[axis_control]

Hi ,

Can a desktop pc be used in place (instead) of a programable logic controller for a cnc 3 axis mill ?

[Al_The_Man]

Usually only when it is used in conjunction with PC based CNC control or PC with CNC motion controller card such as Galil, e.g. Mach for PC based and Camsoft for PC motion card system.
Al.

[axis_control]

so what restricts the axis travel (distance) of a cnc machine?....is it the plc or something else?.

thanks!

[Al_The_Man]

The PLC is usually a separate controller than the CNC controller, although it can be a processor on the same BUS, they both have to communicate with each other.
The CNC controller is the one that controls servo's, the PLC is used for miscellaneous I/O.
Apart from any mechanical restriction, electronically the limit is probabally the mathematical limit of the CNC processor.
Al.

[axis_control]

The PLC is usually a separate controller than the CNC controller, although it can be a processor on the same BUS, they both have to communicate with each other.
The CNC controller is the one that controls servo's, the PLC is used for miscellaneous I/O.
Apart from any mechanical restriction, electronically the limit is probabally the mathematical limit of the CNC processor.
Al.

so does it mean that a 3 axis desktop machine can be controlled with the following combinations :-

1] computer + cnc controller = a complete cnc machine (here the computer does the work of a plc & sends signals to the cnc controller unit to operate the servos )

2] plc + cnc controller = a complete cnc machine (here once the plc is feeded with data from the computer, the computer can be either switched off & the plc can handle the work further.)

am i right ?..just asking as i am not still sure about .

[ger21]

1) Correct. The software control can either send signals directly to the drives, or to a motion control card like a Galil as Al mentioned.

2) Not entirely sure, but I don't think so. Perhaps for simple repetitive jobs. As Al said, typically the PLC is included to handle all the I/O needs of the machine.

3) normally what you would have, is - PC running control software which controls the motion and also uses the PLC for Inputs/Outputs.

[axis_control]

1) Correct. The software control can either send signals directly to the drives, or to a motion control card like a Galil as Al mentioned.

2) Not entirely sure, but I don't think so. Perhaps for simple repetitive jobs. As Al said, typically the PLC is included to handle all the I/O needs of the machine.

3) normally what you would have, is - PC running control software which controls the motion and also uses the PLC for Inputs/Outputs.

Thanks for replying....

1] why to use a motion control card when the control software running on the computer can directly operate the drives? ( if i have mis-understood the work of a motion card) ..or ......does the motion control card take inputs from the computer running control software & amplify the current to the motors simulating the control software program since a pc can,t do that. (so i should guess that there r three main electronic components in a desktop cnc 1] pc 2] motion control card 3] motors)

what basically i want to understand is .....can a cnc 3 axis desktop mill be made using the least amount of electronics?..just to keep the cost down.

[Al_The_Man]

There are mainly two types of PC controlled CNC, the software based system such as Mach, where the servo loop is closed externally, this is not a closed loop system, as per the likes of Galil motion card type.
Galil closes the loop via firmware in the card via encoder feedback direct to the card.
Mach requires semi-inteligent drives to close the loop.
With a motion card, the drives can be passive style, i.e. just condition the command signal and provide amplification, there is seldom position feedback to the drive.
Mach provides the CNC/PLC system in software in the PC.
Galil offer some internal PLC or an external PLC can be utilised.
Once the servo position command is downloaded to the Galil, it does not require the PC per se to operate as can be demonstrated by the stand alone version that has communication to the PC through the serial port, once the command is sent, the serial connection can be removed and the card will continue to operate the servo loop positioning.
If you look at the Mach site and Galil Motion site, there are examples of both.
The cheapest system is probabally going to be Mach based.
The above relates to PC based and not commercial CNC systems.
Al.

[axis_control]

There are mainly two types of PC controlled CNC, the software based system such as Mach, where the servo loop is closed externally, this is not a closed loop system, as per the likes of Galil motion card type.

Galil closes the loop via firmware in the card via encoder feedback direct to the card.

Mach requires semi-inteligent drives to close the loop.
With a motion card, the drives can be passive style, i.e. just condition the command signal and provide amplification, there is seldom position feedback to the drive.

Mach provides the CNC/PLC system in software in the PC.

Galil offer some internal PLC or an external PLC can be utilised.
Once the servo position command is downloaded to the Galil, it does not require the PC per se to operate as can be demonstrated by the stand alone version that has communication to the PC through the serial port, once the command is sent, the serial connection can be removed and the card will continue to operate the servo loop positioning.

Al.

sorry to reply late..i was a bit busy

ok..i think i have started understanding cnc a bit better than when i had joined this forum:rainfro:

i am not into electronics & hence i have to google to search the meaning of words to properly understand ur whole reply.lol! but that is getting all my doubts cleared...i like it.

ok...talking about Mach & Galil

1] Mach is a pc based controller software ...now what about the hardware part? Once mach is installed on the pc & the g code generated, through what media(electronic components) does the command reach the motors?(are any more electronics used between the pc & motors?).....

2] Galil is a motion controller card which fits into the PCI slot of the pc or external card is available.
Is a motion controller card also reffered to as a plc? i think this has been confusing me a lot
In a controller card like Galil .....r the motors directly connected to this card(galil) without any electronic component between these two ?

I hope my questions r not silly as i am confused due to reading about cnc from different sources & i think mostly there r two terms used for the same component & may be thts why i misunderstand it when both sources r refering to the same component.

Thank you for ur reply!...cheers!:rainfro:

[axis_control]

i think i should get the basic knowlege about cnc first by going through the build threads & then ask questions that r more specific...

[Al_The_Man]

Although PLC have advanced functions, they can generally only offer separate servo control positioning for each axis, IOW they normally cannot do interpolated (another good word) moves.
CNC servo control offers interpolated moves over multi axis.
Because a PLC is not generally a 'real time' process, it is relegated to the Miscellaneous control functions of a CNC machine.
Spindle, Tool changing, Coolant. Although not fast enough for servo update, it is considerably faster that mechanical operating devices such as Contactors and relays etc. Also some systems do offer PLC servo control, but have the draw backs listed above, so they are relegated to simple axis control tasks such as tool changers etc.
Both Mach and Galil, and most other CNC systems, require a Servo drive, or amplifier between the control and the motor.
Al.

[axis_control]

Thank you Al & ger21 for ur help. I sincerely appreciate it

for now, i am gonna do some research to get familiar to cnc`s.

Have a nice day..cheers !..

regards,
Pradeep