[psas-airframe] Roll control-wind tunnel
OK. I've been toiling with developing a way to test our fin designs in a high speed wind tunnel: 1) We could build a compressed air wind tunnel and get fast (perhaps supersonic) flows, but only for a brief period and not at one steady flow rate. 2) We could build a small high speed wind tunnel using a 2 stage centrifugal fan driven by a small block chevy (the red-neck in me loves this idea), or perhaps a more manageable surplus 4 cylinder engine from work (50 HP, $30). What about moving the fin in the air? 1) Moving the fin on a test rocket doesn't allow steady state testing, and instrumentation is a problem. 2) The fin travels a long way at 300 meters/second, so a linear track is out of the question. Finally an idea hit me: Move the fin in a circle! (Naturally, this idea came to me while I was working on my helicopter). We can remove the rotor blades from a gas powered helicopter and replace them with carbon fiber tubes (available at local hobby shops). At the end of the tubes, we attach our fin design. With the existing helicopter mechanics, we have precise control of the blade's angle of attack. I happen to have an old helicopter that I can donate to this cause. Naturally, this test would take place behind a shield and/or at a remote location. At typical rotor head speeds of 2000-3000 RPM, a fin mounted at a radius of 600 mm from the center of rotation will see airspeeds of around 125-190 meters/second! That's 280-425 miles/hour! Further, we can evaluate the lift / AOA curve using a scale under the helicopter. This could easily be expanded to a 1 meter swing radius, getting us into the trans-sonic speed regime. Filming the blade with a high FPS camera would show us shock lines if we get there. Fire away with thoughts, check math, etc. ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
[psas-airframe] Roll control
Hey all, About http://psas.pdx.edu/rollcontrol/ If i were doing it i'd use 4 mechanically independent servos. I estimate the reliability of doing this is about 1/2 of the single servo system being proposed. (Can we put the cool linkage drawing on the wiki page?) Despite the reduced reliability, i still think the 4 independent scheme is a winner. * Reliability is still very high * Mechanically ready for full flight control * Mechanically simpler * Same system that must be developed down the road anyway In the 4 independent scheme, the servos must be individually trimmed. To do this i would consider an absolute magnetic shaft encoder: http://www.austriamicrosystems.com/eng/Products/Magnetic-Encoders/Linear-Encoders/AS5311 http://www.rls.si/default.asp?prod=am8192B1 There is firmware involved in slaving the servos, which has reliability implications, however, we need to evolve some best practices on this anyway. Mechanical stops can be added to the servo design to limit the potential for damage in the event of servo or controller malfunction. -- The White Rabbit put on his spectacles. Where shall I begin, please your Majesty ? he asked. Begin at the beginning,, the King said, very gravely, and go on till you come to the end: then stop. -- Lewis Carroll ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
Quoting rq1...@q7.com: If i were doing it i'd use 4 mechanically independent servos. I estimate the reliability of doing this is about 1/2 of the single servo system being proposed. (Can we put the cool linkage drawing on the wiki page?) Despite the reduced reliability, i still think the 4 independent scheme is a winner. * Reliability is still very high * Mechanically ready for full flight control * Mechanically simpler * Same system that must be developed down the road anyway Add higher cost to the list. The reason I was pushing for a linked system is because there were reservations on the team about what could go wrong if the micro messes up and the servos go out of sync. Keep in mind we have NEVER successfully flown an ARM micro in 3 airborne tests. I will be happy to machine both the linked single servo system and the four servo system, so multiplying the work required is not a big problem. I really want to take baby steps on this, and I think the additional insurance is worth the work. We'll do the independent version, but I don't think we should do that one first. In the 4 independent scheme, the servos must be individually trimmed. To do this i would consider an absolute magnetic shaft encoder: Why add a shaft encoder when the existing servo positioning system gives minute of angle precision? I think the additional sensor violates the KISS principle. Modern digital servos are strong, fast, and accurate. I plan to align the fins the same way RC helicopter blades are aligned; using an inclinometer style pitch gauge. ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
