u/Ambitionless_Sad_Gir

Hi all, I have an exam tomorrow and have no clue how to distinguish between an unstable non-linear system and a linear marginally stable system. Here is an example of what I am talking about.

I have been told to design a PID controller for a pendulum attached to a DC motor. Then I need to compare the stability between open loop linear and closed loop linear system. For the open loop non-linear, the step input of 0 to 10V and 0 to 15V have constant oscillations and increase (see below). Am I correct to say that between 0 to 15V the system is unstable?

https://preview.redd.it/afx2mrn1frjh1.png?width=1600&format=png&auto=webp&s=90bfc734868939c110245d7149a9bbd5427f3c36

Secondly, from the same system I need to test if the system works without the gravitational term. This means the sin term is taken out, so can I say the system is linear and therefore marginally stable as the output response is a ramp? See below.

https://preview.redd.it/0654jyedfrjh1.png?width=1600&format=png&auto=webp&s=2a14464d6a89037c1aef04f5d2908cf7d497fb94

I am genuinely confused. How does the stability criteria vary between linear and non-linear systems? How do I identify the stability?

Thank you so much for your help.

Hi guys,

Not sure if this is the correct sub for this. I am in desperate need of help here. I am a 4th year engineering student, and I really need help to figure out what I can do to study better. I have failed the same course 3 times now and if I don't pass this time, I will be expelled. So someone help me figure out what I am doing wrong.

It is an oral exam based on 4 questions that you need to solve beforehand. These questions involve simulations and math. You need to present one of the 4 questions that you randomly pick and then you get general questions about the course and your presentation.

First attempt: Didn't watch all the lecture videos, did not solve any exercises. Did the questions an exam with a friend so I failed.

Second attempt: Watched the lecture videos, solved some exercises. Did the exam alone and did some of the exam questions and reused the previous exam questions. Still failed.

Third attempt: Watched the lecture videos and YouTube videos about the subject. Solved all exam questions. Solved exercises. Still failed.

I have attended lectures and spoke to the professor, who basically said to go through the lecture videos. I just wanted to know if anyone has any unhinged study techniques that worked for them. I have a bit of a procrastination and concentration problem due to ADHD.

Thanks a lot.