It's probably impossible to design a system that couldn't be manipulated once you start disenfranchising people. Who gets to define the relevant "knowledge"? How do we measure " intelligence"?
I propose the MacDonalds test. If someone is too stupid to be able to quickly order something in a drive thru, they're too stupid to be allowed to vote. Seriously, every system can be abused but encouraging stupid, uninformed people to vote promotes the worst in democracy.
Which of course is the whole point concerning things such as mandatory voting, automatic voter registration, not showing an ID at the polls, etc. If someone is too lazy to make even a small effort to vote, they shouldn't.