New Member - Want to rebuild an industrial robot

    Hi All,


    This is my first post on this forum, so I apologize if this is posted in the wrong spot. I graduated from college a few years ago having studied Mechanical and Electrical engineering with a focus in controls. I currently work for a large industrial automation company as a project engineer for motion control (servo) systems. I have too many hobbies already (I have a full machine shop in my basement), but I have wanted to purchase and rebuild an industrial 6 axis robot for some time now. My original intention was to look for a unit which came complete with a controller and teach pendant, however I caught on early that finding one of these used was either rare or prohibitively expensive.


    Not to be discouraged, I decided that with my background in motion control, PLC programming, and access to industrial servo motors/drives, I would be ok buying just the robot itself and interfacing to the existing motors or replacing them entirely. In doing so I realized that I would be writing the control software and would need to teach myself the kinematics. I purchased “Introduction to Robotics: Mechanics and Controls” by John Craig and began studying it along with any college course material I could find that followed along with the book. My goal was to learn the material well enough that I could derive the forward and inverse kinematic equations and program these into a PLC to control the robot.


    Unfortunately after a year or so of working at this, I got busy and lost the kick to keep working through the book. It felt like I was very much on my own and was struggling with a few of the concepts. I passed on a number of opportunities to purchase a robot like I was looking for in the <$1000 range, and kind of put the project away on the shelf.

    For whatever reason I got the itch to pick it up again now and see if I can’t actually make this happen. I don’t know why I never got involved with the forums before but I’m hoping someone can answer a few broad questions below to get me pointed in the right direction:


    1.     Is anyone aware of people trying to do anything along the lines of what I want to do? If so, where can I find information or examples?


    2.     Which forums would be good to get involved with? I’ll be wanting to discuss the math of robot kinematics, interfacing to existing hardware, and just the project in general. I really get motivated by sharing the projects that I do, so I think getting involved with an online community would be the best way to keep me on track.


    3.     My understanding of kinematics for these robots is that there is not guaranteed to be a closed-form solution to the forward/inverse kinematics of a serial robot, but in the special case of a robot with a spherical wrist, there are solutions possible. Is this correct or is it a fool’s errand to try to derive these?


    4.     During all my research, I found very few examples of someone truly working out all the kinematics (the only one I remember was a graduate thesis on the kinematics of a Universal Robots UR5). Does anyone know of an example where someone worked out the kinematics for a 6 axis serial robot with a spherical wrist?


    5.     Does anyone know a good place to look for buying a robot? There is an industrial surplus company not far from me at all that often has used robots come through. They are often in the $2000-7500 range, but sometimes, smaller units come in without controllers and sit until they sell for a few hundred. I’d prefer to find something in better shape than I see there, but I’m hoping to keep the actuator purchase around $1000.


    I feel confident that if I were to derive the forward and inverse kinematic equations AND I were to find a robot in good enough shape to be run, I have the knowledge and resources to interface to/replace the motors and write motion control code in a PLC to actually use it.


    Thanks for your time and thoughts!

    Good morning and welcome,


    I don't think that a PLC will be sufficient, there are many, many things, especially safety, that the robot controller does. Not only that, but also most, if not all, industrial robots are running a Real Time Operating System (RTOS). ABB, Kuka and Fanuc I think use VXWorks to program their operating systems. A VXWorks license costs about $250,000 USD. Rather than go through the pain involved in reinventing the wheel, consider the many hours you would spend times what you value your time to be worth. A few thousand for a used robot should compare favorably. Not to diminish your knowledge or ability, just some food for thought.

    Lemster68,


    Thanks for your reply! I guess I should clarify that this is purely for hobby/educational/home use only. I have not been able to find a small, used, functional robot for less than $10k. I also would find value in being forced to learn the depths of robotic control. If I could find an affordable option, I would definitely get it though.


    If I end up going down the "build your own" control option, the PLC I have available to use would be capable of real time motion control well beyond the requirements of a single robot and should be able to handle the calculations required to operate the robot (although I would need to write these myself). The servo equipment I have access to would allow me to implement a proper Safe Torque Off (STO) safety system. I may not be able to do some of the fancier safety functions found in collaborative robots. This should be enough to get me a robot that moves and stops with the press of an ESTOP.


    I have gone down this path long enough to know that this feels like a somewhat insurmountable task, but I find the topic extremely interesting and I am still interesting in exploring if this can be done.

    Ok, so if you want to pursue this path, it would be useful to know where you live, country, state or province. For example, I know that somewhere near me, upstate South Carolina, there is a used, full size industrial robot (ABB) that would cost $3k USD.

    I live in Cleveland, Ohio and I am not a stranger to road trips.


    I have a basement shop that I am comfortable moving equipment up to 1200 lbs into (although my target size for a robot would be much smaller). I have 240V single phase power available but would be fine to run a phase converter to get 208V 3P. I don't have access to 480 and would rather not run a RPC + transformer.


    Thanks!

    I'll keep an eye out on there!