Quoting Doug Ausmus daus...@gmail.com: What has been the root-cause of each of the three failed ARM flights? I'm pretty sure that two of the failures were attributed to a failed IO pin on the micro. My understanding was that the pin failed in such a way that it looked like it was fine, but actually was not. I think it was a data ready pin on one of the sensors. The first drop test failed to deploy the 'chutes, and the second simply failed to log data from the test (chutes were deployed by a pic based timer). We originally wanted to deploy the chutes based on a timer that starts when the zip line got pulled. As with many projects, we added the additional complexity of the altimeter, and it could have cost us the nosecone. The thing we were testing (NSR and parachute) didn't get proven because of our added complexity. On the third run (last May) the launch was not detected by the board. ...A robust controller solution would seem to be a primary factor for the roll-control project, with either a single- or a multi- servo approach. So how do we measure robustness? There are a number of methods that will organize failure severity and probability (FMEA, some six-sigma stuff, etc.) but we still have to make the choice ourselves. My choice is to chose a mechanism that is not capable of changing the rocket direction regardless of a failed control. The single servo approach makes the controller robustness much less of a factor, and possibly not even a primary factor since the rocket won't spin any faster than it did on the last launch. Four servos is really what I wanted from the beginning, but in retrospect, I feel that the present design is the way to go. ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
(2009.07.08) kirk...@pdx.edu: Quoting rq1...@q7.com: If i were doing it i'd use 4 mechanically independent servos. I estimate the reliability of doing this is about 1/2 of the single servo system being proposed. (Can we put the cool linkage drawing on the wiki page?) Despite the reduced reliability, i still think the 4 independent scheme is a winner. * Reliability is still very high * Mechanically ready for full flight control * Mechanically simpler * Same system that must be developed down the road anyway Add higher cost to the list. Certainly it might cost more. Probably no more than 400 $ extra. Keep in mind that the per-servo torque requirements might be considerably reduced in the 4 independent scheme. The reason I was pushing for a linked system is because there were reservations on the team about what could go wrong if the micro messes up and the servos go out of sync. Keep in mind we have NEVER successfully flown an ARM micro in 3 airborne tests. Yeah, what's up with all the ARM failures? (See Doug's question below.) The linked system is safer with respect to controller failure. A controller failure in the independent system has the potential to make the flight path into a large radius circle, possibly at high spin rate, not good. I will be happy to machine both the linked single servo system and the four servo system, so multiplying the work required is not a big problem. I really want to take baby steps on this, and I think the additional insurance is worth the work. We'll do the independent version, but I don't think we should do that one first. I appreciate what you're saying. Since you're doing the work, you should do it the way that seems best to you. The linkage will be hard to get working well, but if it does work well it will be a thing of beauty. In the 4 independent scheme, the servos must be individually trimmed. To do this i would consider an absolute magnetic shaft encoder: Why add a shaft encoder when the existing servo positioning system gives minute of angle precision? I think the additional sensor violates the KISS principle. Modern digital servos are strong, fast, and accurate. I plan to align the fins the same way RC helicopter blades are aligned; using an inclinometer style pitch gauge. My experience with off the shelf servos is they won't re-point with single degree accuracy but i haven't tried the more expensive digital servos. -- I just checked what i think is the Futaba site http://www.futaba-rc.com I can't find a specification for pointing accuracy, which does not inspire confidence. Ditto for http://www.rc.futaba.co.jp Every servo i've ever taken apart had a sub-50 cent potentiometer in it. Will that A) work. B) work under vibration, C) work reliably, D) work to sub-degree precision? Honestly i think the answers are: Sometimes. No, no, no. Clearly doing anything but buying and using an off the shelf servo is way more work than desirable. I'd like someone to prove me wrong and show that off the shelf servos are just fine. Please. I did notice these guys, who are interesting http://www.openservo.com/ (2009.07.08) daus...@gmail.com: What has been the root-cause of each of the three failed ARM flights? A robust controller solution would seem to be a primary factor for the roll-control project, with either a single- or a multi- servo approach. If i recall, first was either a blown GPIO pin or an odd reset due to a wiring problem. Second was a blown GPIO. Third was a firmware bug. If i were to pick a root cause: inadequate testing. ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
