Background

The 2017-2018 VEX robotics game was about stacking cones on goals. Simple concept, hard to optimize. The game was two scoring objectives adding together: cone stacks (each cone worth 2 points) and mobile goal positioning (worth 5/10/20 points based on zone). Bonus points came from having the highest stack in each zone.

Off the bat, our team noticed that scoring was dominated by two factors:

1. Stack efficiency - how fast you could build towers

2. Goal movement - which zones to target and when

We focused on these two factors throughout our design process. While other teams built increasingly complex mechanisms, we focused on optimizing these two variables.

Lift Design

The first challenge was height. We needed a lift that could:

• Start under 18 inches

• Reach high enough for 10+ cone stacks

• Move fast enough to outpace other teams

• Stay stable while stacking

Our solution was a combination of 3 linkages:

1. Main six-bar for raw height

2. A four-bar on top of the six bar facing in the opposite direction

3. A pneumatically actuated four bar on top to grab and stack cones

This wasn't particularly novel - most competitive teams used similar setups. The main leap came when we asked: "What's actually slowing down stacking speed?"

Gripper Design

Watching match videos, we mapped out the standard stacking sequence of a motorized gripper:

1. Pick up cone

2. Lift over stack height

3. Move forward

4. Open gripper to drop

5. Wait for stack to stabilize

6. Back away from stack

7. Lower and repeat

A full cycle took around 3-4 seconds. However, about half of it was spent waiting around. Teams had to:

• Wait for their powered gripper to release

• Wait again for the cone to settle

• Clear the stack before moving

This seemed like an opportunity. If we could eliminate the "hover and wait" time, we'd dramatically cut our cycle time. The solution came from a basic physics observation: cones naturally nest when dropped vertically but slide off when pushed horizontally.

We prototyped a passive gripper using notched plexiglass (taking advantage of VEX's allowance for 0.070" plastic). Through trial and error, we tuned three key features:

1. Side walls deep enough to grip during movement

2. Front notch shape that would cleanly release onto stacks

3. Back notch angle that could reliably scoop up new cones

After lots of iterations, we got a design that could:

• Release cones by pushing forward

• Pick up new cones on the downward stroke

• Start moving to the next cone immediately

The cycle time improvement was dramatic. As an added benefit, our design also freed up a motor that could better stabilize our main lift.

Results & Failure Analysis

The design worked - we won our regional tournament and qualified qualified for the California State Championship and made the US Open for the first time in team history, reaching quarter finals in the latter. However, over multiple matches, our lift motors started wearing down.

• More lift motors = more speed

• More speed = more stress

• More stress = faster wear

• Faster wear = unstable stacking

Looking back, we should have:

1. Added additional support to our lift

2. Rotated high-load motors between matches