Fall 2012 ME 395 - GSI Josh Lacey

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 Force Measurement Question

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sinalex



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Join date : 2012-09-12

PostSubject: Force Measurement Question   Fri Sep 14, 2012 7:25 pm

What are we comparing for the Instron & Load Cell (do we need Load Cell stuff?) Machine? We have our data in term of voltages, and we have the force data from the DRO, but we dont know what to do with it.
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GSI Overlord
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PostSubject: Re: Force Measurement Question   Sun Sep 16, 2012 1:14 pm

When in doubt, always refer to the task letter about what to do. You have two sets of voltages that the LabVIEW interface recorded, one of which is for the strain gauge, and one of which is for the Instron load cell. The task letter wants you to assess the linearity and uncertainty of each of the measurement devices...so as it turns out, you may actually not need that force measurement. You are essentially just being asked to compare the calibration curve of each device, which is just an input (the weight you hung on it) and and output (the recorded voltage). The mark of a 'good' digital measuring device is something that produces an output which is linear with the input and minimizes error and uncertainty.
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Bob Kalmbach



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Join date : 2012-09-13

PostSubject: Re: Force Measurement Question   Sun Sep 16, 2012 9:07 pm

The spec sheet and the Instron website list accuracy as % of rated output, but the output is given as a force. How can we determine the rated output in voltage?
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tuesday ninja



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Join date : 2012-09-15

PostSubject: Re: Force Measurement Question   Mon Sep 17, 2012 11:02 am

Are we to use the accuracy of the strain gauge load cell as cited in the "Basic Measurements" pdf?

It is cited as "most force transducers (also called load cells) can be calibrated to an accuracy of ±0.3% using known weights."

And the Instron accuracy is cited as "equal to or better than 0.025% of the load cell rated output or 0.25% of the indicated load, whichever is greater." Does this mean the accuracy error is 0.025% of the voltage reading or 0.25% of the force reading?

In addition, where can I find information on the number of AC converter bits and the FSR values of both cells used to measure the two voltages? Or is this not relevant when calculating the resolution error for this part of the lab?
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GSI Overlord
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PostSubject: Re: Force Measurement Question   Mon Sep 17, 2012 2:04 pm

So, the actual output of the Instron load cell device that is being measured by the data acquisition system is the voltage output...that force measurement only appears on the LED screen and is not the physical signal output by the instrument (the conversion from voltage to that force measurement is done internally). Remember what the task letter said...you are actually finding a calibration curve for both the Instron load cell and the strain gauge, so there really are only two potential sources of accuracy error here:

1) If you somehow biased the data (i.e. forgot to zero the instrument at the beginning, but I tried to make sure every group did this part correctly to remove any spurious offset from the data)
2) The test weights have an accuracy associated with them (i.e. 1 kg isn't 'exactly' 1 kg)

So actually, quantifying the accuracy error isn't so difficult as you think.

The information that you'll need to calculate resolution is on CTools in the MBQSpecsheets file that talks about the data acquisition system.
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tuesday ninja



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PostSubject: Re: Force Measurement Question   Tue Sep 18, 2012 3:58 pm

The way I'm going about creating the calibration curve is I will plot the mass of the weights along my x axis and the voltage readout on the y.

As for error, the horizontal axis will only contain the accuracy error of the weights, which was 0.1% of the mass.

The error on the y axis I am still having trouble with. I'm going to assume no accuracy error because we did zero our instrument at the beginning. The precision error I am going to take as twice the value of the standard deviation of the mean because the voltage value outputted is the average of a set of measurements. For the resolution error, the MBQ spec sheet only contains FSR for the Instron Load Cell, ±10 kN. Nowhere is there information on the bit resolution of the instruments used in the force measurement lab. In fact there is no information at all on the strain gauge cell. How am I to calculate resolution error for the two instruments?
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GSI Overlord
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PostSubject: Re: Force Measurement Question   Tue Sep 18, 2012 5:37 pm

You are correct to assume there is no accuracy error on the voltages. You do have a std. dev. that was saved, so you can get a precision error. You can actually check the 'Instron load cell model' topic for discussion regarding the resolution of the load cell and strain gauge.
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