A question for Don


Dan H
 

I saw your reply to Czar explaining the difference between a servo I and a servo II. You stated that the servo II can read a 65 million tics/rev encoder. We are trying to add a Renishaw encoder to the DEC axis to a split ring drive. It obviously can't be a direct drive so I'm working on an offset drive using 3/64 inch cables and pulleys. Not ideal, but better then we have now. Do to space constants it's going to be difficult to drive the encoder at a 1:1 ratio. It looks like it will be closer to a 2:1. We know we will have to use tic management to get the correct tic count. My question is will this increase in ticks per axis revolution of the scope be a problem for the Servo II even though the encoder is only 67 million tics per rev.


Don W
 

Hi Dan,
I don't know.  The Servo II can read absolute encoders at 65 million tics/rev.  I doubt it can read twice that.
Tic Management won't work on Dec - it only works on RA while tracking.  TM has nothing to do with adjusting tic count.
Don W


Russell R
 

Dan & Don,

The limit for a Servo II is 8M to 10M ticks per second.  With absolute encoders, it's impossible to "miss" encoder tick. You read the encoder and that's where it is.  If it's ttl, and the encoder cables are adjacent to the motor cables you could miss ticks, also if the encoder cables are too long, so keep them short as possible.  You can use cascade mode and set backlash to zero.

Cheers,
Russell R.


Dan Hummel
 

Thanks Russell, one of the great thing about the Sidereal system is the vast knowledge of its users group and there willingness to share with those of us with issues.

I misspoke when I talked about using tick management to determine ticks per scope revolution. What I meant to say was using Ticks Determination in servo config. After doing some research into the BISS interface and finding that the data is sent in packets, I had no worries about transition rates. My concerns is if the registers in the controller are large enough to handle the 130 plus million tick value and will it confuse the software trying to handle such a large number. At the moment I'm having a cabling issue with the controller, but I'm going to try to enter the value into the controller and then read it back. That will answer the first question. That still leaves the software issue unsettled. It would be nice to be sure this will work before I invest the time and material to build this mount. Any help out there would be greatly appreciated.



Russell R
 

Hi Dan,

I always understood the absolute encoder was a 360 degree devise.  You would be the first one I know to increase the ratio of an absolute encoder.  I don't know how the software would understand the absolute position at that point.  Maybe someone has done this and can explain it to us.

Best,
Russell R.


Dan Hummel
 

Halfheimer's can be a bugger. Its a running joke with my partner that it's taking so long to finish this project that by the time we get done we will have forgotten how to use it. We installed a Renishaw on the RA axis long ago. As we remember we couldn't get servo config to accept the correct tick count. We thought we had used Tick Determination to get the values entered but that must have been faulty memory cells. I just tried to enter a new altitude value in the Servo Config Auto Tracking window and again couldn't get it to accept the values. For some reason it would accept up to 19,999,999 but not a tick more. I did discover that if I changed the value in a saved config file then load the controller with that file it would accept the new value. It even took the 134,217,728 count for twice the Renishaw resolution.

As for the Renishaw being an absolute encoder, there is a checkbox in the SiTechExe Mount Parms window to select Using Absolute Encoders. It was my impression that if this is unchecked we could use the higher ratio. Am I misunderstanding what that check box is for?

I'm going back to the drawing board and do my best to keep the ratio to 1: 1. It would be a shame not take advantage of two absolute encoders. Retro fitting this split ring has been a pain from the beginning. It always seems that I'm always trying to fit 5 pounds of crap in a 3 pound can.

Thanks again all for your help and patients.



Russell R
 

Hi Dan,

I was talking about how an absolute encoder works, unlike an incremental encoder which just counts ticks, absolute encoders always know it's position within 360°.  My concern is by changing the ratio of the encoder from the axis, how would it know it's position?  Maybe it would work, I just don't know.   I use a 64,800,000 tick Renishaw incremental encoder (413mm diameter) with dual read heads on the RA and built a circuit to use the index signal to initialize the mount in SiTech.  My motor ticks are almost the same.  I have always believed what good are lots of ticks on the axis if the motor ticks are not almost the same,  the accuracy would be no better than the motor ticks.  
65M ticks is plenty, with the cable there is some error and won't fully utilize the accuracy of that encoder.  Optimum is a direct axis mount or 1:1 in my opinion.

