Author Topic: motor current sensing - technical details required  (Read 11513 times)

Offline Armando

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motor current sensing - technical details required
« on: Monday, 25.11.13 - 20:59:45 - CET »
Hi @all,

by opting for small reduction ratio (4:1) on my mount (180T worm wheel) I found that motor speed is not constant.
Maybe the cause is that I'm checking with no load.

Anyway I found that increasing motor current reduces the issue. So I've some doubts: maybe current peak value is not the same on the phases of my motor... The difference is negligible by setting higher current values but is no more negligible if I use lower current values...

I'm referring to RA motor. Obviously the same is valid also for Dec motor.

Current on each phase is sensed by a voltage drop on a resistor in series with the corresponding motor phase...
We have 2 resistors; their resistance value could differ. And we have 2 ADCs to sample the voltage. And they can differ...
I also see there are 2 potentiometers (one for each motor) and an offset (maybe related to the AD transfer functions of both the converters used for the same motor) value for each motor that the user can set.

So I'm tempted to think that each potentiometer is to be properly tuned for each motor...
And then maybe an offset value (for each motor) is to be tuned to take into account an offset related to the couple of ADCs used for each motor...

In LFEP building instructions available online Rajiva indicated that by setting the max current value (with no motors connected) the user had to tune each potentiometer to measure 0.71V... (http://elegance.photo.rajiva.de/)
Maybe I need to finely tune these potentiometers. I hope someone can offer some details about this procedure...

Then I'd like to have some details about the ADC offset values, if known by someone...

Thank you and Clear Skies
Armando Beneduce

Offline Armando

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Re: motor current sensing - technical details required
« Reply #1 on: Monday, 25.11.13 - 23:21:31 - CET »
I just made some attempts: by changing the ADC offset value on RA (by #>xxo to set xx as offset value) I found motor current shown by the handbox changed.
So if the user is able to measure accurately the current in each phase then he can set this offset value accordingly to make the handbox showing the same value...
Playing with this command won't solve the issue.
I also think the potentiometers are used to set a Vref (used to properly measure current on each sensing resistor...).
But if there are only 2 potentiometers (one for each motor) I see no way to scale/tune the current of one phase by keeping the other current (of the same motor) fixed.

Probably there is no issue at all and the variable speed is caused by no mechanical load on the motor...
I'll be able to check as soon as I'll have mounted the motors (a couple of Sanyo Denki) on my mount.

CS
Armando

Offline Tobias

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Re: motor current sensing - technical details required
« Reply #2 on: Tuesday, 26.11.13 - 22:43:50 - CET »
Hi Armondo,
please please post your results!
I want to use an old EQ6-Wormwheel with a 1:4 one-step-gear and some 0.9° Steppers and hoped that the movement is smooth. But with only 1:720 the risk is high... so i'll be anxious for the results :D

CS Tobias

PS Good luck and excuse my English.
PPS I tried the same with a laser-pointer on a nanotec motor and found some stutters on the screen; but also with no load.
früher unter forum.rajiva auch als Tobias unterwegs

Offline Armando

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Re: motor current sensing - technical details required
« Reply #3 on: Wednesday, 27.11.13 - 14:48:27 - CET »
Hi Tobias,

I want to use an old EQ6-Wormwheel with a 1:4 one-step-gear and some 0.9° Steppers and hoped that the movement is smooth. But with only 1:720 the risk is high... so i'll be anxious for the results :D
I also own an EQ-6 and I know that higher reduction ratio could be preferred; this is the reason why I'm also tempted to switch to a 0.9° step angle solution.
But I need to ascertain the issue is not caused by the LFEP itself:
I bought a SynScan motor kit (without the SkyScan handbox) to use it by my LFEP on my old EQ6. But I found really bad tracking on RA (on a real star). Then I removed the motors and gave a look at them while they were rotating with no load and I found variable speed with low current values, no issues with nominal current value.
You know EQ6 mounts moved by SkyScan/SynScan controllers work well with 3.97:1 reduction...

So now I want to test the same SynScan motors (or a couple of Sanyo motors still with 1.8° step angle) by pulley/belt transmission and 4:1 reduction.
If I'll find everything working then the tracking wobble found initially on my mount was related to wrong transfer gear profile...
If I'll find it not working (let's hope this won't be the case! ), then... only LFEP can be the cause of the issue. At that point I'll be obliged to switch to higher reduction ratio and maybe I'll buy a pair of 0.9° steppers to make use of the same belt/pulley kit...
I really hope my mount uses a different transfer gear on the worm axis...

