Numerical differentiation: ChnDeriveCalc

[gemu]

Hello,

I'm using the script command ChnDeriveCalc to differentiate channels in accordance with SAE J1727.

The formula is shown here: http://zone.ni.com/reference/en-XX/help/370859H-01/crash/misc_differentiation/

My Question: How does DIAdem calculate the derived values on the begin/end of a channel, because there
are no values Y[t ± Δt] / Y[t ± 2Δt] available for calculation.

Thanks

gemu

[Ralf_Mueller · Accepted solution]

Hi gemu,

you pointed out correctly that the calculation must be different for the first two and the last two values of the channel. Being N the number of values and dx the distance between two consecutive x-values the algorithm does the following:

At the beginning of the channel DIAdem calculates the forward finite difference of 2nd order:

dy(1) = [ -3y(1) + 4y(2) - y(3)] / 2 * dx

dy(2) = [ -3y(2) + 4y(3) - y(4)] / 2 * dx

At the end of the channel DIAdem calculates the backward finite difference of 2nd order:

dy(N-1) = [ y(N-3) - 4y(N-2) + 3y(N-1)] / 2 * dx

dy(N) = [ y(N-2) - 4y(N-1) + 3y(N)] / 2 * dx

I guess this approach goes back to NHTSA regulations for side impact evaluations.

I'm sorry that the documentation of the algorithm was incomplete. We will add the information above to the DIAdem help.

Best regards,

Ralf

(National Instruments)

[gemu]

Hi Ralf,

that's exactly the information that I need.

Thank you

gemu