Re: true closed loop vs. hobby closed loop

Hello,

In your commercial controls, if one axis has an increasing following
error, the OTHER axes are compensated to keep the tool on path.

The way to think of it is this: With your "big iron" controls, the
loop is closed at the control, and therefore the control ALWAYS knows
exactly where ALL the axes are at any given point in time.

With step-servos, as far as the CONTROLLER is concerned it's driving
high torque, high speed steppers; open loop. The DRIVE is tasked with
carrying out what the controller asks/tells it to do. It does its
best, but will not let the controller know if anything is amiss UNTIL
the FE limit (128 steps in Geckos) is reached/exceeded. So a step-
servo machine can actually be anywhere within the 128 plus or minus
steps an ANY or ALL axes at any given time and the CONTROLLER HAS NO
IDEA...

That's the difference.

In the worst case situation you describe, the step-servo and true
servo are as you say; the same.

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "snagglexr650"
<snaggletto@c...> wrote:

> Hello,
> I work in a machine shop and we have several retrofitted
> lathes/mills using OpenCNC and various motors/drives including
> Yaskawa servo motors/drives, we also have a Haas VF4 VMC. These

are

> all high dollar, industrial level cnc controls, with "true" closed
> loop feedback from the encoders.
>
> It seems to be widely considered that true closed loop actually
> allows the computer control to self-adjust the motors if error in
> positional accuracy has occured. However, all the machines we

have,

> including the Haas (not a retrofit) have an allowable following

error

> (FE) on each axis. If that FE is exceeded, the drive faults out

and

> cuts power to the servo motor. If the positional error falls

within

> that FE range, then inaccuracy does occur, it's happened to me.
> Furthermore, this FE is used in axis crash situations. Say you

crash

> Z into your vice/part/table, as the axis stops moving due to

hitting

> a solid object, FE is exceeded and the drive faults. Attempted
> positional compensation via the control would simply drive the axis
> into the part harder and cause more damage to the machine.
>
> Now, with that in mind, compare the +/- 128 counts 'FE' on the
> Gecko drives. If positional error exceeds 128 counts, then the

drive

> faults out and servos stop.
>
> In either scenario, it appears that the same thing happens, and
> that even in the case of 'true' closed loop controls, there is no
> attempt to correct the positional accuracy, just a fault.
>
> My question is where is the difference? I know software is the
> key to closing the loop. Is it simply that with Geckos, your
> step/direction software will happily continue to run the program
> without knowledge of the drive faults, and that on the Haas
> control/OpenCNC software control the software is aware of the fault
> and stops running the program and displays an 'excessive following
> error exceeded' message?
>
> Is this true? If not, then please explain to me the difference
> between hobby closed loop via Gecko Rutex etc... and
> industrial 'true' closed loop via our Haas VMC and OpenCNC retrofit
> control.