Jagged wavy singal by 9237 at no load condition

[ksrkm]

Dear forum users and NI employees,

I would be thankful to you if you could resolve my following problem. My test specimen is a simple plastic rectangular piece of 1 inch by 0.25 inch cross section with a 8 inch length. I am trying to measure strain (with the help of a general purpose TML, Japan strain gage of 1 mm gage length) in the sample by hanging dead weight to the sample. I am measuring the strain with the help of cDAQ 9178 chassis, NI 9237 strain module with quarter bridge completion accessory NI 9944. For a given dead weight (thus the load applied becomes a static one), the signal (output voltage) should remain constant independent of time. Further, I also expect that when no load is present on the specimen, the above DAQ system is expected to show constant but nearly zero output strain with time. What my problem that after carrying out the proper offset nulling and shunt calibration with the help of DAQ assistant of LABView, the above DAQ system shows a jagged wavy output strain of significant variation between maximum and minimum value even when the sample is at no load condition (the sample simply lying on the table). Further, even after loading the sample with certain amount of dead weight, instead of getting a constant signal, I am still getting a jagged wavy output strain (with more magnitude than zero load position) with time. Please help me in getting constant signal output for the above DAQ system with and without load on the sample.

Thanking you

KSRKM

[johnsold]

Please provide more information:

What is the sampling rate, fs? What is the apparent frequency of the wavy signal? What is the amplitude of the signal? Is the amplitude proportional to the load? Does the frequency vary?

Do you see the same signal if you connect a fixed resistor of similar value in place of the strain guage? Do you see the same effect if the strain gauge is mounted to a different type of material (metal rather than plastic)?

Are there any air currents near the sample?

Lynn

[ksrkm]

Dear Lynn

Thank you for your prompt reply.

1. The aquisition is continuous, no. of samples to read = 100 and sampling rate is =100. Another interesting thing we observed is that whether load (dead weight)  is applied or not the output strain values (with wavy nature) vary if I set the no. of samples to read = 2 and sampling rate is = 2. Please correct me if I am wrong, as the signal should remain a constant with the time (for the type of load given in the present case) the singling rate should not effect the accuracy of the output.

2. The apparent frequency is about 2Hz.

3. The amplitude of the signal at no load condition is around 30 micro strain (minimum valus in the wavy line) to 50 micro strain (peak value in the wavy line)

4. Yes. The amplitude is proportional to the load if load (if applied)

5. To the best of my observation, the frequency is aboutn 2Hz only and does not vary much.

6. I have not observed the behavior by connecting another fixed resistor of the similar value in place of the gage

7. Yes. I see the same effect on the metals also.

8. There are no air currents near the apparatus and I have taken every care to eliminate noise in the surrounding area.

Please let me know if you need further details

Looking forward to your reply

With sincere regards

KSRKM

[johnsold]

1. I suspect that you have a high frequency signal which is being "aliased" into your data.  The most likely suspect is the power line frequency. The way I read the specifications for the NI 9237, you cannot get sampling rates of 100 or 2 samples per second.  With the internal timebase the minimum sampling rate is 1.613 kS/s.  Does your code which sets the sample timing generate any errors or warnings?

2. If the signal is an alias, the actual frequency may be n*fs +/- m*fa. Where n and m are integers, fs is the sampling rate, and fa is the actual frequency of the signal. For example if the actual signal frequency is 60 Hz and the sampling frequency is 100 Hz an alias would appear at 40 Hz. For a 2 Hz alias the interfering signal will be close to the sampling rate or a harmonic.

5. Amplitude proportional to load sounds less like an alias. 

6. I can interpret your answer in two different ways: You have not run an experiment with a fixed resistor. Or when you run it with the fixed resistor the wavy signal does not occur. Which interpretation is correct?

Try running it with the sample rate set to 2000 samples per second. This should result in any frequency less than 1000 Hz being measured correctly without alias.  If you can post the data, I will take a look at it.

Lynn

[ksrkm]

Dear Lynn

1. The sampling rate 100Hz was suggested by NI engineer when he came for demonstration of the equipment. From that time onwards this figure has not been changed in my program. Secondly, I do not get any error/warning massage either with 100Hz or 2Hz.

2. The first interpretation is correct. no experiment has been conducted so far with a fixed resistor.

3. Ok. I will run the experiment with 2000 Hz. In such case can I set number of samples to read to be any convenient number?. I wish to keep less number so that the output file does not become too big to handle.

4. I shall post you both 100 Hz data and 2000 Hz data. Is this ok?

Sincerely

KSRKM

[johnsold]

The number of samples can be any convenient value. The minimum should probably be about 0.1 second (200 samples) and one second is plenty long enough. Posting both data sets might be useful.

Lynn

[ksrkm]

Dear Lynn

Please give me two or three days to get back to you with data.

