The way I understand your quesiton is this: each of those three calls to SecondOrderLowpassFilterTustin should have its own persistent state. That is, each time
sig1 = SecondOrderLowpassFilterTustin(u(1),fn,dn,Ts);
is invoked, it should use the 'xs' from the prevoius call on this line, but not from the other two calls.
If that is your intent, then one way to solve the issue would be to implement a class that contains 'xs' as a property, and make SecondOrderLowpassFilterTustin() to be a method of that class.
classdef MyLowpassFilter < handle
properties
xs
end
methods
function obj = MyLowpassFilter(u)
obj.xs = [u 0]';
end
function y = SecondOrderLowpassFilterTustin(obj, u,fn,dn,Ts)
... all your code here but use obj.xs instead of xs...
end
end
end
function main
persistent filter1, filter2, filter3
if isempty(filter1)
filter1 = MyLowpassFilter(u(1));
filter2 = MyLowpassFilter(u(2));
filter3 = MyLowpassFilter(u(3));
end
sig1 = filter1.SecondOrderLowpassFilterTustin(u(1),fn,dn,Ts);
sig2 = filter2.SecondOrderLowpassFilterTustin(u(2),fn,dn,Ts);
sig3 = filter3.SecondOrderLowpassFilterTustin(u(3),fn,dn,Ts);
end
Another way would be to just move the persistent state to the caller:
function main(u)
persistent xs0, xs1, xs2
if isempty(xs0)
xs0 = [u 0]';
xs1 = xs0;
xs2 = xs0;
end
[sig1, xs0] = SecondOrderLowpassFilterTustin(u(1),fn,dn,Ts, xs0);
[sig2, xs1] = SecondOrderLowpassFilterTustin(u(2),fn,dn,Ts, xs1);
[sig3, xs2] = SecondOrderLowpassFilterTustin(u(3),fn,dn,Ts, xs2);
end
function [y, xs] = SecondOrderLowpassFilterTustin(u,fn,dn,Ts, xs)
...your code here but remove the initialization of xs...
end
