Broken quadrature encoder circuit on TI F281x DSPs? - Page 2

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Re: Broken quadrature encoder circuit on TI F281x DSPs?
On Thu, 25 Aug 2005 16:32:52 -0300, Ricardo

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The quote looks right here, too, though my original post had one more
space on each of the upper lines (and the numbers line).  I'm not sure
where that space went.  Or where it came from, assuming the OP was
correct.

Regards,

                               -=Dave
--
Change is inevitable, progress is not.

Re: Broken quadrature encoder circuit on TI F281x DSPs?
On Thu, 25 Aug 2005 10:17:05 -0700, Chris Carlen

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    The only way to resolve this problem is to post screen captures of what
*we* see when we look at your message.  I've started the ball rolling by posting
"Diatribe.jpg" to ABSE.

    O wad some power the giftie gie us To see oursel's as ithers see us! It
wad frae monie a blunder free us. And foolish notion; What airs in dress and
gait wad lea'e us, And ev'n devotion! (R. Burns)

Jim



Re: Broken quadrature encoder circuit on TI F281x DSPs?

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 Below is the diagram from your original post. I put it into
 a hex editor.  All "s" chars below are where there was a 20H
 (space char) and the 0A char is your newline. I've also
 added a blank line between waveform pairs of lines.

ssssssssss______ssssss______ssssss______ssssss______0A
B:s______ssssss______ssssss______ssssss______0A

ssss___ssssss______ssssss______ssssss______ssssss___0A
A:ssss______ssssss______ssssss______ssssss______0A

ssssssssssssssssssssss______0A
Z:s__________________ssssss________________________0A

 You will see that on all lines where the first character
 is a space, then an extra space has been added. I think
 this was done by your newsreader when you generated the
 post. AFAIK this is a characteristic of some newsreaders.

 Conversely when reading your post, some newsreaders will
 drop the extra space (when it is the first char on a line),
 whereas others will not.

 For example, in Turnpike (same newsreader as JW) your post
 lined up perfectly, whereas in this newsreader (Pluto) the
 one-space-offset is kept and displayed.

 I suspect this is why Fred Bloggs and Win Hill post ascii
 diagrams with dummy chars as the leading chars on each line.

 As a matter of interest, all my typed text here has a leading
 space on each line.  Do you see that space or not?

--
Tony Williams.

Re: Broken quadrature encoder circuit on TI F281x DSPs?
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Yes, I see leading spaces on your text, not on your timing diagrams with
spaces mapped to "s".

I also see that the waveforms are not appearing to you as I see them.
Interesting.  It seems that spaces are not a reliable means of spacing
ASCII diagrams in news posts.

Unless as you mention about Win's posts, one puts a leading non-space
char on each line?

Thanks for the input.  I was begining to think folks were going
overboard in hashing the issue.  But now I have learned something.

Good day!




--
_______________________________________________________________________
Christopher R. Carlen
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Re: Broken quadrature encoder circuit on TI F281x DSPs?

(snip)

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It came out fine on Netscape 7.1.

-- glen


Re: Broken quadrature encoder circuit on TI F281x DSPs?

   ...

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It could have been worse. A dash is a dash, so there was only a little
displacement between successive lines. But consider his labeling:

                   N-1 N  0  1  2  3  4  5 ...
          ______      ______      ______      ______
B: ______      ______      ______      ______

I suppose he meant something like

     N-1     N     0    1     2     3     4     5     ...
          ______      ______      ______      ______
B: ______      ______      ______      ______


Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

Re: Broken quadrature encoder circuit on TI F281x DSPs?

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<snip>
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But you are about to reset, so all you are talking about is a one count
skew in the MAX possible number, just before reset.

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  At least they gave a RESET, and gave you a choice on the mode :)

  Sounds like they tried to "cover the bases", and the first option is
preferrable, if the encoder allows; if it has a wide Z, then you need
2nd option, but as you say, that can have Zero exit errors, but as
long as the edges swallowed inside Z are stable, you should get
stable counts on either side, again with a small +/- 1 error on
'absolute' location.
  Normally, you use much finer quad steps, than your system needs, so
this level of detail should not be a practical issue.

  [ Level sense on Z would probably have been better design, but
maybe they wanted to avoid wide/unknown dead-bands ? ]

-jg


Re: Broken quadrature encoder circuit on TI F281x DSPs?
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Yes.  But actually it is Ok, because there is a period register for the
timer that will ensure it can't ever count beyond the encoder
resolution.  Thus, the if the Z pulse lags and the first revolution
starts with an under-count, then the Z pulse will reset things the first
time and be ignored thereafter.

The only deficiency is that it cannot reset in the reverse direction.
Since our application is mainly one-directional, this isn't a serious
problem.  Nor it's more of a disappointment.  There are times when we
will want to manually move our shaft forward and backward over the zero
point for alignment purposes, so it is actually essential that we can
maintain accurate 1/4 cycle counting throughout.  It seems that is
possible, as long as one forward pass is first made through Z.

Also, I will have to guarantee that Z rising edge occurs after the two
phases are high.  That is what really bugs me.  The correct
implementation would be a condition sensitive reset, when A=B=Z=1.  But
they have Z edge sensitive instead, which also contradicts what they say
in the documentation.

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Yeah, perhaps.  Are there wide variations in the implementation of index
signals on encoders throughout the industry?  Unfortunately, I only have
worked with BEI.  Oh actually, we had an older brand that we've since
replaced which also worked just like BEI.  Z pulse was fairly in line
with a single pulse of one phase.


Good day!



--
_______________________________________________________________________
Christopher R. Carlen
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Re: Broken quadrature encoder circuit on TI F281x DSPs?

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  the above looks correct to me.
   depending on how you want to do it.
  (B) input is good for counting on the raising
edge because (Z) follows it.
  (A) can be used to determine the direction
you are going when (B) raises.

  looks simple to me.



Real Programmers Do things like this.
http://webpages.charter.net/jamie_5


Re: Broken quadrature encoder circuit on TI F281x DSPs?
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It's not quite so simple. Consider what happens when the encoder
oscillates back and forth across a B transition without A changing at
all. The old solution is a state machine. The new solution (mine) with
hardware uses two XOR gates and yields double or quadruple the basic
resolution of the wheel.

Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

Re: Broken quadrature encoder circuit on TI F281x DSPs?

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Not sure I'd call Nov 10 1981 'new' ?
XOR based quad solutions have been around a very long time :
You need either a pulse generator ( hard in an ASIC, or FPGA)
or some FF's, and a faster clock, to do effectively the same thing.
In today's digital fabric, the FF's are cheaper, and simulate
correctly, and avoid issues with clock domain crossing...
to .
-jg


Re: Broken quadrature encoder circuit on TI F281x DSPs?

   ...

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The only pulses are generated by the encoder. I don't know what you mean.

Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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