I think the normal thing is to drop trash down a tower with a forced air cross-current. Glass gets diverted less than plastic by the cross-current.
You can also use various densities of liquid, to catch different densities of plastic floating vs. sinking if you've already crushed things up to avoid air bubbles. However, then you need to later separate the liquid from the shredded garbage.
Seriously. Give that robot some more senses. Echoic, magnetic, infra-ultra, spectrographic analytic, nuclear magnetic resonant... the whole schmeer. It's certainly easier than making them smarter.
