PROSTHESIS: THE ANTI-ROBOT
Fig. 2: The Alpha Leg next to Prosthesis
The most important distinction from its sci-fi counterparts is that it is a real machine, which means there are consequences when something fails or goes out of control. From a hydraulics perspective, this equates to a need for durability, reliability, and stability. Because it is built for sport, however, it has to feel right. This means the controls have to be responsive, light, and ergonomic to deliver the ideal experience to the pilot. Excessive automation and technological intervention are not necessarily the right solution. Prosthesis is a sports machine, and the pilot is the athlete. It would undermine the purpose of the machine to simply hand over the controls to computers. Putting the human in control has several advantages: 1) it will save us from the robot apocalypse, 2) it creates a rewarding challenge for the pilot, and 3) it takes advantage of the most sophisticated and adaptable control system in the world, the human nervous system. This is often referred to in robotics as “human-in-theloop-controls.” For a racing machine, the controls must be repeatable and stable enough to master with practice but require low enough input forces that the machine can be operated for at least 20 minutes without excessive fatigue. To start out, we wanted to keep things simple and direct.
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FIRST-GENERATION INTERFACE PURE MECHANICAL - A TECHNOLOGICAL BASELINE The first incarnation of the control system represents the simplest possible way to achieve the basic positional feedback control system we needed. It used a purely mechanical “hydraulic logic” circuit to control mechanical spool valves, which directed hydraulic power to the working cylinders in the leg. The first-generation exoskeletal interface is shown with the Alpha Leg in Fig. 4. This system used three, small, passive, lowpressure hydraulic cylinders per joint, plumbed in parallel: an input cylinder attached to the exoframe, a feedback cylinder in the corresponding joint in the leg, and a signal cylinder on the lever of the corresponding spool valve. The schematic of the complete system is shown in Fig. 3. When the pilot moved his arm, flow would go from the input cylinder on the exo-frame to the signal cylinder on the spool valve atop the platform and open the valve (essentially remotely operating the valve). This would cause the joint on the leg to move and bring the feedback cylinder with it, which would move the signal cylinder on the valve in such a way as to close
the valve once the leg joint caught up with the position of the exo-frame. If the pilot wanted to keep the valve open to keep the joint moving, he would have to keep moving his arm. This created a very smooth and stable positional feedback control system with essentially only three moving parts (aside from the spool valve). It was interesting to note that if the feedback cylinder (in the joint) was plumbed directly to the signal cylinder (on the spool valve), resonance and shuddering would occur at certain flow rates. By putting the pilot in the middle of the loop, the feedback signal was sufficiently damped before affecting the valve, creating a stable system across all operating conditions. While functionally very satisfactory, the system suffered from three main problems: a large deadband, limited adjustability, and high input forces.
SECOND-GENERATION INTERFACE THE INTRODUCTION OF ELECTRONICS – PERFORMANCE AT THE EXPENSE OF SIMPLICITY The next-generation control system, shown in Fig. 5, addressed all of these problems by switching to solenoid valves and controllers donated by