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The current design provides a step resolution of a little over 0.5 arcs. US Digtal has other 1" optical encoders that offer higher resolution (note: the disk and the sensor need to be bought as a pair). I was wondering if there was any downside in replacing the current 256 encoder with say a 512 encoder. The logic being to have a smaller step size for improved guiding. The Gemini II controller seems to allow for an encoder up to 1024. I would imagine the speed setting would need to be changed, but I'm unsure on that.

Third edit (sorry jim et al,)
1/256/25/180 (teeth on G8 ring gear)=8.7E-7 rotations
*360 degrees *3600 =1.1 arc sec.? Hmm, that seems coarse.
Maybe I messed up the math again.? Don't feel like editing any more

yeah I have tick size as .05625 arc sec's per tick on G11 ...and G8...as it has a 2 prescalar (I think) for Ra gear

Titan 50 was at 0.02 arc sec's per tick which is the most accurate pointing wise.
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Brendan

Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.

This seems rather coarse. We have 2.3 million steps per rotation of the Ra shaft. From talking with Sideral Technologies high end mounts have a greater number of steps. If it is possible to change the encoder used in the motor this could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec per step. This I think should have a significant impact on improving guiding performance, especially in the Dec axis, as right now the minimum correction is 0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any thoughts. Perhaps Losmandy can comment on wether the controller will work with a higher resolution encoder. They cost around $65 per axis to do the upgrade, so well worth running a test I feel.

hmmm my cals might be NQR the Lvl 4 manual says .56 arc sec

but

The encoders act in quadrature giving 4 times the resolution =.145arc sec

Also remember the system has a prescaler divisor for systems division see lvl 4 manual pg 31.? Page 45 mentions step accuracy but doesn't mention quadrature.?

As to the encoders.?? I have often thought of just this but I think there may be an issue with "motor jitter" (especially on the old 3 pole motors) in that rotational movement "jitters" and higher resolution encoders may miss-count because of this jitter.?? I am not sure but this would be my bet.? I do know the new motors (5 pole) jitter less as do the maxon.

The other issue is I'm not sure if the IC devices are fast enough to handle the signal frequency if encoder resolution is increased.?? I suspect it could handle a step to 512K ...but it may induce more jitter.?? I'm sure the G2 DsPIC33s can handle greater resolution ..not so sure of the old G1 PEEL devices and PIC42Cs.

Lastly obtaining encoders with the correct shaft ID is imperative.?? I know Aliexpress sells many encoder wheels (obvious its like a US digital 1" wheel") I have found it hard to find 256 tick encoders but plenty of 512K tick units.? Consider at sidereal rate its (for a G11) 256 ticks in 9.56s...is 27 ticks a second and at 1200X slew that's 32,133 ticks per second the encoder Dspic33 has to read at.

512K in same conditions is 54 ticks per second...@1200X is 64,267 ticks per second the Dspic? has to read at.? Double.? IMHO it can do this...but really unsure of using higher... The PWM freq of the servo loop is around 32Khz which would match the 256K encoder almost 1:1 at max slew.?? I suspect greater resolution above this is not possible. I think this is the key...its PWM sampling drive at 32KHZ matches the 256K encoder at max slew....sounds like they did this for an engineering reason.?? Obviously you can lower max slew.

All in all...in reality I do not know but suspect it may be ok to 512K on the newer G2.?? Rene' may be best to answer this...indeed I'd like to know as well.? FWIW....

I guess the question is why change to higher resolution when seeing is at best 7-8 time (if not more) step resolution


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Brendan

Research of the threads finds...this from Rene' many years back...2001!? wow!

