Modelling & Control

Objectives

Presented as the ultimate objective of any recycling system, the design of a circular system brings with it a distinctive set of challenges: by nature it is potentially unstable, it requires high efficiency from all its subsystems, and it can only really be validated after a large number of successful cycles.

This leads directly to the need for a very detailed characterisation of every process involved, in both static and dynamic modes, and under both optimal and degraded conditions.

Challenges & Approach to the Research

Almost any circular system is built from an assembly of interacting processes. The nature and respective dynamics of these processes are so diverse that they cannot operate successfully as stand-alone elements, without control. Deterministic control is therefore a prerequisite for robust performance.

At its most basic level, this means a triptych: measurement by reliable sensors, a control scheme, and regulation. Deterministic mathematical modelling and simulation of the system’s interacting parts form what amounts to its “brain”.

Since the earliest days of the MELiSSA project, and probably owing to its origins with Claude Chipaux, a MATRA-Airbus engineer, these challenges were recognised early on, and considerable effort has gone into:

Intensive characterisation of all processes at the level of the main chemical elements (C, H, N, O, S, P…).
Mechanistic modelling and simulation.
Predictive control laws.
A systems-engineering approach.

Mathematical simulation of the MELiSSA loop, using the MATLAB Simulink platform.

Flight Experiments

Control and modelling tools are certainly not the elements most affected by the space environment; even so, it remains important to validate engineering performance under realistic conditions. The ARTEMISS flight experiment, for example, demonstrated an excellent prediction of the CO2-to-oxygen conversion on board the International Space Station.

Mathematical prediction of oxygen and biomass production, plotted against results collected on board the ISS.

Ground Demonstration

Despite considerable interest today, surprisingly few circular systems have reached the demonstration stage.

Within the MELiSSA Pilot Plant, the systemic approach is progressively integrated and demonstrated over a period of months. This demonstration of the MELiSSA loop starts from gas recycling (CO2 to oxygen), the fastest of the loop’s dynamics, and progressively integrates the MELiSSA processes upstream from there.

Quality of the recycling loop with three interconnected processes.

MELiSSA’s control strategy already allows three compartments to be connected in continuous mode, recycling CO2 and urine to produce oxygen. The controlled oxygen concentration achieved in the crew habitat is shown below, across four different set points: 21%, 19%, 20% and 21%.

Controlled oxygen concentration in the crew habitat across four set points: 21%, 19%, 20%, 21%.

Terrestrial Applications

The systemic approach developed within the MELiSSA programme forms the baseline for PhiSystem, a tool-based methodology for the design and evaluation of complex systems.

It is now commonly used in terrestrial applications, particularly in the French automotive sector for the control design of vehicle energy systems, improving quality, performance and flexibility. PhiSystem is currently being deployed in autonomous vehicles, and for the overall resource management of circular systems such as smart buildings and eco-districts.

Education & Communication

Owing to its exceptionally high degree of constraint, as well as its visibility, the MELiSSA project is often presented as the ultimate circular system, and today serves as an excellent academic tool.

Over the years, a large number of talks and MELiSSA lectures have been given, driving strong European engagement, PhDs and associated publications.