The Engineering Design Process, Metrics, and Reverse Engineering
Primary Learning Objectives:
Describe the six-step engineering design process in both theoretical framework and practical application.
Understand Key Performance Parameters (KPPs) and Technical Performance Measures (TPMs), and translate customer requirements into testable KPPs and TPMs.
Explain the concepts and methodologies of reverse engineering, including when and why engineers utilize it.
The Six-Step Engineering Design Process
Theoretical Framework (Oakes & Leone, Chapter 12): Engineering design is a structured, iterative process consisting of six fundamental steps executed sequentially, with feedback loops to earlier stages as testing dictates:
Define the Problem: Clearly articulate the core issue or need before jumping to solutions.
Imagine Solutions: Brainstorm a wide range of creative and potential solutions without immediate judgment.
Evaluate Options: Analyze options against constraints, cost, effort, and feasibility to identify the most promising approaches.
Create a Solution: Select and build/develop the optimal candidate design.
Test & Validate: Subject the created solution to empirical testing to check if it satisfies specifications.
Improve & Refine: Analyze test results to make adjustments. Step 6 closes the loop and distinguishes methodical engineering design from lucky guessing.
Iterative Nature: Testing and refinement frequently reveal new constraints or flaws, sending the design back to earlier stages in the process.
Engineering Design in Practice: The Bicycle Example
Scenario: Application of the six-step process to a real-world scenario where an old bicycle makes an abnormal noise.
Step 1: Define the Problem: The old bike makes a grinding noise specifically when pedaling.
Step 2: Imagine Solutions: Consider potential root causes and fixes:
Lubricate the chain.
Replace the chain.
Replace the complete drivetrain.
Step 3: Evaluate Options: Weigh the cost, required physical effort, and the mathematical likelihood of successfully eliminating the root cause.
Step 4: Create a Solution: Select replacing the chain as it represents the most likely root cause with the lowest associated cost.
Step 5: Test & Validate: Install the new chain, ride the bicycle, and evaluate if the grinding noise has been eliminated.
Step 6: Improve & Refine: If the grinding noise persists, inspect the cassette for wear, closing the loop and continuing iteration.
Scoping Engineering Solutions
Importance of Proper Scoping: Matching the scope of a solution precisely to the defined problem is essential to avoid wasting financial resources, time, and stakeholder trust.
Under-Scoped Solutions:
Problem: Cracked bicycle frame.
Fix: Apply oil to the chain.
Implication: The proposed fix fails to address the underlying structural problem. The bicycle remains fundamentally unsafe.
Right Scope Solutions:
Problem: Worn bicycle chain.
Fix: Replace the chain.
Implication: The fix directly targets the root cause of the failure. The problem is fully solved.
Over-Scoped Solutions:
Problem: Squeaky bicycle chain.
Fix: Purchase a brand-new bicycle.
Implication: The initial issue was minor and localized, making the solution excessively expensive, wasteful, and unnecessary.
Customer Requirements and Voice of the Customer
Definition of Customer Requirements: Plain-language statements describing what the user or client wants the final product to achieve.
Methods of Gathering Requirements: Engineers collect data through interviews, surveys, field observations, and focus groups prior to starting design work.
Voice of Customer (VoC): The principle of capturing the fundamental underlying need behind a customer request, rather than taking their literal wording at face value.
Example: When a customer states, "I want a wheelchair that fits in my car trunk," the underlying engineering requirement extracted by the engineer is portability.
Metrics Framework: KPPs and TPMs
First Translation — Key Performance Parameter (KPP):
Definition: The translation of raw customer feedback into a non-negotiable, high-level functional goal expressed in customer-centric language.
Nature: A KPP represents the "line in the sand." If a design fails a KPP, the product is considered a failure overall by the customer, regardless of its other features.
Core Question: "What must be true for this design to count as a success?"
Example: Translating "Easy to travel with" into the KPP of Portability.
Second Translation — Technical Performance Measure (TPM):
Definition: The conversion of a high-level KPP into concrete engineering units that can be quantified, tested, and validated.
Structure: A TPM contains explicit numerical values, units of measure, and thresholds (the minimum acceptable performance level) or objectives (the ideal target performance level).
Core Question: "How will we know, with a specific measurement, that we achieved the target?"
