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Author Topic: RA tracking speed  (Read 1592 times)

darowuk

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RA tracking speed
« on: September 02, 2018, 12:05:40 »

Hi,

Hopefully someone can help with this please. I'm having some accuracy issues with my DIY Astroeq which is set up on an SW EQ-5 mount with custom motors (NEMA14 1.8 step angle) and the pulleys as described on the FAQ section. The problem I seem to have is that the sidereal tracking doesn't seem to be correct. After three point alignment I seem to not be able to track reliably despite polar alignment, setting home position etc. The goto accuracy also then gets worse with time. I have tried switching motors and have also changed the microstepping value to try and help as it was a bit jumpy.

I think it may be moving too fast, it takes just under 14 seconds for the RA stepper to rotate once when at sidereal speed, is this correct please? When I slew with the DEC axis with the same rate the motor turns appears to be slower. I have used the custom mount settings in the config software so the settings should be okay.

Any help would be great.

Thanks in advance

Dan
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darowuk

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Re: RA tracking speed
« Reply #1 on: September 02, 2018, 12:54:22 »

Hi,

Something else to add on the problem, moving the mount in 800x sidereal speed for the RA seems to match the RA scale on my mount well. Slewing 1 hour in Eqmod moves the scale on the mount by one hour. Do it at slower speeds and it is way too fast. Not sure why the faster slew would be more accurate but might help with diagnosis of the problem.

Thanks again.

Dan
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CCphoto

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Re: RA tracking speed
« Reply #2 on: September 03, 2018, 19:28:52 »

Hi, I to have an eq5 mount with an astroeq controller. I was wondering what configuration settings to use.
If anyone is using an astroeq controller on a an eq5 mount with the custom pulleys as per defined bt Tom Carpenter i would be grateful. I am not convinced that mine points to the right object selected in Carte du Ciel.
I am testing in daylight trying to point to sun bit it appears to be way off the mark.
I am using eqascom to drive the controller via carte du ceil.
I am particularly interested what micro step settings to use.
Also I cannot get my head round which way the mount revolves when jogging east west, North south with eqascom. If it could be described as revolving clockwise or anticlockwise when turning in ra and Dec when viewed from counter balance arm when the scope is parked pointing to nip that would be great.
Thanks
Charles Clark
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jslight

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Re: RA tracking speed
« Reply #3 on: September 04, 2018, 19:22:27 »

I think it may be moving too fast, it takes just under 14 seconds for the RA stepper to rotate once when at sidereal speed, is this correct please?

The EQ5 mount has a 144:1 worm gear ratio for both axes.  360 / 144 = 2.5, the angle of mount movement with one revolution of the shaft.  If you are using the 40 and 15 tooth pulleys from the FAQ (with the 40 tooth pulley on the mount shaft), then 2.5 / 2.6666667 = ~0.9375, the angle of mount movement with one revolution of the stepper.

Sidereal rate ≈ 1 / 4min.  So it should take about 3 minutes and 45 seconds for a full stepper revolution at sidereal rate.

The default values of the EQ5-Custom-Pulley.conf are correct for this configuration, make sure you also have the correct Motor Driver IC selected (defaults to DRV882x).

As far as microstepping, it shouldn't make much difference regarding sidereal tracking, it's more about how fine the resolution of nudges can be while guiding.  Ideally you want to use the highest number your Motor Driver IC supports and enable Gear Changing (these are the defaults).

What are the Voltage and Phase Current specifications for your steppers?
Are you using the AstroEQ with Windows/ASCOM or Linux/INDI?
Which version of the AstroEQ firmware are you using?
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jslight

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Re: RA tracking speed
« Reply #4 on: September 04, 2018, 22:57:31 »

Actually, 14 seconds would be 16x too fast.  It sounds like the Stepper Driver ICs are not switching to the correct mode (1/2 instead of 1/32, or full instead of 1/16).  Make sure you have the correct Stepper Driver IC selected in the Config Utility; also verify you soldered the two jumpers on the PCB for the correct model of Stepper Driver IC and the connections from the microprocessor to the mode pins are good.
« Last Edit: September 05, 2018, 00:12:34 by jslight »
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jslight

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Re: RA tracking speed
« Reply #5 on: September 04, 2018, 23:19:41 »

Also I cannot get my head round which way the mount revolves when jogging east west, North south with eqascom. If it could be described as revolving clockwise or anticlockwise when turning in ra and Dec when viewed from counter balance arm when the scope is parked pointing to nip that would be great.
Moving West should always travel the same direction as sidereal tracking.
If it is traveling the wrong direction, reverse the RA axis using the AstroEQ Config Utility.

North and South can be confusing because the direction to rotate depends on which side of the pier the telescope is on.
The best option here is to use the EQASCOM Reverse DEC checkbox when flipping pier sides so that North actually travels North.
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darowuk

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Re: RA tracking speed
« Reply #6 on: September 05, 2018, 07:52:15 »

Thank you for the info jslight. The calculations are useful information the fact that it appears to be a sensible factor out makes sense.

 I did solder the jumpers.

 I think it is my driver on the RA itself as when I swap them on the board the motor speeds also swap.

 I'm using the latest firmware on Windows and the drivers are set correctly in config.

The steppers themselves are 0.8A rated with suggested voltage of 12V. I do find it difficult to set the current limit so the motors can turn with out getting too hot. 0.4V on the vref doesn't always seem to be able to turn it. I could get higher amp rated steppers.

I will update when I have tested with a new driver.

Thanks

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jslight

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Re: RA tracking speed
« Reply #7 on: September 05, 2018, 16:59:07 »

I think it is my driver on the RA itself as when I swap them on the board the motor speeds also swap.
Yep, that's definitely the problem then.

The steppers themselves are 0.8A rated with suggested voltage of 12V. I do find it difficult to set the current limit so the motors can turn with out getting too hot. 0.4V on the vref doesn't always seem to be able to turn it. I could get higher amp rated steppers.
An unmodified AstroEQ has a maximum limit of 1.2A steppers, 0.8A should work well (my steppers are 0.8A).  It might be the 12V rating, that is, if you are powering the AstroEQ with a 12V PSU, then the Stepper Drivers can't perform their current limiting because the stepper needs the full PSU voltage to even draw that current to begin with.  You could test this with a slightly higher input voltage (15V maximum according to this post by Tom https://www.astroeq.co.uk/forum/index.php/topic,259.msg1672.html#msg1672).


P.S.  Additionally, too low of a stepper voltage rating (less than ~50% of input voltage) is known to sometimes cause stuttering/ticking problems with DRV8825 drivers (see https://www.astroeq.co.uk/forum/index.php/topic,80.0.html).  This effect can be diminished by lowering the input voltage (9V minimum), changing the decay mode, or adding the diodes in series with each stepper phase.
« Last Edit: September 05, 2018, 17:38:59 by jslight »
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darowuk

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Re: RA tracking speed
« Reply #8 on: September 20, 2018, 12:09:15 »

Just to update, I replaced the stepper drivers with new ones, actually used A4988s this time and all is good. Just had to set the RA and DEC directions to the opposite to the setting with the previous DRV8825 drivers. Accuracy and sidereal tracking much improved, able to slew to objects after three point alignment and they would be in the field of view, although I think the home position might be slightly off, I have read that the EQ5 setting circles can be a bit of a pain when trying to set this accurately. 
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