Simulink - How to create a conditional "if" block

Question

Ahmad Al-Issa 2024-6-21

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编辑： Ahmad Al-Issa 2024-6-23

采纳的回答： Paul

Dear experts, @Stephen23

I have a simulink model, in this model there are two senarios. (heating and cooling)

1- heating: The 1st eq applies when the tempreature increases from 50 to 70 C

2- cooling: The 2nd eq is applied when the tempreature decreases from 70 to 50 C

So, when the tempreature goes from 50 to 70, I use the 1st eq. Until the tempreature reaches >=70 and start runing the cooling unit and using the 2nd eq. Until the tempreature reaches <=50. and so on.

the initial condition is the tempreature is 50 and start heating.

How can I create blocks for this purpose in simulink.

Thank you.

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Umar 2024-6-21

Hi Ahmad, To implement speed-dependent equations in Simulink, you can use the "Switch" block along with a combination of "Relational Operator" blocks to compare the speed values and switch between equations accordingly.

Here's a high-level overview of how you can achieve this:

Use a "Relational Operator" block to compare the speed signal with the threshold values (e.g., 50 and 70 rpm). Connect the output of the "Relational Operator" blocks to a "Switch" block to select between the two equations. Implement your equations in separate subsystems or blocks and connect them to the "Switch" block based on the condition. By setting up the conditions and connecting the blocks appropriately, you can create a dynamic system in Simulink that switches between equations based on the speed thresholds specified. This approach allows for a flexible and efficient way to model speed-dependent behavior in your simulation.

Ahmad Al-Issa 2024-6-21

Hi Umar, @Umar

Did you notice that many values located in both cases ? like (51, 52, ........69).

for the same value 55 rpm for example I use 1st eq and and then I use 2nd eq.

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Answer 1

Paul 2024-6-21

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Perhaps you can use a Relay block to model the desired behavior based on the heating/cooling analogy. The output of the Relay can be used to drive a Switch the controls which equation is used, or a pair of Enabled subsytems. Probably other options as well.

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Umar 2024-6-22

Hi Ahmad,

you can consider using the "Saturation" block in MATLAB Simulink. The Saturation block allows you to limit the output signal within a specified range, which aligns with your requirement of maintaining one equation until reaching the maximum or minimum values. By setting the upper and lower limits in the Saturation block, you can ensure that the output remains within these boundaries until the threshold is reached. By incorporating the Saturation block into your Simulink model, you can effectively control the behavior of your system and ensure that the equation continues to be applied until the predefined limits are exceeded. This approach provides a flexible and straightforward solution to address the issue with the relay block not working as expected.

Additionally, you may also consider implementing conditional statements or logic within MATLAB Function blocks to achieve similar functionality. By utilizing if-else conditions based on speed values, you can dynamically switch between equations until the maximum or minimum thresholds are reached.

Here is a code snippet that demonstrates how you can achieve this:

if speed >= maximum_threshold

 % Equation for maximum threshold

output = equation_for_maximum_threshold;

elseif speed <= minimum_threshold

 % Equation for minimum threshold

output = equation_for_minimum_threshold;

else

% Default equation when speed falls between thresholds

output = default_equation;

end

In this code snippet, the 'speed' variable represents the current speed value, 'maximum_threshold' and 'minimum_threshold' represent the maximum and minimum speed values respectively, and 'equation_for_maximum_threshold', 'equation_for_minimum_threshold', and 'default_equation' represent the equations to be used based on the speed value.

Hope this will help resolve your problem.

Ahmad Al-Issa 2024-6-22

编辑：Ahmad Al-Issa 2024-6-22

@Umar

Thanks Umar for your contribution.

Umar 2024-6-23

No problem Ahmad, as a team working together, you did achieve your goal. Paul was really helpful throughout this process. Kudos to Paul for his efforts.

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Answer 2

Sam Chak 2024-6-21

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Hi @Ahmad Al-Issa

Firstly, could you determine the rate of change of speed (referred to as "acceleration" in physics)? This information is necessary to ascertain whether the speed is increasing from 50 rpm to 70 rpm or decreasing from 70 rpm to 50 rpm, and to apply the corresponding equation.

If your model does not directly measure acceleration, you may need to estimate it using mathematical functions or Simulink blocks. If you have a specific mathematical function, please provide it.

Strictly speaking, only the sign of acceleration is needed. If acceleration is positive, Equation #1 should be applied; if negative, Equation #2.

There are two additional issues to address:

What happens if the speed remains constant at any value between 50 rpm and 70 rpm (e.g., 60 rpm) at the initial simulation? Is there a third equation for this scenario?
If the speed increases linearly from 50 rpm and Equation #1 is applied, but then decreases from 60 rpm onwards without reaching 70 rpm, should Equation #1 or Equation #2 be used?

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Ahmad Al-Issa 2024-6-21

Hi @Sam Chak

To your question (For instance, if the speed fluctuates between 55 rpm and 65 rpm (not yet drop below 50 rpm and not yet rise above 70 rpm), should Equation #1 or Equation #2 be used?)

Please just look at the figure I uploaded. If the speed fluctuates in the upward curve so I used the first equation. If it fluctuates in downward curves I will use the second equation.

Because I assume from time = 0 that the speed value is 50 so I will use the first equation. And I'll wait until I get 70 and start using the second equation.

Just imagine cooling or heating something. When the temperature reaches 70 I turn on the cooling until it reaches 50 and then turn on the heating until it reaches 70 and so on.

assume the tempreature is 50 at t = 0.

Ahmad Al-Issa 2024-6-21

编辑：Ahmad Al-Issa 2024-6-21

Hello @Sam Chak

I change the text of my question, please look on it again.

yes you are right for this (I have a feeling that your switching logic must maintain a persistent memory of which Equation is being used in the "existing state" when the speed fluctuates between 55 rpm and 65 rpm.)

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