Spider robot with PIC16F84

  
To get enough outputs from the PIC chip, a pair of 74HC164 shift registers was used. These need only two lines from the PIC (data & shift) to produce the twelve signal pulses for the servos. A single bit is shifted into the first shift register, then, after a specific delay, it is shifted to the next servo. The second shift register cascades off the first. This system can be extended for any number of servos. The idea of this design is that no power is needed when the legs are in the full down position. They also create a lot of extra leverage at the bottom of travel. This helps overcome friction as the leg is moving horizontally inward at this time. When the leg is raised, the velocity increases with height making it easy to raise the leg over an inch.
Spider robot with PIC16F84 - schematic

The platform on top holds the downloader board when trying out new code. The full schematic is available for download as is the source listing. As of this point, I have worked out walking forward and backward, and turning left and right. The rest, as they say, is left as an exercise for the student. The HEXBOT is constructed from 12 (cheap) model airplane servos. Each leg moves laterally on one servo and vertically on the other. Conveniently, this type of servo runs on 5 volts. These servos need only a specific width pulse to position it. Once it arrives at its target location, it draws very little power (about 10 ma). Power is provided by a set of four AA metal hydride batteries mounted underneath. Neither the servos or the PIC requires highly regulated power. An earlier version of the hexbot had legs made out of 1/4 inch aluminum square stock with a 90 degree bend. This caused the vertical servos to draw a lot of power just standing still. It could barely walk. The new leg design uses a piece of 1/2 inch aluminum angle stock (see the close-up), and two peices of the 1/4 inch square stock. The parts are joined with #6 screws and those plastic-insert 'aircraft' nuts. The nuts are tightened just enough to take up the slop but still allow movement.



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