R & D Update (October 6, 2015)

As I have been on vacation for the past week or so, I have set myself the goal of completing the major hardware elements (3D printed body parts, possibly electronics) over the next two weeks.  I have focused primarily on leg designs, but I have done some quick research into electronics and the main body design.

Over the past week, I have been looking into different leg designs.  I started with a rather large, bulky design, but I moved on when I decided it was too complex to be reliably and quickly 3D printed.
This design involved placing a servo in every joint, as just about every other hexapod I have seen has done.  I didn't like this, and it proved very difficult to design, so I moved on to another idea.

The finger-inspired design in the up position

For the next design, I decided to draw inspiration from the human finger and create a system with two tendon-like wires.  The idea was that you could have a servo inside the body pull a wire and the leg would contract and move downwards.  The servo could also pull another wire to lift the leg above the body.  These wires would be wound around a spool inside the body, one wound clockwise, the other counter-clockwise.  This winding would ensure that I would be able to tighten one wire and loosen the other with a single servo.

The finger-inspired design in the down position

 When I first came up with this design, I thought it was the solution to all of my design problems, it didn't have complex joints, heavy servos on the leg, and the parts printed very quickly.  However, I soon found that, while these were acceptable as fingers, they were hopeless as legs.  Any load applied from the base would cause the leg to fold upwards, regardless of the tension in the wires.  Because this was a fundamental flaw in my design, I went back to the drawing board...

I had to return to my previous idea of a servo in each joint.  However, I didn't want to create the same heavy and complicated design that I started with, so I revisited the idea of an asymmetric leg.  This was an idea I had a while ago (I may also have seen this elsewhere on the internet) involving a mount to the servo's horn without a mount to the other side, a one-sided axle.  I didn't like this to start with, because I worried about the design resulting in an off-center load and too much weight on the servos' motors, causing damage.

The Asymmetric Leg Design from the front

Despite these worries, I decided that this was the best way forwards.  I created a new design that allowed my servos to be swapped out quickly and attempt to minimize the weight put on the servos' motors.  On the left is the resultant design.

The Asymmetric Leg Design from the back

 From these images, you can see the side-entry bracket style in use and the lack of pivots on the back of the servos (the other design would have included these).

The control board is also visible in these images.  This board is a mostly complete soft (still on the breadboard) mock-up of the StIRLing control system.  It consists of an Arduino Mega and two daisy-chained TCL5940 PWM controllers for servo control.  I'll explain the board more when I get a circuit diagram (and possibly board design) locked in.

Right now, I am just test driving this new design to ensure everything is ready before entering the next stage of the project, leg production and main body design.  To do this, I created a very basic Arduino script that allows me to set servo positions manually, allowing me to check joint limitations and stability.

The asymmetric leg design running a test script.