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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > How to control a servo motor?
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  1. #1
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    How to control a servo motor?

    Because of a problem I'm having in another thread, I want to build my own THC basically. Or more simply put, I want to control a servo motor separately.

    I want to disconnect the Z axis servo motor from my CNC table setup, and somehow control it completely separately.

    That's basically what I'm doing now since I don't have ATHC. I just sit on a chair with a welding helmet on and watch the spark cutting, bringing the head up and down by eye. I'm completely used to it by now and I don't use it enough to upgrade to a new system.

    So how can I manually control this servo motor to go up and down? It has the power cable coming out of it, which is easy to get to, and it has a RJ45 connector going into it at the encoder. Here's a pic:


    What do I need to do to make that happen?

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    The servo motor connects to a servo drive. What type of command input does the servo drive require? Will it take a Step & Direction input?

  3. #3
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    Re: How to control a servo motor?

    Do they sell a simple servo drive for this that I can use completely separately. As in what if someone gave you this servo motor, and you wanted to use it to control say a doggie door or something...

  4. #4
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    Well, it doesn't really work that way. I can see enough of the motor to see that it's a brushed DC motor, so you might be able to control it with a DC motor controller. I'm pretty sure the existing encoder feeds back to the machine controller, but no way to tell from here. You don't really need to buy much hardware to make it work.

    If the servo drive will take a step & direction input, then an $18 MPG encoder from EBay could control it. If the servo drive wants a +/- 10 VDC command signal, then it would be possible to control it with a pot connected to the command input. That is the reason I asked about the existing servo drive. You have everything there that you need to control the motor, you just need to connect it to the correct device that you can have in your hand.

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    Re: How to control a servo motor?

    Quote Originally Posted by Jim Dawson View Post
    Well, it doesn't really work that way. I can see enough of the motor to see that it's a brushed DC motor, so you might be able to control it with a DC motor controller. I'm pretty sure the existing encoder feeds back to the machine controller, but no way to tell from here. You don't really need to buy much hardware to make it work.

    If the servo drive will take a step & direction input, then an $18 MPG encoder from EBay could control it. If the servo drive wants a +/- 10 VDC command signal, then it would be possible to control it with a pot connected to the command input. That is the reason I asked about the existing servo drive. You have everything there that you need to control the motor, you just need to connect it to the correct device that you can have in your hand.

    How do I tell if it'll take a step and direction input?

  6. #6
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    A model number from the drive would be helpful, pictures are good too.

    The US Digital encoder on the motor would indicate that there has been some retrofit work done on the machine. As far as I know, those encoders only come standard on some BLDC motors from Anaheim Automation.

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    Re: How to control a servo motor?

    Quote Originally Posted by Jim Dawson View Post
    A model number from the drive would be helpful, pictures are good too.

    The US Digital encoder on the motor would indicate that there has been some retrofit work done on the machine. As far as I know, those encoders only come standard on some BLDC motors from Anaheim Automation.
    Here's some more pics, hopefully you can read the label good... it's a little messed up. These were factory installed as it came from Practical CNC in Michigan. (this is a CNC plasma cutting table) They are now out of business.

  8. #8
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    That is helpful, but what I really need to figure out how the servo drive is configured is the manufacturer and model or part number of the servo drive. Given that it's a MCG motor, I'm going to guess that it's a DMC6D drive.. But lets confirm that.

  9. #9
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    Re: How to control a servo motor?

    Quote Originally Posted by Jim Dawson View Post
    That is helpful, but what I really need to figure out how the servo drive is configured is the manufacturer and model or part number of the servo drive. Given that it's a MCG motor, I'm going to guess that it's a DMC6D drive.. But lets confirm that.
    Sorry, I thought you meant servo motor, not drive. So you need to see what is on the other end of the cable going into the encoder on the top of the servo motor correct? It goes into a Practical CNC control box. I can open it up and take some pics of it tomorrow if that's what you're looking for.

  10. #10
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    Really more interested in the other end of the power cable on the motor. That will connect directly to the servo drive and that is the device we are really interested in. That should tell us whether the servo drive is analog or step & direction. It might be possible to connect a MPG encoder in place of the encoder on top of the motor, and get manual control that way, but it would be tricky at best.

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    Re: How to control a servo motor?

    Quote Originally Posted by Jim Dawson View Post
    Really more interested in the other end of the power cable on the motor. That will connect directly to the servo drive and that is the device we are really interested in. That should tell us whether the servo drive is analog or step & direction. It might be possible to connect a MPG encoder in place of the encoder on top of the motor, and get manual control that way, but it would be tricky at best.
    OK... maybe I'm getting confused on terminology here... I've had this machine for over 10 years but honestly, this is the first time I've heard the term SERVO DRIVE...
    Here's what I'm thinking, I'll simplify it for my own ignorance, let me know if I'm wrong.

