How do I plot the output of a sensor with respect to the output of a different sensor?

[belae1ka]

I would like to plot force vs displacement. The force is coming from a load cell that is attached in between the shock (my project is to create a shock dynamometer) and the frame as seen below;

Here are a couple pictures that represent the linear displacement, the stroke length is 3 inches;

The max frequency of my motor is about 5Hz. So one revolution takes 0.2 seconds. Peak to peak takes 0.1 seconds. So 3 inches of linear displacement takes 0.1 seconds. 

I would like to plot the force vs displacement in 0.1 inch increments so if I understand correctly it would take 3.33ms for a linear displacement of 0.1 inches if the motor was running at 5Hz.

How do I correlate these values with the selection of a suitable proximity sensor when analyzing datasheets?

Would using a IR proximity sensor be a logical choice?

Thank you.

[Henrik_Volkers]

Again: What hardware do you want to use to read your load cell ?

Usually I would read force with an analog input card (DAQ) ... what do you want to use?

Distance: 0.1 what ??  Ah the distance of the holes in a prototyping perfboard 🙂

Optical (incremental? ) or inductive (LVDT) sensors are quite robust.

Somehow I have the impression that you don't want to spend to much money on industrial grade sensors 😉

A lot of makers use cheap elektronic callipers for their hobby CNCs, don't know how fast they are.

[Kyle97330]

This is the datasheet for your sensor.  Look on page 2; it says "we suggest to use over 60ms measurement cycle".  As such, you're limited to about a 16 Hz measurement if you use that suggestion.  It probably doesn't need as much time to measure a short distance but it's still way too slow for your application, which if you're using a motor at 5Hz and want to take measurements 40 times per second, you need 200Hz.

An IR distance sensor would likely be faster but it would also be difficult to get one that's fast enough.

Perhaps you can get something along the lines of a Rotary encoder to track the current position of your device?  

[belae1ka]

I am using a USB-6002 to read my load cell.

What I meant by 0.1 was that I would like to take 30 samples within the 3 inch linear displacement (peak to peak distance).

3 inches/30 samples = 0.1 inches per sample.

Every time the linear displacement changes by 0.1 inches I would like to take a sample.

As far as proximity sensors go, I am ideally looking to spend around $10-$30.

I like the electronic calipers idea but I can't seem to find how fast they are. Perhaps I can use a linear slide potentiometer and correlate distance that way. Or maybe mechanical gears along with a rotary encoder.

I would like to use a proximity sensor though. Do you think a IR proximity sensor would be fast enough to take 300 samples in a second? If so can you point one out to me?

Thanks.

[belae1ka]

Hi Kyle,

I like the idea of using a rotary encoder, I may have to give it a shot. If possible though I would like to try and use a proximity sensor. Would this one work?; https://www.pololu.com/product/2578

Thank you.

-Kevin

[Kyle97330]

The particular sensor you linked to only goes on and off.  You have to pre-set its range sensitivity.  

Something else you could consider similar to a rotary encoder: get a Photo Interrupter, and get a metal plate that you can attach to the moving part of the tester while the photo interrupter remains stationary.  Cut holes in the metal plate every .1 inch.  As your tester moves up and down, you'll get a pulse on the output that you can sync up to the pressure sensor reading.

[belae1ka]

What a great idea! I really appreciate the feedback Kyle. Do you think it may work if instead of using a metal plate and drilling holes, I use a PCB board cut into the shape I need and attach that to the moving part? Attached below is something similar to what I had in mind.

The parallel white lines across each row represent the cuts that I would make in order for the Photo Interrupter to easily identify what is being recognized as an interrupt.

Thank you.

-Kevin

[Kyle97330]

In theory that would work.  In practice it depends on how wide the beam is of your photo interrupter to ensure that the hole area versus solid area is enough to trigger it.  Since photo interrupters are relatively cheap your best bet may be to order it and experiment.

[belae1ka]

Thank you Kyle.

[Henrik_Volkers]

If you want to use the 6002 :

You need a bridge amplifier for your load cell.

You should look for a distance sensor with an analog +-10 or 0-10V output.

The two analog inputs should be sampled with about 1kSPS hardware timed!

The digital in (and outs)  of the 6002 are only software timed and you will not be able to do your task with USB software timing!

As for a cheap solution: you can use any distance sensor with a ramp  along your ~76mm to 'transfer' to some (5?) mm  🙂  .. and use a CNY70 😄 

Not very linear, but you haven't told us any requirements ... and if you build the ramp from metal sheet  you can bend it  to compensate nonlinearities ....