I am working out what materials I should use, I might use aluminium. Do you have optimised dimensions for M6 x 1.0P threaded rod? Running @ 1RPM? That would be awesome if its not too much to ask!
No probs! Happy to help Russ.
The parameters I'm giving you assume that you are building the type 4 Trott, with the two boards starting parallel and pushing apart. It's not for my mod. My mod involves building something that starts at a negative angle, passes through parallel, then heads out the other way. Easy to mathematically model, easy to say not so easy to make.
You can also use the type 4 principle but make a double tangent arm drive instead of a hinged board type. When I designed the bisymmetric double arm , it was to build it as a high precision 4hr tangent for the RA axis on my old mount. It would end up looking like a complicated Astrotrac arm.
I never built it, I was getting more and more into eclipse chasing and instead my time went in to build a very lightweight EQ mount to carry in my luggage to solar eclipses around the world. Eventually I bought an iEQ45 mount for home observing.
Assuming you are going to build the easier Trott Type 4 with the boards starting parallel and pushing apart, I've optimized the design parameters for that. I have guessed because you asked for M6 and 1rpm that you have a fixed 1 rpm motor/gearbox- perhaps a synchronous motor?
If you saddle yourself with a fixed motor speed, the dimensions need to be very very precise or the errors will blow out really quickly.
An error of 0.025mm on the radius arm will push the error out to 8 arc sec over 2 hrs.
An error of 0.25mm will push the error out to 80 arc sec over 2 hrs.
A variable speed drive will let you fine tune to cancel out any minor construction errors.
If you have a variable speed stepper driver, then you can make fine adjustments to the motor speed to trim out any construction errors at the end.
The construction parameters and error curve are on the excel zip file. The diagrams from the original articles in S&T and show which parameters r, b & c are.
cheers
joe[/QUOTE]
Thanks heaps Joe, appreciate you time & efforts, love the astro community, everyone seems willing to help out each other!
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,481
Quote:
Originally Posted by Steve_C
What would be considered sufficient?
Also, in a two arm design, how do you stop the threaded rod drive gear from moving vertically?
Steve
Quote:
Originally Posted by Steve_C
Is there any advantage using a stepper motor vs a constant speed motor that is powered by a voltage regualted source?
Most constant speed motors, synchronous or dc, need to turn at > 1000 rpm, usually 2000-3000 rpm to have sufficient torque. They then need very high reduction gear boxes(read expensive) to get down to the sort of speeds required for these trackers or worm drives.
You can buy small stepper motors with integral 64:1 gearboxes (sufficient for a 200mm tracker radius arm) and PCB's with drive circuits for them for under ten dollars each on ebay. You can also buy power supply cards. If you are good with electronics then it'll be childs play. It's just a matter of putting them in a box and connecting the power to the drive and the drive to the motor through a connector. There are two types of stepper, bipolar and unipolar and the drive circuits for each are different, just make sure you get the right type of drive.
While Bojan is correct that discard printers etc do have motors, I have pulled a few apart found it a but hit and miss in the past with motors with strange characteristics or high voltages 24V. Also, many printer motors may not have gearboxes.
You also asked
Also, in a two arm design, how do you stop the threaded rod drive gear from moving vertically?
If I understand your Q, you are talking about the curved threaded rod type in Gary's article. The weight of the camera and top board holds the gear down. The rod is not attached to the lower board. There is a large hole there that the curved rod just passes through. It's actually a single arm tracker not a two arm.
Incidentally, we call the type with two boards a single arm and the type with three boards a double arm tracker. Naming is somewhat historical based on the original magazine articles written in the 1980's.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,481
Quote:
Originally Posted by Russman
Thanks heaps Joe, appreciate you time & efforts, love the astro community, everyone seems willing to help out each other!
No problem. If you want to change any construction parameters, let me know. Do PM me. I get busy at time and don't always check IIS.
It's very quick to re-run the optimization on any of the primary parameters - primary radius, motor speed etc and spit out the results. As explained in the Feb 88 S&T article, the ratio of the hinge separation to the slide contact length b/c is critical to the stability and works best kept at 2.186 so I keep that ratio fixed.
