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When we talk about systems in the most general sense, we are talking about anything that takes in a certain number of inputsand produces a certain number of outputs based on those inputs.
In the figure above, the $u(t)$ inputs could be the jets on asatellite and the $y(t)$ outputs could be the gyros describing the"bearing" of the satellite.
There are two basic divisions of systems: static and dynamic. In a static system, the current outputs are based solely on the instantaneous values of the current inputs.An example of a static system is a resistor hooked up to a current source:
At any given moment, the voltage across the resistor (the output) depends only on the value of the current runningthrough it (the input). The current at any time $t$ is simply multiplied by the constant value describing the resistance $R$ to give the voltage $V$ . Now, let's see what happens if we replace the resistorwith a capacitor.
Solving for the voltage in the current voltage relationship above, we have:
So in the case of the capacitor, the output voltage depends on the history of the current flowing through it. In a sense, thissystem has memory. When a system depends on the present and past input, it is said to be a dynamical system.
As seen in voltage-current relationship of a capacitor, differential equations have memory and can thus be used todescribe dynamical systems. Take the following RLC circuit as an example:
In circuits (as well as in other applications), memory elements can be thought of as energy storage elements. In this circuitdiagram, there are two energy-storing components: the capacitor and the inductor. Since there are two memory elements, it makessense that the differential equation describing this system is second order.
In the most general case of describing a system with differential equations, higher order derivatives of outputvariables can be described as functions of lower order derivatives of the output variables and some derivatives of theinput variables. Note that by saying "function" we make no assumptions about linearity or time-invariance.
By simply rearranging the equation for the RLC circuit above, we can show that that system is in fact covered by this general relationship.
Of course, dynamical systems are not limited to electrical circuits. Any system whose output depends on current and pastinputs is a valid dynamical system. Take for example, the following scenario of relating a satellite's position to itsinputs thrusters.
Using a simple model of a satellite, we can say that its position is controlled by a radial thruster ${u}_{r}$ , which contributes to its vertical motion, and a tangential thruster ${u}_{}$ which contributes to its motion tangential to its orbit. To simplify the analysis, let's assume that the satellite circles the earth in a planar orbit, and thatits position is described by the distance r from the satellite to the center of the Earth and theangleas shown in the figure.
Using the laws of motion, the following set of differential equations can be deduced:
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