Algebraic loop in LQR state feedback control system?

Please can anyone help to diagnose the problem with simulink model. I modeled a control system based on LQR control. I want to apply the control to a nonlinear system and i want to use sliding mode observer to estimate the states for full state feedback. However, Eact time I run my simulation, It runs very slowly and when i checked the diagnostics, It shows ALGEBRAIC LOOP in the state feedback gain as shown in the attached picture. I dont know why i get such since i do not need to use any continous block at this point. Thank you

Answer 1

[Delprat Sebastien]

The algebraic loop does not depends on continous ou discrete nature of the process, but on the direct transfert of the input to the output of your system. In your case, I would bet that the system is exactly proper : the output is y(t)=gain.u(t)+something that depends on state. The same thing could happen in discrete time. If the system is linear, then its transfert function has numerator and denominator of the same order.

It is very likely that some dynamics have been neglected in the system modeling because most of physical systems are strictly proper.

You may fix this by adding an actuator dynamics. Alternatively, adding a memory bloc on the output or the control will solve the problem but it is not so nice.

[KaMATLAB]

Thank you for the response and the beautiful insight!!!! I will implement your observations and observe the response.

Answer 2

[KaMATLAB]

Thank you for this beautiful insight. I actually applied the controller to a nonlinear system but giving the algrbraic loop warning. The moment i applied it to the linearized system of the nonlinear system, it gave satisfactory response.

[Delprat Sebastien]

Happy that it worked