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Robert Brown's "MyFocuserPro2" build


JamesF
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Today I finished bench-testing my third dew controller (had to wait for another humidity sensor to arrive as the one I had appears to have been faulty).  So, I think it's time to start looking at Robert's focuser controller.

The Sourceforge site with all the details is a bit overwhelming at first because there are a number of different designs: myFP2E, the wifi version which has actually moved to a separate project on Sourceforge; myFP2N with a touch-screen display; myFP2M which is a ready-made controller with various options and the standard DIY version which also has a number of hardware options.  There being no big friendly "Start here to build your own focuser" button, the documentation here is probably the place to start.

I have a number of OTAs that I'd like to be able to fit a controller to, varying from my Photoline 72 which is about f/4.75 when fitted with a focal reducer, to my C9.25 and PST at f/10 or even my 127 Mak at almost f/12.  Obviously the size of the critical focus zone at f/4.75 is only marginally larger than a gnat's whisker.  In order that I might hang on to what remains of my sanity I think it makes sense to keep the hardware the same, which probably makes something like the Nema17 with a 27:1 gearbox the obvious choice.  Whilst I might get away with the 5:1 gearbox on some of the OTAs, there's no real cost advantage to using the lower reduction ratio gearbox and I think I'd probably still end up needing micro-stepping for the faster OTAs.  I think I'd prefer not to need to keep the motor coils energised if possible.

I did consider not using a gearbox and using timing gears and a belt drive instead, but I'm not sure that really gains me anything other than potentially being able to hang the motor under the focuser on 'fracs and newts rather than have it poking out of the side.

So, for the moment at least, my plan is to use the Nema17 with 27:1 gearbox from OMC across the board, which means I'll be using the DRV8825_HW203 driver and need the appropriate circuit design.

On the electronics side, unlike the dew controllers I intend to build these on strip board.  I do actually have one circuit board for a revision of the controller that is a couple of revisions behind, but I'm thinking I may save that for a combined travel dew/focuser controller box.  Circuit diagrams and layouts are available in the stripboards directory, but it's not immediately obvious which are the relevant reading.  I started with "BOARD OPTION DRV8825 HW203.pdf", which I think is the main document covering most of the DRV8825 builds.  I'm not actually too worried about displays, temperature compensation and suchlike as the focuser will be remotely operated and software will take care of refocusing properly, but having a set of buttons to adjust the focus manually would be good.  The documentation for the buttons (in the OPTIONS directory) says that pressing both buttons together resets the focuser position to zero and beeps to confirm that it is done.  I was wondering what the buzzer was for...

The main build document doesn't cover exactly which options I want, and in fact suggests that the strip board layouts may be out of date when compared with the "everything" design, so one of the first jobs would seem to be to compare the various circuits, work out what does what and remove the bits I don't need or add the bits I do and see how that affects the strip board design.  I think "Stripboard DRV8825-HW203 FULL" is not that far off, though I don't need the firmware programming switch or the temperature sensor.  Some of the other designs appear to have a 1N5408 diode and a 100uF electrolytic capacitor across the supply and ground that I'm not fully sure I understand the reasons for yet.

James

Edited by JamesF
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I've built a number of them, using both the Aisler PCB, and my own PCB layout\build, which to be honest I prefer, as the board orientation has an "in" side and an "out" side, I've also taken into consideration the display\switches, which are housed in a separate enclosure, and a temperature probe that is cabled upto the focuser motor. 

If you want any more info, I'll gladly supply.....

 

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yep it all works with 12v fine,  note that my board doesn't have the 7808 regulator, which is only used for powering the Nano, & I've never had any issues driving it with 12v, which is what they are rated for anyway !

I was considering reworking the board so I may well then add it in.

I've only had one failure, which was caused by a spider getting to a nice warm spot, but shorted out the powerline to the driver module....  since then I've packed the 'open'  end with poly foam to stop anymore getting in....

 

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I hope that this question isn't too off topic, but I have been thinking about building my own autofocuser for some time.  I am totally confused about the classification of stepper motors.

Am I correct in thinking that NEMA17 just refers to the physical size of the motor housing?

Is there a guide to selecting, and buying stepper motors anywhere?  (I've looked, but I haven't understood what I found).

I've currently got a Robofocus on a TakFSQ106, but the step size on this is a bit too big - so I wand to find a stepper motor that is strong enough to lift a fairly heavy camera, and has enough steps, or microsteps.

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18 minutes ago, Dr_Ju_ju said:

yep it all works with 12v fine,  note that my board doesn't have the 7808 regulator, which is only used for powering the Nano, & I've never had any issues driving it with 12v, which is what they are rated for anyway !

Ah.  I was tempted by a 14 rather than a 17 when I was looking, but they all seemed to be 9V (I think).

