Progress 10/22/2014
Conceptual Revisions
Based on further research into existing designs and on conversations with our sponsor, I eliminated the thigh link component of the leg. This allows attachment of a pyramidal adapter (standard prosthetic mounting component) as close to the knee joint as possible. Following the design choices made in the Vanderbilt transfemoral powered prosthesis, I will design the ankle actuator to mount on the back of the calf section and the knee actuator to mount on the front.
I also explored the concept of using coil springs with a linear potentiometer instead of leaf springs with a strain gauge in order to measure torque at the leg joints. These springs would mount in-line with the hollow tube that the ballscrew travels inside, allowing design of the series elastic actuator in a single assembly intended to be mounted on rotational joints at each end. The other advantage of this design is that the linear potentiometer provides a much cleaner signal than the strain gauge. However, finding springs designed to operate in both tension and compression has so far proved difficult. Also, reliably mounting the spring will likely be difficult.
Finally, I identified and obtained two motors (AM Equipment 214 Series Gearhead Motor) and a suitable driver (Sabertooth 2X25) from the MRSD inventory that will allow us to integrate and test our position control algorithm early on, before the ballscrews and motors have arrived. These motors will mount directly to the joint shafts, bypassing the actuation system. With no load, these motors operate at about 65 RPM (390 degrees per second). This nearly satisfies the requirement for the joint rotational speeds, which at their maximum are about 450 degrees per second. Unfortunately, if these motors are attached to our intended ball screws, which have a high reduction ratio, the speeds at the joints will be very slow. Therefore, the torque control testing procedure would be time-consuming and probably ill-suited as a fallback for the final validation experiment.
Leg Component Assembly
After the joint components arrived from Misumi, I revised several rapid prototyped parts that had incorrect tolerances, and then assembled the knee and ankle with encoders (Figure 1).
Challenges
Knee Actuator Proximal Mounting
The proximal mount of the knee actuator must be cleverly designed to allow for 90 degree knee flexion, which happens often during daily life (sitting, driving, etc). Basically, the knee actuator, which is mounted on the front of the calf, must be able to connect to the pyramid adapter mount, which is oriented towards the back of the calf. This can be accomplished by a plate that sits alongside the knee joint and transfers motion parallel (approximately) to the calf to rotary motion on the other side, facing the thigh socket.
Risk of Incorrect Actuator Selection vs. Risk of Late Delivery
There are three main ways in which we could choose an inappropriate actuator system: underpowered, overpowered (and therefore overweight), and incorrect tradeoff of torque vs. speed. The effective gear reduction ratio is determined by the moment arm and the ball screw lead, with the option to incorporate a gearbox at the motor if necessary. The motor power will be supplemented by a brake at the knee and a parallel spring at the ankle, enabling the weight and cost savings that can be achieved with a lower wattage motor.
As we have learned more about all of these options, the risk of incorrect selection has steadily reduced, yet the risk of late delivery has steadily increased. At this point, we feel that the relative magnitude of these two risks has reversed, and we must select and order the actuation components as soon as possible. To somewhat mitigate the risk of late delivery, we will maintain two assembled test-beds. One will incorporate the alternate motors identified above. The other test-bed will allow mounting of the ballscrews and motors as they arrive.