kwatters's blog

Reflections on the resistance of power supply lines to a servo motor

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I recently saw some posts in the InMoov forum talking about how the supply lines are very important to being able to power the servos properly.  My initial reaction was that the resistance of the wires going to the servo motors should be negligable.  I mean, after all , it's wire.. it's copper.. it's a conductor.  When we think about circuits we always consider that the wires don't have any resistance at all.  They're ideal conductors, if you will.


The road to industrial robot control for hobbyests

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My new years resolution is to really get a good understanding of industrial robot arm control with the hopes to come up with some dynamic control to keep robots balanced and borg that into MRL.

I recently picked up a robot arm kit from OWI.  It's a 5 degree of freedom robotic arm.  It took about 2.5 hours to assemble.    

The OWI Robotic Arm Edge Kit

 


Python Robotics Toolkit

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Based on http://www.petercorke.com/Robotics_Toolbox.html

I've been working through getting a kinematic model for the InMoov, recently, I stumbled across a python library based on the matlab robotics toolkit.  It uses NumPy for much of the heavy lifting math operations.  

So far I've been able to create a very small script to compute forward kinematcis of a set of links using this python robotics toolkit


Python easy install on windows

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Here's a quick little page with some notes of how to install python and dependent modules.  This was done on python 2.7

 

1. download and install python 2.7.x from https://www.python.org/downloads/  

2. install python in the default directory  c:\Python27

3. download the ez_setup.py script from  https://pypi.python.org/pypi/setuptools   (save this file in c:\Python27 )

4. open up a command prompt 

5. type   cd %PYTHON_PATH%

6. type   "set PYTHON_PATH=c:\Python27"


3D printed leg from 3DPrinterOS

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Here's a fun little project and was open sourced at https://cloud1.3dprinteros.com

This leg is about 1/2 the size that the inmoov would need, but it's interesting as a start.  The parts were reasonably easy to print, though some did need to be printed with supports. 


Center-tap pot and the oscope

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I've tried connecting up the center tap of the servo pot to an analog input of the arduino and measured it with the oscope of MRL.  fun...

 

here's some images of the with and without the ground line connected.. i'll leave it up to you to see which is which.  (I was manually turning the pot myself.. watching what happened on the analog input pin 0 of the arduino that i connected up...)

 


D-H Parameters and the InMoov for a Kinematic Model

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Update:   I'm kinda geekin' out about this right now...  It looks like all those matrix multiplications do something useful...

 

 

Did a quick simulation with MRPT of what I believe is a resonable approximation of the InMoov arm.

 

 


Closing the Loop on Hobby Servos with MRL

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This is really a page to talk about how to measure the actual angle that a servo has reached, rather than the last position the servo was told to move to.  The aim is to be able to allow the servos to automatically configure their actual max/min angles.  In addition, it should allow for quicker response to the stop() method.

The stop() method stops the sweeper that is currently running for the servo.  However, this only tells it to stop the sweeper, the servo will continue to move to the last position it was told to go to.


Using OpenSCAD to edit STL files

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Here's a simple example of how to use OpenSCAD to edit a pre-existing STL file.  This is useful if you want to add or subtract areas to the existing file.  

OpenSCAD is an open source CAD program that allows users to programmatically build up models using basic geometries.  The below example is based off of the part 

Download OpenSCAD from http://www.openscad.org/   (It's free)

Download the STL file that you want to edit. For this example I used http://www.thingiverse.com/download:89523