Nature of RF pickup signal for flow meter

[michalk]

I bought a used flowmeter that has an RF coil rather than inductive pickup in order to measure very low flow rates. This type of pickup does not introduce cogging that happens with the magnetic pickup.

I have never interfaced to this type of sensor. It looks very similar to LVDT type of pickup, and it would be very nice if I could use an off the shelf IC, but there's no differential return signals to compare.

I assume the turbine vanes cause a varying inductance to occur in the coil, which damps a varying magnetic field. Regardless, could someone point me to a sample schematic, or IC, or at least describe the nature of the circuit needed for this?

Thanks.

 

[chuckey]

I would think that there two types of turbines, one with ferrite loaded vanes these would, as you say, change the inductance of the coil. the other sort would have copper turbine blades these would change the inductance a little and its Q a lot. What does this mean?, if the coil was part of an oscillator circuit, when a blade was near to the coil, the Q of the coil would fall and the oscillations would stop, if the blade rotated so a gap was by the coil oscillations would re-start. The oscillations could easily converted into a voltage pulse with a diode.
If I were designing a system the change of inductance would require careful temperature correction while the on/off Q system looks a lot simpler. Can't you get the manufacturers data on this gizmo?
Frank

 

[michalk]

I did look it up. Sorry, I don't have the flowmeter with me, but specs on this sort of equipment are very difficult to find. Just discovering the fact that it's RF pickup was about all I could find.

I believe their interface circuit costs about $350, and I'm very certain it's just as simple as you've stated. When I get back to the meter this weekend, I'll write down the numbers. If I recall correctly, it's made by FTI.

There's high margin and lots of trade secrets with vendor lock-in with this industrial equipment. I think this meter cost about $1500 when new.

---------- Post added at 11:44 ---------- Previous post was at 11:27 ----------

Okay, I can do better than my last post. Let's assume it's for one of these.
**broken link removed**
and let's pick the RF4. Are they modulating in the MHz range? Can a simple RC circuit with a diode -> amplifier get me in the ballpark?

---------- Post added at 12:03 ---------- Previous post was at 11:44 ----------

My, what a big difference a small change in Google search makes. I just found this gem:
**broken link removed**
It appears that they are exciting at 50KHz. However, it's not completely clear if they are working with an amplitude modulated response, or a frequency modulated response. I would prefer the on/off as you mentioned.

 

[chuckey]

Right I now understand how that "Sponsler" kit works. Its exactly halfway between my two methods!!! What they do is to use the RF coil as part of an oscillator circuit, but as the turbine rotates the effect they are looking for is the change of level as the ferrite/copper/???? turbine passes the coil. So its not stopping the oscillation but putting an added amplitude modulation on them. So using the diode will recover this component and measuring the frequency of this component gives the exact rotational speed of the turbine. I guess you will have to do something to this low level voltage to make it do something useful. The actual oscillator is sort of a 6 -12V supply a transistor and a handful of component. If you just want to actuate a switch at a certain flow, an extra IC + relay is required, if you want to meter the output, then a frequency to voltage converter would be required and either an analogue meter or a digital voltmeter.
Frank

 

[michalk]

Actually, I just want the vane crossing events to be CMOS compatible. I have source code for a magnetic flow meter.
The signal drives an interrupt which counts the pulses, accumulates and calculates rate and sends it via SMBUS.

Almost all of my experience is with digital circuits, and I've never done anything in RF. Could you point me to a schematic that would get me close to a solution? I can breadboard and modify until I get the response I'm looking for.

 

[michalk]

Would the schematic in this evaluation board work for the Sponsler pickup coil?
https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATA_BRIEF/CD00190840.pdf