Delay system in feedback path throws a method error

[jamestjsp]

The following code throws a method error. This can be easily reproduced by placing a delay system in the denominator of any expression.

s = tf("s")
L = 5; # Delay time
P = 10/(10*s + 1) * delay(L);
Kp= 0.1;
Ti= 10;
C = Kp*(1 + 1/(Ti*s));
T = C*P/(1+C*P);
t =100;
plot(step(T,t))

Work around.
Use: T = feedback(C*P, 1);

_MethodError: /(::DelayLtiSystem{Float64, Float64}, ::DelayLtiSystem{Float64, Float64}) is ambiguous. Candidates:
/(anything, sys::DelayLtiSystem) in ControlSystemsBase at ~.julia\packages\ControlSystemsBase\CKedD\src\types\DelayLtiSystem.jl:93
/(sys1::LTISystem, sys2::LTISystem) in ControlSystemsBase at ~.julia\packages\ControlSystemsBase\CKedD\src\types\Lti.jl:5
Possible fix, define
/(::LTISystem, ::DelayLtiSystem)

Stacktrace:
[1] top-level scope
@ In[16]:7
[2] eval
@ .\boot.jl:368 [inlined]
[3] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
@ Base .\loading.jl:1428_

[baggepinnen]

Thanks for the report! We do not support inverting delay systems so you will have to use the feedback function for this, but we should try to improve the error message to indicate this.

[baggepinnen]

In general, using hte feedback function is always recommended even for cases when we do support inversion, the inversion will not give you a minimal realization and you'll thus have to rely on numerical approximation through minreal to obtain a minimal realization after forming the feedback loop. The feedback function creates the minimal realizatoin directly.