[ZeroBacklash]

I have a few questions about closed loop CNC.
1. Do existitng softawre / hardware handle closed loop CNC. Or are they running in open loop mode like just send steps and hope that the motors reach their position?
2. In case of DC servos with encoders attached, how do the drives use the encoder information. Because AFAIK the even the DC servo drives have step / dir inputs. So if a servo drive receives 3 steps will it map it to degrees and rotate the motor to go to that absolute position based on the encoder feedback?
3. Do motion controllers handle encoder feeback or is it the drives responsibility to do that?

I am asking this because I need to used high power motors for at least 2 of my axis. The motors would be rated at least 1.5 to 2 KW so I'm not sure whether you get such high power steppers. I'm currently considering to go in for either a regular AC motor with VFD. But for this I would need a VFD with position control and I don't know how well VFD's handle position control or possibly I'm thinking of going in for the DMM Ac servo's / drive combo.

Thanks in Advance

[RCaffin]

First of all, I would question whether you really need that much power on the axis drives. Seriously. You should experiment a bit with one axis and a load.

Secondly, do not try to compare modern digital CNC systems with those from (say) 30+ years ago. The older designs were a rats nest of technologies, every one different.

1: Different SW packages handle closed loop in different ways. Modern digital CNCs tend towards either open loop steppers or closed loop (ie encoder) servo motors.
With steppers, you just have to 'calibrate' your machine parameters: peak velocity and peak acceleration. Stay within those limits and you should be OK.
With encoder-controlled servo systems, the motor driver handles the 'closed loop' bit. If all is well, then all is well. If the motor lags too far the driver senses this and flags the controller with a fault. The system stops. You do not need the controller itself to handle the closing of the loop.

2: Modern CNCs are digital. They do not use analog control; they use digital controls. And that falls neatly into the Step/Dir paradigm. The controller does the mapping, and tells the motor driver to go 100 steps that-away. If you are moving 2 (or more) axes together, the controller handles the synchronisation.

3: see above.

You have a good range of servo motors to call on. Brushed DC motors, BrushLess DC motors, AC servo motors, and probably more.

Cheers
Roger

[ZeroBacklash]

Roger, Thanks for the reply.
Apart from Dynomotion, Kflop / Kanalog system are there motion controllers available that close the loop. I mean close the loop at the motion controller level and not at the drive level ?

[RCaffin]

There may be, but I do not know of any.
The motion controller does not need to close the loop itself; it only needs to know that its path commands have been obeyed.

Cheers
Roger

[ZeroBacklash]

There may be, but I do not know of any.
The motion controller does not need to close the loop itself; it only needs to know that its path commands have been obeyed.

Cheers
Roger

Does the controller know that its paths / commands have been obeyed in current motion controllers like Smoothstepper or UC400ETH?

[RCaffin]

Yes, emphatically so.
If they are not obeyed the motor driver will fault - fast.

This is how thousands of CNCs, including most hobby ones, work every day.

Cheers
Roger

[ZeroBacklash]

Yes, emphatically so.
If they are not obeyed the motor driver will fault - fast.

This is how thousands of CNCs, including most hobby ones, work every day.

Cheers
Roger

How would the controller know that the Drives / motor combo has obeyed the command unless three is an encoder feedback connected back to the controller. I have not often seen encoders being connected to Steppers at least at the hobby level.

[Bdrangdo]

I have used CSLabs CSMIO/a in the past. AFAIK they only work with Mach3/4. They are easy to set up and accept the encoder signals thus closing the loop, The "a" implies an analogue demand signal. I believe they do digital versions too but they are not cheap but they do work right out of the box without the need for programming etc apart from setting the plug in parameters for servo tuning and other factors. You could also look at Galil's offerings most of which work well with Mach3 and 4. They are usually even more expensive unless you can buy secondhand. Often in the UK industrial controllers such as Fanuc Heidenhain and Fagor amongst others come up for sale but you need to do your homework on these.