Also remember that no matter how accurate you make it, you will need to tune out the construction errors by trimming the motor speed at the end.
I contacted Profilium today about those hinge fittings. Thanks for the heads up on that. Is that the brand you use? how solid are the screw clamped joints you use? Do the boards bounce much? There a fair cantilever there.
Threaded rod is easily sourced from your local nut & bolt shop. and hinges from hardware.
The Ebay stuff looks right, Jaycar gears not so sure. The smaller doesn't appear to have any shaft and grub screw to mount. The larger one would need to be drilled out to take the rod. Littlebird seems to have the relevant gears, but the larger one from them would need a hub, which they have. Re the ebay controller, you would want to possibly replace the pot with a trim pot so you don't accidentally bump the setting.
I do have some RC servos I might be able to press into service, although I doubt they would have the torque.
I have already looked into other nuts and bolts source, one thing I found out is that a 5mm (M5) brass rod is 32TPI, so fits Gary's design well. I do know you can change dimensions to suit different tpi.
I qualified as an electronic tech in the 1960's, but have never played with this newer stuff, been out of the game for around 10 years.
I ventured out into the yard to give my single arm barndoor and new DIY mount a go. It was pretty easy to get it aligned, hardest part was adjusting the speed of my motor, eventually got it as close as I could to 1rpm and was happy with results so I started taking shots. Unfortunately some thin cloud started to roll in so I called it quits and came inside.
3rd photo is a shot of my DIY setup in action. 1st is a quick edit in Nebulosity of approx 12 images total exposure time 16mins with darks & flats subtracted, and a bit of processing in Neb and PS. I will need to get some more practice in on Nebulosity as I am pretty new to it, I like the program a lot, a little bit time consuming but with some patience and some good stock images I think it will produce amazing pictures.. 2nd shot is a 5min 26 sec exposure @ 50mm to show example of trailing.
Is there any advantage using a stepper motor vs a constant speed motor that is powered by a voltage regualted source?
I think there is.. especially if used with longer lenses, because the speed is controlled by crystal, which is more accurate.
Also, if you are using processor to drive the stepper motor (via appropriate drivers of course), you can pre-program the motor speed such that you go not need anything more that basic barndoor mechanic.
Also, have a look at Sounstepper (at sourceforge), this app can do this (it is running on PC)
You would need a stepper motor driver module as well wouldn't you?
I don't have the necessary tools at home to make an accurate device, however a new Men's Shed will be opening up near me in September, so hopefully I will have access to some decent machinery.
No problem. If you want to change any construction parameters, let me know. Do PM me. I get busy at time and don't always check IIS.
It's very quick to re-run the optimization on any of the primary parameters - primary radius, motor speed etc and spit out the results. As explained in the Feb 88 S&T article, the ratio of the hinge separation to the slide contact length b/c is critical to the stability and works best kept at 2.186 so I keep that ratio fixed.
Also remember that no matter how accurate you make it, you will need to tune out the construction errors by trimming the motor speed at the end.
I contacted Profilium today about those hinge fittings. Thanks for the heads up on that. Is that the brand you use? how solid are the screw clamped joints you use? Do the boards bounce much? There a fair cantilever there.
cheers
Joe
I think mine came from RCS Products, not too sure though as I these were given to me. Once clamp lever is tightened they lock up very well, I can not make it move when I apply force. The boards are pretty solid, I haven't used a telephoto lens yet, that'll show how solid it is. My cousin has a 300mm f2.8 I might try on it tonight if he lets me borrow it!
You would need a stepper motor driver module as well wouldn't you?
I don't have the necessary tools at home to make an accurate device, however a new Men's Shed will be opening up near me in September, so hopefully I will have access to some decent machinery.
Regards,
Steve
One of the links is a stepper motor controller, as far as I am aware that's all I need. I don't really know much about this stuff, but I am quite cluey so I'll nut it out..
