Where are the x-ray scanners located-on the platform levels or on the main floor? I keep getting conflicting information from different websites. Also, using Google earth for looking inside the station, there is an walled off(with clear glass) area with a sign saying "Access Control" on the main floor. What is that for? I am just trying to figure out if I need to stop on the main floor for any reason or just go directly to the platforms(as some websites say).
... what do you intend to do in Sants? do you need to purchase a ticket? which sort of train are you taking?
Catch the AVE train to Madrid with pre-purchased tickets and luggage.
Just head to the centre of the station, there you'll find the security scanners. You'll be entering from Plaça dels Països Catalans, and the area you're looking for is the green area, behind 13 (http://www.adif.es/en_US//infraestructuras/planos/estaciones/plano_71801.pdf) Although the whole security process is quick, be there a while in advance, just in case.
Just follow the signs for the AVE trains; this will take you to the security check, then to the waiting area for the trains.
If you are lost or confused for any reason, just ask (there are uniformed helpers around the station).