Capacitive Sensor for detecting plastic objects

HI guys, I'd like to know if someone has already done a capacitive sensor for detecting plastic objects (range 1-4cm), my intention is to build a robotic arm that holds plastic objects if it detects one, I'd be very grateful if you could show me a diagram that you may have used on any projects you have developed.

Thanks in advanced.

Plastic vs. what?

What do you mean by "vs what" exactly?

What do you mean by "vs what" exactly?

Wood, iron, copper, steel, eggs, tomatoes, zucchini, etc.

Are you trying to make a picker than can discriminate plastic from other materials? If so, from what other materials?

Basically it'd be aluminum.

There is certainly no problem discriminating aluminum from plastic -- an inductive proximity sensor will do that. Here are some commercial examples (but you can make your own): Tubular Sensors | Allen-Bradley