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Fullerscopes MKIV tracking or GoTo help?


Dave1
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Hi guys,

So I've bought an old Fullerscopes MKIV mount. For those that don't know, these mountings were made over 30-40 years ago, in the UK. And have a very high capacity. The quoted information is what the previous owner has told me.

" The mount comes with Beacon Hill 6 1/2" worm gears and worms. The information I have at the moment is that it has 12v DC geared motors. The RA drive is geared to run at Sidereal rate. Connect 12v and it should rotate at 1 rev per day. The motor is geared down a great deal and drives the RA worm through a 90° gearbox.

The declination drive is another 12 v geared motor that uses a timing belt to drive the worm. The worm and wheel are 360:1, 1 rotation of the worm moves the worm wheel by 1 tooth. The motors are reversible so some form of speed control will allow you to adjust the rotation to whatever you would like. "

 

For now I would be happy with a tracking mount, without GoTo if that is the cheaper option. What I need help with is figuring out some form of safe power source? And coming up with some way of controlling this set up?

 

David

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Edited by Dave1
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Simplest would probably be swapping the motors for steppers driven by an arduino? That way you’d not need to worry about servo loops with encoders and could run it off a power tool battery or power bank

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Nice one Dave, I have just bought one as well there is a thread in the DIY section. Mine came with a really old gearbox and motor set up, which was way past it's sell by date, so I scrapped it. What I'm going to do with mine is fit stepper motors and a kit from a manufacturer like Onstep, EqDrive, or EqStar. 

http://instein.eu/onstep.htm

http://astroproservice.com/index.php/product-category/telescope-control-systems/

https://astro-gadget.net/gadgets/category/control-of-telescopes/eq-control

Looking at your mount it would be very easy to do as you have all the brackets and pulleys there already. I've also noticed that your brass gears and worms have been removed and new ones fitted to a different position.

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I'm sure the SGL collective will have some good advice. I don't know a solution but I really like the way that it's been engineered so far (beauty being in the eye of the beholder!)

Hope you find a solution that retains all but the 12v motors. 

Keep us posted!

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Hi @markse68 thats a very good idea you have there. I think unless these motors have servos or encoders, it will put the tin hat on using them. The only way I can figure to use them is to make a form of control that would have adjustable voltage control to speed up or slow down the motors by varying the voltage the motors receive. I think those motors are rated between 4.5-15v. 

Thanks @Doc yes I've been watching your thread on your Fullerscope MKIV mount with keen interest. Thanks for the links and information, I will fully investigate. My MKIV is the standard version I believe, and not the deluxe. From what I can see Doc yours is the deluxe version with the brass worm gears and wheels, with the brass setting circle. The brass worm gears and wheels were an expensive upgrade.

Dave

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2 hours ago, Doc said:

Nice one Dave, I have just bought one as well there is a thread in the DIY section. Mine came with a really old gearbox and motor set up, which was way past it's sell by date, so I scrapped it. What I'm going to do with mine is fit stepper motors and a kit from a manufacturer like Onstep, EqDrive, or EqStar. 

http://instein.eu/onstep.htm

http://astroproservice.com/index.php/product-category/telescope-control-systems/

https://astro-gadget.net/gadgets/category/control-of-telescopes/eq-control

Looking at your mount it would be very easy to do as you have all the brackets and pulleys there already. I've also noticed that your brass gears and worms have been removed and new ones fitted to a different position.

Hi Doc,

I have looked at all of the links, there does seem to be some very good options there. What I will do, is when the mount arrives. I will count the teeth on the 6 1/2" Beacon Hill worm gears. Let the three different suppliers know that the stepper motor's and and gearing needs to be able to deal with forces of 1.8-3 Nm of torque, and be either 2:1 or 3:1 ratio. See what they all say, and who comes up with the cheapest option. I do like the look of the On step option the best so far.

David

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9 hours ago, Dave1 said:

Hi Doc,

I have looked at all of the links, there does seem to be some very good options there. What I will do, is when the mount arrives. I will count the teeth on the 6 1/2" Beacon Hill worm gears. Let the three different suppliers know that the stepper motor's and and gearing needs to be able to deal with forces of 1.8-3 Nm of torque, and be either 2:1 or 3:1 ratio. See what they all say, and who comes up with the cheapest option. I do like the look of the On step option the best so far.

David

Cheapest is not always the best Dave, Onstep is the most expensive if I remember correctly. Both Eqdrive  and Eqstar both make kits for large Eq mounts that can hold between 40-60 Kg which you will need I presume.

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Yeah I know On Step isn't the cheapest. But when I read what it can do ,  it's specification I was impressed. 

If money is no object, then there is the bolt on kit designed for fullerscopes MKIV mounts, by AWR. Full kit will cost £950. And of course AWR is in the UK.