Sincerely

KSRKM

[ksrkm]

Dear Lynn

I want to post 24 files of various types in response to your previous reply. However the this web page is not allowing me to post more than 3 files and moreover, I am also unable to upload a rar file. Please suggest me.

KSRKM

[johnsold]

The Forums will accept zip files. A less desireable method is to add an acceptable extension (such as .txt) manually. If you do that please explain what the real fiel format is.

Lynn

[ksrkm]

Dear Lynn

Please find test data (zipped with extension *.rar but appended extension *.rar.txt to send the files) as discussed and note that

1. Experiments have been conducted for two cases: Case A: sampling rate = 100 Hz and number of samples to read = 100 and Case B: sampling rate = 2000Hz and number of samples to read = 500.

2. For each case, output from two strain gages have been measured and stored when the specimen (a) is under no-load condition (ie., just after offset nulling and shunt calibration is carried out) and (b)  is loaded with weights of mass 3.092Kg and 5.092Kg.

3. The first column in the data sheets is a voltage singal which we use it for simultaneous force measurement from the loadcell. The second and third columns in each data sheet corresponding to the data for two strain gages. You may please omit the details in the column one as we did not connect the corresponding measurement module to the loadcell.

4. The following are the various results files corresponding to the Case A i.e., when the sampling rate = 100 Hz and number of samples to read = 100. The names and the corresponding content of the these files are as follows:

• CaseA-NoLoad-Calb-100Hz-100samp.jpg:                  Screenshot at the end of offset nulling and shunt calibration when load is zero
• CaseA-NoLoad-Signl-100Hz-100samp.jpg:                 Screenshot of form of two strain signals when the specimen is in no-load condition
• CaseA-NoLoad-raw-100Hz-100samp.lmv:                    Raw output data for 22 seconds during no load condition
• CaseA-NoLoad-Sorted-100Hz-100samp.xls:                Excel datasheet of the above file without unneccessary text material
• CaseA-NoLoad-avg-100Hz-100samp.xls:                      Excel datasheet of the above file with averaged data for each second
• CaseA-Load3.092Kg-Raw-100Hz-100samp.lmv:         Raw output data for 30 seconds when load on the specimen is 3.092 Kg
• CaseA-Load3.092Kg-sorted-100Hz-100samp.xls:       Excel datasheet of the above file without unneccessary text material
• CaseA-Load3.092Kg-avg-100Hz-100samp.xls:             Excel datasheet of the above file with averaged data for each second
• CaseA-Load3.092Kg-Signl-100Hz-100samp.jpg:         Screenshot of form of two strain signals when the specimen is loaded with 5.092 Kg
• CaseA-Load5.092Kg-Raw-100Hz-100samp.lmv:          Raw output data for 26 seconds without any modifications
• CaseA-Load5.092Kg-sorted-100Hz-100samp.xls:        Excel datasheet of the above file without unneccessary text material
• CaseA-Load5.092Kg-avg-100Hz-100samp.xls:             Excel datasheet of the above file with averaged data for each second

5. The following are the various files for Case B:

• CaseB-NoLoad-Calb-2000Hz-500samp.jpg:                 Screenshot at the end of offset nulling and shunt calibration i.e., when load is zero
• CaseB-NoLoad-Signl-2000Hz-500samp.jpg:                Screenshot of form of two strain signals when the specimen is in no-load condition
• CaseB-NoLoad-Raw-2000Hz-500samp.lmv:                Raw output data for 26 seconds during no load condition
• CaseB-NoLoad-sorted-2000Hz-500samp.xls:              Excel datasheet of the above file without unneccessary text material
• CaseB-NoLoad-avg-2000Hz-500samp.xls:                   Excel datasheet of the above file with averaged data for each second
• CaseB-Load3.092Kg-Raw-2000Hz-500samp.lmv:      Raw output data for 30 seconds when load on the specimen is 3.092 Kg
• CaseB-Load3.092Kg-sorted-2000Hz-500samp.xls:    Excel datasheet of the above file without unneccessary text material
• CaseB-Load3.092Kg-avg-2000Hz-500samp.xls:         Excel datasheet of the above file with averaged data for each second
• CaseB-Load5.092Kg-Raw-2000Hz-500samp.lmv:      Raw output data for 40 seconds when load on the specimen is 3.092 Kg
• CaseB-Load5.092Kg-sorted-2000Hz-500samp.xls:    Excel datasheet of the above file without unneccessary text material
• CaseB-Load5.092Kg-avg-2000Hz-500samp.xls:         Excel datasheet of the above file with averaged data for each second
• CaseB-Load5.092Kg-Signl-2000Hz-500samp.jpg:     Screenshot of form of two strain signals when the specimen is loaded with 5.092 Kg

I hope the above files may provide you the complete information regarding my problem.

Sincerely

KSRKM