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# Sensor for Magnetic Levitation

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#### mysore

##### Newbie level 2
Hi,
We are doing a project on magnetic levitation of a small steel ball. We were thinking of using an LDR as part of a simple voltage divider circuit and a light source. But the problem is if the ball is equally above or below the centre of the LDR within its cross section, the resistance of the LDR would be the same, and it would not be possible to control the current to the electromagnetic coil as desired. Is there any other way of using the LDR to overcome this problem or is there any other sensor which can be used?

hello

Where is the problem ?

Can you show us your project drawing ?

the LDR can be masked continuesly, direct light , minimum value of LDR,
completly masked , LDR > 1Mohm ,
or we can suppose than the ball diameter is less than the LDR diameter ?

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Very roughly our project is like: Sensor(resistance) --- Corresponding voltage through voltage divider network---- Controller--- Required current to the coil to either pull the ball up or down.

My question is: I your drawing, there will be a certain resistance of the LDR and finallly the ball has to be pulled up. However if the ball is above the centre(in your drawing), the resistance of the LDR will be the same, but the ball has to be pulled down...

hello,

it is only a point of control adjustment,
LDR value represents the feedback of the ball positionning
if the coil is upper the ball
example : 100% light on LDR => maximum of curent in the coil
the ball will go up.....
around 50% of the light on LDR =>the ball stay in equilibre
LDR value <10% => curent becomes nul the ball fall.

so no blocking situation,but control loop
to adjust with a PID controler
Proportional (gain),Integral (integration),Derivee(like anticipation)

the mechanic part is very important,because included in the
closed loop of control

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