you're modelling gets more responsive input than a real TC can give? No, not more respinsive than possible, but I thought that the damping in FlightGear _and_ in real world was only for display purposes. So maybe there would be a possiblility to get the signal before it was damped. After reading the article on the AVWeb site and noting this: The instrument also contains a dashpot in order to slow down the movement of the gimbal ... and The dashpot is replaced by a viscous dampener ... It seems that since the gimbal is dampened it can not output a more responsive signal. -- Roy Vegard Ovesen ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
On Wed, 04 Feb 2004 17:18:33 -0800, Andy Ross [EMAIL PROTECTED] wrote: Roy Vegard Ovesen wrote: David Megginson wrote: Originally, the TC responded instantly -- I had to do a fair bit of work adding the slight lag to make it work like a real TC. The lag smooths out the indication a bit. So I shouldn't touch the responsiveness then?!. But rather add a new property with better responsiveness. What are you trying to model? Real autopilots don't have perfect instrumentation. If David is right about the behavior of the turn coordinator, then a real C172-class aircraft simply won't have the fidelity to drive your autopilot. Are you sure you're not trying to fix a bug with the real world? It was not my intention to do something that wouln't be possible in the real world, and this discussion has brought me to the conclusion that the TC is damped by design, and that I need to tweak my controller tuning. -- Roy Vegard Ovesen ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Roy Vegard Ovesen wrote: No, not more respinsive than possible, but I thought that the damping in FlightGear _and_ in real world was only for display purposes. So maybe there would be a possiblility to get the signal before it was damped. After reading the article on the AVWeb site and noting this: The instrument also contains a dashpot in order to slow down the movement of the gimbal ... and The dashpot is replaced by a viscous dampener ... It seems that since the gimbal is dampened it can not output a more responsive signal. Exactly. The article went on to state that the damping was added specifically for autopilots. Consider the alternative -- in rough air, the TC is bouncing back and forth from a medium left turn to a right turn every half second or so, and the AP is flexing the ailerons left and right violently trying to compensate. It's critical that you test your AP in light and moderate turbulence and not just in smooth air, since turbulence is the norm for small planes flying below 8,000 ft or so, especially on a summer afternoon. I think that more modern APs, like the STEC, do their own filtering as well -- I've heard people say that they're the first low-end autopilots that you don't have to disengage in light or moderate turbulence. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Faster responsiveness on the turn indicator
David Megginson writes: Roy Vegard Ovesen wrote: No, not more respinsive than possible, but I thought that the damping in FlightGear _and_ in real world was only for display purposes. So maybe there would be a possiblility to get the signal before it was damped. After reading the article on the AVWeb site and noting this: The instrument also contains a dashpot in order to slow down the movement of the gimbal ... and The dashpot is replaced by a viscous dampener ... It seems that since the gimbal is dampened it can not output a more responsive signal. Exactly. The article went on to state that the damping was added specifically for autopilots. Consider the alternative -- in rough air, the TC is bouncing back and forth from a medium left turn to a right turn every half second or so, and the AP is flexing the ailerons left and right violently trying to compensate. It's critical that you test your AP in light and moderate turbulence and not just in smooth air, since turbulence is the norm for small planes flying below 8,000 ft or so, especially on a summer afternoon. I think that more modern APs, like the STEC, do their own filtering as well -- I've heard people say that they're the first low-end autopilots that you don't have to disengage in light or moderate turbulence. I don't know if this has been been incorporated into Aircraft autopilot's but on any *good* marine autopilot the amount of damping is adjustable so as to be able to tune the AP for the current enviroment Cheers Norman ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Roy Vegard Ovesen wrote: 1) Increase the responsiveness of the turn indicator. I'm not a pilot and I've never seen a turn indicator in action so I don't know how resposive these instruments are, so maybe increasing the responsiveness isn't a good idea. Originally, the TC responded instantly -- I had to do a fair bit of work adding the slight lag to make it work like a real TC. The lag smooths out the indication a bit. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn
From: Roy Vegard Ovesen [EMAIL PROTECTED] I am currently in the process of implementing the Bendix/King KAP 140 autopilot. This is a rate based autopilot, it uses the turn rate and rate of climb as its primary inputs. The turn indicator instrument implements a low-pass filter so that the indicated turn rate output from this instrument is a bit sluggish. This sluggishness is bad for controller performance because it adds a time delay. I see two possible solutions: A lot of autopilots have a rate-of-turn hold input, not just the KAP140, so this is a generic problem. Avoid any hacks specific to this device. It is possible that the low pass is too strong, but I'd have to study it. The turn indicator is a gyro instrument and, unlike the VSI for example, doesn't actually have an inherent low pass that we _have to_ model right. The low pass is primarily due to the fact that both the display routine and the underlying FDM are running in observable timestep increments. If we don't filter the data then the instrument looks different to the pilot because the increments actually modulate subtle changes in the indication so they become easier for the pilot to see and act upon. As a result, the aircraft becomes unnaturally easy to fly on instruments. However, there are other human-corrective hacks we can do to the data. 1) Increase the responsiveness of the turn indicator. I'm not a pilot and I've never seen a turn indicator in action so I don't know how resposive these instruments are, so maybe increasing the responsiveness isn't a good idea. Because the low pass is computed digitally without any noise contribution, you can back it out in the AP algorithm. I'm not suggesting you use a filter with a carefully-placed zero to recover the raw signal though. Instead, I suggest you put in a stronger differentiator term in the loop and/or use a separate roll rate feedback loop from the roll angle feedback. Bear in mind that the TC signal is a composite of rate-of-turn and of rate-of-bank because the gyro is mounted at an angle, so the instrument can indicate a standard rate of turn when the nose has not moved at all. Thus, your feedback loop might be responding to the bank data component. 