Re: What makes a motor a servo?

I have been following this thread with great interest - plenty of
good technical information and solid reasoning. There are a couple
of points that need to be made though that seem to be getting
glossed over...

Why are there still steppers around if servos are so much better.
Well, that's like asking why are there still Porches and Celicas,
both appeal for different reasons. A stepper motor and drive is
cheap compared to a servo and driver. A stepper works great in
situations when it stalls, it doesn't scrap out a $10,000.00 piece
of material being machined. While I may not understand the technical
aspects, I can easily understand why I have steppers on my router
while wishing I has servos - my pocketbook being primary.

As for steppers loosing steps being a bad thing - well, frankly it
is a very bad thing - at least when it is not known. If the stepper
looses steps blindly, every movement thereafter is off by "Skipped
amount" - most probably resulting in scrap. For a pick and place
robot or a printer, the final dollar loss might be measured in
pennies. However, on a machine tool, the financial loss can be very
great. If the servo is overburdend, there is a way to measure the
overload - ammeter measuring current, or even heat generated by the
motor. Or, perhaps the servo board will fault if the motor cannot
keep up with the cutting forces. Atleast with a servo system, there
is a higher probability of getting good product from the machine
tool - there is feedback in several different forms (amps, heat,
stalling, faulting reported by the driver, etc.).

Another way of looking at steppers versus servos, years back, many
NC machines were programmed in Incremental coordinates, that lost
favor to Absolute coordinates for very similar reasons to the
problems with steppers. Steppers being like Incremental coordinates.


I have noticed that some of the newer printers are using both
steppers and servos in them. The last two epson printers I tore
apart had both. I suppose there are good reasons in both cases as
to why they are in the printer.

I agree with the post someone else made earlier, if servos were as
cheap as steppers, we wouldn't be seeing too many steppers anymore.
Industrial machine tools (with the exception of some EDM machines)
all use servos - I have not seen steppers on any serious machine
tools with the exception of some small "Half breed" machines.

So, until I find a way to get servos on my router for the same price
(motor and driver) as steppers, I will continue to use steppers.

> Couple of points:
>
> 1) Everyone gets hung up on torque when it's power that matters.
> Power is speed times torque, gearing changes the mix of speed to
> torque to match the load. Think Watts, not Nm or in-oz.
>
> 2) Stepper low-speed running torque is equal to the holding torque
> when microstepping. Why? No torque is invested in resonating the
> motor. Endless dyno tests prove that out.
>
> 3) An open-loop stepper should be derated to 50% of what it's

capable

> of. But why blame the motor when the fault is the way it's being

run?

> That same motor run closed-loop (a "50-pole AC servo motor" now)

can

> do considerably better.
>
> 4) You build in a 50% derating for steppers but you do not for
> servos. What happens as wear sets in and the load on the servo
> increases? A step motor is smart enough to shed a load if it is

too

> large. You call it stalling like it's a bad thing. A servo is too
> dumb. It will keep trying to move it while its operating point

moves

> into the self-destruct range. Gotta keep the playing field level

to

> compare fairly.
>
> -----------------
>
> There was a larger point I was trying to make. I don't think I did

a

> good job because it was missed so here goes again:
>
> Steppers (50-pole AC servos) are ideal for applications that have

two

> distinct operating modes; high-torque/low-speed and low-

torque/high-

> speed (cutting feedrates and rapids). The motor's speed-torque

curve

> naturally matches this type of application.
>
> On the otherhand, servo torque is independent of speed, making its
> efficient operating range speed very narrow. This makes correct
> gearing very important.
>
> Mariss
>