Hello everyone,

I'm trying to figure out what is the best configuration for the old CI-700. Currently it uses 1.8 degree uni-polar steppers and a 8:1 gearbox. The planned upgrade is via belt&pulleys and will have a 1.8 degree bipolar stepper (Nema17 17HS4401N) rated at 1.7A. OnStep still will sport LV8729s so at most micro stepping is going to be 128x (now is 64x). If I use a 1:2.5 reduction ratio I get the following form the OnStep configuration spreadsheet:

AXIS1_STEPS_PER_DEGREE	32000.00000	200	128	2.5	180
AXIS2_STEPS_PER_DEGREE	32000.00000	200	128	2.5	180
	<= 61200		AXIS2_DRIVER_	A planetary	A worm/wheel
			MICROSTEPS	gear-box for	for example.
AXIS1_STEPS_PER_WORMROT	64000		128	example.
			Tracking resolution (in arc-sec)
			Calculated	Estimate
SLEW_RATE_BASE_DESIRED	1.50	°/s          Axis1	0.11	0.70	<= 1.25
		Axis2	0.11	0.70	<= 1.25

If I use a 1:2 reduction ratio the following:

AXIS1_STEPS_PER_DEGREE	25600.00000	200	128	2	180
AXIS2_STEPS_PER_DEGREE	25600.00000	200	128	2	180
	<= 61200		AXIS2_DRIVER_	A planetary	A worm/wheel
			MICROSTEPS	gear-box for	for example.
AXIS1_STEPS_PER_WORMROT	51200		128	example.
			Tracking resolution (in arc-sec)
			Calculated	Estimate
SLEW_RATE_BASE_DESIRED	1.50	°/s          Axis1	0.14	0.88	<= 1.25
		Axis2	0.14	0.88	<= 1.25
	↓		(servo, etc.)	(stepper drive)

Considering that the mount is going to be used for imaging (mostly deep sky and planetary when time will come) what is the best path to follow?

Thanks for your answer,

Andrea

On Thu, Nov 26, 2020 at 02:48 PM, andrea tasselli wrote:

I'm trying to figure out what is the best configuration for the old CI-700. Currently it uses 1.8 degree uni-polar steppers and a 8:1 gearbox.

Are you having issues with this setup?
What are they?

The planned upgrade is via belt&pulleys and will have a 1.8 degree bipolar stepper (Nema17 17HS4401N) rated at 1.7A. OnStep still will sport LV8729s so at most micro stepping is going to be 128x (now is 64x). If I use a 1:2.5 reduction ratio I get the following form the OnStep configuration spreadsheet:

If I use a 1:2 reduction ration the following:
Considering that the mount is going to be used for imaging (mostly deep sky and planetary when time will come) what is the best path to follow?

Either way, you should not be relying mainly on the microstepping in lieu of actual gear reduction.
Using the full 1/128 microsteps of the driver will not be as accurate as 1/32.
The spreadsheet tells you that in the "estimate" column.

From what I have seen others do with the G11/CI-700, it seems that it can have larger pulleys on the
worm wheel, you are better off using 4:1 or 5:1 (e.g. 60T/12T). With 1/32, it will be:

200, 32, 5, 180 = 16,000 steps per degree, 0.23" calculated, and 0.56" estimate.
You can then experiment with 1/64 and see if it has any benefits.

Note that these motors can be driven by the LV8729, but only up to a point. The current that you
should start with is 1.7A X 1.41 X 0.4 = 0.96A. But if you find out that you need more current,
check the LV8729 specs first, and be at 20% below the maximum.

On Thu, Nov 26, 2020 at 02:48 PM, andrea tasselli wrote:

I'm trying to figure out what is the best configuration for the old CI-700. Currently it uses 1.8 degree uni-polar steppers and a 8:1 gearbox.

Are you having issues with this setup?
What are they?

There is a lot a backlash. A LOT. PHD2 complains loudly about that. This is the main reason. Also a lot of noise from the spur gears, you can see it tracking the PE.

The planned upgrade is via belt&pulleys and will have a 1.8 degree bipolar stepper (Nema17 17HS4401N) rated at 1.7A. OnStep still will sport LV8729s so at most micro stepping is going to be 128x (now is 64x). If I use a 1:2.5 reduction ratio I get the following form the OnStep configuration spreadsheet:

If I use a 1:2 reduction ration the following:

Considering that the mount is going to be used for imaging (mostly deep sky and planetary when time will come) what is the best path to follow?

Either way, you should not be relying mainly on the microstepping in lieu of actual gear reduction. Using the full 1/128 microsteps of the driver will not be as accurate as 1/32.The spreadsheet tells you that in the "estimate" column.

How is that derived? What parameters affect it?

From what I have seen others do with the G11/CI-700, it seems that it can have larger pulleys on the 
worm wheel, you are better off using 4:1 or 5:1 (e.g. 60T/12T). With 1/32, it will be:

200, 32, 5, 180 = 16,000 steps per degree, 0.23" calculated, and 0.56" estimate. 
You can then experiment with 1/64 and see if it has any benefits. 

Note that these motors can be driven by the LV8729, but only up to a point. The current that you 
should start with is 1.7A X 1.41 X 0.4 = 0.96A. But if you find out that you need more current, 
check the LV8729 specs first, and be at 20% below the maximum.

