what are the suitable sensors that detect the robot movement in gazebo world? so we can use this signal to affect on the robot movement?

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I have a gazebo world and it contains 4 different stages (1- normal walk 2- slow walk 3- climbing up the stairs 4-climbing down the stairs ), with an exoskeleton for lower limbs as a robot.

I want to make the robot changes its movement mode automatically and to do that I have to implement sensors between those four stages (not on the exoskeleton), so when the robot cross in front of those sensors, it will change his movement mode of the robot according to the stage that it will walk through it )

what are the suitable sensors to accomplish this task?