Example: Translating the KPP of Portability into the TPMs of Mass \n\le 20\,\text{kg}\n and Folded Dimensions \n\le 80 \times 50 \times 30\,\text{cm}\n
Cross-Industry Metric Terminology
Different industries utilize unique terminology to express the core concepts behind KPPs and TPMs:
General Engineering:
High-Level Customer Metric: KPP (Key Performance Parameter)
Low-Level Engineering Metric: TPM (Technical Performance Measure)
Business Sector:
High-Level Customer Metric: KPI (Key Performance Indicator)
Low-Level Engineering Metric: Performance Target (a specific, measurable goal)
Technology & Startups:
High-Level Customer Metric: OKR (Objectives & Key Results)
Low-Level Engineering Metric: Key Result (the specific number proving objective achievement)
Manufacturing Industry:
High-Level Customer Metric: CTQ (Critical to Quality)
Low-Level Engineering Metric: Spec Limit (upper and lower dimensional bounds)
The Engineering Design Chain
The Seven-Step Execution Chain: To deliver a functional product, engineering teams execute a continuous 7-step translation chain from concept to validation:
Customer expresses a requirement in human language.
Identify the implicit KPP within the statement.
Define a quantifiable TPM to make the requirement testable.
Develop a design specifically to meet the defined TPM.
Build the prototype or solution.
Measure actual performance metrics under test conditions.
Establish a clear, unambiguous Pass or Fail outcome.
Significance of Step 7: Step 7 completes the chain; without an explicit, measured pass/fail determination, the engineering design chain is incomplete.
Four-Tier Structural Progression: \n\text{Customer Need} \longrightarrow \text{Key Performance Parameter (KPP)} \longrightarrow \text{Technical Performance Measure (TPM)} \longrightarrow \text{Design Requirement}\n
Customer Need: Plain-language description of user desires.
Key Performance Parameter (KPP): Customer-level definition of success.
Technical Performance Measure (TPM): Quantitative engineering metric tracking progress toward the KPP.
Design Requirement: Specific physical or system constraint necessary to satisfy the TPM.
Worked Example: Wheelchair Engineering Design
Customer Statements Collected:
"It must be easy to fold and fit into a car trunk."
"I need it to last a full day without recharging."
"It should handle small curbs and uneven sidewalks."
"The controls must work with limited hand strength."
Extracted KPPs:
Portability (foldable, compact, light enough for an individual to lift)
Battery Life (minimum continuous operation)
Maneuverability (ability to negotiate obstacles and tight turning radii)
Ease of Control (low-effort actuation interface)
Complete Design Chain Matrix:
Row 1:
Customer Need: Easy to fold & fit in trunk
KPP: Portability
TPM: Folded dimensions \n\le 80 \times 50 \times 30\,\text{cm}\n
Design Requirement: Hinged frame equipped with quick-release latches
Row 2:
Customer Need: Lightweight to lift
KPP: Portability
TPM: Total weight \n\le 20\,\text{kg}\n
Design Requirement: Frame constructed from aerospace aluminum combined with a modular battery pack
Row 3:
Customer Need: Last a full day
KPP: Battery Life
TPM: Energy storage capacity \n\ge 400\,\text{Wh}\n
Design Requirement: High-density Lithium-ion battery system managed by an integrated power management circuit
Row 4:
Customer Need: Handle uneven sidewalks
KPP: Maneuverability
TPM: Obstacle climbing threshold \n\ge 5\,\text{cm}\n curb height
Design Requirement: Dual electric motors coupled with of suspension travel
Reverse Engineering
Core Definition: The process of disassembling and analyzing a pre-existing finished product or system to deduce its underlying design choices, manufacturing constraints, and operational logic.
Three-Step Reverse Engineering Procedure:
Step 1 — Extract: Disassemble the physical hardware, make precise observations, and execute measurements to determine component relationships and interconnections.
Step 2 — Model: Construct an explicit theoretical representation of the system (such as a 3D CAD assembly, an electrical schematic, or a functional block diagram).
Step 3 — Review: Document all technical findings, detailing identified design choices, trade-offs, material selections, and physical constraints.
Primary Applications in Engineering:
Learning and benchmarking against existing commercial solutions.
Ensuring cross-system interoperability and physical/electrical compatibility.
Repairing, maintaining, or upgrading legacy hardware systems lacking original technical documentation.