[ZeroBacklash]

I have used CSLabs CSMIO/a in the past. AFAIK they only work with Mach3/4. They are easy to set up and accept the encoder signals thus closing the loop, The "a" implies an analogue demand signal. I believe they do digital versions too but they are not cheap but they do work right out of the box without the need for programming etc apart from setting the plug in parameters for servo tuning and other factors. You could also look at Galil's offerings most of which work well with Mach3 and 4. They are usually even more expensive unless you can buy secondhand. Often in the UK industrial controllers such as Fanuc Heidenhain and Fagor amongst others come up for sale but you need to do your homework on these.

Thanks for the info Bdrangdo. Would you consider Kflop also closing the loop at controller level, currently that's on my list of to buy stuff.
On another forum post I found that only EMC can close loop dircetly in software with special hardware. Can Kflop close the loop similar to EMC, what I like about Kflop is that they provide API's to link to their hardware and we can write programs in C, C# to interface with them.
The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so.. but from what I see on these forums is that many hobbyists are asking questions on Kflop and Dynomotion products.

[samco]

The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so..

No - the documentation is always getting updated. There is just a lot of it... Sometimes hard to find what you want.

I am guessing you are referencing this thread. https://forum.linuxcnc.org/32-docume...d?limitstart=0

Linuxcnc is constantly checking actual following error and will stop if it is violated (you set the following error limits). Also - it has path following tolerance.

[ZeroBacklash]

No - the documentation is always getting updated. There is just a lot of it... Sometimes hard to find what you want.

I am guessing you are referencing this thread. https://forum.linuxcnc.org/32-docume...d?limitstart=0

Linuxcnc is constantly checking actual following error and will stop if it is violated (you set the following error limits). Also - it has path following tolerance.

I was actually referring to this post https://www.cnczone.com/forums/gecko...html#post36415 which says about LINUX EMC. I can't find the post right now regarding lack of documentation of Kflop.

[Bdrangdo]

Thanks for the info Bdrangdo. Would you consider Kflop also closing the loop at controller level, currently that's on my list of to buy stuff.
On another forum post I found that only EMC can close loop dircetly in software with special hardware. Can Kflop close the loop similar to EMC, what I like about Kflop is that they provide API's to link to their hardware and we can write programs in C, C# to interface with them.
The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so.. but from what I see on these forums is that many hobbyists are asking questions on Kflop and Dynomotion products.

I have only dabbled with Kflop, they do can close the encoder loop dependent on the model. If you are using an analogue demand signal I believe you need a separate board which bumps up the price. You will need to have a basic understanding of C to get the most out of them. I have no experience of EMC or if Kflop supports it. Many EMC users speak of using Mesa cards. You have many options open to you and that can make it even more confusing. One thing that the CS labs unit has going for it is the fact that is a much more robust unit with a more industrial feel with well made interface units on it rather than the basic circuit boards made by Dynamotion. Horse for courses I guess.

[ZeroBacklash]

I have only dabbled with Kflop, they do can close the encoder loop dependent on the model. If you are using an analogue demand signal I believe you need a separate board which bumps up the price. You will need to have a basic understanding of C to get the most out of them. I have no experience of EMC or if Kflop supports it. Many EMC users speak of using Mesa cards. You have many options open to you and that can make it even more confusing. One thing that the CS labs unit has going for it is the fact that is a much more robust unit with a more industrial feel with well made interface units on it rather than the basic circuit boards made by Dynamotion. Horse for courses I guess.

