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.
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. 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. > 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. > > 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. 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. > > > 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. > > thanks, > > > Ron > > > > _______________________________________________ > 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
