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The DLR Crawler - an Actively Compliant Walking Hexapod Robot

Fig. 1: The DLR Crawler within the gravel testbed

For planetary exploration as well as for search and rescue missions on Earth groups of hexapedal walking robots seem to be a promising alternative to wheeled or tracked vehicles. They offer several advantages like, no need for a path of continuous ground contact, the ability to step on or over obstacles, to climb rock formations and to walk along steep slopes with varying ground substrates. Furthermore, they are inherently redundant and can handle the loss or damage of several legs being more or less impaired, but still able to move. Depending on the design, the legs can also be used as manipulators in order to collect material samples or to hold and operate tools like a drill. Nevertheless, this versatility comes for the price of a higher complexity in mechanics and control, which poses many interesting questions that are subject of active research.

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.

Fig. 3: DLR Crawler within the gravel  testbed	Fig. 4: Stereo vision based navigation map  of the gravel testbed and walked path

Videos

The DLR Crawler walking in the testbed

References

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 for
Actively 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.