Brickthus Posted October 11, 2009 Posted October 11, 2009 A big Thank You to TLG for listening to AFOL requests for pneumatics and producing set 8049 for 2010. To celebrate, here are the pictures of my latest pneumatic system. It allows the cylinder to be commanded to any position. - not just the two ends - not just two ends and one position in the middle - yes, any position within its travel! The variable input is at the red sliding beam. The variable output is the position of the turquoise lever. Techniques: dithering of valves (with the motor crank) overcomes their hysteresis and stiction offsetting the valve levers removes most of the deadband in the middle feeding back the output allows closed loop control of cylinder position This uses no more pneumatic or PF parts than the contents of sets 8049 and 8293, so the parts will soon be quite accessible. How it works might take a little more understanding! More detailed description / info here. Folder when moderated. PLMKWYT Mark Quote
Milan Posted October 11, 2009 Posted October 11, 2009 Hmm, I have general idea of how it works, but video will greatly help! Can you upload small video of it? It looks quite ingenious! Quote
CP5670 Posted October 12, 2009 Posted October 12, 2009 This is a great concept and you've explained it well too. I like how the mechanism is fairly simple given what it does, and can be realistically used in an actual model. Quote
Conchas Posted October 13, 2009 Posted October 13, 2009 Well Mark,... you want to know what we think. Don't know about the others, but what I think is that you are one of the few, who knows how to take LEGO pneumatics to the limits. I mean it in the sense of building extremely complex functions which are not easy to understand just in a glance, or even two, three,... Also they should be not easy to explain. And this is another I should find the time to try, before I can really understand how it works. I agree that one video would be a great addition to this contribution, and if it would be made available, I know also where to highlight it... Quote
Brickthus Posted October 13, 2009 Author Posted October 13, 2009 Well Mark,... you want to know what we think.Don't know about the others, but what I think is that you are one of the few, who knows how to take LEGO pneumatics to the limits. I mean it in the sense of building extremely complex functions which are not easy to understand just in a glance, or even two, three,... Also they should be not easy to explain. And this is another I should find the time to try, before I can really understand how it works. I agree that one video would be a great addition to this contribution, and if it would be made available, I know also where to highlight it... I'm glad you want to try it. I guess it's like that with a lot of innovations - it has to be repeatable by an independent person and it takes time to sink into the minds of those who can understand and think of ways to use it. Understanding quicker by doing is why I have made models of a few concepts I deal with at work. People at work really enjoyed seeing the models and I had 50 people turn up to see them one lunchtime! Unfortunately I can't share pictures of them on the web This system arose from my growing understanding of how hydro-mechanical units work in the control of fluids in real world engineering (my first degree was electronics, not mechanical engineering). The way it works is similar for cars, planes, trains, ships and many other applications. An electronic controller sends a variable signal to the hydro-mechanical unit. The signal moves a motor that controls a valve, which regulates the pressure on each side of a piston that moves a larger valve, which regulates the flow of fluid to an engine or other device. A feedback sensor detects the main valve position and feeds it back to the electronic unit. The electronic unit uses the error signal (the difference between the demand and the actual valve position) to determine what signal to send. The system would be inherently unstable (it would go to one end stop or the other) except for the closed loop feedback. A further, outer, loop is made by sensing the effect on the driven equipment, so that the electronic unit can work out what valve position to demand. I'll have to add that in another experiment. It was a pleasant surprise to find that it works well with pneumatics as well as with hydraulics. Hydraulic fluid (oil or fuel) is not very compressible (proper hydraulic fluid is even less compressible) but air is very compressible. The important thing here was to use the same air pressure for both sides of the cylinder. Then performance under load is proportional to the supplied pressure, within the limits of LEGO parts (30psi). I picked a cylinder with least air flow restriction and a good seal at the top (I sorted my cylinders into different bags depending on seal integrity and air flow resistance). A benefit of the system over the mid-stop pneumatics of a few years ago (apart from needing fewer components and being smaller) is that the system can compensate for the unequal piston areas at the two ends of the cylinder. The mid-stop system needed two opposing cylinders to equalise the areas, otherwise it would drift to the extended position. I knew this would be OK because a real system valve has to drift shut for safety reasons, so the shutting-side area of its piston is greater than the opening-side area. I have been thinking about making a video since Milan mentioned it. I was puzzling how to pump air, control the red input beam and hold the camera all at once! I'll see how it is with the car tyre air compressor - a bit noisy, but it frees up a hand! My camera needs a lot of light to do a movie, so lamps too, and I must think of a script that won't confuse everyone! Mark Quote
