Analog Output signal too high for first second of output. - Pxi-8336

[MikeHahn]

I'm having trouble with the analog output signal from our PXI-8336.

The analog output should be a smooth signal (which is set by a labview program). Right at the beginning of the output the analog output gives 5 V  or more for about a second. After 1 second the output begins to decay to the programmed signal and by about 2.5 seconds matches the programmed signal.

The labview program generates an output signal, which should be a smooth signal, say a ramping analog output from 0 - 5 V. Using Debugging tools I've examined the waveforms generated by the program and shown that the signal is correct up to the point where the signal is input into the "task". Also, the program worked correctly in the past and has not been changed. It seems to me that the problem cannot be software related.

Any ideas on how to resolve this problem?

Thanks,

Mike

[Simran_K]

Hi Mike,

Can you please tell me what analog output device you are using?

Thank you,

Simran K

[MikeHahn]

the analog output device is the PXI-6071E

my mistake in posting the title as pxi-8336, i looked at the wrong card.

Everything is in a pxi-1042 chassis.

[Corby_B]

Hello MikeHahn,

Thanks for your post. I see that you have a program that used to work and now does not function as you expect. How much time should your voltage ramp that your program makes take?

So if you go into Measurement and Automation Explorer and open a test panel for the 6071E. If you set a voltage output level to say 1 volt and then move the slider to 5 volts, do you see the same slow response that you do in your program? Does this same thing happen on other AO channels? Let us know your results as this might lead us to a better solution!

Cheers!
Corby_B

[MikeHahn]

Thanks for your response,

The amount of time the programmed voltage ramps is variable, usually around 10 - 100 seconds.

The issue is not so much the slow response or the exact form of the programmed voltage as the fact that the initial voltage on the analog out is high. Using Measurement and Automation Explorer I have set the voltage to be 0. If I move the dial around, say from 1V to 5V it responds quickly. The same thing happens on other analog out channels on the same card (pxi - 6071E ).

I have noticed that the hight output voltage from the analog out is the same that it was set to at the end of the last run of a program. A partial solution to the problem seems to be to modify the labview program so that the output signals always go to zero at the end. This solution is unsatisfactory for several reasons - first I don't always want to have to drop the output to zero at the end of the program, and second if there is some other error so that the program must be stopped or a computer error occurs the voltage of the analog out will not be reset to zero.

It seems to me that in the absence of a programmed input the voltage at the analog output should be the default voltage set in Measurement and Automation Explorer (in my case zero Volts). The analog output responds to changes from Measurement and Automation Explorer, but apparently is not retaining those settings.

[Corby_B]

Hello MikeHahn,

Thanks for your post back!

You are right and the best way to ensure that your output channels are not at the last voltage you set, for example 5 Volts, is to clear it by writing 0 volts to the port when ending the program. The reason for this is that when you set an AO channel to a voltage you are simply updating the channel so if there is no communication from the computer to tell it to change it does not. LabVIEW is just simply sending commands to the hardware to tell it what to do. A similar situation would happen if the computer were to crash or hand, there would be no communication to the device to tell it to reset all ports. So resetting the device in MAX or ending your program with a value of 0 on the AO ports is the best way to ensure that your outputs are not at a voltage level that is undesired. 

Please post back if you have any other questions!

Cheers,

Corby_B
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