I do understand the feelings of the users who experienced the accuracy problems caused by the wrong cables, but I'm sure this will be transient.The motors are DC motors with encoders attached. Mechanical gear ratio is 360:1 (worm gear) times 25:1 (motor gears), resulting in 9000:1.Since a DC motor is no stepper, the resulting "step" size derives fromthe encoder attached to the motor (256 ticks), giving 2,304,000:1 (0.5625) arcsec resolution. This "step" size is fully maintained by the Gemini main CPU software, allowing positioning of the motor axes to givethis 0.5625 arcsec accuracy. Considering the play in the motor gears and the coupling, about 20 arcsec repeatable pointing should be the real world accuracy value possible.For smooth tracking, the servo processors internally work at 4 times thenominal resolution (.14 arcsec) resulting in very smooth, vibration-freetracking at very low current consumption.There is a huge difference between stepper motor control and servo motor control. Rene'

In short yes .56 arc sec step for G11 but in quadrature.....is at 1/4 this resolution = 0.14 arcsec per step

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Brendan

Sadly not true. If you step the motor it will be 0.56 arcs. There is no micro stepping with a servo motor. The whole purpose of the encoder to ensure the motor moves one step in a closed loop fashion. You cannot intruct the motor to move a quarter step as far as I'm aware.?

yes but he's saying the sampling accuracy of this .56 is at 1/4 electronically controlled..

You can instruct the motor to do lots of things in the settings.?? But no better than .56 without changing encoders....and while its interesting I've yet to be 100% sure any better tick accuracy would benefit.....and the question of how many ticks at fastest slew the system can compute.
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Brendan

No soldering required. You can buy a 512 or 1024 encoder with a 1/8th inch shaft plus the associated sensor for $65. The sensor is a plug in component. I spoke with a technician at US Digital and the current sensor will not works only with the 256 encoder wheel. The question I have is will the Gemini II support a higher resolution encoder? I do not thing it an issue with clock frequency but what the Genimi II software will allow you to do.?

I am experimenting with McLennan gear boxes from RS components in the U.K. Funny they cost the same as the much inferior gear box from Losmandy but are unfortunately metric. I failed miserably to enlarge the pinion gear hole from 2mm to the required 1/8" for the 25:1 gear box. I just bought a chucking reamer and a new pinion gear to have another go at it, but I might have to go to a machine shop to do the work. The 9.2s pinion gear error jumped from under 0.1 arcs to a whopping 0.6 arcs that is how sensitive things are. There error appeared close to a sine wave on the PHD2 graph. ?It seems much easier to change out the encoder that the gear ratio of the gearbox for the reasons you stated.?

@Brenden

I think you might be onto something with the slew rates. If you change the encoder to say 512 do all the speed rates have to be halved? While the gear settings are per axis this does not seem the case the speeds. This implies one would have to do both the RA and Dec motors. You may be right about not being able to increase encoder resolution. There has to be a reason why they did not use a higher res encoder in their design.

It's really the step control accuracy we are after here. I do think finer control is key to better guiding if you are shooting for sub arc second performance. I think I'll just have to buy two encoders and give it ago as this does not appear to have been done before?

During periods of good seeing, I can have very good guiding in DEC, due to the DEC axis not having to do anything.? (Kind of a back-handed compliment, LOL)? The RMS value is.. I dunno, less than 0.5 arc-sec?? But then I would occasionally see a jump in DEC from +0.5 to -0.5, or something like that.? I figured that it must be stiction in the axis, but maybe it was the discrete step sizes after all?? This is for a G8 axis)
At the same time, during these periods of good seeing, the RA guiding is always a bit worse than DEC.? Could it be the axis banging away at 15/0.56=27 times a second?? (G11 RA axis)? Nah, that seems too fast.

Just thinking out loud:

Will adding a more precise encoder on the motor really do anything for better guiding on these mounts? This will only better tell the computer brain where the rotational axis of the motor shaft is, but will that translate to a better known position of the RA (or Dec) axis? In between motor axis and sky coordinate axis you have so many meshing surfaces. On the G11, there is motor pinion gear, gear box gear #1 outer gear, gearbox gear number #1 inner gear, gearbox gear #2, spur gear #1, spur gear #2, left and right Oldham coupler mating surfaces, and finally worm to ring gear contact. The number of contacting surfaces here is at least 7 if I am counting right.

