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Hexapod_I, ready for the next step
PREAMBLE

Hexapod_I was inspired by Flik, the fastest RCX walker I've seen. Flik uses two RCXs -- I wanted to see whether a similar articulated hexapod could be built with a single RCX.

MECHANICS

Hexapod_I is built in three sections and uses five motors. Each section has a motor to shift from one leg to the other, the other two motors are used to twist the two 'articulation points' where the sections are connected. Hexapod_I's step rate (about one step every three seconds) is slower than Flik and the reasons are interesting.

Let's number the legs of a hexapod like this


1--|--2
   o
3--|--4
   o
5--|--6

[The 'o's are the articulation points.]

The fastest gait (walking pattern) for a hexapod is the 'alternating tripod' where the weight is supported on legs 1-4-5 for the first step, then on legs 2-3-6 for the second step, then back to legs 1-4-5 for the next step, and so on cycling between the two 'tripod' sets of legs for alternate steps.

As Flik starts up, it rises to its full height with all six legs fully extended. To walk, it lifts the tripod that will be free for this step (1-4-5, say) leaving the support tripod (2-3-6) fully extended and twists the articulation points to take a step. At the end of the step Flik fully extends the free legs to form a stable hexapod with the fully extended support legs, and then lifts the support legs for the next step. The advantage of this is that, once the hexapod is at its full height, little energy or time is required to shift from one set of legs to the other. The disadvantage is that each leg requires a motor (two leg motors in each section).

By contrast, Hexapod_I uses a rack through the center of each section to shift from one leg to another -- only one leg motor is required in each section. However, it also means that the bot must 'sit down' in the middle of each step while it is shifting weight from one leg to another. The result is that it must 'climb' back up from a gravitationally stable position each time it shifts weight from one tripod to another, and this takes more time and energy than the method used in Flik's design.

Note on walking

The ground covered by an articulated hexapod design such as Hexapod_I is proportional (all other things being equal) to the angle through which the articulation points move. If you are contemplating this kind of design you should

  • design for the tips of adjacent pairs of legs to pretty much meet when the articulation is twisted at its maximum angle.
  • design that the distance from the articulation point to the point where legs exit the robot is the same for each segment.
What's nice about Hexapod_I is that it is a *big* robot with a big turn angle on the articulation points -- it's pretty menacing-looking when it's walking around the room.

Note on turning

All in-line hexapod robots have difficult turning, and Hexapod_I is no exception. The turning radius is large and uses a fairly ugly 'skid-steer' approach.

CONTROL

Control was primitive -- two 'whiskers' at the front led to touch sensors. The program walked forward until the whiskers detected an obstacle and then the robot backed and turned to try and walk around it.

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