Re: Jaw Motion with Servo?

What you are looking for is called a four bar linkage. (see
http://www.flying-pig.co.uk/mechanisms/pages/linkage2.html ) The
vertical part of the jawbone is one bar. The servo horn is another
bar. A piece of rod between the servo horn and the jaw is the third
bar. The solid structure of your animatron's head what the servo and
the jaw hinge are attached to is the fourth bar.

The easiest way to make a four bar linkage is to make the length of
the jawbone bar the same length as the servo horn bar and have the
center of the servo axis on a horizontal line directly behind the axis
of the jaw bone. With the jaw closed, the servo horn should be
pointing downward, parallel to the vertical part of the jaw. The
connecting rod going from the servo horn to the jaw will be horizontal.
To open the jaw, the servo is activated to move the bottom of the horn
away from the jaw. Since the connecting rod can not stretch, the jaw
has to rotate to keep the maintain the same distance between the points
on the servo horn and the jaw.

One thing you have to be careful about when designing a linkage is that
you don't make a structure that will bind - i.e. get into a
configuration where it can not move without some part of the linkage
bending. In the old days, before solid modeling, linkages were tried
out using cardboard cutouts and a type of push pin where the shank was
made of two pieces of brass that could be bent around (any of you old
guys remember what these are called? I still have a box of them in my
desk at work, but I won't be back there until next week). Try to make
a model of your linkage in cardboard before you start drilling holes in
your animatron. You can also try to learn how to use the linkage
modeler available at: http://iel.ucdavis.edu/chhtml/toolkit/mechanism/

Paul Pawelski

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