I am trying to run a closed loop system on simulink as shown below.
When I run it it get this result.
As you can see, there is a slight time delay in getting the step function up to 1. The closed loop output also doesn't start until around 1.5 sec. I understand that this is what would occur in the real world, but I was wondering if there was a way to get rid of this time delay and make the output show 'ideal' results.
Thanks
Edit
Just thought i'd add a bit more info. The step input is a standard step input and the only things I have changed in terms of settings is the simulation time is 8 seconds, and the solver is a fixed-step ode1 (euler).
You are using a fixed step solver, so the step size defaults to (stop_time-start_time)/50, which in your case equals 0.16. Hence you do not have a time step at exactly 1s. At the 6th time step = 0.96, the step is 0. At the 7th time step = 1.12, the step is 1. That is exactly what is being shown, and correct for the simulation parameters you are using.
With a fixed step solver, if you want the step to occur at exactly 1s then you need to specify a step size so that the model takes a time step at 1s.
You do that by going to the Solver panel of the Simulation Parameters pull down menu and change the Step Size to something appropriate. (Note that the plot will still show the step starting at 1, but finishing one time step later.)
Alternatively you can use a variable step solver.
(This would display the step as being exactly vertical at 1s.)
Regarding the time delay, you have a 3 more poles than zeros so will have a 3 step time delay when using fixed-step Euler.
The only way to change that is to use a different solver.
Related
I state that I have searched for a long time but I can't find an answer to my problem.
I don't find a Simulink block that has the function of giving in input one step that is worth an x (defined by me) for 5s and 1.1x from 6 to 50s at two models (one linear and one non linear systems)
Usually i use the Step Block to start an initial and final value of the step but the change is almost instantaneous. What block can I use to do this?
Option 1
Use multiple step blocks and add them up. Simple as that.
Option 2
Use the signal builder block.
I am having troubles understanding the output of my scope in this simple simulink model:
I am using a fixed step solver (tried with ode3 and ode8).
Pulse type of the puls generator is set to Sample based and I varied the Period and Pulse Width.
First I set the simulation time to 10 and set the puls generator to Period = 10 and Puls width = 5. The output of the scope is as expected:
But when I tried with simulation time 10,000 and the puls generator with Period = 1,000 and Puls width = 500 it seems my scope is wrong:
Why is the first falling edge at 5,500? I used the Autoscale button every time.
Using sim time 100,000 and Period = 10,000 and Puls width = 5,000 I don't even get a single falling edge:
Even with longer simulation time there seems to be a single rising edge at the end of the scope window.
What am I doing wrong? Is the scope not suitable for such long simulation times using fixed step solver? Or is it not "safe" to use the Autoscale button?
All of the plots you show are correct. Simulink is fine with long simulation times. It is "safe" to use the Autoscale button.
By default a scope is set to only display the last 5000 simulation time steps. Since your model is taking a step size of 1s (this is based on using the default step size of the Pulse Generator, which is 1s), in your second plot you are only seeing points from t=5000 to t=10000 (so the first down step in that time period is at 5500), and in your third plot you are only seeing points from t=95000 to t=100000 (which is a period in which the value of the pulse is low/zero).
To see all simulation times, open the Scope block's parameters (by clicking the button with a picture of a cog on it), go to the History tab, and deselect the Limit data points to last: check box.
Then rerun your simulation and press the autoscale button. You'll then see what (I think) you are expecting.
I want to set a variable step size for every solver step by using the command in the S-function like:
dT= ... % calculate the dT from the inputs of Block and the parameters of S-function
set_param(gcs,'...',num2str(dT));
However, the Matlab does not provide us with a assignable parameter like 'Step' for specifing the solver step size by using the command "set_param()" above. The callable and assignalbe parameters for the solver step size are only 'MaxStep' and 'MinStep'. Therefore, the following two commands are acceptable and executable in Simulink:
set_param(gcs,'MaxStep',num2str(dT1));
set_param(gcs,'MinStep',num2str(dT2));
Thus, I was trying to assign a same value to dT1 and dT2 in order to get the a certain step size, but there was immediately a error report indicating that the max. step and min. step cannot be the same.
So my question is how to specify a step size to the solver in the script of S-function?
The solver settings (used by variable step and fixed-step blocks) are set on initialization and can't be changed using the simulation.
And I assume the fixed-step solver suggestion in the comments won't work for you as you seem to indicate that you want to change the step size during the simulation.
Generically there is no real mechanism for you to have that sort of control over defining (on a step by step basis) the step size that Simulink takes during the simulation.
Nominally that's what the Simulink solver does for you automatically based on the settings during initialization.
You can do it on a block by block basis if all the blocks are S-functions and have a variable step size.
And you could do it by running the simulation over a single time step, saving the SimState, determining the next sample time, running for one time step, saving the SimState, etc., but that would be very inefficient.
What is the magnitude of delay in sec offered by unit delay (simulink block) for following condition?
Configuration parameters are variable step with ode45 solver.
Max step size is 1/60 and min step size is auto.
In delay block sample time is kept as 0.5s.
Input processing is set as 'Elements as channels (sample based)'.
Thanks
I hope you are using discrete blocks as you have mentioned "Unit Delay". In that case, the delay caused is your sample time. ie here it is 0.5s.
Usually Matlab will go for a discreet solver unless you have explicitly mentioned 'ode45' solver. Also, stepsize of solver wont affect the delay
Simply your sampling time. But If you have some informations that we do not know I will give you a way to measure it.
In you model add 2 Scopes. Assign one of these Scopes' input as output of the Delay Block. Other Scope's input will be input of the Delay Block.
Run the simulation and open the graph results of Scopes. There you go. You can see how much delay your Delay Block caused.
I'm using the simulink block From Workspace to read in some audio data provided by a script. I have formatted the data in a matrix with 2 columns, the first is the timestamp and the second is the data.
In the configuration paramaters, I have specified Fixed-Step and Discrete solver. The Start time and Stop also need to be configured manually and don't seem to come from the data.
Also, in the From Workspace block configuration, I need to specify the sample time (1/44100) or I get a warning if I specify -1, to inherit from the data and then get strange sample times.
So, how can I get simulink to use only the sample times in the matrix and use the first and last timestamps as the start and stop time of the simulation?
You should be able to do what you want by doing the following:
Firstly note that your problem is by definition not fixed step, hence you cannot use a fixed-step solver, which by definition is ... fixed-step.
You must use a variable step solver.
Assuming your (2 column) input data is called simin then set the start and stop times to be simin(1,1) and simin(end,1) respectively.
In your From Workspace block set the sample time to be 0 (which should have been the default).
Also de-select the Interpolate data option; and set "Form the output after final data value by:" to zero (you won't be using anything past the end of your data set so this should be OK.
Then you need to tell the solver to take additional steps to those that it would naturally want to take.
Do this on the Data Import/Export pane of the Model Configuration Parameters.
Near the bottom of the pane there is a selection box and an edit box for doing this.
Note however that this does not prevent the solver from taking steps at other time points, it just forces it to take additional steps at the times you specify.
But because you have your From WOrkspace block to not interpolate this shouldn't be a problem either. You should put simin(:,1) in here so that the solver is guaranteed to take steps at the time points in your input data.
Note that if you want an input block that only samples at the time points in the simin time vector then the only way to do this is to write an S-function that uses the mdlGetTimeOfNextVarHit method to tell the solver what the next sample time (for this block) should be.