Is this what you see when using the Balance function?

[w0mbat]

Hi all,

I am trying to improve my all position balance on my DDM60 Pro. I have been having problems getting sensible results and it has occurred to me that what I am seeing may not be normal. Before I ask ASA for help I would like forum members advice as to how what I see compares with your experience.

As I understand it the red line is an average of the current drawn by the motor during the balance movements. My question is how much variation do you see in the peak of the yellow bar as the average is calculated? On my system the top of the yellow bar moves up and down around the red line average by around several hundred milliamps when the average is about 1 Amp. In other words, the variation in the peak current would be approximately 20 to 30% of the average current. It suddenly occurred to me that in trying to achieve good balance I am actually trying to achieve the same current draw in all positions and that becomes rather pointless if the current drawn is varying so much as the axis rotates! The pattern of variation I see is repeated in exactly the same way during each cycle of the balance movements.

Soon after I received the mount from my ASA dealer I asked ASA why I could feel an obvious variation in resistance to movement when turning the Dec axis by hand. I was advised that this was most likely due to what they called "magnetic remanance" and not to be concerned. I am now concerned that the resistance variation I was feeling back then is actually a problem that explains what I am seeing now during balance.

How does this compare with your experience?

The prospect of having to return the mount to Austria from Australia is not a nice feeling!

Thanks in advance and regards to all,

Ian

 

[astrobry]

Soon after I received the mount from my ASA dealer I asked ASA why I could feel an obvious variation in resistance to movement when turning the Dec axis by hand. I was advised that this was most likely due to what they called "magnetic remanance" and not to be concerned. I am now concerned that the resistance variation I was feeling back then is actually a problem that explains what I am seeing now during balance.

How does this compare with your experience?

 

Hi Ian,

 

What you write lookas familiar to me...  I had the same experience when i turn the DEC-axis by hand, the resistnace was not equal and somtimes it blocked when i move the axis by hand left/right, several times...  When the telescope was on the mount, I had no problems with it. What the current through the motor was, I have no experience with it, I've not analyzed. Later, when observing, I noticed that the results were not good sometimes. tracking was not alway good , about 70% of the images were OK. Sometimes it was so sudden that the mount left his pointing, just went away and then came back to his point. This sometimes several times during a single shot (long exposure) This was not normal!! In the log file I saw that the current was up to about 5Amps and then left his position to go here over and over again, and again to go to hios intitial position. Apparently this is so in the software programmed, i don't know, but it seems to? The power(current) fall back to the acceptable value of about 1.3 amps after this short sweep. After that the current was going up to 5 AMP. Having contacting support of ASA and reported the problem, they have asked me to send for review. Mount came back after service and upgrade,the problem was that the bearing was replaced and DEC-axis problem was resolved. This is now about 6 months ago and mount still works perfectly. Whether you have the same problem, I can hardly say, I suggest you contact ASA-support. I understand that it's not fun when the mount must return to ASA.I must wait about 2 months.... I also had problems with it, I was not happy about it, but have conceded, and afterwards you're happy with it. She is fully serviced. I read that you have a new mount? Then this is and strange phenomenon, my mount is now nearly 4 years old. I suggest to bring this back to the dealer and ask another one.

 

good luck !

Erik

[MarkS]

Hi Ian,

 

In my experience, it is the settling position of the red line that counts. The yellow bar can move around quite a bit, but if I let the system run for several cycles it settles somewhat. (If you're not on a very stable base,though, it can kick around a lot.) Frankly, I've never worried about it.

 

For me the critical issue for balance is whether or not the motor currents stay below about 1.4A (DDM60) when tracking over a long period. If they don't, a Position Error is on the cards.

 

Because my set-up is mobile, and I must therefore rebalance every session, I only attempt to get balance good enough to stay within that current guide of   I have never found tracking performance to be compromised by this approach.

 

I monitor the current with a digital ammeter in the power supply lead to the mount, and, whilst it may not be essential, I recommend it for keeping track of the current draw, rather than looking at the log after a disaster has occurred!

