Thank you for you're replies,
Gardners ide is nice but the turntable is not going to move constantly the people need to interact with it.
Justone idea Is really cool and if I can not arrange somehow with ready encoder I will need to try you're idea to create an optical rotary encoder in the bottom of the table.
Raschemmel ide seem to be the most possible even though I did not understand completely. Do you mean to make kind of canal in the side of the turning plate so the encoder wheel will fit on it ?
Thank you all