Linear Guide Shafts
After consulting the open-source SEA developed by Yobotics, I decided that our intended SEA design was insufficiently robust under bending moments. Under compression from each end, the actuator would have buckled, damaging the ball screw. Therefore, I added steel linear shafts (Figure 1) to serve as guides for the ball nut and spring module, and to support the end of the ballscrew. These shafts do add weight to the design, so I am looking at replacing them later on with aluminum or ceramic-coated aluminum linear shafts, although those are likely to be expensive.
SEA Spring Module
The spring module of the SEA (Figure 2) incorporates two pairs of two springs. One of the two pairs is under compression when the actuator is subject to compressive force. The other pair is under compression when the actuator is subject to extensive force. When not compressed, each pair of springs floats between plates, secured in the center by a shaft or spacer. A linear potentiometer is mounted to measure the deflection of the central element of the module relative to the outside plates. The force exerted by the SEA can be calculated based on this deflection and the spring constant.
Part Fabrication and Assembly
I continued fabricating parts using the Makerbot Replicator 2X. I cut the linear shafts using the abrasive chop saw at Techshop. I found instructions for cleanly cutting threaded rod and created a jig to hold the rod while cutting it. I greased the bearings, and inserted the brass thread protectors provided with the bearing nuts. Then, I assembled the ball screw mounting hardware and tightened the bearing nut to hold the assembly together. The brass thread protector prevents the bearing nut set screw from marring the thread on the ball screw. Finally, I mounted limit switches to prevent the ball nut running off the ball screw in each direction. These limit switches will be hooked up to a relay module that will cut power to the motors if the travel limits are reached. These limit switches are not triggered during normal operation.
Short Ball screw Stroke
When selecting the ball screws, we chose a stroke length equal to the expected range of motion of the moment arm attachment at the joint. When we were making this choice, I didn't know that the easiest way to mount springs in an SEA is apparently to mount two pairs of springs in line with the ball nut. One set of springs sticks out to each side of the ball nut, so this design necessitates a ball screw stroke that is longer than my estimate by about twice the spring length (Figure 3). In retrospect, we should have just selected ball screws with a long enough stroke to accommodate this design, as the knee ball screw, which we had the option of choosing the length of, has a low mass/length. The ankle ball screw was ordered from a secondhand vendor and we had a limited choice of lengths available.