Re: Servo controllers for table top machines.

While I am more of a fan of Servos than steppers, I can see where
both have purpose and application. While not always true, most
stepper driven systems are completely open loop - in other words
blind.

The motor to sent to a given position, it may or may not actually
get to the destination.

Servo systems are, for the most part, closed loop. So, the motor is
told to move to position X, if it gets there, great, keep working.
If it can't get to it, alarm out and prevent further damage.

There are many aspects of a machine tool or machining process that
can prevent an axis from reaching it's intended position:

Too fast of rapid rates
Too fast of acceleration/deceleration rates
Screw-nut not being lubricated
Ways not getting lubrication
Chip build-up on the cutter increasing cutting forces
Re-cutting of chips during a cut
Tools dulling and not being replaced in time
Overloading of a cutting operation causing too high of cutting forces

The list goes on and on. Closed loop systems prevent further damage
to a part by preventing the operation to continue when something is
wrong. Many folks will state that a properly designed system will
have enough power to overcome all problems, well, that is fine if
you believe in miracles. Murphy's laws seem to always overrule the
best of engineering.

Servos are natively a closed loop system whereas steppers are an
open loop system. Stepper systems can be designed as closed loop
systems, but the complexity does rise dramatically to achive the
same reliability to that of servos.

What I have found very interesting and fascinating about all of this
is in basic ink-jet printers. Some Epson printers use a very cool
closed loop servo driven system. Rather than a rotary encoder on
the motor, they use a linear encoder system for the utmost
in "closed loop" accuracy.

All methods of motion control have their advantages and
dissadvantages. So, I don't really think one system is supperior to
the other unless you consider the application which includes
economics and simplicity in our application.

Just my 2 cents

Chris





--- In CAD_CAM_EDM_DRO@yahoogroups.com, William Carr <Jkirk3279@...>
wrote:

>
>
> On Dec 17, 2006, at 7:53 PM, enytned1 wrote:
>
> > It sounds like a micro stepping system will provide the same
> > repeatability and precision as a Servo system like the one from
> > camtronics. Then why are there so many companies saying that

servo

> > will be a better system?
>
>
> I have a Precix 9000 that uses stepper motors. There is a

problem

> with the z axis if the z axis speed is too high, the motor height
> starts to slip.
>
> I recall I was doing a 1/2 inch deep 2" x 2" cutout in foam for
> practice and was puzzled to see it started 1/2 inch deep and

ended

> 1/4 inch deep at the end.
>
>
> I was told to turn down the z speed. But that converting to

servo

> motors would also solve the problem.
>
> So I second the question, why do companies say that servos are
> better? Marketing?
>
>
> William Carr
>
> [Non-text portions of this message have been removed]
>