Re: [CAD_CAM_EDM_DRO] Re: stepper vs servo

Ian Wright wrote:

> Hi,
>
> This is something I've been pondering for a while and wonder if someone can
> answer one point at least. While it is obvious that stepper motors do just
> that and have discrete and easily seen steps, do servo motors not also have
> irregular or somewhat stepping motion?

No, they exhibit VERY smooth motion at very low speeds.

> As the normal DC servos are basically
> a DC motor with a set number of field coils and a certain number of segments
> on an armature, surely there will be points in the revolution which have
> greater torque than others.

No, by proper winding patterns of the armature, the torque ripple you mention
(and velocity ripple, too) can be eliminated. Cheap, non-servo motors do
exhibit these deficiencies.

> I know that in high precision servos, DC motors
> are not used and they use a solid rotor which is driven by induced currents
> which does give a very smooth but not very powerful motion but surely, in
> servos using DC motor technology as are usually found on machine tools,
> there is some 'cogging' action relating to the number of segments in the
> armature.

I have a number of wound-armature DC servo motors where I challenge you
to detect any torque ripple or velocity ripple! When built right, there is
less torque ripple form the motor winding than there is due to errors in
the grinding of the bearing balls!

> I also wondered about the starting performance of servos. In a stepper, the
> initial current produces as powerful a torque as when it is running at speed
> but, in a DC servo, presumably the current must build up the torque until
> the motor can overcome the friction - a more gradual effect if still only
> taking microseconds. Does this have any actual effect on a machine's
> capabilities or does it mean you need more powerful servos than you would
> steppers?

This is the job of the servo amp, and that is why it usually has 2 closed loops

inside the positioning loop. There is a DC tach or other velocity feedback
device that compares actual velocity to commanded velocity. Inside that
loop, there is a current loop that compares actual armature current to
commanded
armature current, as determined by velocity error. This allows current to be
raised very rapidly in a controlled manner, until the commanded velocity is
reached. The armature inductance of a typical servo motor is a lot less than
the inductance of a typical stepper, therefore the armature current can rise
more quickly. that is why small servo motors can run at tens of thousands
or RPM, while few steppers get up to 1000 RPM.

What difference does a microsecond make to a machine with a 400 Lb cast
iron table? The motor, leadscrew, belt drive, etc. can't transmit transients
in that range anyway.

Jon