CARL - the Complaint Robotic Leg

Contact: Steffen Schuetz, Atabak Nedjadfard


Further Information


This research thread is pursuing the ambition to bring the Bio-inspired Behavior-Based Bipedal Locomotion Control (B4LC) to a physical platform. Based on the experiences gathered by the deployment of B4LC to the simulated biped, the requirements on a physical system could be well defined. In order to render natural-looking and energy-efficient bipedal walking, a main concept of B4LC is to exploit the passive dynamics of the underlying system. Thus, the system should as far as possible imitate its anthropomorphic counterpart - regarding e.g. the actuation, the kinematic layout, and the weight distribution.

As a first iteration, the Compliant Robotic Leg (CARL) has been developed. It is a planar robotic leg that features mono- as well as bi-articular actuation. As the actuation is a key component in a robotic leg, a series of linear Series Elastic Actuators - the RRLab SEAs - has been developed. The design was mainly driven by two requirements: the capability to act as a force/impedance source and a inherent tolerance against impact forces. Each RRLab SEA is encapsulated by a dedicated FPGA-based system. For more information see the dedicated page linked above.

Within the leg the SEAs are acting on the joints either by direct or by four-bar linkages. This resulted in redundant system in which all five SEAs are coupled. Motivated by the scientific evidence that human amputees are well capable of producing natural walking using a combination of a SEA and a off-the-shelve prosthetic foot, an ots product is used for the leg as well.

To enable a walking motion with a single leg a test rig incorporating a treadmill and a lifting mechanism has been developed. Especially the latter is important as it allows for the imitation of a second, virtual leg.

After validating the low-level impedance and force control even the case of the fully coupled system, a first walking motion could be generated. Therefore, a subsystem of B4LC has been ported to the leg.