DragonBot has two continuous rotation servos, one on each side; this allows it to drive forwards, backwards, and turn on a dime. A large, high torque servo found in the middle of the base is used to spin a rod that functions as a pulley. This pulley has a string tied to it that is hooked to one of the bars in the four-bar mechanism. The four-bar mechanism consists of four bent strips of steel, and is supported by two 16" lengths of aluminum box extrusion. By using the pulley to draw in or let out string, the grabber on the end of the four-bar mechanism can be raised and lowered. The four-bar mechanism allows for the grabber to remain parallel to the ground as it goes up and down. The grabber operates using a continuous rotation servo with a gear on the end. The servo is on a linear bearing so that as the gear spins, the whole thing moves along a rack, allowing it to pinch objects between two platforms. If that was a bit confusing, just take a look at the photos below.
In terms of things still left to to, the most important one is to machine grooves for e-clips on each rod so that everything doesn't slide around. I could also come up with a more permanent and secure fix for mounting the grabber on the four-bar mechanism. I also would like to add rubber pads on the rack and pinion grabber to increase its grip, allowing it to pick up heavier and more complex shapes.
All in all, I think DragonBot is a great success. The grabber works perfectly and the the lifting arm is extremely strong. Even so, if I attached the string on the pulley further back on the four-bar it would increase the mechanical advantage by about 3x! Check back soon, I want to get a video of DragonBot in operation!