Laplace Transform Engineering Applications | RLC & Control Systems

Laplace Transform Engineering Applications — PinoyBIX EE ECE ME Board Exam Reviewer

This is where everything from Parts 1 through 4 comes together in the format board examiners actually use. Nobody gets asked to transform a bare function for its own sake on exam day. They get asked to analyze a circuit, a vibrating beam, or a control loop — and the Laplace Transform is the computational engine that makes the algebra possible. This post covers the three physical system types that dominate EE, ECE, and ME board questions involving Laplace Transform: RLC circuits, spring-mass-damper mechanical systems, and control system transfer functions. All 10 worked problems are written in the style of actual board questions, complete with physical context, system equations, and full solutions. This is Part 5 of the Laplace Transform Series.


📋 BOARD EXAM RELEVANCE

  • EE (Electrical Engineer) — Very high frequency. RLC circuit transient analysis and s-domain impedance problems appear almost every exam cycle. This post’s RLC problems represent the exact type and difficulty of what the Professional Regulation Commission tests.
  • ECE (Electronics Engineer) — Very high frequency. Transfer functions and system response questions are core ECE board content. Every block diagram problem that asks “find the output given this input” is solved by the method in this post.
  • ME (Mechanical Engineer) — High frequency for the spring-mass-damper problems specifically. Vibration analysis, damping classification, and natural frequency extraction from the characteristic polynomial of the ODE are standard ME board items.
  • CE (Civil Engineer) — Low frequency. Structural dynamics electives cover spring-mass systems, but the core CE board rarely asks for a full Laplace-based solution.
  • ChE, GeE, MetE, MinE, Naval Architect and Marine Engineer — Low to rare. Process control in ChE touches transfer functions, but it is not a primary board exam topic for the other boards.

Bottom line: This part completes the series. Every technique you learned in Parts 1 through 4 — pairs table, partial fractions, derivative transforms, and shifting theorems — gets used again here, now dressed in an engineering context. Master the pattern once and it applies across all three system types.


The Three Systems and Their Governing Equations

SERIES RLC CIRCUIT (EE / ECE)

    \[L\ddot{q} + R\dot{q} + \dfrac{1}{C}q = v(t)\]

    \[\text{s-domain impedance: } Z(s) = Ls + R + \dfrac{1}{Cs}\]

Kirchhoff’s Voltage Law around the loop. Working in charge q(t) rather than current i(t) eliminates the integral term in KVL — differentiate once at the end to recover i(t) = \dot{q}(t) if needed.

SPRING-MASS-DAMPER SYSTEM (ME)

    \[m\ddot{x} + c\dot{x} + kx = F(t)\]

    \[(ms^2 + cs + k)X(s) = F(s) + \text{IC terms}\]

Newton’s second law applied to the mass. The structure is identical to the RLC circuit above — same second-order ODE, same solution method, just with different physical labels. Underdamped when c^2 < 4mk, critically damped when c^2 = 4mk, overdamped when c^2 > 4mk.

TRANSFER FUNCTION (ECE / EE)

    \[G(s) = \dfrac{Y(s)}{X(s)} \quad \text{(zero initial conditions)}\]

The ratio of the output transform to the input transform, assuming all initial conditions are zero. This definition underlies every block diagram problem on the ECE and EE boards. The output to any given input is simply Y(s) = G(s)X(s), then invert to get y(t).


10 Worked Board Exam Problems


Problem 1. Find the s-Domain Impedance of a Series RLC Circuit

Problem: A series RLC circuit has resistance R = 4\,\Omega, inductance L = 2\,\text{H}, and capacitance C = 0.25\,\text{F}. Find the total s-domain impedance Z(s).

Given: R = 4\,\Omega, L = 2\,\text{H}, C = 0.25\,\text{F}

Find: Z(s)

Solution:

Step 1: Apply the s-domain impedance formula for a series RLC circuit.

    \[Z(s) = Ls + R + \dfrac{1}{Cs}\]

Step 2: Substitute the given values.

    \[Z(s) = 2s + 4 + \dfrac{1}{0.25s} = 2s + 4 + \dfrac{4}{s}\]

Step 3: Write as a single rational expression over a common denominator.

    \[Z(s) = \dfrac{2s^2 + 4s + 4}{s}\]

✓ ANSWER: Z(s) = \dfrac{2s^2+4s+4}{s}

Examiner note: In the s-domain, an inductor L becomes Ls (increases with s), a resistor stays R, and a capacitor C becomes 1/(Cs) (decreases with s). Series combination adds all three impedances exactly like series resistors in DC circuits.


