Leg Design Improvements
After consulting with Ben Brown, I moved the series elastic actuator attachment points to be in line with the center of the actuator (Figure 1). This nearly eliminates bending moments on the actuator. Bending moments are bad because they increase the friction on the linear slide, potentially reducing the accuracy of the force measurement.
To accomplish this, I offset the shank, using the shank and the end attachments to bear the bending moment. This redesign also completely eliminated the previous 80/20 aluminum shank material. As a result of these design changes, the leg reduced in weight by about 0.75 kg.
Most of the parts in the series elastic actuators in the leg have only 2D or 2.5D geometry, making them well-suited for cutting on the waterjet at TechShop. Therefore, I have been testing different speed vs. accuracy tradeoffs on the waterjet. I will have to ream out bearing and bushing holes, as well as the holes to mount the linear guide shafts. I will cut these slightly undersized on the waterjet. I confirmed that screw tap and clearance holes come out accurately.
Finally, I took the RI Machine Shop safety training from Chuck, so I can now access the RI shop. While I plan to cut the outline of the parts and cut the tap and clearance holes on the waterjet at TechShop, I will try to do milling and CNC milling at the RI shop. The CNC milling may not be necessary for most of the parts now that the design has been simplified.
I selected appropriate flat stock (7075 aluminum, 12”x12”x0.5”) and ordered it. In addition, I found appropriate fan+heatsink combinations and ordered those.
Leg Stump Simulator
I obtained the stump simulator from Professor Collins’ group and have unlimited access to it until mid-March. At that point, I will work with a PhD student in their lab to establish a week-by-week schedule for borrowing it. To determine the appropriate overall design length for the leg, I made two sizing devices that fit into the bottom of the stump simulator and extend rigidly to the ground. I also identified the dimensions and fastener types used to attach to the bottom side of the iWalk knee brace device.
Improved Actuator Design
This redesign was a substantial amount of work. I had been using the 80/20 stock as a crutch, and so now I had to redesign nearly all of the components in some way so that they would fit the new necessary dimensions. One of the biggest difficulties was due to the limited stroke of the ballscrew for the knee. During the early design phase, we didn’t leave enough extra stroke on the ballscrew to accommodate fasteners, attachments, etc.
The mechanical design and fabrication is taking longer than expected, so I am trying to think of creative ways to reduce the time necessary. I’m also trying to decide how to keep the existing (FVE) version of the leg intact as long as possible in order to let John and Jessica test the control algorithms.