DIY motorized horseshoe mount - help please

[Osprey]

Here's a photo op with the scope resting on top of the cradle.  Still don't have the rollers right and need to purchase the 1" bearings so I can mount the scope proper.  getting close, can taste it 😛

[Andy_ZH]

Hello Osprey,

wow, nice pictures! This will not be a horseshoe mount. This will be a Dinosaur-Shoe mount! My misunderstanding before was that I thought the declination axis will be at the horseshoe level, but as I can see now it is not, and therefore the eye piece end (in your case "camera end") is more accessible.

Just some additional thoughts on the threaded bar: Possibly it is a good idea not to leave all the weight on the threaded bar, but have 2 roller wheels to take the main weight, and the bar / screw is pressed by a spring or something similar to generate pressure/tension onto the horseshoe. And you could try to initialize the mechanics with a second threaded bars: To the first one, you could grind a bit of roughness into the surface with (fine!) sand paper in order to create the first thread, and later on you could replace this by the smooth one.

As I can see, you are making quite some progress with your horseshoe. I should add a disclaimer to the offer of my raspberry pi stepper driver source code: My project is still at a quite early stage, and currently it contains just the code to drive the 2 motors and get an accurate step count via an interrupt kernel module.

I did quit my job some days ago in order to take care of my kids for a while (the little one is now 1.5 years). Though I will have more free time to spend with my hobby, my progress with the software might not be as fast and predictable as your hardware is advancing. I guess that I still need some 2-3 months to get basic goto features to work. So, in case you plan to have everything ready earlier (and more reliable / tested), Onstep should be your preferable software.

> The choice of whether to drive a stepper or a servo is purely about your ability to measure and manage the speed, in your case either will do fine, for geared or friction drives. There is no difference in 'drift' susceptability.

When I mentioned the possible slip with the friction drive, I had in mind as "servo" the closed loop between the DC motor and the horseshoe itself. And then in this case, you would need encoders on the horse shoe. With the thread or any drive that involves teeth, the encoder on the horseshoe would actually not be necessary, but the encoder can be on the motor shaft, or you could use a stepper instead. Of course, in case you compare a servo motor with an integrated encoder, they are more or less equivalent.

The scope that you are using and any discussion about is now a bit off-topic: I am still scared about your refractors strong chromatic aberration. Did you already try your scope in combination with a camera or visually at a star? My experience is that with an eyepiece, the human eye is quite forgiving and can adapt its focus, but when you use finally a camera, there could be some disappointment. And I think it would be a pity if you build a fine mount and your optics are quite limited at the end.

However: In case a H-alpha filter (or any narrow-band filter) is used, the problem should be gone immediately. Therefore, I am asking myself why people are even using APO's for the sun and H-alpha? A very simple single element lens should be perfect for any solar scope. Of course, there will be other aberrations (spherical, koma, astigmatism), as far as I know the biggest problem of refractors is their chromatic aberration.

best

Andy

 

Edited February 4, 2020 by Andy_ZH

[Osprey]

11 hours ago, Andy_ZH said:

This will not be a horseshoe mount. This will be a Dinosaur-Shoe mount!

@Andy_ZH LOL.  It weighs quite a bit already.  I tried to turn a roller on the lathe the other day and while I did get it flattened, it still doesn't work so I just grabbed a prototype idler pulley off my printer (41 hour job) and will clean it up and fit the bearings in once I get the magic elixir of life in my system (coffee for the lay person).

11 hours ago, Andy_ZH said:

Did you already try your scope in combination with a camera or visually at a star?

Yes, but ehhhhh...... Yes, I did test it on a star buuuuuuuuuttttttttt...the star was 93 million miles away. I have not taken the solar filter adapter out to test it on anything else.

[skybadger]

Hi 

Here is the drive I described. The motor sits on the gearbox towards the bottom and drives the central pulley through a pair of 4:1 reductions. The two idlers either side provide tensioning and the timing belt then  loops over the two idler pulleys supporting the bottom of the  horseshoe and stretches around the curve of the horseshoe between them. That way the drive pulley works on the toothed side and the horseshoe sees the smooth side. 

Regs

Mike

[Osprey]

@Andy_ZH Good morning from a balmy Michigan!!  Any chance I can get your code to load on my Pi Zero W?  I have a high torque NEMA 17 motor and spare drivers, working on the motor housing currently...revising the screw locations and such but am closing in on getting ready to test the RA side of the system.  Thanks and hope all is well on your end

 

Bill

[Osprey]

@fwm891 Getting back to the calculations....

I bought a NEMA 17 high torque motor with 11.9 mm diameter 20T pulleys.

According to the math you've shared with me, here is what I come up with:

86,164.0905 seconds per sidereal day

86,164.0905 / 360 deg = 239.344696 seconds per 1 deg.