Problem 2. Find the Current Response of a Series RL Circuit With Step Input

Problem: A series RL circuit has R = 3\,\Omega and L = 1\,\text{H}. A step voltage of v(t) = 12\,\text{V} is applied at t = 0 with zero initial current. Find i(t).

Given: R = 3, L = 1, v(t) = 12\,u(t), i(0) = 0

Find: i(t)

Solution:

Step 1: Write KVL in terms of current: L\dot{i} + Ri = v(t).

    \[\dot{i} + 3i = 12\]

Step 2: Transform. The right-hand side 12 transforms to 12/s.

    \[[sI - i(0)] + 3I = \dfrac{12}{s} \implies (s+3)I = \dfrac{12}{s}\]

Step 3: Solve for I(s) and decompose.

    \[I(s) = \dfrac{12}{s(s+3)} = \dfrac{A}{s} + \dfrac{B}{s+3}\]

    \[s=0: A = 4, \quad s=-3: B = -4\]

Step 4: Invert.

    \[I(s) = \dfrac{4}{s} - \dfrac{4}{s+3}\]

✓ ANSWER: i(t) = 4(1 - e^{-3t})\,\text{A}

Examiner note: The constant term 4 is the steady-state current — what the circuit settles to as t \to \infty, which checks out: V/R = 12/3 = 4\,\text{A}. The e^{-3t} term is the transient that decays with time constant \tau = L/R = 1/3\,s.


Problem 3. Find the Charge Response of a Series RLC Circuit With Step Voltage

Problem: A series RLC circuit has R = 3\,\Omega, L = 1\,\text{H}, C = 0.5\,\text{F}. A step voltage v(t) = 10\,\text{V} is applied at t = 0 with q(0) = 0 and q'(0) = 0. Find q(t).

Given: R = 3, L = 1, C = 0.5, v(t) = 10, q(0) = 0, q'(0) = 0

Find: q(t)

Solution:

Step 1: Write KVL in terms of q: \ddot{q} + 3\dot{q} + 2q = 10.

Step 2: Transform. All initial conditions are zero.

    \[(s^2 + 3s + 2)Q(s) = \dfrac{10}{s}\]

Step 3: Solve for Q(s) and factor the denominator.

    \[Q(s) = \dfrac{10}{s(s+1)(s+2)}\]

Step 4: Decompose using Case 1.

    \[s=0: A = 5, \quad s=-1: B = -10, \quad s=-2: C = 5\]

Step 5: Invert.

✓ ANSWER: q(t) = 5 - 10e^{-t} + 5e^{-2t}\,\text{C}

Examiner note: Both roots s = -1 and s = -2 are real and negative, meaning the circuit is overdamped — no oscillations in the response. If the roots had been complex conjugates, the response would contain exponentially decaying sine and cosine terms instead.


Problem 4. Find the Transient Current From Charge Response

Problem: Using the charge q(t) = 5 - 10e^{-t} + 5e^{-2t} found in Problem 3, find the current i(t).

Given: q(t) = 5 - 10e^{-t} + 5e^{-2t}

Find: i(t)

Solution:

Step 1: Differentiate q(t) since i(t) = \dot{q}(t).

    \[i(t) = \dfrac{dq}{dt} = 0 - 10(-1)e^{-t} + 5(-2)e^{-2t}\]

✓ ANSWER: i(t) = 10e^{-t} - 10e^{-2t}\,\text{A}

Examiner note: Working in q(t) first eliminates the integral term in KVL and reduces the circuit ODE to a standard second-order form. Differentiation at the end to get i(t) is always cleaner than carrying the integral through the entire transform process.


Problem 5. Classify the Damping of a Mechanical System

Problem: A spring-mass-damper system has mass m = 1\,\text{kg}, damping coefficient c = 4\,\text{N·s/m}, and spring stiffness k = 4\,\text{N/m}. Classify the damping and find the characteristic roots.

