How to compare a wave form against a master wave

[Joe_John]

I have a press force monitoring application which records distance (mm) vs. force (NM) and plots it on a graph. 

Before the test is run a master is run and the distance vs. force is recorded. A +/- tolerence is added to the master graph and all tested parts must fall within these limits or the test fails.

I need to compare these two curve based on distance. There is an express VI called in SignalExpress: Analysis: Test and Compare:Limits Test, It does everything I need except I'm unsure if they both can be referenced to the same axis.

I am using 8.5.1 Both curves are formatted into two coloumns Force in one and Distance in the other. Does anyone know how to make sure a curve fails within an upper and lower limit. I need to reference all the data to distance.

[Joe_John]

I'll add a little more information.....

The problem I'm running into is the fact that the all the curves may have different array lengths and time between readings. for all tests the distance starts at 0 mm and will increase to 30 mm and during that time the press force will increase. However, if the press is a bit slower for one part compard to another then the test that too longer will have more data points and the curve will be stretched. If I take both curves and plot them against distance, the they should both look normal. I dont care how long the test takes, I care about the force and specific distances. I hope that clears things up.

[johnsold]

Joe John,

The additional information was helpful.

Although you may acquire the data on a timed basis, convert it to a force vs distance curve and compare that to the limits. I have never used that Express VI so I am not sure how it works internally. You can open the front panel of an Express VI and then look at the block diagram.

One way of dealing with the differing lengths is to interpolate points to fill in the gaps. Suppose you get data at 0, 7, 14, 21, and 28 mm and have limit specifications at 0, 10, 20, and 30 mm. If you interpolate both arrays to generate values at each mm you will then have two arrays of 31 elements each. Then add the +/- tolerances (scalars if the same at all points or an interpolated array of 31 points otherwise). The comparison functions (greater than and less than) are polymorphic and can accept arrays. The comparison mode can be set to Compare Elements or Compare Aggregates, depending on whether you need a single pass/fail or information about which points failed. The In Range and Coerce function can be used to compare both limits simultaneously if you do not need to know which limit was exceeded.

Lynn