PID Control

[VNR]

Hi,
Why some machines have:

1) PID control between the driver and the motor (through the encoder).
Here the driver have an active function.

2) PID control between the CNC and the motor (through the encoder).
Here the driver have a passive function.

The second case seems to be cheaper (less expensive drive) but the first case is widely used.

Thanks in advance, and excuseme for my poor english.
Victor

[fkaCarel]

Can you explain what you mean with the second case? Oh, and your English is good enough.

Carel

[Al_The_Man]

I believe you are refering to in 1) is the type of control that is common here, used in through the parallel port systems, where the loop is not closed back to the controller.
However for the major commercial systems and PC based motion cards like Galil etc, the loop is closed back to the controller.
Item one allows for an economical way of using software only (non-Hardware based) control system.
Simply put. the second case costs more.
Al..

[VNR]

Originally Posted by fkaCarel Can you explain what you mean with the second case? Oh, and your English is good enough. Carel

The first case is:

CNC====>DRIVER====>MOTOR
..<======.......<=====ENCODER

<---PID--->.......<---PID--->
Widely used in real production environments.
Usually drivers Siemens, Fanuc, Yaskawa, Parker, etc.
Possible with EMC but I have not seen it in Retrofitting's, I looked for in Internet.

The second case is:

CNC====>DRIVER====>MOTOR
..<================ENCODER
Most retroffiting's that i saw with EMC.

<-----------PID------------->

Most common case in PC BASED CNC's (third case):

CNC===========>DRIVER====>MOTOR
............................<========ENCODER

(No PID, Open Loop) <----PID------->
Common with Mach3.
Widely used in routers.

In the first case there is a "intelligent" driver (PID included, more expensive), in the second case there is a "passive" driver (NO PID, less expensive), in the third case there are cheaper drives for hobby machines.

I have to do a retrofitting for a production machine (not for hobby) using EMC and i'm asking me why some machines use two PID instead of one.

With EMC i can use first and second case but i consider two PID as excessive, redundant and more expensive. So if there is no reason to use two PID i will use the second case but if there are a reason that i should consider i will want to know it !!



Thanks you,
Victor

[fkaCarel]

A servo motor has to close the loop somewhere. Normally it is done at the driver level. For a pulse/direction driver with the PID standalone you can have a look at the DIY servo on this forum. Double PID is used on machines with an encoder on the motor and an encoder on the machine for measuring the real position. For final positioning the two have to be combined. So there is a PID for the motor and a PID for the system.

Carel