Re: Using CNC as DRO to save $$$

Wow...that's what I call a reply...thanks a bunch.

John

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <info@t...> wrote:
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Roy J. Tellason"
> <rtellason@b...> wrote:
> > On Wednesday 24 March 2004 06:37 pm, caudlet wrote:
> >
> > > In order to use the Software Controller as a DRO you need two
> > > things. You need a encoder mounted on your equipment (it
could
> take
> > > the form of the encoders on your servos if they can be
> installed) or
> > > even linear scales on the table. The encoder output needs to
be
> dual
> > > channel.
> >
> > Two questions on this part: Are you saying that this can only
> apply to servo
> > and not stepper setups? And, what do you mean by "dual
channel"
> here?
>
> Let me address the first question about using linear scales for
> encoder feedback for CNC control. The answer is yes and no (like
> that approach?). It would appear logical that if you put the
> encoder on the table itself and measured the position of that,
which
> is after all what you are really interested in, and commanded the
> motors to move to that position, you would have the best and most
> accurate system. The flaw in that approach lies in the method of
> postional feedback and the dreaded feedback loop. Most of the
hobby
> grade stepper to servo systems adjust postition like this: A
number
> of steps are commanded from the controller program. The encoder
has
> lines that are read by a optical sensor (actually a pair but that
> will answer question two later) that outputs pulses as it moves.
> These are distinct transitions (logic pulses) and are digital in
> nature. The feedback loop samples the commanded pulses from the
> computer and compares them to the pulses from the encoder. Any
> mismatch and an error voltage is developed. The job of the motor
> controller is to keep the error voltage as low as possible. If
the
> error becomes to great the drive will fault (somethings wrond and
we
> need to stop). Keep in mind that the controller software has no
> knowledge of any of this an "assumes" the machine has moved where
it
> was commanded. Only a fault condition will alert it to an issue.
> In this tyoe of system the loop is closed in the motor controller
> itself. Examples of this are Gecko 320/340 and Rutex 9XX series.
>
> What has this got to do with the type and placement of the
encoder?
> well the digital nature of the feedback loop is such that it
always
> has a tiny error and "hunts" between encoder lines for
equalibrium.
> This creates a condition where the motor rapidly oscillates
between
> two encoder lines (provided the encoder is connected directly to
the
> motor shaft) and the result is a impeceptable movement back and
> forth of the shaft and a "hum" from the oscillation. The amount
of
> movement is determined by the encoder count and the gain and
> dampening of the feedback loop controls (adjustable on all servo
> systems). Under conditions where the coupling between encoder and
> motion is tight (virtually no backlash) the oscillation presents
no
> problems. If we move the encoder to a position where there is
> backlash present we setup a condition I call the "blindfolded
> driver" syndrome. Imagine driving a car blindfolded and all that
> guides you is are two tones: high for right, low for left. Now
> image introducing a steering system that has a 1/2 turn of the
> wheel "backlash". Remember there are two tones. No tone is not a
> valid condition. What you get is overcompensation and it
amplifies
> itself and causes violent movements to attempt to correct much
like
> microphone feedback. Depending on the mass of the machine and the
> strength of the motors it might well walk itself across the floor
> and perform an e-stop by disconnecting itself from power!
>
> The solution is to eliminate as much backlash as possible which
> means the addition of precision drive componets like zero backlash
> ballscrews. Of course it that point you have remove most of the
> error in the system and the more closely reflects the actual
> position of the machine. So once you have the tightness on place
to
> actually use linear scales as encoders, you technically don't need
> them. Can you use them? Sure.
>
> Another thing for discussion is being able to close the loop in
> software at the controller (the loop is closed in software). The
> only hobby grade (read: affordable) software closed loop system is
> Jon's product that works with EMC. I will leave it for him to
> address how they would handle system backlash and how much they
can
> compensate for in the software loop.
>
> Answer for the dual channel question.
>
> The dual channel encoder is used in a configuration
> called "quadrature" output to increase resolution. Two sensors
are
> placed such that the signals are sensed 90 degs apart. As the
lines
> move past the sensors a line passes the A sensor and it goes high
> and stays there until it is completely through then it goes low
> again. Halfway through the line passing through A, another line
> passes through B causing it to turn on. You now have two
> transitions for the same amount of movement so you have doubled
your
> resolution. If we also count the transiton of A going from high
to
> low as the backside of the line passes, and do the same with B we
> have 4 transitions to count and we have four times the number of
> pulses....quadrature. That is why 200 lines create 800 pulses. By
> having two sensors we can determine which count comes first, A or
B
> and from that we can determine direction. The encoder just
> processes the signal and sends the pulses out of the channel
wires.
> It is up to external circuitry to figure the count and direction
of
> travel from the information.