How do I convert an internal variable into frequency

[HenrikOlsson]

Ioannis,
Really, I think we are basicly saying the same thing here, however.... You wrote:

Well the derivative is a number (no units associated with).

I think this is where you are wrong. The derivative is the rate of change. But you cant just say that the rate of change is 7. You have to say 7 this per that otherwise it doensn't make sense IMO. For example 1.5V per second or 25degrees per hour or 15m/s per second etc.

I found this on Wikipedia:

In mathematics, a derivative is the rate of change of a quantity. A derivative is an instantaneous rate of change: it is calculated at a specific instant rather than as an average over time. The process of finding a derivative is called differentiation. The reverse process is integration. The two processes are the central concepts of calculus and the relationship between them is the fundamental theorem of calculus.

Again, I believe we're meaning the same thing but without assigning units to the numbers they just doesn't make sense.

/Henrik Olsson.

[Ioannis]

From what I remember the derivative is the tangent of the curve described by the function and shows the gradient of the tangent line. So a tangent cannot have units.

This is how far I can go...!

I understand what you are trying to say as a rate of change (like the slew rate of an op-amp). But on functions, a derivative I am sure is just a number, showing the direction of the curve at that point.

Maybe a math guru is listening here to give a hand?

Ioannis

P.S. Since all the maths I learned was in greek language, maybe the terms I am using are not the proper ones, so.... I am sorry for that!

[schlaray]

The sensor connected to the A/D normally produces a constant , fixed output. The A/D output is therefore also fixed at this point in time. When a flow is present, the sensor produces a sin wave output whose frequency is dependant on the flow rate. The A/D output produces a series of varying numerical values if plotted against time would look like a sin wave. I have a program that will detect the numerical peak of each sin wave and I can count these peaks. Right now, I am using a timer (TMR0) with interrupts to generate the time base. This timer is activated when flow starts. The count /time will give me the frequency. The program will have a variable trip value i.e. when f >= trip then alarm. My program is almost operational with a few bugs remaining to be worked out.

I have built this circuit using Op Amps etc., and now I would like to keep up with the times and reduce the number of components using a PIC.

I wish to thank all those who have contributed and still might contribute to this thread.
R.W.S.

[Ioannis]

Since your sensor produces a sine wave as a signal I would propose for the conversion part, to do the following:

Drive a cheap 4046 PLL with the sensor signal. From the VCO control voltage drive a buffer op-amp (optional but suggested) and then the A/D of your PIC. Now you have a clean analog voltage that follows your flow converted to a digital.

If you want to do the opposite, drive a voltage to the VCO of the 4046 and now you have a voltage to frequency converter (very linear I might say).

Ioannis

[schlaray]

It was my error to mention dv/dt. Although I would like to measure this also, the frequency is what I am looking for.
Thank you IOANNIS for your suggestions but my thrust right now is to try and do it all with the 16F877A.

The program I have right now runs just fine when it is compiled and programmed under ICD. Using the real time compiler and programmer it doesn't work?????

Any suggestions?

[Ioannis]

Hm, that's another story. What compiler, what code, what programmer, what...

Please post everything you have to look at.

Ioannis

[schlaray]

After playing with the program over the weekend I discovered two registers (intcon and option_reg) were incorrectly configured. The "Clock" generating ticks was not turning off . Steping through the program with the registers displayed the MCSP revealed the defect. Thank you IOANNIS for you offer to provide further assistance. I think I have a good start now with other obsticles yet to overcome.