Given: m = 1, c = 4, k = 4

Find: Damping classification and characteristic roots

Solution:

Step 1: Write the governing ODE: \ddot{x} + 4\dot{x} + 4x = 0.

Step 2: Check the discriminant: c^2 - 4mk = 16 - 4(1)(4) = 16 - 16 = 0.

Step 3: Since the discriminant equals zero, the system is critically damped. The characteristic polynomial (s^2 + 4s + 4) = (s+2)^2 gives a repeated root.

✓ ANSWER: Critically damped. Characteristic root: s = -2 (repeated).

Examiner note: Critically damped is the boundary case between overdamped and underdamped. It produces the fastest return to equilibrium without oscillation, which is why it is the design target for door dampers, vehicle suspension systems, and many control systems.


Problem 6. Solve an Underdamped Mechanical System IVP

Problem: A spring-mass-damper system has m = 1\,\text{kg}, c = 2\,\text{N·s/m}, k = 5\,\text{N/m}. The mass is displaced x(0) = 1\,\text{m} with \dot{x}(0) = 0. No external force is applied. Find x(t).

Given: m=1, c=2, k=5, x(0)=1, \dot{x}(0)=0, F(t)=0

Find: x(t)

Solution:

Step 1: Check discriminant: c^2 - 4mk = 4 - 20 = -16 < 0. Underdamped — complex roots.

Step 2: Write and transform the ODE \ddot{x} + 2\dot{x} + 5x = 0.

    \[[s^2X - s(1) - 0] + 2[sX - 1] + 5X = 0\]

    \[(s^2+2s+5)X = s+2\]

Step 3: Solve for X(s) and complete the square in the denominator.

    \[X(s) = \dfrac{s+2}{(s+1)^2+4}\]

Step 4: Split the numerator to match shifted cosine and sine pairs.

    \[X(s) = \dfrac{(s+1)+1}{(s+1)^2+4} = \dfrac{s+1}{(s+1)^2+4} + \dfrac{1}{2}\cdot\dfrac{2}{(s+1)^2+4}\]

Step 5: Invert using the first shifting theorem in reverse (a = -1, k = 2).

✓ ANSWER: x(t) = e^{-t}\cos(2t) + \dfrac{1}{2}e^{-t}\sin(2t)\,\text{m}

Examiner note: The e^{-t} envelope decays exponentially while the \cos(2t) and \sin(2t) oscillate at the damped natural frequency \omega_d = 2\,\text{rad/s}. This decaying oscillation is the defining signature of an underdamped system.


Problem 7. Find the Transfer Function of a Mass-Spring-Damper System

Problem: A mass-spring-damper system has m = 1\,\text{kg}, c = 3\,\text{N·s/m}, k = 2\,\text{N/m}. Find the transfer function G(s) = X(s)/F(s) assuming zero initial conditions.

Given: m=1, c=3, k=2, zero initial conditions

Find: G(s) = X(s)/F(s)

Solution:

Step 1: Write the governing ODE: \ddot{x} + 3\dot{x} + 2x = F(t).

Step 2: Transform with zero initial conditions.

    \[(s^2 + 3s + 2)X(s) = F(s)\]

Step 3: Divide both sides by F(s) to get the transfer function.

    \[G(s) = \dfrac{X(s)}{F(s)} = \dfrac{1}{s^2+3s+2}\]

✓ ANSWER: G(s) = \dfrac{1}{s^2+3s+2} = \dfrac{1}{(s+1)(s+2)}

Examiner note: The transfer function is always evaluated at zero initial conditions. If initial conditions are not zero, the transfer function alone cannot describe the full response — you need the complete IVP solution from Part 3 instead.


Problem 8. Find the System Response to a Ramp Input Using a Transfer Function

Problem: A system has transfer function G(s) = \dfrac{1}{s+2}. The input is x(t) = t (a ramp). Find the output y(t) assuming zero initial conditions.

Given: G(s) = \dfrac{1}{s+2}, input x(t) = t

Find: y(t)

Solution:

Step 1: Find X(s) = \mathcal{L}\{t\} = \dfrac{1}{s^2}.

