Clarification on acados s-function sampling time option

User Questions
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Hi,

Let’s say I want to use acados for the following scenario:

  • Simulink Configuration: fixed-step of 1 second.
  • OCP Configuration: 2 steps for prediction horizon with a fixed step size of 0.1 seconds (ocp.solver_options.N_horizon = 2 and ocp.solver_options.tf = 0.2).

This should result in time steps for acados like [0, 0.1, 0.2] at simulation time 0 and [1, 1.1, 1.2] at simulation time 1 (internally, the solver’s shooting nodes remain at [0, 0.1, 0.2], but with inputs coming from the corresponding Simulink time step).

Based on the snippet from simulink_example_advanced.m:

simulink_opts.samplingtime = '-1';
% 't0' (default) - use time step between shooting node 0 and 1
% '-1' - inherit sampling time from other parts of the Simulink model

it appears that setting simulink_opts.samplingtime = '-1'; should make the S-function inherit the sampling time from the rest of the Simulink model.

Yet, when running the model with these settings in Simulink, I encountered the following errors:

  • “Invalid setting for fixed-step size (1.0) in model ‘simulink_model’. All sample times in your model must be an integer multiple of the fixed-step size.”
  • “The sample time period (0.1) of ‘simulink_model/acados_sfunc’ is not an integer multiple of the fixed step size (1.0) specified for model.”

Am I misunderstanding the purpose of the ‘-1’ setting, or is there an additional configuration required for the described scenario?