Swapping motors

[Guest]

I have a problem with RA on my HEQ5. The mount works ok but I get a periodic disturbance (120sec) in RA which I'm trying to eliminate. I've looked at various things and now I'm wondering about the stepper motor itself. Are the two stepper motors in the HEQ5 identical? My plan is to swap them over and see what happens.

Cheers

Steve

 

[Cornelius Varley]

The PE is probably due to manufacturing deviations in the worm drive so swapping the motors will have no affect.

[vlaiv]

They probably are the same and my guess is that you won't see any difference.

~120s period is one single revolution of stepper gear. If you have significant wobble in that period - try swapping stepper gear instead of stepper to see what happens.

Mind you - these are very difficult to remove - you need pinion extraction tool for Heq5 for that (it comes with Belt mod, but could possibly be purchased separately).

[vlaiv]

Come to think about it - yes, since these stepper gears are on shaft of the motor itself - swapping motors will swap gears as well, and since motors are the same - it is simple way to do it without needing pinion extraction tool.

[Guest]

Thanks, I have an extraction tool because I installed the rowan belt drive which is when my problem started. The belt mod works well except that on RA every 120 seconds I get a disturbance in RA which I can see in the PHD2 graph. The disturbance only lasts for about 25 seconds so it's not PE related to the worm cycle. I've attached a chart. The period of the disturbance is related to the period of the motor cog. I've checked everything I can think of and checked that the belt installation is ok. That's when I started thinking about swapping the motors.

The picture below looks like the pulses are increasing in size but actually they stay more or less the same just sometimes a bit higher or a bit lower.

I suspect that I had the problem before the belt mod but the periodic disturbances in RA were hidden by the general crappiness of the guiding in RA. It might not look it but after the belt mod the guiding in between the disturbances is a lot better than it was with the gear train.

[vlaiv]

In that case - I would examine pulley on RA motor shaft. It has same number of cogs as original gear - which means that period of single cog is 13.6s (13.6s * 9 cogs = 122.4s).

If your disturbance lasts for 25s - that would be roughly twice that period of 13.6s - which points to single or pair of cogs doing something funny.

Take magnifying glass and look at it close up to see if you can identify the problem. I've found that looking at the mount while tracking sometimes helps to identify the problem. I sorted tension related issue that way (it was happening each 13.6 because of improper belt / pulley meshing).

[malc-c]

I seem to recall suggesting the issue might be with the motor pulley in your other post 

Quote

I have to agree that it's too frequent to be on the main gear, but it could be worm, but more likely the motor pulley.  However the Rowan doesn't use a custom 4:1 ratio, it uses the stock 47:9 ratio.   

The RA gear ratio of an HEQ5 is 705:1, so if I've got the math right, and bear with me here, 24 hours is 86,400 seconds (24*60*60).  86,400 divided by 705 to get the period for a single revolution of the RA motor results in 122.5 seconds (rounded).  This is very close to the 120s frequency of the spikes in the tracking graph.  So either the pulley has a fault or there is something going on with the stepper motor

Now you mention that this became noticeable when you fitted the Rowan kit, so my best hunch would be that the machining of the motor pulley is slightly off.  The simple way to confirm this is to swap items.  Start with the two belts.  Remove the belt from the RA and place it on the DEC and vice versa.  If that makes no difference then swap the two 9t pulleys.  If that makes no difference, then just swap the steppers... If the 120s pulses migrate to the DEC axis then this confirms the cause.  If after swapping the steppers, belts and the pulleys the spike is still percent then the issue may be down to the controller board...but it would be very strange for the controller  to glitch on the RA at 120 sec intervals, suggesting to me that this is more mechanical rather than electrical.

[vlaiv]

11 minutes ago, malc-c said:

If the 120s pulses migrate to the DEC axis then this confirms the cause

Not sure there is sensible way to test that though.

[markse68]

Could be a damaged tooth (could be carefully filed smooth?) or just too tight mesh and it’s binding on one tooth - can you adjust the gear mesh for the motor? 

[malc-c]

36 minutes ago, vlaiv said:

Not sure there is sensible way to test that though.

My logic was that any spike caused by the pulley would show up on his PHD trace, but on the DEC axis rather than RA ??

[vlaiv]

13 minutes ago, malc-c said:

My logic was that any spike caused by the pulley would show up on his PHD trace, but on the DEC axis rather than RA ??

Therein lies the issue - DEC normally is not tracking and will stay still for most of the session - only slightly deviating from its starting position (very slow drift due to PA misalignment). In ideal case - DEC does not even need tracking and can stay still for the whole session (very stable mount - no wind issues or similar and excellent polar alignment).

120s of sidereal rate means that DEC needs to move at least 120 * 15 = 1800 arc seconds = 30 arc minutes = 0.5°. That simply does not happen with even very poor polar alignment - if you are 1° away from Polaris (two moon diameters) - drift rate will be 15"/minute, so you would need about two hours to get 0.5° of drift and move DEC axis for "120s" total.

Only way you can detect is if you are lucky enough to start with DEC close to problem point and that there is enough drift to move it past that problem point.

[malc-c]

Yeah, thinking about it logically, DEC would be small pulses to maintain the position, remaining stationary in between...

Well the simple proof would be if the spike no longer appears once any of the three items have been swapped...