Step 2: Multiply in the s-domain.

    \[Y(s) = G(s) \cdot X(s) = \dfrac{1}{s+2} \cdot \dfrac{1}{s^2} = \dfrac{1}{s^2(s+2)}\]

Step 3: Decompose — repeated linear s^2 and simple linear (s+2).

    \[\dfrac{1}{s^2(s+2)} = \dfrac{A}{s} + \dfrac{B}{s^2} + \dfrac{C}{s+2}\]

    \[s=0: 1 = 2B \implies B = \dfrac{1}{2}\]

    \[s=-2: 1 = 4C \implies C = \dfrac{1}{4}\]

    \[s^2 \text{ coeff}: 0 = A + C \implies A = -\dfrac{1}{4}\]

Step 4: Invert each term.

✓ ANSWER: y(t) = -\dfrac{1}{4} + \dfrac{1}{2}t + \dfrac{1}{4}e^{-2t}

Examiner note: For a ramp input to a first-order system, the steady-state response eventually tracks the ramp with a constant error (the -1/4 term). The e^{-2t} transient decays and the output converges to t/2 - 1/4 as t \to \infty.


Problem 9. Find the Response of a Second-Order Control System to a Step Input

Problem: A control system has transfer function G(s) = \dfrac{4}{s^2+4s+4}. A unit step input is applied. Find the output y(t).

Given: G(s) = \dfrac{4}{s^2+4s+4}, x(t) = u(t)

Find: y(t)

Solution:

Step 1: Transform the unit step: X(s) = 1/s.

Step 2: Multiply in the s-domain.

    \[Y(s) = \dfrac{4}{s(s^2+4s+4)} = \dfrac{4}{s(s+2)^2}\]

Step 3: Decompose — simple factor s and repeated factor (s+2)^2.

    \[\dfrac{4}{s(s+2)^2} = \dfrac{A}{s} + \dfrac{B}{s+2} + \dfrac{C}{(s+2)^2}\]

    \[s=0: A = 1, \quad s=-2: C = -2\]

    \[s^2 \text{ coeff}: 0 = A + B \implies B = -1\]

Step 4: Invert all three terms.

    \[Y(s) = \dfrac{1}{s} - \dfrac{1}{s+2} - \dfrac{2}{(s+2)^2}\]

✓ ANSWER: y(t) = 1 - e^{-2t} - 2te^{-2t}

Examiner note: The repeated root at s = -2 means this is a critically damped second-order system. The te^{-2t} term is the signature of critical damping. As t \to \infty, the output settles to 1 — the steady-state equals the input magnitude, as expected for a unit step into a unity-gain system.


Problem 10. Full RLC Circuit Analysis With Underdamped Response (EE Board Style)

Problem: A series RLC circuit has R = 2\,\Omega, L = 1\,\text{H}, C = 0.2\,\text{F}. A constant EMF of e(t) = 5\,\text{V} is applied at t = 0 with q(0) = 0 and i(0) = \dot{q}(0) = 0. Find the charge q(t) and classify the circuit response.

Given: R = 2, L = 1, C = 0.2, e(t) = 5, q(0) = 0, \dot{q}(0) = 0

Find: q(t) and damping classification

Solution:

Step 1: Write KVL in terms of q: \ddot{q} + 2\dot{q} + 5q = 5.

Step 2: Check discriminant: R^2 - 4L/C = 4 - 4(1)(5) = 4 - 20 = -16 < 0. Underdamped.

Step 3: Transform with zero initial conditions.

    \[(s^2 + 2s + 5)Q(s) = \dfrac{5}{s}\]

Step 4: Solve for Q(s).

    \[Q(s) = \dfrac{5}{s(s^2+2s+5)}\]

Step 5: Complete the square: s^2+2s+5 = (s+1)^2+4. Decompose.

    \[\dfrac{5}{s((s+1)^2+4)} = \dfrac{A}{s} + \dfrac{Bs+C}{(s+1)^2+4}\]

    \[s=0: A = 1, \quad s^2 \text{ coeff}: 0 = A+B \implies B=-1, \quad s^1 \text{ coeff}: 0 = 2A+C \implies C=-2\]

Step 6: Rewrite and match to shifted pairs.

    \[Q(s) = \dfrac{1}{s} - \dfrac{s+2}{(s+1)^2+4} = \dfrac{1}{s} - \dfrac{(s+1)+1}{(s+1)^2+4}\]

    \[= \dfrac{1}{s} - \dfrac{s+1}{(s+1)^2+4} - \dfrac{1}{2}\cdot\dfrac{2}{(s+1)^2+4}\]

Step 7: Invert using the first shifting theorem (a = -1, k = 2).

