# Disk Margin and Smallest Destabilizing Perturbation

This example shows how to interpret the `WorstPerturbation`

field in the structure returned by `diskmargin`

, the smallest gain and phase variation that results in closed-loop instability.

### Disk Margin as Range of Allowable Gain and Phase Variations

Compute the disk margins of a SISO feedback loop with open-loop response `L`

.

L = tf(25,[1 10 10 10]); DM = diskmargin(L);

The disk-based margins define a range of "safe" gain and phase variations for which the feedback loop remains stable. The `diskmarginplot`

command lets you visualize this range as a region in the gain-phase plane. As long as gain and phase variations stay within the shaded region, the closed-loop system `feedback(L,1)`

remains stable.

diskmarginplot(DM.GainMargin)

`diskmargin`

models gain and phase variations as a complex-valued multiplicative factor `F`

applied to the nominal loop transfer `L`

. The set of `F`

values is a disk whose intersection with the real axis is the interval `DM.GainMargin`

. (See Stability Analysis Using Disk Margins.) `diskmarginplot`

can also plot the `F`

disk.

`diskmarginplot(DM.GainMargin,'disk')`

`diskmargin`

also computes the smallest variation that destabilizes the feedback loop, returned in the field `DM.WorstPerturbation`

. This variation is returned as a state-space model that realizes the destabilizing gain and phase variation. When you multiply L by this perturbation, the resulting closed-loop system has an undamped pole at the frequency returned in `DM.Frequency`

.

WC = DM.WorstPerturbation; CL = feedback(L*WC,1); damp(CL)

Pole Damping Frequency Time Constant (rad/seconds) (seconds) -1.49e+00 + 7.93e-01i 8.83e-01 1.69e+00 6.70e-01 -1.49e+00 - 7.93e-01i 8.83e-01 1.69e+00 6.70e-01 -2.17e-19 + 1.96e+00i 1.11e-19 1.96e+00 4.61e+18 -2.17e-19 - 1.96e+00i 1.11e-19 1.96e+00 4.61e+18 -4.19e+00 1.00e+00 4.19e+00 2.39e-01 -9.46e+00 1.00e+00 9.46e+00 1.06e-01

Verify that the gain and phase variation of the destabilizing perturbation mark a boundary point for the range of "safe" gain and phase variations. To do so, compute the gain and phase of `WC`

at `DM.Frequency`

.

hWC = freqresp(WC,DM.Frequency); GM = mag2db(abs(hWC))

GM = 1.7832

PM = 180/pi * abs(angle(hWC))

PM = 23.1695

diskmarginplot(DM.GainMargin) line(GM,PM,'Color','k','Marker','+','MarkerSize',8,'LineWidth',3,'HandleVisibility','off') text(GM+.1,PM+1,sprintf('Gain and phase of WC\n at f = %.5g',DM.Frequency))

### Nyquist Interpretation

The statement that the perturbation `WC`

drives the closed-loop system unstable is equivalent to saying that the Nyquist plot of `L*WC`

touches the critical point at the frequency `DM.Frequency`

. (See Stability Analysis Using Disk Margins.) The following plot shows the Nyquist plots of `L`

and `L*WC`

. The crosses on each plot mark the response at `DM.Frequency`

, and confirm that the response of `L*WC`

is –1 at this frequency.

figure(2), clf hL = freqresp(L,DM.Frequency); nyquist(L,L*WC), title('Open-loop response') legend('L','L*WC') axis([-2 2 -2 2]) line(-1,0,'Color','r','Marker','+','MarkerSize',8,... 'LineWidth',3,'HandleVisibility','off') line(real(hL),imag(hL),'Color','k','Marker','+',... 'MarkerSize',8,'LineWidth',3,'HandleVisibility','off') text(real(hL)+0.05,imag(hL)-0.2,sprintf('f = %.5g',DM.Frequency)) line([real(hL) -1],[imag(hL) 0],'Color','k','LineStyle',':',... 'LineWidth',2,'HandleVisibility','off')

The perturbation `WC`

is dynamic and its Nyquist plot hugs the boundary of the disk of `F`

values. The point of contact is the frequency `DM.Frequency`

where the disk margin is weakest. The following plot uses `diskmarginplot`

to render the disk of allowable gain and phase variations on the Nyquist plane, superimposing the response of the perturbation `WC`

. The black cross again marks the response at `DM.Frequency`

.

hWC = freqresp(WC,DM.Frequency); diskmarginplot(DM.GainMargin,'disk') hold on nyquist(WC) hold off axis([0.6 1.6 -0.6 0.6]) line(real(hWC),imag(hWC),'Color','k','Marker','+',... 'MarkerSize',8,'LineWidth',3,'HandleVisibility','off') text(real(hWC)+0.02,imag(hWC)-0.05,sprintf('f = %.5g',DM.Frequency))

Equivalently, this frequency is where the Nyquist plot of `L`

touches the exclusion region associated with the disk margins `DM`

. The following plot shows the disk of variations with the Nyquist plot of `L`

. The black cross marks the response of `L`

at `DM.Frequency`

.

diskmarginplot(DM.GainMargin,'nyquist') hold on nyquist(L) hold off axis([-2 0 -1 1]) line(real(hL),-imag(hL),'Color','k','Marker','+',... 'MarkerSize',8,'LineWidth',3,'HandleVisibility','off') text(real(hL)+0.05,-imag(hL)+0.05,sprintf('f = %.5g',DM.Frequency))

Thus, the disk `F`

represents a region in the Nyquist plane that the response of `L`

cannot enter while preserving closed-loop stability. At the critical frequency `DM.Frequency`

, the frequency at which the gain and phase margins are smallest, the Nyquist plot of `L`

just touches the disk.

## See Also

`diskmargin`

| `diskmarginplot`

| `wcdiskmargin`