Maximum and Minimum values from a waveform chart

[Neil Sampson]

Hi there,

The problem I have is displaying the maximum and minimum power from a waveform chart. I have a Labjack U12 interface card, where I take in two voltages. One of the input voltages is converted to current by simply dividing the value by the small sense resistor in the external circuit that the voltage is across. I then calculate the power by multiplying the voltage by the current (P = VI) in LabVIEW. This power is then plotted with time on the waveform chart.

What I would therefore like to do is display the minimum and maximum power in an indicator, as the voltage and current are changed by varying the load in the external circuitry I have connected to the interface card.

I have tried using th
e Max&Min block from the Comparison function, but the values of the maximum and minimum power are the same.

Please find attached the LabVIEW version 6.1 file.

Thanks very much, Neil.

[Oleg]

Hi,
actually I have LV6.0 and I couldn't open your VI, but it sounds like you need to use "Function->Array->Array Max & Min" to determin maximal and minimal values from your power meter.
Another thing is if you use waveform GRAPH to represent your data you must wire it directly to "Array Max & Min", because GRAPH is simillar to array. But if you use CHART then you must wire its history to "Array Max & Min", because CHART is represented as scalar. To read chart history you must create its property node and select "Properties->History data" from its drop down menu.

Good luck.

Oleg Chutko.

[Dennis_Knutson]

What you need to do is accumulate the power into an array. Create a shift register and use the build array function to add the last power measurement to the array of all power measurments. Then use the Array Max & Min function on the array palette. Input the power array and the outputs are what you want to display. I've attached a modifid VI.

[Dennis_Knutson]

One problem that would arise if using history data is that it is limited to the chart history length. If the length is not long enough and the program runs for a while, early data gets dropped.

[chutla]

Salutations, Neil!

What you need to do is save the data points as a WAVEFORM, and then use the Waveform Max and Min node. YOu can get both the max and min values as well as their times with this node. (Times are referenced to data points)

ERic

[Neil Sampson]

Hi Dennis,

Thanks very much for taking the time out to help me on this problem. Your solution is perfect! Keep up the good work,

Best Regards,

Neil.