Gene, Belts are a good idea for the Ax and El axis but in the Eq axis he needs something on the order of 100:1 reduction
The time tested traditional method is a synchronous AC moter and a VFD that can do 60 Hz +/- about 5 percent. This axis turns only 1 rev per day but the step need to be less then the eye can see Belts can go to about 6:1 at best a double belt reduction only goes to 36:1 wich would be about right for Az, El. We don't know if he is doing photography One more thing: I think I read he is using 48 volts. That is the wrong voltage for this use case. I'd go down to half that. He is doing only about 10 Hz. On Thu, Jul 9, 2020 at 5:35 PM Gene Heskett <ghesk...@shentel.net> wrote: > On Thursday 09 July 2020 19:05:45 R C wrote: > > > Hello Gene, > > > > > > To start with the end of your reply, I actually am a mathematician > > :) > > > > On 7/9/20 4:24 PM, Gene Heskett wrote: > > > On Thursday 09 July 2020 14:23:57 R C 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?) > > > > > > I think the gear ratio is as important as the microstepping ratio > > > which I would probably set to 8 or 16 with the dip switches on the > > > dm542's. Any higher than that and the higher speeds need stepper > > > pulses faster than the opto's in the DM542 inputs which will limit > > > your top speed to just north of 200 kilohertz for a step rate. > > > > Correct, the gear ratio and stepper resolution are important, so is > > leverage/torque.. and well.. space to put it. To move the mount > > > > on it's vertical axis, azimuth, and to move the newtonian tube, the > > latitude, U use 360 teeth gears, because they are bigger and have more > > "leverage", so > > > > I can move them easier/faster (somewhat, that is a trade off too). > > > > > Next is the resolution of your mirror, so you would want it to move > > > about 4 microsteps to see it visibly move in the eyepiece. > > > > The one I am doing this with is not that big, it is an 8" main > > mirror. > > > > > Then if you are well balanced and running on ball bearings for low > > > friction on all 3 axis's, you may want to experiment with the motor > > > current to see if a certain current gets you a more even movement, > > > some drivers, and I haven't asked, nor tried to prove that 1/8th or > > > 1/16 step gives the same movement as it depends both on the currant > > > mapping used by the DM542 driver and the magnetic (iron) properties > > > of the motor. > > > > Yes they all have ball bearings, a big "lazy susan" style one betwen > > platform and mount, pillow/pocket bearings for the > > > > elliptical friction "runners". However, kepping the Azimuth > > bearing/joint and latitude bearing/joint steady for a dobsonian is > > more important.. they kinda need to be "locked" when on a target. > > > > The equatorial platform needs to run smoothly and steady, no or little > > "jerking" > > > > > > That is something I want to experiment/try/figure-out if there is a > > micro stepping setting that would be best to use. The > > smoother/constant/consistent I mover the EQ > > > > platform the better. That is where micro stepping and speed comes in I > > think. If I can continuously can keep moving without stopping, slowing > > down, speed up, the better it is probably. > > > > >> 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). > > > > > > Steppers aren't pwm driven, so ignore that. But a 48 volt supply is > > > pushing the ratings of a DM542 which is 50 volts max, and you would > > > be wise to adjust it a few volts lower. I have a pair of 7.5 amp 48 > > > volt supplies running my 11x56 Sheldon lathe, but they are turned > > > down to 42.5 volts, much safer, and it still marches at way faster > > > speeds than needed for a good cut. > > > > I can do that, I can put a voltage regulator between them. What do > > you mean by "safer" btw? > > > > >> 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. > > > > > > LinuxCNC has "adjustment knobs" for that, but are set with a text > > > editor and forgotten once done. Slewing to another star can be done > > > with a second or more to get to speed, and a slowdown in approaching > > > the new target so "jerk" can be very low. > > > > I am not too worried about that part, zooming in on an object, that > > is fine, I could do that manually, well remotely. and once "locked > > in" keep it steady and start the EQ platform. > > > > I am also terribly worried about the "Go To" features of the > > telescope. I am planning on a good camera on the telescope, and a > > "cheap" camera on the finder scope. Ultimately I want to use > > Stellarium, and maybe even use the Stellarium connection features to > > connect to the telescope (this project is going to take a while). > > > > > The problem I see in moving a Dob is in the kinematics conversion > > > because its axis's for az-el need to be mapped to the location on > > > the planet for the base motors movements. A newtonian on a polar > > > mount cancels 99.99% of that because the 2 axis's are 90 degrees to > > > each other, the Dob mount is not. > > > > That is correct, but that would be a "way later" thing to play > > with.. that part would definitely need some geometry and math > > trickery. > > > > All the "pivoting" points are offset to eich other... when the > > platform moves, the whole telescope is tilting, etc etc etc... > > > > >> sorry if totally of topic.... > > > > > > Not totally. Might be an interesting diversion. :) But I'll let > > > some of our better maths people comment on the kinematics. > > > > Cool, well I am glad you see it like this. Basically what I am > > thinking is, OK, my mill has 3 stepper drivers/motors, and that > > telescope does too (once I built it like that)... and .. same > > hardware, same motors, same stepper-drivers... So "these emc-user" > > guys must know how to work that hardware.. because both my > > Sherline mill and lathe are working just fine using it with the same > > hardware :) > Well, I don't see a problem, except your fascination with PWM which isn't > normally used to drive steppers. Thats not saying it won't, because it > will, but normal wiring TO the DM542 has a fixed 5 or 3.3 wired to all > the + input terminals. The controlling logic is wired to the - > terminals. Why, sound upside down, but most logic circuits can sink > around 10 times the currant when driving a logic zero than the can > source driving a logic one, so you get a much solider driving signal > into the opto-isolator in the DM542. Then, again generally speaking this > logic zero is the step command on its falling edge, and because the opto > stuff is slow, the pulse is usually at least 3.5 u-secs just to make > sure its recognized, else shorter times, like the 2.5 u-secs you quoted > someplace, may occasionally miss a pulse, playing hob with the accuracy > of your tracking over the length of a photo exposure. Thats the last > thing you need. > > The last time I looked at stellarium, it had some sort of an interface to > drive the scope, And I have a 10" Meade Newtonium that really needs > better mirrors. But where I live, the horizon is at a 10 to 25 degree > up angle, so I didn't set it up in a permanent house, or try to hook it > up to stellarium. I can just barely get down to polaris to align it. If > I had another 20 years which I don't as I'm now 85 with a chest full of > replacement parts, I might have considered it. > > One thing I might consider is belt drives, using very fine toothed > pulleys like a 3d printer uses, but since your motions are under 180 > degrees, I'd use smooth turned wheels, fastening the belt in slots in > the far side of a good sized wheel. Cheap, zero backlash, unlike an > expen$ive worm drive. Those belts are generally kevlar or dacron > backed, and zero stretch under long term tension. Buy them by the foot. > Use the money you save to buy your suds come your birthday. ;-) > > > > Ron > > > > >> thanks, > > >> > > >> > > >> Ron > > >> > > >> > > >> > > >> _______________________________________________ > > >> Emc-users mailing list > > >> Emc-users@lists.sourceforge.net > > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > Cheers, Gene Heskett > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > Cheers, Gene Heskett > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > _______________________________________________ > 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
