Differences in simulation results in state space models using matlab

Hi,

I'm writing some code to simulate a relay dynamically. I've derived differential equations, linearised them around an operating point and put into state space form. Firstly, I checked the impulse and step response at the operating input using the state space function in matlab, the response was ok. I then implemented the same equations in code (see attached) and the impulse response is different, in fact unstable - any ideas anyone?

Cheers,

Andy

Answer 1

[Suneesh]

You are using continuous SS coefficients while you are (discrete) simulating your system in:

% Perform the system simulation:
x = x0;                % Set initial state
for n=1:Ns-1           % Iterate through time
  y(n) = C*x + D*u(n); % Output for time n-1
  x = A*x + B*u(n);    % State transitions to time n
end
y' % print the output y (transposed)
%plot(y')

Instead use the discrete coefficients, which you already have available in 'sysd':

discA = sysd.a;
discB = sysd.b;
discC = sysd.c;
discD = sysd.d;

Then try:

x = x0;                % Set initial state
for n=1:Ns-1           % Iterate through time
  y(n) = discC*x + discD*u(n); % Output for time n-1
  x = discA*x + discB*u(n);    % State transitions to time n
end
y' % print the output y (transposed)
%plot(y')

[Andrew Wileman]

Hi,

Thanks Suneesh for your help, works fine now.

Once again many thanks,

Andy