At left a spring/mass/dashpot system is shown, driven by a piston 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 plunger (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. A grey vertical line measured by a red segment indicates the time lag \(t_0\) (which is also read out in red at the bottom of the screen, below a readout of the period \(P\) in cyan).

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 time value is set using a slider under the window. The [>>] key starts an animation. The [<<] key resets \(t\) to \(t = 0\).

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 \(\frac{k+ bi\omega}{p(i\omega)}\) (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 horiziontal yellow line to appear relating the amplitudes in the three top windows, along with a readout of the amplitude. 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.

© 2001 H. Hohn and H. Miller