For high value inputs, you use the terminal window.

All commands must be in CAPS.

Open Servo Config and select the correct COM port.

Click Terminal Window

A window should pop up. Make sure you are communicating with the controller by pressing ENTER on the keyboard.  A string should appear in the format: X# Y# XZ# YZ# XC# YC# V# T# X[AM] T[AM]

Do not use commas or parentheses when typing ticks numbers

 

For Az/RA motor encoder ticks type: XXV(Ticks) and press ENTER 

Type XXV and press ENTER, you should see XXV(Ticks)

For Alt/Dec motor encoder ticks type: XXU(Ticks) and press ENTER

Type XXU and press ENTER, you should see XXU(Ticks)

For Az/RA mount encoder type: XXZ(Ticks) and press ENTER

Type XXZ and press ENTER, you should see XXZ(Ticks)

For Alt/Dec mount encoder type: XXT(Ticks) and press ENTER

Type XXT and press ENTER, you should see XXT(Ticks)

 

To save configuration, type XW and press ENTER.

Sometimes, SiTech don't like the input and you will have to ignore the warning.

I think it's great Dan, send some pics!!!

Clear Skies!
Russell R.


Don W
 

Hi Dan H,

Absolute encoders as Russell has described do not issue tics, instead they issue an address.  So SiTech either counts tics (from incremental encoders) OR it read the address (the angle from a zero point).  Sitech has to know what kind of encoder signal to "read", so the check box in the SiTech tells whether to count tics or read the BISS data.

If you use an absolute encoder at anything except 1.000000 to 1.000000, then the reading will be false.  For instance if you use 2:1 ratio, the encoder will report two identical angles per revolution.  I am sure that would confuse all your software.

Don W


Dan Hummel
 

Thanks Don and Russell for clarifying the absolute encoder issue. You save me a lot of grief, anguish and confusion. I'm a little embarrassed that I hadn't worked this out myself. If your a shoot from the hip engineer, you can end up shooting yourself in the foot. Looks like I'm committed to a 1:1 encoder drive. With the limitations of the axis shaft diameter, bolt head clearance and bolt head access I have little wiggle room, but I thank I can make this work.

This brings up a new question. We are using a Renishaw on the RA axis now. Without checking the Using Absolute Encoders (plural) check box are we actually taking advantage its resolution? I was under the impression that you needed absolute encoders on both axis to have this checked.

Russell, here are a few images you requested.

What started out as a drag and track scope is now a remotely operated goto. Being a split ring drive, the declination axis can't be directly driven. We use two 50:1 stacked harmonic drives to get the needed reduction. Two matched diameter drums, one mounted on the drive output and one one the dec axis, are linked with .010 inch by .5 inch stainless steel feeler stock tensioned with modified guitar tuners. The RA drive is a single 150:1 harmonic drive. The 2 inch drive wheels against the 36 inch split ring adds another 18:1 to reach the needed reduction. It had to be mounted outboard and back from the original drive to make clearance for the large harmonic drive. The original dec encoder was low resolution and was driven with plastic gears. I'm trying to mount a pulley and cable system to drive the Renishaw encoder in the gap between the dec axis clamp and the dec bearing housing where the original gear was mounted. These just are a few of the many modifications we've had to made over the years.



Jeff Ross
 

On 4/26/21 10:26 AM, Dan Hummel wrote:

Thanks Don and Russell for clarifying the absolute encoder issue. You save me a lot of grief, anguish and confusion. I'm a little embarrassed that I hadn't worked this out myself. If your a shoot from the hip engineer, you can end up shooting yourself in the foot. Looks like I'm committed to a 1:1 encoder drive. With the limitations of the axis shaft diameter, bolt head clearance and bolt head access I have little wiggle room, but I thank I can make this work.

