novel robotic joint design

This work was performed while I was employed at the Italian Institute of Technology. After completing the COMAN design, I was tasked with developing the next generation of high-performance humanoids for the Advanced Robotics dept. of IIT.

This concept combines two antagonistic series elastic actuators. A high-power, high stiffness actuator drives the joint directly while on the other side a bungee chord is used to store a large amount of energy, which can be regulated using a second motor which pre-tensions the bungee. The bungee can be tightened to store energy for explosive movements such as jumping, or can be adjusted to match the frequency of the system to conserve energy during periodic movements such as walking or bouncing. I designed and built this single-degree-of-freedom leg to demonstrate the concept. I completed the entire robot, from concept to finished device, in two months. The system was lightweight (under 4kg for the whole leg) and was shown to be extremely robust (see video below). By adjusting the tension of the bungee during periodic hopping, the system used only 20% of the energy than with no bungee attached.

For full details see the published paper: "An Asymmetric Compliant Antagonistic Joint Design for High Performance Mobility", N.G. Tsagarakis, S. Morfey, G.M. Cerda and D.G. Caldwell, IEEE IROS 2013.

The main drive motor of the leg (left) uses a titanium torsion bar (stiffness 10KNm/rad) to provide some elasticity to protect the transmission during impacts. Peak torque is 250Nm. Torque sensing is done by measuring the deflection of this torsion bar. This torque sensor worked extremely well: it had a resolution of 0.02Nm and gave an extremely clean signal with high repeatibility and no hysteresis.

The second motor, which tightens the bungee to store energy, needed a two-way overrunning clutch so that it did not require power to hold the bungee under tension. The picture below shows the design I came up with. The rollers lock against the inside of the housing when the motor is backdriven, instantly preventing motion. Load cells are used to measure the tension in the bungee.

The main actuator of this prototype leg eventually became the design that was used in the Walkman robot, which was built shortly after I left IIT.