It would be nice if someone just had a paint shaker, to start with. We also have been known to whirl things around on long ropes... Bart In message 307640010907081713n6a953bbbv4993aaeffce06...@mail.gmail.com you wrote: Do we have access to rigging/machinery to help us simulate the g-forces in the proper axis also while in test-harness mode? ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
Since you're doing the work, you should do it the way that seems best to you. :) The linkage will be hard to get working well, but if it does work well it will be a thing of beauty. I don't think it will be hard, but we'll see. My experience with off the shelf servos is they won't re-point with single degree accuracy but i haven't tried the more expensive digital servos. Remember, our linkage ratio is not 1:1. The servo travels (180,,) degrees. The fin requires (,36,50) degrees. That's how we get our torque AND our accuracy. Every servo i've ever taken apart had a sub-50 cent potentiometer in it. Will that A) work. B) work under vibration, C) work reliably, D) work to sub-degree precision? Honestly i think the answers are: Sometimes. No, no, no. I think you were working with crap $12 servos. I'll bring some real servos for you to inspect. Do you think this guy could do this with an unreliable vibration-prone servo? http://www.youtube.com/watch?v=AqZ-mCd0HhM Clearly doing anything but buying and using an off the shelf servo is way more work than desirable. I'd like someone to prove me wrong and show that off the shelf servos are just fine. Please. I'll try. I've been putting RC servos in cars, planes, boats, and helicopters since I was a young child (counting... wow, has it really been over 25 years?). I'm certain they will work fine as long as we can determine the requirements and choose the servo. I did notice these guys, who are interesting http://www.openservo.com/ That's interesting. I doubt they get better performance than Futaba, but it's neat that they have position FB. I2C still sucks... ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
Re: [psas-airframe] Roll control
(2009.07.08) kirk...@pdx.edu: [..] My experience with off the shelf servos is they won't re-point with single degree accuracy but i haven't tried the more expensive digital servos. Remember, our linkage ratio is not 1:1. The servo travels (180,,) degrees. The fin requires (,36,50) degrees. That's how we get our torque AND our accuracy. Yes, increased angular resolution is an advantage of the external linkage. I know you've been working on anti-backlash designs, so i expect the actuator control will be precise. In the single servo design the collective accuracy is a function only of the linkage. A DC offset in the servo neutral will presumably be handled by the controller, so there is no problem. On the other hand, 180/50 ~4:1 so the per-servo torque requirements are still about the same. However, i'm willing to give up on this and concede defeat. Let us be linked in ;) Every servo i've ever taken apart had a sub-50 cent potentiometer in it. Will that A) work. B) work under vibration, C) work reliably, D) work to sub-degree precision? Honestly i think the answers are: Sometimes. No, no, no. I think you were working with crap $12 servos. I'll bring some real servos for you to inspect. Do you think this guy could do this with an unreliable vibration-prone servo? (One of the servos was 35 $, still 30 cent pot.) http://www.youtube.com/watch?v=AqZ-mCd0HhM That is a very impressive video. I don't think the helicopter vibration environment is as harsh as ours. The question of servo quality is still interesting. Potentiometers tend to be noisy. They get noisy all of the sudden. They temperature drift like carbon resistors. They get noisy in high vibration environments. They are not reliable compared to standard semiconductors. I would feel better if Futaba made a servo with an optical or magnetic encoder, but i have not been able to find one. I would feel better if any hobby servo manufacturer specified pointing accuracy, but i can't find that either. -- As far as pointing accuracy goes, if the servo is only a degree or two off, the controller can compensate. The maximum penalty will be somewhat increased drag. The only serious problem then is servo failure. So i guess i should figure out how long a hobby servo has to operate while on a shake table to prove it's a reliable device, and then try it. ___ psas-airframe mailing list psas-airframe@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
