model.options.IMODE defines the problem type. Each problem type treats variable classes differently and builds equations behind the scenes to meet the particular objective inherit to each problem type. The modes are:
|Non-Dynamic||1 Steady-State (SS)||2 Steady-State (MPU)||3 Steady-State (RTO)|
|Dynamic Simultaneous||4 Simultaneous (SIM)||5 Simultaneous (SIM)||6 Simultaneous (CTL)|
|Dynamic Sequential||7 Sequential (SQS)||8 Sequential (SIM)||9 Sequential (CTL)|
Differential equations are specified by differentiation a variable with the dt() method. For example, velocity v is the derivative of position x:
m.Equation( v == x.dt() )
Discretization is determined by the model time attribute. For example, m.time = [0,1,2,3] will discretize all equations and variable at the 4 points specified. Time or space discretization is available with Gekko, but not both. If the model contains a partial differential equation, the discretization in the other dimensions is performed with Gekko array operations as shown in the hyperbolic and parabolic PDE Gekko examples.
Simultaneous methods use orthogonal collocation on finite elements to implicitly solve the differential and algebraic equation (DAE) system. Non-simulation simultaneous methods (modes 5 and 6) simultaneously optimize the objective and implicitly calculate the model/constraints. Simultaneous methods tend to perform better for problems with many degrees of freedom.
Sequential methods separate the NLP optimizer and the DAE simulator. Sequential methods satisfy the differential and algebraic equations, even when the solver is unable to find a feasible optimal solution.
Non-Dynamic modes sets all differential terms to zero to calculate steady-state conditions.
Steady-state simulation (IMODE=1) solves the given equations when all time-derivative terms set to zero. Dynamic simulation (IMODE=4,7) is either solved simultaneous (IMODE=4) or sequentially (IMODE=7). Both modes give the same solution but the sequential mode solves one time step and then time-shifts to solve the next time step. The simultaneous mode solves all time steps with one solve. Successful simulation of a model within GEKKO helps initialize and and facilitates the transition from model development/simulation to optimization. In all simulation modes (IMODE=1,4,7), the number of equations must equal the number of variables.
Model Parameter Update (IMODE=2) is parameter estimation for non-dynamic data when the process is at steady-state. .. This mode implements the special variable types as follows:
Moving Horizon Estimation (IMODE=5,8) is for dynamic estimation, both for states and parameter regression. The horizon to match is the discretized time horizon of the model m.time. m.time should be discretized at regular intervals. New measurements are added at the end of the horizon (e.g. m.time[-1]) and the oldest measurements (e.g. m.time) are dropped off.
timeshift enables automatic shifting of all variables and parameters with each new solve of a model. The frequency of new measurements should match the discretization of m.time.
Fixed variables are fixed through the horizon.
STATUS adds one degree of freedom for the optimizer, i.e. a fixed parameter for fit.
FSTATUS allows giving a fixed measurement.
Manipulated variables are like FVs, but discretized with time.
STATUS adds one degree of freedom for each time point for the optimizer, i.e. a dynamic parameter for fit.
FSTATUS allows giving a measurements for each time.
Controlled variables may include measurements that are aligned to model predicted values. A controlled variable in estimation mode has objective function terms (squared or l1-norm error equations) built-in to facilitate the alignment.
If FSTATUS is on (FSTATUS=1), an objective function is added to minimize the model prediction to the measurements. The error is either squared or absolute depending on if m.options.EV_TYPE is 2 or 1, respectively. FSTATUS enables receiving measurements through the MEAS attribute.
If m.options.EV_TYPE = 1, CV.MEAS_GAP=v will provide a dead-band of size v around the measurement to avoid fitting to measurement noise.
STATUS is ignored in MHE.
Real-Time Optimization (RTO) is a steady-state mode that allows decision variables (FV or MV types with STATUS=1) or additional variables in excess of the number of equations. An objective function guides the selection of the additional variables to select the optimal feasible solution. RTO is the default mode for Gekko if m.options.IMODE is not specified.
Model Predictive Control (MPC) is implemented with IMODE=6 as a simultaneous solution or with IMODE=9 as a sequential shooting method.
Controlled variables (CV) have a reference trajectory or set point target range as the objective. When STATUS=1 for a CV, the objective includes a minimization between model predictions and the setpoint.
If m.options.CV_TYPE=1, the objective is an l1-norm (absolute error) with a dead-band. The setpoint range should be specified with SPHI and SPLO. If m.options.CV_TYPE=2, the objective is an l2-norm (squared error). The setpoint should be specified with SP.