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Toggling IEPE on/off for 9234 voltage

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I am using this hardware: NI USB9162 or cDAQ9172 with NI 9234 module. I want to be able to select wether to use regular accelerometers or IEPE accelerometers with the 9234. I have built a LabVIEW Vi where I want to be able to configure the accelerometer channel as a voltage channel and later input sensitivity data. Here are my experiences:

  1. If I use "Custom voltage with exitation" I get the message "No supported devices found". This happens in MAX "create task", labview DAQassistant or "DAQmx create channel"+"custom voltage with exitation".
  2. If I use "DAQmx create channel" (with or without exitation) and set exitation source and value as a property I still have the same problem
  3. If I do the same as in 1, but with acceleration I have no problem setting Iex source and value. But this is not what I want.
  4. If I use "IEPE enable" inside the test panels of MAX I have no problem using the IEPE accelerometer and get the voltage
Can somebody help me figure out how I could enable an IEPE accelerometer as a voltage source?
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Message 1 of 12
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The NI 9234 in DAQmx does not support the "Custom Voltage with Excitation" channel type.  Available options are Voltage, Sound Pressure:Microphone, Accelerometer, and Position:Eddy Current Proximity Probe. 

 

If you want to configure a voltage channel and then enable/disable excitation later, you can do this.  First create a voltage channel.  Next you'll need to configure the excitation source (AI.Excit.Src in the Labview API) and the excitation value (AI.Excit.Val).  You will probably want to configure the coupling as well (AI.Coupling). Coupling on the NI 9234 for a voltage channel defaults to DC.  If you enable excitation you will have to configure the coupling to be AC.

 

I don't believe the DAQ Assistant gives you enough control over the excitation property to soley use it to configure your task but it's a good starting point.

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Message 2 of 12
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I don't use daq assistant and I have ruled out votage with excitation (1 in my list)

 

"If you want to configure a voltage channel and then enable/disable excitation later, you can do this.  First create a voltage channel.  Next you'll need to configure the excitation source (AI.Excit.Src in the Labview API) and the excitation value (AI.Excit.Val).  You will probably want to configure the coupling as well (AI.Coupling). Coupling on the NI 9234 for a voltage channel defaults to DC.  If you enable excitation you will have to configure the coupling to be AC."

 

This is what I was what I was talking about in 2): Voltages source without exitation, with properties settings AI.Excit.Val=internal, AI.Excit.Val=0.02, AI.Coupling=AC and AI.Excit.VoltageOrCurrent=Voltage.

 

I get this message:

----------------------

Error -200077 occurred at Property Node DAQmx Channel (arg 3) in FRF filtrert.vi

Possible reason(s):

Measurements: Requested value is not a supported value for this property.

Property: AI.Excit.DCorAC
You Have Requested: AC
You Can Select: DC

Channel Name: Dev2/ai0

Task Name: _unnamedTask<11>

----------------------

If I change do DC instead I get:

----------------------

Error -200077 occurred at Property Node DAQmx Timing (arg 1) in FRF filtrert.vi

Possible reason(s):

Measurements: Requested value is not a supported value for this property.

Property: AI.Excit.VoltageOrCurrent
You Have Requested: Voltage
You Can Select: Current

Channel Name: Dev2/ai0

Task Name: _unnamedTask<9>

---------------

 

This is strange for a device like 9234, which should be able to measure current. I find it even more strange since I am (as I said in 4) able to set IEPE in test panels and get the voltage.

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Message 3 of 12
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A little misprint - The 9234 should be able to measure voltage
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Message 4 of 12
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The method I suggested in my previous post is correct.  You're trying to manipulate different properties, which is why you are seeing errors.

 

From your last post:

 

Property: AI.Excit.DCorAC
You Have Requested: AC
You Can Select: DC

 

Property: AI.Excit.VoltageOrCurrent
You Have Requested: Voltage
You Can Select: Current

 

The actual IEPE excitation is 2mA, which is why the excitation needs to be set to current and AC/DC needs to be set to DC.  But for the 9234 you don't have to manipulate those properties--the defaults are fine and you can't change them anyway.

