Now you could hardwire a wobot to have some non rational purpose...but you better be real careful what it is. Obviously you tell the robot that it's purpose is weaving baskets, then it will simply spend all its time weaving baskets and will never even appear to be sentient.
You could try telling the robot it's purpose is human DNA replication...trying to be clever. But the robots solution to that problem will probably be drawing blood from people and mass cloning them or something. The robot will not exhibit behavior informed by millions of years of trial and error.