u/Known_Fan5765

I have tried modeling this a few times with my limited knowledge of hydraulics and kept getting very weird and ugly state space equations. I am not sure what I am doing wrong so I figured I would post it here (I sincerely apologize if I am breaking the sub rules by posting this).

I have been tasked with controlling the following system with the goal of minimizing the 'ripple' on Q_jet. In other words, ideally Q_jet would look like some constant non-zero value despite changes in the system and environment.

Context of my assumptions for this system:

-Ideal Check Valves

-Qacc ~ (V_o/nP_o)*dP_acc/dt (V_o, P_o initial gass pressure and volume of accumulator, n as poly-tropic index)

-All pressures are absolute

-Incompressible Fluid

-Local restriction equation is: Qjet = C_d*A_n*sqrt(2(P_acc - Patm)/rho) (C_d: discharge coefficient, A_n: restriction area, rho: liquid density)

-A_n is our control input

-Q_pump ~ Asin(2pi*f*t):

4.25e-5 m^3/s < A < 4.25e-3 m^3/s

0.1 Hz < f < 0.6 Hz

System:

https://preview.redd.it/iimigg7c32jh1.png?width=2419&format=png&auto=webp&s=00cc974e80a780f4d7fdbe0421ce6ef659aeace7

Q_jet_Reference from Simulation:

https://preview.redd.it/ig37daqjn2jh1.png?width=1962&format=png&auto=webp&s=d2932549737775df75a489d121442dd50bd6360e

I’m going into my senior year of Uni, I have done very well in every class, I have a very good GPA and I felt like I really did understand most topics. However, I just had an interview and got completely obliterated. The question was simple look at the step response of the circuit and describe some waveform. I started and realized how much I didn‘t know, even with a simple undamped LC LPF, I knew there was oscillation obviously but I could not reason exactly how it was oscillating; whether there was a DC off set, etc. I didnt realize how much I was just relying on math and concrete algorithms to ‘solve’ these circuits, now I am worried I have no idea what I am doing. I tried the same question exercise with some other random passive circuits and I can’t do it. My intuition is always wrong and it makes me feel like I don’t know anything. So with that being said, has anyone else experienced this, I feel like a complete phony.

I have attached a picture below of the non-linear model which describes my system. I wish to linearize this system, however, I only know what one of the equilibrium points would be and the rest are not immediately obvious although they technically don't matter for my control objective.

My goal is to essentially maintain x1 at it's equilibrium point, which for my system is the average value of x1. u2 is the input I can actually control and u1 is uncontrollable.

My guess is that I could maybe set x1 to it's equilibrium position, then maybe find some function where I can express u2e = f(x2, u1) and maybe even use this function to do some sort of gain scheduling where I linearize across different x2, u1 combinations for the different operating states of my system.

Also, one last thing of note, this state space model doesn't quite feel right, so please tell me what I am doing wrong in the comments if possible, this is my first real project so I am fairly new to this.

https://preview.redd.it/ef9ol29o0uhh1.png?width=2458&format=png&auto=webp&s=48ad452edb13f05129811d68543a61241ea0e924

State Space Representation of

As you can see from the image above I managed to find a state space model of my system for the project I am working on. Before I move on to finding a way to control this thing I wanted to ask some of ya'll on here, as this is my first real project and I'm not sure what the best course of action is.

For context this system is a mechanical-hydraulic system and x3 is the volumetric flow rate, from some experimentation x3 is pretty much some average value plus a sinusoidal ripple. The goal of the system is to essentially force x3 to it's average value, to completely eliminate the ripple. The input u1(t) is my control input and u2(t) is an uncontrollable external force driving the system.

My original plan was to just linearize the bottom equation around my desired operating point (zero ripple) but I am not sure how I would actually do that. My current guess is that I could use some sort of small signal approximation treating x3, x1, and u1 as some average value plus a small perturbation and then just setting the small perturbation part of x3 to zero and finding some function between between the average value of x3, x1 and u1 which make my desired operating condition true. However, I have no idea if that is even a mathematically valid way to linearize this thing and also I may even need to implement gain scheduling since this is pretty dang non-linear.

