how much torque required?

[Demonperformer]

Right, first things first.

In terms of DIY knowledge, assume that I don't have any and you are probably still giving me more credit than I deserve. However, I can use google and so may use terms which may suggest more proficiency than I have. So please don't use long, complicated words (or jargon).

I have been re-reading an article in last April's AN (pp58-61) and am wondering what sort of torque figure (in Ncms) would a motor require to turn a collimation screw? I have been looking at stepper motors and have come to the conclusion that, if I am to give this a try, this figure is the first requirement. Once I have that, I can try to find something that would be (dimensionally) small enough to fit. If I can find something that meets both requirements then I can look at the next problem. If I can't, then I might as well stop before I go any further.

Thanks.

[Robert72]

Morning Demon,  this is an interesting question.  Something seemingly simple, but frustratingly difficult to measure.  I am sure someone on here has a good answer, and probably better than mine, but i'll give it a go.  The best way I can think of to actually get a rough value is to turn that screw with a low range torque wrench/screwdriver at a low setting of torque.  Make sure it torques out and then repeat with a slightly higher setting.  Repeat until it stops torqueing out and that's your value.  You may have to temporarily replace the screw with one that is capable of being driven by the wrench/screwdriver, in which case the different thread will give a false reading, however you should be in the ballpark.

My school physics teacher taught a good way of visualising force (and therefore torque).  Newton is associated with the falling apple, and the mass of an apple is about 100g (0.1Kg).  Force =mass x acceleration. F = 0.1 Kg x 9.81 m/s2 = approx 1N.  So the weight force of an apple in your hand is roughly 1N.  Torque is force x distance applied from pivot.  So 1Ncm would be 1 apple at 1cm from the pivot, although estimating this is, as you have eluded to, more difficult.

To answer your original question, I cannot help with actual numbers of Ncm, in your particular application.  Perhaps others may know ballpark figures to look for from previous projects.

Don't forget you can perhaps use a gearbox to increase the torque (and reduce the step size) if required or if space permits.

[Gina]

I think a NEMA17 stepper motor should be able to drive it directly.  I presume you can turn the collimation knobs by hand and the torque these stepper motors are capable of is greater than that, I think, from my experience (though there are various sizes/lengths of NEMA17 and varying torque capabilities).

[Demonperformer]

Thanks. I'll check them out and see if they will fit in the (limited) space.

[Dr_Ju_ju]

What size of OTA do want auto-collimation for, as I suppose it would require 6+ motors ?? (3 primary \ 3 secondary) For the 3 primary motors, maybe NEMA 14 (smaller) or 17's would probably do, but I'd recommend getting motors with planetary gearboxes to prevent slippage e.g. https://www.ebay.co.uk/itm/5-1-Planetary-Gearbox-Nema-17-Stepper-Motor-1-68A-4-Wires-3D-Printer-CNC-Robot/392161126938?epid=19025208142&hash=item5b4e9fea1a:g:hx0AAOSwQqJb1ppA

As for the secondary, something small e.g. https://www.ebay.co.uk/itm/10pcs-mini-Micro-Small-8x9-5mm-2-phase-4-wire-stepper-motor-with-copper-gear/282228460926?epid=506086420&hash=item41b6208d7e:g:P6YAAOSwKfVXIgwa&frcectupt=true ??

It would also depend on where\how you were thinking of mounting them & then connecting to the adjustment screws...

[Demonperformer]

The first one I am going to look at is the 8SE, so it would only require secondary motors. At the moment, it still has the original (phillips head) screws, but I have a set of bobs knobs that I can install. At the moment I don't know how I am going to get them to drive the bolts, which is another reason I was looking at what motors are available first. As the BKs have teeth around the top, I suppose they would be good, added to which their larger diameter would allow smaller corrections. It looks as if the motors you suggest have a toothed bit. The article does not explain how he got them to drive the bolts and he only used two, but looking at your suggestion, it would be easy enough to fit three in the space available and that, surely, would be better. And, although I hate buying things on ebay from China having had some disasters in the past, I reckon at £2.80 for ten motors I can hardly go wrong.

Thanks.

[RolandKol]

If you're going to test Nema17PG5,

I do recall they produce a small version... 

Will try to find later... Traveling now

[Gina]

Those tiny stepper motors are so tiny I wonder that they can do anything.  The can't produce much torque, surely!  I use 28BYJ-48 stepper motors with built-in gearbox for focussing and they don't give very much torque - they're a lot bigger than those very tiny motors.

[RolandKol]

This one is large PG5

and this is smaller PG5

I am not sure, but I would test a cheaper Nema 14 with 0.8A per coil...  Something like this one.... It should have enough torque, but not sure about the precision...

(but i you will use driver boards with options for 1/2 or 1/4 step, - you may have quite enough of precision)

And I am almost sure PG5 will be overkill in terms of precision... As I recall it has around 1000 steps per circle.

And the connection... Only the direct, -  using a coupler (something like this), comes to my mind...

With Pulley and belts you can get more torque of-course, but will have a larger backslash and more moving parts to look after...

[RolandKol]

P.S.

In most collimation manuals l actually recall statements, that it's enough to adjust 2 collimation bolts to center the primary...

Never tried it as l can always easily reach 3 bolts and naturally, l used them all...

[Demonperformer]

5 hours ago, Gina said:

Those tiny stepper motors are so tiny I wonder that they can do anything.  The can't produce much torque, surely!

The description does not give a value, so I have emailed the seller. I will report back if/when I get a response.

