I can not measure strain gauge

[immortal]

I'm trying to work for strain gauge measurement with scc-sg04, sc2345 signal conditioner and ni-6223 DAQ card. Internal +2.5V supply I have used as excitation voltage. I want to get the data via Measurement & Automation explorer and I have created a task for strain measurement. I have selected the SCC1Mod1/ai0 channel for this purpose and adjusted task timing to Continuous mode. When I have pushed the Test button, Analog Input Test Panel is shown and observed strain value is -0.0021. This value do not changes while I pushing my finger to the load cell.

With +2.5V excitation voltage can I see the any changes for output (note that manufacturer recommends 10V excitation voltage).

Message Edited by immortal on 11-12-2007 02:11 AM

Message Edited by immortal on 11-12-2007 02:12 AM

[Nick_F1]

Hi Immortal,

Thank you for posting on the National Instruments forums.

With a load cell, it needs the excitation voltage that the specifications states or it will not behave properly.  Is there a reason you cannot provide a 10v excitation to the load cell? 

When you are testing the load cell, you should also go into the task and click on the run at the top.  That way it will take into account the excitation.  If the load cell is working, then the voltage should change in the Test Panel, but you will get a better reading by looking at the output from the task.

[Wilfried_71]

The supply voltage is not the problem : 2.5 V is suffisant to see the variation, the accuray is just a little bit smaller. Keep in mind that you measure strain, so it's a very small values. Generally, we speak about microdeformation and not deformation (we multiply the values by 1 million) to have big enough variations .... Moreover, if you see the range of your signal, it's +/- 0.001 ! (1m to -1m) ... When you have a strain of 1 % (so 0.01) , it's already a big value ...

Bye

[immortal]

thank you for your reply

I use 2kg load cell in my work and pushing it with my finger intensively. But there is no change in the obtained value. Should I take the change in the observed data from load cell in this case?

Note that, I have seen that the value of -0.0021 is not produced by load cell that I used because I have broken the part of SC2345 off from 6221 and the same value has encountered.

I'm waiting your precious replies.

Kemal

[Wilfried_71]

You didn't see the variation at all ? I repeat that the changes must be in this range : from around -0.001000 up to 0.001, maybe less !

And be carefull with your sensor : if the maximum load is 2 kg, you can damage it with your fingers !!

[Nick_F1]

Hi Kemal,

Can you verify that the voltage that you are reading from the DAQ card is correct? Can you take a handheld multimeter and measure the voltage on the load cell and make sure that you are getting the same readings as from the DAQ card.  It's possible that you are connecting the load cell incorrectly.  Are you using differential, RSE or NRSE for the channel terminal  configuration?  If you are using Differential and reading off of channel 0, you will have to use channel 0 and channel 8 for the two channels of your load cell.  If you are using RSE, you will need to connect one side of the load cell to channel 0 and the other side to AI GND.  If you are using NRSE, you will need to connect one side of the load cell to channel 0 and the other side to AI SENSE.

Please check these things and let us know what you find.

[immortal]

 Dear Nick and Wilfried,

I have not worked on this subject for a long time. Sorry for I haven't answer to you earlier. I have seen strain data at last. My problem was encountering from Load cell connection failure. I've added a figure that shows the observed data. As I told you before, my excitation voltage is 2.5V and amplitude of changes is very low. Recommended excitation voltage of our load cell is 10V and tmaximum is 15V. I want to supply load cell from external voltage source with 10V excitation. I'll connect output of the voltage supply as input to load cell. SCC-SG04 will only have ACH(-) and ACH(+) inputs. Is this configuration correct for external excitation?

The other question is, How can I write the acquired strain via Cont Acq Strain.vi in Labview to a text to use with other softwares.

Ant the other :), is there any option to change the input configuration (diff., RSE or NRSE)?

Thank you for your attention.

[immortal]

I've connected 10V DC supply to load cell but I see 50Hz electric line frequency this time and amplitude is very high. I want to use a  high pass filter to prevent this defect, but I'm not sure this will solve my problem.

[Hani R]

Hi,

Connecting the strain gauge using external excitation will work.  You will have to connect the power supply across the bridge (VEX- and VEX+) as shown in the image.



If the external excitation source is not ground referenced, connect its negative terminal to VEX-. If the external excitation source is ground referenced, don't connect the negative terminal to VEX- because a ground loop could damage the SCC-SG04.

The easiest way for you to save data into a text .lvm file (which can be opened in any text editor) is to use the Write to Measurement File VI.  Place the VI outside of the While Loop if you do not need to process the data as it is being acquired. 

Lastly, you cannot change the input configuration setting of the SCC-SG04.

I hope this helps.


Message Edited by Hani R on 12-12-2007 06:05 PM

Message Edited by Hani R on 12-12-2007 06:06 PM

[immortal]

Dear Hani,

Thank you for your reply firstly.

I connect external power as you give but there is some problems I have encountered. I connect cables of DC voltage supply to input of the load cell (DC supply is not running) but acquired data (there should be only noise ideally in this state) is fluctuating (See q3_1.jpg). After, I run DC supply and there is a sinusoidal wave in this phase (See q3_2.jpg). If I push my finger to load cell, DC value of this sinusoid changes only. So, I have a big problem with my external excitation supply :(.

The other problem I've encountered is Vex value of external excitation. I change Vex source to external and Vex=10V and my task is not working and gives an error as "requested value is not supported value for this property" (See q3_3.jpg). I couldn't understand why this error occurs. 

Waiting your reply...