Equatorial Platform - New Build

[Stub Mandrel]

TLDR: I found that arrangement very critical in term,s of positioning and worried that the PLA might 'creep' so I lost drive. Instead I've come up with an arrangement more tolerant of my woodwork skills!

 

 

I've had a rethink!

The problem with the angled sectors is that although they have a simple, circular cross section is it makes the positioning of the support/driving rollers critical. I found that this made getting a good drive to the sectors difficult, and a slight twist (just a couple of mm) in the board was enough to make a difference.

I'm also wary that the PLA could 'creep' under a sustained angular load despite the hefty design.

The elliptical design avoids this by having a vertical load but needs long rollers as the contact point moves in and out.

An ideal design MIGHT be to make the sectors part of the surface of the 52-degree cone so they track across the same point like my original angled sectors, but can use almost horizontal rollers angled in line with the pivot point.

Hard to do in wood without a 3D jig but easy in CAD. The front curve of this new runner follows the same curve as my MK 1, but instead of havinga steeply angle rolling surface it has an almost horizontal one. It also means the upper surface can be supported by the upper board, even though the fixings will be set back. Importantly, as the load will all be acting vertically that small twist in the board effectively becomes irrelevant as the position of the rollers is no longer critical. Instead of having to set them so the sector falls at the right place on the roller, the roller can be made over-length without any criticality in positioning. Finally, almost all the weight of the scope will be keeping the roller in contact with the sector instead of about 60%:

The downside is that it means the aluminium strips may not be feasible. I can get a decent surface finish without steps using careful orientation and support material (on a non-working surface). Any residual irregularities will, hopefully, be too small to cause problems and partly compensated by give in the rollers.

Edited June 8, 2020 by Stub Mandrel

[Chriske]

I might have come up with a completely 'new' EQ-platform, First need to do some more brainstorming...🤔

[markse68]

53 minutes ago, Chriske said:

I might have come up with a completely 'new' EQ-platform, First need to do some more brainstorming...🤔

I was thinking of making a shallow alt az platform with computer control to create a virtual polar axis. You could enter your latitude and use it anywhere in the world. At the pole it would just use the azimuth turntable and at the equator it’d use only the altitude rocker, side on, and in between a combination of the 2. I think it could work but a bit more complicated than a simple rocker platform...

Edited June 8, 2020 by markse68

[Chriske]

1 hour ago, Stub Mandrel said:

TLDR: I found that arrangement very critical in term,s of positioning and worried that the PLA might 'creep' so I lost drive. Instead I've come up with an arrangement more tolerant of my woodwork skills!

 

 

I've had a rethink!

The problem with the angled sectors is that although they have a simple, circular cross section is it makes the positioning of the support/driving rollers critical. I found that this made getting a good drive to the sectors difficult, and a slight twist (just a couple of mm) in the board was enough to make a difference.

I'm also wary that the PLA could 'creep' under a sustained angular load despite the hefty design.

The elliptical design avoids this by having a vertical load but needs long rollers as the contact point moves in and out.

An ideal design MIGHT be to make the sectors part of the surface of the 52-degree cone so they track across the same point like my original angled sectors, but can use almost horizontal rollers angled in line with the pivot point.

Hard to do in wood without a 3D jig but easy in CAD. The front curve of this new runner follows the same curve as my MK 1, but instead of havinga steeply angle rolling surface it has an almost horizontal one. It also means the upper surface can be supported by the upper board, even though the fixings will be set back. Importantly, as the load will all be acting vertically that small twist in the board effectively becomes irrelevant as the position of the rollers is no longer critical. Instead of having to set them so the sector falls at the right place on the roller, the roller can be made over-length without any criticality in positioning. Finally, almost all the weight of the scope will be keeping the roller in contact with the sector instead of about 60%:

The downside is that it means the aluminium strips may not be feasible. I can get a decent surface finish without steps using careful orientation and support material (on a non-working surface). Any residual irregularities will, hopefully, be too small to cause problems and partly compensated by give in the rollers.

Could explain this a bit more in an assembly drawing please, not sure I understand, sorry...

[Chriske]

8 minutes ago, markse68 said:

I was thinking of making a shallow alt az platform with computer control to create a virtual polar axis. You could enter your latitude and use it anywhere in the world. At the pole it would just use the azimuth turntable and at the equator it’d use only the altitude rocker, side on, and in between a combination of the 2. I think it could work but a bit more complicated than a simple rocker platform...

As a matter of fact, you've re-invented a fork mount...😉

[Gina]

I have an EQ fork mount for widefield imaging on the books together with a micro-observatory.  Both with many 3D printed parts.  That mount will have remote PA adjustment using stepper motors.

Edited June 8, 2020 by Gina

[Stub Mandrel]

2 hours ago, Chriske said:

Could explain this a bit more in an assembly drawing please, not sure I understand, sorry...

Photos will make it clear when I have some!

The sector is normally modelled on a section through an imaginary cone with its axis pointed at the pole and its lower edge horizontal. To create a bearing surface, the  sections are typically extended into short prisms of constant cross-section.

A section at 90-degrees to the cone axis is circular.

• Its contact point (where you would place a roller) is stationary as it rotates (convenient, allows rollers to be compact).
• Its bearing surface is angled to match the elevation of the pole star (awkward, and easy to lose position/drive).

A vertical section is elliptical.

• Its contact point moves towards or away from the pivot (cone apex) as it rotates (awkward, requires longer rollers).
• Its bearing surface is horizontal (convenient).

My idea is for the section at 90-degrees to be extended as a short truncated cone - a true slice of the imaginary cone.

