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pythonsimulationgekko

Why does my gekko ARX simulation start with unreasonable values?

I used gekko sysid to model a process with 5 MV's and 4 CV's. I want to simulate the real process and it does fairly well, except at the beginning. Even with a good initial value for CV's, the plots always look something like this:

CV Simulation

I have tried different inputs for the MV's but I always see this initial dip before returning to reasonable values. I am using na=2 and nb=4 but it doesn't seem to matter. Is there an explanation or fix?

Solution

  • Perform steady-state initialization as a first step. When calculating the steady state, ensure that the correct MVs are used to get the desired output. This is demonstrated in the TCLab F example Python GEKKO System ID with ARX Model

    TCLab Example

    The initial point in your plot is the initial guess value. The states of the model also need to be initialized with a stead state calculation:

    # create control ARX model
y = m.Array(m.CV,2)
u = m.Array(m.MV,2)
m.arx(p,y,u)

# rename CVs
TC1 = y[0]
TC2 = y[1]

# rename MVs
Q1 = u[0]
Q2 = u[1]

# steady state initialization
m.options.IMODE = 1
m.solve(disp=False)

    Then the MPC application can be configured with those steady-state values:

    # set up MPC
m.options.IMODE   = 6 # MPC
m.options.CV_TYPE = 1 # Objective type
m.options.NODES   = 2 # Collocation nodes
m.options.SOLVER  = 3 # IPOPT
m.time=np.linspace(0,120,61)

    Correct initialization should solve the problem.