Quote
PPS I tried the same with a laser-pointer on a nanotec motor and found some stutters on the screen; but also with no load.
I already made exactly the same test!  :)
You can notice that using greater current (e.g. using the nominal current value for your motor) still with no load the wobble disappears...
This observation is the reason why I hope the wobble found on my mount (on real stars) was not related to this motor rotation wobble detected with no load.

I'll let you know. But I need to ask one of my friends to enlarge the bore on the pulley to be able to fit it inside my mount (on the worm axis)...

CS
Armando

Offline the_lizardking

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Re: motor current sensing - technical details required
« Reply #4 on: Saturday, 30.11.13 - 11:02:05 - CET »
Mmmm dont know but this wobble seems to be cngruent with the problems I found on my mount. Changing the current braight some changes on the peak to peak value of this wobble,... however I cannot go too low with the current otherwise the mount stucks moving,... I have a heavy load with around 25Kg.

First of all I was playing around with the ratio because I saw an old articel in the old forum where someone played around with had minor success and stopped then doing it. But this did not solve my problem or it solved the problem only for certain exposure times. Which indicated that this is not a continous problem. I thought then it must be some waving or wobbling problem that comes and goes with different amplitude - so I was playing with the current. This brought some effects that where messureable and reproduceable but did not solve the problem at all,... working with 2000mm fl by making 2000 sec exposures even guided shows this probplem very clear. It even seams that LFEP does messure the difference in moving as the position seems to be correct. However you even feel this problems when using the pointing of LFEP - you can see a always constant encreasing value of misspointing to stars that is too large after a while to be related to the gears.

However I could never find out where this problem comes from and I am still searching,... today as we have fine weather I will have a test in the evening trying the new drift compensation. However as I think that this is not a continous problem I do not think this will solve the issue,...

CS

Offline Armando

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Re: motor current sensing - technical details required
« Reply #5 on: Saturday, 30.11.13 - 21:21:49 - CET »
Hi the_lizardking,

I think your issue is different.
I was speaking about motor speed oscillating around the nominal (sidereal) value with 50 oscillating periods/revolution (and my motor step angle is 1.8° - 200 steps/revolution). The oscillating frequency increases linearly with the reduction ratio so that I see 50 oscillating periods on each full motor rotation whatever the reduction ratio in use...
Probably the issue is only related to the motor not loaded at all: as already stated I limited to detect the issue by giving a look at the motor removed from the mount.
Increasing motor current surely reduces the amplitude of the oscillation.

At higher reduction ratio I think this oscillation (at higher frequency) is hardly  detected and in any case it doesn't affect the mean value of the tracking speed at all.

If I'll find this issue on a real star then an higher reduction ratio is definitely required by LFEP.

As for your issue, altering the reduction ratio isn't obviously a valid solution.
As you already deduced, If you see a wrong mean tracking speed (that can occur simply because of a not perfect alignment of the mount - maybe your LFEP is defective) drift compensation should be of help.

CS
Armando

Offline the_lizardking

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Re: motor current sensing - technical details required
« Reply #6 on: Sunday, 01.12.13 - 07:48:37 - CET »
Well I am really not sure what the problem is however it is not only a drift in one direction,... first of all I thought it is relatated to a higher speed,... I was working on alignment as well as current and ratio. I could improve this issue a bit by using a different current value but only for one fixed exposure time and only for 600mm fl. When I started to expose lower or higher this effect popped up again. First of all I thought it is related to drift in only oine direction, in my case I thought it moves too fast - but this does not seem to be true as even when I lower the ratio massive it drifting,... it more seems to be an issue that cause moves in both direction during movement like swinging forward and backward all the time. It would be maybe helpul to read out the encoder information during traking and compare it with the one the controller gives,... I meanwhile have the renishaw on it that is very high reading. Only in this case we could really see which problem it is as the correction do not help. The stars only show very straight spaghetties,...

So it also cannot be related to the alignment as in this case the star would not show a line after 20 minutes exposure, it would show a move up or down or more a "u" turn,...