 

Regards,

Mark

[w0mbat]

Thanks Erik and Mark,

I have to say I am really confused by this balance issue. Some previous posts in this forum suggest that an imbalance equivalent to the weight of a coin (an Australian 50 cent piece was mentioned) is a problem. But Mark, your post suggests to me that it is not so critical. If the current varies as much as I see from watching the balance function doing its job then that suggests to me also that it is not so critical. Also I recall Philip Keller (I think) saying somewhere that the mount can easily handle a 1 Amp variation. My mount is permanently mounted on a 270 mm diameter steel pipe pier which is filled with concrete and buried in roughly 1 cubic metre of concrete in a Sirius observatory so I don't think there are any issues there. Also claims are made that a mount out in the wind can deal with wind gusts. Surely the varying loads induced by wind gusts will be more of an issue than a small constant load caused by a small imbalance. So I simply don't know what to believe!

What I would really like to know is whether others see significantly varying current when running the Autoslew balance function. If this is normal then what is the point of precise balance in all positions?????

My concern is that the constant current variation as the balance routine runs is that it is a sign that there is a problem with the bearings in the Dec axis. As I mentioned, uneven mechanical resistance is easily felt when moving the Dec axis by hand.

I really wish these things were clearer than they seem to be!!!

Ian

[lukepower]

Hi Ian,

 

what I saw during my odyssey with the defective electronics is the following, maybe it helps you:

• The balance function might jump and generally be "nervous" when the motor PID settings are not optimal. Adjusting them improved the situation for me.
• Balancing is crucial. Not in the sense that 10 grams make a difference in every situation, but with large scopes (having a diameter of the tube of 60cm like mine), an imbalanced weight on one edge of the tube can make a big difference in certain situations. I guess with smaller scopes this doesn't always apply.

So maybe you could try and check your PID values in different positions (as you would with the balancing) and see if they keep the scope calm (no sudden small movements).

 

Good luck

[MarkS]

Hi Ian,

 

I'll have a close look at it next time I do a balance.

 

In the meantime, I do use 50c pieces for the final DE balance. It DOES help to get it as good as possible, and doing this beats messing with the OTA in its rings!

 

I've seen wind gusts shifting the current between 0.5A and 1.7A  but that is only transient, of course. The real problems occur when the current slowly rises while tracking - until the dreaded PE occurs...

 

Regards,

Mark

[Waldemar]

The third axes balance (radial scope rotation) may be the critical factor: A focus motor at one side of the scope, a filterwheel not centered, or a finderscope off axes may really make it very difficult to achieve a good balance and get PID settings right in all directions. These balance off sets need to be compensated for.

As far as I noticed, when PID settings are off in different positions, third axes balance is the cause.

Balance, and PID settings are in a dynamic relationship, so both settings have to be repeatedly adjusted, until a proper result is achieved.

Off course weight is a factor, too but smaller then balance

Changing camera's introduces an other challenge. I put weight on the lightest one until it reaches the same balance (not weight for focal distance is a balance factor also), as the heavy one, in that way settings will be the same

and the payload to the focuser also.

I noticed the same “magnetic remanance” effects, when nothing is attached to the Mount. Seems to be very common.

 

Waldemar

Edited July 24, 2014 by Waldemar

[w0mbat]

Thanks to you all, you have given me a lot to think about.

A couple of other things I have noticed.

1. There is a setting in balance called time which is set to 5 seconds. If I change this to say 10 or 20 seconds the balance result changes. To my mind if changing the time over which the averaging is done changes the result it can only be that the motor is encountering a significantly varying resistance to movement so that an accurate average cannot be determined in the default time of 5 seconds.

2. If I slew the dec axis throughout its full range several times the balance result also changes slightly. I don't know what is happening here (maybe lubricant being redistributed in the bearings???). Before anyone suggests it I have VERY carefully made sure that every screw, nut, clamp or whatever in the whole system is fully tightened.

 

I have only done automatic PID adjustments, both normal and high performance. I believe the only time I have ever seen position errors is when I tried high performance tuning before the standard tuning. Doing the normal one first and then high performance worked fine.

 

Perhaps I should just ask ASA for a remote session and see what they think.

Thanks again to you all,

Ian