Tuesday, May 08, 2012

The Symbrion swarm-organism lifecycle

I've blogged before about the Symbrion project: an ambitious 5-year project to build a swarm of independently mobile autonomous robots that have the ability - when required - to self-assemble into 3D 'multi-cellular' artificial organisms. The organisms can then - if necessary - disassemble back into their constituent individual robots. The idea is that robots in the system can choose when to operate in swarm mode, which might be the optimal strategy for searching a wide area, or in organism mode, to - for instance - negotiate an obstacle that cannot be overcome by a single robot. We can envisage future search and rescue robots that work like this - as imagined on this ITN news clip from 2008.

Our main contribution to the project to date has been the design of algorithms for autonomous self-assembly and disassembly - that is the process of transition between swarm and organism. This video shows the latest version of the algorithm developed by my colleague Dr Wenguo Liu. It is demonstrated with 2 Active Wheel robots (developed at the University of Stuttgart - who also lead the Symbrion project) and 1 Backbone robot (developed at the Karlsruhe Institute of Technology).

Let me explain how this works. The docking faces of the robots have infra-red (IR) transmitters and receivers. When a 'seed' robot - in this case the Active Wheel robot on the left - decides to form an organism with a particular body plan, it broadcasts a 'recruitment' signal from its IR transmitters, with the 'type' of robot it needs to recruit - in this case a Backbone robot. The IR transmitters then act as a beacon which the responding robot uses to approach the seed robot, and the same IR system is then used for final alignment prior to physical docking.

Once docked, wired (ethernet) communication is established between robots, and the seed robot communicates the body-plan for the organism with the newly recruited Backbone robot. Only then does the Backbone robot know what kind of organism it is now part of, and where in the organism it is. In this case the Backbone robot determines that the partially formed organism then needs another Active Wheel and it recruits this robot using the same IR system. After the third robot has docked it too discovers the overall body plan and where in the organism it is. In this case it is the final robot to be recruited and the organism self-assembly is complete.

Using control coordinated via the wired ethernet intranet across its three constituent robots, the organism then makes the transition from 2D planar form to 3D, which - in this case - means that the 2 Active Wheel robots activate their hinge motors to bend and lift the Backbone robot off the floor. The 3D organism is now complete and can move as a single unit. The process is completely reversible, and the complete 'lifecycle' from swarm -> organism -> swarm is shown in this video clip.

It is important to stress that the whole process is completely distributed and autonomous. These robots are not being remotely controlled, nor is there a central computer coordinating their actions. Each robot has the same controller, and determines its own actions on the basis of sensed IR signals, or data received over the wired ethernet. The only external signal sent was to tell the first robot to become the 'seed' robot to grow the whole organism. Later in the project we will extend the algorithm so that a robot will decide, itself, when to become a seed and which organism to grow.

The Symbrion system is not bio-mimetic in the sense that there are (as far as I know) no examples in nature of cells that spontaneously assemble to become functioning multi-cellular organisms and vice-versa. It is, however, bio-mimetic in a different sense. The robots, while in swarm mode, are analogous to stem cells. The process of self-assembly is analogous to morphogenesis, and - during morphogenesis - the process by which robot 'cells' discover their position, role and function within the organism is analogous to cell-differentiation.

While what I have described in this blog post is a milestone following several years of demanding engineering effort by a very talented team of roboticists, some of the ultimate goals of the project are scientific rather than technical. One is to address the question - using the Symbrion system as an embodied model - of under what environmental conditions is it better to remain as single cells, or symbiotically collaborate as multi-celled organisms. It seems far fetched but perhaps we could model - in some abstract sense - the conditions that might have triggered the major transition in biological evolution of some 1000 million years ago which saw the emergence of simple multi-cellular forms.