Building block diagrams in MATLAB - Step-by-Step Execution

Concept Flow - Building block diagrams

Define blocks

↓

Set block parameters

↓

Connect blocks

↓

Simulate system

↓

Analyze output

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Modify blocks or connections if needed

↩Back to Define blocks

This flow shows how to create a building block diagram by defining blocks, setting parameters, connecting them, simulating, and analyzing output.

Execution Sample

MATLAB

G1 = tf([1],[1 1]);
G2 = tf([2],[1 2]);
sys = series(G1,G2);
step(sys);

This code creates two transfer function blocks, connects them in series, and plots the step response.

Execution Table

Step	Action	Variable/Block	Value/Result	Notes
1	Create transfer function G1	G1	1/(s+1)	Block G1 defined
2	Create transfer function G2	G2	2/(s+2)	Block G2 defined
3	Connect G1 and G2 in series	sys	2/((s+1)(s+2))	Blocks connected
4	Simulate step response	step(sys)	Plot generated	Output shows system response
5	End	-	-	Simulation complete

💡 Simulation ends after step response plot is generated

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
G1	undefined	1/(s+1)	1/(s+1)	1/(s+1)	1/(s+1)
G2	undefined	undefined	2/(s+2)	2/(s+2)	2/(s+2)
sys	undefined	undefined	undefined	2/((s+1)(s+2))	2/((s+1)(s+2))

Key Moments - 3 Insights

Why do we create transfer functions before connecting blocks?

Because each block must be defined with its parameters first (see steps 1 and 2 in execution_table) before we can connect them (step 3).

What does the 'series' function do in this context?

It connects two blocks one after another, multiplying their transfer functions as shown in step 3.

Why do we call 'step(sys)' after connecting blocks?

To simulate and visualize how the combined system responds to a step input, as in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'sys' after step 3?

A 1/(s+1)

B 2/((s+1)(s+2))

C 2/(s+2)

D Undefined

💡 Hint

Check the 'Value/Result' column at step 3 in execution_table

At which step is the step response plot generated?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for 'Plot generated' in the Notes column of execution_table

If we swapped the order of G1 and G2 in the series connection, what would change in variable_tracker?

A The value of 'G2' would change

B The value of 'G1' would change

C The value of 'sys' would change to 2/((s+2)(s+1)) but is equivalent

D No variables would change

💡 Hint

Series connection multiplies transfer functions; order changes expression but not meaning (see variable_tracker final values)

Concept Snapshot

Building block diagrams in MATLAB:
- Define blocks with transfer functions (tf)
- Connect blocks using series, parallel, or feedback
- Simulate with step or other commands
- Analyze output plots
- Modify blocks or connections as needed

Full Transcript

This visual execution shows how to build block diagrams in MATLAB by defining transfer function blocks, connecting them in series, simulating the step response, and analyzing the output. First, we create two blocks G1 and G2 with specific transfer functions. Then, we connect them in series to form a combined system 'sys'. Next, we simulate the system's response to a step input using the step function, which generates a plot. The variable tracker shows how each variable changes after each step. Key moments clarify why blocks must be defined before connecting, what the series function does, and why simulation is done after connection. The quiz tests understanding of variable values and simulation steps. This process helps visualize system behavior step-by-step.

Practice

1. What is the primary purpose of building block diagrams in MATLAB Simulink?

easy

A. To write MATLAB scripts faster

B. To create 3D plots of data

C. To visually represent and simulate system behavior

D. To debug MATLAB code line by line

Solution

1. Step 1: Understand the role of block diagrams

   Block diagrams in Simulink are used to visually model systems by connecting blocks that represent components.

2. Step 2: Identify the main use

   They help simulate and analyze system behavior, not for scripting or plotting.

3. Final Answer:

   To visually represent and simulate system behavior -> Option C

4. Quick Check:

   Block diagrams = Visual system simulation [OK]

Hint: Block diagrams show system flow visually, not code or plots [OK]

Common Mistakes:

• Confusing block diagrams with MATLAB code scripts
• Thinking block diagrams create plots
• Assuming block diagrams debug code

2. Which of the following is the correct way to add a Gain block in a Simulink model using MATLAB command line?

easy

A. add_block('simulink/Math Operations/Gain', 'model/Gain1')

B. add_block('simulink/Sinks/Gain', 'model/Gain1')

C. add_block('simulink/Sources/Gain', 'model/Gain1')

D. add_block('simulink/Commonly Used Blocks/Gain', 'model/Gain1')

Solution

1. Step 1: Identify the correct library path for Gain block

   The Gain block is located under 'simulink/Math Operations' in Simulink library.

