Data acqusition with Labview using these specific cards

[Akash Kumar]

hi,

I am working on a machine of expansion and compression, I need to interface this machine with the computer using Labview. The machine consists of sensors to measure the force and the amount of extension or compression. Well for the acquisition of data I am using a
1. Card PCI 6220 ( inside the computer)
2. SCXI  1520
3. SCXI 1000
4 SCXI 1349
 
The last three cards are used to gather data from the sensors and are then connected to PCI 6220

Well I dont have any idea how should I start building my acquisition in Labview, It would be really gratefu if you could guide me for building up an acquistion diagram on Labview.
There are 6 sensors 2 for measuring the length and 4 for Force.
Out of which we use one sensor for length and one sensor for force at one time.

Any help from your side would go a long way.............................

Thanking You,
Akash

[smercurio_fc]

You may want to start here.

There are also lots of examples that ship with LabVIEW, and it also has a DAQ Assistant that can create code for you.

[Akash Kumar]

Hi, I went throgh that link and I made a labview program, However the problem that I getting is that the data the labview is showing is very unstable it is showing weird values and secondly I am using an internal exitation for SCXI 1520 I made the relavant changes in program, However I observe that 10V does not appear at the SCXI terminals,  and I believe that is one of the reasons for getting weird values.

Well could you please suggest me a solution to this problem of internal exitation..................................

Thanking You

Akash

[smercurio_fc]

Without knowing more about the way you've set it up, and exactly what "weird results" means, I can only speculate. You may want to look over these documents in terms of the excitation voltage: [links below removed; articles no longer available]
The Excitation Voltage from My SCXI-1520 Does Not Match Its Configured Value 
How Can I Test the Voltage Excitation Level of a SCXI-1520? 
Reading a Voltage Signal With the SCXI-1520 

[Akash Kumar]

Thanks a lot for all these links, I am quite confused at this point of time I have certain issues which might sound really silly but these have hv kind of totally irritated me

1. When I test the device using MAX explorer, and after that I test the internal exitation voltage using my voltmeter it is never 10 V rather it is some millivolts.

2. When I use the following VI : initialization_acq.vi ( as suggested by someone ), there is an exitation of 10V but the values it reads from the sensors are very unstable  and vary a lot.

3. If just after that I use this VI called untitled1.vi which is nothing but slight change in above VI I observe no exitation voltage.*

4 Now the interesting part is if now again I run the VI in part two i.e. initialization_acq.vi  I observe no exittation voltage.

 But if I run this after say 30min it shows the exitation voltage.

I have no idea whatsoever what is happening and it would be really grateful if you could solve this problem for me......

 Please find attached the two VI's initialization_acq.vi  and untitled1.vi

[Akash Kumar]

Hi, After my previous mail I also used a DAQ Assistant, It does create an internal exitation of 10V but the problem is that the output values vary a lot. I think one reason for such a case is tht the value at input that is output of sensors is in mV and it is because of that that there is variation in the output.

I know that SCXI 1520 is capable of adjusting the gain but I dont know how should I do it in Labview. The manual says that is possible to adjust the gain programabally but I dont know exactly how........ I am sending you my VI and please suggest me what changes could I do to make my output signal more stable

Akash Kumar

[smercurio_fc]

Unfortunately, I just don't have hardware in order to be able to help further. Your first set of VIs seems OK, other than you're not actually acquiring data, but I understand that you're just using them to configure the module. I didn't see anything obvious with the DAQ assistant, but I'm using a simulated SCXI-1520, so that's why it works for me. The only other thing I can suggest is to verify your wiring and to make sure you're not inadverdently causing noise to be added onto the lines. I.e., do the standard stuff of twisted pairs, etc.

In terms of your gain question, with DAQmx the gain is set automatically based on your input range.

NI frequently checks these boards, and I hope someone from the DAQ group chimes in here. You may want to move this over to the DAQ forum.

Sorry I can't be of further help.

[Jeremy_M]

Hi Akash Kumar,

Fisrtly, I would like you to verify the way you wired your sensors. Are you really in differential mode?

Secondly, could you explain precisely what you are using as sensors and what kind of measure you want to do.

I have changed your VI even if it has the same effect. You don't need to put a read task function after a DAQ assistant which already do it.

What is the purpose of the subVI?

Regards,

[Akash Kumar]

Hi,

Well I am using two sensors which are of the type full bridge and I am quite sure that the output I am from these sensors is in the differential mode. One sensor measures the force upto IKN and the othe sensor measures the expansion or contraction and both sensors generate a voltage of the order millivolts I have very well confirmed the functioning of the two sensors they produce very stable and accurate results.

Well my first problem is the internal exitation, which I dont know is solved or not because some times I see 10 V signal  when I check with the voltmeter and sometimes I dont when I run the same VI that I had sent to you. and my second query I have sent in as another thread but I would like to reqoute here:

"I am using SCXI 1520, I believe that it has an automatic gain adjustment. The problem is that I am using two channels out of the eight channels in the multiplexed mode. And the other six channels are just left opened. My problem is that when I receive the signal the out put of these two channels is very small and is very unstable whereas the output of the unconnectd channels is stable and is quite well amplified.

My question is that if I am not using the other six channels is it OK if I leave them open??, or if not that way is it possible that I give 0 exitation voltage to these six channels and and 10 V  to the two channels which I am using. ( bcoz exitation voltage is configurable but I dont know if for different channnels we can give different voltages)

What I see as a problem that the open circuited six channels generate very large input voltage compared to the two useful channels ( which receive signal from sensors in mv) and the gain adjusts itself according to these channels and and the useful channels remain unamplified.

Please tell me if my assumptions regarding SCXI 1520 are right or wrong??? and provide me with a solution so that I can obtain a good stable amplified signal  from the two useful channels.

Well the application software that I am using for the same is Labview."

Thanks,

Akash Kumar

[Jeremy_M]

Hi Akash Kumar,

Thank you for these informations. If you are sure that your are in differential mode, the other channels have no effect. I advise you to make the calibration of your SCXI 1520 module. Secondly, may you verifiy in your task configuration the range of input you set? Decreasing this setting will increase your amplification. And you can change the cut-off frequency of the low-pass filter to eliminate noise.

May you try to verify those parameters and make the calibration of you module?

Regards,