Choose a Mechanism You Can Defend

Mechanism design is a tradeoff problem disguised as a building problem.

A team can make almost anything move on a workbench. The competition question is whether it can perform the required task repeatedly, legally, safely, within weight and packaging limits, and without consuming the entire build season.

Start from the task

Write the task as a measurable requirement.

Weak:

Build a fast intake.

Stronger:

Acquire the game piece from the floor, center it into the robot, and confirm possession without jamming during repeated cycles.

Now the mechanism has jobs that can be tested.

Compare architectures

Suppose three ideas can acquire the same game piece:

• compliant rollers;
• rigid wheels with a funnel;
• a pivoting arm with rollers.

Create a tradeoff matrix.

Use a simple 1-5 score only after you define what the numbers mean. Otherwise the matrix becomes decorative arithmetic.

Prototype the riskiest assumption

Do not prototype the entire robot if the uncertain question is whether a roller material grips the game piece.

Build or simulate the smallest test that can answer the question.

Examples:

• Does the roller grip under compression?
• Can the elevator lift the expected load?
• Does the arm clear the frame perimeter in the intended motion?
• Does the mechanism recover after a jam?
• Does the sensor still detect the piece at speed?

The point of a prototype is evidence, not appearance.

Design review example

Claim:

We should use the 6:1 ratio because it is faster.

Review response:

• What load was tested?
• What current behavior was observed?
• What is the required cycle time?
• What happens when battery voltage is lower late in a match?
• Can the mechanism stop accurately?
• What breaks first if it jams?

A design survives review when it has answers better than “it worked once.”

Build your mechanism decision sheet

Include:

1. measurable task;
2. at least two candidate mechanisms;
3. a tradeoff matrix;
4. one gearing or speed calculation;
5. the riskiest assumption;
6. a bounded prototype or simulation test;
7. observed result;
8. one design change based on evidence.

If you have no hardware, use diagrams, CAD screenshots, motion sketches, motor data, or a small simulation. The reasoning standard does not change.

Reliability is part of design

A mechanism that scores one extra game piece but requires constant repair may reduce total match performance.

Access to fasteners matters. Cable routing matters. Sensor mounting matters. Replaceability matters.

These details are not boring extras added after the “real engineering.” They are the real engineering once the prototype has to survive competition.

Keep the decision sheet. In Unit 9, you will use it as evidence during the final design review.