Dear all,

Symbolic linearization is not working. Numerical linearization works as expected, but the symbolic one yields an incorrect result.

The system I'm using for testing is a simple harmonic oscillator. The equation of motion is

x'' + w0^2 * ( x - gnd ) = 0, with resonant frequency w0 = 2 rad/s.

The state space representation is easily calculated by defining the variables q1 =x and q2=x'. The state space vector and matrices are

q = [ q1 ]
       [ q2 ] ,

A = [ 0 ,  1 ]
       [-4 ,  0 ] ,

B = [ 0 ]
       [ 4 ] ,

C = [ 1 , 0 ] ,

D = 0.

with

Input: gnd, position of the ground
Output: q1 = x, position of mass.

The symbolic method yield the correct values for A and B, but not for C. Namely, it yields

C = [ 0 , 1 ].

In more elaborate examples, it can be seen that the value for D is also wrong, but not in this simple one.

I'm using Windows 10, OpenModelica v1.18.1 (64-bit) and OMShell.

I guess there is still a chance I'm misunderstanding the usage of the symbolic linearization, but this seems to be a bug. Is the correct forum to post this, is GitHub  more appropriate?

Any help in solving this will be appreciated.

Regards,

Fabian

Numeric linearization:

loadFile("HarmonicOscillator.mo")

clearCommandLineOptions() 

linearize(HarmonicOscillator)

readFile("linearized_model.mo")

Symbolic linearization

loadFile("HarmonicOscillator.mo")

setCommandLineOptions({"+generateSymbolicLinearization"})

linearize(HarmonicOscillator)

readFile("linearized_model.mo")

Model

model HarmonicOscillator

    input Real gnd = 0.03 "Position of the ground" ;

    Real q2 "Velocity of the test mass" ;

    constant Real w0 = 2 "Resonance frequency" ;

    output Real q1 "Position of the test mass" ;

initial equation

    q1 = 0.03 "Initial position of test mass" ;

    q2 = 0.0 "Initial velocity of test mass" ;

equation

    der( q1 ) = q2 ;

    der( q2 ) = - w0 * w0 * (q1 - gnd) ;    

end HarmonicOscillator ;