Motion Coordinated Linear Move

Oboy

Member
Join Date
Apr 2005
Location
Paducah KY
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24
Does anyone have a simple example using the MCLM instruction in Control Logix? This job basically requires moving a piece of equipment from point A to point B using a VFD and encoder position feedback. The MCLM instuction seems perfect for the application. The Rockwell help file seems a little vague. I found an example program on their web site but it is moving a piece of equipment in 3 dimensions. I just need one axis. I'm really confused about creating a virtual axis.

Any help would be greatly appreciated.
 
First of all, it sounds to me like you simply need the Motion Axis Move (MAM) instruction. This will allow a single axis to perform a point to point move, which is what it sounds like you are looking for.


The MCLM instruction will coordinate the motion of two or three orthoginal (X, Y, Z) axes to perform a linear motion in two or three dimensional space. This instruction only works if the axes are orthoginal. Many application don't need the motions coordinated. Many pick and place applications, for example, don't need the X and Y direction motions coordinated. Each can finish as soon as it can and things are OK. You don't care about the actual path the item takes as long as it ends up where it is supposed to.

However, consider a machine that needs to draw diagonal lines on a paper. To do this given the X and Y direction setup you need to run both x and y motors in a coordinated fashion since the path of the item through space is what defines the diagonal line. In this application you would use coordinated moves. You specify the desired Cartesian (X-Y) destination and the linear speed along the item path. the motion controller takes care of controlling both motors to follow this path.

Like I said, you probably want to look into the MAM instruction. That should take care of what you want to do.

Keith
 
I am guessing a bit here and may be wrong in what I am saying. You wish to move an item from point A to B using a VFD and an encoder for position feedback. Now I assume you do not have a motion card in the PLC ie an MA2?? What I think you can do is create a virtual axis for the move and tell the VFD to mirror the virtual axis commands.
If it is only moving something from A to B there are maybe easier ways to do it. Regards Alan Case
 
i think you have to have motion card to create and use virtual axis...
at least this was requirement a while ago. i asked what is virtual then and
an answer was that you don't need amplifier, motor and encoder but
to use it you still need motion controller for processing (no overhead for cpu).
fortunately i had at the time one axis spare but in case someone
knows this is possible without *******ler please confirm...
 
Last edited:
I will talk to the local AB rep tomorrow about the virtual axis without the motion control card. I would like to try some of the new instruction sets with Control Logix. I realize it could be done other ways, but this looked like it would be pretty clean.

Thanks for your replys
 
No motion card is necessary to create and use virtual axes.

I use this functionality regularly to test / debug gearing and camming logic.

using a VFD and encoder position feedback
You should use a motion card to control this. A 1756-MO2AE will provide +/- 10 volt analogue output and accept the encoder feedback.
 
The problem I have with using the card is both the VFD and Encoder communicate via DeviceNet. Would this not be a good application for the MAM instruction?
 
If they both communicate with DNet then no, it is not a good way to do it. You would have to program in an intermediate layer in you PLC to allow the 2 units to communicate.
The motion instuctions are meant for the motion cards ie MO2AE or HYD (sercos card also). Theoretically it may be possible to use the MO2ae card and map the analog out and in through DNet to the drive and encoder but it would be messy. ie The card expects an encoder connected, you may even have to SSV the encoder position into the cards tags, force errors of no real input to off etc etc. Then again I may be talking out of my butt and it is easy but i doubt it.
Regards Alan Case
 
I think loop closure is actually performed on the motion cards. A virtual axis will perform all the course command side calculations (position command, velocity command, feed forwards, etc) but will not close a position loop.

So, as far as I know, the best you could do without a motion card is use a motion command on a virtual axis to provide the motion profiling and commands and use plc-based PID functions for loop closure. If your application isn't overly demanding this may work for you. But, caveat emptor. I have never done this myself.

Keith
 
You can close the loop in the PLC using very simple math to generate a speed command that is proportional to your position error and limited to a maximum speed. It won't be as responsive as a servo, but can be very accurate. I have done this with a PLC5/40, AB160 series V/Hz drive and resolver feedback from the driven device. I was able to repeat my motions within one or two resolver counts (out of 1024) which equated to about 0.050".

See this thread for more details
 
Thanks Okie, The logic the custemer has now basically moves the equipment at full speed until it gets close to the target and then goes into creep speed until it reaches the setpoint. They are wanting an accuracy of +/- 0.5 mm and are not getting it. Another point is speed isn't crucial to the process.
 

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