Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: mmontev on June 11, 2013, 05:35:28 PM

Title: Mach3, servos and no motion controller
Post by: mmontev on June 11, 2013, 05:35:28 PM
Hello,

I'm thinking about the construction of my first router. A gantry type, about 6x3x1. As versatile as possible regarding usage.
I live in Argentina, here is very difficult to obtain and import equipment so I must choose very well.

I'm considering Mach3 and I have some questions.

As I understand Mach3 works as a trajectory generator, open loop. So Mach3 does not care about the motor-drive combo as long as they accept pulse & dir. It could be open loop steppers, closed loop steppers or servos. Is that correct?

If the above is correct I'd like to use servos (closing the loop only in the drive, no motion controller), and I see some AC servo & drive economic options. I am considering the following alternatives, and would like some feedback on them and other options in the same price range.

At the moment my preferences are:

AC servo & drive
http://www.americanmotiontech.com/Products/BrushlessServos/Sets/default.aspx?type=1
http://www.cnc4pc.com/Store/osc/index.php?cPath=97_99

closed loop stepper
http://www.americanmotiontech.com/Products/HybridServos/Sets/

open loop steppers
gecko

I am new in this area and any advice is welcome.
Thanks and regards,

Martin
Title: Re: Mach3, servos and no motion controller
Post by: bollga on August 14, 2013, 02:44:29 PM
I'm also very interested in this topic.
I'm hoping to use linear encoders.

I understand that backlash will throw a monkey wrench into the system, but let's say for the sake of argument that there is no backlash in the system.

If I set Mach3 to move 1000 steps per mm and I enter G-code to move an axis 1mm at a certain feed rate.
The servo driver will get 1000 step commands at the set feed rate, correct?
Will the servo driver verify through linear encoders that it has move the 1mm specified by 1000 steps?

Hoping I can either verify this before dropping coin on upgrades.

Thanks for any input.
Title: Re: Mach3, servos and no motion controller
Post by: ofg on March 29, 2014, 01:07:09 PM
Has anybody got more info on this? Im looking to build a mill with servos rather than non encoded steppers, and want to make sure its worth the effort.
Title: Re: Mach3, servos and no motion controller
Post by: ger21 on March 29, 2014, 01:18:57 PM
What is your question?

The above mentioned servos will work fine with Mach3.
I plan on using the 400watt Leadshine servos from AMT in the first link for the new router I'm building, but I haven't purchased them yet.
Title: Re: Mach3, servos and no motion controller
Post by: Hood on March 29, 2014, 01:26:10 PM
You can and have always been able to use servos with Mach when using the parallel port.
There are a few things to bear in mind however, first is that your drives have to accept Step/Dir signals and second is your steps per unit are constrained by the kernel speed of the parallel port and thus you will either have to use lowish count encoders or your drives will have to be capable of electronic gearing.
With servos that use Step/Dir commands the loop is closed in the servo drive and feedback is to the drive.
Feedback usually takes the form of the motors encoder but you could also use linear encoders although any backlash at all would make this very difficult to tune if you did use linear encoders rather than the motors. Some servo drives can accept dual feedback but they are not common and certainly not in the lower end of the scale.

I have servos on all my machines but I use external controllers on all. Originally I used the parallel port on the big lathe but that was because there was no viable controller at the time. When using the parallel port I had to use the electronic gearing in my servo drives as realistically 45 to 65KHz is the most you are likely to get reliably from the parallel port.
 I am now moving over to external controllers that can control analogue command servo drives as you not only have closed loop in the external controller but you also have update of DRO position in Mach from the encoder information.

Hood
Title: Re: Mach3, servos and no motion controller
Post by: ofg on March 29, 2014, 05:39:09 PM
Those leadshine items look like a fairly reasonably priced solution. So, the combos that come with the motors, and drives would be a good match for mach3 control? With separate outboard servo drives, is here still need for a control box with break out board,, or just a hub for power supply?
Title: Re: Mach3, servos and no motion controller
Post by: ger21 on March 29, 2014, 06:08:39 PM
Motors and drives need to be a good match for your machine, not Mach3. Anything that takes step/dir will work with Mach3.

How you wire it and what components you need is entirely up to you.
Title: Re: Mach3, servos and no motion controller
Post by: ofg on March 29, 2014, 07:32:06 PM
Well, Im trying to match servo motors output shaft size to the readily available ball screw size. This is determined by what is included in the cnc retrofit kit(s). Some are 8mm, and some are 1/4"  Of course, a nema23 size frame in the mid range power and wattage wize is about right for a small mill(bf20). So, making everything match up physically, and electronically is actually a bit complicated. My router works great on stepper motors, but it seems like servos are the way to go when making heavy hogging cuts. The router loses steps if pushed too hard. When I mention heavy hogging,, I mean in terms of a 300 pound bench mill,, so I know, not real heavy,, but presumably much deeper than the aluminum framed router can handle.
Title: Re: Mach3, servos and no motion controller
Post by: ger21 on March 29, 2014, 07:42:31 PM
You can get couplers that connect shafts with different diameters.

The 200W Leadshines have an 11mm shaft I think.
The 400w has a 14mm shaft.
Title: Re: Mach3, servos and no motion controller
Post by: HimyKabibble on March 29, 2014, 08:29:54 PM
Well, Im trying to match servo motors output shaft size to the readily available ball screw size. This is determined by what is included in the cnc retrofit kit(s). Some are 8mm, and some are 1/4"  Of course, a nema23 size frame in the mid range power and wattage wize is about right for a small mill(bf20). So, making everything match up physically, and electronically is actually a bit complicated. My router works great on stepper motors, but it seems like servos are the way to go when making heavy hogging cuts. The router loses steps if pushed too hard. When I mention heavy hogging,, I mean in terms of a 300 pound bench mill,, so I know, not real heavy,, but presumably much deeper than the aluminum framed router can handle.

Servos are NOT inherently any better than steppers at anything.  For machines in the size range you're talking about steppers are perfectly capable of performing every bit as well as servos under any conditions the machine itself can handle IF the stepper system is properly designed.  Of course, a badly designed servo system is also quite capable of performing very badly.  My "big" machine, a Novakon Torus Pro runs steppers, and is capable of 350 IPM rapids, and taking "hogging" cuts a BF 20 could not even dream of.  Servos add absolutely nothing to the real-world performance, other than slightly less noise.

Regards,
Ray L.
Title: Re: Mach3, servos and no motion controller
Post by: ger21 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.
Title: Re: Mach3, servos and no motion controller
Post by: Hood 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
Title: Re: Mach3, servos and no motion controller
Post by: HimyKabibble 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.
Title:
Post by: txpto 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.

Enviado desde mi GT-N7100 mediante Tapatalk
Title: Re: Mach3, servos and no motion controller
Post by: Hood 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