I have a Matlab function (created by me) that must be evaluated only at a given rate. I would like to sample the value of a signal, give to this function (discrete values) and then, the calculated output must be hold until the next value is available. Is there a way in simulink to do this? All answers I have found use quantizer + ZOH but in this case I still get "a continuum" (or almost it) of points to be evaluated by thsi function which is really slow. Changing the rate of simulink's solver is also not an option as the result of this function will be given for a continuous time system.
Any help will be highly appreciatted!
Thank you
Assuming by Matlab function you mean a MATLAB Function block, then it sounds as if all you need to do is make the block discrete. Do that by right-clicking on the block, going down to Block Properties and then in the resulting dialog enter your required sample time.
The block will then sample its input and generate an output (which is held between sample times) at each sample time.
Related
I have a MATLAB function block embedded in a Simulink model. In my initFnc callback I set up some vectors which I need to use in my function block inside the simulink model. The vector is 1x10000 and contains setpoints for a robotic arm. The function block simply needs to read the next value at each sample iteration.
So far I've tried using "From Workspace" and "evalin()" but they all throw various errors when building the model (I'm using code generation which doesn't play nice all the time).
What would be a good way to read in that vector in Simulink and feed the cells one by one into my function block? Something like repeating sequence stair but without the repeating part.
I got it to work by reading in the vector into a constant block and feeding that into my embedded function. I then use a persistent iterator which is a 1x1 matrix (ones(1)) and incrementing it every time the value from the vector is read.
I need to use a Matlab function inside of a Simulink model. I know how to use a Matlab function to do simple stuff. But what I need now is a little bit more complicated. Let me give you a basic example.
Assume that I need to have a block to generate a sine wave to be viewed directly on the scope (I know that there is already a sine-wave block, I'm just taking that as an example). If I'm writing in Matlab NOT in Simulink, I would do something like:
t = [0:1/30000:0.2];
A = 1;
f =10000;
y = A*sin(2*pi*f*t);
plot(t(1:100),y(1:100))
How can I build the same function inside a Simulink matlab-function block and see the results directly on the Scope?
Remember: The Matlab Function Block has two ports, u and y. Which represents input and output respectively. In the above-given example, a sine-wave generator doesn't need an input.
Perhaps it's just that you haven't chosen a very good example, but there are several things to be aware of when translating the code you've given into Simulink.
The easiest way to get the simulation time into a MATLAB Function block is by feeding a Clock block into an input port (which as #Daniel indicates, are optional, but in this case I suggest would be used for t). So I think you do want an inport in this example.
Your use of plot in your example only plots the first 100 points, where as a Simulink Scope rolls through the data being displayed. There's no concept of only displaying the first X points in a Scope when the simulation runs for longer than that.
You need to remember that Simulink generates data one simulation time step at a time, so you can't generate them all (as per your MATLAB code) and then plot them all. (Well... you can if you want to use frame based signals, but I assume that's not what you're asking here.)
So, to implement the equivalent of what you have would involve doing the following.
write a MATLAB Function block containing the following code (although you might want to make A and f input parameters rather than hard coding them)
function y = myCustomSineWave(t)
A = 1;
f =10000;
y = A*sin(2*pi*f*t);
Feed a Clock block into the above block, and have a Scope block on its output
I would like to make a discrete filter, where the sampling rate can be controlled by an input. I am trying to understand how the discrete filter block looks, "under its own mask." Is there anyway to retrieve the code behind this block so it can be modified for my use?
You can use an user-defined function as a filter, pick the filter transfer function, translate it into a difference equation (the discrete-time equivalant to the diferrential equation), implement that difference equation in a function and feed the sampling rate as an input (the sampling rate will appear as a constant in your difference equation).
The block is too complex and has too many options to simply be able to look under the mask. Your best option is to have a look at the documentation, which does show some detailed implementation of the block in some articular cases to get an idea and then try to recreate the discrete filter you want from basic building blocks, using a constant sample time at first, until you can validate your own implementation against the Simulink library block. Only then, start considering how you will change the sample time. Your main problem though is that the filter coefficients will change with the sample time, so you need to be able to re-calculate them on the fly. It's not an easy problem, I don't even know if it's possible.
I believe I am doing something fundamentally wrong when trying to import and test a transfer function in Simulink which was created within the System Identification Toolbox (SIT).
To give a simple example of what I am doing.
I have an input which is an offset sinusoidal wave from 12 seconds to 25 seconds with an amplitude of 1 and a frequency of 1.5rad/s which gives a measured output.
I have used SIT to create a simple 2 pole 1 zero transfer function which gives the following agreement:
I have then tried to import this transfer function into Simulink for investigation in the following configuration which has a sinusoidal input of frequency 1.5rad/s and a starting t=12. The LTI system block refers to the transfer function variable within the workspace:
When I run this simulation for 13 seconds the input to the block is as expected but the post transfer function signal shows little agreement with what would be expected and is an order of magnitude out.
pre:
post:
Could someone give any insight into where I am going wrong and why the output from the tf in simulink shows little resemblance to the model output displayed in the SIT. I have a basic grasp of control theory but I am struggling to make sense of this.
This could be due to different initial conditions used in SimuLink and the SI Toolbox, the latter should estimate initial conditions with the model, while Simulink does nothing special with initial conditions unless you specify them yourself.
To me it seems that your original signals are in periodic regime, since your output looks almost like a sine wave as well. In periodic regime, initial conditions have little effect. You can verify my assumption by simulating your model for a longer amount of time: if at the end, your signal reaches the right amplitude and phase lag as in your data, you will know that the initial conditions were wrong.
In any case, you can get the estimated initial state from the toolbox, I think using the InitialState property of the resulting object.
Another thing that might go wrong, is the time discretization that you use in Simulink in case you estimated a continuous time model (one in the Laplace variable s, not in z or q).
edit: In that case I would recommend you check what Simulink uses to discretize your CT model, by using c2d in MATLAB and a setup like the one below in Simulink. In MATLAB you can also "simulate" the response to a CT model using lsim, where you have to specify a discretization method.
This set-up allows you to load in a CT model and a discretized variant (in this case a state-space representation). By comparing the signals, you can see whether the discretization method you use is the same one that SimuLink uses (this depends on the integration method you set in the settings).
I'm running a SimEvents simulation using Variable-step discrete solver. I save a signal data using a 'to Workspace' block, but the plot that I obtain is different from the one shown in the 'Scope' block inside the model.
The original signal remains constant between t=64[h] and t=65.4[h] (and this seems to be correctly done also in the matlab plot), while elsewhere it is like the plot command and the 'scope' block are working with different "sample times".
I'd like to obtain a plot showing the typical "step shape" of a discrete signal rather than a "nearly continuous" signal.
I've used the Scope 'save data to workspace' as well, but I didn't solve the problem.
I would have attached some screenshots in order to make the question more complete, but this is my first question so I have not enough reputation to post images. If you need, I can send them to you via e-mail.
Thank you in advance!
There are no screenshots to look at, but my guess would be that the signal is discrete, and the Simulink scope block, knowing this, is only changing at the discrete time points.
However, if you are plotting the data dumped to MATLAB using the standard plot function then it treats the data as continuous and joins successive points with a (non-constant) line.
If that's the case, you most likely want to be using the stairs function to do your MATLAB plottig.