This brings up a new question. We are using a Renishaw on the RA axis now. Without checking the Using Absolute Encoders (plural) check box are we actually taking advantage its resolution? I was under the impression that you needed absolute encoders on both axis to have this checked.

Russell, here are a few images you requested.

What started out as a drag and track scope is now a remotely operated goto. Being a split ring drive, the declination axis can't be directly driven. We use two 50:1 stacked harmonic drives to get the needed reduction. Two matched diameter drums, one mounted on the drive output and one one the dec axis, are linked with .010 inch by .5 inch stainless steel feeler stock tensioned with modified guitar tuners. The RA drive is a single 150:1 harmonic drive. The 2 inch drive wheels against the 36 inch split ring adds another 18:1 to reach the needed reduction. It had to be mounted outboard and back from the original drive to make clearance for the large harmonic drive. The original dec encoder was low resolution and was driven with plastic gears. I'm trying to mount a pulley and cable system to drive the Renishaw encoder in the gap between the dec axis clamp and the dec bearing housing where the original gear was mounted. These just are a few of the many modifications we've had to made over the years.


Hi Dan,

Wow that's a lot of upgrades to an NGT-18!  Thanks for the photos--other than replacing the drive system when the original died mine is pretty much stock yet.

Jeff Ross
Townsend, MT



Russell R
 

Hi Dan H,

From what I see, if you shoot from the hip, I definitely don't want to get in a gun battle with you! 

Regarding the absolute check box, I'm not sure, maybe Don knows.  If it was me, I would tick the box and see what happens. I live on the edge.
 
I keep looking at that Dec clamp....... I'm assuming it is to stop the Dec axis from freewheeling?   It would be good if you could put that Renishaw in that spot and somehow make a brake on that stacked harmonic drive.
Now look at who's shooting from the hip. ;-)

I always wondered if split rings have a problem with eccentricity on the ring, is that an issue?

Well done and thanks for sharing those photos!!

Russell R.


Dan Hummel
 

As for the DEC clamp, it's essential for dismantling this scope. To counterbalance all the gear we have strapped to the secondary cage we have over 70 pounds of steel bolted to the base of the mirror bucket. This scope didn't lend itself well to modifications. Once the secondary and trusses are removed, the tub is extremely bottom heavy. To lift the mirror assembly out of the bucket requires either a super human effort to lift it straight up or tilt the bucket as close to horizontal as possible. The feeler stock has no chance in supporting that much force. Without that clamp we would need a mobile crane.

And yes, how accurately can you bend a steel pipe. And will it stay bent that way. We depend heavenly on mount modeling and hope we don't run over a mosquito . To do it again we would salvage the optics, scrap the drive and start from scratch.



Dan Gray
 

The absolute encoder checkbox should not be used unless both encoders are absolute.  The only effect on the system is when you start SiTechExe, it reads the encoders and saved offsets and starts up initialized, no need to unpark or sync on a star or something.
Dan


On Mon, Apr 26, 2021 at 8:01 PM Russell R via groups.io <rem.64=yahoo.com@groups.io> wrote:
Hi Dan H,

From what I see, if you shoot from the hip, I definitely don't want to get in a gun battle with you! 

Regarding the absolute check box, I'm not sure, maybe Don knows.  If it was me, I would tick the box and see what happens. I live on the edge.
 
I keep looking at that Dec clamp....... I'm assuming it is to stop the Dec axis from freewheeling?   It would be good if you could put that Renishaw in that spot and somehow make a brake on that stacked harmonic drive.
Now look at who's shooting from the hip. ;-)

I always wondered if split rings have a problem with eccentricity on the ring, is that an issue?

Well done and thanks for sharing those photos!!

Russell R.


Dan Hummel
 

Thank Dan for your help. Glad we have someone with your knowledge of the system to give us a hand.