Parax Posted October 14, 2009 Posted October 14, 2009 I have been thinking about making a video since Milan mentioned it. I was puzzling how to pump air, control the red input beam and hold the camera all at once! I'll see how it is with the car tyre air compressor - a bit noisy, but it frees up a hand!My camera needs a lot of light to do a movie, so lamps too, and I must think of a script that won't confuse everyone! Mark Try making a lemonade bottle reservoir, no noise and enough air for a demo! and rope in a camera person assistant. P. Quote
Brickthus Posted October 14, 2009 Author Posted October 14, 2009 (edited) Try making a lemonade bottle reservoir, no noise and enough air for a demo! and rope in a camera person assistant. P. A lemonade bottle is an interesting suggestion. Is that how people power their pneumatic engines in cars (e.g. V8 Mustang), to make them untethered? Here's a 10-minute tutorial covering most aspects. Hope it's small enough Milan! Conrinuously Variable Pneumatics Video I used free Prism software to convert the .mov file to .mp4 - 58MB rather than 290MB! I think my voice is sufficiently audible over the compressor. I'm glad it works at 20psi because that's at the lower end of the range for pneumatics, prolonging their life. This system has a small risk of valve seal wear but it's a lot slower than the engines! I didn't write a script in the end, so maybe there's a few too many "um"s, "er"s or "so"s in there! (Not as many as some of my teachers at school though - 768 in a 40-minute lesson once!) An old computer power supply (12V at 6A) that I used to use for the compressor decided it was time to expire. Probably one of the power diodes on the mains side. The smell was awful. Glad I unplugged it before any overheating became serious! At least I also had the smaller 3 Amp supply that I use to charge my car battery. Enjoy! Mark Edited October 14, 2009 by Mark Bellis Quote
Milan Posted October 14, 2009 Posted October 14, 2009 A lemonade bottle is an interesting suggestion. Is that how people power their pneumatic engines in cars (e.g. V8 Mustang), to make them untethered? Industrial compressor is the best source of air for Lego pneumatic engines (LPE). Alternately, people does use two bottles with compressed air, which they put it in with bicycle pump. Of course, they have to modify the bottle(s) cap to fit the pump. WOW That really works as linear actuator. Race is on. I watched it few times to absorb all of the information. As I said earlier, it is ingenious! Truly, you have to be really clever and persistent to create such a revolutionary mechanism. As I watched it, one zillion ideas started to appear in front of my eyes, using this system for operating the cylinders, really options are limitless, and LAs are no more in top of the pneumatics regarding precision. Pneumatic fans will certainly appreciate that. I will and I congratulate you for such achievement! I am sorry for your compressor failure! Quote
CP5670 Posted October 20, 2009 Posted October 20, 2009 I just got around to looking at the video. The piston is remarkably precise, much more so than I guessed from the pictures, and once again you have explained its working principles clearly. I'm going to try building it myself from your description in the video. Don't know about the others, but what I think is that you are one of the few, who knows how to take LEGO pneumatics to the limits. I mean it in the sense of building extremely complex functions which are not easy to understand just in a glance, or even two, three,... Yes, I've learned a number of things about pneumatics from just browsing his Brickshelf gallery in the past. This sort of stuff is what Technic is all about to me, creativity with the mechanisms. Quote
Brickthus Posted October 20, 2009 Author Posted October 20, 2009 I just got around to looking at the video. The piston is remarkably precise, much more so than I guessed from the pictures, and once again you have explained its working principles clearly. I'm going to try building it myself from your description in the video.Yes, I've learned a number of things about pneumatics from just browsing his Brickshelf gallery in the past. This sort of stuff is what Technic is all about to me, creativity with the mechanisms. Thanks Some people make the best-looking MOCs. Others make the stuff that's hidden inside to make them move. I'm really glad to have made something that others want to build. The simplest things that this mechanism should be good for driving are: - vehicle steering, either Hand of God or PF remote with motor worm drive to the red sliding beam. - one or more pneumatic valves, stopping in the middle. Either the red beam or the blue beam or both could be swinging rather than sliding, which might be useful for a steering mechanism. The cylinder precision needs the dither and compressor (if used) to be at sufficiently high frequencies so that their cycle times are shorter than the response time of the cylinder to a change in air pressure. Lower frequencies might make the cylinder oscillate, but this could be useful for particular applications, such as intentional chaotic motion, which might make an exhibition model appear more life-like. The closed loop system also needs excess air. I realise compressed air is harder to come by in LEGO models because we usually use tiny compressors, but this follows the industrial principles. Industrial closed loop oil and fuel systems follow this principle and have excess fluid pressure, feeding back the excess. Feeding back is less easy