Posts by mcardoso

    Physical position knowledge and electrical position knowledge are two different things (unless you take steps to correlate them).

    I’m familiar with Allen Bradley motion control so I’ll use their products as an example. A motor offered with a “low resolution” 2000 line Quadrature encoder will have separate wires in the encoder for UVW commutation signals.

    The drives expect this and typically won’t work without it. The workaround to using a motor without commutation data would to apply DC voltage across the windings to snap the motor to the nearest pole and measure incrementally from there. The downside of this is that it requires time and movement every time you power the drive on to find this offset, and and misposition of the shaft (due to loading) will make the motor inefficient and run hot or not work at all.

    The “high resolution” servo offering (Hiperface and Hiperface DSL) uses magnetic disks and sin/cos signals. There are no commutation outputs, however the commutation data is transmitted over a secondary serial line at startup (over the encoder cable). If the encoder is removed by a customer, then the commutation offset will be ruined and the motor would need to be thrown away or sent in for factory repair and calibration.

    AC synchronous motor will usually have 2,4,6, or 8 poles (has to be a multiple of 2). I will have to verify, but I’m pretty sure all of the AB standard motors are 4 pole.

    With your absolute encoders, you might be able to measure this offset once manually and save it in the controller. As long as you don’t move the encoder you’re ok. i think?

    Maybe you’re doing something different with your motor control algorithm but I believe this information is required to properly control the motor (based on my knowledge of commercial servo drives).

    Perhaps you might consider a commercial servo drive for your application. They are purpose built to do this job and have lots of protective features that would be hard to do on your own. I think Granite Devices makes a nice line of servo products for small applications (and open source). Also AMC has component servo drive offerings that might solve your motor control requirements while still maintaining a distributed and open control system.

    The commutation encoder is a bit more complicated than a traditional ABZ encoder. It also has UVW tracks which help the controller know where the motor is (electrically) within the magnetic poles. This drives the particular output phasing for the 3 phase AC to the servomotor.

    This type of feedback is necessary with synchronous 3 phase AC motors and position control of brushless DC motors, but not for AC induction motors, stepper motors, or brushed DC servos.

    I see the issue with trying to read the serial bus. You need to know what to write to the encoder to get it to respond. Could you share any pictures of the encoders or servos? I'll try to do some research.

    That sucks you couldn't get info on it. Guess you are stuck with doing it yourself!

    I think that if you are replacing the encoders, you'll want to have an encoder with commutation signal outputs. It depends on exactly how you're planning on driving the motors, but unless you are planning on self sensing the commutation angle, I think you'll be stuck without it.

    US Digital makes a relatively low cost encoder with commutation outputs. You'll need a scope to align it (which I see you already have).…ders/incremental/kit/EC35

    If your scope has data bus decoding, I wouldn't be opposed to trying to interface with the current encoders even if just to see what they put out. Unfortunately unlike some analog/serial encoders (Hiperface for example) those appear to be entirely serial communications. Couldn't hurt to do a little digging before you remove them.

    Yaskawa is an entirely different company, but when I was looking at a Mitsubishi Melfa robot, Mitsubishi sent me all kinds of documentation (including stuff that looked like internal engineering documents) when I asked for some info. Maybe the robot was so old they didn't care? Either way, it can't hurt to ask.

    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!

    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 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.

    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!