Mount slewing acceleration/deceleration profile


Craig Young
 

Hi Roland,
I have a 10A 13.8V Laboratory Bench Supply powering the mount.  It is located at the base of the pier and uses the same power cable that came with the mount.

I had a productive video call with Mike and Dave this morning and we were able to resolve all of the issues.  Most importantly, Mike was able to make a couple of CP4 tweaks which greatly improved the stopping performance of the mount with this heavy load.  We also confirmed that slew ramping is indeed working but it is much faster than other mounts.  It is also slew rate dependent so at 240X you cannot hear it, but at 600X it is definitely there.  I asked Mike if he would bring up the subject of adding a user configurable ramping time for the mount given its heavy payload, or at least be able to have AP tweak it.  Maybe specifying the load on the mount would automatically calculate the best ramp speed.

A big thank you to Mike and Dave for their excellent support.  At this point it appears everything is working okay now.

Craig


Roland Christen
 

There is one thing that could cause a sudden stop at the end of a slew, and that is a low voltage at the terminals of the CP4. This can be caused by the power cord being too long, or by the power supply not delivering at least 12.5 volts at the CP4 terminals. What happens is that the motors cannot slew fast enough to keep up with the commanded speed and fall behind. At the end of the slew the motors are still behind and are still trying to catch up, even though the commanded rate is slowing down during the ramp down phase. The motors will then stop abruptly when the commanded position is reached. This is by no means a normal situation and needs to be resolved at the power supply.

I have seen this more than once in customer observatory setups where the 12 volt power was located some distance away and a large voltage drop occurred in the 12 volt cable feeding the mount controller.

One thing to try is to increase the voltage to the mount to 16 or 18 volts, which will reduce the current being drawn by the controller and which moves the voltage away from the 12 volt limit where problems can occur.

Roland


-----Original Message-----
From: Roland Christen via groups.io <chris1011@...>
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Tue, Apr 19, 2022 7:03 pm
Subject: Re: [ap-gto] Mount slewing acceleration/deceleration profile

What color is your light on the CP4 controller? Does it turn yellow when it finishes slewing?

Roland

-----Original Message-----
From: Craig Young <craig.young.m8@...>
To: main@ap-gto.groups.io
Sent: Tue, Apr 19, 2022 7:02 pm
Subject: Re: [ap-gto] Mount slewing acceleration/deceleration profile

I have a conference call with Mike and Dave early tomorrow on another issue which may be related.  I will bring this up at the same time.  Thanks Roland.

Craig

--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics


Roland Christen
 

What color is your light on the CP4 controller? Does it turn yellow when it finishes slewing?

Roland

-----Original Message-----
From: Craig Young <craig.young.m8@...>
To: main@ap-gto.groups.io
Sent: Tue, Apr 19, 2022 7:02 pm
Subject: Re: [ap-gto] Mount slewing acceleration/deceleration profile

I have a conference call with Mike and Dave early tomorrow on another issue which may be related.  I will bring this up at the same time.  Thanks Roland.

Craig

--
Roland Christen
Astro-Physics


Roland Christen
 

The mount has a ramp-up and ramp-down and does NOT stop instantly. This is an error of some kind that we will figure out.

Roland

-----Original Message-----
From: Craig Young <craig.young.m8@...>
To: main@ap-gto.groups.io
Sent: Tue, Apr 19, 2022 6:59 pm
Subject: Re: [ap-gto] Mount slewing acceleration/deceleration profile

This is definitely a configuration item we should have available.  For light payloads it is not that big an issue, but with much heaver payloads it becomes quite significant.  Setting the ramp up and ramp down times is a tradeoff between convenience (faster slewing times) and stress on the gearing system (when accelerating and decelerating).  Recommendations from AP would definitely be helpful in setting these times given the payload and mount.

Craig

--
Roland Christen
Astro-Physics


Craig Young
 

I have a conference call with Mike and Dave early tomorrow on another issue which may be related.  I will bring this up at the same time.  Thanks Roland.

Craig


Craig Young
 

This is definitely a configuration item we should have available.  For light payloads it is not that big an issue, but with much heaver payloads it becomes quite significant.  Setting the ramp up and ramp down times is a tradeoff between convenience (faster slewing times) and stress on the gearing system (when accelerating and decelerating).  Recommendations from AP would definitely be helpful in setting these times given the payload and mount.

Craig


Roland Christen
 

Call AP tomorrow, ask for Mike. Tell him what's happening and we'll try to figure out how to do a remote session to find the error.

Our mounts have always had a ramp-up and a ramp-down. Never an instant stop. Something is definitely wrong with the electronics. Is this a new mount, and has this happened right from the start?

Roland

-----Original Message-----
From: Craig Young <craig.young.m8@...>
To: main@ap-gto.groups.io
Sent: Tue, Apr 19, 2022 6:49 pm
Subject: Re: [ap-gto] Mount slewing acceleration/deceleration profile

Yep, definitely stops instantly, which is why the motor box shakes a bit and the OTA oscillates.  If it takes a few seconds to ramp down then I should notice that.
Craig

--
Roland Christen
Astro-Physics


Craig Young
 

Yep, definitely stops instantly, which is why the motor box shakes a bit and the OTA oscillates.  If it takes a few seconds to ramp down then I should notice that.
Craig


Roland Christen
 

There is a slowdown ramp in the 1600 mount and the motor does not stop instantly, even at 1200x. If it stops instantly, then there is something wrong with the CP4 software, or perhaps you have a problem with the encoder connection. The slowdown ramp of the 1600 occurs over a few seconds, the motor does not stop instantly. We can easily change the up-down ramp via software, but first you have to be certain that you have a problem. Check again to see if the motor stops instantly.

Roland Christen
Astro-Physics Inc.



-----Original Message-----
From: Craig Young <craig.young.m8@...>
To: main@ap-gto.groups.io
Sent: Tue, Apr 19, 2022 5:30 pm
Subject: [ap-gto] Mount slewing acceleration/deceleration profile

I have a Planewave CDK20 on an AP1600GTOAE mount.  The OTA + instruments weigh about 160lbs and the mount is spec'd for 220 lbs, so the payload is well within the capacity of the mount.  But when I slew at 600x from point a to b the motor stops instantly and the OTA rocks in the cradle until settled.  Looking at other mounts with large payloads they use a noticeable acceleration/deceleration slewing profile to prevent this.  I did not see any place in APCC setup to set the ramping profile.  Is this not supported on AP mounts?  For the moment I have changed the slew rate from 600x to 240x (1 deg/sec) which is a bit slow getting around the sky, but it still rocks somewhat when slew is finished.  This has got to be hard on the drive system when this happens.

Craig

--
Roland Christen
Astro-Physics


Craig Young
 

I have a Planewave CDK20 on an AP1600GTOAE mount.  The OTA + instruments weigh about 160lbs and the mount is spec'd for 220 lbs, so the payload is well within the capacity of the mount.  But when I slew at 600x from point a to b the motor stops instantly and the OTA rocks in the cradle until settled.  Looking at other mounts with large payloads they use a noticeable acceleration/deceleration slewing profile to prevent this.  I did not see any place in APCC setup to set the ramping profile.  Is this not supported on AP mounts?  For the moment I have changed the slew rate from 600x to 240x (1 deg/sec) which is a bit slow getting around the sky, but it still rocks somewhat when slew is finished.  This has got to be hard on the drive system when this happens.

Craig