 

You want the coupling of the module, not the excitation coupling.  This is AI.Coupling.

 

You also want to be able to enable and disable the excitation.  Use AI.Excit.Src and AI.Excit.Val.

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Message 5 of 12
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Here's a quick snippet of code explaining the properties I'm talking about.

Message Edited by MarkGrot on 04-21-2009 11:13 AM
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Message 6 of 12
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What I was describing was exactly how you are showing it in your picture, the only difference being that I have added the AI.Excit.VoltageOrCurrent which is voltage default.

 

When I try running the VI I get this message:

Possible reason(s):

Measurements: Requested value is not a supported value for this property.

Property: AI.Excit.DCorAC
You Have Requested: AC
You Can Select: DC

Channel Name: Dev2/ai0

Task Name: _unnamedTask<20>

 

What I think might be the problem is that I use ai0 and ai1, where ai1 is the channel I want to use as IEPE. I thus put the property node after the Create Channel node for ai1, see picture. But the properties seems to be reflected to ai0 as well...

Do you have any solution to this?

 

 

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Message 7 of 12
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Solution
Accepted by topic author Even

The problem is that you are using AI.Excit.DCorAC.  This is not the right property.  AI.Excit.DCorAC has to be set to DC for the 9234, the default.

 

If you want to configure the coupling of the channel, you need to use AI.Coupling.  You can get to this by Analog Input->General Properties->Input Configuration->Coupling

 

If you want to configure each channel independently, use Active Channel in your property node to select a channel.  All properties you configure in that node will only apply to the specified channel.

Message 8 of 12
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That made everything work - Thank you very much for your patience!
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Message 9 of 12
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Hello, Im having similar problems, the main difference is that Im using 3 PCB IEPE Piezotronics pressure sensors (112A21) and 1 PCB IEPE accelerometer.

The setup is similar to the one presented in the forum by using DAQ chasis and NI9234 and NI9237 cards:

 

AI voltaje (NI9234) +-5 volts pseudodiferential

AI.coupling : AC

AI.Excit.Scr: internal

AI.Excit.Val: 0.004

AI.Excit.VoltajeOrCurrent : current

Complete setup in the attached figure.

 

There are a few coments and questions:

 

Comments:

1. Im trying to measure pressure inside a tank due to the movement of the tank (harmonic motion) sometimes there are waves which impacts on the tank walls and generates impact pressure signals composed by a hidrostatic and dynamic pressure. Besides the PCB sensor there are another Piezoresistive sensors (Kulite Ktel-90) connected to NI9237. The sensors without water must report 0 mbar pressure and then increase and decrease when the wall impacts on it. The point is that the piezoresistive (Kulite) are able to catch perfectly the pressure but the PCB ones, which are working with the detailed configuration are reporting weird results. This results ,as can be appreciate in the attached figure, are:

- There is a strong bias in the pressure signal (green signal), zero pressure (dry sensor) is not correct.

- There is an weird signal during the time the sensor is dry and its magnitude is more than 60 mbar .

- Bias is not constant, differs in each cycle

- By applying an offset to a portion of the signal the magnitude of the pressure peak is similar to the one catched by the kulite (piezoresistive) sensors.

 

2. If I understood correctly, when you measure IEPE sensors and the phenomenon is slowly the DC coupling is the best.  Im using AC coupling because its necessary excitate the sensors between 2-20 mA. NI9234 doesnt permit activate IEPE and activate DC couplin, but I still dont know if the DC coupling will solve the problem of the BIAS etc.

 

3. Chasis is connected correctly to the ground, accelerometer sensor is reporting a good measure of the vibration wihtout bias, zero is perfect, etc.

 

Questions:

 

1. How I can get a signal similar to the one registered by the piezoresistive sensors?

2. There is something wrong in the setup?

 

Thanks in advance for your help.

 

 

 

 

   

 

 

 

 

 

 

 

 

 

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Message 10 of 12
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