I am building an autonomous robot for robot swarm reasonably small in size (say 30mm x 30mm). I was analyzing such robot's locomotion and found that coin vibration motors are used widely as opposed to generic DC motors with wheels. I get that the the coin vibration motors are very cheap and can keep the overall size of the robot fairly small, whereas with DC motors, wheels will be involved which will make the overall body size of the robot bigger compared to using vibration motors.
My question is how do coin vibration motors create forward motion. Say a robot uses two of these motors. How can it be determined which direction the robot will travel and WHY? Is this something to do with centripetal forces?