239.344696 sec/deg divided by 60 sec = 3.989078 min/deg

The stepper motor is 400 steps per revolution @ .9 deg / rev.  I want to micro-step it down to 1/4 which should produce 1,600 steps per .9 deg thus it would take 1,777.777778 steps to make 1 degree (1600/.9)

The ratio of the horseshoe diameter to the stepper pulley is 102.45:1 (1219.2 mm diameter to 11.9 mm diameter).

This produces circumferences of 3,830.117 mm and 37.384953 mm respectively

I figured out that the amount of travel the horseshoe can move is 346.8 degrees (total of 13.2 degrees taken out for telescope cut) so in theory, I can achieve 3,689.7 mm of travel (3,830.117 mm * 96.3333%)

 

Pulley diameter is 11.9 mm

11.9 * pi = 37.384953 mm circumference

37.384953 mm * 3.989078 min/deg = 149.231493 mm of travel per deg

149.231493 mm * 360 degrees = 53,687.33762 mm total travel required?

 

At this point, I get lost in where I am going.  53.6k mm of travel is far greater than 3.8k mm.  Help from here would be appreciated

 

Bill

[fwm891]

7 hours ago, Osprey said:

@fwm891 Getting back to the calculations....

I bought a NEMA 17 high torque motor with 11.9 mm diameter 20T pulleys.

According to the math you've shared with me, here is what I come up with:

86,164.0905 seconds per sidereal day

86,164.0905 / 360 deg = 239.344696 seconds per 1 deg.

239.344696 sec/deg divided by 60 sec = 3.989078 min/deg

The stepper motor is 400 steps per revolution @ .9 deg / rev.  I want to micro-step it down to 1/4 ……….

 

Bill

Bill, I think this is where you're getting confused:

Your motor gives 0.9° per step. You want to micro step x4 steps = 0.225° per step. OK with that? your reducing the step size so the angular movement per micro step will also be reduced.

Divide 360° / 0.225° = 1600 steps per 360° rotation of the motor.

Your Horse shoe is 1219.2 diameter giving 3830.22976... circumference

Your pully is 11.9 diameter giving 37.38495... circumference 

Your motor needs to turn 102.453788... times per sidereal day

Total micro steps 102.453788 x 1600 = 163,926.061583.... in a sidereal day

There are approx. 86,164 seconds per sidereal day (ignoring decimals)

Therefore you motor needs to run at 1.9025 steps per second to drive your 1219.2mm horse shoe at sidereal rate.

I think you've got timing and distance measurements crossed. 

At approx. 2 steps per second that will appear jerky in an eyepiece so you may want to either increase the micro stepping rate, or add a reduction gear to increase the number of steps per second.

Old movies used 16 frames per second as a minimum rate as below that rate visual persistence showed flicker.  Hence why its common now to use 25, 30 and higher rates to smooth out motion.

I hope that helps

Francis

 

[Cosmic Geoff]

I'll just throw out some ideas here which may or may not be helpful: 🙂

Just because the mount is supported by rollers under the horseshoe, that does not mean you have to drive it via the rollers.  If you drive it via a metal gear and worm placed at the lower end, then the drive train can have the same sort of worm wheel and worm, motor drive train, electronics and software as in mounts such as the CPC800, EQ-6 etc.  The CPC series mounts rotate a fork on a turntable driven by a large worm wheel and worm. The same remarks apply to the declination drive.  In fact you might be able to recycle the whole system from a scrapped mount.  So no need to invent your drive train, electronics and software from scratch.

I have not seen mention above of the requirement to have high speed slew rates (several hundred times the sidereal rate) which are a standard feature of GoTo mounts.

You do not necessarily have to use stepper motors. IIRC some mounts use DC servo motors.  The SLT mounts have a simple encoder on the end of the motor/gearbox, obviously intended to count the motor revolutions.

[Osprey]

On 26/02/2020 at 00:11, fwm891 said:

I think you've got timing and distance measurements crossed. 

LOL...by the time I got done typing all of that...I think my eyes were crossed too!!!  Will rework the numbers on paper so I understand...if not,  I'll yell again

[Osprey]

@Cosmic Geoff What isn't shown is the new track system (multi piece c channel for lack of better description) I am printing to keep the belt from slipping off the aluminum band around the larger wheel.  Right now, I am trying to understand how to calculate and then code my own testing software before downloading the real deal stuff (either someones custom software or onstep).  I would like full operational capabilities of both my observatory and scope(s) from a web based browser.  This is my very first scope build so I am cutting my teeth in every way...I expect a lot of mistakes and generous feedback   Thanks for the ideas too!!  Worm gearing would be nice but I haven't figured out how I would make that happen at this point in the game...and would it be cost effective for a first build?

