Hi everybody, my name is Carlos Marcano, I am an
electronics engineer from Venezuela (south america), I
am new to the list and I hope to be helpfull as
posible. Right now I am working on a little experiment
wich involves a PID controller working on a 16F84.
Ok, here is the thing: I want to keep a metallic ball
between two bands and I am using an electromagnet to
make the ball "float" between those bands; if the ball
passes the lower band the magnet should increase its
magnetic field and if the ball passes the upper band
the magnet should decrease its magnetic field. I am
getting the position of the ball using a couple of
LDRs, one for the upper band and other for the lower
band. They are polarized by an adjusting circuit in a
way to get 5 volts from each when the ball is between
the bands, that is letting the LDRs get full light.
When the ball is attratcted to the magnet it will
avoid certain amount of light to get to the top LDR
and the voltage across it will drop. The sums of the
voltage from the LDRs will give an aproximation of the
position of the ball. After the sum there is an
adjusting circuit wich responds to: Position= 5-
[(V1-V2)+5]*0.5 where V1 and V2 are the voltages
across the top and bottom LDRs respectively. This will
produce a
voltage from 0 (the ball completely over the bottom
LDR and living full lighted the top one) to 5 Volts
((the ball completely over the top LDR and living full
lighted the bottom one). This means that when the ball
is between the LDRs ("floating") the resulting voltage
will be: Position= 5-[(5-5)+5]*0.5 = 5-2.5= 2.5 V.
That means that 2.5 is de ideal setpoint for my
system. I did this using just analog electronics a
couple of years ago, but I lost my schematic circuits
and annotations. Recently I came with the idea of
using a PIC to control de system, implementing the PID
on it. I would use an ADC to convert the position
voltage and get it into the PIC, calculate the
response with the PID, and then use a DAC to actuate
the electromagnet. Well, this is pretty much
everything, PLEASE FORGIVE MY ENGLISH, it is not my
first language and I am still learning, and as ussual
any sugestion will be welcome...

Carlos Marcano.
(proud venezuelan).


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