How to find velocity using a displacement sensor

[Sully1071]

I have a very basic knowledge of labview but as part of my college project I am going to have to find the linear velocity of a sliding rail. I am being supplied with a linear displacment sensor which i presume will have an otput between 0-10 Volts. How can i use this sensor to find veocity?

Any help is appreciated.

[mikeporter]

You will need two pieces of information:

1. The calibration for the displacement sensors. It will be stated in something like voltage for a given distance - like 3V = a 5 cm displacement. Of course depending upon how tricky your prof is you might need to come up with that too...
2. How long it takes to move a measured amount.

After that it's just some simple math.

[Sully1071]

Thanks.

I have calibrated a thermocouple on Labview so i presume calibrating the displacment sensor is possibe for me to do.  If i differentiated the displacment coming in with respect to time would it be possible to plot this on a graph or is there an easier way of doing this?

[Sully1071]

Thanks.

I have calibrated a thermocouple on Labview so i presume calibrating the displacment sensor is possibe for me to do.  If i differentiated the displacment coming in with respect to time would it be possible to plot this on a graph or is there an easier way of doing this?

[mikeporter]

Nope, that would be the way to do it...

Mike...

[Sully1071]

I have built my labview program now but i need to input the displacment into a formula node but it keeps coming up as an error. I have attached the labview program. Im not sure how to correct the error. The formula is linking the position to the force at that point.

[mikeporter]

Two problems: First you don't have the X input connected to a source. Second, the B variable that you use is not defined - to correct that simply create an output labeled B. You don't have to use it, but it needs to be there.

Also from the standpoint of readability it might be helpful to throw in a parentheses or two...

Mike...

[Sully1071]

Thanks.

I have sorted the output but now i am having trouble putting the value for displacment in. I think the problem is the value oututted from the graph is a voltage and time. I need to input a pure displacment value into the formula node. Is there a way to solve this problem?

[mikeporter]

The problem is that the output of the express VI is an array of values where as the formula node is looking for a scalar value input. My next recommendation is to get rid of the express VIs. The problems are many, but to start: They hide too much fucntionality. They are inherently inefficient. They use the dynamic datatype which can cause a lot of ambiguity.

Right click on each and select to open their front panel. This will convert them into normal LV VIs.

MIke...

[Sully1071]

Thanks Mike. I have now gotten rid of the sub vi's and replaced them with simple differentiation terms. To be honest i am now fairly lost with this set up. I simply just want to convert a analogue signal to give me displacment and velocity. I seem to be making quite a chore of it though.