In order to reach our future goal of a highly mobile, legged exploration robot, the DLR Crawler is a first experimental platform to test various control, gait and navigation algorithms. It uses the fingers of DLR Hand II as legs. These are well suited for this prototype due to their broad range of proprioceptive sensors measuring joint and motor angles as well as joint torques and foot forces.
Fig. 2: Connection schematic of leg coordination rules of the DLR Crawler
This set of sensors allows to implement different active compliance control algorithms as well as many reactive behaviors. The robot comprises 18 active degrees of freedom, 3 per leg, which are actuated by permanent magnet synchronous motors in combination with harmonic drive gears and tooth belt transmissions. Since it is a laboratory testbed, the Crawler uses an external 24 V power supply and external computation. This allows testing different algorithms with varying computational complexity without caring about hardware limitations at this stage of the research. In order to achieve a fast 1kHz control loop, the Crawler employs a serial SpaceWire-based communication. Nevertheless, future designs will have onboard computation and onboard power supply.
The gait algorithm of the DLR Crawler draws inspiration from nature and implements findings of stick insect experiments conducted by biologist Holk Cruse and co-workers. It uses a decentralized approach, where neighboring legs influence each other by inhibiting or exciting stepping motions. Thus, the gait of the Crawler emerges and varies from a slow wave gait via a tetrapod to a fast tripod gait. Additionally, the gait algorithm allows omni-directional motion and the use of reflexes. Elevator, stretch and search reflex enable the robot to autonomously negotiate obstacles within its walking height. Another interesting feature of the algorithm is its ability to immediately adapt to the loss of a leg without getting unstable. In order to navigate in unknown terrain a stero camera head is mounted on the Crawler. Using the stereo vision algorithms developed at the institute, the robot is able to assess the terrain traversability and to plan a path to a desired goal soley based on the information acquired by its own sensors.
M. Görner and G. Hirzinger, “Analysis and Evaluation of the Stability of a Biologically Inspired, Leg Loss Tolerant Gait for Six- and Eight-Legged Walking Robots”, in IEEE 2010 International Conference on Robotics and Automation, 2010, pp. 1525 – 1531.
A. Chilian and H. Hirschmüller, “Stereo Camera Based Navigation of Mobile Robots on Rough Terrain”, in IROS, International Conference on Intelligent Robots and Systems, 2009, pp. 4571–4576.
M. Görner, T. Wimböck, and G. Hirzinger, “The DLR Crawler: evaluation of gaits and control of an actively compliant six-legged walking robot”, Industrial Robot: An International Journal, 36(4), (2009), pp. 344–351.
M. Görner, T. Wimböck, A. Baumann, M. Fuchs, T. Bahls, M. Grebenstein, C. Borst, J. Butterfass, and G. Hirzinger, “The DLR-Crawler: A Testbed forActively Compliant Hexapod Walking Based on the Fingers of DLR-Hand II”, in IEEE/RSJ 2008 International Conference on Intelligent Robots and Systems, 2008, pp. 1525 – 1531.