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[Julianus]

Hey,

I'm gonna post this here, maybe it'll be useful for someone.
a few years ago i made a version that still has a step-up power supply on 35V. This makes it possible to run my EQ6 with up to x1000 speed. As driver I use TCM 2208 which is pin compatible to the A4988 drivers at 1/16 and 1/2 steps. But it only works if you enable 1/16 and gearing to 1/2. By interpolating to 256 microsteps the mount now runs silently and the problem is solved that 1/16 steps do not run smooth enough for astrophotos because 1/16 steps are about 0.5" and therefore the stars are +-0.5" bigger. Also with Goto with x1000 the drivers interpolate the 256 steps internally, which makes the pans (at least with belt drive) almost silent. Gearstepping seems to be necessary for x1000, because I suspect that the CPU is not fast enough to generate fast enough steps at 1/16.

mfg Julian

[ecc2000]

Hello and thank you for the info!

How you wire these drivers compared to the 8825, do they have the same pins as the 8825,

Otherwise for the 1/16 and 1/2 I did not understand the handling!

Thank you

Pierre

[Julianus]

The wireing is almost the same. only real difference is, that the 2208 has only 2 pins MS1 and MS2 for Microstep selection.

The Mocrosteps with the TCM2208 are:
MS2  MS1  uStp
GND   GND   1/8   1/256   
GND   VIO   1/2   1/256   
VIO   GND   1/4   1/256   
VIO   VIO   1/16   1/256   

and with the DRV8825:
MODE0    MODE1    MODE2    Microstep Resolution
Low            Low            Low       Full step
High    Low            Low     Half step
Low            High    Low            1/4 step
High    High    Low            1/8 step
Low            Low            High    1/16 step
High    Low      High    1/32 step
Low            High    High    1/32 step
High    High    High    1/32 step

the Problem ist, that the Combination needed for the TMC2208 is not implemented.

but The combination for the A4988 wich has exactly the same pinout as the 8825 is:
MS1    MS2    MS3    Microstep Resolution
Low            Low            Low     Full step
High    Low            Low            Half step
Low            High    Low            Quarter step
High    High    Low            Eighth step
High    High    High    Sixteenth step

so because both the TMC 2208 and the A4988 use MS1 HIGH, MS2 LOW for 1/2 Step and MS1 HIGH, MS2 LOW for 1/16 Step its possible to choose A4988 as driver in the AstroEQ Controller and use uStepGearing because then for FAST Mode 1/16 and for SLOW Mode 1/2 is used.

mfg Julian

[ecc2000]

OK, thanks !

Note that the more micro steps, the less linear the displacement!

In France there was the Picastro at the time, we still find the files, and if Tom takes over the codes one day it can be useful!

Before mounting the motors on the equatorial mount it was necessary to put a laser pointer on the axes, on a sheet of paper fixed to the wall it was necessary to measure the differences between the micro steps, and adjust them on a small software to balance them!

The small software made it possible to run the motors a micro step per second or less time to point on paper!

The small software then generated a file that had to be saved in the Pic, and the microphones were not equidistant!

Attached the link for Tom, it will make him read, the Picastro was the must for equatorial mounts, halas we can no longer find the printed circuits!

https://www.google.com.tw/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwjqsby2kJDnAhU78uAKHVLuAHYQFjABegQIAhAB&url=http%3A%2F%2Fastrolabo.com%2Ftag%2Fpic-astro%2F&usg=AOvVaw3IncoFoeAYH4kHzadmQQUq

Pierre

[Julianus]

Interesting subject. So far I assumed that stepper motors have only one load-dependent angle error, which can be +-1 step static and +-1 step dynamic at maximum holding torque, independent of the microsteps. But after reading a little bit deeper into the matter I have to state that there are a lot of other effects with the position accuracy with microsteps, and it is not yet done with the compensation of the nonlinearity of the microsteps, because in addition to the load dependent backlash there is also a microstep dependent nonlinearity in the torque, which together with the load dependent backlash also leads to position errors. I'm not sure if it is possible to compensate all these errors by measuring the static displacement only. In a scientific paper on the subject they even come to the conclusion that compensating for such errors usually only leads to a deterioration due to the complexity of the error. In reality it seems to be the case that encoders are mainly used when accuracy is required.
The other question is however, where I am not sure how this affects the reality, that in a mount no such high absolute accuracy is needed. Rather, it is important that the speed of the motor remains constant within one cycle of the guiding camera, and here, the response of the motor with a sine wave through many microsteps seems to be advantageous in any case. But for this I would have to do some more research first.

In any case, thanks for the information and the food for thought,

mfg Julian

[ecc2000]

Hello,

In fact for a mount, instead of putting a motor pulley and a pulley for the worm, it would be better to put a double reduction with four pulleys and two belts and run the motor with less micro pitch!

Another solution is to moin a train of two gears and a train of two pulleys for reduction, and lower the pickups!

The main problem factor is equatorial mounts that are unbalanced or that force too much!

Trains with four or more gears are not bad too, but you have to adjust them well!

The back-up speed on the ST4 port is also important!

Pierre

[Julianus]

Yeah many it would also be possible to use a planet gear reduction as a drop in solution, but accuracy seams to the e pretty high, but the problem is, on the other hand you lose as much goto speed. That's why I didn't tried a 1:5 or 1:10 reduction so far. And also I don't know any commercial mounts using reduction rates that big. So it must be possible with microstepp. I try to lube my mount with low stiction greas next because I think reducing the load could be key. On the other hand I am not sure if it's even worth the hussle with my mount because I thing the main contributor to bad guiding is the +-15" pec 

Mfg Julian

[ecc2000]

Hello,

On two sets of pulleys you can make huge reductions, but also small reductions, you have to make an average so that the ratio is not excessive either!

I have seen for years that both in the world of HIFI audio and in astronomy many people nitpick and look for the little beast to the point that he changes equipment every six months because they are looking for a gral they impose themselves and they enter a circle of hellish galley!

The most important thing on an equatorial mount is mechanics, and this from the beginning of the instruments!

For example, when we see that an equatorial mount like the AZEQ5 has been mounted with an aluminum gear wheel and a steel worm, mechanically these metals are incompatible, normally we put a bronze or brass gear, and a stainless steel or hard steel worm gear, when the Chinese start to think about our quality may not be !!!

When the loads advertised on the frames make me laugh, when we announce 15 or 30kg it is better to put than half to be quiet!

To grease the frames, evening lithium grease, or tallow!

Regards,

Pierre