dynamic strain measurement

[indu]

Sir,

My experiment involves the measurement of strain on the specimen on the application of torque.

I'm using SC-2043SG for the quarter bridge ,with an internal excitation of 4.13V.

I'm using PCI-MIO-16E4 DAQ device.

The DAQ Driver used is NI-488.2 2.30

Measurement and automation explorer version is 3.1.1

The problem is that when i apply 1.5 Nm and 2Nm torque, both are showing the same strain, 243 microstrain.

Whereas in the digital multimeter they show .7mV and .9mV respectively.

The strain is supposed to vary , which is not happening for every 0.5Nm. The multimeter is showing a

variation of .1mV for every 0.5Nm increase in torque. But this is not being shown in the strain output. Only

when it reaches 3Nm it shows 486 Nm. But this remains the same even for 4Nm.

external signal conditioning is also not helping.

The automation explorer test run is always showing the same value about some -2.Its not at all changing inspite of the different toques applied

thanking u

[Neil_S.]

indu,

Thank you for posting to the NI Forums.  I am not sure exactly what the problem you are seeing is related to since there are many factors that could be involved.  However, I do have a few things you can try to troubleshoot the setup:

1. For quarter bridge completion you must insert a resistor into the SG-2043 SG in the corresponding socket of the channel you are using.  The value of the resistor should match the strain gauge's resistance.
2. Assure all fuses on the SC-2043 SG are currently operating correctly.
3. Verify that all jumpers are set to the correct positions for your application.  Refer to the manual (http://www.ni.com/pdf/manuals/371201a.pdf).
4. Confirm that the cables are and other pins are not bent or broken
5. Check to see if you are able to read voltages correctly on the PCI-MIO-16E-4 device.  Use a battery or some other known voltage to confirm this.

The SC-2043-SG is supported in both in NI-DAQmx and Traditional NI-DAQ (Legacy) drivers.  This may be a software configuration problem but from the symptoms it sound more like a hardware configuration problem.  I would first confirm that connections, settings and resistor values are correct.

You DAQ driver version is shown under Software in Measurement and Automation Explorer (MAX).  NI-488.2 is actually a GPIB driver and not a DAQ driver.  Could you look in MAX and see what DAQ driver you have installed (Traditional NI-DAQ or NI-DAQmx).

Regards,

Neil S.
Applications Engineer
National Instruments

[indu]

The DAQ device i m using is PCI -MIO-16E4

The DAQmx Device driver version is 7.5 0f 1

the initial offset voltage that i m getting without applying any torque is .2mV.But the output strain is showing 0.  The problem is that the changes in the torque are not giving a constant output strain. its fluctuating between two values. im supposed to properly show the change in strain with the torque applied.

how will i do it?

its supposed to properly discriminate between the .5 Nm change in torque

im attaching the program i made for my application.kindly see through it

thanks a lot

[indu]

hi Neil,

Does it mean that i don t need to make the bridge completion separately?

actually i had already made a full- bridge using 3 more strain gauges , with the fourth one being the active strain gauge...So if i connect the bridge completion resistor for quarter bridge connection to the socket, i don't need the external SG s right?

thanks

[indu]

Hi ,

i have only the 120 Ohm Quarter bridge completion resistor from NI.

But the active strain gauge that i have used for my measurement is a 350 Ohm resistor. Is it alright to use   a  350 Ohm resistance by series connection using the normal resistances for this purpose?

thanking ,

indu

[TheWoost]

Hi indu,

For information about the proper connections for quarter-bridge completion, please refer page 3-9 of the SC-2043-SG User Manual found here.  The completion resistor will need to be as close to the value of your strain gauge as possible.

You mentioned that you are using NI-DAQmx 7.5, but in your VI you are implementing the Traditional NI-DAQ function calls, and they are not fully configured.  I would recommend using the NI-DAQmx DAQ Assistant as a starting point for configuring your strain gauge measurement task.  This is found in the LabVIEW functions palette under Measurement I/O>>NI-DAQmx.  One thing to make sure of is that you have the SC-2043-SG configured as the accessory that is connected to your PCI-MIO-16E-4.  You can do this by opening Measurement and Automation Explorer and right-clicking on the device under Devices and Interfaces>>NI-DAQmx Devices and selecting "Properties".

Before you begin programming in DAQmx, I would highly recommend checking out the Developer Zone article Getting Started with NI-DAQmx: Basic Programming with NI-DAQmx.

Regards,
Andrew W
National Instruments

[indu]

Hi Neil,

I' ve used the traditional NI DAq version 7.4 0f0.

I had first connected to active strain gauge to a bridge completion circuit using 3 other SGs to form a full bridge.  And then i have tried the method specifeid in the SC-2043-SG manual  by following these steps.

•   SW1 for power supply
• W1 and W3 For excitation
• W8 for excitation
• W2 for channel 0 input
• W7 for HB for channel 0
• placed  the 120 Ohm bridge completion resistor in the socket R52
• connected the lead wires from the active strain gauge to the +EX0 and +CH0 screw terminals.

I have connected nothing additional to what i have specified here.

I'm getting the same kind of output for both methods.

The outputs are fluctuating between values.

I don't know what is wrong in my configuration.Plz help. 

 i'm attaching my program once again and the results of the torques applied in the order 1Nm,1.5Nm,2Nm,2.5Nm,3Nm,2.5Nm,3Nm,3.5Nm,4Nm

Thanks a lot

[indu]

Hi Neil,

I' ve used the traditional NI DAq version 7.4 0f0.

I had first connected to active strain gauge to a bridge completion circuit using 3 other SGs to form a full bridge.  And then i have tried the method specifeid in the SC-2043-SG manual  by following these steps.

•   SW1 for power supply
• W1 and W3 For excitation
• W8 for excitation
• W2 for channel 0 input
• W7 for HB for channel 0
• placed  the 120 Ohm bridge completion resistor in the socket R52
• connected the lead wires from the active strain gauge to the +EX0 and +CH0 screw terminals.

I have connected nothing additional to what i have specified here.

I'm getting the same kind of output for both methods.

The outputs are fluctuating between values.

I don't know what is wrong in my configuration.Plz help. 

 i'm attaching the results of the torques applied in the order 1Nm,1.5Nm,2Nm,2.5Nm,3Nm,2.5Nm,3Nm,3.5Nm,4Nm

Thanks a lot

[TheWoost]

Hi indu,

All of your connections seem fine, but I have a few things to point out:

1.  You have indicated that you are configuring SW1 for an external power supply.  Have a verified that this external supply is providing a stable +5V level and that you have connected that supply correctly to connector J8?
2.  You mentioned earlier that your strain gauge is 350ohm, but you have placed a 120ohm completion resistor in socket R52.  The quarter bridge completion resistor must be equal in value to the external strain gauge element.
3.  In your first post, you say:  "I'm using SC-2043SG for the quarter bridge ,with an internal excitation of 4.13V."
Using the onboard excitation only provides up to 2.5V excitation, so it is correct that you have configured jumpers W1 and W3 for an external excitation source.  Have you connected the external excitation source to connector J7 and verified that it is providing a stable 4.13V excitation voltage?

Regards,
Andrew