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Newbie question- using PM-DC motor as servo
« on: September 20, 2010, 07:15:32 PM »
I may be asking a ridiculous question, but could a pair of 90 vdc PM-Brush motors with quad encoders installed be operated from MACH. I know that a stepper motor is supposed to synchronize a specific turn angle per pulse, but have not figured out how a servo is kept synchronised. I thought that it was simply turned on till the encoder count matched the proper travel, but in reading the MACH manual I see all this talk of step pulses and such and am totally confused. Since a straight dc motor will turn a given distance entirely dependent on current, drag, pulse length and so many other variables, how is it actually being synchronized to a digital count. Do the step pulses continue until the encoder reaches a predetermined count based on the exact coordinates programmed into the tool track, or would changes in speed and drag potentially cause the motor to stop travel in an unsynchronized position. Also since these particular motors are high voltage/current and controllers like the gecko can't operate at those limits I suppose standard scr motor drives would have to be employed. In this case how would they be affected by a pulse based drive signal? If anyone has attempted this and found a solution a block diagram and material list would be of great assistance in assessing how to proceed. Thanks, Les  ???
Re: Newbie question- using PM-DC motor as servo
« Reply #1 on: September 20, 2010, 10:36:14 PM »
Yes, you can do that, but you need a DC servo controller, like a Gecko G320/340 to operate it.  There are folks on here who are running their mills using the ubiquitous DC treadmill motors as servos, by simply adding a digital quadrature encoder, and running it with a Gecko.  It may not be quite as good as a real servo motor, but is plenty good enough for hobby use.

Regards,
Ray L.
Regards,
Ray L.
Re: Newbie question- using PM-DC motor as servo
« Reply #2 on: September 21, 2010, 09:10:29 AM »
Thanks Ray,
       I was hoping it could be done. I am still a bit concerned about the voltage limits of the gecko units though. They have a max voltage of 80, and these motors I have are 90 which isn't a great amount but if a treadmill motor were employed for spindle drive it would be 130. Does a person just have to sacrifice rpm at the top, or could the output of the geckodrive be fed to the voltage following inputs of a secondary motor drive like the KB units. I wonder how the Geckodrive is actually controlling motor speed and acceleration. If it is varying voltage then the voltage folower inputs of the kbsi 240d isolater could track that and send an appropriate signal for any motor controller to follow. If it is pulse width modulation that is taking place it might be a little trickier. Of course in any case a person would have to work out a method of reversing the motor since that part of the loop would be lost in translation between the gecko and the signal isolator. How much loss of efficiency would a person see using the gecko at its upper limit and just losing that top end voltage I wonder?
     Regards, Les ;D

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Re: Newbie question- using PM-DC motor as servo
« Reply #3 on: September 21, 2010, 09:54:14 AM »
I wouldnt go above the Geckos rating, although they will likely stand a bit more its not worth the risk IMO.
Two options, put up with the slightly slower speed or look at other drives such as the Dugong from http://www.shop.cncdrive.com/index.php?productID=166
I have never used them but have heard good reports.
Hood
Re: Newbie question- using PM-DC motor as servo
« Reply #4 on: September 21, 2010, 10:02:53 AM »
Hi Hood,
     Actually I am probably a bit more conservative then most having spent years as a electronics service tech. That is my main concern about the gecko since I would probably want to operate it at no more than 70% of it's highest rated voltage. That would mean a substantial speed loss and force the motors into a range where heating could be a secondary problem. The ideal would be a servo controller designed for the voltage of the motor. I will check out the link you sent. Maybe they have something suitable. Thanks for the input.
     Regards, Les

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Re: Newbie question- using PM-DC motor as servo
« Reply #5 on: September 21, 2010, 10:31:18 AM »
I have heard that the Geckos use the same power transistors that other higher rated drives use but how much truth there is in that I dont know.
The Dugong drives linked to are 160V 35Amp and if you look here simpson36 did a lot of testing of drives. http://www.machsupport.com/forum/index.php/topic,12233.0.html

Hood
Re: Newbie question- using PM-DC motor as servo
« Reply #6 on: September 21, 2010, 11:03:44 AM »
Hi Hood, I looked at the dugong everything but price looks great. Even at the price though it is looking like a very easy solution. I will check out this other forum post and see if there is more info to be had there. Thanks for all your input, it is greatly appreciated and has helped immensely in getting closer to a solution.
     Regards,
           Les

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Re: Newbie question- using PM-DC motor as servo
« Reply #7 on: September 21, 2010, 11:13:22 AM »
Sad fact with servos I am afraid, voltage costs and features cost :(
I have never used Geckoos servo drives but from what I have heard they are a bit lacking in features, their stepper drives are the best but their servo drives dont seem to be in the same league.

There is a guy over on the Yahoo group that uses treadmill motors on his lathe but not sure what drives he uses, have it in my mind its Geckos but not positive. Have a search  there for  his user name CNC 6-axis Designs

Hood
Re: Newbie question- using PM-DC motor as servo
« Reply #8 on: September 21, 2010, 11:47:33 AM »
Hi Hood,
     And again you come through with great info. I am presently studying all the info on the last recommended forum post and really starting to lean toward the Dugong. compared to the others reviewed it doesn't look as high as it did compared to the Gecko, and appears to be able to actually do what I need. I will check out the yahoo group once I finish reading all this. I am actually beginning to see a little daylight on this idea and believe I might make it actually work thanks to your great input.
     You might find my purpose interesting. I am trying to set up a cnc wood lathe to turn Tractrix horn speakers from hardwood. I built a set by hand which was a tremendously tricky and time consuming turning project, but the end result was an unbelievably clear high quality sounding speaker. they are not commercially available except as junky fiberglass castings, and I think well turned hardwood units could be a very marketable product to audiophiles. Trouble is to buy a cnc machine and gamble that amount against a possible market is a bit too scary. If I can get a homebrew up and running at a minimal cost it might turn into a very proffitable enterprise.
     Thanks again for all your help, it is proving invaluable.
          Regards,
                 Les

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Re: Newbie question- using PM-DC motor as servo
« Reply #9 on: September 21, 2010, 12:01:02 PM »
Sounds great and its good to hear of  another unusual use that Mach has been put to.
Hood