Re: [CAD_CAM_EDM_DRO] What makes a motor a servo?

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