NEQ6 pro stalling at low speed

[BobKinsman]

I have an old NEQ6 mount that I picked up secondhand last year. I've just started using it with a guide scope via ST4 and PHD2. Up until last night, it was working well with rms errors of 1 - 1.5 arc seconds. Then phd2 reported that the corrections it was making didn't seem to be having any effect. Sure enough, when I tried phd2 manual guiding this morning, the motors were silent when I pressed the east button, so I opened up the covers and watched the motors. Basically, whenever I slew at rate 0 or 1 in 3 of the 4 directions, nothing happens. Everything works fine at rate 2 or above. Unfortunately guiding appears to use rate 1, so itis not working. The signals on the ST4 connector are operating correctly and the power supply (nominally 13.8v) never falls below 13.3v, even when slewing both axes at rate 9. Does anyone have any brilliant ideas on the likely cause? I don't know whether to strip the whole thing down, or start replacing electrical bits (if available).

[malc-c]

Can you give details of your set up.  Presumably you are using an EQDIR cable between the PC and mount, and possibly EQMOD to control it, or are you linking via the handset with a PC Direct cable, or using some other hardware to interface between the two ? 

[BobKinsman]

Hi Malcolm,

The guide camera (ASI120MM mini) connects to the mount using ST4. The PC connects to the imaging camera via USB3 and the imaging camera's usb hub connects to the guide camera via usb2. The mount has the orginal v3.23 handset, and no connection to the PC . There is no EQMOD (yet), I'm trying to keep it simple while I get the hang of PHD2, APT, etc. The mount, imaging camera  and PC are powered independantly. As far as I aware, this is all working fine. The ST4 signals all get waggled in response to phd2 manual guiding commands, so I suspect it is either some swarf / sticky grease or other contaminant in the drive train, or a faulty motor control board.

Bob

[malc-c]

Bob, thanks for the clarification.  I didn't want to mention things that were not applicable.

I'm not familiar with your guid cemara, nor with ST4 guiding, but if I understand it you have something like this  (ignore the makes / models, it's just the wiring I'm referring to)

 

I personally doubt that the board is faulty as they either work or don't, rather than work at max slew rates but not at low ones.  The thing is the pulses will be tiny when guiding, so you probably would struggle to see them although you can normally hear the motors make a song as they are applied.

To be honest, if you are using the set up as shown I would suggest you invest in an EQDIR cable for your mount, download ASCOM and EQMOD and do away with ST4 altogether.  The combination of EQMOD and PHD2 works so well, and both will give you the indication you need to confirm its working.

Sorry having internet issues and this uploaded before I had finished... 

EQMOD is simple to use.  If you can use PHD2 you can use EQMOD.  Both have simulator options if you want play rather than run live you can...

Edited February 28, 2021 by malc-c

[gajjer]

Hi Bob. Here are my thoughts. The drive motors are stepper motors so I would have thought that if they move, it wouldn't matter what speed you are demanding. I don't know anything about ST4, I drive mine through a Lynx Astro cable. When I have had motor stalling problems it has usually been when fast slewing and it sounds horrible, like gears are being damaged. It's not as bad as it sounds, it's just the stepper not stepping. The motor wont over current when stalled. Not like a conventional brushed motor.

If you have the handset try it with that. 

There is a very good tutorial by Astrobaby that tells you how to strip the mount down and tune it up. The bit I would have a look at is the adjustment of backlash. The backlash is adjusted by moving the worm closer to, or further from the main gear wheel. It is possible that it is fine in one position but binds at others as the gear rotates. A product of the eccentricity of the gear. The adjustment screws move the whole casting holding the worm gear. This is true in both axes.

Personally I would try to establish if there is something in the ST4 drive that is not operating correctly first. If it is stalling you will hear it. If it's not getting the drive signal you wont. Then I would look at adjustments before doing a strip down. I would recommend an adjustment anyway. If it's secondhand you don't know what has been done to it.

Hope that helps.

cheers

gaj

[BobKinsman]

Thanks for your replies guys.  I have exactly the same setup as per your diagram Malcolm, and it worked well for a couple of evenings. Whilst I will inevitably move over to EQDIR to be able to do plate solving etc in the future, I wonder if it would address this particular problem. The motors don't run at low speed (in 3 of the 4 possible directions) no matter whether they are driven by the handset or the ST4 port. EQDIR will use the same port as the handset., so unless ASCOM / EQMOD requires the motors to run at a higher speed, I suspect the result will be the same.

I think the next step is to look at the stepper motor drive voltages on an oscilloscope. That should definitively show if it is the motor controller board or something downstream. I can't imagine that both motors are faulty, so if not the MC board, it will probably be the mechanics. If that is the case, then it might be a good time to do a belt drive conversion. 