✓ ANSWER: q(t) = 1 - e^{-t}\cos(2t) - \dfrac{1}{2}e^{-t}\sin(2t)\,\text{C} — Underdamped circuit response.

Examiner note: The steady-state charge q(\infty) = 1\,\text{C} checks out: q_{ss} = Ce = 0.2 \times 5 = 1\,\text{C}. When the answer gives a constant plus decaying oscillatory terms, always verify the steady state from the circuit parameters directly — it is a fast integrity check that catches sign errors before you finalize.


Common Mistakes and Examiner Traps

❌ Common Mistake ✅ Correct Approach
Writing KVL with current i(t) and carrying the capacitor integral through the transform Work in charge q(t) — KVL in q produces a standard second-order ODE with no integral term. Differentiate once at the end to recover i(t) = \dot{q}(t) if needed.
Using the transfer function formula when initial conditions are nonzero G(s) = Y(s)/X(s) only holds for zero initial conditions. If y(0) \neq 0 or y'(0) \neq 0, use the full IVP method from Part 3 — the transfer function alone cannot capture those effects.
Misclassifying damping by checking only one condition Use the discriminant c^2 - 4mk (or R^2 - 4L/C). Negative means underdamped, zero means critically damped, positive means overdamped. Check it before solving — it tells you what form the final answer will take.
Treating parallel impedances the same as series impedances Series impedances add: Z_{total} = Z_1 + Z_2. Parallel impedances use the reciprocal rule: 1/Z_{total} = 1/Z_1 + 1/Z_2. The s-domain rules are identical to DC resistor rules.
Forgetting to complete the square when the characteristic polynomial has complex roots If the discriminant is negative, the quadratic in the denominator must be put in (s+p)^2+q^2 form before inversion. Trying to invert (s^2+2s+5) directly, without completing the square, will not match any basic pair.
Not verifying the steady-state value from physical parameters For a constant input, the steady-state output can always be checked from the physical parameters. For the RLC circuit: q_{ss} = Ce. For the mechanical system: x_{ss} = F/k. This is a fast cross-check that catches sign errors before finalizing.

Board Exam Quick Tips

  1. RLC circuits and spring-mass-damper systems solve exactly the same way. The ODE structure is identical — L \leftrightarrow m, R \leftrightarrow c, 1/C \leftrightarrow k. Master the method once and you have both.
  2. Work in charge q(t) for RLC circuits, not current i(t). This eliminates the integral term in KVL and converts the circuit equation to a standard second-order ODE.
  3. Check the discriminant before solving. Negative discriminant means complex roots, complete the square, expect e^{pt}\sin and e^{pt}\cos in the answer. Positive discriminant means real distinct roots, expect exponentials only.
  4. A transfer function assumes zero initial conditions. If the problem gives nonzero initial conditions alongside a transfer function question, it is asking for the full response, not just the forced response — use the complete IVP method.
  5. Every applications problem is a Parts 1–4 problem in disguise. Strip the physical labels, identify the ODE, apply the four-step pattern from Part 3. The engineering context does not add any new math.

Frequently Asked Questions

Q1. How do I convert between circuit parameters and the standard second-order ODE form?

The series RLC KVL equation is L\ddot{q} + R\dot{q} + (1/C)q = v(t). Divide through by L to get the standard form \ddot{q} + (R/L)\dot{q} + (1/LC)q = v(t)/L. The natural frequency is \omega_n = 1/\sqrt{LC} and the damping ratio is \zeta = R/(2\sqrt{L/C}). These two parameters fully characterize the circuit response type.

Q2. What is the steady-state value of the response for a constant input?

For a constant input applied to a stable system, the steady-state value can be found by setting s = 0 in the transfer function: y_{ss} = G(0) \cdot x_{input}. For the RLC circuit with constant voltage V: q_{ss} = CV. For the mechanical system with constant force F: x_{ss} = F/k. These steady-state checks should always be used to verify the final time-domain answer.

