Re: [CAD_CAM_EDM_DRO] Re: stepper vs. servo

At 22:43 10/02/2004, you wrote:

>Les,
>
>Well then let's just confine ourselves to the second half of my
>statement. Steppers, once tuned--and I disagree with your assertion
>that "tuning" is required--need no further attention. (steppers DO
>need proper "selection". But not tuning.)
>
>Now let's address your second assertion, in a fun way:
>
>No matter how carefully you set up SERVOS, one day you will hit a
>particular set of circumstances that will make them lose steps. It is
>most likely to happen if you are demonstrating the machine or just
>finishing a very expensive and time consuming job..."
>
>But you won't know it! Only when the part is rejected because it
>doesn't meet spec will you find that the following error ALWAYS
>PRESENT with servos, has caused the actual toolpath to be other than
>intended... Because the g320 and similar can be off by up to 128 steps
>(encoder counts, really) before a fault is issued...
>
>If you're going to defend things by using worst case scenarios, let's
>apply them across BOTH systems being considered...

It is worth remembering that with a servo system, if the motor is moving,
it is doing so *because* there is a discrepancy between the current
position and the position that it is required to be in. So with a servo
system, the only time you know that there is no positional error is when
the motor is stopped. As you increase the load that the motor has to shift
(e.g., by increasing depth of cut), the size of any positional error will
tend to increase.

In contrast, with a stepper-based system, as long as the system doesn't
lose steps (i.e., as long as the motors are sized correctly for the applied
load), there is no change in the accuracy of the system as the load is
increased.


Regards,
Tony