I'm planning a robot vehicle and plan to get one or more motors that have encoders built in.
How many encoders do I need I really need in order to gather solidly accurate information about speed and distance? I'll get four if it makes a difference but I'm having trouble seeing much benefit beyond one. Would averaging across two or four encoders help to mitigate the impact of slipping wheels?
There's about a $20 price difference with and without the encoder, so it would be nice to minimize the number of motors with encoders.
EDIT: This is for a four fixed wheeled robot.