James

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4 minutes ago, don4l said:

I hope that this question isn't too off topic, but I have been thinking about building my own autofocuser for some time.  I am totally confused about the classification of stepper motors.

Am I correct in thinking that NEMA17 just refers to the physical size of the motor housing?

Is there a guide to selecting, and buying stepper motors anywhere?  (I've looked, but I haven't understood what I found).

I've currently got a Robofocus on a TakFSQ106, but the step size on this is a bit too big - so I wand to find a stepper motor that is strong enough to lift a fairly heavy camera, and has enough steps, or microsteps.

I can't claim to be an expert, but...

The "17" refers to the size of the cross section of the motor as far as I understand it, so "17" is 1.7 inches and "14" is 1.4 inches.  I don't know if it's generally true, but the larger size motors seem to run at higher voltages.

The length of the motor appears to vary with its power, so a short motor won't generate as much torque as a longer one of the same cross section.

The documentation for Robert's controller (linked in my first post) has some very useful information on what you need to be able to achieve in terms of step size etc. as it relates to the critical focus zone of the OTA.  That might be worth a read as it will presumably apply in the general case.  You can choose to use microstepping or not, but it apparently reduces the torque available and means that the motor has to be powered all the time rather than only when stepping, which can generate more heat.

From there I think it's really a case of finding a match that produces sufficient holding torque and runs at a voltage you can supply (and perhaps is of a physical size that suits your application).

Robert suggests a holding torque requirement of something like 60Ncm (could be 70, not sure without checking), but I have been wondering how much less I could get away with.  Presumably it must be possible to estimate from the weight of the kit on the end of the focuser tube, but I'm not sure how.

James

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1 minute ago, Dr_Ju_ju said:

stepper motors are current driven, not voltage, so as long as the driver is able to provide ample current pulses, all will work ok....

Oh, right.  When looking at the specs I assumed the voltage was the intended operating voltage.  I guess perhaps it still is, but there's a bit more freedom there than I was assuming.

James

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Its more of a safety thing in that higher voltages have the potential to short windings etc. so putting 100V into a thing wire, will probably make it act like a fuse & blow.....

BTW, I get my motors from OMC (https://www.ebay.co.uk/str/OMC-StepperOnline?_trksid=p2047675.l2563) they are actually good quality and reasonably priced, just make sure you get motors that are sourced in the UK, else you could be waiting a while for the delivery from China....

 EDIT:  I also use motors with gear-boxes as these not only give finer control, they also hold position without having power applied, which, over time can heat the motor....

Edited by Dr_Ju_ju
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23 minutes ago, Dr_Ju_ju said:

 EDIT:  I also use motors with gear-boxes as these not only give finer control, they also hold position without having power applied, which, over time can heat the motor....

I own a NEMA 17 motor (17HD34008-22B) I did not know about this gearbox advantage, this model does not seem to have a gearbox.

I will be using an RPI4 with a Waveshare Stepper Motor HAT. This is neat as I only have to provide 12V to the HAT for the motor and it also provide the 5V to the RPI, so only one power source for both. Someone wrote an INDI driver for this and what the driver does is stop on a full step.

"only stop on full steps, it only uses microstepping to make things smoother and quieter. A stepper motor can't hold position at one of the microsteps without power applied, and even then, it's just an approximation. It can hold position at a full step even without power, which is what I do, I kill power to the motor when it isn't actively being driven."

So that would mean that using a motor without a gearbox makes you lose 32 microsteps, maybe this is ok on a short focal refractor that has a larger Critical focus zone (CFZ) ... I need to dig deeper into this, I might have to buy a gearbox motor then...

This is the thread on CN

https://www.cloudynights.com/topic/688855-building-myself-a-stellarmate-gadget-any-advice/

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4 minutes ago, Vox45 said:

So that would mean that using a motor without a gearbox makes you lose 32 microsteps, maybe this is ok on a short focal refractor that has a larger Critical focus zone (CFZ) ... I need to dig deeper into this, I might have to buy a gearbox motor then...

Faster OTAs such as short focal length refractors often are have a smaller CFZ don't they?  An alternative to buying a motor with a gearbox if you already have a non-gearbox motor might be to use a drive belt and pulley to get a reduction, depending on how much reduction you need.

James

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Short focal lenght haver smaller CFZ ? I thought it was the opposite ... 

Here is the formula for the CFZ from the arduino document: [CFZ in microns = focal_ratio * focal_ratio * 2.2]

So in my case that would be :

CFZ = (((600*.85)/80) * ((600*.85)/80))*2.2 = 89.41 microns

Now let's change the focal lenght to 1000

CFZ = (((1000*.85)/80) * ((1000*.85)/80))*2.2 = 248.35 microns

By George ! you are right :)

But let's put real number from a real optical formula like a newton 200/1000

CFZ = (((1000/200)*((1000/200))*2.2)) = 55 microns

So the diameter also plays a role not only the focal lenght, but really the focal ratio.