David

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Hi guys,

So I've now come to the conclusion of what route I will go down with regards to stepper motor for my Fullerscope MKIV mount. Its not as easy as you'd think. Its a steep learning curve. And one has to be careful to match the stepper motors to the micro stepper control box. Use a stepper motor with too high current rating and some of the drives  in certain micro stepper control boxes wont support the current of the motor, and will get very hot and break. 

So you really have to look carefully at the off the shelf options with regards to the Micro Stepper control boxes. We will come back to this later. 

The obvious aspect you really need to decide on first, is what actual motors will you use? How does somebody go about figuring that out?

This can be tricky, its good to have a base line to work from. Fortunately we have two sources for a base line. Although those two sources don't exactly agree. Fullerscopes themselves say there synchronous motors were rated at 8 Lb/in. AWR state you need between 0.9 Nm 2 Nm depending on gearing.

So I've just mentioned gearing. If you have worm gears and worm drives. You need to find out how many teeth your worm gear has, either from the manual or well count them. With my Beacon Hill gears thats 360 teeth. 1 complete revolution of the worm drive will turn my worm gear by 1 tooth. 1 degree. 

I'm not sure if Fullerscopes use a form of planetary gearing between the motor and worm drive. So from now on we will only use AWR as a base line. AWR use 1.5 Nm for direct drive to the worm drive. This is our base line for figuring out our gearing, then in turn we can choose our stepper motor, which will then allow us to look at the hardware used in the micro stepper box.

So how do we figure out the torque requirements for further gear reductions. Commonly used gear reductions are 2:1, 3:1, 4:1. By using some math. So for example for 2:1 reduction. You take 1.5 divide it by 2 which equals 0.75. This means you'd need a stepper motor that produces 0.75 Nm. For 3:1  1.5/3=0.5.

One consideration that also needs to be taken into consideration when selecting the stepper motor and gearing. Is tracking and slew speed. ( the physical size of the telescope to be mounted should be taken into consideration from a safety perspective ). One helpful figure to know here is the overall reduction. This is worked out by taking the worm gear teeth and multiplying it by the gearing reduction. 360*3=1080. 

Figuring out slew speed isn't an exact figure its variable. As we need to use RPM and RPM changes. For our example lets take 300 rpm. For this to be useful we need to convert from rpm to revolutions per second. The formula is 300rpm divide by 60 equals 5rps. 

Now its useful to convert rps to degrees a second. This is done by taking the rps number 5 and multiplying it by 360 which equal 1800 degree a second at the stepper motor. So how does the gearing come into this? Well for a 1:1 drive 5rps=5deg/sec. For 3:1 gearing take 5 and divide it by 3 which equals =1.66 deg/sec.

So armed with the above information we can now start to look at selecting an stepper motor.

If you intended to use 1:1, then you need a motor that produces 1.5 Nm. That puts your firmly in NEMA23 territory. For 3:1 gearing you can use NEMA17.

But there is other considerations when considering stepper motors and GoTo systems. One is the accuracy of the motors themselves. There is two choices here, 1.8 deg/sec or 0.9 deg/sec, which translates as 200 step or 400 step stepper motors. This consideration also affects stepper motor choice, as it limits what choice you have. Another consideration that needs to be accounted for is loses of torque through the worm gears, as worm gears are not very efficient. So choosing a motor that just produces 0.5 Nm of torque isn't a good idea, choose a motor with more torque to account for the losses. 

Now its a good idea to, look at each potential stepper motors torque curve. Which can be downloaded from a good online retailer. Why is this important? Simply each motor produces different torque curves due to the different ampage and voltage of each individual motor. Within its class NEMA17 or NEMA23, no two NEMA17 motors necessarily have the same torque curve. What you are looking at on each torque graph, is the generic shape of the curve, and for a given rpm how much torque is produced. We know from certain calculations which we went through earlier that its a good idea to produce a decent amount of torque at 300rpm. I would also suggest having a good amount of torque available from 120rpm. 

The next important consideration is the ampage of the motor, this has to be matched with the correct drivers in the micro step control box.

Matching the stepper motor to the correct driver.

So now we need to talk about ampage of the motors and the drivers ability to handle ampage. Drivers are used to micro step each axis on the telescope. That's RA and DEC axis. So a typical driver used is TMC2130. The type of measurement we need to look at is amps RMS. The A RMS rating for the TMC2130 is 1.2A RMS. So that limits us to using a motor that uses no more than 1.2A. So now we have a hard limit for use with the TMC2130 driver. For my use because of torque requirement of my mount and gearing choice. That rules out most NEMA17 motor when combined with TMC2130 because of the amps required to drive an stepper motor with the required torque, unless I go for higher voltage, higher impedance. I have been advised though, that the lower the impedance the better. By now it probably should be obvious that a different driver with a higher RMS amps rating is needed. One answer is the TMC5160 driver, this has a A RMS rating of 3A RMS. This allow us to pick and choose from a wide range of NEMA17 and NEMA23 motors.