• Its contact point is stationary (convenient).
• Its bearing surface is close to horizontal (horizontal at lowest point, a few degrees for practical sector designs) (convenient).

 

 

 

[Stub Mandrel]

Here we go, these are the new rollers. You can see they are a much simpler shape and how the 'new' sectors allow them to be horizontal. Instead of a precise location, they just need to be placed in contact with the supported and unloaded platform and slightly angled to get line contact and avoid any risk of the sector hitting the box.

I've decided to stick with the simple 5: ratio for now,as slightly smaller rollers will give me a step size of about 0.016m and I suspect that for a simple platform rather than a  precision engineered mount a resolution smaller than this is pointless.

At my sector diameter of 788mm, one arc-second is almost 0.002mm, so I will have 8 arc-seconds per step, 550 milliseconds per step.

Obviously these are best estimates to be fine tuned in use. I will be able tune to +/- 0.2% using the millisecond counter in Arduino.

The new parts fill the old upper platform but it doesn't take advantage of the extra surface on the new sectors:

I'm going to make a new platform, with a more rounded shape that will also allow me to move the pivot point further forward again. I will be getting close to the axis through CofG ideal by accident rather than design!

I've just got to wait for some ball races to replace the ones I lost for the tension idler.

 

[Stub Mandrel]

Well... the last few weeks have been a crash course in C++ which I always said I wouldn't bother with.

So where am I?

I have an all-new set of sectors and rollers but need to remake the upper platform.

I will need feet, two of which at least are adjustable for polar alignment.

I need to refine the control box and also print some bracket/boxes for the limit switches. I've actually used small (good quality) push buttons rather than microswitches as exact repeatability isn't needed. The main change to the control box will be adding a housing for a little piezo sounder.

Quite pleased with the control system so I'll summarise it:

Arduino Uno with a16x2/buttons shield I've had in a draw for a few years.

Stepper driven via a DRV8825 in 32 microstep mode. The ~12mm diameter roller rotates in about 37 minutes, but this will no doubt have to be tuned, software allows for this. Then limit switches and sounder.

Starts up and makes a pretty close approximation of a BBC Micro beep 😉

If eeprom is untainted it stores the default microsecond and millisecond delays for three rates (sidereal, solar, lunar).

Then loads these values into RAM.

Then loads up sidereal rate and starts tracking.

When in track mode it displays the rate and delay length to 0.1ms (default is about 68ms). Also up down arrows to show what to press to change the delay in 0.1ms steps.

A mode select button cycles through track and three other modes. Tracking continues unless overidden by another movement.

Next mode is to choose the rate, just cycles through the three and pause.

Move mode has up and down buttons to make small movements (will need fine tuning) to help with centring an object in RA or polar aligning.

Left and right buttons get the platform to move to the end of travel as determined by the limit switches. When 'homed' it starts tracking again, but f it reaches the other end it just stops unless manually moved off the limit switch.

Final mode is the clever bit, you can save the current rate into eeprom as a new default, or you can restore the default for the current rate from eeprom. A third button allows you to restore the 'factory' values if you have totally muddled them up. Unlike the other modes which are 'sticky' this one shows a  brief confirmation message then drops back into tracking.

Other refinements are a 'beep' on saving data or hitting a limit switch.

So quite excited really and looking forward to building it all up in time to catch Saturn and Jupiter as they approach opposition, and, of course, Mars later in the year.

 

[callisto]

This goes waaaaaaay over my head......that's why I purchased one instead 😀

[Stub Mandrel]

I've just done a test, using a leisure battery instead of the telescope. Not as heavy, but placed right at the front to put all the weight on the rollers and way below the CofG so much less balanced.

All went well, except near one extreme (probably beyond the range I actually need) it stalled but the motor current was limited at only about 200mA, I upped it to 400mA and it worked perfectly. The stepper is a 900mA one so I've got plenty of headroom if needed.

Left to sort:

• Adjustable feet (printing one now)
• limit switch arrangement.
• Properly wiring up

[Stub Mandrel]

And now done a test with the scope (passed) and made two adjustable two feet for the north end and paddle for the limit switches, now printing a first try at a switch housing.

I'm going to have a fixed foot at the south end (with a curved foot) so (to a first approximation) if the two feet are turned by the same amount it will adjust the polar alignment in AZ only, turning them the same amount in opposite directions will adjust it in azimuth only.

I feel child-like anticipation!

 

[Stub Mandrel]

D'oh!

Tilting the platform from side to side is pretty pointless... it needs to be slewed not tilted for azimuth adustment.

Never mind, a single adjustable foot at the pivot end would be tricky to implement anyway as the pivot is in the way.

Slewing will have to be physically moving the platform, which should be fine in practice.

 

[Stub Mandrel]

It's finished!

All I need now is a clear evening to test it.

Photos to follow.

[Gina]

Well done

[Chriske]

How about it Neil...

[Stub Mandrel]

I don't want to put it on wet grass... to photograph it.

Perhaps tomorrow evening?

[Stub Mandrel]

Here we go:

 

 

Edited July 5, 2020 by Stub Mandrel

[Chriske]

Nice...!!

[Chriske]

Were are you at °N btw...??

[Stub Mandrel]

52° 78', but I made the platform for 52 degrees as I am likely to be using it further south. It has a few degrees of adjustment built in.

[Stub Mandrel]

That unshielded bearing is annoying me...

[Chriske]

I'm @ 51.8' so that is a design I could use...😉
Do you share that file..?

[happy-kat]

What about sitting the whole thing on a lazy suzanne for slewing azimuth.

Great build you've finished it so fast.

[sploo]

Great job.