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Amphibionics 04 3/24/03 8:23 AM Page 54
FIGURE 4.4
Spring-loading
mechanism with limit Amphibionics
switch sensor.

When both servos have been positioned so that the springs are
loaded and the legs are in their jumping position, the microcon-
troller gives both servos the command to move forward. This
moves the lever past the position where the spring is loaded, at
which time the spring quickly pulls the upper leg mechanism
downward, giving the legs enough energy to leap the frog forward.

R/C Servo Motors

The R/C servo is a geared, direct current motor with a built-in
positional feedback control circuit, as pictured in Figure 4.6. This
makes it ideal for use with small robots because the experimenter
does not have to worry about motor control electronics.

A potentiometer is attached to the shaft of the motor and rotates
along with it. For each position of the motor shaft and poten-
tiometer, a unique voltage is produced. The input control signal is
a variable-width pulse between 1 and 2 milliseconds (ms), deliv-
ered at a frequency between 50 and 60 Hz, which the servo inter-
nally converts to a corresponding voltage. The servo feedback cir-

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