Root locus analysis in Simulink - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a transfer function.

Simulink

sys = tf([1], [1 3 2]);

Drag options to blanks, or click blank then click option'

A1

B[1 5]

C0

Ds

Attempts:

3 left

💡 Hint

Common Mistakes

Using 's' as numerator causes error.

Using a vector instead of scalar for numerator.

The numerator of the transfer function is a scalar 1, representing a simple gain.

2fill in blank

medium

Complete the code to plot the root locus of the system.

Simulink

rlocus([1]);

Drag options to blanks, or click blank then click option'

Aplot

Brootlocus

Csys

Dtf

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a plotting function name instead of system.

Passing transfer function constructor instead of system variable.

The rlocus function requires the system transfer function as input.

3fill in blank

hard

Fix the error in the code to compute the characteristic equation.

Simulink

char_eq = poly([1]);

Drag options to blanks, or click blank then click option'

As

Brlocus

Ctf

Dsys

Attempts:

3 left

💡 Hint

Common Mistakes

Using rlocus instead of system variable.

Using symbolic variable s incorrectly.

The poly function requires the system or its poles as input to get the characteristic polynomial.

4fill in blank

hard

Fill both blanks to create a root locus plot with grid and title.

Simulink

rlocus([1]);
grid [2];

Drag options to blanks, or click blank then click option'

Asys

Bon

Coff

Dplot

Attempts:

3 left

💡 Hint

Common Mistakes

Using grid off disables the grid.

Passing wrong variable to rlocus.

Use the system variable for rlocus and turn the grid on with grid on.

5fill in blank

hard

Fill all three blanks to compute poles, zeros, and plot root locus with title.

Simulink

poles = pole([1]);
zeros = zero([2]);
rlocus([3]);
title('Root Locus Plot');

Drag options to blanks, or click blank then click option'

Asys

Dplot

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for poles and zeros.

Using plotting function name instead of system variable.

Use the system variable sys for poles, zeros, and root locus plotting.

Practice

1. What does the root locus plot primarily show in control systems analysis?

easy

A. The noise level in the system output

B. The frequency response of the system

C. How the system poles move as the gain changes

D. The time response of the system to a step input

Solution

1. Step 1: Understand root locus purpose

   The root locus plot shows how the locations of system poles change as the gain parameter varies.

2. Step 2: Differentiate from other plots

   Frequency response and time response plots show different system behaviors, not pole movement.

3. Final Answer:

   How the system poles move as the gain changes -> Option C

4. Quick Check:

   Root locus = pole movement with gain [OK]

Hint: Root locus tracks poles as gain varies [OK]

Common Mistakes:

• Confusing root locus with frequency response
• Thinking it shows output noise
• Mixing it up with time response plots

2. Which Simulink command is used to plot the root locus of a system with transfer function G = tf([1], [1, 3, 2])?

easy

A. rlocus(G)

B. step(G)

C. bode(G)

D. pzmap(G)

Solution

1. Step 1: Identify root locus command

   In Simulink and MATLAB, rlocus() is the function to plot root locus.

2. Step 2: Differentiate from other commands

   step() plots step response, bode() plots frequency response, and pzmap() plots poles and zeros but not root locus.

3. Final Answer:

   rlocus(G) -> Option A

4. Quick Check:

   Root locus plot = rlocus() [OK]

Hint: Use rlocus() to plot root locus in Simulink [OK]

Common Mistakes:

• Using step() instead of rlocus()
• Confusing bode() with root locus
• Using pzmap() which shows poles but no gain variation

3. Given the transfer function G = tf([2], [1, 4, 5]), what is the number of branches in the root locus plot?

medium

A. 1

B. 2

C. 3

D. 4

Solution

1. Step 1: Determine number of poles

   The denominator polynomial [1, 4, 5] is second order, so there are 2 poles.

2. Step 2: Relate poles to root locus branches

   Root locus branches equal the number of poles, so there are 2 branches.

3. Correction:

   The denominator polynomial [1, 4, 5] is of order 2, so there are 2 poles, but the number of branches in root locus equals the number of poles, which is 2, not 3.

4. Final Answer:

   2 -> Option B

5. Quick Check:

   Branches = number of poles = 2 [OK]

Hint: Count denominator order for branches [OK]

Common Mistakes:

• Counting zeros instead of poles
• Confusing numerator order with poles
• Assuming branches equal zeros

4. Identify the error in this Simulink command for root locus: rlocus(tf([1 2], [1 3]))

medium

A. No error, command is correct

B. The numerator vector is invalid

C. The transfer function is not proper

D. The denominator vector length is less than numerator

Solution

1. Step 1: Check transfer function order

   Numerator order is 1 (degree 1), denominator order is 1 (degree 1). Transfer function is proper if denominator order >= numerator order.

2. Step 2: Analyze properness

   Here numerator and denominator orders are equal, so transfer function is proper, not improper.

3. Step 3: Check command validity

   The command is syntactically correct and will run without error.

4. Final Answer:

   No error, command is correct -> Option A

5. Quick Check:

   Proper TF and syntax = no error [OK]

Hint: Check numerator and denominator orders for proper TF [OK]

Common Mistakes:

• Assuming numerator length must be less than denominator
• Thinking equal order is improper
• Confusing vector length with polynomial degree

5. You have a system with transfer function G = tf([1], [1, 2, 2, 1]). You want to design a controller gain K so that the system poles lie on the left half of the complex plane for stability. Using root locus analysis, which gain range should you choose?

hard

A. K = 0 only

B. Any positive K value

C. K < 0 only

D. K > 0 and less than the gain where poles cross imaginary axis

Solution

1. Step 1: Understand root locus stability

   Root locus shows pole movement as gain K changes. Poles in left half-plane mean stable system.

2. Step 2: Identify gain limits for stability

   Poles start stable at K=0 and move as K increases. Stability is maintained until poles cross imaginary axis at a critical gain.

3. Step 3: Choose gain range

   Gain K must be positive and less than the critical gain where poles cross imaginary axis to keep system stable.

4. Final Answer:

   K > 0 and less than the gain where poles cross imaginary axis -> Option D

5. Quick Check:

   Stable poles = gain below crossing point [OK]

Hint: Pick gain before poles cross imaginary axis [OK]

Common Mistakes:

• Assuming any positive gain is stable
• Choosing negative gain for stability
• Ignoring pole crossing points