07 February 2016

2.77 Seek and Geek #1

Seek and Geek #1: Apple Lathe

I discovered this wonderful tool in my living group's kitchen. It's used to simultaneously peel and core an apple. 

The lathe can be attached to any smooth surface using a suction base. By turning the handle, a channel underneath the lathe pulls up and creates negative pressure that keeps it against the surface. 

Here, my girlfriend models the machine in action. The apple is held by three skewers at the end of a screw. The screw is actuated by a handle at the back end. By turning the screw, the apple both spins and moves forward, enabling the spring-loaded cutting tool to peel the apple, and the blade at the far left to remove the core. 

This spring-loaded tab disengages the screw and enables it to be pushed back without having to turn. 

By letting go of the tab, the screw engages and must turn in order to move linearly. 

St. Venant's principle in action! The screw is supported by two linear bearing blocks. The distance between them is equal to at least 3-5 screw diameters! No jamming occurs, the motion is smooth. 

The blade at the end that removes the core also allows the apple flesh (the desired output of this machine) to spiral out in one long piece. 

The cutting tool for peeling is normally spring-loaded against the apple to apply cutting force. In order to insert the apple, however, it must be positioned out of the way. There is a small tab near the bottom, with an adjustment screw, to block the cutting tool from coming down as you insert the apple. 

Pushing it aside once the apple is inserted brings the cutting tool to its ready position. 

The cutting begins!

The pitch of the screw matches the width of the "chips" coming off of the apple peeler. 

Such lovely chips! The apple begins to pass through the blade that will remove the core and remove the flesh in a spiral. 

Nearly done with the operation! The spring-loaded cutting tool compensates for a wide range of apple sizes and diameters by being constantly loaded onto the apple with a spring. Note that proper alignment of the apple on the skewers is vital for removing the core cleanly. This is the only failure mode I've seen, sometimes the apple will be pushed off of its skewers if sufficiently misaligned. The blades seem to be removable for easy replacement or cleaning. 

2.77 PUPS #1


(See below for peer-review version.)

Peer Reviewed Work

20 April 2014

MIT MechE deFlorez Competition Entry: Underactuated Robot Gripper

An Open-Source Low-Cost High-Strength Rapid-Prototypeable Underactuated Robot Gripper
by Daniel J. Gonzalez - dgonz@mit.edu
MIT MechE deFlorez Competition Entry
(Hi, Professor Hover!)

 Here is a solid model of my gripper. All of the components can be readily found on McMaster, Trossenrobotics, and other online retailers for under $500.00 total. The parts can be fabricated using only a waterjet, or using online waterjetting services such as bigbluesaw.com. With the solid model released as open-source, just about anyone can put this thing together for use in their robot manipulation project: no expert machining experience required!

The design of this gripper is inspired by the WillowGarage Velo Gripper (Work by Matei Ciocarlie): 

and the design optimization work of Prof. Aaron Dollar of Yale and Prof. Robert Howe of Harvard:

(link to paper: http://biorobotics.harvard.edu/pubs/2010/journal/Dollar_IJRR2010.pdf). 

The design of the drive mechanism is underactuated. Specifically, one motor drives both fingers, each of which has 3 DOFs. Passive compliance between each DOF allows the finger to envelop any given object that fits within its grasp without having to perform complicated grasp planning calculations in realtime. 

As of this writing, the fabrication process is nearly complete, and it should be 100% functional by the middle of next week (~4/23/2014). An Arduino will receive a grasp command from a host computer (or a button for demonstration) and command the servos to either grasp or let go of an object. The Dynamixel servo is torque-controlled and provides torque feedback, so we can apply the proper amount of pressure for a given object.
Here is an example demonstrating the underactuated closing behavior of the gripper that allows for asymmetric grasps. Using constraint tendons, the gripper acts as a standard parallel gripper until the proximal link encounters resistance. The two distal links then close around the object for a snug grasp. 
Here is a cutaway showing the direct-drive spool of the Dynamixel MX64 servo (which can output 7+Nm of torque), the cam lock mechanism actuated by an EMax digital hobby servo, and the main drive cable (Yellow). 

Here is a detailed view of the various active and passive tendons and other components within one of the fingers. Each finger is identical. 

Thank you for considering my project in the MIT MechE deFlorez Competition!