The concept of system is widely used on engineering. There is no unique definition of a system, how it should work and which are its components. A system is an abstract concept, it can be anything.
The user define what the system is. To define a systems we need to define its behavior and boundaries. Everything outside the boundaries is called the surroundings.
Image: System abstractization
A system can be defined as a region which contains energy and/or matter, separated form its surroundings by boundaries defined arbitrarily. The systems boundaries is regraded as a closed surface, surrounding the system, through which energy and mass can be exchanged. Everything external to the system is the surroundings.
Let’s take as example an internal combustion engine with reciprocating pistons. For it, we can define two systems:
- thermodynamic system
- mechanical system
The thermodynamic system in enclosed in the combustion chamber. For this system we can define the thermodynamic laws which govern it and the exchange of energy (heat) and matter (air and fuel) with its surroundings.
The mechanical system is composed by the crankshaft, connecting rod and piston. It’s governed by laws of motion, it doesn’t exchange matter with the surroundings but only energy.
Image: Thermodynamic and mechanical systems with boundaries
Depending on the type of exchange with its surroundings (mass and/or energy), a system can be:
- open system: both energy and matter can be exchanged with the surrounding
- closed system: only energy can be exchanges outside boundaries
- isolated system: neither energy or mass can be exchanges with the surroundings
For example, hot coffee in an open cup is an open system because it releases energy (heat) to the surrounding and we it can exchange matter (coffee) with the surroundings by taking out (drinking) or putting in (pouring). Hot coffee in a closed cup is a closed system because it can only exchange energy (heat) through the cups walls. Hot coffee in an isolated (thermos) bottle is an isolated system because it can not exchange energy nor matter with its surroundings.
Image: Types of systems – open, closed and isolated
A system is also defined by its state. The state it’s the condition of a system at a given moment of time. In the same time, the state of a system is function of the properties of the system. The property can be regarded as a quantity with a numerical value, determined by some type of physical operation or test.
Two or more states can be the same (identical) only if theirs properties are also identical. When the properties of a system changes in value, it means that the system is in a process.
The current state of the system depends on the previous state and the inputs of the system. The inputs are external variables which can influences the way the system behaves. Function of the inputs and states the system can provide some output variables. The outputs are an image of the state of the system at a particular time, function of the inputs.
Image: Dynamic system representation
Some examples of inputs and outputs for several types of systems:
| System type | Input | Output |
| Electric | Voltage | Current |
| Mechanical | Force | Displacement |
| Hydraulic | Flow | Pressure |
Systems are the backbone of control engineering. Both the controller and the plant are systems. The controller is the system which performs a regulation/control action on a plant. The plant is also a system which has its states regulated by the controller.
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