Subtraction between the current sample value and the one several sampling times before

[chalkwu]

Hello everyone,

I have a device which generates a series of numbers at sampling times, let's say every 1s.

The series of numbers is denoted as x0, x1, x2, ..., xk. I want to get the result at each sampling time Res = xk - x(k-n). If n=4, that is Res4 = x4 - x0, Res5 = x5 - x1....For the first 4 numbers, Res3 = x3 - x0, Res2 = x2 - x0, and Res1 = x1 - x0.

My codeing idea is that 

Res1 = (x1 - x0) +       0     + 0 + 0;

Res2 = (x2 - x1) + (x1 - x0) +0 + 0;

Res3 = (x3 - x2) + (x2 - x1) + (x1 - x0) + 0;

Res4 = (x4 - x3) + (x3 - x2) + (x2 - x1) + (x1 - x0);

Res5 = (x5 - x4) + (x4 - x3) + (x3 - x2) + (x2 - x1);

...

Because n is supposed to be a variable, I can not use shifters directly. I try to use the for loop with the loop count N being variable n. The attached file is a test vi.

Can you help me please? Thank you.

Happy New Year!

Qun 

[apok]

having not looking at your code.....

• put the series of numbers into an array
• feed into a for loop
• numbers go into a double pulled down shift register
• take the difference
• feed that number to another shift register with the addition function
• oputput of that shift register will give you the last value

note: manipulate the array to start at the first or last element, also feed the n# to the for loop to run

[GerdW]

Hi,

Why use loops and shift registers?

All you need to do is collecting your samples in an array…

[chalkwu]

The array is aquired online instead of a given array. And the sum of difference is not calculated by all elements, it is calculated by the latest n elements. At each sample time, I get one result. I am still working on it. Thank you. 

[chalkwu]

Is this what do you mean? (Bg is a subvi to obtain data from device at sampling times). Thank you.

[GerdW · Accepted solution]

Hi chalkwu,

why do you put the whole array into the shift registers?

[chalkwu]

Hi Knight. GerdW,

I thought there should have some subvi with the function storing certain length of some squence data. And that is Data Queue. Thank you very much.

Cheers,

Qun