Catapult

Our mechanical design for the thrower is a spring-loaded torsion catapult. The arm is attached to a live axle via a 40-tooth L-series timing belt pulley, which is press fit on the axle. This axle also has several large torsion springs, the quantity of which we will adjust as-needed, to provide the spring tension force. The live axle is captured by a pair of 1/2" ID pillow block bearings. The 40T timing-belt pulley is driven by a smaller 10T timing-belt pulley, which connects to the window motor via an electronic clutch.

Design Rationale: The catapult was chosen for its mechanical simplicity. The catapult was also chosen because it would throw objects in a highly repeatable manner.

Clutch

The clutch mechanism was switched from an electromagnetic clutch to a mechanical-only dog clutch on a custom-made 1/2" steel shaft. The mechanical dog, between the red bearing block and the black belt, rides on the hex portion of the clutch shaft. It is pinned through the clutch shaft, which connects it to the shifter block on the right of the black pulley. By moving the shifter block, the dog can be engaged or disengaged from driving the black pulley. When disengaged, the black pulley spins freely on the shaft and is captured by an e-clip. Two bearings capture the shaft and resist the forces from belt tension. The two bearings are now in a sturdier unified holder, as the two individual holders had significant deformation under belt tension. This caused the belt to slip and the catapult to launch prematurely. Added a tensioner (see week 6 notes for design iterations) to counteract slipping.

When the clutch is released, the torsion springs will attempt to return to rest, and thus throw the arm without requiring the window motor to be backdriven. As the window motor used a worm drive, backdriving it is impossible.

Design Rationale: A clutch was needed to separate power transmission from the throwing arm, allowing the stored energy in the spring to be transfered to the bean bag.

Chassis

The chassis is built on iRobot Create 2. This model provides motors, sensors, and battery, and can be easily controlled through an Arduino.

Design Rationale: Using an off-the-shelf base means that we simply need to interface to an already working system, rather than build a robot base for ourselves. The interface is well-documented and easy to use.

Reloading

The reload mechanism will be mounted to two 80-20 aluminum posts on the sides of the catapult. Each will hold a wood cup, which will be above the launch arm when it is fully pulled back. The wood cups will be attached to a servo, which will rotate when that bean bag is needed.

Design Rationale: The loading mechanism was chosen for its mechanical simplicity, ease of manufacturing, and repeatability.

Controller: Arduino Mega

• Connected to RC receiver (Rx) via PPM (single-wire RC-style pulses in serial)
• Connected to iRobot Create 2 via serial (iRobot Create open interface)

Design Rationale: Arduino Mega retains the ease of use of the smaller Arduino Uno as a simple microcontroller, but has four hardware serial ports instead of one. As we need three fast serial ports in our design (iRobot Create 2, RC receiver, connection to computer for programming / debug), this was a necessary upgrade.

Battery: 14.4V NiMH Roomba battery in custom holder

Design Rationale: Using the same battery as the Roomba for the non-Roomba pieces is a boon in that we can both use the Roomba to charge the battery and we only have to have a single battery type.

Sensors

• Battery-powered wifi web camera: Apple iPhone 6

Design Rationale: Completely self-contained and capable of video calling to a laptop.

Motors/servos:

• (2x) high-torque RC servos for beanbag loading mechanism
• DC motor with gearbox
• Linear actuator for engaging/disengaging clutch

Arduino Mega (C derivative)

Design Rationale: Simplicity of coding and programming combined with existing libraries for controlling the iRobot Create and interpreting the RC PWM signals meant that code was relatively turn-key.

Interpret RF signals from COTS controller

• Able to control Roomba in real time - tested up to 200' away with no lag
• Switch to change between drive and throw controls

Movement:

• Utilize base code from this tutorial
• Convert RC signals (1.0-2.0us pulses) from joysticks into -255/255 digital
• Send -255/255 as raw motor commands to iRobot Create over serial
• Control Roomba with two joysticks (tank style)

“Throw” command:

• Start with main cup loaded
• Left joystick controls winching down arm
• Release clutch with right joystick to throw loaded beanbag
• Winch down motor until at desired position
• Control loading order (load beanbag 2 or beanbag 3)

Windows laptop:

• View wifi webcam for teleoperation