How to change Voltage to Pressure or Pascal (Pressure) in Labview

[camerond]

I thought you meant air pressure. What you have to do, then, is to construct a platform which will exert all of its weight (pressure) directly and evenly on your force sensor (if it rests on anything else because it's unbalanced, etc., then all bets are off). But you still have to be able to read your sensor with a known force (measured with some other instrument) to calibrate it.

Then tweak my second sentence and beyond to read: "Then exert force A on your platform, take your measurement with your sensor, repeat for force B, force C, etc., until you have exceeded your anticipated range or the sensor shows no further response with increasing force. For your setup, this should be pretty quick, so take a lot of points all along the range. One thing you do know from the company literature is that there is likely to be a curved region and a straight region in your calibration curve (maybe a curved region at the end)."

Pressure is simply force divided by the area it is exerted on, as pjr1121 said. As for your feet, they will obviously exert different amounts of force at different spots, so you'd better have a lot of sensors or you will just have a bathroom scale.

By the way, are you SimpleSam or HEAP08? IMHO, it's not particularly polite to change your name in the middle of a thread (and multi-monikers can give people pause thinking about your motivations for doing so).

Cameron

[camerond]

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@HEAPO8 wrote:

well lets say my weight is 12 stone... I step on the mat and that gives out 5V. How do I get the force from that? I hope i am not frustrating you guys? Lol

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If you have positioned it correctly, the average force is 12 stone divided by the area of your pressure sensor (or, by extension, the average force on the mat is 12 stone divided by its area). The only relevance of your 5V reading is that it can be used as one point in your calibration.

Cameron

[pjr1121]

Force = Mass*acceleration (newtons = kg * 9.8 m/s^2).  so start with a set of known weights (say from a lab).  Each weight will give you a voltage reading which you record.  You will want to use at least 10 different weights.  The more you use, the more accurate your calibration curve.

[HEAPO8]

Right got my recorded data! The force have been converted to Newtons and I am guessing that it Force against Voltage for the Curve?Is this posible to do so in Labview straight away? If so could you guide me through which VI to use and how to do it. Thanks for your help guys

ps camerond sorry about the different names- I forgot the password to my other email account)

[pjr1121]

You don't need any special vi's just go in the numeric pallete and use basic math operations.

remember, F=ma  so 10kg * 9.8m/s^2 = 98Newtons, easy.

[HEAPO8]

It is easy to you, but not to me! I have my readings (see below)

1. 4.90N= 2V

2. 2.45N= 0.9V

4. 19.61N= 2.75V

5. 9.8= 2.35V

Am I meant to plot this in a calibration graph? The graphs that I have on my code can show Voltage against time- I want to change that to force against time

[pjr1121]

You have 5 numbers, which is a good start.  Since the curves show in the manual a non-linear, I would recommend collecting more.

Use these numbers to construct a calibration curve.  Plot them in excel, and fit a polynomial curve to them (a polynomial of high enough degree can fit anything).  There are probably other equations which will fit with fewer coefficients but I'll keep this generic for now.  This will give you an equation to convert voltage to force for your sensor.  Now you can calculate the Force for any voltage reading given by your sensor with reasonable accuracy.  

To improve the accuracy, you should masure a lot of data points accross the full detection range available.

[camerond]

What's your experimental setup look like? Can you attach a picture? Your data look like you have a major problem, since the graph is not even monotonic.

Cameron

[HEAPO8]

Right okay I have done that now. The Polynomial I got is Y=0.0001x2 + 0.0923x+1.1667. Now gow do I put this in Labview (fingers crossed lol)

(ps camerond i am not using that menor sensors in my array, so it is very very basic experiment. It is not going to be an accurate foot mat by any stretch of the imagination..!)

[camerond]

What I mean is that it looks like your mat which is carrying the weight is not supported only on your sensor - one or more corners is sitting on the floor (bench, table). Probably also shifting around between each reading. This would make your readings totally invalid. If you cannot get a reading which increases (or decreases, one or the other, not both at different points) with weight, then your setup is useless.

Cameron