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Author Topic: Mach3, servos and no motion controller  (Read 6647 times)

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Offline ger21

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Re: Mach3, servos and no motion controller
« Reply #10 on: March 29, 2014, 08:38:59 PM »

Quote
Servos are NOT inherently any better than steppers at anything.

Servo's are better at going fast. If you're building a router that cuts at 1000ipm, then you start to reach a point where you need to trade resolution for speed, due to the limited rpm range of large steppers.

Other than that, though, yes. If you use the right steppers, you're machine should run perfectly fine.
Gerry

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Offline Hood

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Re: Mach3, servos and no motion controller
« Reply #11 on: March 29, 2014, 09:22:55 PM »
I would also add that servos have another benefit over steppers, that is that they can have both acceleration and velocity.
The point however, as has been mentioned, , a badly designed servo system is worse than a well designed stepper system and vice versa.
If the machine requires a certain amount of power/torque then your system has to be able to deliver that, whether that is servo or stepper does not matter, if it cant deliver that then you will loose steps in the case of steppers  or fault out with an over current error in the case of servos.
There are machines that a servo is a waste and machines that a stepper is not really suitable for, as examples a coil winder and a small positioning stage I have made do not require blistering velocity and acceleration and thus the only benefit to using servos on it would be the lack of whine/singing (whatever you want to call it) that a stepper has. The Chiron VMC,  and the Large lathe I have do require a servo system as there is a need for velocity and acceleration on them and they are simply too large for that to be even a remote possibility with a stepper. The large knee mill I have is sort of in between, it could be used with steppers if I was willing to compromise on velocity and acceleration but the "hogging" ability of a sero or stepper system properly designed would be identical. The small lathe, I have, I chose to put servos on, not because it really needed them but because I wanted it to be quieter than it would have been with steppers and if I am honest I do prefer servos.
 
Hood
Re: Mach3, servos and no motion controller
« Reply #12 on: March 29, 2014, 11:35:29 PM »
There is a "cross-over" point in power requirement where servos start making more sense that steppers - something around 400W, IIRC.  Below that point, for "typical" applications, like a milling machine, a stepper will do everything a servo will do, performance-wise, and at a lower cost.  Above that point, steppers can no longer deliver the performance, and servos start to make more sense, albeit at a hggher cost.  But, 400W is FAR more than is required for a BF20. 

There are always applications at both ends of the spectrum for which that rule does not apply, because of unusual requirements, like extreme speed, or something similar.  The trick is to design the drive system to fit the application, not just blindly mix and match components.  It takes actual engineering to do it right.  But, for milling machines and routers, the rule works pretty well.  Yet, most people still seem to believe that lost steps are an inherent characteristic of stepper motors, and servos will ALWAYS perform better, and never lose steps.  Just ain't so....

Regards,
Ray L.
Regards,
Ray L.

Offline txpto

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« Reply #13 on: March 31, 2014, 05:25:00 PM »
Is possible make closed loop servos controller with mach plus pokeys plus encoders 1024pprev. plus inverter in gearing mode.

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Offline Hood

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Re: Mach3, servos and no motion controller
« Reply #14 on: April 01, 2014, 02:57:10 AM »
If your servo drives can accept Step/Dir signals then you can use Mach via the PoKeys to drive them.
The PoKeys however, if still the case, is only capable of a max pulserate of 25KHz. What that means is, for example, with a 1024ppr (4096 as Mach sees it)  encoder then the max rpm of that motor would be 366 rpm, so if connected directly to a ballscrew of 5mm pitch the max velocity would be 1831mm/min (72 IPM).
As mentioned earlier many servo drives can utelise electronic gearing and thus it might be possible for you to get more velocity, this however reduces the resolution of the axes, as an example if you had 1:5 gearing your steps per unit would be 164 instead of 819 for the previous example.

Hood