Input disturbance in a level control system

Hi All

I have a problem with a controller i am modeling it is for a pressure vessel which uses steam to heat up water tubes then the steam condenses to water and the outlet flow of the water is controlled by a valve working from a level transmitter. i have built a first order with time delay model but i need to add disturbance to simulate the steam coming in. I am a little stumped on how to go about this i understand i need to add a sum block after the controller but how do i represent the disturbance all help would be greatly appreciated

Thanks

Dan

[Mischa Kim]

Daniel, can you attach your current model?

Answer 1

[Daniel]

Kim please find attached my two models im not sure which is correct as i think model 1 is output disturbance and not input disturbance. I am also unsure what to put in the tf for the disturbance ive done so much reading on this but basically all i can seem to find is that i should put the same tf in as my plant tf and then just alternate the step input to show the different responses is this correct

Answer 2

[Mischa Kim]

First, how and where (in terms of a block diagram) a disturbance enters the systems is typically dictated by the physics of the system, the way you model it (do you get noisy measurements from a sensor? are there uncertainties in the plant model? etc.). In other words, disturbances can enter the system in various locations. The particular approach you need/should take is probably in your problem description.

Having said that, you might remember from your Dynamics or Controls course (modern, linear) that you can re-structure your block diagram, e.g., to simplify your analysis. As an example, you can have a disturbance input right after the plant model (just like in your models), or, have that same disturbance enter the system right before the plant model. In the latter case you need to pre-multiply the disturbance with the plant transfer function in order not to change the system. That's probably what you are referring to.

Does that make sense?