Mechanical

Mechanical Overview

NIMBY utilizes a quad 35A durometer compliant wheel intake and ejector system and is timing belt driven with 12V DC motors. It is a three-wheel robot consisting of dual front wheel drive and a rear caster wheel. NIMBY also boasts a sloped ramp storage system that can store up to 5 game pieces and positions game pieces for smooth and rapid ejection. The chassis consists of two levels, allowing for mounting of mechanical systems and electrical boards. The intake ramp also serves as mounting area for three opto-reflectance sensors, an IR phototransistor, and an IR LED. Above the upper level, the superstructure contains the indicator servo, the beacon IR phototransistor, and both the power switch and the on/off button.

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Dimensions

Isometric view

Side view

Intake & Ejection System

NIMBY's utilizes a 35A durometer compliant wheel system for both intake and ejection of game pieces. This dual functionality simplifies the mechanical system and actuator requirements, as well as saving space to allow for more game piece storage. By spinning the compliant wheels different directions, we can activate either intake or ejection modes (demonstrated in the figures below). The system is capable of complete ejection of five game pieces in ~1 second, as well as complete tree intaking of five pucks in ~7 seconds, giving NIMBY a significant competitive advantage by allowing for enough match time for free puck search in the open field.

Intake Spinner Direction

Ejection Spinner Direction

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Open Field Intake

Able to intake pucks approaching from a big range of angles due to spinner nudging pucks into the center. Able to eject pucks at decent speed to get through the fence.

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Tree Intake

Belt driven quad intake allows for under the tree intake up the storage ramp. Video shows tree intake and ejection in ~10 seconds. Spinning in the ejection direction also nudges pucks out of the way to prevent jamming when approaching tree for intake.

Design Process for Intake System

Adjustable Jig for Spacing

Since spacing is essential to a compliant wheel intake system, we first started with an adjustable intake arm jig that allows us to adjust spacing with three degrees of freedom (X, Z, angle of intake w.r.t ground plane).

Testing and Iteration

We tested the puck intake with updated arms and set dimensions. The intake system works reasonably well with free pucks, but due to a steep angle w.r.t. the ground plane, pucks under the tree become jammed during intaking. After intaking, the puck blocks the view of the IR sensor behind the ramp since the puck did not slide up far enough. We came up with a timing belt driven system with another set of spinners to keep the puck at higher position after intaking and to mitigate the tree jamming issue.

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Testing with timing belt system

With new timing belt driven system, puck intaking from tree yielded vastly improved results. We found that since pucks fall from the tree on top of the spinners, it is better to implement a "jostling" routine to prevent jamming that consists of the drivetrain moving back and forth a set amount to consistently intake all the pucks in the tree.

Breakdown of Intake System

Note: Shaft coupler, shaft collar and wheel-shaft adapter are 3D printed high infill PLA with 2-56 heat set inserts to ensure parts can be properly fixed to D-shaft with set screws. Bronze bushings are embedded in the mounts, interfacing with the rotating d-shaft for lower friction instead of direct contact with PLA.

Puck and Tape Sensor Bracket

Embedded in the ramp (in black) are two brackets (in white) that serves as both IR LED-phototransistor sensor and optoreflectance sensor housing. The IR LED-phototransistor pair are used for puck sensing. When a puck is in front of the ramp, the emitted IR by the LED is reflected back and can be sensed by the phototransistor. The opto-reflectance sensors are used for tape following, with two in the middle and one on the right. The two middle opto-reflectance sensors are spaced a tape's width apart, allowing us to adjust back to the tape depending on which sensor loses sight of the tape. The right opto-reflectance sensor allows us to detect when we have hit a tape intersection, which is useful for stopping for trees and locating during free puck search.

Sensor Bracket

A Sensor bracket holds one IR phototransistor and one IR LED, which are placed in separate channels to prevent direct incidence of the IR output on the transistor. The tapered front of the sensor directs the IR signal into a narrow beam and also prevents stray IR signals into the photo-transistor. Two opto-reflectance sensors sit on top of the bracket behind the IR LED-sensor pair, with an opening below to allow the opto-reflectance sensor to peek through. It is attached by screwing the sensor onto heat set inserts in the bracket.

Sensor Bracket in Ramp Assembly

The sensor bracket snaps into the ramp slots. We ended up using 1 IR LED-phototransistor pair and 3 opto-reflectance sensors, so only 2 sensor brackets were needed. There are openings in the ramp that allow the IR LED and phototransistor pair to transmit and receive IR signals to and from pucks on the game field.

Sensor Placement

The sensor bracket is placed where the IR LED and photo-transistor can still transmit and receive IR signals even when storing puck. This feature increase puck search efficiency since NIMBY can find and intake multiple pucks from open-field puck search before ejecting them. The bracket also allows for optimal spacing between the opto-reflectance sensors and the floor to make tape following reliable. It is placed at the axis of rotation of the robot, which allows it to stay on the tape during rotation.

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Puck sensing demonstration

The video on the left shows the the intake motors activating when puck is placed in front of IR LED-phototransistor pair.

Storage Ramp and Extension

The storage ramp consists of a 3D printed ramp lip that contains integrated mounting of the aforementioned sensor brackets. Between the lower and upper chassis decks, the ramp lip is joined into a laser cut Duron ramp with side walls and bottom guide, with additional corner braces and a top wedge for game piece jamming prevention. At the rear of the chassis, the ramp extends on a hinge into a drawbridge-esque ramp extension, which is supported in tension by two pairs of strings. One pair mounts to the superstructure mounting points above, while the other pair mounts to the rear vertical steel shafts on the chassis. This ramp extension is leaned inwards a few degrees, and passively deploys at the start of the match using angular momentum. The entire ramp is covered in PTFE film tape to decrease friction against sliding game pieces.

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Ramp deployment

The video on the left shows the passive extension ramp deployment as the robot rotates at the start of the match for extra puck storage, as regulations require the SQRL to be 32x32 cm before match start and can extend to 48x48 cm during match time.

Chassis

The chassis consists of two main laser cut Duron decks spaced apart by vertical steel shafts. The height between the two decks is set by 3D printed hollow cylindrical spacers, while the top and bottom of the shafts are constrained by 3D printed shaft collars with 2-56 heat set inserts and set screws. The top deck mounts the majority of the electronics boards, including the power distribution board, two PIC32 boards, and four H-bridge boards. The bottom deck mounts the custom opto-reflectance sensor board, the puck IR phototransistor Universal Signal Conditioning board, and the two batteries with custom brackets.

Drivetrain

The drivetrain consists of two 12V DC motors mounted onto custom 3D printed motor brackets, which are mounted onto the lower deck of the chassis and extending out the side. Each motor shaft has a traction wheel (Polulu 3690 Multi-Hub Wheel) on the external end, while the back end of the motor output shaft has an quadrature magnetic encoder mounted with custom 3D printed bracket. In the center rear of the lower deck, there is a free-spinning caster wheel mounted below the chassis that acts as the third point of contact with the game field.

Superstructure

The superstructure consists of a beacon IR phototransistor with 3D printed directional shield, indicator servo, on/off button, power switch, and a mounting section for the beacon Universal Signal Conditioning board. Additionally, there are 2 mounting points for the drawbridge ramp extension's support strings as well as inset wire paths for the servo wires on one side of the main superstructure and the Universal Signal Conditioning board on the other side.

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