How to tell whether a continously number is increase or decrease?

[fourteen]

I was using a differential linear encoder to measure position for dual loop control. but i this encoder have a problem, the result returned by the encoder seems to be wrong. The data returned by the encoder shows difference when the platform move from right to the left compare with when it move from right to left.

I do not know why this happening, but inorder to set the number to be 0 when the platform returned to the same position, i have to use different scale. so i want to determine weather this plat is moving from right to the left and so on.

this maybe a simple question, but for a labview beginner, i really needs your help. Please help me. Thanks a lot.

[KateB]

This is most likely the noise issue.

I've an experience where encoder uses external supply and cause a lot of noise.

In such case, use +5V supply from NI card solve the problem.

If it does not solve your problem, please show us how do you connect your encoder to NI card, and your code.

Also tell us how does an error look like?