Mechanical Design

Chassis and Appendage Design 



So, if you have a fair amount of patience and want an excellent finish, aluminium will do. 

Tools you will need:

• A band-saw or hack-saw
• Drill press
• Bench grinder (assists filing)
• Flat-faced file
• Rounded file 
• Sandpaper (400, 800, 1600 grit is what I used)

These mechanical components require significant planning because of the various factors which need to be taken into account. When designing recreational/hobby quadrupeds, you will find that most chassis configurations have the following common traits which impact your design:

• Base servo's are mounted equidistant of each other to simplify gait generation.
• Quadrupedal leg-joint anatomies are similar for each individual leg. This implies that the low-level algorithm developed to control one appendage, when duplicated, will control all appendages individually.
• The adoption of a reptilian stance in frontal pattern lowers the center of gravity and provides easier control of stability
• Similar femur and tibia-fibula lengths for three degree of freedom appendages facilitate greater flexibility, as well as greater movement efficiency. Your quadruped should be able to "stand" without your servo motors being powered if your design is sound and your servomotors are of suitable spec.
• Increased femur and tibia-fibula lengths increase the knee and hip servo torque requirements
• The hip servo will experience the highest torque levels and should be of a higher spec. than the base and knee servos.  
• More mass on your appendages means lower torque requirements since your quadruped's mass is situated nearer the joints this way (Moment = Distance x Force).

I will post a section on basic static, and perhaps, dynamic quadruped mechanics at some point. For now, I cannot over-emphasize the need for accuracy when drilling the holes for your servo and servo-horn mounting positions. These holes determine the symmetry and rigidity of your quadruped's stance once assembly is complete. Truth be told, symmetry and rigidity are key and seperate the  awe-inspiring from the standard contraption.




Materials, Materials


Hi everyone!
 
So, it has been a while since I've been able to drag myself away from my quad and share some progress. Here is where I'm at. I've decided on a preferred platform and appendage design. These 2mm aluminium sheets have been laser cut and bent to 90 degrees by a local company. As you can see, I may have to get very friendly with my metal file for the desired shaping of these sheets.

When selecting a material for the chassis of your robot it is of utmost importance to select a material based not on it's affordability only, but for maximum rigidity during your quad's operation. Aluminum provides just this.

I was fortunate enough to have access to my university's band saw - this reduced dreaded days of metal file fumbling to a bare minimum. You could also use a hacksaw, but ensure the blade you use is as stiff as possible for accurate results.

Expect the final shaped mechanical components in my next post!

- Mart