It is not unusual (away from this website, anyway) to see CELSS defined as a controlled environmental life support system. I personally prefer closed, because 1) I think that is the more critical characteristic to strive towards, and 2) controlled gives the impression of sitting in front of a console and "driving" the system. Incidentally, I also prefer ecological to environmental, as it implies a biological basis to the system, rather than just a fancy HVAC system, which won't produce food.
In thermodynamic systems, closed means that matter cannot be exchanged from without the system, but forms of energy (heat, light, electricity, etc.) can. By contrast, an isolated system cannot even exchange energy; and a thermodynamically open system can exchange both matter and energy (think of a jet engine, with air and fuel going in and faster air, thrust, and heat coming out).
A CELSS habitat is a closed thermodynamic system made of up of many controlled subsystems.
In the control system diagram, you'll notice the little circle where the external input is modified by the feedback. There is a minus sign there denoting a negative feedback. Negative is good. Positive feedback loops quickly spin out of control, and are inherently unstable. This may be useful when the goal is a chain reaction, but negative feedback control systems are what engineers usually are referencing when discussing the subject of control.
In lay terms, control (as in "remote control"), often implies manual operation, or active control. In control systems engineering however, the goal is typically automation, i.e., passive control (think robotics). The other thing that folks often assume with a controlled system is that it is inherently technological, either mechanically or electrically controlled. This seems to be the direction NASA has taken, but Nature has been doing control systems chemically, geologically, biologically, and ecologically for over 4 billion years on Earth, with global stability generally measured on the scale of millions of years, and local ecosystem stability on the order of hundreds or thousands of years. The question here is how far can this snapshot of Earth's evolved system (so crucial to human existence) be scaled down in packaged form, and still retain stability over the timescale of multiple human generations? Clearly, system size plays a role in stability: more mass means greater reserves of chemical resources and also dampens external thermal fluctuations; more diversity of biomes leads to greater biodiversity which makes a system more robust and stable. It can therefore be assumed that a scaled down version will be inherently less stable than the Earth's ecosystem. On the other hand, an off-Earth system might benefit from a lack of geological perturbations such as volcanism and extreme weather.
This is not to say that the ideal CELSS is completely controlled passively. As the apex consumer in an EcoArk, the humans will be uniquely in control of the system, if nothing else, by when, what, and how much they choose to consume. I would likewise argue that CELSS should ultimately be designed not only for humans, but to require humans, at least for long term stability and reproduction.
The measure of control system effectiveness is how quickly it reaches the desired value. The selection of gain terms for each type of control is known as tuning. For complex systems, tuning is not a trivial endeavor and must often be done iteratively.
It should be noted, that many biological and ecological response mechanisms do not fit neatly into the PID categories the way, say a spring represents a proportional response in a mechanical system, or an inductor a derivative response in electronics
Biosystems are inherently complex, but are generally well tuned by nature through the iterative process known as evolution. The complexity of biosystems is due to the chemical control systems used which are temperature and pressure dependent and usually nonlinear. The field of ecology is ripe for quantifying and characterizing control responses mechanisms experimentally using CELSS.
Definitions
Control systems and (thermodynamically) closed systems are both engineering terms, and in the engineering sense, neither term is wrong as applied to the kind of habitats that are the goal of this community. A controlled system is, nominally, a stable system. A thermodynamic closed system must be stable, and must therefore be controlled. A control system can be open or closed, not necessarily in the thermodynamic sense. Below shows an open control system driven by an external input, r, outputting from the system, y. A closed control system would take no external input, and all the output would be internal.In thermodynamic systems, closed means that matter cannot be exchanged from without the system, but forms of energy (heat, light, electricity, etc.) can. By contrast, an isolated system cannot even exchange energy; and a thermodynamically open system can exchange both matter and energy (think of a jet engine, with air and fuel going in and faster air, thrust, and heat coming out).
A CELSS habitat is a closed thermodynamic system made of up of many controlled subsystems.
In the control system diagram, you'll notice the little circle where the external input is modified by the feedback. There is a minus sign there denoting a negative feedback. Negative is good. Positive feedback loops quickly spin out of control, and are inherently unstable. This may be useful when the goal is a chain reaction, but negative feedback control systems are what engineers usually are referencing when discussing the subject of control.
In lay terms, control (as in "remote control"), often implies manual operation, or active control. In control systems engineering however, the goal is typically automation, i.e., passive control (think robotics). The other thing that folks often assume with a controlled system is that it is inherently technological, either mechanically or electrically controlled. This seems to be the direction NASA has taken, but Nature has been doing control systems chemically, geologically, biologically, and ecologically for over 4 billion years on Earth, with global stability generally measured on the scale of millions of years, and local ecosystem stability on the order of hundreds or thousands of years. The question here is how far can this snapshot of Earth's evolved system (so crucial to human existence) be scaled down in packaged form, and still retain stability over the timescale of multiple human generations? Clearly, system size plays a role in stability: more mass means greater reserves of chemical resources and also dampens external thermal fluctuations; more diversity of biomes leads to greater biodiversity which makes a system more robust and stable. It can therefore be assumed that a scaled down version will be inherently less stable than the Earth's ecosystem. On the other hand, an off-Earth system might benefit from a lack of geological perturbations such as volcanism and extreme weather.
This is not to say that the ideal CELSS is completely controlled passively. As the apex consumer in an EcoArk, the humans will be uniquely in control of the system, if nothing else, by when, what, and how much they choose to consume. I would likewise argue that CELSS should ultimately be designed not only for humans, but to require humans, at least for long term stability and reproduction.
The measure of control system effectiveness is how quickly it reaches the desired value. The selection of gain terms for each type of control is known as tuning. For complex systems, tuning is not a trivial endeavor and must often be done iteratively.
Biological Systems
In many ways the operation of a CELSS is analogous to the physiology of an organism, so we can borrow some terminology.- Homeostasis is defined as the maintenance of a constant or unchanging ânormalâ internal environment during unstressed conditions.
- Steady state is also defined as a constant internal environment, but this does not necessarily mean that the internal environment is at rest and normal. When the system is in a steady state, a balance has been achieved between the demands placed on the system and the systemâs response to those demands.
It should be noted, that many biological and ecological response mechanisms do not fit neatly into the PID categories the way, say a spring represents a proportional response in a mechanical system, or an inductor a derivative response in electronics
Biosystems are inherently complex, but are generally well tuned by nature through the iterative process known as evolution. The complexity of biosystems is due to the chemical control systems used which are temperature and pressure dependent and usually nonlinear. The field of ecology is ripe for quantifying and characterizing control responses mechanisms experimentally using CELSS.