2) Add another output property from the turn indicator instrument with higher responsiveness. The lazy solution is to ignore the property associated with the instrument and feed directly off the raw body data. The problem with doing that is (a) it is not intuitive when working on the XML configuration files (b) doesn't give the correct behavior for instrument failure situations ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Roy Vegard Ovesen [EMAIL PROTECTED] said: On Wed, 04 Feb 2004 16:37:49 -0500, David Megginson [EMAIL PROTECTED] wrote: Roy Vegard Ovesen wrote: 1) Increase the responsiveness of the turn indicator. I'm not a pilot and I've never seen a turn indicator in action so I don't know how resposive these instruments are, so maybe increasing the responsiveness isn't a good idea. Originally, the TC responded instantly -- I had to do a fair bit of work adding the slight lag to make it work like a real TC. The lag smooths out the indication a bit. So I shouldn't touch the responsiveness then?!. But rather add a new property with better responsiveness. Hmmm...does the KAP140 run right off the gyro? And is the lag in the realworld turn coordinator by design for smoother indication? Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn
On Wed, 4 Feb 2004 13:48:37 -0800 (PST), Alex Perry [EMAIL PROTECTED] wrote: From: Roy Vegard Ovesen [EMAIL PROTECTED] I am currently in the process of implementing the Bendix/King KAP 140 autopilot. This is a rate based autopilot, it uses the turn rate and rate of climb as its primary inputs. The turn indicator instrument implements a low-pass filter so that the indicated turn rate output from this instrument is a bit sluggish. This sluggishness is bad for controller performance because it adds a time delay. I see two possible solutions: A lot of autopilots have a rate-of-turn hold input, not just the KAP140, so this is a generic problem. Avoid any hacks specific to this device. Noted. My next project will probably be the S-TEC System Twenty/Thirty series. Which is also rate-based. I figure the control system wil be very similar. It is possible that the low pass is too strong, but I'd have to study it. The turn indicator is a gyro instrument and, unlike the VSI for example, doesn't actually have an inherent low pass that we _have to_ model right. So in the real world a responsive turn rate indication would be available, right? The low pass is primarily due to the fact that both the display routine and the underlying FDM are running in observable timestep increments. If we don't filter the data then the instrument looks different to the pilot because the increments actually modulate subtle changes in the indication so they become easier for the pilot to see and act upon. As a result, the aircraft becomes unnaturally easy to fly on instruments. However, there are other human-corrective hacks we can do to the data. 1) Increase the responsiveness of the turn indicator. I'm not a pilot and I've never seen a turn indicator in action so I don't know how resposive these instruments are, so maybe increasing the responsiveness isn't a good idea. Because the low pass is computed digitally without any noise contribution, you can back it out in the AP algorithm. I'm not suggesting you use a filter with a carefully-placed zero to recover the raw signal though. Instead, I suggest you put in a stronger differentiator term in the loop and/or use a separate roll rate feedback loop from the roll angle feedback. I could try that, but the PID algorithm also includes a low-pass filter on the derivate error, so that might kill this approach. Since the KAP140 (and S-TEC) do not get any input from the roll indicator, I will try to avoid that if possible. Bear in mind that the TC signal is a composite of rate-of-turn and of rate-of-bank because the gyro is mounted at an angle, so the instrument can indicate a standard rate of turn when the nose has not moved at all. Thus, your feedback loop might be responding to the bank data component. Yes, but that would also be the case for the real world KAP140 and S-TEC, right!? 2) Add another output property from the turn indicator instrument with higher responsiveness. The lazy solution is to ignore the property associated with the instrument and feed directly off the raw body data. The problem with doing that is (a) it is not intuitive when working on the XML configuration files (b) doesn't give the correct behavior for instrument failure situations Point (b) is one of my goals to avoid. I'm thinking that adding a second indicated-turn-rate property that is filtered with a higher bandwidth would be a good solution. -- Roy Vegard Ovesen ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Roy Vegard Ovesen wrote: So I shouldn't touch the responsiveness then?!. But rather add a new property with better responsiveness. Out of curiosity, why do you think that the responsiveness should be better? I've flown briefly behind two small-plane autopilots (one newer, one older) and they were both extremely jerky things. Do you have any reason to believe that the AP you're modelling gets more responsive input than a real TC can give? All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Jim Wilson wrote: Hmmm...does the KAP140 run right off the gyro? And is the lag in the realworld turn coordinator by design for smoother indication? I remember reading that it was, but I no longer remember the source. It might have been an article about gyros on AvWeb (but then again, it might not have been). In real life, my TC (at least) does lag a little, and I think that they all do -- that's one of the reasons that they recommend steering only with the rudder pedals after a vacuum failure, to avoid overbanking the plane chasing the TC indications. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Faster responsiveness on the turn indicator
Roy Vegard Ovesen wrote: David Megginson wrote: Originally, the TC responded instantly -- I had to do a fair bit of work adding the slight lag to make it work like a real TC. The lag smooths out the indication a bit. So I shouldn't touch the responsiveness then?!. But rather add a new property with better responsiveness. What are you trying to model? Real autopilots don't have perfect instrumentation. If David is right about the behavior of the turn coordinator, then a real C172-class aircraft simply won't have the fidelity to drive your autopilot. Are you sure you're not trying to fix a bug with the real world? Andy ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
