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PMC-Eight - configure for other quality mount


 

I would like to explore using the PMC-Eight controller system on a high quality Rob Miller GEM (genesis of the AP900 designs) because it easily tracks at <1as, is a proven performer and I have an attachment to this old mount that uses simple steppers that directly attach through couplers the worms, has clutches and no encoders.
The RA is 6" 360 Teeth
The DEC is 5" 240 Teeth


Assuming I update the steppers to those used on the G11 (NEMA 17 frame motor), is there a configuration in the software or firmware that would work for these gears?


This would make an interesting DIY upgrade project. 

Thank you and just exploring possibilities...


Chris




hubbell_jerry@...
 

Hi Chris,

I am sure that the system would not have any problem working with this mount. There are only 2 parameters to change between any mount system and that is the total motor counts for each axis and the motor current needed. The PMC-Eight is designed to provide motor currents up to 1200 mA for each motor, currently the G-11 motors are set to 900 mA and the EXOS 2 motors are set to 600 mA.

The equation for motor counts in the PMC-Eight as shown in the Programmer's Reference is:

Wheel Teeth x Spur Gear Ratio x Motor steps/rev x 32

G-11 Counts = 360 x 1 x 400 x 32
                     = 4,608,000

Of course, this upgrade assumes that you already have the PMC-Eight system. Currently, you cannot buy the PMC-Eight as a separate DIY upgrade kit, but that doesn't stop you from using your purchased PMC-Eight system on any other mount you want. 

The only other piece of information you need is the wiring diagram for the motors. You can figure that out by looking at the wiring on the G11 or EXOS 2. Just 4 wires.

Currently the ASCOM driver does not have a way to manually enter the Total Motor counts but I can make that change in the driver setup to select a CUSTOM value for the RA and DEC axes.

I would need to work with you to provide you some custom firmware if you discovered you needed to have a different motor current to support your mount, otherwise the 900 mA may work fine for you. This may or may not void your warrantee if you use a higher motor current, I would need to discuss this with Scott Roberts.

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.


 

Thank you Jerry, this is all very encouraging and really appreciate the detail given.

I was interested in buying the exos2-gt and then on occasion use the PMC-Eight controller on my quality GEM, at least that was the idea.  The G11 is very appealing but I'm not ready to move on (just yet) from my proven GEM.

I just checked the existing stepper motors (7.5 degrees step angle) that are geared (not sure of the ratio), and they are 6 wire rather than 4.  It would of course be ideal if I could use the existing steppers.  I have some more research to do this end to give out the motor current and gear ratio.  These are McLennan/Airpax stepper geared stepper motors.

Is there any difference between the PMC-Eight controller on the exos2-gt versus the G11 ?
If I go the replacement motor option,  is there anything I need to know about ordering NEMA 17 motors - are they all the same?  Is there a separate gear box like on my existing steppers?

Many thanks,

Chris


hubbell_jerry@...
 

Hi Chris,

The EXOS 2 and G-11 use the same controller, there is no difference. Basically, you are getting a free mount with the controller when you buy the EXOS 2 at $899. The PMC-Eight is designed to control bipolar stepper motors (4 wire). There are different configurations for the NEMA 17 frame size. The motor we use, and that I would suggest, on the G11 is the 17HM19-1684S. Here is the link to that motor: 

There is no gear box just the helical coupler between the motor and the worm. These are 0.9 degree/step (400 step/rev) motors.

Do you have the model number of the existing motors so that I can try and find them to see what the output torque rating is for those motors?  What is the weight capacity of the mount?

Thanks

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.
 


 

Hi Jerry, the capacity of the mount is about 45 Lbs and weights only 28Lbs.  It's beautifully machined aluminum with precision ball bearing raced throughout.
The max slew rate is 150x sidereal (Original controller has accelerator circuitry) - that used to be fast in those days:-)

Here's the motor information I have:
The actual stepper motor is labelled as Airpax 9904 112 32101 3191 (made in Belgium).  6 wires suggest to me they are middle tapped.
The gear box is labelled McLennan P5 32-ST101- (can't seen the rest of it).
Uses Oldham Couplings I think.

I agree, the EXOS 2 is awesome value.

Many Thanks,  Chris


Robert Hoskin <r_hoskin@...>
 

Well, Jerry...

This is interesting, and an option I really hadn't considered before.  You just upped the value proposition considerably.

I bought my EXOX2/PMC-8 as a portable (well, luggable...) AP platform.  It's simply too soon in the journey for me to want to put anything permanent in the back yard, and I have a dark-ish site available perhaps a half hour down the road.  So portable-for-retiree becomes a requirement.

Now, when (and if...) I get my skills and imaging time up to the point where it makes sense to put something permanent in the back yard (pier, etc.), my PMC-8 can swing between a 'home' mount and an 'away' mount as needed.

- Bob
 





From: "hubbell_jerry@... [ESPMC-Eight]"
To: ESPMC-Eight@...
Sent: Sunday, March 4, 2018 11:01 AM
Subject: [ESPMC-Eight] Re: PMC-Eight - configure for other quality mount

 
Hi Chris,

I am sure that the system would not have any problem working with this mount. There are only 2 parameters to change between any mount system and that is the total motor counts for each axis and the motor current needed. The PMC-Eight is designed to provide motor currents up to 1200 mA for each motor, currently the G-11 motors are set to 900 mA and the EXOS 2 motors are set to 600 mA.

The equation for motor counts in the PMC-Eight as shown in the Programmer's Reference is:

Wheel Teeth x Spur Gear Ratio x Motor steps/rev x 32

G-11 Counts = 360 x 1 x 400 x 32
                     = 4,608,000

Of course, this upgrade assumes that you already have the PMC-Eight system. Currently, you cannot buy the PMC-Eight as a separate DIY upgrade kit, but that doesn't stop you from using your purchased PMC-Eight system on any other mount you want. 

The only other piece of information you need is the wiring diagram for the motors. You can figure that out by looking at the wiring on the G11 or EXOS 2. Just 4 wires.

Currently the ASCOM driver does not have a way to manually enter the Total Motor counts but I can make that change in the driver setup to select a CUSTOM value for the RA and DEC axes.

I would need to work with you to provide you some custom firmware if you discovered you needed to have a different motor current to support your mount, otherwise the 900 mA may work fine for you. This may or may not void your warrantee if you use a higher motor current, I would need to discuss this with Scott Roberts.

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.




hubbell_jerry@...
 

Hi Chris,

I found the following Airpax motor catalogue but really couldn't match the numbers up.


I also found the exact motor you had identified on ebay


But of course, it didn't have much information about it.

I did find this datasheet from the gear motor id... It seems this is the motor gearset that you have identified on the last page


It is listed as P5 32 ST  and motor 9904 112 32101

Getting closer!

Jerry Hubbell
Director Electrical Engineering
Explore Scientific, LLC.
 


 

Hi Jerry, thanks for the research.

I found a 2 pager datasheet for the motor with all sorts of unintelligible information on it for the 9904 112 32101 motor. Looks like the current per phase is 220mA and 21ohms. Never studied these before even way back in my EE days...


Getting closer still?

Thx,  Chris