You may be refering to Kanalog right? yeah that provides the analog interface to Kflop. I haven't really understood the analog part. In the previous post you also refered "CSMIO/a" which you refered to as analog. Are these meant for the older servo drives that accept +/-10V input. AFAIK current servo drives support STEP / DIR inputs.
I also see that 6 axis motion controllers from CS Labs are all EUR 550+

[Bdrangdo]

You may be refering to Kanalog right? yeah that provides the analog interface to Kflop. I haven't really understood the analog part. In the previous post you also refered "CSMIO/a" which you refered to as analog. Are these meant for the older servo drives that accept +/-10V input. AFAIK current servo drives support STEP / DIR inputs.
I also see that 6 axis motion controllers from CS Labs are all EUR 550+

Many earlier servo amplifiers used an analogue demand signal as opposed to the now more common step-dir signals. Usually this implies a voltage that varies between 0V and 10V. There are two commonly used type of control ie torque control or velocity control. 0v represents zero speed or torque and 10v is the maximum. I believe it is still used to a limited extent but the attraction to this system is that most second hand servo drives of any age will use this method and are usually cheap to buy. Nearly all brushed DC servo drives and many brushless DC will be controlled with an analogue signal. One important thing to remember should you go down this route is that the encoder signals go back to the motion controller and not the servo drive unlike the digital step-dir control system which incidentally is referred to by most as positional control. The CS labs digital controller should be cheaper than the analogue version.

[greggv]

Delta Tau can close the loop in the controller. I imagine Galil can do that also. I think there are plug-ins for Galil

[ZeroBacklash]

The ones I use are PC based Galil motion, they close the encoder loop back to the card, encoder capable of 12Mhz reading resolution.
They also allow the use of some simple transconductance (non-intelligent) drives such as A-M-C, Copley, Aerotech etc.
The only commercial system using Galil AFAIK is Camsoft.
The Kanalog comes close to this.
The other card is Acroloop, but they seem to have dropped out of sight unfortunately after being taken over by Parker.
Al.

Delta Tau can close the loop in the controller. I imagine Galil can do that also. I think there are plug-ins for Galil

Thanks Greg and Al, i checked out both of these, they seem to be industrial grade motion controllers.Al, would the kanalog work with Step/dir servo drives or would a converter be needed?

The position control is all done in the motion controller. The analog drives use the +/- 10 V command signal for velocity control (or in some cases torque control). The positioning is done in encoder pulses from the encoder feedback to the motion controller, the motion controller sends the command signal to the drive proportional to requested speed and position error. Overall an extremely accurate system. The encoder can be linear on the load (table) or rotary on the motor or lead screw. So let's say you have a rotary axis, and a 1:1 ratio, if the encoder is 10,000 pulses/revolution, then the resolution is 360/10,000 or 0.0036 degrees/encoder pulse. Typical resolution on a linear axis would be in the range of 0.0001 inch or better.

Above you asked about motion controllers, I use Galil Motion Control products on all of my projects. Full industrial class motion controllers at reasonable prices and easy to program. I am using DMM servos on my lathe, and am very happy with the result. I am just getting ready to order 2 more for my next project.

Wonderful explanation Jim, that's an eyeopener regarding the way analog servos work. Also checked ou Galil and DMM servos they are awesome. They seems to be pricey though, but I guess you get what you pay for.

It sounds like you are very concerned with following error tracking, If I were as concerned as you seem, I would probably use a dual loop feedback with the rotary encoder feeding the servo drive and a linear scale reporting back to the motion controller. I haven't done this myself and probably never will since It isn't at all necessary for my tasks, but who knows. I may get bored and do it for grins someday. It does interest me so I have done much research on it in the past and LinuxCNC is definitely capable of this.

Amazing info 109jb, dual loop feedback is something I didn't know about...

[Al_The_Man]

.Al, would the kanalog work with Step/dir servo drives or would a converter be needed?

..

The Kmotion does work with step dir, if analogue is used, the Kanalog card is needed.
Al.

[samco]

Linuxcnc is a closed loop pc based cnc controller. Free and open source.

LinuxCNC

Because the motion controller is inside the computer the loop is closed there.

sam

[ZeroBacklash]

Linuxcnc is a closed loop pc based cnc controller. Free and open source.

LinuxCNC

Because the motion controller is inside the computer the loop is closed there.

sam

That post was referring to Linux EMC. Are Linux EMC and Linux CNC the same products?

[samco]

Yes - Linuxcnc changed its name from emc a few years ago.

That post was referring to Linux EMC. Are Linux EMC and Linux CNC the same products?