[malc-c]

So are you saying that regardless of the speed setting (1-9 on the handset) the mount will only move in three of the four rotations, ie the RA azis will move back and forth, but DEC will only move in one direction ?  If so, does the same happen with the ST4 removed from the mount?

I've never tried scoping a board, but if what I described is the case then it could be that one of the four driver chips has blown.  It won't be the PICs as you've not reported any messages on the handset about an axis not responding.  If both axis move in both directions when the fastest slew speed is selected then it won't be a faulty motor board...

Is the scope balanced and the CofG correct?

The suggestion to try EQMOD and an EQDIR cable would be to rule out the handset as being the cause.  If you find that with EQMOD set to 1 or 2 fo the four speed settings and slew the mount moves then it would point to the handset or the cable.  

Good luck

[Ricker]

What PSU are you using?

[BobKinsman]

I had a look at the currents in the stepper motors today. Using the handset buttons, they are fine at all speeds except Rate 0 and 1.  At Rate 1, there are no microstepping currents in one direction only, just the holding currents that are there when no button is pressed. At Rate 0, the motor moves in the same direction whether left or right buttons are pressed !!

When using PHD2 via ST4, I don't understand the East / West behaviour below, though it may be correct. The buttons referred to are those in the manual guiding window. The microstep cycle referred to is the is time to complete one motor step, through the sequence of microstep currents. A couple of pictures of this are shown below. Motor CW direction corresponds to CW rotation of scope, i.e following the stars.

PHD2 manual guiding, Mount tracking off, PHD2 guide speed 0.1 x sidereal.

        East button, 0.8s / microstep cycle : Motor CW
        West button, 2.4s / microstep cycle : Motor CW    ?? Same as East!!
        North button, 2.4s / microstep cycle : Motor CCW
        Southbutton, 2.4s / microstep cycle : Motor CW

PHD2 manual guide, Mount tracking sidereal 0.5x, PHD2 guide speed 0.5 x sidereal

        No button, RA motor: 1.2s microstep cycle : Motor CW
        East button, 0.8s / microstep cycle : Motor CW
        West button, 2.4s / microstep cycle : Motor CW    ?? Same as East!!
        No button, DEC motor: no movement & only holding current
        North button, 2.4s / microstep cycle : Motor CCW
        Southbutton, 2.4s / microstep cycle : Motor CW

This is the behaviour with no scope on the mount, i.e. slight unbalanced in RA, but good in DEC.  The PSU is a Maplin 13.8v 7.5A mains unit

I am now not sure whether guiding should work or not. Unfortunately, we had 100% cloud last night so I couldn't try again. There is obviously an electronic fault related to Rate0 &1 on the handset or MC board, but that would not justify replacement unless it upsets EQMOD guiding at some point. Perhaps it is time to get a cable and find out.

[malc-c]

The fact you got traces for both axis suggest the motor board is functional.  Can't say I understand the results of scoping the steppers, but as I mentioned, using EQMOD, set the pulse guide rate to x0.90 and see how PHD2 performs (enable the log files). 

If you get the same behaviour then it would suggest the issue is with the mount, so possibly reflashing the firmware might resolve the problem, or the issue is something physical on that axis.  I would also check the soldering around the pads where the wires from the ST4 port connect to the PCB just in case there is any stray whiskers or flux that could be causing the problem.

[gajjer]

Hi Bob. Sorry but I can't make out what the traces are. Are they current? Are you using a current probe? What are the scales on the first two?

I wouldn't have expected a sinusoidal waveform at all. I did some current probe measurements on my NEQ6 and I don't recall anything sinusoidal. But then it was a while back and I was mostly interested in current consumption at different speeds.

I would agree with Malcolm about checking the connections. Bad connections are the main cause of problems. Seems very odd that it works in some directions and speeds but not others. Also check continuity and isolation of cables. If there is anything DIY made: then give that extra special attention.

cheers

gaj

[BobKinsman]

Sorry I should have said. On the first 2 images, the horizontal scale is 100ms per division, the last one 10us per division. The vertical scale is winding current for the 2 RA motor windings (DEC is basically the same). These are measured  at the un-earthed end of the winding current sense resistors where they go into the appropriate driver chip ( 2 windings & chips for one motor). The peak current is about 0.5v/0.68ohms or about +/- 0.75A max.  I think there are 64 micro steps per motor step, the current ramping up and down with a sinusoidal envelope over the course of 1/200 of a rev. The amplitudes and phasing of the 2 traces in the last picture varies considerably over the course of the micro-step cycle