Design and assembly notes
Because of the simple mechanics, except for the 3D-printed parts, the design requires only the servos, ball bearings (where plastic parts move against each other), a battery, and a simple circuit board with pico-controller (Fig. 2.1). The standard version also includes a small OLED display showing the state of the robot, and three front-mounted distance sensors.
A complete list of parts can be found here; alternatives are listed under modification and sensors.
| Fig. 2.1 - Overview |
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All .stl files are (now) oriented as they should be printed. The following parts need support: rear_bottom.stl, rear_bottom_lipo1000.stl, board_carrier.stl, sensor_adapter_tof_pololu.stl and front_bottom.stl.
I used a Dremel 3D45 (default settings "Medium Quality - 0.2 mm; gradual infill") and ECO-ABS as printing filament. PLA usually works as well; however, my impression is that the parts are more brittle and you have to be more careful when bending parts up a bit to assemble them.
In all places where you can put a screw through, M3 through-bolts (i.e. machine screws with nuts) - except when mounting the board, where you need M2 through-bolts. Where something is screwed into a plastic block (e.g. the servos), you need small pull-through screws; here I use the screws that usually come with servos. For the "hinge" on the sensor arm, I also use small pull-in screws.
Deep groove ball bearings (6 x 3 x 2.5 mm MR63ZZ) must be used at the front bottom (left and right) and at the rear top and bottom (left and right) so that the legs fit well and move easily.
The servo horns should be fitted so that they sit as close to the center of the servo range as possible when the respective pair of legs is in the neutral position.
The front and rear legs of one side are connected at the bottom with a rod to move them together with a servo. There are three different length rods; depending on the rod length, the basic position of the legs is slightly different (more perpendicular to the sides or angled slightly forward and back). Which bar you choose is a matter of taste; however, the longer the bar, the more the range of motion of the legs is restricted. Deep groove ball bearings should also be used here.
To make the feet slip less, you can put a short piece of shrink tubing over them (tip from Carsten Wartmann) or alternatively use "rubber shoes" (e.g. these; tip from Carsten Otto).
WARNING: The socket J2 was designed for the 5V step-up voltage converter module Pololu U1V10F5. However, this module is no longer available. An almost equivalent replacement is the module U3V16F5. But there the pins for VIn and VOut are swapped, i.e. the replacement module must be plugged in "head down" (=components towards the board)!!
Lead the cable of the middle servo down through the hole in the base plate on the right side (seen from behind). Now you can thread all three servo cables from below through the central hole in the base plate to the top; when you have soldered the connections for the servos to the bottom of the board, almost all the wiring is inside. The cable from the battery, however, I lead around the outside, because you have to be able to change the battery easily. Depending on the length of the cables, you have to shorten them a bit.
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