Need Help With Derivative

[maxofmilions]

hey i need some help from you guys...i am want to use the derivative object i suppose...correct me if i'm wrong...to read the flow rate when a weigh hopper empties so i can control the gate and thus the flow rate....i believe the ones on batch systems we use now look at the flow every two seconds...thanks ahead....randy

[Mike@DTSI]

Sounds more like you need a PID instead.

http://zone.ni.com/devzone/cda/epd/p/id/3982

That should get you started.

[maxofmilions]

thanks mike...you know i thought that also...but i can't even get the deriv to work yet....let me explain a little more what i'm doing.....i'm designing a concrete batch control for ready mix....now i've always worked in the concrete block side of our company and have designed and built several batch systems for those plants...but this is way different.....the main thing i guess is in block we dump all the rock in the mixer...add some water....dump all the cement in the mixer...add a little more water and your done.....in ready mix the batch sizes are about 6 or 7 times larger so the batching is a little different but the biggest diff is the discharge into the truck.....if you start with say 32000 lbs of rock and sand and around 4700 lbs of cementitious material.....you begin discharging the rock first...after 3 or 4 thousand pounds you start adding cement.....now a general guideline is to have about 12000 lbs of rock and 2000lbs of cement at midpoint...then when all your cement is in have around 2000 lbs of rock left.......we use flow control valves so the discharge gates take about 2-3 seconds to open and close so you can bump them with 1/2 second pulses to control the discharge flow rate....a little more advice would be appreciated....randy