Amplitude and Phase: Second Order II

At left a spring/mass/dashpot system is shown, driven by a dashpot at the bottom.

The equation governing this system is displayed in yellow at the top. Mass is set to \(1\); \(b\) is the damping constant, \(k\) is the spring constant, and \(\omega\) is the circular frequency of the sinusoidal motion of the piston.

To the right, the position of the top of the dashpot (the input signal \(\cos(\omega t)\) is graphed in cyan, and the position of the mass (the system response \(x\)) is graphed in yellow. Diamonds indicate the current values \(\cos(\omega t)\) and of \(x\), and a vertical white line between them indicates the extension of the spring. The time lag \(t_0\) is read out in red at the bottom of the screen, below a readout of the period \(P\) in cyan. If \(t_0 \gt 0\), it is represented by a grey vertical line measured by a red segment.

Rolling the cursor over the graphing window produces crosshairs and a readout of the values of \(t\) and \(x\). Depressing the mousekey suppresses the crosshairs. The horizontal crosshair line is continued in the window displaying the system.

Use the slider handle below the graphing window to select a value of \(t\). Animate the system using the [>>] key. During the animation the key changes to [||], and selecting it will stop the animation. At the end of the animation the key changes to [<<], which resets \(t\) to zero.

Grab the \([b], [k],\) or \([\omega]\) slider to vary those parameters.

The [Bode plots] key toggles display of two windows on the right side of the screen. The top window displays the amplitude \(A\) of the sinusodial response as a function of \(\omega\). The window below it displays the negative of the phase lag \(\phi\) as a function of \(\omega\).

The [Nyquist plot] key toggles display of a window at lower right, showing a portion of the complex plane. On it, a grey curve traces the path traversed by the complex gain \(\frac{bi\omega}{p(i\omega)}\) (where \(p(s) = s^2 + bs + k\) is the characteristic polynomial) as \(\omega\) varies over positive values. A yellow diamond marks the value of this complex number for the chosen value of \(\omega\). A yellow line segment connects it to the origin. The length of this segment is the amplitude \(A\), and the angle up from the positive real axis, marked by a a green arc, is \(-\phi\).

Roll the cursor over the amplitude window to cause a horizontal yellow line to appear in that window and in the graphing window, marking the maximal displacement, and a readout of that maximal value.

Roll the cursor over the phase shift window to cause a readout of the phase shift.

Note: These are not quite truly Bode or Nyquist plots. A Bode plot graphs \(\log(A)\) vs \(\log(\omega)\) or \(-\phi\) vs \(\log(\omega)\). A Nyquist plot displays \(\frac{k}{p(i\omega)}\) as \(\omega\) ranges from \(-\infty\) to \(+\infty\) it has a portion above the real axis which is symmetric with what is drawn.