how do i check for limits on a waveform chart?

[NewMachine]

i know this is a simple task and i remember learning about it in the labview basics class but couldn't find anything just now.....so i just have charts in a vi that are acquiring data from sensors and i would like show what the limits of those sensors are and take action such as limiting the voltage (to a drill) if something did reach a limit. can someone show me how to do that? thanks!

[AlejandroZ]

Hello.
I am assuming you have the data you are acquiring connected to a waveform chart which has the y-axis set to auto-scale, so you are expecting to be able to get the maximum value of your data by getting the maximum value of the range of the y-axis of the graph.

If this is what you want to do, you need to right-click on the graph, and select Create > Property Node. With the selection tool, click on the property node and select Y-Scale > Range > Maximum. Connect an indicator to the output of that property node, and you will get the numerical value you are looking for.

You must note, however, that this value is just an approximation of the data point with the highest value (the maximum range is almost always a bit higher than the maximum data poin
t), and it is not always a very close approximation. Thus, it seems like a better idea to monitor the data directly, instead of the range of the chart.

If you are getting the data in an array (which I don't think so, since you are using a chart instead of a waveform graph, but anyway, just in case) you can use the "Array Max & Min" function located in the third row of the array functions sub-palette.

If you are getting the data one point at a time, you can have a shift-register with the highest point until now, and with each iteration compare the contents of the shift register with the new data point, using the "Max & Min" function located in the 3rd row of the comparison sub-palette, and connecting the "maximum" output of this function to the right side of the shift register. I am including an image showing what I mean.

Hope this helps.

AlejandroZ

[Tom Whitaker]

The most obvious way is to check the chart's history data (create a property
node for the chart and look under properties; its the last item listed).

TJ Whitaker


"NewMachine" wrote in message
news:506500000008000000BCEF0000-1079395200000@exchange.ni.com...
>i know this is a simple task and i remember learning about it in the
> labview basics class but couldn't find anything just now.....so i just
> have charts in a vi that are acquiring data from sensors and i would
> like show what the limits of those sensors are and take action such as
> limiting the voltage (to a drill) if something did reach a limit. can
> someone show me how to do that? thanks!

[NewMachine]

I don't understand how connecting the indicator signals to the user that the limit for the sensor has been reached. I most likely didn't explain that in my question when posting........what I want to do is check the sensor's data against limits that are pre-determined. Like if the current goes above 4 amps, also as shown on the graph then the graph goes above a red line and also can signal an alarm. Can I do this too? Thanks!

[NewMachine]

I wrote this on the comment above to the other person that helped but it also applies to your suggestion: I don't understand how connecting the indicator signals to the user that the limit for the sensor has been reached. I most likely didn't explain that in my question when posting........what I want to do is check the sensor's data against limits that are pre-determined. Like if the current goes above 4 amps, also as shown on the graph then the graph goes above a red line and also can signal an alarm. Can I do this too? Thanks!

[AlejandroZ]

Hello.
OK, I am attaching a VI in version 7.0, which you can hopefully open. If not, I am also attaching an image of its block diagram, where it is shown how you can compare data points (which are simulated as random numbers in the VI) with predetermined limits. The comparison nodes produce boolean values which you can then use to trigger whatever action you need to given the breach of the limits.
Hope this helps.
AlejandroZ

[johnsold]

I think what you are asking for is a multi-plot chart. Simply put the limits into a cluster bundle and feed them to the chart. I have attached a simple example.

I did not include any test to see if the data exceed the limits. That can be added with a simple > or < function.

Lynn

[NewMachine]

i only have labview 7...would you mind sending the vi in a LV7 format...if you can, i would like to look at it. thanks!

[johnsold]

Here is a LV7.0 version.