A robot powered on a diet of flies
One important factor for robots is that of energetic autonomy. Robots will be required to extract energy from the environment. In many ways robots will face the same problems as animals. An earlier phase of our work centred around the use of slugs as a bio-fuel. This resulted in the construction of the 'SlugBot' - a robot which could identify and pick up slugs to be used in an anaerobic 'digester'.
The main objective of our work is to build energetically autonomous robots. We believe that microbial fuel cell technology is a good way forward, as the robot will incorporate in its behavioural repertoire actions that involve search and get hold of food and also remain inactive until energy is sufficient to do the next task. This will be a paradigm shift in the way action selection mechanisms have been designed so far.
Our current work is now focusing upon the use of plant material as a source of energy. In this work we are exploring the use of microbial fuel cell technology - 'bug power'. The project is code-named EcoBot and the first stage of this investigation is now completed. This involved the construction of a proof-of-concept sugar-eating robot, named EcoBot I, that follows the light.
EcoBot II Project
The next stage, involves the construction of a robot, which is called EcoBot II that also performs phototaxis but at the same time reports the temperature remotely. It is powered by Microbial Fuel Cells (MFCs), containing a flora of microorganisms originating from sludge and fed with dead flies or rotten fruit.
In contrast with its predecessor, EcoBot I, which was the first robot in the world to acquire all its onboard power from MFCs (i.e. it carried no batteries) utilising refined fuels (sugar), EcoBot II utilises raw foodstuffs such as flies or rotten apples. It is also the first in the world to employ the gas (O2) diffusion cathode, which in terms of autonomy is extremely important. What is novel about our work is the integration of MFCs fed with raw substrate and utilising O2 from air with a small scale robot.
In the MFC anode, bacteria found in sludge, act as catalysts to generate energy from the given substrate (flies or rotten apple). In the MFC cathode, O2 from free air acts as the oxidising agent to take up the electrons and protons to produce H2O. This closes the circuit and keeps the system balanced. In a different cathode configuration, ferricyanide (K3Fe3-[CN]6) acts as the oxidising agent to close the circuit. Both of these systems have been tried with similar success.
A total of 8 MFCs wired up in a series configuration have been used to power the robot which moves in a ‘pulsed’ mode. This means that the robot movement is discontinuous i.e. when a low threshold level is reach, the robot becomes ‘inactive’. In the mean time energy from the MFCs is accumulated in a bank of 6 capacitors until a second (higher) threshold level is reached. In this case the robot resumes power and moves towards the light whilst transmitting wirelessly the value of temperature at that point in time. The whole concept of the experiment is shown in the schematic diagram of Figure 1. An electronic circuit switches between the two thresholds and directs power to either or both motors (depending on the reading from the photodiodes) and the wireless transmitter.
Figure 1. EcoBot II with the O2 cathode MFCs moving towards the light whilst transmitting the temperature to a base-station. The maximum (indoors) range of transmission is 30m.
Relationship with other work
EcoBot II is not the first robot in the world to use bacteria. The first was Wilkinson’s Gastronome (Chew-chew) in 2000, which employed chemical Fuel Cells to charge up a bank of Ni-Cd batteries. Power was generated by E. coli fed with refined sugar, and a synthetic mediator (HNQ) enhanced the electron transfer process to the chemical fuel cells.
As far as MFCs are concerned, we are not the first group in the world to exploit sludge, and certainly not the first in the world to use the O2 cathode. To the best of our knowledge the first sludge MFC reported in the scientific literature was from Habermann and Pommer back in 1991, in which case they had a stack of MFCs running continuously for 5 years. In later years, Park and Zeikus (2002) had done some significant experiments with sludge, E. coli and neutral red. And more recently, but certainly before us, Logan’s group at Penn Sate, have illustrated power generation from sewage sludge and more importantly with and without using a proton exchange membrane.
Figure 2 is a picture of the EcoBot II with the wireless transmitter onboard, powered by MFCs with the ferricyanide cathode.
Figure 2. EcoBot II fully assembled powered by MFCs with the ferricyanide cathode
Figure 3 (a) is a graph of the distance travelled by the robot versus time and (b) is a graph of the average temperature transmitted versus distance. Both graphs show the average values from the 5 times that the experiments were repeated.
Figure 4 is a snapshot from the EcoBot II powered by MFCs with the oxygen (O2) cathode performing phototaxis.
Figure 4. EcoBot II performing phototaxis powered by MFCs with the O2 cathode.
Figure 5. Labelled schematic representation of EcoBot II.
Microbial fuel cell versus alkaline battery
A single MFC is no match to a standard battery or a solar panel. Below is a comparison between a standard AA size alkaline battery and a typical MFC, after 1 year of continuous operation.
|Alkaline Cell-AA||1.5 V||2.8 Ah||4.2 Wh||25 g||604 J/g||£0.30||Will run out|
|MFC||0.8 V||1.05 Ah (~120uA for 1 Year)||1 Wh (100uW for 1 Year)||10 g||360 J/g||<£1||Will keep on going|
Although the origin of both technologies dates back to Luigi Galvani, batteries have had centuries of development, whereas MFCs are still in their infancy stages. However, the most important difference between the two technologies is the fact that an MFC can provide continuous energy supply for as long as the bacteria can be kept alive. Potentially this can be years in a continuous flow system, where there is continuous inflow of key ingredients and continuous outflow of waste products.
- Dr Ioannis
Tel: +44 (0) 117 32 86318
Tel: +44 (0) 117 32 86322
BioEnergy and Self Sustainable Systems
Bristol Robotics Laboratory
University of the West of England
Bristol, BS16 1QY