Virtual Master

So I've been referencing a lot of code in RSLogix and have noticed something that I would like to have cleaned up.

The implementation of a virtual axis essentially allows you to send motion instructions to a virtual axis that responds exactly how you want it to. However, I've noticed that a lot of other physical axes reference this virtual axes.

Now I'm working on a design that featuring two axes: one that winds and one the rewinds. I was intending to gear the two axes together to control a specific winding tension, but I'm curious as to whether or not it is better to gear both axes to a virtual master, because that is what I've seen done before.

So my question is, is gearing an axis to a virtual master generally better practice? Is there any reason to not gear together two physical axes? What is the general advantage of have a master virtual axis instead of simply defining a physical axis as the master axis for a machine?

tomizzo11 said:

So I've been referencing a lot of code in RSLogix and have noticed something that I would like to have cleaned up.

The implementation of a virtual axis essentially allows you to send motion instructions to a virtual axis that responds exactly how you want it to. However, I've noticed that a lot of other physical axes reference this virtual axes.

Now I'm working on a design that featuring two axes: one that winds and one the rewinds. I was intending to gear the two axes together to control a specific winding tension, but I'm curious as to whether or not it is better to gear both axes to a virtual master, because that is what I've seen done before.

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So the virtual axis would be the line speed? As the winding continues the source speeds up and the destination slows down? You need to be changing the gear ratio of the slaves to the virtual master continuously. This can be done. I don't see why it wouldn't work.

So my question is, is gearing an axis to a virtual master generally better practice?

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It is personal preference. Can you do this without cam tables?

Is there any reason to not gear together two physical axes? What is the general advantage of have a master virtual axis instead of simply defining a physical axis as the master axis for a machine?

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I don't see a problem using a virtual master.

tomizzo11 said:

Now I'm working on a design that featuring two axes: one that winds and one the rewinds. I was intending to gear the two axes together to control a specific winding tension, but I'm curious as to whether or not it is better to gear both axes to a virtual master, because that is what I've seen done before.

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How are you intending to determine the diameter of the unwind and rewind ?

L D[AR2 said:

How are you intending to determine the diameter of the unwind and rewind ?

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Ultra sonic sensors.

I would suggest using a virtual master as opposed to a real master in this case. A real master can sometimes be a little bit of a challenge because any master axis instability will be transferred directly to the slave axes. So you end up having to add filtering to the real master, which introduces latency. In your case it may not make that much difference, though. Your application probably isn't that dynamic.

So you are going directly from a driven unwind to a driven rewind? Are these center-driven axes or surface driven axes? If they are surface driven you should be able to gear them together as you propose and it should work for you as long as you have good speed command resolution. If center-driven with no intermediate isolating axis you will very likely not be able to simply gear one to the other unless you are dealing with a very stretchy material, like elastic. Even then you will likely have issues. Unless you are dealing with very high performance sensors, an ultrasonic sensor will not provide the measurement stability or accuracy you will need to gear two dynamic diameters directly together. Do you have any feedback devices in the web path between the two axes; something like a loadcell roll or a dancer?

Keith