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turmite
07-22-2003, 10:28 AM
Another newbie. I have posted a couple of times so I am not totally new I guess.

I am going to build a new cnc machine this fall and am trying to round up all the parts I am need ahead of time. This machine will be considerably heavier than most of the machines I have seen here since I am building it to do heavy production on. I have been in touch with Mariss at Gecko and he was most helpful. He did however, bring up a point I need clarification on. My plans initially were to use brand new micro steppers with the Mach 2 controll program but I found a good deal on some servos and started researching the use of the servos.

Here is what Marris told me. In my application either would work fine. There is really no advantage of the servo over the stepper in the mid range machine[ midrange = power needs]. He also told me that the rotary encoders on servos do nothing more for feedback position that do the steps on the stepper motor. If I wanted to get absolute position I would have to have some sort of linear scale. Not to disparage Mariss, but is this correct.

turmite
www.turnercustoms.com

HomeCNC
07-22-2003, 12:33 PM
As far as TRUE positional feedback you would need linear scales on the ways. What Mariss may or may not have told you, there is a partial feedback when using servos and his G320/340 drives. I really like knowing that I can't loose steps with the servo system if I push it just a little too much. If the steps get too far behind, the drive will fault. With a stepper setup you will loose steps and not know until you go an measure your part!

turmite
07-22-2003, 01:20 PM
Jeff thanks for the reply. If the driver faults will it stop motion or is that part of the controller's job? The losing of steps, and in my case the momentary blip of the power lines cause me the most headaches and that is exactly waht I was trying to get away from.

turmite

balsaman
07-22-2003, 01:35 PM
The gecko will fault and stop. It's posible to have the geckos notify the PC via an input that something went wrong, so the program stops as well. There is an output from the gekco for this purpose, altho it's a little tricky to implement. I am working on this for my machine.

With the servo's you can not make a bad part without knowing it.

Eric

turmite
07-22-2003, 01:41 PM
Thanks balsaman. That is exactly the ifo I was looking for. Now i know which way I will go. I will post some pics as soon as I get started.

turmite

balsaman
07-22-2003, 03:40 PM
I think the gecko will allow the motor to be 128 steps behind before faulting. This a fixed following error built into the drive. I have 120 line encoders so that's around 1/10th a revolution. The father behind the motor is the harder the gecko will push it to catch up. There is a gain pot which adjusts the stiffness of the drive, or just how hard it will push to be right "on step". One can make the drive very stiff, at the price of smoothness accelerating. I set mine quite stiff and use accel in the software to smooth it out a little. A high gain setting can make it overshoot, or rock violently around the commanded position. Gecko 320 drives are quite a beautiful unit for the money.

Eric

Eric

HuFlungDung
07-22-2003, 04:57 PM
Originally posted by balsaman
~snip~

With the servo's you can not make a bad part without knowing it.

Eric

Oh ya? ;)

balsaman
07-22-2003, 05:10 PM
Well, I guess you can, but it won't be the servo's fault...:)

Eric