Hello friends, welcome to the second video of our predictive control course
MPC based on non-linear model (NMPC) Today we will see how it is that
control is resolved by Combinations Linear My name is Sergio Castaño and
you know that all this information the you can find on the website of
https://controlautomaticoeducacion.com/
if you want you can increase the speed of the video so you can make the explanation much faster and more dynamic
Another type of solution strategies for our NMPC controller is
using linear combinations. Here, what we are going to do is solve the
control problem using or using several linear models,
near a break-even point or a work point of the process where
we are working. This type of solution is a little more familiar to
We have used it a lot on the page "automatic control education"
basically, if we represent in our system with a
static graph where we are representing the output
in relation to the input variable, which tells me the static behavior
of my system. The system will have a maximum output and a minimum output
where I also have a minimal control signal and
a maximum control signal. Inside this box would be all my
region of operation. If I do the static graph, I will see that this system
It has that behavior that is really non-linear. If it were linear then
we would see a straight line inside my work zone. But we see
that we have a non-linear system and with linear combinations
we can do another type of NMPC control that consists of taking
several points, for example this point here and linearize the point. We can
take this other point and linearize around that point
this other point and this other point. For example here take four points
In those four points linearized and four linear models are obtained
This linear model 1 serves for work points within this region
(inside the circle) and between further
this point, the model is no longer valid in that region.
The model of this point serves for
regions of the process that are within this area. The linear model 3 for
that are within this area and the linear model 4 for those who are
within this area. With these four linear models you can
Apply four linear predictive drivers
As we saw on the page. For example, we can
apply here a DMC1, DMC2, DMC3 and DMC4. We have 4 DMC to be
applied depending on the region where our process is located.
If the process is located in this static gain, we use the DMC1
if we are in some
point close to model 3, we use the DMC3. We are changing
the MPC that is going to be used. With this strategy they combine
the solutions found to generate the control to be applied. but which one are we going to use?
For that there are several strategies to select which of the
four drivers use. They can be for example strategies using
fuzzy logic or using weights by distance. For example, if I am
here, then I pondered the distance from the model to
this point. A weight is made including the next models
and so on
What happens if there is a point in the middle of two models?
Which of the two do you use?
the linear model 1 or 2? then we could by
think about making a linear combination of the two models that
I'm going to establish with some strategy that can be an example
with fuzzy logic or simply doing linear weights.
If we use linear weights then
the control DMC1 is calculated to obtain the control law 1 and also
the DMC2 control is calculated to obtain the control law 2. Then
a line weighting is made based on the distance with any parameter
(alpha) to determine the global control that is equal to
alpha * u1 + (1-alpha)
alpha is a parameter that is between 0 and 1 that ponders linearly
the 2 controls that were calculated. For example, if alpha = 1
will only apply u1 then it means that this point
It really is much closer to this solution, to this point. So
only DMC1 is used, if alpha = 0.5, weights half of the control action
u1 and u2 and so on. I hope you understand the idea of
the linear combination between the two control actions
calculated in this example. Another alternative way to calculate this driver
by linear combination is using a linear combination of the models
involved depending on the point where the process is located to obtain a single model and
with that model, the calculation of the linear control is done, for example the DMC
Basically what you would do is:
Calculate the predictions for
output 1 using this linear model 1 and
Calculate the predictions for the model linear 2.
Then a general prediction is made
using the weighting of the two outputs.
This weighted prediction
is formed by a linear combination of the two outputs and with that
output then it can already be applied in a linear MPC to
calculate the control law
With that friends
we come to the end of the second non-linear predictive control video NMPC
remembering then that in the next videos we will see other ways of
resolution of this algorithm and I remind you that in the last two or three
videos of this series, let's see how to implement the algorithm in matlab
I am going to explain some codes step by step, that I am preparing so that
You can reproduce them in your homes or universities. With this then
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