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robot fall over gyro accelometer aibo rumble tilt

Making my robot tell when it's fallen over

Hi everyone! I have a question about sensors for making my robot realize it's fallen over. I see it now in a couple of robots and semi robots, where a bot can determine if it's fallen over. I did a bit of research and found a couple of options, but i'm not sure which one would be best. I saw an acellerometer, buthose are expencive, tilt sensors look more like what I'm after cuase they seem cheap and are not as dangerous as mercury switches. I'm not exactly sure what would be best or if I'm missing something.