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  • Flow control

    O'Fithcheallaigh

    Member

    Updated: Oct 25, 2024
    Views: 937
    Hello,

    I am hoping to get some help if I can.

    I have a class this year in Control Engineering, and we have been set a problem involving keeping a water take at a set level. We have to break the model down into it's respective parts - signal amplifier, motor, valve, and plant, or tank, and we need to work out the respective transfer functions.

    During class we worked out the model and TF for everything bar the plant.

    I know the plant/tank relates the input flow to the output flow via the 'level' in the tank. I know this, but I don't know how! I don't know how to work out the transfer function for the relationship between the input flow and the output flow of the tank.

    Can anyone shine any light on this process?

    I am not looking to be told the answer ...I want to understand it myself rather than just been told.

    Thanks in advance.

    Seán
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  • Voltaire

    Member • Sep 20, 2010

    I am not looking to be told the answer ...I want to understand it myself rather than just been told.
    Good man! That is the way it should be asked!!!
    Ok, please explain your question just a bit more - I'm old and forgetful 😀
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  • O'Fithcheallaigh

    Member • Sep 20, 2010

    Hello 😀

    The question involves having a tank which has to be kept at a specified level of water ...so water can flow out, and in to the tank. The level is determined by a sensor, if it gets low, water flows in when a motor opens the required valve ...if the level is too high, the motor opens a different valve and water flows out, allowing the tank to reach the required and correct level.

    I have the model for the motor and the valve, but not for the tank ...the lecturer said the input (water flowing in) to the tank and the output (water flowing out) are related by the level in the tank ...but I just don't know how to model this ...I don't know where to start getting the TF for it.

    Thanks in advance.

    Seán
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  • Voltaire

    Member • Sep 20, 2010

    Ok
    Let's make a start somewhere then we take it from there
    Assuming dr for the tank is zero we can say that dV = rdh (since dV = d(rh) = rdh + hdr)
    The inflow rate dV/dt = rdh/dt = Q*. The same goes for outflow
    Am I on the right track here? Is this where you wanted to go?
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  • O'Fithcheallaigh

    Member • Sep 20, 2010

    Hey,

    Thanks for the reply ...I am not sure ...in a way I think it is ...generally, our Transfer Functions involve Laplace operators etc, so I think what I need to do is use your information to move forward.

    I'm in class at the minute, so won't really have time to think about this until I get home! But thanks again ...I actually think this might set me on the right road 😁

    Seán
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  • Voltaire

    Member • Sep 21, 2010

    Great! 😀
    Try linearity with one-to-one proportionality so that
    1. L(ah) = aH and L(h + k) = H + K
    2. L(h) = L(k) i.e. h = k
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  • O'Fithcheallaigh

    Member • Sep 21, 2010

    Thanks 😀

    I will go from here ...and if I have any issues, I will be back.

    Cheers
    Seán
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  • O'Fithcheallaigh

    Member • Sep 23, 2010

    Hey,

    Just wanted to let know you, you were right ...I had to use the flow out and flow in, but also a function to represent the tank itself ...but we got there in the end.

    So, again, thanks for your help! 😎

    Seán
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  • icehari

    Member • Sep 27, 2010

    so, you are going to create a closed loop control system... am i right?.

    you can use a float as a level sensor and the level of the water will be proportional to the resistance output in the float and that resistance output can change the speed of the motor proportionally....

    got any starting idea now.....
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