OK, got it a "Dob, on a Ea, platform and the platform on an Az, El mount.
Again PWM is "Pisle Width Modulation" Steppers and stepper drivers don't use PWM. They use what we cal "Pulse/Direction" PWM uses a constant square wave rate like that is always between 60 Hz and maybe up to 1,000 Hz. What changes is the "duty cycle" or the ratio of "on" to "off". But it just runs at about 60Hz. PWM uses just one wire. Pulse/Direction uses two wires. One sends a single pulse each time the motor needs to move a step and the other wire is either "1" or "0" to indicate the direction, either backward or forward. With this type of system I don't think you worry about minor things like atmospheric refraction of structural flex. You can just run at a constant rate. I am wondering why you selected a Raspberry Pi and not some micro controller. Perhaps you are planning a user interface that is complex? All you software needs to do is generate plusle each time the motr needs to step. For the motor that runs the EQ platform this step rate is fixed forever. For the other two they can step at whatever rate do want. Why encoders? If the motors are proerly sized they will never skip steps and you can count steps to know where you are. If you put on encoders you just need to encoder steps On Thu, Jul 9, 2020 at 1:48 PM R C <cjv...@gmail.com> wrote: > there will be worm gears, and those will just gear all the axis/axes > down down. > > On 7/9/20 1:23 PM, Chris Albertson wrote: > > Telescopes are something I know a little about, What you don't tell us > and > > what matters quite a lot is the mechanical gearing. How are the > > motors connected to the mount? > > there will be worm gears, and those will just gear all the axis/axes > down down. the 3 motors are going to be connected to a stepperdriver > (DM542 or so) and the stepper drivers are 'driven' by an raspberry pi > (mounted in the mount) and I am using a quadrature encoder on all 3, on > the shaft that drives the main gear on each motor. > > > > > If this is an equatorial mount what are there three motors? and why care > > much about if declination moves smoothing as it is only for slewing to a > > new target. > > > It is not exactly an equatorial mount, it is an equatorial platform, > aligned with polaris (northern star) and it turn oposite the rotation of > the earth, to compensate for rotation. > > it only has one motor, because it only has to tilt to compensate for > rotation, at a steady speed. The telescope, with a dobsonian mount is > standing on top of that ... and > > it has an azimuth (rotate horizontally, with respect to the platform) > and and an elevation mount, to tilt the telescope. > > > So one motor for the platform, one for rotating it (azimuth) and one > motor for altitude, tilting the tube.. (the latter two are for > "convenience" just to point the telescope at an object and precission > is not "that" big a deal because once the object is in sight, those two > motors 'hold'. after that the plaform is tilting, to compensate for > rotation of the earth). > > > > > > But if this is Az, El then yes I see why three motors, You have Az, El > and > > Feild rotation and all most move smoothly and in exact synchonized > motion. > > But an equatorial drive needs only one moer to move smooth. > > > Exactly.. you got the idea there. once the object is "locked in" only > the equatorial platform has to precisely move, without "jerking" > > > > > > As for PWM. PWM is nothing at all to do with stepper motors. You can > > forget about it. Unless you decide to not use steppers. > > > I am using stepperdrivers, which are running the stepper motors, and > the stepperdrivers use PWM to tell them what to do? > > > > > > > > The way to work this is to choose a gear ratio, typically a very large > > reduction, andchoose a step size that is literally "invisable" given you > > angular resolution. Of course angular resolution depends on aperture or > > maybe the pixel size and focal length if this is purly a photographic > > 'scope. The steps need to be smaller then 1/2 the angular resolution > and > > conservative designers might go even smaller, Hence the use of really > big > > worm/wheel reduction systems > > > I already have figured out the mechanical part so that even with full > steps, I can still accurately move t=it and track rotation of "the > field" aka rotation of the earth. > > > > > > Example. (1) You have a 180 tooth gear and a 1.8 degree step size. > That > > works out to 36 arc seconds per step. This might be good enough for a > > small scope used in poor seeing condidtions. > > (2) 360 tooth gear and 1/8th microstepping now you are at roughly 2 > > arcseconds per step. This is much better > > I have two 360 gear sets (for alt/azi) but they are big.. and a 60 > teeth gear/work set. ratio 60:1 > > (one rotation of the gear needs 60 rotations of the worm, with full > stepping that means 12,000 steps for > > one rotation of the shaft that turns the platform. and I only need to > tilt the platform 15 degrees in an hour > > which should be doable. I would only need 14-15 