How to change ramp rate of the signal multiple times during simulation ?

Hi,

I have tried to implement a cold ramp rate for a certain plant in Simulink. Meaning that when the plant has been shut down for more than 3 hours, the start up will take longer. I have implemented this with If- and If action -blocks that include dynamic rate limiter -block. However, there is a problem with the simulation, it only changes the rate of change of the signal for the first cold start up, and not all of them. I have attached figures of the system behaviour. In this example, there are two time instances where the plant should go through a cold start-up. In the first picture, there's the if action block that causes the issue and its contents. The following figures are screenshots of the scopes that are visible in the if action block. Finally, there's a figure of the plants load throughout the simulation. In that figure, you can see that at timestep 51, there's slower increase of load than at timestep 82, even though these should look similiar. Any suggestions on how to fix this?

[Sam Chak]

@Elisa Konttila

If possible, please provide a mathematical function that describes the ramp rate signal.

Otherwise, can you sketch desired the ramp rate signal versus the time?

We might be able to suggest a piecewise function for the cold start and warm start conditions.

[Elisa Konttila]

Hi @Sam Chak,

thank you for the response.

I used the mathematical function for the ramp rate from Mathworks website:

Here the u(i) is the value that the model suggests and y(i-1) is 0. t(i) is 2 as I want the signal to take two time steps to reach the value that is suggested by the model. t(i-1) is 0.

From the last figure you can see the sketch for the desired ramp-up at time step 51. Wheras at time step 82, the signal goes from 0 to the suggested value at one time step, which is not the desired result.

Answer 1

[Sarvesh Kale]

Hi Elisa Konttila,

You can use the signal builder block or the signal editor block to change the ramp rate of the signal in a deterministic manner