Adding OnStep to a Cave Model A mount


rpasken
 

I had a window of a few days to work on my OnStep project. I have a Cave Model A mount with both a Declination and Right Ascension worm and wheel. Originally the DEC was a direct connect from a 1rpm synchronous motor to a 144 wheel. I replaced the 1rpm motor with an Arduino and stepper motor shield and simple push button north-south motion. Originally the RA was a 1rpm motor with a 10tooth 48 pitch gear on the motor connecting to a 144 tooth 48 pitch gear connecting to worm of a 100 tooth wheel. I also replace the 1rpm  synchronous motor with an Arduino and stepper motor shield turning the stepper at 1rpm with a push button for east-west motion. Since the stepper motors with the Arduino shields worked extremely well I hoped that I could just replace the electronics I built with those from OnStep. I have a MaxESP3 board with tmc2209 stepper drivers., Config.h has #define AXIS1_STEPS_PER_DEGREE       6400  (A 1.8 deg stepper, 8 microsteps, 14.4 GR1 and 100 GR2) and #define AXIS2_STEPS_PER_DEGREE      640.0 (     1.8 deg stepper, 8 microsteps, 1 GR1 and 144 GR2)   #define SLEW_RATE_BASE_DESIRED        2.0. When I start OnStep I can set the time, turn on tracking and use the slow motions to move the scope around and everything works as expected; smooth quiet movement on both axis. However when I try to align or goto the DEC axis moves as expected, but the RA stepper chatters loudly. I can replace the 10-tooth/144 tooth pair with a belt drive but I would prefer to keep everything as original as possible. I can restore everything to original condition with the existing configuration by simply swapping the steppers for the 1rpm motors.Any suggestions??


John Petterson
 

The loud chatter is most likely the sound of a stalled stepper motor - trying to move the mount too fast with either too high or too low a current or too low a voltage.  Or it is driving an unbalanced mount and does not have the torque to move it at higher speeds.  Is there a telescope on that mount and is it balanced well?  A GEM will not be turned easily with these small motors if it is not balanced.

Have you set the vRef?

First step would be to check if the motor does the same thing when it is not connected to the mount.  Can it run at high speed without stalling?  Then connect it again and set the slew rate to "slowest" and see if it works that way.

There are limits to how fast each combination of steppers - drivers - and mount can operate.  You can improve that a bit with some settings.  Rebuilding and re-lubing the mount might help as well if it is not smooth and frictionless.  More powerful steppers might help.   And I am not familiar with the 2209 drivers.

The particular stepper motors you are using could be causing this as well - which ones are they?


rpasken
 

Thanks for the suggestions. The RA stepper turns as it should when not connected to the mount and I can easily turn the 10tooth gear with my fingers to move, albeit slowly the RA axis. I don't have a scope or counter weights attached to the mount at this point. Based on your response I'm guessing the stepper is stalling. Checking the voltage and current is next. I was hoping it was something as simple as the slew rate being to high. TIme to check current and voltage. But given final exams are in a week I won't have time check for a while


John Petterson
 

So reducing the slew rate will likely get you to a point where it will work, but first try putting a scope on the mount and balancing it well, and see how that works.  All bets are off with a GEM that is not balanced.

You can reduce the slew speed  for go tos- With the SHC menus, Settings - Configuration - Goto Speed.  With the Wifi  or Ethernet  browser use Settings - Maximum Slew Speed.

You can also set the speed of your push button movements (guide rate) to higher levels to see when those stall - on the SHC, the two bottom buttons change that speed faster (the right button) and slower (the left button), and on teh browser there are buttons at the bottom of the screen that has the directional arrows..

When you get a chance, let us know what model motors you are using.  And good luck with the finals...


rpasken
 

The steppers are recycled from old scientific equipment. All the steppers I have are NEMA-17 and most are made by minebea, best I can do on the worn label is 17xxx1024-g2v. I purchased two new 1.8 deg steppers but they worked so poorly (very coarse motion with definite steps even with 64 microsteps) they were unworkable


Howard Dutton
 

On Sun, May 2, 2021 at 05:47 PM, rpasken wrote:
I have a MaxESP3 board with tmc2209 stepper drivers
Using these drivers with a MaxESP3 (and I'm assuming you have the correct TMC2209's per the Wiki advice.)

Axis1 and Axis2 must be in the same micro-step mode, and their AXISn_MICROSTEPS_GOTO must be OFF.
You should be able to switch between stealthchop and spreadcycle though (TMC2209_QUIET.)


Howard Dutton
 

On Sun, May 2, 2021 at 05:47 PM, rpasken wrote:
I can replace the 10-tooth/144 tooth pair with a belt drive
1440:1 overall reduction should be enough to move around a fairly stubborn mount with NEMA17 motors.

Does the RA motor get good and warm when tracking is on for say an hour?  Generally speaking a fully powered hybrid motor like this gets "warm" enough you don't want to grasp and hold it for an extended period.  Not saying you want to operate it at those power levels but it sets a benchmark to judge by lacking specifics.  Another way is to look at Oriental Motors website (wide selection and easy to view detailed specs) and find a motor of similar frame size and body length and coil resistance.