arc second precision > really, per second. > > > > > > How fast do you need to step? Assume the Earth turns once per day ;-) > > That is 1,300,000 arc seconds per 86,000 seconds. Or about 15 > arcseconds > > per second. So the motor in case number two steps only about 7 or 8 > times > > per second. So it looks like you could do 1/16th step or even more. > > > I need to tilt the platform 15 degrees per hour, earth rotates 360 > degrees in 24 hours, that is 15 degrees per hour, or 15 arc mins per > minute which is 15 arc secs per second. .. which is pretty slow, but > the need it to do it smoothly. > > > > > > But you trade off slewing speed if the microsteps are too small. So you > > want them to be invisible at your optical resolution but no smaller. > This > > depends on the aperture of the scope and the camera's angular pixel size > > right, like that and yes, taking lot's of pics, so vibration is a no > no (although that can be fixed afterwards) but ideally, smooth moving > and accurate, for compensation, would be best. > > > > > But is this az, el or equatorial? And what is the gearing? I just made > > up those examples from thin air. > > equatorial, the equatorial platform is most important. > > > The idea is. set the platform ready (at 15 degrees "towards" rotation. > then move the scope to the object and keep it there. so AZ and Alt are > holding. the EQ starts moving, in order to track. > > > > > One thing you really want is FEEDBACK from a guide star or even from the > > main camera. A guid camera can detect feild drife and change the speed > of > > sure, maybe some day.. for now I am using rotary encoder .. I have a > few 400ppr quadrature encoders lying around, they are very small.) > > > > the motor compensate. Drift is caused by (1) changes in > > atmospheric refraction as the target moves and (2) the telescope mount > > flexing as the scope moves. Some sophisticated software tries to model > > this and calculate the effect but a close loop is best and now days cheap > > to do. > > > > Last time I did this we used NTP to keep the controller's clock > > synchronized to actual time. We used a local GPS receiver to create a > > local NTP server > > > > Yes, one could use LinuxCNC to drive the motors it might be easier to > just > > use a microcontroller. > > > That is what I thouht of when I saw an older post about using CNC for > an rc model (a plane or so?) > > > > > > So I have the mechanical parts figured out, and how to do some > reasonable tracking. What I am after is how to best make the stepper > drivers work. > > > With PWM signals I can 'drive' the stepperdrivers and I can move the > stepper motors, I am just trying to figure out what the best method9s) > are. for example what the connection is between pulses that are > longer.. or shorter pulses with larger gaps between them. > > > Ron > > > > > On Thu, Jul 9, 2020 at 11:26 AM R C <cjv...@gmail.com> wrote: > > > >> Hello, > >> > >> this is (probably) off topic, been seen that happen. If it is please > >> ignore it. > >> > >> > >> I am building a "motorized" telescope mount (dobsonian) with what is > >> called an equatorial platform, it has 3 axis which I am going to drive > >> with stepper motors. > >> > >> > >> The stepper motors I use with a stepper driver, those common DM542 ones, > >> the stepper motors themselves are 2A and 1.8 degrees per step. > >> > >> > >> What I want to accomplish with the equatorial platform) (it compensates > >> for the rotation of the earth) is that, the start and end position > >> accuracy is not that important, smooth and constant/consistent movement > >> is. for the azimuth/altitude precision is not a really big deal, but > >> you'd want to move these 2 axis somewhat swift. > >> > >> > >> So there are a few factors to decide. > >> > >> > >> I probably want micro stepping, what settings on the driver for pulses > >> per rev, is best to use (or is that just trial and error?) > >> > >> > >> As with PWM itself, I am probably just not too familiar with it. From > >> what I understand, the voltage I use for the motors determines how fast > >> I can go (I am going to use a 48V switching power supply). > >> > >> > >> as for PWM, I can of course change the length of the pulse itself > >> and, independently, change the time between two pulses. What is the > >> relation ship there? WHat does a longer width of the pulse itself do? > >> and what exactly does a longer gap between the pulses do (of course the > >> wider the gap between two pulses the slower the motor turns). > >> > >> > >> for, especially, the equatorial platform, I want to avoid "jerking" it, > >> meaning starting and stopping the stepper motor as little as possible > >> and just go at a 'slow' constant speed. > >> > >> > >> > >> sorry if totally of topic.... > >> > >> > >> thanks, > >> > >> > >> Ron > >> > >> > >> > >> _______________________________________________ > >> Emc-users mailing list > >> Emc-users@lists.sourceforge.net > >> https://lists.sourceforge.net/lists/listinfo/emc-users > >> > > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
