Do nothing in a double case structure

[slimy]

Hi everyone,

I´m having a little issue that I cannot manage to solve, maybe one of you could be kind enough to provide some help.

As you can see on the joined picture, I´m using a double case structure and changing a voltage accordingly. What I would like is to keep the voltage unchanged in the False case of the internal one.

I saw two things: some developers suggested to implement a functionality that keeps the previous value for an unwired output but this hasn´t been implemented yet. Someone suggested using a shift register to keep the value unchanged. I´m new to this method and can´t seem to get a hold of it with my two cases program.

Thank you for your answers.

[pincpanter]

The simplest method is to stick with the "do nothing" philosophy 🙂

Enclose the voltage setting vi in a case structure; the function should stay in the False case.

Wire a True constant out of the inner False case of your nested case structure; check the "use default if unwired" option for all the unwired tunnels.

As a result, the output value will be True for the "do nothing" case, False for all the other. Wire this value to the Voltage setting case structure input.

[slimy]

Thanks for your answer but I´m not sure I got what you´re saying. IF I measure something equal to 0,05, I need to change the voltage. But the rest of the time I´ll be in the other case, where I want to keep the same value. Both cases will occur and if I leave a use default if unwired, I´ll probably get zero as a voltage. Could you please send a print screen of your simple solution?

[thoult]

Something like this, you mean?

Question: what happens to values that are 0.0499999999? Do you care about measurements that are equal to your setpoint plus some tolerance? If so, have a look at In Range and Coerce.

[slimy]

Thanks for your amswer. The problem in your solution (maybe pincpanther´s one too) is that my case condition doesn´t concern the voltage I´m changing. At the entrance of the cases is a conductance measured somewhere before that is independent on the voltage I want to change.

About the tolerance, you´re right and I had a superior or equal condition before instead, maybe the equal will cause me trouble if the voltage is not dropping continuously as I would like it to do.

[thoult]

That may be a little clearer for me.

So you're taking a measurement A and selecting an output B based on the value of measurement A.

If the current value of A is the same as the previous value of A, you don't want to change B? Or if the current value of A is greater than(or less than, whichever) a constant, you don't wish to change B from its previous value?

You can do this using shift registers to retain the value from the previous iteration. Here's a simple example of the second case: A is compared to the set point, and if it's less than the set point, the previous iteration's B value is used. If it's greater than or equal, a new set point is used. You can adapt this approach of using shift registers to retain the previous value for the capacitance, if that's what you meant instead.

[joaopam]

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thoult escreveu:

That may be a little clearer for me.

So you're taking a measurement A and selecting an output B based on the value of measurement A.

If the current value of A is the same as the previous value of A, you don't want to change B? Or if the current value of A is greater than(or less than, whichever) a constant, you don't wish to change B from its previous value?

You can do this using shift registers to retain the value from the previous iteration. Here's a simple example of the second case: A is compared to the set point, and if it's less than the set point, the previous iteration's B value is used. If it's greater than or equal, a new set point is used. You can adapt this approach of using shift registers to retain the previous value for the capacitance, if that's what you meant instead.

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Based on the snippet, wouldn't a select function be suitable for this situation?

I'm asking because I would do that in my first try. The result would be the same, right?

[thoult]

It would if you only have one comparison and two output cases: if less than, do this, else do that.

If you have several comparisons and/or outputs, then you're probably best off using nested case structures.

[joaopam]

I don't want to hijack this thread; but I see the point. I knew that.
I asked because looking at the snippet, a select function would be completely okay.

BUT, I must add that I haven't seen the initial VI, so let me clarify that I was talking about your code, thoult 😄 (I will edit my previous post to quote yours and make this clearer).
Hope you understand me.

Regards

[pincpanter]

You missed the point: in the selected condition you would not set the voltage at all. See the following image.