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.
Related
I want to be able to externally have inputs for the lower passband edge frequency and higher passband edge frequencies for the butterworth filter block in the simulink signal processing toolbox in matlab. How can I achieve this. Currently you'll have to click the block to specify these frequencies and this is not possible at runtime.
Regards,
Alfred
Basically you are asking for a filter that has time varying parameters. The Butterworth filter block does not allow for this, and cannot be modified to do so, so you are going to have to roll your own. This can be achieved in several ways:
Determining the difference equations that you need to implement, then creating a filter out of fundamental blocks (product, summation and unit delay blocks) where the "parameters" you want to change are fed into the product blocks as signals.
Using a block such as Transfer Fcn Direct Form II Time Varying. (This assumes you can parameterize the changes you need as a gain-scheduled signal.)
Write an S-Function (or perhaps a MATLAB Function block) to implement any detailed/specific functionality.
How to use rate limiter with square signal and variable step size in Simulink?
Here's a screenshot of the model I'd like to set up:
model:
I feed a customized rectangular signal to a rate limiter to avoid vertical slopes.
Unfortunately that doesn't seem to work. I'm using ode15s, it's a requirement. Here's the error message Simulink throws:
Error: Input signals to Rate Limiter '.../Rate Limiter' are neither
discrete nor continuous sample time signals. Only discrete or
continuous input signals are supported
Quite surprisingly I found a workaroud by adding an integrator directly followed by a derivative. This works:
workaroud:
But it's ugly and I'm getting some very annoying stability issues in some cases. And I doubt very much that it is considered "good practice".
So how is one supposed to use this rate limiter block in such a situation?
John
Thank you both for your answers.
I forgot to say I had already checked the sample time with the colour display. It was "Fixed-in-Minor-Step".
Actually it was quite simple. If I get it right, the sample time was not specified or specified in a wrong way in my subsystem. Specifying Continuous in the rate limiter dialog box solved the problem!
thewaywewalk, I'll keep your suggestion in mind. Since I'm using steps a lot it might be useful.
Try displaying the sample time colours in your model to check what sample times your signals are using.
Introducing an integrator block will force the signal to become continuous, hence why it works. Maybe using a Signal Specification block or a Rate Transition block with a sample time of [0, 0] (for continuous signal, see Specify Sample Time in the documentation) will achieve the same thing and be slightly more elegant (using the derivative block is not considered good practice).
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 have set up a model using SimMechanics. It outputs data at the times where the solver steps to. Is there any possibility to have some kind of dense output such that it is possible to interpolate these data to get the solution at arbitrary points without losing the high order of the integrator?
In Matlab this is easily possible using the function deval after the integration of one of the built-in ODE integrators.
In SimMechanics I can select these integrators, too. Is there some kind of analouge way to deval?
Yes, it's possible, although it's a Simulink functionality, not specific to SimMechanics. In the Configuration Parameters of the model, you can set the model to Produce Specified Output Only (see http://www.mathworks.co.uk/help/simulink/gui/data-import-export-pane.html#bq9_fhw-1), under Data Import/Export. This way, only the outputs you specify will be produced regardless of the time steps taken by the solver.
I have face a problem for last few months where solution to it seems like a dream to me! :D
Work i followed...
Initially i high pass a certain signal to remove dc shift of it using filtfilt function of matlab. The results were as intended.
But when i implement the same high pass filter in real time with matlab fdatool (butterworth filter with order2) filter response changes dramatically.
Seems like filtfilt function implementation is perfect possibly due to zero phase.
It's also important to notice phase plays a important in my filtering process?
Can anyone help me to design a real time filter which gives matlab filtfilt performance?