Proportional Control of Airflow

[willcharlton]

I am designing a device that regulates the flow rate of oxygen with a solenoid-type, pwm controlled proportional valve. This valve is a great product, but it's not as linear as I would like it to be. I have been searching for a device that I can put in series with the valve that can monitor the flow rate and provide an electrical signal, analog or digital, back to my control device (PIC24) in order to recalculate the duty cycle of the pwm port and more precisely tune in the flow rate. Does anyone know of a device that I can put in-line with my proportional valve to electrically monitor that actual flow rate? I think my problem is, for instance, I give the valve a 10% duty cycle and I just expect it to be 10% open, but I don't think the flow rate curve is that linear. So what I plan to do is use this electrical readout of the actual flow rate to readjust the pwm signal. Any ideas? Thanks.

 

[srizbf]

do you have the characteristics curve of the valve from the manufacturer?

 

[willcharlton]

**broken link removed**

The part number is 930112.

The input psi is unknown at this time. I thought of fitting these curves, but thought a pid approach to controlling the open-ness of the valve would be a better approach in case the input psi changes for whatever reason.

 

[srizbf]

one option to monitor the flow from the valve , for further adjustment from microcontroller , is the mass flow sensor for air.
it gives its output in either serial or spi format.

another , one , which may look odd , is using the mass air flow sensor used in automobiles for fuel injection .it gives output in PWM or 0 to 5v .
the only problem may be to find the sensor for your flow rate range.

 

[willcharlton]

Thanks for the tip srizbf! I looked into some mass airflow sensors from honeywell and it was exactly what I was looking for. However, after much debate and discussion we opted for the cheapest method possible: to write a calibration subroutine using an oximeter. That way more components don't have to go into the production of the device. We're going to have a manufacturers recommendation of recalibration every couple years or so. Need to gather more data, but I think this will work very well. Thanks again!