2. Step 2: Match the command syntax

   The command add_block('simulink/Math Operations/Gain', 'model/Gain1') correctly adds the Gain block to the model.

3. Final Answer:

   add_block('simulink/Math Operations/Gain', 'model/Gain1') -> Option A

4. Quick Check:

   Gain block path = Math Operations [OK]

Hint: Gain block is under Math Operations library [OK]

Common Mistakes:

• Using wrong library path for Gain block
• Confusing Sources or Sinks with Gain block location
• Incorrect syntax in add_block command

3. Consider this MATLAB code snippet for a Simulink model:

open_system('myModel');
add_block('simulink/Sources/Step', 'myModel/Step1');
add_block('simulink/Sinks/Scope', 'myModel/Scope1');
add_line('myModel', 'Step1/1', 'Scope1/1');

What will this code do?

medium

A. Create a model with a Step input but no connections

B. Create a model with a Step input connected to a Scope block

C. Create a model with a Gain block connected to a Scope block

D. Produce an error because add_line syntax is incorrect

Solution

1. Step 1: Analyze blocks added

   The code adds a Step block (input) and a Scope block (output) to the model 'myModel'.

2. Step 2: Analyze connection

   The add_line command connects output port 1 of Step1 to input port 1 of Scope1, linking input to output.

3. Final Answer:

   Create a model with a Step input connected to a Scope block -> Option B

4. Quick Check:

   Step block connected to Scope = Model created [OK]

Hint: add_line connects blocks by port numbers correctly [OK]

Common Mistakes:

• Confusing block types added
• Assuming no connection was made
• Thinking add_line syntax is wrong

4. You try to connect two blocks in Simulink using:

add_line('model', 'BlockA/1', 'BlockB/2');

but get an error. What is the most likely cause?

medium

A. add_line requires port numbers as strings, not numbers

B. BlockA does not have an output port 1

C. The model name is incorrect

D. BlockB does not have a second input port

Solution

1. Step 1: Understand port numbering

   Port numbers must exist on blocks; if BlockB/2 does not exist, connection fails.

2. Step 2: Check common error

   Most blocks have input port 1, but not all have a second input port, causing error.

3. Final Answer:

   BlockB does not have a second input port -> Option D

4. Quick Check:

   Invalid port number causes add_line error [OK]

Hint: Check if destination port exists before connecting [OK]

Common Mistakes:

• Assuming port numbers can be any number
• Thinking add_line syntax requires strings for ports
• Ignoring model name correctness

5. You want to create a Simulink model programmatically that sums two signals and then multiplies the result by 5. Which sequence of commands correctly builds this model?

hard

A. add_block('simulink/Math Operations/Sum', 'model/Sum1');
add_block('simulink/Math Operations/Gain', 'model/Gain1');
set_param('model/Gain1', 'Gain', '5');
add_line('model', 'Sum1/1', 'Gain1/1');

B. add_block('simulink/Math Operations/Gain', 'model/Gain1');
set_param('model/Gain1', 'Gain', '5');
add_block('simulink/Math Operations/Sum', 'model/Sum1');
add_line('model', 'Gain1/1', 'Sum1/1');

C. add_block('simulink/Math Operations/Sum', 'model/Sum1');
add_block('simulink/Math Operations/Gain', 'model/Gain1');
set_param('model/Gain1', 'Gain', '5');
add_line('model', 'Gain1/1', 'Sum1/1');

D. add_block('simulink/Math Operations/Sum', 'model/Sum1');
add_block('simulink/Math Operations/Gain', 'model/Gain1');
set_param('model/Gain1', 'Gain', '5');
add_line('model', 'Sum1/1', 'Gain1/2');

Solution

1. Step 1: Add Sum and Gain blocks correctly

   Sum block adds two inputs; Gain block multiplies by 5. Both blocks are added properly.

2. Step 2: Set Gain parameter and connect Sum output to Gain input

   Gain is set to 5. The output port 1 of Sum connects to input port 1 of Gain.

3. Final Answer:

   Correct sequence with Sum output connected to Gain input and Gain set to 5 -> Option A

4. Quick Check:

   Sum -> Gain with Gain=5 is correct [OK]

Hint: Connect Sum output to Gain input; set Gain parameter after adding block [OK]

Common Mistakes:

• Connecting Gain output to Sum input (wrong direction)
• Using wrong port numbers for connections
• Not setting Gain parameter correctly