for us because we don't have sealed 4-port valves. I had thought of using 4 valve switches, but it would double the dither motor load, slowing it down. The extra switches might help in making a sealed system, feeding expelled air back to a reservoir, but it is the differential pressure that moves the piston, so that could work against the objective. A better use for 2 more valves would be for a separate air loop to a separate cylinder, with the feedback from both cylinders being added or averaged back to the blue sliding beam. I was asked whether mini compressors could be added to the dither motor crank. These would be fixed to the red beam, with the whole lot sliding by 2M. If so, a pair should be used like a flat-2 engine so that air pressure peaks equally on both halves of the dither cycle. Otherwise the uneven pressure would bias the cylinder one way or the other. An air supply that is uneven and synchronised to the dither cycle could be used as a fail-safe, as in industry, to bias a function to a safer state. Given the excess air required, a separate compressor of larger capacity is preferable. If more cylinders are driven from the same air output (2 pipes), the feedback cylinder should be at the far end of the pipes. Otherwise the performance of the extra cylinders is not so good. Movement of extra cylinders will creep to one end or the other unless their position is fed back to the valve positions. With 2 cylinders both fed back, equal pipe distances are recommended. Easier to control is a single larger set of cylinders using just the one feedback connection. The 8421 crane jib could work well with this system, as long as the feedback used the correct proportion of jib height (~2M full travel). A set of parallel cylinders could lift a heavier load with the same control system, giving greater amplification and showing how a simple low-power input moving the red beam by 2M could drive something more powerful. Mark Quote
Brickthus Posted November 5, 2009 Author Posted November 5, 2009 (edited) Genious! A really great idea! Many thanks to Conchas for blogging my video, along with some kind comments I've tried the mechanism with a small pneumatic cylinder. It works OK, not quite as well as with a larger cylinder, but overshoots more when supplied from a hand pump. This is because the hand pump capacity is more than the capacity of the small cylinder and the air arrives all at once, which is at just one point in the dither cycle. To stop the overshoot, either a mini compressor should be used (with capacity smaller than the small cylinder) or the air supply should be spread over at least one dither cycle (about 0.25 second for 240-300Hz dither). A smoother air supply should be OK. The other thing to note is that the stiction of the small cylinder is greater in proportion to its size than that of the larger cylinder. The advantage with a small cylinder is that the turquoise lever of the first mechanism is not required because the blue feedback beam sliding range of 2M is the same as the small cylinder extension. The other advantage is that a small cylinder lends itself to making a pneumatic steam engine for L-gauge track (surely one of the holy grails of Technic Trains!). I've built a basic steam engine with two of the feedback systems, quartered onto the wheels like those on a normal 2-cylinder steam engine. My aim was to lay it out in a way that would facilitate using BBB wheels on the track. At the moment it's too tall for a railway layout because of the long valve lever extensions, and the size of the mechanism forward of the driving wheels doesn't leave room for the front bogie of the engine, but I wanted to get the mechanism working before making too many modifications for the space envelope. From the original mechanism I fed the output (from the blue beam with small cylinder) to the wheels. This is connected to a valve gear crank axle via a +/-20ish degree advance/retard gear mechanism, whose cranks move the red input beam. That's enough to make the steam engine go one way or the other. The advance/retard mechanism is a simple one with gears because Walschaert's valve gear or Stephenson Link Motion required sliders, which proved to add too much play to the mechanism, as well as causing size and torque difficulties for the mounting arrangement. An expansion link for a demo of steam engine valve gear is one thing, one for real feedback to drive pneumatic valve levers is quite another! The combination of a proportion of cylinder position and a proportion of advanced or retarded wheel crank position to produce the valve position is exactly what happens on a steam engine. I don't expect it to have the performance or speed of commercial LEGO pneumatic engines, but there might be an advantage in its slow speed control, such as moving a fraction of one wheel revolution. I have some more testing and tweaking to do before taking pictures but I'll keep you posted. I hope there might eventually be some proportion between speed/torque output and the amount of advance/retard angle if I can get rid of all unwanted effects and tune the proportions of each signal (as a real steam engine with Walschaert's valve gear uses about 1/4 cylinder position and 3/4 advanced/retarded wheel position) There are other, simpler, steam engine mechanisms that I've doodled as well. I'll see which ones are best for what. My first 2-cylinder uni-directional one was in 1995, soon after I had assimilated the 8868 truck mechanism, which was the original 1991 LEGO pneumatic steam engine, being the first pneumatic repeating mechanism. Mark Edited November 5, 2009 by Mark Bellis Quote
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