Screw vs Express SoftMotion function blocks

Hello

Are there differences between express screw SoftMotion function blocks? I intend to drive a servomotor through a NI 9505. Examples AND show how using function blocks, but I've been advised to use the screw Express instead, work around odd behavior when using a simulated axis (http://forums.ni.com/t5/LabVIEW/SoftMotion-quot-Update-Buffer-quot-VI-refuses-to-take-more/m-p/22508...

Thank you!

Hi JKSH,

If I booted up a new application, I would use screw Express on function blocks. Here is a list of things to consider when choosing:

By default, function blocks are asynchronous. They run on the rising edge of the entry "run" and are carried out when the output "done" is high. They do not block and function block architecture will almost always call the same function blocks again and again in a loop. You control the flow of execution with the 'run' and 'done' entry and exit. Screw Express, on the other hand, are synchronous by default. They don't have an entry not "run" and instead start running as soon as they are called. They block also until the end screw Express can be converted to asynchronous by right-clicking on them and by changing the "calendar model. When you change the calendar in asynchronous model, the Express VI will now act exactly like a function block.

In general, you should never mix asynchronous and synchronous API. Your application should be one or the other. If it is possible to have safe synchronous and asynchronous calls, you must be very careful and really know what you're doing.

To my experience, write simple applications is easier with a synchronous API, and more complex applications are better suited to an asynchronous API. Since blocking synchronous calls, you need more loops in your application and it can become difficult to maintain.

There is also the property node / API call, on which are based the functional blocks and screw Express. You can do a right click on a function block or Express VI and convert it into a Subvi to see the underlying code. Then the function blocks and screw Express can be pleasant to work with, you must sometimes lower level control that is offered by the property and invoke nodes.

I looked at your other post and it seems that you meet one of the intricacies of asynchronous calls. In point 1 above, I mentioned that function blocks increase sensitive edge on the execute line. In your modified sample and the original sample, the 'Update buffer' function block is executed when space becomes higher or equal to the "size of the Contour data. In the original example, the size of the table is 40, but in your example, it is 100. We will follow the stream through the example:

We do nothing in the first round since we do not see a rising edge of the signal "run" "Initialize the buffer" until the second iteration.

At the beginning of the second iteration, we initialize the table with 20 points, leaving 20 spaces available. This function block can take more than one iteration to complete, so that the loop can run several cycles before the release of "Done" is set to true. In this case, the next block runs.

Then we start the move of contour. No point is taken in the buffer zone This occurs in the part of the user rather than the scan code.

Then, we see that there are 20 spaces available, so we add more than 20 points to the buffer, leaving us with 0 beds available. This means that in the next iteration of the loop, the signal "run" function block 'Update buffer' will be false. This is essential to the functioning of the example.

Since there is no abandonment called, we have a loop around back.

Each scan, SoftMotion takes a point in the buffer zone. Finally, the number of available points will be 20. When this happens, the entry "run" will switch from false to true on the 'Update buffer' function block and 20 points more will be added to the buffer. This cycle continues until you stop.

So, what was wrong with your changes? You did the size of the table 100 points. If you look at the steps above, you will see that in the example, we have 40 points in the buffer before taking points. If you increase the size of the table to 100, it means that there are 60 places available. However, 60 is always higher than the "size of data contours" (which is 20) and so the entry "run" function block 'Update buffer' will never wrong; We have always more available space than the "size of data contours. Since "run" is sensitive edge up, we are never really new data in the buffer, and so you get only two cycles that you initially put in. I agree that it is difficult, but it's exactly as function blocks are supposed to work and are certainly something to think about when you use the asynchronous API.

Thank you

Tags: NI Hardware

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