Robo-Rats Locomotion: Hexapod

Six-legged locomotion is very common in nature--most insects use hexapod locomotion.  The benefit of six legs is superior static stability.  Using the tripod gate, insects can move three legs while being supported by the other three legs.  Consider the example hexapod below in which each leg can pivot forward 30 degrees and backward 30 degrees:

Now consider the following sequence of diagrams which illustrate the tripod gate:

In the above diagram, green "feet" are touching the ground, red "feet" are not touching the ground.  The left diagram shows three of the feet have been lifted off the ground--the remaining three feet form the three-legged "tripod" structure which gives the gait its name.  In the second diagram from the left, the legs which are touching the ground have pivoted backward, moving the body of the robot forward (as can be seen by the position of the blue body vs. the gray outline of the original position), while the legs that are off the ground have pivoted forward to prepare for the next phase in the gait.  The third diagram shows that the legs have swapped their contact modes--the legs that were off the ground have been lowered into contact and the legs that were on the ground have been raised.  Now the same pivot motion can occur with opposite legs, resulting in the rightmost diagram.  Note that at all times a minimum of three legs in a triangle arrangement were in contact with the ground.  This guarantees static stability throughout the gate.

Straight-line motion is quite easy using the tripod gate, but turning is more difficult.  It can be done, but odometry is difficult because turning requires a distortion of the standard step size on different sides of the robot and usually causes some foot slippage.

Below is a photo of the Robot II hexapod developed at the Biorobotics Laboratory at Case Western Reserve University.  Hexapods are the most common legged robot in the robotics field but they are limited to exploration/locomotion tasks because they usually have no manipulators.

Motors:

³ 2 per leg - Each leg needs to move forward/backward and to be raised/lowered.  Some designs use three motors per leg.

Pros:

Stability - Good static stability.

Versatility - A legged system can traverse rough terrain.

Cons:

Complexity - At least 12 motors are required--not worth it unless the terrain demands it.

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