Edited by Vox45
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3 hours ago, JamesF said:

Faster OTAs such as short focal length refractors often are have a smaller CFZ don't they?  An alternative to buying a motor with a gearbox if you already have a non-gearbox motor might be to use a drive belt and pulley to get a reduction, depending on how much reduction you need.

James

Yes I am planning on changing my focuser to a Baader steeltrack with a built in pulley between the 2 knobs (coarse and fine) I find this very elegant, placing the motor underneath the focuser instead of sticking out on the side.

steeltrack.JPG.13fdea0a1f2e56a12a0f48a64e853294.JPG

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I've done a bit of reading and messing about with GIMP this evening and this is what I think I plan to build:

focuser-schematic.png

I believe I understand pretty much all of the circuit now.  The resistor network to the left of the Nano is just for the buzzer, manual movement buttons and LEDs.  C1, C2 and the 7808 provide the 8V regulated supply for the Nano.  D3 stops the Nano trying to push 5V backwards through the regulator if there's no 12V power.  R14, R16 and D2 form a power detection circuit (so the code won't try to step the motors and get confused about its position if there is no 12V power).  D1 is reverse voltage protection and C4 is apparently required across VMOT and GND on the DRV8825 for it to work.

James

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9 hours ago, Vox45 said:

I own a NEMA 17 motor (17HD34008-22B) I did not know about this gearbox advantage, this model does not seem to have a gearbox.

I will be using an RPI4 with a Waveshare Stepper Motor HAT. This is neat as I only have to provide 12V to the HAT for the motor and it also provide the 5V to the RPI, so only one power source for both. Someone wrote an INDI driver for this and what the driver does is stop on a full step.

"only stop on full steps, it only uses microstepping to make things smoother and quieter. A stepper motor can't hold position at one of the microsteps without power applied, and even then, it's just an approximation. It can hold position at a full step even without power, which is what I do, I kill power to the motor when it isn't actively being driven."

So that would mean that using a motor without a gearbox makes you lose 32 microsteps, maybe this is ok on a short focal refractor that has a larger Critical focus zone (CFZ) ... I need to dig deeper into this, I might have to buy a gearbox motor then...

This is the thread on CN

https://www.cloudynights.com/topic/688855-building-myself-a-stellarmate-gadget-any-advice/

This is not correct. I must take issue with this. A NEMA motor, operated at full steps, CANNOT hold a position at a full step even without power. ONLY if it is a geared motor like a PG27. If it is a standard NEMA non geared motor, then it has insufficient holding torque for most "imaging" trains especially when pointing towards zenith [which must be the true test of holding power]

regards

Robert

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5 hours ago, brown_rb said:

This is not correct. I must take issue with this. A NEMA motor, operated at full steps, CANNOT hold a position at a full step even without power. ONLY if it is a geared motor like a PG27. If it is a standard NEMA non geared motor, then it has insufficient holding torque for most "imaging" trains especially when pointing towards zenith [which must be the true test of holding power]

regards

Robert

Interesting, I was planning to go with a planetary gearbox as there are many other pros to them and you just gave me more reasons to.

Now I may be overthinking this and maybe there is some overkill in getting this kind of motor but for the price, I'd rather futur proof my motor if I ever have to attach heavier equipment to my focuser :)

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7 hours ago, brown_rb said:

This is not correct. I must take issue with this. A NEMA motor, operated at full steps, CANNOT hold a position at a full step even without power. ONLY if it is a geared motor like a PG27. If it is a standard NEMA non geared motor, then it has insufficient holding torque for most "imaging" trains especially when pointing towards zenith [which must be the true test of holding power]

Thank you for that clarification, Robert.  I was also under the impression that the coils didn't need to be powered at full steps for the motor to hold its position.  I must have misunderstood comments I have read elsewhere, or read comments from someone else who had done similarly.

James

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A slight variant on Roberts view is explained here https://engineering.stackexchange.com/questions/9111/stepper-motor-holding-torque-without-coils-energized

Bottom line is if the "Cogging"(Detent) torque is enough then you are laughing - BUT geared models obviously have the highest "cogging" torque( holding effect ) even if the stepper itself doesn't power or not!

Plus finding "Detent Torque" value on specs for stepper motors might be a bit hard - std OMC spec for Nema 17 - I couldn't find it in this spec unless they have called it something else or I just missed seeing it (more than likely 🙂 )

https://www.omc-stepperonline.com/download/17HS15-1504S-X1.pdf

For the small difference in price its a no brainer IMO - go geared !

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