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Offline ART

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More On Motors...
« on: October 17, 2006, 10:31:00 PM »
Well, I guess theres allot to be said about motors. Stepper or servo. I dont htink we'll ever cut throguh everything one woudl want to know, but let me blog a few points,
usually more information is better than less.

 I get alot of questions about analogue servos. Theres quite a few of them out there, and its common for an owner to have no idea what they are.

An analogue servo, takes a voltage from +10 to -10 volts to make it work. Basically, the control unit or computer sets a refernece count for position internally, it then
looks at how far out the encoder count from the servo is from the ref point, if the motors encoder is way behind, the computer puts out a voltage which is about +10 volts,
this drives the motors amplifier to put out a high current to the motor to make it spin fast forward. As the distance in counts of the encoder gets closer to the computers set point,
the voltage drops, making the motor go slower and slower until the count of the encoder is the same as the computer. If the encoder was far ahead, the voltage goes negative making it go
the other direction. Thats an analogue servo in a nutshell. Since the computer is calculating how much voltage to apply to the motor based on the actual encoder position, this is refered to as
closed loop. Mach3 is open loop, so to use an analogue servo, the loop must be closed somewhere else. All servos are closed loop of sorts, just a question of where the loop is closed at.
In a Gecko 320 driver, for example, the servo loop is closed between the motor, and the driver.
  For some applications where you have an analogue amplifer and motor, you could use a step/dir convertor, such as those at http://www.skyko.com/products/ , they basically close the loop
at the convertor. But often, if your motor specs will match a Gecko driver, Id install a G320 and be done with it, much more reliable i think..
  Then there the question of using a non-servo motor as a servo. Im asked that allot. I personally prefer someone ask Mariss about such things, he's the guru, but Im of the "If it works, its a good solution" school,
so if you have an encoder on a DC motor, Id hook it up and go, (but then...thats just me..).

    In the end , with either motor, you worry about speed. Speed in servos (as in all motors) is tied to resolution. If a servo has a 500 line encoder on it, thats 2000 counts in quadrature for every rotation. So if one
rotation is one inch, then at 25Khz maximum Mach3 output, you'd get 25000 / 2000 = 12.5 rotations or 12.5 inches per second output speed, at 1/2000 of an inch resolution. Its a pretty easy calculation. (at least it is when I word it that way. :) )
 The general rule is you try to keep the servo going almost full out when your table is going full out, so you should pay attention to the details of the max speed of the servo vs the max speed of the table. Take note many servo driver
packs allow such options as gearing and ratios so the max speed of MAch3 may not matter in your calculations. Many , however, siimply go with the greatest oomph they can, and rework it if it doesnt work. All depends on how much you like to tinker and how picky you are with the end result.  It important to understand people build things much as they drive a car, some do it right , (me) some do it wrong. (everyone else..). CNC is like that. In the end, if it works, its right, maybe not as good as it could have been, but you learn form your mistakes. This will be my third table, so Ill try to learn from some of my mistakes previously.. I have th eluxery of not needing this for commercial use, just personal, so keep in mind that if your building this for the boss, think
three times about everything, and check your facts with others to make sure you get as much agreement as possisble.  I intend to spend less than 2000.00 or so, its likely to cost you much more as I already have alot of parts laying about..
so the more important your tabel is to your particular enterprise, the more planning  you should put into it..


  Well, thats it..another long day.. Time to put this blog to bed for the night..

to be continued....
Art


 





             

Offline khalid

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Re: More On Motors...
« Reply #1 on: June 22, 2010, 12:01:01 PM »
keep continuing..very nice writeup..

Offline TomR

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Re: More On Motors...
« Reply #2 on: April 24, 2012, 05:27:19 PM »
Very interesting and looking forward to your follow-up.  I am considering converting my router from stepper to servo, so you really have my interest.

Thanks, TomR

Offline TomR

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Re: More On Motors...
« Reply #3 on: April 24, 2012, 05:36:06 PM »
Art:

A question popped into my mind after making the above post.  Are you at familiar with ABB servos (a Swiss Company) and if so, are there any recommendations/comments you could provide.

Thanks, TomR