Discretization of the transfer function

Oct 13, 2025 by wbrum.org 84 views

Understanding the lsim function in Matlab

The lsim function in Matlab is used to simulate the response of a continuous-time or discrete-time system to arbitrary inputs. However, when dealing with continuous-time transfer functions and using the lsim function, Matlab performs a crucial step that can sometimes lead to confusion. As far as I know, when you give Matlab a continuous-time transfer function and want to use lsim, what it normally does is it extracts the sampling time from the time vector and discretizes the transfer function. This process is essential for accurately simulating the system's response.

What is the lsim function?

The lsim function is a part of the control systems toolbox in Matlab, which is used to simulate the response of a system to a given input. The function takes in the following parameters:

sys: The system to be simulated, which can be a continuous-time or discrete-time transfer function.
t: The time vector at which the system's response is to be simulated.
u: The input to the system.
y: The output of the system (optional).

How does lsim handle continuous-time transfer functions?

When you use the lsim function with a continuous-time transfer function, Matlab performs the following steps:

Sampling time extraction: Matlab extracts the sampling time from the time vector t. This is done by finding the difference between consecutive time points in the vector.
Discretization of the transfer function: Matlab discretizes the continuous-time transfer function using the extracted sampling time. This is done using a zero-order hold (ZOH) or a first-order hold (FOH) method, depending on the Ts parameter.
Simulation of the system's response: Matlab simulates the response of the discretized system to the given input u.

Common issues with the lsim function

While the lsim function is a powerful tool for simulating system responses, there are some common issues that can arise when using it with continuous-time transfer functions:

Incorrect sampling time: If the sampling time is not correctly extracted from the time vector, it can lead to incorrect simulation results.
Discretization errors: The discretization process can introduce errors in the system's response, especially if the sampling time is not small enough.
Input and output mismatch: If the input and output signals are not correctly matched, it can lead to incorrect simulation results.

Troubleshooting common issues with the lsim function

To troubleshoot common issues with the lsim function, follow these steps:

Verify the sampling time: Check that the sampling time is correctly extracted from the time vector.
Check the discretization method: Verify that the correct discretization method is used (ZOH or FOH).
Match the input and output signals: Ensure that the input and output signals are correctly matched.
Use the Ts parameter: Use the Ts parameter to specify the sampling time and discretization method.

Example code for using the lsim function

Here is an example code snippet that demonstrates how to use the lsim function with a continuous-time transfer function:

% Define the continuous-time transfer function
num = [1 2];
den = [1 3 4];
sys = tf(num, den);
% Define the time vector and input signal
t = 0:0.01:10;
u = sin(t);
% Use the lsim function to simulate the system's response
[y, t] = lsim(sys, u, t);
% Plot the system's response
plot(t, y);
xlabel('Time (s)');
ylabel('System Response');
title('System Response to Sinusoidal Input');

In this example, we define a continuous-time transfer function sys using the tf function. We then define the time vector t and input signal u. Finally, we use the lsim function to simulate the system's response and plot the result.

Conclusion

Q: What is the difference between the lsim function and the step function in Matlab?

A: The lsim function and the step function in Matlab are both used to simulate the response of a system to a given input. However, the step function is used to simulate the response of a system to a unit step input, whereas the lsim function can be used to simulate the response of a system to any type of input.

Q: How do I specify the sampling time in the lsim function?

A: You can specify the sampling time in the lsim function using the Ts parameter. For example:

[y, t] = lsim(sys, u, t, Ts);

This will discretize the system using the specified sampling time Ts.

Q: What is the difference between the ZOH and FOH discretization methods?

A: The ZOH (Zero-Order Hold) discretization method assumes that the system's input remains constant between sampling periods, whereas the FOH (First-Order Hold) discretization method assumes that the system's input changes linearly between sampling periods.

Q: How do I troubleshoot common issues with the lsim function?

A: To troubleshoot common issues with the lsim function, follow these steps:

Verify the sampling time: Check that the sampling time is correctly extracted from the time vector.
Check the discretization method: Verify that the correct discretization method is used (ZOH or FOH).
Match the input and output signals: Ensure that the input and output signals are correctly matched.
Use the Ts parameter: Use the Ts parameter to specify the sampling time and discretization method.

Q: Can I use the lsim function with a discrete-time system?

A: Yes, you can use the lsim function with a discrete-time system. In this case, the lsim function will simulate the response of the system to the given input without discretizing the system.

Q: How do I plot the system's response using the lsim function?

A: To plot the system's response using the lsim function, you can use the following code:

[y, t] = lsim(sys, u, t);
plot(t, y);
xlabel('Time (s)');
ylabel('System Response');
title('System Response to Sinusoidal Input');

This will plot the system's response to the given input.

Q: Can I use the lsim function with a MIMO (Multi-Input Multi-Output) system?

A: Yes, you can use the lsim function with a MIMO system. In this case, the lsim function will simulate the response of the system to the given input for each output channel.

Q: How do I use the lsim function with a transfer function that has a delay?

A: To use the lsim function with a transfer function that has a delay, you can use the following code:

num = [1 2];
den = [1 3 4];
sys = tf(num, den, 'InputDelay', 0.5);

This will create a transfer function with a delay of 0.5 seconds.

Q: Can I use the lsim function with a transfer function that has a transport delay?

A: Yes, you can use the lsim function with a transfer function that has a transport delay. In this case, the lsim function will simulate the response of the system to the given input, taking into account the transport delay.

Conclusion

The lsim function in Matlab is a powerful tool for simulating system responses to arbitrary inputs. By understanding the basics of the lsim function and troubleshooting common issues, you can ensure accurate simulation results.