Velocity

CJC504

Can anyone think of a clever way to read velocity accurately? I am trying to make a velocity meter by using a small dc motor to generate voltage such as in the link:

http://www.amazon.com/gp/product/B0002WZRKE/ref=s9_asin_title_1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=0W3GACJDGV7ZT0ZT707D&pf_rd_t=101&pf_rd_p=278240301&pf_rd_i=507846

I have the two leads of the motor connected to a voltage meter so I can gauge the velocity by the output voltage, but I have no idea what the actual velocity is in terms of FPM (feet per minute). So what I am interested in is a base point such as:
.5V = 10 FPM
1.0V = 17FPM
2.0 = 32 FPM
etc

Once I have those values, I can then interpolate the rest of the missing numbers.

I am open to any ideas including using a differential pressure gauge to obtain a pressure reading, which can be converted into velocity. Or maybe even using the Parallax Flexiforce pressure sensor to gauge the Force by the air. Then using
F = mass flow rate x Velocity
to solve for velocity if I can figure out what the affected area on the Flexiforce sensor is.

Any ideas?

VbGuru

What about a hall effect sensor and a magnet to detect the rpms? you could use some simple math and get alot of data from it.

here is a pdf for something simmilar... hope it helps


http://www.basicmicro.com/downloads/docs/RRPM.pdf

VbGuru

May I also offer you take a look at the articles here I have found many answers there along with a few inventive ways of doing what I have already done


http://www.parallax.com/Resources/NutsVoltsColumns/tabid/272/Default.aspx

VbGuru

One more that might be of help

http://www.parallax.com/Portals/0/Downloads/docs/prod/sic/SmartSensors-v1.0.pdf

Page 65 of the pdf

jmalaysia

Do you have an Anemometer available for comparison? Or are you even measuring wind speed...

VbGuru

http://www.instructables.com/id/Hall-Effect-Latching-Switch-as-a-Turns-Counter-Sen/

CJC504

No, I do not have an anemometer to compare it to. That is what I am trying to avoid buying.

I do suppose by knowing the rpms I could calculate the tip velocity with
v = wr where w is the angular velocity,v is velocity, and r is the radius.

Has anyone ever tried to use the flexiforce sensor? Could I set the sensor up to apply P = F/A where
P = pressure
F = force
A = area

Then once I have the Force (F), solve for velocity by F = mdot*V where
F = force (from above)
mdot = mass flow rate of air (1/density of air)
V = unknown

Thank you for your suggestions.