SIMULINK error: Unable to determine a fixed step size based on the sample times in the model

Friends,

I get the following error while running my SIMULINK model (closed-loop control feedback):

'Unable to determine a fixed step size based on the sample times in the model 'normal_force_input', because the model does not have any discrete sample times. Picking a fixed step size of (12.0) based on simulation start and stop times...

In Solver pane, here are the settings:

Start time: 0.0 Stop time: 10.0

Type: Fixed-step Solver: ODE3 (Boga...)

Fixed step size(fundamental sample time): auto

periodic sample time constraint: unconstrained

Tasking mode ...: auto

Automatically handle ....: unchecked

Higher priority: unchecked

What am I missing? Please help. Thank you.

Mehdi

Answer 1

[ES]

Immediate fix is to choose a variable step solver(in the solver type) if you really don't need a fixed step solver. If you intend to use a fixed type solver, then all the subsystems should have a sample time that is multiple of other subsystems. That is, an Highest Common Factor (HCF) should exist between the sample times of all subsystems. Else make the subsystem inherit the sample time by putting -1 in the Subsystem Properties. More on Solvers here

[Mehdi]

Thank you for your reply. I chose variable-step solver and I don't get that error any more! However, I still don't get the desirable results. The Control book in which this model is presented says that the simulation has to be carried out with a fourth-order Runge-Kutta scheme. Is ode45(Dormand-Prince) what I should choose?

Thank you for your continued help.

Mehdi

[ES]

Explicit Continuous Variable-Step Solvers and ode45 must be your choice of solver!