Ideas for a Home Made Mount for my WF Triple Imaging Rig

[Gina]

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[Tinker1947]

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Sorry for got to link them.......

http://www.amazon.co.uk/RioRand-Stepper-4-Phase-5-Wire-ULN2003/dp/B0089JV2OM/ref=sr_1_1?ie=UTF8&qid=1420819981&sr=8-1&keywords=28BYJ-48

[Gina]

I thought so   Yes the very same.  I've also used the 12v version. 

[Tinker1947]

That's a give away, provided they work..so how are they controlled......

[Gina]

They work well - I have several in use for remote focussing and various other things.  Using the driver module supplied, you just connect the 4 data lines to 4 digital outputs on an Arduino and the power to 5v and Gnd/earth/0v.  Then write an Arduino sketch to do what you want.

[Tinker1947]

Can you recommend a Main Board as there appear to be loads (processor type ect)....

[hughgilhespie]

Gina, Glad to hear you are recovering, albeit more slowly than you would like.

On steppers, I read recently - but can't find the link - that the available torque falls off very quickly with microstepping and if you need anywhere the rated motor torque then 4 micro-steps is about the maximum you should consider.

Regards, Hugh

[Gina]

Can you recommend a Main Board as there appear to be loads (processor type ect)....

Like I said, I use Arduinos.  Mainly Nano (for one to three steppers) or Mega for controlling four steppers at once (or more).

[Gina]

Gina, Glad to hear you are recovering, albeit more slowly than you would like.

On steppers, I read recently - but can't find the link - that the available torque falls off very quickly with microstepping and if you need anywhere the rated motor torque then 4 micro-steps is about the maximum you should consider.

Regards, Hugh

Thank you Hugh

Yes, I think the torque does fall with microstepping but I don't know by how much.  I would have though that once past 2x or 4x there would be no further loss of torque as the drive would approximate to a sine wave regardless of the number of steps per cycle.  The only reason I can see for reduced torque is that a square wave of a given peak amplitude contains more energy that a sine wave of the same peak amplitude.  In fact 2x microstepping using square waves gives more power than single stepping where only one winding is energised at a time (though with more heat generated).

It will be interesting to see how the 28BYJ-48 behaves when adapted to bi-polar working and driven by the RepRap/RAMPS/A4988 system

[Tinker1947]

Like I said, I use Arduinos.  Mainly Nano (for one to three steppers) or Mega for controlling four steppers at once (or more).

Thanks for that Gina, i buy a book or 2 and see if its something i want to play about with....

[Tinker1947]

Like I said, I use Arduinos.  Mainly Nano (for one to three steppers) or Mega for controlling four steppers at once (or more).

Ordered one of these today, have some books Sketches take me back to Dbase IV similar sort of thing, have some odds and ends coming from China, still not sure i will have a proper use for a Nano board but its cheap, hope its the right type..??

http://www.amazon.co.uk/gp/product/B00CG6KQO6?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00

[Gina]

Should be fine   The feedback on Amazon is good.  I have only used genuine Arduino products so far but these clones look alright and are a lot cheaper.  The UNO and Nano work the same but the Uno can be used with the plug on boards called "shields" which are very useful and easy to use saving you building your own extra bits.  The Mega version is the same as the Uno but longer and with more I/O pins for more complicated/bigger projects.  There are plenty of threads on here for Arduino projects and the Arduino web pages are very helpful and full of information   Also, I'm very happy to answer any queries you might have as are many other SGL members Enjoy!

There are other Arduino versions smaller than the Nano with reduced functionality but I won't muddy the waters with those for now...

[Tinker1947]

There is a auto focusing thread i scanned through and will attempt to make the electronic side as a project not doubt there will be question along the way.....

[hughgilhespie]

Hi Gina,

Found the link re loss of torque with microstepping - it's http://www.micromo.com/microstepping-myths-and-realities.

This is something called ''incremental torque' not sure what that is exactly but the fall off in torque is amazingly dramatic!

Regards, Hugh

[Gina]

Thank you Hugh   That does look like it could be a problem for a small stepper motor.  I understand that there is a lag of the output v the input phase shift due to the load.  This is always the case with synchronous motors.  I had forgotten that aspect of it.

In this application I could stand a decrease in slewing rate.  When an imaging session of several hours is envisaged a slew time of maybe a minute or two would be acceptable to get from the park position to the direction of the object.  Putting some figures on this - if we want to slew from horizontal (parked) to vertical at 3 degrees per second this would normally take 30s so 4x would not be so awful   Also, that figure of 2" of arc for the accuracy could be relaxed a bit. 

In practice I usually get ready for an imaging run before dark so an extended initial slewing time would not be a problem.  Nor would it matter at the end of a session.  The only time it would matter is in the event of rain detection and an emergency shut down.

[skybadger]

That's not a bad idea . I already have the machine. What is missing is the measuring and feedback apparatus.

