Robotics 2010-09-05

Today the remainder of the insectronic body went together. Lesson of the day: angled aluminum is somewhat unforgiving when it comes to less than accurate cuts.

First up was making the parts for the middle leg servo assembly, putting them together, and then getting this sub-assembly mounted to the previously fabricatedĀ chassis. After the initial drilling and cutting, I had to go back and “cheat” on a couple of the holes, ultimately making them into ovals, to get the screws to mount in a way that made putting a nut on them possible.

middle_leg_servo_system_parts
middle_leg_servo_system_parts_assembled
middle_leg_servo_system_attached_to_chassis

Up next was cutting and drilling the servo horns to be used for mounting the legs to the servos. This entailed a trip to the hardware store to acquire a pair of end cutter pliers that could generate more torque than the pair I already had. Seventeen dollars! Whatever… I also acquired white lithium grease while there (for lubricating the nylon washers in a later step), but failed to acquire rubber tips (for the feet that go on the ends of 1/4″ square aluminum bar) or protective wire covering (function unclear). The girl working at the store helping me find the various parts was suitably fascinated by my rather atypical project.

servo_horn_before_modification
servo_horns_after_modification

Making the parts for the legs required a lot of cutting and drilling. Angled aluminum serves as the upper leg pieces, square aluminum as the lower leg pieces, and rectangular aluminum bar serves as the mechanical linkages that join the forward and rear legs. One of each of the upper servos gets attached to a front leg, each of which is joined to a rear leg via the aforementioned aluminum bar, causing them to swing forward and backward as a unit. A single long-ish piece of aluminum bar serves as the upper piece of the middle leg, this being attached to the lower servo and serving to tip the “insect” such that its other leg pairs can provide forward motion.

leg_parts_and_mounts

I wish that I had taken more intermediary pictures during assembly, but the finalized body looks like this.

assembled_body_front_view

assembled_body_side_view

It has definitely taken on a bug-like appearance. For the most part, the assembly seems successful. The various pieces aren’t flawlessly symmetric, which means that not all the ends of the legs aren’t flush to the ground, but that issue will find resolution in the application of the rubber tips (feet) which can be slid up or down to counter these imperfections. More concerning are issues with the mechanical linkages.

Firstly, the front/back leg pairs can’t swing as far back as the servos would allow because the sides of the mechanical linkages eventually bump into the chassis. I am wondering whether I should saw back the chassis or figure out an elevation solution. The latter might be quite tricky due to the highly coupled nature of the myriad parts.

mechanical_linkage_range_limitations

Secondly, on the left front/back leg assembly, on return from the full-forward position there is a collision between the corners of the chassis and the mechanical linkage. This is the most problematic issue as it will cause unpredictable jams and possible servo burn out. Hopefully a solution to the first mechanical linkage problem will solve this one as well.

mechanical_linkage_collision_on_return

My success criteria for this project is that the bug can walk reasonably well. If there are slight limitations in the range of motion that the legs can accomplish, that’s fine. I’ll have learned enough from the first bug to build a better second one. In any case, some minor fixes are in order before proceeding, but that’s a project for another weekend. As an epilogue, I tried playing a few games of StarCraft 2 after today’s work, but was too brain damaged to be any good.

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