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NickK

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Everything posted by NickK

  1. Yes. Temperature changes and the effects on the system would be factored in continuously. The hydraulic fluid temperature would vary due to the pump and valve pressure changes causing changes in temps too. There's two ways to do this: a) complex, measuring all the known variables to make a completely predictable system (way way way too many variables and things to sense). super simple by understanding that there is a natural slow deviation and then compensate on the fly. As the system is simple the speed can be very quick to correct deviations. The problem is that you need to have a very precise measure of the result. You measure the delta Rate and compare against the expected (and desired delta). If the force is too high, reduce it a small amount. If it's too low then increase it. When this simple logic is applied in rapid cycles causes the system to naturally solve itself. This places a requirement on how fast the valves can be altered (rate of change) and the speed of the program. An initial calibration exercise would just allow the mount to have basic starting point. The down side is that the mount would not be good if it remained static (ie not tracking) for periods of time as the degree of error on starting to move would be slightly higher initially.
  2. Thanks Rusty, taking in what you've said, it looks like I should be treating hydraulics in terms of force and loading (as if the pump is providing force and the valves providing an opposing force). With a counter balance, the only force required is to overcome it's current state of equilibrium and then continue moving (still requires power) and then finally the amount of force required to bring the scope to a stop. The impact I see on this is that the valves available would have to provide very small amounts of force (ie high deltaP) depending on the cylinder and radius where it's attached. I think the sticking point would be a challenge although I think this may be more of a problem on initial start/stop, however on a continuous track the telescope's own momentum may help (although this drag would cause lack of smoothness). I think the technique would work and it may be worth building a single axis to learn from. Load it with a dummy counterbalanced load and see what it can do. I also see that the system would have to effectively learn a complete force mapping each time the setup is changed (ie changing equipment and if the scope is reattached differently). I think this could be done as a startup test (whilst the scope is cooling down before viewing). The mount could perform a set of movements to gauge the initial loading and it's impact on tracking. It would still have to sweet tune and continuously alter based on the changing conditions in realtime. I have an idea for providing a exceptionally hi res positional feedback, so I see that as the easier part. Optical mouse sensors provide high resolution optic flow analysis without contact. These can be used on the plate where the cylinders attach. Gaming mice can be up to track at 4000 per inch. How that translates into arc sensitivity depends on the distance from the centre of the plate. The natural grain of the metal would be enough to track against. Also I wouldn't just use feedback on the mount itself for tracking - star tracking is really the only accurate way of doing it so it would have to have some feed back too. Once again - thank you for your suggestions.
  3. I know hydraulics are a pain but they can deliver an exceptionally smooth transition unlike pneumatics which would suffer from bounce. I've been attempting to source small components which will work at about 50psi. For example - you can get an electrically driven gear pump that provide oil for the turbos for racing drag bikes. These are 12V and operate at 50psi with a smooth delivery which would be more than enough for a telescope! The pump and power pack can be placed a distance away from the mount with only the out and return pipes connected to the mount. Deep sea subs use a bladder to even out the flow further - hydraulic fluid is an incompressible fluid so if the pump flow rate varies then the flow rate through the entire system would vary if it's not controlled. How to control that pressure! Well there's three options here: a) use electrically-operated variable ball (or gate) valves to control the flow rate (and thus the tracking rate). This results in a very analogue control over a small flow range. use a set of electrically-operated on/off valves in parallel with different flow constrictions to provide different movement speeds. Open a small valve for a preset flow rate, open a large valve for a larger preset flow rate. This is very digital and works for a series of stepped larger flow ranges. The danger is that this square wave would cause telescope bounce or warping of the tube. c) use a combination of a & b. If you want to move quickly then open a large bore on/off valve. When you want to move slowly then use a small ball valve. A further switching valve can then define which direction the cylinder would move (simplifies the design). Naturally this could throw the telescope around as if it was a rag doll, so we would need to program the controller to be soft start & end using the ball valve. This would reduce the stresses. We may be able to use a single flow controller for both cylinders to reduce cost. Next up - pistons. Small pistons are rather rare too but both realistic model makers use these however I think the best performance would be from large surface area cylinders. This would smooth out the pressure further and would allow very accurate transitions. Why pistons? Well you can get rotary actuators but the majority are rack and pin with pistons moving the rack. Some are finned. We're attempting to get away from errors caused by gaps between the gears/worm or rack. So it's possible to make a plate and attach a couple of pistons to it - the axis drives the scope. With a controller it's possible compensate for flow rates to provide a smooth transition. So, in theory, it's possible to maintain a constant flow to provide constant accurate tracking without gear error. It's still possible to use encoders to provide an accurate feedback. If safety was an issue then putting a proximity sensor to kill movement (and a full pressure release button in case anything gets trapped). As a controller - well there are so many options. You could use a basic microcontroller but another option is to directly couple a Blackfin 537 to a camera and then allow the blackfin to perform optical flow and feature tracking, control a set of servos to manipulate the valves and monitor the encoders. A single blackfin would be overkill for this but hey.. The blackfin boards I have are a bit over kill, each has an omni vision camera, 32MB RAM, 4MB EEPROM and a blackfin 537 (500MIPS). Attach it to a MatchPort WiFi chip and you've got a 2Mbit connection for offloading images and control. Hmm.. the ideas...
  4. NickK

    Hello!

    I'm Nick, a complete beginner to astronomy located near Reading. I'll pop along to the local reading club meets in the new year before buying a scope, but for now I'm busy reading up. Principally interested in imaging DSOs so I get the feeling I'll be buying a Vixen VC200L. Something I may be able to bring to the forum is my experience of software, embedded devices and computer vision (I helped put together a stereoscopic rover with omnivision cameras using dual Blackfin microcontrollers). I have over 14 years of developing software (Assembler/C/C++/Objective-C). So I'll probably frequent the DIY section. Also interested in developing an idea for a tracking mount using a blackfin to provide optic flow processing So I look forward to some banter!
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