In conclusion

For a DIY GoTo mount you need to decide if it will be for visual or imaging. For a imaging mount its recommended to go for 400 step stepper motors ( 0.9 deg/sec ). This limits the choice of stepper motors even more so.

I have decided on TMC5160 drivers. This allows upto 3 A RMS.

I have decided on 3:1 reduction driven by belts.

The motor's I will use will probably be NEMA23 1.26 Nm 0.9deg/sec 2.8A 2.5V.  https://www.omc-stepperonline.com/nema-23-stepper-motor/nema-23-bipolar-0-9deg-1-26nm-178-4oz-in-2-8a-2-5v-57x57x56mm-4-wires.html

The torque is way more than I need, but all other criteria's are an excellent match. Amps match within TMC5160 drivers specification. The voltage is low for low impedance. The torque curve is perfect match for the operating parameters taking into consideration the rpm range. Having such a big amount of torque, more than makes up for any losses of torque through the worm gears.

I would not of been able to have come to this conclusion without @markse68 . Mark has the patience of a saint. Thanks Mark.

David

Edited by Dave1
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Well guys an update on this. My Fullerscopes MKIV should be with me on the 14th. I have been waiting for the mount to arrive. So that I can see all the existing hardware for the motors. So given my last update. I was thinking to make my own micro controller box. There is a man in the USA, that does kits. But his communication skills are lacking. So I have ruled that out. I then proceeded, to look at building it from scratch. Getting all the components together myself. Including having the PCB made as I had the Gerber plans for it. I could get the PCB made in America for $5. Could find nowhere in the UK that could get anywhere near that price. So that was the end of that.

I decided in the end to go with an off the shelf option. Having compared all the Options @Doc posted. I went with Astro Gadgets. Alexander who runs Astro Gadgets, is quick to answer to questions, has great customer service. He answered all my questions and didn't swerve any of them. The same could not be said of Instein, his customer service is patchy. I ruled out Astro Pro Service, as I could not find one of there off the shelf micro control boxes with a big enough Amps RMS rating. 

So today I put an order in for https://astro-gadget.net/gadgets/control-of-telescopes/eqpower

Best Regard

David

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Nice one Dave. I think it's the safest way to go. How long do they take to arrive from the Ukraine? Are you using those stepper motors we talked about or are you going with one with larger amps as this one can take up to 4 amps. I would imagine I'll get this kit as well, let me know how you get on.

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I don't know how long it will take to arrive from the Ukraine. I wasn't exactly sure where it was coming from. I did ask if he had the products in stock. He said he had everything in stock to build it. So I think he builds to order. He tests them before sending them. Which is reassuring that he tests them. Nice to get that type of quality control in this day and age.

I will probably use those stepper motors we talked about yes. I'm waiting on my mount to turn up to take measurements. I'm also going to phone Beacon Hill, as they sell motors, hopefully they sell the gears and pulleys. Reason for this I'm hoping for a matched set, to make assembly easier. Although I could get the gears wheels for the motor and worm wheel, built cheaply in China. But given the current situation I'm not sure that is a good idea.

David

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So guys, 

An update, the mount arrived today, see here https://stargazerslounge.com/topic/305552-what-did-the-postman-bring/?do=findComment&comment=3859130

I haven't had much of a chance to measure yet. But I have a rough idea how I will mount the stepper motors. There is also a slight difference from the description. Even though I was told the worm gear and drives were 360:1. They are in fact 287:1. Which means the resolution achieved with 3:1 reduction is slightly less. A way around that and regaining that resolution is to goto a 4:1 reduction. But I'm not sure if that might be a bit slow. Maybe I should by both.

Here is an idea of off the shelf pully systems from China, https://www.aliexpress.com/item/32855945814.html?spm=a2g0o.detail.1000023.42.6895323fPx97QP

David

 

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So guys,

I've done some more work on my Fullerscopes mount today. I've put the worm gears and worm drive on.

See the attached photo. In the photo what you can see is a worm gear, worm drive, combination of bearings, springs, and a screw device to vary the pressure asserted on the spring. Quite a clever bit of engineering, as it allows the worm gear to turn the DEC axis with out the worm gear being directly attached to the Dec shaft. So if for some unforeseen reason the telescope got caught on something, or big amount of force got put onto that axis. The worm gear would just spin around the Dec axis, which would prevent damaging the worm gear teeth, and probably the telescope and any other associated part.

There was a somewhat similar set up on the RA axis but minus the screw device to vary the pressure applied in the system, a minus a bearing. The reason for the screw device being omitted, is simple the RA axis isn't long enough to accommodate one. Which is a shame. Because of the different set up, it doesn't really work. So I've had to bolt the RA worm gear directly to the RA shaft. So no fail safe here. Which is a shame as the RA shaft will need far more energy and power to turn as its moving a lot more weight. The solution would be to get a longer shaft, and copy the set up on the DEC axis. I will probably do this one day.

 

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