Design Notes

SolidWorks Files

Contact David Moore, Andrea Terassi, Nick Szczecinski, or myself for access to the MantisBot SolidWorks files.

Solidworks Files for servos and accessories can be accessed via: http://www.robotis.com/xe/dynamixel_en#d04

Purposeful Design Decisions

                A general outline of the mechanical design of MantisBot’s legs is provided in my thesis (Chapter 4), and should be consulted before making any changes. Below is a summary of what is presented there, as well as some additional thoughts.

  • Points were chosen for the end of MantisBot’s legs to maximize ground friction (assuming MantisBot would be walking on carpet).
  • The coxa of the middle and hind legs are designed to place the rotation axes of the ThC1, ThC3, and CTr joints as close together as possible.
  • The relative placement of the joint angles are chosen to represent a scaled version of Nick’s mantis model.
  • The 3D printing of the AX-12A accessories as a part of the femur was done to eliminate slop in those interfaces. There was some concern with early design that connecting a femur to a purchased OEM frame using fasteners could easily come loose and result in undesirable deflections.
  • The aluminum mounting for the MX-64T FTi joints were purchased for convenience. A redesign of the femurs could include a piece such as this as a part of the design.
  • The tabs on the inside of the femur and tibia were placed there to mount springs to assist the FTi joint. This was a bad idea, and no longer necessary after replacing those servos with MX-64Ts.
  • The unique design of the trochanter serves the purpose of 1) protecting the strain gage, 2) providing multiple options (in 45 degree increments) for mounting the trochanter (for reading loads in different axes), and 3) being a very simple shape to allow for easy gage mounting and easy load analysis.
  • The coxa of the front leg is specifically NOT made to minimize inter-joint dimentions, but instead to mimic the scaling of the coxa from the middle and hind legs.
  • The curve in the trochanter/femur of the front legs serves the purpose of aligning the axes of the CTr and FTi joints of the front legs while maximizing each of their ranges of motion.
  • The curved pads on the front legs are so that maybe one day MantisBot may be able to grab and carry objects.
  • The strange (non-planar) geometry of the middle and hind leg femurs were made to imitate the geometry present in Nick’s mantis model. However, the non-planar nature of this model was an error, and became more pronounced when the model was scaled. It was later found that this non-planar nature is beneficial in the hind legs, helping the tibiae to meet the ground without the CTr joints touching the ground. It may or may not be beneficial to fix this and make the femur planar. Currently it is inconvenient since none of the parts faces are parallel or perpendicular to each other, making the piece very hard to machine manually should alterations be needed.

 

                If you do any more 3D Printing . . .

  • The trochanter part file was lost when Case upgraded Solidworks. The file currently in the assembly is a place holder and should not be printed. If you wish to print an exact copy of the current trochanter piece, there is a “Con4.stl” file available. However, I believe it may be worth the time to re-create a part file for this piece, particularly one in which there is a hole so that the cables can be accessed more easily.
  • The piece that connects the main section of the body to the mounting piece for the rearing servo was lost in Case’s Solidworks upgrade. This piece will need to be re-made in Solidworks should the body need to be reprinted.
  • Should at any point the body need to be reprinted, this could provide an opportunity to raise the electronics mounting board, allowing for a higher mounting of the ThC1 joints, and thus a shorter coxa, which could more closely unify the scaling between the coxa and the rest of the leg (see my thesis – chapter 4). This would represent a serious overhaul that would be a big improvement, but would also require a lot of work, and alterations to the AnimatLab body model.
  • The little tabs on the inside of the femurs and tibiae of the middle and hind legs should be removed.
  • The middle femurs were not strong enough , and needed carbon fiber reinforcement. Should they be reprinted again, it would be more convenient to strengthen them so that this is not necessary.
  • It has been determined that the femurs need to be fully hollow from end to end in order for the scaffold to dissolve away in the bath.
  • Only use polycarbonate for load bearing parts. They will try to talk you out of it. They will be wrong.
  • Currently it is possible (not easy, but possible) to reach every bolt necessary for assembly and disassembly. Be very careful if you do redesigns not the jeopardize this.