Re: Low cost Servo Controller

It seems that most lower cost CAM software is designed for stepper use,
and due to the simplicity of the stepper system, that is where most first
time CNC'rs start out at. It sounds like your idea's are in the line of
designing
a servo system to be controlled by pulses like those supplied to the
stepper
systems. I would be interested in your cost goals for a pulse incremental
servo drive, like those systems that are readily available.

Steve Carlisle

----------
From: David Howland <dhowland@...>
To: 'CAD_CAM_EDM_DRO@onelist.com'
Subject: [CAD_CAM_EDM_DRO] Low cost Servo Controller
Date: Friday, September 10, 1999 12:24 PM

From: David Howland <dhowland@...>

I am working on specifications for a Servo driver board which will allow
any low or medimum cost DC motor to be converted to a Servo motor and
connected to a parallel I/O board. I hope this electronic assembly will
allow high performance motion control for a reasonable cost. Steppers +
Stepper controllers seem to be less expensive and easier to use than Servos
+ Servo controllers. Many of us use steppers and put up with speed -
torque limitations. I know that many low cost DC motors will not perform
as well as a low inertia high quality servo motor, however the board
development I am considering will out perform most steppers for less or
equal system cost.

I want to use a PC Parallel I/O board even though it will cost more than
the parallel printer port, for two reasons. First I want some room to
configure several motors into servos and handshake limit switches and other
functions to the board and the PC. Next I want to get a parallel I/O board
with a counter / timer so that my program will have some reasonable timing
to pace things along.

The board design concept will allow the servo loop to be closed at the
motor shaft in the traditional manner with an encoder or from an external
feedback means from the actual position of the moving components (after
backlash), such as from an encoder mounted on the table itself.

The questions I have, are about how such hardware drivers are patched
into canned application programs. After writing some test routines through
the new board, I'll likely get around to making it run with some CAM
software which I haven't selected and know nothing about at this point.
I'm sure there are multiple formats between application programs, through
interface drivers, resulting in motion. No doubt that can be as simple as
step and direction and get more complex from there. What makes since in
terms of communication from the interface hardware that fits the kind of
software a hobby CNC person may find within budget? If you wanted to hold
down the cash outflow, and were in the stepper motor budget, which software
might you patch to your stepper motors through the step motor driver? How
do you glue the hardware and software together without 22 e-mails and
the clock striking one?

Has anyone seen high performance Servos with Servo controllers which
competed in price to the stepper and stepper controler counter parts? I've
picked up a few surplus servos to investigate their controller
requirements. The simple ones seem to be a brush type DC motor tied to an
encoder wheel. These will work out of the box with the new board. The more
complex servos, seem to be multi phase brushless motors and more complex
encoders. They are certainly in budget at the surplus stores, but the
missing controllers and drivers may not be. It seems to make since to
create an electronic interface which is low in cost and will allow the use
of Servo technology on a reasonable budget. If that makes since, perhaps
Servo like performance can be obtained near the same cost as stepper
performance.

Will someone make a list of stepper controller costs vs. Servo controller
costs plus motor costs, with a little performance data one might get at
each cost? Aside from the main stream sources such as Minarik or the
likes. What does it look like as you travel up the performance - cost
curve for "underground" motion controller components and difficulty with
CAM software interfacing? Even more important, What kind of CNC
performance might one end up with along the way? Frankly, I hate
dissapointment, even when it's my own fault for not spending enough money.



David Howland


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