My other thought was to approach this from a much more classical control perspective. I have a simulation of my model in simulink and I could probably just do some system ID to get an approximate transfer function and go from there.

So, if you were given this model and the objective stated above, what would you think to do? Do any controllers/ strategies come to mind? Is it too non-linear for a linear approximation? etc..

Some context: I am in a very unique situation with my 'career'. I am a rising senior in EE, currently interning at a small research lab. As of now I have VERY limited controls experience; I have taken a single class covering basic state space controls and have done a good bit of self learning on classical control theory. I am taking many more controls classes in my senior year and even doing my capstone in control, but as of right now I know my experience is severely lacking. Despite this lack of experience I was able to land an internship at a small research lab where I am currently working on the control system for one of their projects. The reason I was able to land this internship is that despite my lack of expertise I am the only one in the lab who knows anything about controls (and thus really have no guidance which is why I am here). I am currently working on 1/3 of the project, however, I am the only person working on it. The original plan was for me to finish that 1/3 this summer and be done, however, I suppose I have done something to impress my boss as he offered to fund me through my Master's utilizing the current project I am working on as my thesis. This is where I need advice.

I am torn between two courses of action:

1.) Use the rest of this summer to simply research and learn, hold off on doing anything practical until next summer when I come back with a bunch more practical and theoretical experience and can really look at the entire system from a better perspective. The downside of this is that it feels like I sort of 'waste' this internship by essentially producing nothing. I was expected to deliver this 1/3 of the project and not doing that would be fairly disappointing even if my boss is fine with it (which they most likely are) since I'd be completing it over the 1-2 years my Master's takes.

2.) Learn a bit more about the options of controllers I have beyond just what I currently know (PID, Pole Placement, LQR) and just go for it. The issue with this is what if I go for it, commit to a design choice, and end up regretting it. The other issue with this, is that I have never done a controls project, I don't really know the workflow or the best way to go about things and I really have no guidance to do so.

I thought about reaching out to my intro controls professor but she hasn't ever done anything practical and is purely interested in theoretical stuff.

That brings me here, I would absolutely love any advice ya'll have on the matter and would really appreciate if any of you control gods would let me pick your brain a bit about some other questions I have, I am definitely flying blind here lol. Please feel free to DM me or just comment, any advice is greatly appreciated!

I am going into my senior year of university and I am extremely passionate about power electronics engineering and also renewables. I was wondering if anyone here works in/knows of jobs where you are designing power electronics for renewable. If so I would love to discuss with you/hear your thoughts as that is my absolute dream to design power converters for renewables.

I tried coding one up my self, building it with native components, etc. to no avail. Curious if anyone has tried and succeeded in doing this or if they have any other idea since I am lost. Thank you in advance!

Yeah, I don’t feel very good right now, no idea what to do, I feel like I have a pretty good grasp on the concepts, but I just can not seem to do well on controls exams. For reference I have a 3.99 GPA in my major classes, I usually do really well on exams, but controls is a different animal. Despite feeling like I know the content decently well I got a 68 on my midterm and will probably get less than that on my final. I want to go into grid scale power electronics and a big part of that is control, but now I’m doubting if I could ever even do that. I really am doubting my entire life right now, maybe I’m not cut out to be an engineer. Anyone been in a similar situation and still been successful or is it completely over for me, should I drop out and cut my losses?

I am closing out my third year of Uni, studying Electrical Engineering. This quarter I decided to take a control class as it is a prerequisite I desperately needed and it fit into my schedule. This has been the most hellish, terrible, brutal class I have ever taken. I have never felt more stupid or defeated. It is like no matter how much effort I put into this course it is not enough, I really don't feel like I can control anything despite all of the work I have put in. I realize how crucial controls is to my future career (I want to go into designing and integrating grid scale power electronics so a lot of inverter control and stuff) but I am now worried I just won't have what it takes to ever be able to understand this stuff. This class has made me so mad that out of frustration I am taking 2 additional controls classes and doing my schools controls capstone, but I am honestly worried those classes won't help me understand this stuff any better. Am I just a lost cause? Has anyone else ever felt this way and found a way to 'figure it all out', I am so scared.