RE: [CAD_CAM_EDM_DRO] Ordinary AC motors & position control

Any servo positional drive control will dither between each edge of signal
from it's encoder.

On a proper PID control system it's the PID parameters that effect this all
of which should be adjustable.

There are three types of common control motors used in CNC:-

1 Stepper, these do not use any form of PID control, you tell them to step
forwards or backwards with a pulse. A step is normally something like 1.7
degrees, but depending on the controller they can be made to microstep, i.e.
have steps within this 1.7 degree angle, but they will losse the micro
position is power is lost, and if the load or acceleration are too hi they
can miss complete steps. Normally they work blind, so you feed steps to them
and hope they don't loose any, there is not normally an encoder to feedback
any error. They also have issues of resonance at some frequencies of
operation.

2 DC servo motors, also referred to as brushed servos, which use a DC servo
control card that should have a PID control loop which feeds a current
control loop. These use an encoder so the control circuit is fed with a
position (or velocity) command, then the motor is powered to come to that
position or velocity. For DIY use we use step direction signals and a
control card with the PID control built in. For commercial CNC machines they
have an analogue signal which relates torque or speed command to a control
PCB, but the positional control is done within the CNC control cards.

3 AC Servo, also referred to as brushless servo's. These are like a three
phase AC motor, and most have an absolute positional sensor on the motor so
the angle of the stator is known. If you know this then you know how to
energise the phases to control that position. Most modern machines use
these. They require a special controller, though over the past few weeks
Granite Devices have released a step direction control card that works with
AC or DC servo's and could well set a new standard in DIY CNC systems.



AC and DC servo's have this dither issue, and will always come to position
if they are set up right.



I've tried to keep this simple, and hopefully it gives enough info to start
you off on the basic differences.



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of maxnc15
Sent: 19 June 2007 03:58
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Ordinary AC motors & position control



I'm asking for some basic knowledge. I've seen some motors that
normally spin free lock up pretty solid on a position by use of
a "dither" frequency to equalize both directions of rotation. Slight
adjustments can move the motor one way or the other. Now can this
principle be applied to ordinary AC motors? I understand it works with
DC motors.

If not, is there a way to easily convert an AC motor to DC?

The reason I ask is new AC motors can be inexpensive relative to servo
motors of similar power. In my application I'm more tolerant of
backlash than expense.

Thanks in advance.





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