[Ajohn]

i've been tempted to build a mount many times. One thought that struck me was that worm wheels are usually gashed rather than having "rounded teeth" hobbed into them so why not use a gear and angle the worm. It's sometimes possible to get a gearbox lathe to cut worms of some pitch with suitable gearing added. A spread sheet can be handy to see what can be done that way as the ideal setting might crop up at any gearbox setting. i also thought about making a rotary table like that but went from a small hobby miller to a dore westbury and the guy that sold that to me threw in a small table and dividing head that had been built from kits thinking cheap Chinese stuff would be better,

If a super accurate gear / worm wheel is needed they can be lapped.  The idea is to think prime numbers and several sizes of lapping gears so that the lapping is even. Astrophysics didn't even worry about that when they worked on and rebadged Meade mounts. Toothpaste might be ideal for that sort of thing. A gear would need something a bit coarser initially.

Fork mounts tend to be lighter and more stable on a decent tripod.  An SC fork mount would just need some sort of tube with the scope rings in the centre. The bigger ones that would leave clearance for a small scope plus camera aren't that light though.

Encoding - an optical mouse will resolve around 1000 dpi, not sure if they will work against curved surfaces such as a gear or what ever. They might against a flat portion of a belt but I would worry about give in the belt with varying load.  Sort of extension of the old idea of using optical encoder out of a mouse.

I'm fairly sure Mel what ever he was called came up with an idea to fix up alt az mounts so that they will track correctly - the camera has to be rotated as well. The info might still be about some where.  Now there's a project for and Arduino or two.  A pi might be a better idea probably with plenty to spare.  I have no idea how well this might work.

Worst of all if some one gets tempted - there is plenty of info about on the web concerning casting aluminium at home. I've managed to avoid the temptation so far but did ask one person about their propane burner design and how they got on with it. There is also a special sand available for use round the pattern that leaves a much better finish than ordinary sand.

John

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[Gina]

I'm thinking about this again now that I have a ZWO ASI1600MM-Cool plus ZWO FW mini and SLR camera lenses, giving a relatively lightweight and small widefield imaging rig.  I plan to use all belt drive with two stage reduction using MXL timing belts and pulleys.  The imaging rig should be around 1Kg and a fork with a depth of around 120mm should suffice and width of something like 70-80mm.

[happy-kat]

Those dimensions sound very tidy.

[Gina]

I think I would use NEMA17 stepper motors or maybe NEMA14 both easy to get and used for 3D printers, drivers likewise.  I think the main construction will be aluminium or possibly wood - 3D printed plastic might be too prone to creep though by far the easiest material to use.  Not sure about ball or plain bearings as yet.  I was going for ball bearings but not sure now.

After deciding how to make it I guess the next thing is accuracy requirements.  So here are some calculations... 

Longest FL lens I shall use with this mount will be 200mm and the camera resolution is 3.8microns per pixel.  Pixel resolution will be fine.  So angular resolution is given by tan(θ) = 3.8 / 200x1000 = 0.000019 giving θ as about 0.001°.  NEMA17 steppers have 1.8° per step but using 16x micro-stepping = 1.8 / 16 = 0.1125°.  I think we can call that 0.1° so to get a resolution of 0.001° we need a "gear" ratio of 100:1.  This will need a two stage reduction drive ratio.

I think MXL size timing belts and pulleys will easily handle this application and I have used these extensively for various projects in the past.  I have 1 100t MXL timing pulley with 5mm bore plus some others - I'll have to see what I've got.  The smallest pulley with a 5mm bore has 15 teeth so the first step down belt drive will be with 15t motor pulley and 100t on an intermediate shaft giving a ratio of 3:20 (1:6.67).  The final drive can have a 15t pulley on the intermediate shaft and a large pulley sized to give an overall ratio of 1:100.  This would have 100 x 3 / 20 x 15 = 225 teeth.  The pitch diameter works out at about 145mm which seems quite reasonable.

Since mount axes only have to rotate 180°, the belt could simply be attached by its ends to points on the pulley, saving the need for teeth and meaning the pulley could simply be turned from a 150mm diameter blank on the lathe.  I've done this size pulley before on my little Chinese heap of a lathe quit successfully.

With 16x micro-stepping is the most obtained by A4988 stepper drivers as used for 3D printers I did consider others that give up to 32x micro-stepping but I think 100:1 is reasonable for the 2 stage belt drive step-down ratio and 145mm is reasonable for the final drive pulley.

One further point is that with good PA there is no need for guiding with this scale of FOV and lens aperture so only sidereal tracking is required of the mount.  This still needs the above resolution and needs accurate calculation for tracking accuracy for the RA drive but the DEC drive would be used only for slewing and would be fixed during imaging runs so a lower resolution would be adequate - the limit being just framing rather than tracking/guiding.