Q3. Why does the critically damped case have te^{at} terms in the response?

Critically damped means the characteristic polynomial has a repeated root (s+p)^2. From Part 2, \mathcal{L}^{-1}\{1/(s+p)^2\} = te^{-pt}. The extra factor of t comes directly from the repeated root structure — it is not a coincidence but a mathematical consequence of the partial fraction decomposition for a squared factor.

Q4. Can I apply the Laplace method to circuits with multiple loops?

Yes. Apply KVL to each loop separately, transform each equation, then solve the resulting system of algebraic equations for the unknown currents I_1(s), I_2(s), and so on. This is Mesh Analysis in the s-domain, and it works exactly the same as DC mesh analysis but with impedances instead of resistances.

Q5. How is the transfer function used in feedback control systems?

In a feedback control loop, the closed-loop transfer function is G_{cl}(s) = G(s)/(1 + G(s)H(s)), where G(s) is the forward path and H(s) is the feedback path. The denominator 1 + G(s)H(s) = 0 is the characteristic equation — its roots determine stability and response type using exactly the same discriminant analysis covered in this post.


Series Complete — What You Have Covered

This completes the PinoyBIX Laplace Transform Board Exam Reviewer Series. Across five parts and 50 fully worked problems, you have covered every technique that appears in Laplace Transform questions on the EE, ECE, and ME boards:

  • Part 1 — The definition, seven basic transform pairs, linearity, first shifting theorem, and change of scale
  • Part 2 — Inverse transforms and all three partial fraction decomposition cases
  • Part 3 — Derivative and integration transforms, the second shifting theorem, and the four-step IVP method
  • Part 4 — Dirac delta impulse functions, periodic waveform transforms, and the convolution theorem
  • Part 5 — RLC circuit analysis, spring-mass-damper systems, and control system transfer functions

→ Visit the Laplace Transform Series — Complete Solutions Post (50 Problems)

→ Back to the Laplace Transform Series Index


Published by PinoyBIX.org — Engineering Education for Every Filipino Student. Electronics · Mathematics · Board Exam Review · Free for Everyone.

Please do Subscribe on YouTube!

P inoyBIX educates thousands of reviewers and students a day in preparation for their board examinations. Also provides professionals with materials for their lectures and practice exams. Help me go forward with the same spirit.

“Will you subscribe today via YOUTUBE?”

Subscribe
What You Also Get: FREE ACCESS & DOWNLOAD via GDRIVE

TIRED OF ADS?

  • Become Premium Member and experienced complete ads-free content browsing.
  • Full Content Access to Premium Solutions Exclusive for Premium members
  • Access to PINOYBIX FREEBIES folder
  • Download Reviewers and Learning Materials Free
  • Download Content: You can see download/print button at the bottom of each post.

PINOYBIX FREEBIES FOR PREMIUM MEMBERSHIP:

  • CIVIL ENGINEERING REVIEWER
  • CIVIL SERVICE EXAM REVIEWER
  • CRIMINOLOGY REVIEWER
  • ELECTRONICS ENGINEERING REVIEWER (ECE/ECT)
  • ELECTRICAL ENGINEERING & RME REVIEWER
  • FIRE OFFICER EXAMINATION REVIEWER
  • LET REVIEWER
  • MASTER PLUMBER REVIEWER
  • MECHANICAL ENGINEERING REVIEWER
  • NAPOLCOM REVIEWER
  • Additional upload reviewers and learning materials are also FREE

FOR A LIMITED TIME

If you subscribe for PREMIUM today!

You will receive an additional 1 month of Premium Membership FREE.

For Bronze Membership an additional 2 months of Premium Membership FREE.

For Silver Membership an additional 3 months of Premium Membership FREE.

For Gold Membership an additional 5 months of Premium Membership FREE.

Join the PinoyBIX community.

DaysHoursMinSec
This offer has expired!

Add Comment

THE ULTIMATE ONLINE REVIEW HUB: PINOYBIX . © 2014-2026 All Rights Reserved | DMCA.com Protection Status
This content is protected. Subscribe to Premium to unlock full access.