Rob Davis

Siphonophora

A collaborative work between Usman Haque, Haque Design + Research and myself.

Siphonophora, a class of marine invertebrates resembling jellyfish, are the inspiration for this project. A Siphonophora is actually a colony of Siphonophorae, they consist of cells that could not survive on their own, but require the funtional differentiation of neigboring cells in cooperation to survive. Certain classes of Siphonophora are also able to emit light by bioluminesance, in some cases red light in order to attract fish. These elements have been incorporated into the installation at Osier marsh to create a parallel evolution, in a sense, to the Siphonophora organisms found in the wild. The installation consisted of three main components, centred about the lake at Osier Marsh, Gunpowder Park, a human-lake interface at the bird hide, web interface for logging behavioural and environmental data, and a collection of analogue electronic devices floated on the lake able to move, sense and interact with their environment.

The Bird Hide

Situated at a bird hide to the edge of the lake was a pair of headphone sockets driven by amplified signals from hydrophones immersed at the lake edge. Here visitors could listen to the combined sounds of the lake water, bubbles rising from lake plants, the motions of aquatic insect life and the sounds from the cellular devices floated in the lake water.

Also positioned at the bird hide was a hand cranked generator that supplied energy to an oscillator feeding an underwater speaker, the frequency of which was similar to the sounds emitted by the cellular entities, this provided a way for visitors to signal, or stimulate the floating components.


Headphone socket and hand crank mechanism attached to bird hide at Osier Marsh, Gunpowder Park


Web Interface

Using pachube an online data feed from sensor and positional information from the floating platform was arranged. The sensor package consists of power supply, sensor circuitry, energy storage, and interface circuitry to a modem. This allowed sensor data to be transmitted in the form of text messages to an SMS gateway and rendered on the pachube web site. Measurements are taken every 20 minutes with transmissions occurring every 4 hours. The sensor data consists of 5 instrument channels and positional information as detailed below:

Energy Storage was via 3.5Ah maintenance free sealed gel lead acid battery under cyclic charge from a 14.5V shunt regulator fed from a 4.5W polycrystalline solar panel. The charging circuit also included a low leakage diode to prevent losses into the solar panel at night.


pachube sensor package (foreground) and cellular entities (background) floating on lake surface


The sensor package enclosure was constructed from reclaimed acrylic sheet approximately 13mm thick, this had then been solvent welded watertight and any unwelded joints filled with silicone sealant. Nylon bolts were used throughout in order to account for thermal expansion of the plastic and to resist corrosion. In all places throughout the design an attempt has been made to preserve the transparency of the structure, this has the effect of lessening its visual impact as a collection of technological artifices and to imply a more biological presence, becoming almost cellular, having a transparent cell membrane enclosing a collection of functional blocks, much like a cell nucleus. It also enabled the structure to visually coalesce with the water surrounding it making the structure visually ambiguous and slight.

Cellular entities

The cellular entities were of two types, sensory cells and motor cells. The sensor cells served to monitor environmental conditions of motion/vibration, temperature and illumination and convert them into sound, temperature controlling the rate at which the tone pulsed, illumination controlling the pitch of the emitted tone, and motion the modulation and intonation of the tone burst.

The motor entities propelled the cell array in response to acoustic signals, they were also able to sense, using pulses of red light, objects and boundaries that they might collide with and avoid them. This is a reflex action inbuilt by design. Not all of the responses of the motor cells are fixed, but able to adapt over time in a self determined way.


Sensor cell – clear formed PETG with encapsulated circuit board and solar cells


To avoid the use of batteries and toxic electrolytes super capacitors where chosen as the main energy storage device. Charge regulation was by shunt regulator using a light emitting diode and a string of signal diodes. During periods of high sunlight, when the battery or capacitor might be at full charge the shunt regulator dissipates the excess energy converting it into heat. It effectively shorts out the photovoltaic to reduce the voltage across it. As it does not switch the output of the photovoltaic it does not introduce any loss that might be associated with a transistor switching scheme. During periods of high sunlight and low charge the regulator lies dormant until the voltage across the energy storage device exceeds a critical value, then its resistance drops. By placing this circuit behind a low leakage diode, with respect to the charge storage device, any possible currents back from the charge storage device to the solar panel were minimised.

Construction

The cellular devices were constructed from two symmetric halves that have been vacuum formed on a purpose built vacuum forming bed and solvent welded together to form a lenticular shell. The material itself being PETG, a thermoplastic that vacuum forms easily and can conform to tight detail, approved for contact with food it offers no particular toxicity to the aquatic environment. It is also optically clear, again so that photovoltaic power might be harvested and so that it might appear more in keeping with a biological entity than a collection of technological artefacts. Each cell was hermetically sealed so that signals to and from each cell must be transmitted as light, motion or sound through the cell membrane. Each cell has a volume of around 750cc, approximately half of which is filled with circuitry and an optically clear silicone encapsulant. The encapsulation further protects the circuitry from water and also lowers the centre of gravity of the cell so that it floats half submerged as the encapsulant has a density of approximately that of water. It also allows sound to be transmitted efficiently to and from the cell and to some degree, owing to its thermal inertia, to stabilise the temperature of the circuitry.

Operational Behaviour

The behaviour of the sensory cells is fixed in the way that it responds to environmental parameters, while the behaviour of the motor cells is not. The motor cell behaviour is free to adapt to certain environmental parameters with two caveats:

  1. That the cell is primarily phototropic when the energy storage is depleted. That is that it has a propensity to close distance with light sources. As a photovore the cell would have evolved this type of behaviour via selection as without it the cell would be unable to persist in its other behaviours – effectively electing to ‘starve’ during periods where light was short or a different behaviour than ‘feeding’ was favoured
  2. That it will move aside when there is a perceived boundary ahead. The motor cell has innate collision avoidance behaviour. To avoid the platform becoming beached this behaviour has some precedence above other behaviours that the cell might exhibit.

The remainder of the motor cell behaviour is centred about phonotropism, that is the tendency to track and follow sounds. All of the above behaviours also included a degree of down regulation during periods of over excitement, which is that when over stimulated in a particular way the cells behaviour changes to regulate out the source of this stimulus, becoming exhausted.

The cells are excited by novel events when other behaviours are not so strong as to override them. The circuitry that determines the behaviour of the motor cells is analogue in design and is an implementation of a spiking neural network, a parallel to the processing that occurs in the nervous material of biological entities. There is no digital processing, no conventional computer or microcontroller involved in determining the behaviour of the motor cells, all processing is parallelistic and continuously valued in either time or intensity. This affords the neural circuitry a more biologically plausible mode of operation and behaviour. The neural network approach to the design of the circuitry allows the device to exhibit behaviours that might not be predicted or indeed predictable by the designer, this allows greater freedom for the design to find its own tolerances, tendencies and habits, rather than the designer to impart these by deliberate action. This is the core idea that allows the cellular platform to evolve its own behaviour patterns. As the components used are all of slightly different values, as a result of the manufacturing process, individual differences are inherent in the construction and the behaviour of each of the cells, this is true of both the sensory cells and the motor cells as they are both of analogue construction. If the dynamics of an analogue information processing system are even the most trivial of functionality, there will always be small differences between what appear to be the same circuit by design as the components used will all vary a little from there stated value. Most resistors and capacitors, the most basic of passive electronic components are commonly of plus or minus 5% of their stated value, so too varies the performance with temperature and the forward voltage of a simple semiconductor diode, or the gain of a transistor, making these systems quite sensitive to their environment. If a system contributes some data back into the environment from which that data were obtained then there is the distinct possibility that the system might develop strongly nonlinear dynamics.