Re: [CAD_CAM_EDM_DRO] Servo Motors.

james owens wrote:

> From: "james owens" <wotisname@...>
>
> Hi All,
>
> Can anyone explain servo motors and their controls to me in enough
> depth so that I can put together an axis.

Whew! Not a small topic!

If you mean, find an appropriate motor and servo amp and hook them up,
maybe I could guide you through that. If you mean, design your own
servo amp, that would be difficult, indeed! I have done it, but it
wasn't
easy, and I had to learn a lot along the way.

The simplest servo system uses a brush-type DC permanent-magnet motor,
a DC tachometer, and a position encoder (either linear, or on the
leadscrew).
The CNC system provides a desired velocity command to the servo amp
as a -10 V to +10 V signal, with zero volts being "don't move", and the
extreme ends of the range meaning move at maximum speed one way or
the other. The encoder provides position feedback to the CNC control.
Based on difference between the current position and the desired
position,
and the speed of the move currently in progress, a new velocity command
is computed every CNC cycle to minimize the position error.

The servo amp has two nested control loops. The outer loop takes the
velocity command from the CNC control, and compares it to the
actual velocity as read by the DC tachometer, and produces an
error signal. This signal is limited by a current limit circuit, and
fed
to the inner loop, which compares this error signal with the actual
motor
armature current. A signal is produced that varies the voltage applied
to the motor armature, either by a PWM (Pulse Width Modulation)
scheme, or by a linear amplifier.

The above describes the classic servo system used on machine tools,
industrial robots and other positioning systems.

Jon