One of the links is a stepper motor controller, as far as I am aware that's all I need. I don't really know much about this stuff, but I am quite cluey so I'll nut it out..
If you look further down the controller link page, it mentions you need another board. That is basically what I am referring to. I am new to this as well and also quite cluey. I can't see where on the one pcb you would connect the motor.
I think this would be better, a number of sellers have them.
If you are going to look for a board for Astro purposes, alwys consider Micro-stepping capability. The drive board in the link does not have micro-steeping in the description.
It may still work but is always a better option for both smoother operation plus can reduce the speed even further if required.
Quote:
Originally Posted by Steve_C
I think this would be better, a number of sellers have them.
Not sure if this is the type of info you need. You will need a board to generate the stepping pulse and forward/reverse logic, such as a microprocessor or 555 timer. These signals are sent to a driver board (such as an eady driver) which provides the correct voltage and energises the coils sequentially. The step and direction inputs are marked on the board - hook these up to the logic outputs.
A microprocessor can be used in two ways. 1. Send individual coil pulses to a darlington array, such as a ULN2008, which becomes the driver board (very small footprint) 2. Send a single stepping pulse and direction logic to a driver board, such as an easy driver.
Not sure if this is the type of info you need. You will need a board to generate the stepping pulse and forward/reverse logic, such as a microprocessor or 555 timer. These signals are sent to a driver board (such as an eady driver) which provides the correct voltage and energises the coils sequentially. The step and direction inputs are marked on the board - hook these up to the logic outputs.
Maybe I'm reading this controller spec wrong, but it seems to me it has one mode which generates the pulse and drives the motor. It does have a 4Mhz crystal on board, why would it have that if not to generate the pulse? It also seems to have a mode where you can use an external pulse. I found a more detailed spec sheet, http://www.kc-kwan.com/supports/menu...troller_01.jpg. Biggest issue is if it can generate the correct speed. Price is around $16, so not much out of pocket if it does not work.
I still might go with Gary's original design, using a gearmotor driven through an LM317 regulator. The regulator will compensate for minor variations in input voltage and should provide good output voltage regulation as long as input/output voltage ratio is high enough. There will of course be variations due to temperature. I'll have to see how that goes. I don't particularly want to fiddle with Arduino or microprocessors. Unless the pulse is generated by a crystal, I doubt if would be any more accurate than the regulated motor method, especially as the motor is geared down from a higher rpm. I might be missing some basics here, I haven't fiddled with electronics at this level for a long time.
Starting again - perhaps I pitched my comments a bit low. It sounds like you know what your about.
If speed control is the only consideration then microprocessors might just be personal preference.
Most, if not all tangent arm builders will use a DSLR or similar, and staggering shots by turning the drive off for a few seconds between images, a sort of dithering, is very beneficial with significant gains in image quality.
Using the microprocessor as an intervalometer can be a nice way of automating the whole thing, along with the short delays between images. A neat level of sophistication.
Not being an electronics expert a microprocessor was the simplest route. A ULN2008 makes a simple interface for unipolar motors and an L298 for bipolars, with some flyback protection if necessary.
Starting again - perhaps I pitched my comments a bit low. It sounds like you know what your about.
I'm TAFE qualified in electronics (1969) and have an Advanced class amateur radio licence. I have not played with electronics much in recent years, my career moved in a different direction.
If you look further down the controller link page, it mentions you need another board. That is basically what I am referring to. I am new to this as well and also quite cluey. I can't see where on the one pcb you would connect the motor.
I think this would be better, a number of sellers have them.
Hi Steve, Thanks for pointing that out! I will have to get that as well!
I think I am going to give up on the stepper motor design and stick with DC voltage controlled for now. Most research I have done seems to indicate it will work well. Also look at http://www.ronsongears.com.au/ for gears. Best I have found so far and reasonably priced.
For accurate polar alignment, I think I can use a compass and an inclinometer app to set elevation = to latitude. Saves fiddling trying to line up with a non existent point in the sky.
to drive this thing?
