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Systems with memory

"what is a dynamical system?"

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.

Generalized system

Generalized System

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:

Resistor connected to a current source

Resistor connected to a current source
V t R i t

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.

Simple capacitor connected to a current source

Simple capacitor connected to a current source
I t C t v t

Solving for the voltage in the current voltage relationship above, we have:

v t v t 0 1 C t t 0 t i t

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.

"describing dynamical systems"

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:

Rlc circuit: 2nd order

RLC circuit: 2nd order

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.

t 2 y t 7 2 t 1 y t 9 y t 6 u t

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.

"planar orbit satellite"

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.

Simple planar orbit satellite example

Simple planar orbit satellite example

Using the laws of motion, the following set of differential equations can be deduced:

t 2 r t t 1 r t 2 u r k r 2
2 t 1 r t t 1 t r t 1 t u
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Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
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Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
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"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
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progressive wave
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A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
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Source:  OpenStax, State space systems. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10143/1.3
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