Precision and uncertainty of video-extensometer
Precision and uncertainty of video-extensometer
(OP)
Hi all
This is my first post after a few years absence from Eng-Tips. Hopefully I'm posting in the correct forum...
I'm new to extensometer measurements so I wanted to find out more about the measurement uncertainty of my video-extensometer.
Performing the uncertainty calculations described in a paper available from the optics manufacturer of the extensometer system, I get an measurement uncertainty in the order of 1 millimeter for measuring the position of an object.
The supplier of the extensometer system states the precision to be in the order of 10 micrometer for the strain measurement.
How this is possible?
Am I way off believing that the uncertainty needs to be at least one tenth of the measurement precision?
Maybe there's another paper "out there" explaining this?
Is it all due to the extensometer software performing some image processing techniques which reduces the measurement uncertainty?
Any input on this is most welcome.
Regards,
Joacim
This is my first post after a few years absence from Eng-Tips. Hopefully I'm posting in the correct forum...
I'm new to extensometer measurements so I wanted to find out more about the measurement uncertainty of my video-extensometer.
Performing the uncertainty calculations described in a paper available from the optics manufacturer of the extensometer system, I get an measurement uncertainty in the order of 1 millimeter for measuring the position of an object.
The supplier of the extensometer system states the precision to be in the order of 10 micrometer for the strain measurement.
How this is possible?
Am I way off believing that the uncertainty needs to be at least one tenth of the measurement precision?
Maybe there's another paper "out there" explaining this?
Is it all due to the extensometer software performing some image processing techniques which reduces the measurement uncertainty?
Any input on this is most welcome.
Regards,
Joacim





RE: Precision and uncertainty of video-extensometer
As far as uncertainty, no - you can have the precision of the instrument be much greater than its uncertainty. One might round the readings, or better, report the uncertainty, when reporting/analysing results. When calibrating the instrument, however, you should have the calibration standard have an uncertainty better than the instrument's uncertainty, and generally better than 1/10.
RE: Precision and uncertainty of video-extensometer
What size is the object and what is the resolution of the camera?
TTFN

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RE: Precision and uncertainty of video-extensometer
TTFN

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RE: Precision and uncertainty of video-extensometer
btrueblood,
I agree with you on this. But how do I compensate for the error given by the angle from the line of sight?
See the attached picture.
As the material is stretched the beta angle increases and the end of L2 approaches the boundary of the FOV. Then the measured distance, y, is smaller than the actual distance, L2.
Using L=0.5 m, L1=10 mm and L2=150 mm I get an error, L2-y, of approx. 7 mm. If I'm calculating this correct, it's a very large error.
Is this kind of error usually compensated for by the extensometer software?
IRstuff,
At a distance (L) of 0.5 m my resolution is 160 micrometers/pixel.
RE: Precision and uncertainty of video-extensometer
Your picture doesn't make a lot of sense for operation as an extensometer, i.e. making a strain measurement in a tensile test. I.e. (based on your figure) you are measuring L1 and then L1+L2 in a tensile test, right? And typically, L2<<L1, so right off the bat, the parallax error could be ignored for small strains. To be more specific, the error in measurement for L2+L1 and L1 could be expressed as a constant factor, like:
L1 = L1measured * f1
L1+L2 = (L1 + L2)measured * f2
and for small enough displacements, f1 ~ f2
Thus when calculating strain, the "f's" would cancel out.
But let's say your figure is a close approximation of the measurement you want to make. There are corrections that can be applied to the measurements, if you know (or can approximate) the angles involved.
Second, if you know where the displacement is going to happen, you can adjust the camera field of view to minimize the parallax error for that area.
RE: Precision and uncertainty of video-extensometer
> Even if it were true, you could certainly halve the error by placing the the LOS at the virtual center of your setup. Additionally, you could certainly calibrate out the error by using a reference scale like a steel ruler.
> Finally, if the measurement system is ostensibly insensitive to any distance within its operating specifications, then it is sufficiently telecentric to do the measurement accurately.
TTFN

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RE: Precision and uncertainty of video-extensometer
IRstuff's suggestion that you calibrate to a known standard is an excellent one.
RE: Precision and uncertainty of video-extensometer
RE: Precision and uncertainty of video-extensometer
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RE: Precision and uncertainty of video-extensometer