[computernc]

Hi folks,

I try to understand how the entire cnc process works. From a CAD drawing until
the final servo motor movements, there are a few steps I have not understood yet.

I read about the basics of servo motors and stepper motors and want to use servos
for my machine.

My first questions are about how the G-Code, that was generated in my PC,
goes to the controller.
Will it be send as a file in a whole or instruction by instruction?
How is the controller connected to my PC (usb, ethernet) if it is not a internal PCI card?
Is there such thing as a "industry standart" for sending G-Code to the controller?

Once the G-Code is in the controller, how is the servo motor controlled by the
controller? In other words. Is there a protocol or rule about how the controllers
make the servos/amps turn?
Will any controller work with any servo/amp?


CAD-drawing ----converted----G-code
G-code----------sent-to------controller
controller------???????------amp-of-servo

What is happening where the questionmarks are?

The reason why I am asking this is:
I have a cnc router build half way. Now I want to drive the X-axis with a servo
motor for some tests. That I want to do AS CHEAP as I can. I dont want to buy
the final/nice controller, amps and servos quite yet at this time.

I ONLY want to buy the final servo for the X-axis now. But all the rest to
actually drive it, I want to buy as scrap on ebay. Just scrap parts that will not
be used eventually. Just so I can run the real servo on my X-Axis for testing.

Later I want to be able to use the servo on a "real/good one" controller/amp.
But for that to happen I need to now how thing communicate with each other so
I know what dependencies there might me.

Thank you all in advance

[Al_The_Man]

You may want to get a grasp on the many different forms for interfacing between the basic components, In this respect it is hard to come up with one simple definition.
Very basically there is an HMI interface, Motion controller, PLC or general I/O control and Servo stepper drives or amplifiers.
Initially you may start with a CAD drawing, this is converted using CAM program that converts the CAD to machine profile, tool offset, feed rate etc, this is converted in the CAM to G/M/S/T code using a Post Processor for that particular CNC.
This is where the similarity ends, the CNC controller takes the G code and converts to the required trajectory requirements to produce the signal for the drives to perform the programmed motion.
Controllers such as Mach, output the signal via Step Dir means out of the parallel port and leave it up to the drives to complete the motion, IOW there is no feedback to Mach, even with servo encoder feedback.
In true closed loop systems such as most high end commercial, the motion controller calculates the trajectory and controls the motion via Analogue/PWM/Digital drives or in some cases step dir. with encoders back to the trajectory controller
And as mentioned, the CNC or motion is controlled by a common dedicated controller for G or position control, the M, S & T are usually handled separately and both CNC and PLC communicate with each other.
This is only a very cursory explanation as they are varied in nature.
Al.

[computernc]

@Al_The_Man
You say: "G-code is generated using a Post Processor for that particular CNC" .
What exactly is the CNC in that case? The particular controller?

Also you say: "both CNC and PLC communicate with each other."
If PLC is the controller, what is the CNC in that case? It can not be the PC since that does not play a role after G-code generation.

What I want is a true closed loop system. So what/where is the CNC part in my "drawing".


CAS -> CAM -> G-code -> PCL -> Servo -> Encoder |
| |
<-----------------------

Thanks


BTW. What is a gear or belt breaks. Then the "closed loop" is still intect but the machines position is wrong? I there a way to get TRUE closed loop systems?

[Al_The_Man]

The post processor has to know what controller you are using as there are differences in methodology, as just an example, the method used for circular interpolation is different between some systems, i.e. Radius/Dia and Relative/Absolute, so the Post.P. has to know which is in use in the machine in question, many other examples.
The machine CNC control is basically the machine motion control system, lathe/mill/router positioning, etc, the CNC side is considered the part of the machine that takes care of the servo/stepper motion, the general machine control M/S/T code does not require high speed updating so it is usually handed off to either another process in the software or a separate processor entirely.
There are so many variations, it is hard to come up with a simple definition.
If you want near to, or actual closed loop servo, then something such as EMC with motion card, Dynomotion, Galil Motion etc.
As for motors, steppers are generally not closed loop, for this you require servo's with their PID loop feedback.
Your example of a system where the encoder is on the motor and a drive belt breaks, then no, the system does not know this, you would need and encoder on the final operator.
Al.