Coupled Oscillator: Coupled Mass-Spring System with Damping

Problem Statement

For the below system set up a set of state variable equations, and then solve. Assume all motion takes place in the vertical directions.

Fig. 1

Solution

Initial Values

For the upper mass:

$m_{1}=80kg{\frac {}{}}$

$k_{1}=60000{\frac {N}{m}}$

$b_{1}=0.1{\frac {}{}}$

And for the lower mass:

$m_{2}=400kg{\frac {}{}}$

$k_{2}=120000{\frac {N}{m}}$

$b_{2}=0.2{\frac {}{}}$

Find the Force Equations

First we need to sum forces in the y-direction for each block.

For mass 1:

$+\uparrow \sum F_{y_{1}}=m_{1}{\ddot {x}}_{1}\Rightarrow \ m_{1}{\ddot {x}}_{1}=-2b_{1}{\dot {x}}_{1}-k_{1}x_{1}+k_{2}(x_{2}-x_{1})+m_{1}g$

For mass 2:

$+\uparrow \sum F_{y_{2}}=m_{2}{\ddot {x}}_{2}\Rightarrow \ m_{2}{\ddot {x}}_{2}=-2b_{2}{\dot {x}}_{2}-k_{2}(x_{2}-x_{1})+m_{2}g$

So if we put the equations above into the correct form we have:

${\ddot {x}}_{1}=-{\frac {2b_{1}}{m_{1}}}{\dot {x}}_{1}-({\frac {k_{1}}{m_{1}}}+{\frac {k_{2}}{m_{2}}})x_{1}-{\frac {k_{2}}{m_{2}}}x_{2}+g$

and

${\ddot {x}}_{2}=-{\frac {2b_{2}}{m_{2}}}{\dot {x}}_{2}-{\frac {k_{2}}{m_{2}}}x_{2}+{\frac {k_{2}}{m_{2}}}x_{1}+g$

Note: Many people include the original length of the springs in the above equation, however if we take our initial reference point to be where the mass is in equilibrium, we do not need to include the initial spring length.

State Space Equation

The general form for the state equation is as shown below:

${\underline {\dot {x}}}(t)={\widehat {A}}\,{\underline {x}}(t)+{\widehat {B}}\,{\underline {u}}(t)$

Where ${\widehat {M}}$ denotes a matrix and ${\underline {v}}$ denotes a vector.

If we let $x_{1}{\frac {}{}}$, ${\dot {x}}_{1}{\frac {}{}}$, $x_{2}{\frac {}{}}$, and ${\dot {x_{2}}}{\frac {}{}}$ be the state variables, then

${\begin{bmatrix}{\dot {x}}_{1}\\{\ddot {x}}_{1}\\{\dot {x}}_{2}\\{\ddot {x}}_{2}\end{bmatrix}}=$ ${\begin{bmatrix}0&1&0&0\\-({\frac {k_{1}}{m_{1}}}+{\frac {k_{2}}{m_{2}}})&-{\frac {2b_{1}}{m_{1}}}&{\frac {k_{1}}{m_{1}}}&0\\0&0&0&1\\{\frac {k_{2}}{m_{2}}}&0&-{\frac {k_{2}}{m_{2}}}&-{\frac {2b_{2}}{m_{2}}}\\\end{bmatrix}}{\begin{bmatrix}x_{1}\\{\dot {x}}_{1}\\x_{2}\\{\dot {x}}_{2}\end{bmatrix}}+{\begin{bmatrix}0\\1\\0\\1\\\end{bmatrix}}{\begin{bmatrix}g\\\end{bmatrix}}$

I am leaving the C matrix set to zero for now in case I add an input force.

We don't need to include gravity here if we allow are initial conditions for the spring to be zero with gravity accounted for.