    Do you (or anyone) have thoughts on some of the questions I posted above? Some won't be applicable if I get a fully operational unit, but I am curious on a theoretical level regardless.


    Thanks!

    I don't sorry. I goofed off too much in my High school math and had none in college. I have a degree in Film and Video production, btw. This would be the right place to ask about kinematics, but some parts of you inquiry might be addressed to the general category. Also, you could look into ROS, that is a project for controlling industrial robots. Other than that, some of the other users might pick up on this thread.

    Bit late on replying to this, hope this helps.

    Quote from mcardoso

    1. Is anyone aware of people trying to do anything along the lines of what I want to do? If so, where can I find information or examples?

    Chris Annin made a arm using stepper motors:

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    Link to his GitHub on the arm:

    https://github.com/Chris-Annin/AR2


    Kris Temmerman also build his own arm, but is a bit lighter on the details.

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    He mentions that the source code for his arm is on his website, but I am unable to find it.


    Quote from mcardoso

    2. Which forums would be good to get involved with? I’ll be wanting to discuss the math of robot kinematics, interfacing to existing hardware, and just the project in general. I really get motivated by sharing the projects that I do, so I think getting involved with an online community would be the best way to keep me on track.

    This one is pretty good (maybe some bias there). There is also the robotic stack exchange.

    Quote from mcardoso

    3. My understanding of kinematics for these robots is that there is not guaranteed to be a closed-form solution to the forward/inverse kinematics of a serial robot, but in the special case of a robot with a spherical wrist, there are solutions possible. Is this correct or is it a fool’s errand to try to derive these?

    Yes, this is correct. The wrist center point plays a key role which allows you to separate the problem into two easier problems. With the XYZ position of the wrist center point known, you can solve for J1, 2, and 3, and with the rotation of the destination point known, you can solve for J4, 5 and 6.

    Quote from mcardoso

    4. During all my research, I found very few examples of someone truly working out all the kinematics (the only one I remember was a graduate thesis on the kinematics of a Universal Robots UR5). Does anyone know of an example where someone worked out the kinematics for a 6 axis serial robot with a spherical wrist

    Chris Annin documents how he derived the inverse kinematics for the arm he built.


    Here is the start of the series:

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    Quote from mcardoso

    5. Does anyone know a good place to look for buying a robot? There is an industrial surplus company not far from me at all that often has used robots come through. They are often in the $2000-7500 range, but sometimes, smaller units come in without controllers and sit until they sell for a few hundred. I’d prefer to find something in better shape than I see there, but I’m hoping to keep the actuator purchase around $1000.

    Pretty much bid spotter is your best bet for industrial arms. Down side is they are usually huge, and/or old as hell, and to top it off use proprietary servos. Occasionally you will see a deal on ebay.


    Also there is a kickstarter going on right now for a stepper arm based on an abb.

    Check out the Fanuc position converter I wrote here! Now open source!

    Check out my example Fanuc Ethernet/IP Explicit Messaging program here!

    I have felt very identified with your situation, for years I have played with robots as a hobby, and I always looked at the industrialists with curiosity, every now and then I looked on ebay for a cheap one, even if it doesn't have a controller, and this Christmas I got it for 200 € !!! It is small, but a robot with 6 degrees of freedom suitable to have in a house :-), when I have it working minimally I will take it to the makerspace where I will be able to use it at 100% of its capacities...


    My idea is not to use a PLC but everything with open hardware, based on arduino for real time control and raspberry pi for ROS, the control drivers I am doing it myself with MOSFET transistors.



    He already lives! :smiling_face:

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    I am incredibly impressed! This is exactly what I am looking to do / hoping to find!


    I haven't found one yet, but I was bidding on a Fanuc LrMate 200iD that sat at $400 until the last couple of minutes. I sure got my hopes up on that one!


    What brand of robot is that? Are you using the original servos? If so, what is you interface to controlling them?


    Thanks for sharing and awesome job!

    Yes, i am using the original servos, but without reading the encoders, just using the joints that have little inertia, to make the interpolation i use a version of the GRBL arduino firmware modified for 8 motors at once (project Thor https://hackaday.io/project/12989-thor ), and each motor i move it with a own code that i have made to generate the 3 PWM sine waves to activate the transistors, STEP pulse signal advances or retreats the phase, according to the state of the DIRECTION pin.


    The original GRLB (https://github.com/AngelLM/grbl) does not include brake control, that will have to be modified, now use a relay connected to the ENABLE_MOTORS pin.

    I'm waiting for the components to be able to move all the servos at once, and start documenting the project in the arduino blog.

    The complete project I think will be 1 arduino MEGA for real time control, 6 arduino nano for PID servo, and another 6 arduino to read the encoders directly on the joint and send everything by a data bus. I prefer a didactic and modular system, that can be understood easily, without black boxes.


    When everything works well with encoders, I will integrate it into ROS.

    ( next week start a free course https://www.edx.org/es/course/…os-robot-operating-system)


    I hope you have luck and soon you get your used robot to experiment !! and you join to The Frankensteinbot Club, reviving old robots around the world :smiling_face:

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