Each gear to gear mating is going to have its own slight backlash and periodic error, though worm and ring will have the largest contribution. Even if the motor turns more precisely will that motion be transmitted proportionally to the RA axis without be absorbed by the gear slop? If anything, there should be a high precision encoder directly on the RA axis itself. I think some older mounts tried these but they did not work that well. Perhaps the tech has changed to the point where a new encoder disc and sensor could be added there for better control.

Just re-watched this video on the new?Skywatcher EQ8Rh-Pro:?

Although this is a much more expensive mount at $7000 just for the mount head, perhaps some of this tech could be transferred to the Losmandy mounts in the future. Firstly get rid of the present gear system and add a high torque motor and belt drive directly to the RA (and Dec) axis. If I am understanding how these Skywatcher mounts operate, the tracking is ultra-precise without even using a guide camera; the computer just knows where the scope is pointing right from the encoders. Wouldn't that be nice for a Losmandy mount? But of course anything for a price.

You can buy AVAGO HEDS Broadcom encoders...and I think even though they are stamped different numbers It'll fit ok.

like 9100#F00 for 256tick (for Gemini motors) ........I find it hard to believe that you have to replace the "#F00"? optical pick-up when changing to a 512Ktick? encoder disc "9100#I00".? I mean as a manufacturer.....why would you limit or make them different its all about costs...I'd suspect the optical pick-ups are much the same they just change the code wheel

Pretty sure I suggested this some time back to someone who needed an encoder pick-up and could not get the "F00" so I said get the "I00"....

Would be interested if it works for you Aliexpress is a source of these Broadcom parts.....though 256 tick at 2mm shaft appears to have disappeared.? The Min shart is 3mm these days.

cheers
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Brendan

On Sat, Jun 12, 2021 at 11:17 PM, John Kmetz wrote:

Will adding a more precise encoder on the motor really do anything for better guiding on these mounts? This will only better tell the computer brain where the rotational axis of the motor shaft is, but will that translate to a better known position of the RA (or Dec) axis? In between motor axis and sky coordinate axis you have so many meshing surfaces. On the G11, there is motor pinion gear, gear box gear #1 outer gear, gearbox gear number #1 inner gear, gearbox gear #2, spur gear #1, spur gear #2, left and right Oldham coupler mating surfaces, and finally worm to ring gear contact. The number of contacting surfaces here is at least 7 if I am counting right.

Each gear to gear mating is going to have its own slight backlash and periodic error, though worm and ring will have the largest contribution. Even if the motor turns more precisely will that motion be transmitted proportionally to the RA axis without be absorbed by the gear slop??

Well, yes, it does matter.? (Like in my case)
Let's assume that you have a very very slight polar misalignment so that the DEC generally drifts in one direction.? And also that seeing is pretty good so the axis is not constantly correcting back and forth.? In that case, the DEC motor will give a pulse every x minutes in the same direction.? It will take up all the backlash slack and elasticity.? Eventually, the telescope will move the same amount as required for every step.

The Gemini L4 manual states:

Michael,
I have a G11 with Gemini 2 and a McLennan gearbox on the RA axis. I have programmed PEC using 3 methods: built in PEC training, PEMPRO and my own PEC programming software, and did not run into the issue you described. Do you have a link to another post where you discuss the issues with programming PEC on your G11T?

Eric

I agree with your analysis. The gearbox from the pinion gear through to final drive gear generates most of the high frequency noise with the exception of seeing that PHD2 tries to eliminate. When I miss drilled the pinion gear for the 1/8th motor shaft I had a visably noticeable wobble, that generated at least a 0.5 arc sec guide error ?with a period of 9.2s. It was a very distinct sine wave on the guide chart. You can imagine my guiding was pretty bad, but no worse than 1.3 rms arc sec or so. I gained a whole new appreciation for these higher frequency noise components through just that error on my part. Really, I'd like to get ride of the gear box. These high frequencies can not be corrected for.?With a step size resolution of 0.56 arc sec guiding is actually worsened I feel.