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
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