Closed Loop CNC

[ZeroBacklash]

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.

Interesting info! How do these analog servo drives do position control then? If I remember correctly even Kanalog seems to be for the older analog servo drives. so does 0 to 10V map to 0 to 360 deg for position control?

[109jb]

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.

[Jim Dawson]

Interesting info! How do these analog servo drives do position control then? If I remember correctly even Kanalog seems to be for the older analog servo drives. so does 0 to 10V map to 0 to 360 deg for position control?

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.

[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...

[Bdrangdo]

Interesting info! How do these analog servo drives do position control then? If I remember correctly even Kanalog seems to be for the older analog servo drives. so does 0 to 10V map to 0 to 360 deg for position control?

The simple answer is they don't. At the risk of oversimplification the feedback loop is closed by servo's that are controlled digitally and are by their very nature are positional control, they behave much like stepper motors in many ways. Analogue servo's are controlled by the motion controller which closes the loop. Many people refer to type of servo as servo amplifiers because that is basically what they do, ie they amplify the analogue signal from the motion controller to give enough power to drive the motor. In reality they can be much more complicated but in principal it is true. It all boils down to how much money you want to spend, I have Bridgeport miller with brushed DC motors that were almost free. It would be easy to pay a couple of thousand pounds for AC motors and servo's to achieve the same thing. The motors I am using came off a Kondia which is Spanish copy of a Bridgeport Interact, they are 3.8 NM (roughly .75 KW) and drive using a 1 -1 ratio and they are more than powerful enough. Early Bridgeports and some Interacts actually used stepper motors. The reason I mention this is because it is easy to overestimate how much power you actually need. Nearly all servo amplifiers have the ability to overload the motor for short periods of time and this is often exploited to give good acceleration during G0 moves. Just to throw in a curve ball, many industrial servo controls are connected to the motion controller by propriety methods, sometimes Profibus and Sercos to name but a few are used along with others such as fibre optics all of which implies that the CNC control and the servo/motor is actually a complete package. My advice based on hard and painful experience is "Keep it simple and slow". You can add whistles and bells and more power later, and don't underestimate how much power there is in a speeding machine, it may look impressive until something goes wrong and often things break and it may be you.

[Al_The_Man]

Interesting info! How do these analog servo drives do position control then? If I remember correctly even Kanalog seems to be for the older analog servo drives. so does 0 to 10V map to 0 to 360 deg for position control?

They are simply amplifiers used in the torque mode, rather than the older velocity mode drives originally used in commercial CNC systems.
I came in to CNC from the other end, so to speak, retrofitting Fanuc and Mitsubishi systems so I was spoilt to some degree, so when I found Galil and Acroloop in the late 80's they were a smooth transition for me.
The drives that Galil mention on their web site and used to sell are the A-M-C versions, in the torque mode these are simple trans-conductance amplifiers where a the current out is directly proportional to the analogue voltage in.
They know nothing of position, just do what they are instructed to do by the controller .
These controllers are as simple as you can get!
Al.

[Al_The_Man]

On the question of the dual loop, there is an instructional video by Dr Jacob Tal on the question of using encoder/scale type dual feedback where with servo's the PID loop has to be split between the encoder and scale particularly where there is any backlash existing between motor and motion.
Al.

[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.

[ZeroBacklash]

You can add whistles and bells and more power later, and don't underestimate how much power there is in a speeding machine, it may look impressive until something goes wrong and often things break and it may be you.

Thanks for the fabulous info and sharing your experience!

They are simply amplifiers used in the torque mode, rather than the older velocity mode drives originally used in commercial CNC systems.
I came in to CNC from the other end, so to speak, retrofitting Fanuc and Mitsubishi systems so I was spoilt to some degree, so when I found Galil and Acroloop in the late 80's they were a smooth transition for me.
The drives that Galil mention on their web site and used to sell are the A-M-C versions, in the torque mode these are simple trans-conductance amplifiers where a the current out is directly proportional to the analogue voltage in.
They know nothing of position, just do what they are instructed to do by the controller .
These controllers are as simple as you can get!
Al.