The largest pulley you can put in on the worm is the 40-teeth one I am planning to use. And the smallest to fit a 5mm bore is 16 which gives you 2.5. So basically I'm stuck. A even reduction ratio would be nice for PEC but not essential. 

On Thu, Nov 26, 2020 at 03:38 PM, andrea tasselli wrote:

There is a lot a backlash. A LOT. PHD2 complains loudly about that. This is the main reason. Also a lot of noise from the spur gears, you can see it tracking the PE.

Did you try configuring OnStep to handle the backlash?
Example: I have backlash too.
It is 161" on RA and 33" on DEC.
I tuned it out manually using a terrestrial object, until I had no jump or lag when reversing directions.
PHD2 works fine.
You can do it from the Android app (3 dot menu, then Backlash)

Either way, you should not be relying mainly on the microstepping in lieu of actual gear reduction. Using the full 1/128 microsteps of the driver will not be as accurate as 1/32.The spreadsheet tells you that in the "estimate" column.

How is that derived? What parameters affect it?

I am no expert on stepper motors, but the spreadsheet has the formula.
Howard did that over a year ago.
He also conducted some tests, and wrote an article on it.
Maybe you can understand it better than I do ...

https://onstep.groups.io/g/main/wiki/16270

The largest pulley you can put in on the worm is the 40-teeth one I am planning to use. And the smallest to fit a 5mm bore is 16 which gives you 2.5. So basically I'm stuck. A even reduction ratio would be nice for PEC but not essential. 

Well, maybe you should try the above fix for backlash and see how it goes, before embarking on a major redo.

On Thu, Nov 26, 2020 at 02:38 PM, andrea tasselli wrote:

The largest pulley you can put in on the worm is the 40-teeth one I am planning to use. And the smallest to fit a 5mm bore is 16 which gives you 2.5. So basically I'm stuck. A even reduction ratio would be nice for PEC but not essential. 

After adjusting the backlash as Khalid suggests, if you still want to increase your reduction, here are 5mm bore 12T pulleys. Not sure how long for delivery, but I have bought from this store.

Also, if you can find 48T pulleys without flanges, they may fit in the space. Some pulleys apparently have pressed on flanges, so a few users have simply removed the flanges. Or you can chuck up the pulley and file or sand down the flange to see if it will fit. Just a thought.

-Mike

Thanks everyone for their suggestion.

I don't think it is wise or even feasible to try correct my backlash in DEC via software. I reckon is upward to 20'. Massive, as I said. I think that where there is a chance to fix mechanical issues by fixing the mechanics that's the first port of call.
I've ordered the 12T pulley from Aliexpress (apparently they are rarer than hen's teeth over here), but may have to test the system with 2.5:1 ratio if it is late in the delivery. Note, however, that the same system was used by Eric Coustal (https://onstep.groups.io/g/main/topic/28030295#5924), so it must be working.

Changing tack, here is another configuration I need advice for i.e. the upcoming conversion of the Gemini G-41, now sporting the ageing FS-2 controller. Here is what you might have, reusing the existing setup:



The stepper drivers will be TMC5160s on a BluePill as the mount is heavy and is rated for 45 kg payload. As an alternative I might use a MaxPCB (once I build it, that is).

Any advice?

Andrea

On Sun, Nov 29, 2020 at 01:25 PM, andrea tasselli wrote:

I don't think it is wise or even feasible to try correct my backlash in DEC via software. I reckon is upward to 20'. Massive, as I said.

Did you try it first?
OnStep has the ability to compensate for backlash at very fast speeds, so it can be used in combination with autoguiding.
The only limitation is that backlash is symmetric in both directions, or nearly so.
Then PHD2 will do the rest of the work.

I would say try it first, then go for the mechanical fixes later, since it is easy to test.

Here is an example of a large CGE mount using geared motors. More images.
The difference is that these are precision planetary, not spur gears, like yours.
But mine are also spur gears, 18:1 and still very much usable.

Here are examples of 5 minute guided images, with bad seeing. No processing
apart from color stretching: M42 and M33.

I think that where there is a chance to fix mechanical issues by fixing the mechanics that's the first port of call.

If you are so inclined then go for it.
Others, me included, subscribe to : perfect is the enemy of good enough ... so I would
test what is easy to test first, then reassess based on results whether to go for other
solutions or not.

Changing tack, here is another configuration I need advice for i.e. the upcoming conversion of the Gemini G-41, now sporting the ageing FS-2 controller. Here is what you might have, reusing the existing setup:

The stepper drivers will be TMC5160s on a BluePill as the mount is heavy and is rated for 45 kg payload. As an alternative I might use a MaxPCB (once I build it, that is).

The same comment that we had earlier: relying on microstepping at extreme microstep levels, rather than physical gear reduction.
This is a setup that would be better with 400 step motors (0.23" with 1/32 microsteps).
Again, the proof is in the pudding: if you already have the parts and can easily install them, then test it the 200 step motor with 1/128, and see how it performs, then decide ...

On my CGE, the original bearings felt "gritty" despite limited use.  I replaced them with quality ceramic bearings and that greatly improved the performance.