    The SERVO MOTOR is the cylinder shaped black thing that spins a gear in circles and makes a WHIRRRRRRRING noise.
    The ENOCDER is the little black plastic thing screwed to the top end of the SERVO MOTOR and is labeled US DIGITAL.
    The RJ45 CONNECTOR looks like a big fat phone plug, and is plugged into the encoder, lets call this end #1.
    The DATA CABLE is the 30 foot long blue thing that looks like a super long anorexic snake, and plugs into the encoder. It has an RJ45 CONNECTOR on both ends. lets call them end # 1 and #2.
    The POWER CABLE is also about 30 feet long, but is thicker, black and is wired directly into the SERVO MOTOR at end #1 with no removable plug. It has a red and a black wire inside.
    Both the POWER CABLE and the DATA CABLE run from the SERVO MOTOR to, in my case... a CONTROL BOX.
    The CONTROL BOX is a steel box, in my case about 12"X 12"X 4", that is where ends #2 of the POWER and DATA CABLE get plugged into. I've never opened up it up before, but I assume is either full of all kinds of electronical type things, a family of field mice, or magic smoke... possibly all three.
    Here's the part I'm mostly confused about...
    The SERVO DRIVE is inside the CONTROL BOX, and is some sort of smaller circuit board, mixed in with the other circuit boards in there, and is what the POWER CABLE and the DATA CABLE get wired in to.

    Did you want me to unscrew and open up the CONTROL BOX so I can find and take a picture of what I'm guessing is the SERVO DRIVE...?

    Sorry for my ignorance, but I want to make sure I'm not wasting your time taking pictures of the wrong thing anymore than I already am. I really appreciate your help!

  12. #12
    Community Moderator Al_The_Man's Avatar
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    Re: How to control a servo motor?

    There are basically two possibilities, you have a CNC controller that closes the loop, i.e. the encoders goes back to It and the drive just receives a signal from the controller to position the motor, IOW there is no feedback to the drive itself.
    The Other is the encoder goes back to the drive which is then this that controls position, based on a signal from the CNC controller.
    So it would help to see more info, drawings or pics of the present set up.
    Trace the encoder back to where it ends up and this also will help decide what system you have.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  13. #13
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    I think you have the terminology down just fine. And your description is correct.

    Yes, you are going to have to take the cover off of the control box and get a picture or two. The only thing that confuses me at this point is the size of the box. That size would indicate that it is a junction box, just full of terminal strips, rather than the machine controller. But it's a good place to start. I would expect that there is a much larger electrical box attached to the machine somewhere.

  14. #14
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    Re: How to control a servo motor?

    This web page, especially the block diagram, may help. And there are links to other information. They key point is that the servo driver (the thing that is wired to the motor) must match the motor. The encoder and servo control are replaceable, although they are often integrated with the driver.

    techref.massmind.org/techref/io/servo/index.htm

    http://www.cnczone.com/forums/attach...d=373964&stc=1
    Attached Thumbnails Attached Thumbnails ServoBlock.png  
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  15. #15
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    Re: How to control a servo motor?

    In my opinion it is important to know weather you want to manually jog this motor or to position it on command

    Sent from my Le X522 using Tapatalk

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    Re: How to control a servo motor?

    Quote Originally Posted by Jim Dawson View Post
    I think you have the terminology down just fine. And your description is correct.

    Yes, you are going to have to take the cover off of the control box and get a picture or two. The only thing that confuses me at this point is the size of the box. That size would indicate that it is a junction box, just full of terminal strips, rather than the machine controller. But it's a good place to start. I would expect that there is a much larger electrical box attached to the machine somewhere.

    Ok I took some more pictures, and I also want to clarify that all I want to do, is use this controller I made to manually move the servo motor up and down while the plasma torch is cutting on the table.
    Yes, this controller is pretty jankety... but the plastic one it came with broke after a bit, and I had that little box laying there from a previous project, so I just drilled a couple holes in it, popped some switches in it, and it's been working great for almost 10 years.
    Attached Thumbnails Attached Thumbnails pendant.jpg  

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    Re: How to control a servo motor?

    Anyways, here's some picture of what I've been calling the CONTROL BOX. The inside, back, and outside pics. (Tape measure included for size reference)
    I know I told you that I've never opened that box up before... well, I was wrong on that.

    Upon opening it, I noticed a couple Stakon wire connectors that I use all the time, in the left side of the first pic, and I now remember that I removed the main power switch, put white electrical tape over the hole, (visible on the left side of the 3rd pic) and relocated the switch to right next to my CNC table in the other room. That way I didn't have to walk back and forth between rooms just to turn it on and off.
    I also said earlier that I had never heard the term SERVO DRIVE, but yet, there it is... in big, bold, CAPITALIZED, even italicized RED letters.......... SERVO DRIVE. Just goes to show you how long it's been since I've even seen this thing.

  18. #18
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    Re: How to control a servo motor?

    Here are some inside pics of the right side, a close up of what I assume is the "actual" servo drive boards, and the left side.
    Attached Thumbnails Attached Thumbnails servo drive.jpg   right side.jpg   left side.jpg  

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    Re: How to control a servo motor?

    And here's another brighter picture of the back with the cover removed.

    Anything other picture requests?
    Attached Thumbnails Attached Thumbnails back 2.jpg  

  20. #20
    Community Moderator Jim Dawson's Avatar
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    Re: How to control a servo motor?

    We are getting really close. The servo drives are the 3 devices mounted on the aluminum angle. Good pictures but I can't identify the drives. They don't look like the DMC6D drives that I would expect. Are there any other identifying markings on those or the board below them? We need to track down the manufacturer or other part numbers. This should allow us to figure out what the command signal input needs to look like.

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