Suba,

Thanks for the clarification. I think you should add an example of making an open loop pole zero plot to the RootLocusPlot documentation because there are a few subtle options that need to be set and explained:

1. PoleZeroMarkers option takes a list of the {open-loop poles, closed-loop poles, open-loop zeros} and by putting "" for the closed loop poles, it no longer draws any closed loop poles.
2. The value for k must be small so that the root locus that is drawn is not visible on the plot. In the example above, k=1 is small compared to the pole/zero values.

But in this documentation example, it would leave colored loci lines on the plot:

RootLocusPlot[
 TransferFunctionModel[{{(k (10 + s^2))/(20 - 4 s + s^4)}}, s, 
  SamplingPeriod -> None, SystemsModelLabels -> None], {k, 0, 1}, 
 PoleZeroMarkers -> {Style["\[Times]", 30], "", 
   Style["\[SmallCircle]", 40]}, PlotRange -> {{-2, 2}, {-6, 6}}]

Which can be removed with setting a tiny value for k:

RootLocusPlot[
 TransferFunctionModel[{{(k (10 + s^2))/(20 - 4 s + s^4)}}, s, 
  SamplingPeriod -> None, SystemsModelLabels -> None], {k, 0, 0.0001}, 
 PoleZeroMarkers -> {Style["\[Times]", 30], "", 
   Style["\[SmallCircle]", 40]}, PlotRange -> {{-2, 2}, {-6, 6}}]

To get:

Regards,

Neil

Neil: The way to get RootLocusPlot to plot a pole-zero plot is the following.

RootLocusPlot[ k tfm3[s], {k, 0, 1}, 
 PoleZeroMarkers -> {Style["\[Times]", 30], "", 
   Style["\[SmallCircle]", 40]}, PlotStyle -> None, 
 PlotRange -> {Automatic, {-1000, 1000}}, AxesOrigin -> {0, 0}]

Koushik: The following should have worked, but it does not. This will directly change the size of the default markers. I will commit a fix for this.

Control`PoleZeroPlot[tfm3, 
 PoleZeroMarkers -> {{Automatic, 20}, {Automatic, 20}}, 
 PlotRange -> {Automatic, {-1000, 1000}}, AxesOrigin -> {0, 0}]

For now, the workaround is what Neil mentioned.

Control`PoleZeroPlot[tfm3, 
 PoleZeroMarkers -> {Style["\[Times]", 30], 
   Style["\[SmallCircle]", 40]}, 
 PlotRange -> {Automatic, {-1000, 1000}}, AxesOrigin -> {0, 0}]

You can use the same options as RootLocusPlot:

PoleZeroMarkers -> {Style["T", Large, Background -> Pink]}

Regards

You can also use Control`PoleZeroPlot.

Control`PoleZeroPlot@ButterworthFilterModel[12]

This is an undocumented function, but it's very straightforward. Given a TransferFunctionModel tfm, its pole zero plot can be obtained as

Control`PoleZeroPlot[tfm]

You are probably confused with the prefix operator @.

Control`PoleZeroPlot@tfm

also gives the same result.