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I would agree with what @stevemacenski states. Start with an RPlidar or similar an get gmapping to work. This should be fairly easy as this has been done countless times before.

Typically the stairs/hole problem is solved differently and depends on your robot (is it holonomic, i.e. can it generate a velocity in any direction or is it differential drive?). You can decide to put a second lidar on the robot on an angle and manually detect any sudden terrain changes, or similarly using a depth sensor to do the same.

These things you learn by experience. I would recommend you to go ahead and get started as opposed to spending countless hours researching what to do. A lot of problems will become clear just by trying something. Happy tinkering!