math issue

acadienjcc

Hello all,

what would be the math function to reverse a value?

i.e. the value is high when there is less pressure, and it gets lower as pressure is increased. what math algorithm would I use to obtain the reverse? (higher pressure gives a higher number & lower pressure gives a lower number)

the sensor is a flexiforce sensor, sample code: FlexiForce_Simple.bs2

Bean

Just take the negative of the value. Or subtract the value from a constant.
For example if the value goes from 10 to 100 just do "value = 110 - value" and the value have the same range.

I hope this helps,
Bean

stamptrol

Its not so much a math function, but the application of the standard "scaling and offset" equation used with sensors and signals. You'll find it written as y=mx+b.
Just Google "signal scaling equation" or "signal scaling and offset" and check out a couple of articles.

Briefly, to reverse the response of the sensor, you start with the maximum possible value and subtract the sensor output from it. Say the sensor goes from 0 - 256. For normal readout, you'd expect the number to go from 0 up to 256. For reverse readout you'd say (256-sensor)=value.

Cheers,

acadienjcc

thanks guys, that was very helpful.

I'll see if I can get it to work now and post if I run into problems.

stamptrol

Just to follow up, here's an article showing how it is done: http://kb.mccdaq.com/KnowledgebaseArticle50048.aspx

Cheers,

Tracy Allen

Keep in mind that there are other functions that can satisfy the notion, "the value is high when there is less pressure, and it gets lower as pressure is increased". One common such function is K/x, or a variation thereof, such as K/(x+B). I suspect that this sensor has a dependence of the form of R = K/F, roughly speaking, where F is force or pressure, R is resistance, and K is a constant. The same applied to an RCTIME measurement, R*C = C*K/F.

Tracy Allen

From the flexiforce user manual, resistance proportional to 1/F: