Re: What makes a motor a servo?

> as soon as it lags more than 180 degrees (2 full steps) the torque
> violently reverses,
> and the motor slams to a stop

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Alex Holden wrote:
>
> >On 6 May 2005, at 8:30 pm, Pete Brown ((YahooGroups)) wrote:
> >
> >
> >>What about steppers with encoders then?
> >>Such as:
> >>http://www.maxnc.com/page13.html
> >>
> >>
> >
> >It has closed loop control so I'd say it's a servo, but I don't
> >understand what they gain by using a stepper instead of a DC motor-
I
> >thought the only reason you would use steppers is so you can use
open
> >loop control and avoid the complexity of an encoder and loop
> >controller. Surely this system combines the worst of both worlds?
> >
> >
> >
> Right. The only advantage is that you can DETECT lost steps. But,
without
> exotic control software, like Mariss at Gecko is developing (the
> "unstallable"
> stepper drive) it doesn't really solve the problem. The big
problem with
> steppers is the torque falls off rapidly with increasing speed, due
to the
> high inductance and the high pole count (50 on a standard 200
step/rev
> stepper). Two-pole DC brush motors only reverse the current in two
sets
> of coils
> each time a commutator segment crosses the brushes. Each coil on
the
> armature
> is pretty small, so the current can be reversed easily. When a DC
brush
> motor
> stalls, it gives full rated torque right down to zero speed. When
a
> stepper stalls,
> as soon as it lags more than 180 degrees (2 full steps) the torque
> violently reverses,
> and the motor slams to a stop, and produces no usable torque until
the
> step rate
> is brought down close to zero. The stepper just "gives up" while
the DC
> brush motor
> continues to give its maximum torque against the load.
>
> Another difference is that steppers produce great heat at idle
unless an
> idle current reduction
> scheme is used. They also suffer from a lot of iron heating when
run at
> high speed (over
> 1500 RPM, say). They have no torque reserve, they just put out
whatever
> torque is available
> at the current speed. Servos can be set up with a continuous
current
> (or torque) limit, and
> a peak limit. They can run all day near the continuous limit, but
still
> have a reserve of 4 to 8
> times greater torque for sudden accelerations.
>
> Jon