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ARX Hand Project MK0S

Servo variant of the ARX MK0

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ARX Hand Project mainly started as an interest in creating a robotic hand using 3D printing to see what I could be achieve with just using my RepRap Huxley 3D printer. From being inspired by the e-NABLE community, I had a desire to create a capable and yet low cost robotic hand using 3D printing.

Currently, I've made numerous developments over the years with many interesting techniques and methods I had had to use to within the project, so worth looking through my project logs as it may help with other projects as well.

ARX Hand Project currently has 3 branch designs based on the MK0 hand, so I've split the branches into individual pages for clarity between design specific developments.

The project is still a working in progress so some things may not yet be documented or finished. I've yet to release the design, as there are quite a number of things that I would like to fix before making it publicly available.


ARX MK0S

This is the servo variant of the ARX Hand Project which focuses on being an easy to develop robotic hand using common RC servos that can be easily sourced and controlled. 
 

ARX MK0 Variants 

  • MK0M - https://hackaday.io/project/169585
    An advance fully mechanically driven design with wippletree actuated fingers and 3 or 2 times mechanical advantage pulley system
  • MK0F - https://hackaday.io/project/173565
    Figure variant which is non motorised and static design. Mainly focused for artistic uses such as in stop motion, display purposes, sketching, scale modelling, etc.

Design Intentions

This design is more of a fun low cost design I made that may be used in STEM education for inspiring future engineers and makers. I may work further on the design for other possible purposes.

Design Aims

  • Uses commonly available servos
  • Can be printed in with only PLA or PETG
  • Easy to print - No supports needed nor any fancy materials to be printed
  • Easy to assemble - Roughly 15-30 mins to assemble without the need for drilling, tapping or part cleanup
  • Materials are easy to source - Uses common screws, fishing line, elastic cord and 3D printer filament
  • Maintain good balance of functionality, ease of assembly and aesthetics

Design Overview

The design brings 5 degrees of actuation using servos along with 11 degrees of freedom in joints. It uses 5 micro/mini RC servos which can be controlled using an Arduino, a PWM driver or any servo driver. The use of servos brings the ability to have the fingers actuated to a set position through standard servo control. The servos can also be modified if there's a need for greater control and feedback of the fingers. Lifting and holding capabilities are dependent on the servos used, so ideally a high torque servos should be used if possible. Currently, the design will be compatible with the SG90, MG90S or any similar sized servos. Hand parts can be printed using any printer capable of printing PLA, and can be printed with or without supports. Additional materials required are some self tapping screws, 3mm nylon filament, 1mm diameter elastic cord and 0.5mm to 0.8mm diameter braided fishing line.

Hand Design

  • V3.4 Finger Design

    V3.4 builds on top of the initial design for V3 finger design. The aesthetic design remains the same, but many optimizations have been made to the design to improve overall printabilty and assembly.

    Subtle features have been added within the model to reduce effects of 3D printing imperfections from impacting the fitting of joints. Rounding to edges have been added to reduce overshooting artefacts on surfaces from ghosting, ringing and nozzle pressure build up. Seam adjustments have been added to avoid seams being placed between joint surfaces. Clearances on the joints are still fairly lenient to allow for easier printing, but clearances may be reduced down to 0.1mm if required.

    For a two joint finger, fingers require the use of 1.75mm and 3mm nylon filament to act as pins between joints. Nylon should ideally be used as it provides a low friction, low wear joint. The flexibility of nylon also allows for a compliant spring effect of joints, as well as to aid with friction fit joints through deformation of the material. 

    For closing actuation, fingers are actuated through a single cord of 0.5mm to 0.8mm diameter braided fishing line attached using a screw. For opening actuation, fingers are opened using an elastic material attached on the back side. The method of attachment of the elastic material allows for a number of options that available or easily accessible. Elastic cords can be used through tying knots at ends, and elastics can be printed or made through casting/injection moulding. Detachable cords also allow for easy replacement or disassembly of fingers. Currently, 1mm...

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  • Second design

    Supercell08/25/2020 at 15:20 0 comments

    [To be added]

  • Servo Motors Options

    Supercell07/10/2020 at 15:56 0 comments

    For now, I'm aiming for the design to use the more common SG90 or MG90S servo motors. These servos are fairly popular for their small size and affordability, and can be purchased for a few dollars each. Unfortunately, with the popularity of these servos, many manufactures create similar looking servo motors under the same naming but with subtle differences in how they're made. Some SG90/MG90S servos have different casing, horn height, horn dimensions, potentiometer construction and some even used a digital control system instead of an analogue system. Adapting the design to suite all different variations would be quite difficult but they still at least maintain roughly the similar dimensions of around 12x23mm.

    For servo case gear height from mounting, the variation between same models can vary quite drastically between vendors (or even same vendors). Horn height also has slight differences which adds to things to work around. 

  • temp

    Supercell04/30/2020 at 18:25 0 comments

    temp

  • Temp

    Supercell02/03/2020 at 01:02 0 comments

    Temp

  • First Servo Based Prototype

    Supercell01/23/2020 at 11:43 0 comments

    First experimental servo prototype derived from the ARK MK0 prototype. This prototype used 5 SG90 servos that I had purchased a few years back that I never got around to using. The servos aren't of great quality, so I would recommend using modern MG90S servos which are more durable with the metal gears and has a digital system. Overall initial impressions weren't that great with the use of the SG90 servos due to lack of torque and feedback, however, it does show a promising development for an educational/development tool. I will be developing some methods of improving feedback along with using some slightly more powerful servos in the next version.

    Features:
    - Servo based design
    - Easy control with servos
    - 5 actuation motors
    - Low cost at around $30
    - Added experimental geometry features to help with assembly

    Specs:
    - 5 SG90 servos
    - Used a PCA9685 PWM driver board for easier control through an Arduino.

    Further Work:
    - Adjustments needed for screw locations for swap able fingers
    - Subtle changes needed for cord location and overall adaptability for different servos
    - Possible additions of new elastic cord system
    - Better cable management for servo wires
    - Use of higher torque servos
    - Use of feedback for servo position and load

View all 5 project logs

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