Would that mean that these analog servo drives are simple porportional amps (no I and D) and the output stages (transistors) run in linear mode instead of PWM mode?

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

So If the Kanalog can work with modern as well as legacy servo drives. The advantage of using legacy drives is that I can get full closed loop at the controller level as well as not much PID tuning need per drive. Am I right with this deduction?

[Al_The_Man]

I don't consider Analog 'Legacy' the older Velocity drives that used it, maybe.
The AMC etc run PWM Mosfet outputs for the DC or BLDC motors.
The drives have virtually no tuning involved in them.
The BLDC drives will also run a DC brushed motor by setting the commutation to 120deg.
Al.

[samco]

For analog - linuxcnc hardware is pretty inexpensive. 6 axis analog, 6 encoder (ttl/diff) 48 i/o and expandable for $279. There is an ethernet solution that would be about the same price.

I have used mesa and am very happy with it.

https://mesaus.com/index.php?route=p...&product_id=79

sam

[bhurts]

For step/dir drives you do not need kanalog. K-flop does step/dir without the need for kanalog. Kanalog does not output step/dir signals at all only analog. If you want to use step/dir drives just buy k-flop by itself. If you need more or higher voltage inputs and outputs add a konnect.

Ben

Sent from my E6810 using Tapatalk

[Al_The_Man]

I made a misspelling in post #28, it is now corrected for Kmotion info.
Al.

[ZeroBacklash]

For step/dir drives you do not need kanalog. K-flop does step/dir without the need for kanalog. Kanalog does not output step/dir signals at all only analog. If you want to use step/dir drives just buy k-flop by itself. If you need more or higher voltage inputs and outputs add a konnect.

Ben

That sounds promising. So does K-flop do encoders too and more importantly can K-flop close the loop?

[Al_The_Man]

Kflop closes the loop, servo's require a PID loop, unlike steppers they require a closed loop either back to the drive or the controller.
Al.

[ZeroBacklash]

Kflop closes the loop, servo's require a PID loop, unlike steppers they require a closed loop either back to the drive or the controller.
Al.

So how does this workout?
If my servo drive has encoder feedback would I just skip that and just connect the encoder to Kflop for it to close the loop. Or do I need to connect the encoder to both the drive and Kflop?

Secondly I have read on this and other forums that analog drives don't need any tuning. So how difficult is it to tune a Digital drive stepper or servo?

[ZeroBacklash]

For analog - linuxcnc hardware is pretty inexpensive. 6 axis analog, 6 encoder (ttl/diff) 48 i/o and expandable for $279. There is an ethernet solution that would be about the same price.

I have used mesa and am very happy with it.

https://mesaus.com/index.php?route=p...&product_id=79

sam

I'm impressed with LinuxCNC. What is your system like? I have to control a couple of Huanyang VFD's that don't have encoder feeback. Do I just wire up the encoder to Linux CNC and it would do the closed loop control for the VFD's?

[samco]

This was the first real conversion with linuxcnc. (amc analog drives and toshiba spindle vfd) this is all mesa and all i/o is controlled within linuxcnc (linuxcnc also has integrated ladder logic)



You can hook a encoder up to the spindle to do spindle synced motion (rigid tapping and such) Some have actually done orienting - like for tool changing and such with a vfd. (using a pid loop around the spinde vfd and encoder. The connection would have to be analog to the vfd because modbus isn't realtime enough to do it.

We also converted a mid 80's matsuura reusing the spindle and servo drives



older

[mactec54]

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.

That is not correct for analog control it is +/-10v for the control voltage, for servo applications, PWM is 0-10v for VFD Control

[mactec54]

I'm impressed with LinuxCNC. What is your system like? I have to control a couple of Huanyang VFD's that don't have encoder feeback. Do I just wire up the encoder to Linux CNC and it would do the closed loop control for the VFD's?

If you are trying to control a regular AC motor with a VFD, then if you think by adding an Encoder, Plus software you are going to get some accuracy, like a servo motor and drive, then you are wasting your time