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From: als@bohra.cpg.oz (Anthony Shipman)
Newsgroups: comp.robotics
Subject: Re: Help with Bresenham line DDA and XY tables: they don't work together!
Summary: easy in theory
Keywords: bresenham DDA CNC stepper motor
Message-ID: <1991Mar22.120627.14537@bohra.cpg.oz>
Date: 22 Mar 91 12:06:27 GMT
References: <2237@wet.UUCP>
Organization: Software Division, Computer Power Group
Lines: 25


I think what you want is to have the X and the Y motors run at constant speeds
during the table motion.  Then the path of the table will be a straight line
(to within the resolution of the steppers).

The ratio of the motor speeds determines the angle of the table's path. If you
want to be able to move it at any angle you will need continuous speed control,
i.e. an analogue speed controller.

If the motor speeds are discrete then the path angle can in general only 
approximate the desired angle on average.  This means the motor speeds will
have to keep changing as the path angle is corrected.  You can correct the 
angle frequently like the DDA algorithm does to get a smooth line but then your
motor speed flutters.  You can correct less often to go easier on the motors
but then the path has greater deviations and corrections.


I think you can only get what you want with continuously variable speed control
on your motors.  The equivalent of this in the graphics world is the vector 
graphics display.  The DDA is a discrete approximation for raster graphics.
-- 
Anthony Shipman                               ACSnet: als@bohra.cpg.oz.au
Computer Power Group
19 Cato St., East Hawthorn,
Melbourne, Australia
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