[Osprey]

On 26/02/2020 at 00:11, fwm891 said:

At approx. 2 steps per second that will appear jerky in an eyepiece so you may want to either increase the micro stepping rate

Should I try for 1/8 or 1/16 steps?  I think the 4988 boards I have will do 1/8...not sure of the 1/16, will have to research that one.

 

And so I understand right, you're advocating adding gearing to reduce this even further?

Edited February 27, 2020 by Osprey
clarification

[fwm891]

7 hours ago, Osprey said:

Should I try for 1/8 or 1/16 steps?  I think the 4988 boards I have will do 1/8...not sure of the 1/16, will have to research that one.

 

And so I understand right, you're advocating adding gearing to reduce this even further?

Only if you can't increase the stepper pulses by finer micro steps

[skybadger]

In driving a telescope you are looking for a high dynamic range, from fine guiding to degrees per second slew. 

Using steppers you have a top frequency y limited by resonance and a bottom one through diminishing returns in micro stepping. 

Using DC motors you have a different problem of a top speed limited by DC current and a bottom one managed by positional servoing. So you have to consider what ranges you want your motor to operate over and pick accordingly. 

Note that the large friction drive mounts have small DC motors which are geared down in the motor but offer a large dynamic range due to their high internal rpm . 

Consider that, if you want to use a stepper, in the eyepiece you may want 1/16 steps, 8 per second, resulting in a step rate of .5 per second, but each ustep is 2". Most people aim for fractions of " per ustep. Then you also have to drive stepper at >1KHz to get useful slew rates using full steps. So you have to pick your stepper carefully. 

 

[sploo]

15 hours ago, Osprey said:

Should I try for 1/8 or 1/16 steps?  I think the 4988 boards I have will do 1/8...not sure of the 1/16, will have to research that one.

 

And so I understand right, you're advocating adding gearing to reduce this even further?

Vlaiv gave me some useful info in this post; which may be helpful for calculating required gearing and microsteps: stargazerslounge.com/topic/348545-motor-for-ra-axis-on-modified-dobsonian-mount/?tab=comments#comment-3791469

[Andy_ZH]

On 22/02/2020 at 15:36, Osprey said:

@Andy_ZH Good morning from a balmy Michigan!!  Any chance I can get your code to load on my Pi Zero W?  I have a high torque NEMA 17 motor and spare drivers, working on the motor housing currently...revising the screw locations and such but am closing in on getting ready to test the RA side of the system.  Thanks and hope all is well on your end

 

Bill

Hello Bill,

 

sorry for the very late reply. The flu got me and the whole family. I have been out of business for quite some time, but now slowly recovering. Which drivers do you have? When I read about the 1/8 ... 1 / 16 micro steps, my first thought is: stop, don't do that! Use the new TMC drivers, they have a much better micro stepping resolution up to 256. Before anyone comments: Yes, I know that the 256 resolution does not result in the same gain of position resolution. But whenever you tried these (e.g. TMC2209), you will never go back to the Polulu or TI drivers. With the 2209, you will not be able to feel any stepping at low speeds, and you will hear no noise. They are just quiet. The new drivers cost about 10-20 USD per piece, but I strongly recommend to use these drivers.  Even more, they can handle higher currents (if you need the torque).

> Using steppers you have a top frequency y limited by resonance and a bottom one through diminishing returns in micro stepping.

The stepper drivers maximum speed depends (nearly proportionally) to the driving voltage. This is similar to any other motor, and depending on the motor characteristic (and voltage), you could expect 600-2000 rpm (max speed).

The code that I have (it is not yet very far progressed) is only meant for 2208 or 2209 drivers, since it communicates with the driver with UART, i.e. my PiZero Code tells the driver via the serial interface:
How fast to move (there is an oscillator on the driver),  the current settings, operating modes ("stealth chop" for low speeds). And in case of a nonsinusoidal motor design, you could even save a lookup table on the driver which corrects for nonlinearities.

So, I am not trying to convince you to use my code (you can have it, though). Just in case you are trying to continue with steppers (and not servos), please use the luxury variant of the driver, and not these shaking and vibrating 4988 drivers. The TMC drivers are pin compatible and give the same functionality as the legacy drivers.

 

Andreas

[skybadger]

On 28/01/2020 at 20:01, lenscap said:

Correct. Onstep is a stepper motor controller

No longer correct, onstep will support servos as a flavour of onstep X

On 28/01/2020 at 20:01, lenscap said:

I use a toothed belt, smooth side facing the horseshoe and toothed side facing the motor sprocket.

I put the smooth side around the large wheel facing inwards and have two jockey wheels that take the smooth belt , between them is the drive pulley, if you insert the drive pulley between the two jockey wheels, you are pushing on the toothed 'outside' of the belt.