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Servo-Stock

A Kossel derivative to show off a new PID control framework and command system called the Bowler Communication System.

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This project was created on 04/29/2014 and last updated 4 months ago.

Description
We have developed a new system for high precision motor control. We have replaced stepper motors with DC motors and a new kind of encoder called a hall effect orientation encoder. The best part is we can get 4096 'tick' per revolution measurements and a controller that can drive the motor to within +-2 ticks.
Each axis is a small board holding the encoder chip, a 3 pin connector for the servo, and a ribbon cable carrying the SPI and servo signal, a small magnet and a continuous rotation servo.
The controller board has connectors for 8 axis of PID control, all of which can be coordinated in the kinematics engine. There are 3 heater/sensor pairs for up to 2 hot ends and a heated bed.
The control framework is a high speed protocol called Bowler. We can send packets and get a response back to Java in 4ms. Most of the complex features can now be handled at the high level. We also support a full configuration storage system, so the print configurations stay with the printer.
Details

So here we have the first set of parts for the ServoStock, a derivative of the Rostock using Closed-Loop control, a whole new com architecture, Servos for drive and NO STEPPERS. Pictured is the first attempt of the single axis using the Vitamins from a Printrbot.

Software/Communications:
The system we developed over 4 years and have now applied to a 3d printer is an extensible, namespaced communication protocol. Think of it like a domain specific language over a generic serialized communication link. The protocol allows devices to report to the computer their capabilities, then the computer can generate packets based on that report. We have complete implementations in Java and C99. Clojure, Jython, Jruby and Matlab have been tested working with this library. There is a partial implementation in pure Ruby as well.

Electronics:
We have redesigned the entire control system from the ground up. From the get-go we started using magnet orientation encoders plus low cost continuous servos. The motherboard is a Pic32mx440f128h with each axis receiving a servo pulse and an SPI channel for the encoder. The motherboard uses a 7x2 ribbon cable to connect to each axis. Each axis has a small board with the encoder chip and the connector for the servo.

Firmware:
The control loop runs at 20 ms for the servo pulse calculation, interpolation and forward/inverse kinematics. The device identifies itself as having the PID control namespace, the kinematics namespace, and the 3d printer configuration namespace. Through the communication layer you can change kinematics models, change PID constants and store all the Slic3r/Cura configurations. The configurations are stored non-volatile in Flash and travel with the printer to be read at runtime. NO MORE CONFIG FILES!!

The firmware also has auto-config system. The servo dead band is measured at boot time. The end stops are measured using the encoder with no limit switches. I am thinking about doing a bed calibration as well, but that is future work.

Bootloader:
a (new to you guys) bootloader that i developed for the DyIO controller. It is a serial bootloader that uses the same communication protocol as the printer or any other device for that matter. The bootloading interface is just another namespace that any device can implement. The toolchain for generating bootloader file is all written in Java and attaches to any Makefile build chain.

Hardware:
We have taken the idea of the Rostock and started from scratch using a full set of parametrics. A single file with measurements from the vitamins you are using is all that is needed to generate a whole new printer.

We have also made a library set for vitamins in OpenSCAD. This means we have special SCAD files that are only used to cut out voids for real vitamins. New ones can be added using the same model and committed back the the GIT. I have found that designing with this Vitamin Cutout system really speeds up development of our new printer. I can not wait to finish and try another design using that framework.

Project logs
  • Peek-A-Bot

    4 months ago • 1 comment

    This the Peek-a-Bot, he is installed on Front street in Worcester MA, across from the City Hall. Paula Rudy made the puppet and I used the ServoStock frame on its side as a puppet holder. Those rotated free arms are helping keep it stiff under a lot of load. I think, however, i will need the third upper arm...

    <iframe width="560" height="315" src="//www.youtube.com/embed/aIcjLfX-kZ0" frameborder="0" allowfullscreen></iframe>

  • I resolved the tilt from the video

    5 months ago • 0 comments

    In the video you can see the task space axis tilt to one side as it moves. This was caused by low rigidity in the vertical arms. I added stiffness to them and made the bottom thicker. I also shrunk the pulley for the linear axis to increase the achievable task space resolution.

    https://github.com/Technocopia/ServoStock-hw/commit/2baef74771851634ed8c7448e6b900df52d9fbd5

  • Extruder versioning and position control

    6 months ago • 0 comments

    The extruder had a few features the interfered with the free arms and needs the design tweak, printing will have to wait for that to be done. 

    In the mean time I found a long standing bug in how the centering value was calculated and applied the center calculation. It was being calculated as the middle of the 2 hysteresis bounds. This turned out to be ok, but it was being applied after the bounding values. This meant that the upper value was always high, and the lower was always low. I am now getting much faster settling times with lower I gains.

View all 4 project logs

Discussions

Jasmine wrote 3 months ago null point

Hello mad.hephaestus and Servo-Stock team, it looks like you've not updated the project for a while. Now is the time to add a few more details to your project to give it the best chance of going through to the next round of The Hackaday Prize.

By August 20th you must have the following on Hackaday Projects:
- A video. It should be less than 2 minutes long describing your project. Put it on YouTube (or Youku), and add a link to it on your project page. This is done by editing your project (edit link is at the top of your project page) and adding it as an "External Link"
- At least 4 Project Logs
- A system design document
- Links to code repositories, and remember to mention any licenses or permissions needed for your project. For example, if you are using software libraries you need to document that information.

You should also try to highlight how your project is 'Connected' and 'Open' in the details and video.

There are a couple of tutorial video's with more info here: http://hackaday.com/2014/07/26/4-minutes-to-entry/

Good luck!

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regiscruzbr wrote 4 months ago null point

Finally somebody did it...

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benroberts wrote 5 months ago null point

this is very impressive work. I particularly like the extra strut you have added to the effector platform. I know it's not part of the drive/control project per se but it addresses one of the issues I have with delta printers having too much play at the effector. Can you show us the DC motors you're using ?

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mad.hephaestus wrote 5 months ago null point

The DC motors we are using are a continuous rotation servo. It uses the standard servo package, but is shipped from the factory as a velocity servo rather than a position servo. They are also called "360 servo" in china.

The links to the specific ones are in the BOM.

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mbasecnc wrote 5 months ago null point

Are you willing to share schematics and BOM?

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mad.hephaestus wrote 5 months ago null point

The project links on the side have both. The schematics are in Eagle and the BOM is a google doc.

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ronnied wrote 5 months ago null point

Keep it up. Sometimes the only real way to progress is to knock down the foundations and start over.

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mbasecnc wrote 5 months ago null point

Nice work!
Could use your system for one of my 3D printers, are you willing to share more information?

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mad.hephaestus wrote 5 months ago null point

Sure, what do you need?

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Lightning Phil wrote 5 months ago null point

Great work

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jeshua wrote 5 months ago null point

Perfect!!! How how can I get a mobo and some servos?

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mad.hephaestus wrote 5 months ago null point

We are planning to have a Kickstarter up by August after we do robustness testing. I post here with updates, so keep an eye out!

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matt venn wrote 5 months ago null point

wow, great work everyone! Very impressed!

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