MAJ vs MDS

[ASF]

Hi all,

Got my first proper foray into servo drives coming up. I have an application where two conveyors need to speed match exactly to avoid deforming the product. Each conveyor will be on a Kinetix 5500, talking back to a 1769-L18ERM over Ethernet/IP.

I've been talking to my drives rep and got a basic idea of how to go about it. If I use a MAG (Motion Axis Gear) instruction on the second drive, then all I have to do is run the first drive at whatever speed I want, and the second drive will follow it. My rep suggested the MAJ (Motion Axis Jog) instruction to run the first drive. However in reading through some manuals to get a handle on things, I've also come across the MDS (Motion Drive Start) instruction, and I can't help but think that makes more sense, at least inasmuch as the name of the instruction. To me, "jog" has something of a temporary, "press-and-hold-to-run" connotation to it. While "start" is more of a "run this conveyor until told otherwise" sort of instruction. The MDS instruction also seems to be much simpler, with less parameters to fill in. The motion instructions manual says "Use the Motion Drive Start (MDS) instruction to activate the drive control loops for the specified axis and run the motor at the specified speed" which seems like exactly what I want to do.

Can anyone give me some tips as to how the MAJ and MDS differ, and where each of them might be preferred in place of the other?

 

[chavak]

Never used MDS before, but I think you had control of your acce/dece is an advantage if your application call for it when MAJ is used.

 

[janner_10]

I would just use a Virtual Axis and MAG both conveyors to that.

MAJ is not what it seems, it will jog until the end of time until you tell it to stop.

 

[ASF]

I would just use a Virtual Axis and MAG both conveyors to that.

So you would MAJ the virtual axis and MAG the two physical ones?

Why would you do this instead of MAJ the first axis and MAG the second? To my (inexperienced) mind that just seems to be adding an another layer of complexity.

Also, does the virtual axis count toward the axis limit? The 1769-L18ERM only supports two axes.

 

[janner_10]

So you would MAJ the virtual axis and MAG the two physical ones?

Why would you do this instead of MAJ the first axis and MAG the second? To my (inexperienced) mind that just seems to be adding an another layer of complexity.

Also, does the virtual axis count toward the axis limit? The 1769-L18ERM only supports two axes.

Yes, Jog the VA and then just MAG the other 2 to it.

This is useful because you can play around with the VA during simulation. But like anything, there are more ways to skin a cat.

I don't believe it takes up an axis slot, I can create a project with a L18ERM and add 2 physical axes and a VA and it doesn't moan at me.

 

[ASF]

Thanks. Thinking about it now I can see how that might be helpful because then each (real) axis is independent and you could run one axis with the other out of action. In my particular application that's not so relevant because the conveyors are a top and bottom type arrangement - it's conceivable that they'd want to run the bottom conveyor without the top, but never the other way around, there'd be no point. So if I MAG the top to the bottom I'll be covered. I might play around with the virtual axis regardless, it sounds like a useful technique!

Anyone else able to comment on the difference between the MAJ and MDS? I did wonder if I can only use the MDS in a velocity loop application, not a position loop application?

 

[janner_10]

You will need to do a bit of manual trawling or googling, but I though the MDS was used in conjunction with a SLATs application.

For bog standard speed control I use MAJ or Torque just enable it and give it the torque value.

From memory the MDS sounded good, but there was lots of baggage that came with the instruction too.

Found it here: pg55 onwards: https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/motion-rm002_-en-p.pdf

 

[ASF]

Yep, that's the manual I've been reading. You can disable SLAT if you don't need it.

 

[Soggy Canuck]

You should be able to create a virtual axis without disturbing your processor axis limit.
There is one distinct advantage to slaving your two conveyors to a virtual axis. The difficulty is in the master/slave relationship, any time a drive is following another drive there will be a response lag in the slave drive. This lag is most noticeable in accel/decel, but may also occur during steady state operation.

With careful tuning of the drives you should have a much easier time getting the results you are looking for.

 

[ASF]

That's good to know, thanks for that feedback!

 

[Soggy Canuck]

Peter Nachtwey has written on this site about the master/slave vs virtual axis extensively.

I tend to listen hard to anything he has to say on the subject of motion.

Check out the following as a start point.

http://www.plctalk.net/qanda/showthread.php?t=53988