[Demonperformer]

I think the biggest problem with some recommendations would be space. The distance from the centre of the collimation screw to the outside of the secondary holding is about 11mm. So with a circular footprint, if it is going to drive the screw directly, it needs to be no bigger than 22mm diameter, square ones obviously need to be that much smaller. Or I have enough space to fit a 24mm diameter motor between the (potential) BKs.

Very rough diagram:

[Demonperformer]

6 hours ago, Gina said:

I use 28BYJ-48 stepper motors

I've had a look at these and was initially quite impressed. I like the price (£15 for 5). This data sheet quotes ">34.3mN.m" as the torque value, no idea if this would be enough. And I initially thought the off-centre driveshaft would make things a bit easier. But the main problem again is size. Excluding the 'wings' I could fit two of them in the space I have (and the original article only uses two), but I can see no way of fitting two with the wings attached.

My secondary is about 33mm radius and the screws are 36mm apart (so equidistant on a 21mm radius circle). The drive shaft is 3mm from one edge of a 28mm diameter circle, but the wings add an additional 7mm on either side in a line at right angles to the direction of the drive shaft. I suppose it would not be impossible to cut off the wings and find some other way of attaching them, but the phrase 'heath robinson' starts to spring to mind!

[Gina]

Guess you could hold them with clips.

[RolandKol]

Misread... 

it is for secondary.... not primary... Challenging...

In fact I do have one 28BYJ-48 motor left with no use, - can make a picture how it would look like on the secondary of my 130PDS, which is probably more or less the same size as yours.

Or even post or meet and give it to you, as I am sure I will not use it in any project.

But 28BYJ-48 are not powerful at all... they are usually sold with Arduino Starter Kits  to learn the basics....

I guess, Gina uses belt and Pulley to get more torque... 

P.S.

I would suggest to post for advise on some Robotics or Arduino Robotics Forums...

[Demonperformer]

Thanks for the offer, but it looks like I would not be able to fit them on the secondary. I am currently awaiting a response as to the torque of the mini motors, although I am guessing it will not be a lot.

I like the Gagarin quote ... pretty much my reaction to DIY really ...

[RolandKol]

Yep.. In similar micro motor description, purpose is described as "internal camera motor or for small devices"...
Only 5V in description and nothing about current consumption...

Just an idea,

not a very nice one, but maybe will help in somehow:

A bit larger motors, but a bit further away from the secondary, to reduce the central obstruction effect.

Not sure how you take Flats, - may be a bit challenging if you do not use any solid dew shield..

P.S.

yep, it sounds like Gagrarin was not a very well outspoken person  

I am almost sure, if he said something at that dangerous point, - it was even shorter.... and only because of it's "shortness", nothing was left on any transcriptions and records

 

[Demonperformer]

2 hours ago, RolandKol said:

a bit further away from the secondary, to reduce the central obstruction effect

I don't see how that would work. I can see that it would be so for a light source close by, but the light coming from astro-objects is effectively parallel, so a 100mm obstruction would effectively be blocking 25% of the light to a 200mm mirror at whatever distance, wouldn't it?

[RolandKol]

I am not sure about it....
With mirrors it works somehow a bit different...

Mathematically, you should see the shadow of the secondary anyway, but you don't... it just dims illumination of the primary as light from the stars fall into the scope at the different angles

and the further the obstruction from the primary is, the more shadow from it falls out of the primary area, isn't it?

In other words, the shadow becomes more transparent/spread out.

I am not really cleaver in this field...

I may speak a complete nonsense here...

[Demonperformer]

That is not my understanding.

The light from a single star spreads out as it leaves the star. By the time it reaches the front end of my telescope, an 8" circle of that light is effectively parallel rays, certainly for the next two feet that comprise the tube to the primary mirror. So light from that single star forms an 8" cylinder of light, which is then focussed by means of the mirrors. It is all coming in at the same angle, otherwise it would not be possible to focus it properly (and hence why telescope tubes are baffled).

Now, the secondary blocks some of that cylinder of light, but because there is light from the star entering the whole of the tube, it merely reduces the amount of light reaching the primary, hence making the image dimmer. How much dimmer is proportional to the ratio of the areas of the primary mirror (effectively the total area of the cylinder of light) and the secondary (the area that is blocked). Because all the rays are parallel, however far that blockage is, it is blocking the same percentage of the primary from the perspective of the star.

Now we don't often move secondaries enormous distances, but to give a practical example of a similar case.

I have a small diameter solar filter, made with the film covering a 40mm diameter hole in a cardboard cap that sits over the end of my 4" refractor. I can put this directly over the end of the refractor and get a 40mm diameter "cylinder of light" from the sun. I also have a long dew shield for this scope and can put this between the solar filter cap and the scope. The blockage (which in this case is blocking the outside ring) is still the same size but a lot further away from the primary (in this case) lens, but I am still getting the same 40mm diameter cylinder of light entering the scope.

[NEM]

On 02/04/2019 at 11:44, RolandKol said:

P.S.

In most collimation manuals l actually recall statements, that it's enough to adjust 2 collimation bolts to center the primary...

Never tried it as l can always easily reach 3 bolts and naturally, l used them all...

If you use all three bolts for primary collimation, there's a danger that you'll repeatedly turn them all the same direction and eventually 'walk' the mirror off the bolts.  I'm not sure if that applies to the secondary as well.

Noel

[Demonperformer]

On 02/04/2019 at 15:24, Demonperformer said:

The description does not give a value, so I have emailed the seller. I will report back if/when I get a response.

Well it is now nearly a week since I emailed the seller to get info on these motors and there has been a deafening silence. Regrettably, I can't say that I am particularly surprised, given my previous experience of ebay-sellars in China.