load cell related question..

I have a load cell which is measuring a product discharging into a bin. the rate at which the bin fills is variable at the start and end of the cycle and I get the following readings of load (M = Mass, t = Time) at each given time in seconds in the cycle.

t = [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ] M = [0 0.5 0.6 0.9 2 3 4 5 6 7 8 9 10 10.3 10.4 10.5 ]

I can plot this to show the output from the load cell with no problems.

How do I process the data, to give me the mass flow rate in Kg/sec at any given point in time I want to have a graph showing Kg/s Vs time so that at any given time in the cycle the MFR can be found.

I'm pretty sure that this has something to do with finding the gradient at any given point on the curve?? there must also be some differentiation inthere somewhere too...

can someone guide me to an answer??

-- replace webmaster with simon to mail me.

You need to differentiate the readings, but in real systems the key is getting sensible data to use appropriate filtering to use a filtered derivative.

If you just take the quantity over the last second you get

Mdot = [0 0.5 0.1 0.3 1.1 1 1 1 1 1 1 1 1 0.3 0.1 0.1 ] kg/s

To apply a simple first order filter take only a fraction of the latest difference.

Mdotnew = (1-beta)* Mdotold + beta * Mdot

beta is a small variable linked to the time constant of the filter

e.g with beta = 0.6, Time constant = 2 seconds

[0.0 0.3 0.2 0.3 0.8 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.6 0.3 0.2 ] kg/s

When you implement online you will need to force the number to zero during bin changing or you'll get lots of spurious spikes!

Hope that helps

Michael