Modlab The Modular Robotics Laboratory at the University of Pennsylvania

The XBot system is a lattice style modular self-reconfigurable robot that uses external actuation to deterministically reconfigure XBot modules. Using the principle of external actuation facilitates module miniaturization as modules do not require motors or servos to reconfigure.

XBot module


A significant goal of Modular Self-Reconfigurable (MSR) robotics is realizing systems with a large number of small modules. Increasing the number of modules increases flexibility and redundancy. Miniaturizing the modules allows for access to smaller objects and environments and higher resolution approximations of 3D shapes.

Typically a module in a MSR robot requires some internal mechanism to change its position in the robot during reconfiguration. This mechanism inhibits miniaturization due to its space, weight, and power requirements.

The XBot system demonstrates external actuators can provide the necessary force to move modules for reconfiguration. Unlike stochastic modular robots that assemble configurations in probabilistically bounded time, the form of external actuation can be tuned to reliably reconfigure modules in a known time.


An XBot consists of two ABS frames assembled in an ‘X’ shape. Each of the four ‘legs’ of the ‘X’ has a pair of neodymium magnets. Each magnet mounts to a compliant arm actuated by a Shape Memory Alloy (SMA) wire.

An XBot module bonds to another with two pairs of magnets. To reconfigure, a module breaks one magnet pair connection to its neighbor by heating the SMA wires and retracting the compliant arms attached to the magnets.

The XY stage provides the external actuation to move modules in the form of interial forces. One XBot is fixed to the XY stage table; other modules are added to this fixed module to form a connected configuration. After the reconfiguring module breaks its magnet bond, the XY stage moves in an ellipse profile. The inertial forces cause the module to rotate 180 degrees about its remaining magnet bond to it neighbor. After completing its rotation, it forms a new magnet pair bond with its neighbor.

5 XBots on XY stage table

Experiments demonstrate the ability of the XY stage to reconfigure pendulums consisting of one or two modules. Using these pendulum motion primitives and constructing configurations with meta-modules of XBots, any configuration of XBots can reconfigure to any other.


Related Projects:

RATChET: The RATChET system demonstrates gravity can be used as an external actuation force.

Related Papers:

  • P. J. White and M. Yim, “Scalable modular self-reconfigurable robots using external actuation,” in Proceedings of ieee/rsj international conference on intelligent robots and systems, San Diego, CA, 2007, pp. 2773-2778. doi:10.1109/IROS.2007.4399606
    [BibTeX] [Download PDF]
    author = {P.J. White and M. Yim},
    title = {Scalable Modular Self-reconfigurable Robots Using External Actuation},
    booktitle = {Proceedings of IEEE/RSJ International Conference on Intelligent Robots and Systems},
    address = {San Diego, CA},
    pages = {2773-2778},
    year = {2007},
    url = {},

  • P. J. White and M. Yim, “Reliable external actuation for extending reachable robotic modular self-reconfiguration,” in Proc. of intl. symp. on experimental robotics, Athens, Greece, 2008. doi:10.1007/978-3-642-00196-3_3
    [BibTeX] [Download PDF]
    author={P.J. White and M. Yim},
    title = {Reliable External Actuation for Extending Reachable Robotic Modular Self-Reconfiguration},
    booktitle = {Proc. of Intl. Symp. on Experimental Robotics},
    address = {Athens, Greece},
    url = {},
    year = {2008},
    doi = {10.1007/978-3-642-00196-3_3},

  • N. Ayanian, P. J. White, A. Halasz, M. Yim, and V. Kumar, “Stochastic Control for Self-Assembly of XBots,” in Proc. of asme intl. design engineering technical conferences & computers and information in engineering conference idetc/cie, New York, NY, USA, 2008. doi:10.1115/DETC2008-49535
    [BibTeX] [Download PDF]
    author={N. Ayanian and P. J. White and A. Halasz and M. Yim and V. Kumar},
    title={{Stochastic Control for Self-Assembly of XBots}},
    BookTitle = {Proc. of ASME Intl. Design Engineering Technical Conferences \& Computers and Information in Engineering Conference IDETC/CIE},
    url = {},
    address={New York, NY, USA},

  • P. J. White and M. Yim, “Reliable External Actuation for Full Reachability in Robotic Modular Self-Reconfiguration,” The international journal of robotics research, pp. 13-23, 2009. doi:10.1177/0278364909351942
    author = {White, P J and Yim, M},
    title = {{Reliable External Actuation for Full Reachability in Robotic Modular Self-Reconfiguration}},
    journal = {The International Journal of Robotics Research},
    volume = {},
    number = {},
    pages = {13--23},
    doi = {10.1177/0278364909351942},
    year = {2009},

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