Prev: Running MIPS on Altera board
Next: Finally, Death of the 3.5 inch floppy disk
From: -jg on 26 Apr 2010 16:59
On Apr 27, 6:19 am, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat>
wrote:
>
> If you only care about speed and not direction, one edge on one channel may be sufficient.

A purist could argue, that for position, all edges should be used,
(you can easily discard precision later) but for speed readings, only
the same edges should be considered, as quadrature encoders are NOT
guaranteed to have 50.00% duty cycles (or even 90.00' alignment).

-jg


From: Magnum on 27 Apr 2010 05:17
"princekcs" <princekcs(a)n_o_s_p_a_m.gmail.com> wrote in message
news:Lp6dnSYNcdAENUjWnZ2dnUVZ_jidnZ2d(a)giganews.com...
> why is it necessary to clock on rising and falling edge when reading a
> quadrature encoder?

Because if the encoder changes direction, you will clock on the
wrong edge and slowly lose registration.

Try it for yourself as a paper exercise; keep changing direction
over a very few steps and watch your position counter slowly drift!

And it doesn't have to be a deliberate change of direction, either.
Torsional
vibration when the encoder is sitting on one of the edges will cause a
steady creep in one direction.

Let me try to phrase that a little better.

Let us suppose that you are operating bi-directionally, clocking
from the rising edge, and the encoder is sitting over what would be
your falling edge. Each vibration backwards will give you what appears
to be a rising edge, causing a downwards count, but each vibration forward
will not be registered because it is sitting on the falling edge which you
are
ignoring.

And therby hangs a tale! In a recent project I wa controlling a stepper
motor
with such an encoder and was losing accuracy for the reasons stated above!

The algorithm to avoid the problem comes from a much larger state machine
than
is normally used ... you have to consider what you are going to do if you
receive
two opposite transitions from the same input without there being a
transition on the
other input.



From: Jim Stewart on 27 Apr 2010 13:23
-jg wrote:
> On Apr 27, 6:19 am, Spehro Pefhany<speffS...(a)interlogDOTyou.knowwhat>
> wrote:
>>
>> If you only care about speed and not direction, one edge on one channel may be sufficient.
>
> A purist could argue, that for position, all edges should be used,
> (you can easily discard precision later) but for speed readings, only
> the same edges should be considered, as quadrature encoders are NOT
> guaranteed to have 50.00% duty cycles (or even 90.00' alignment).

Machine-tool grade encoders had damn well better be
guaranteed to have accurate angular alignment.

From: Spehro Pefhany on 27 Apr 2010 13:58
On Tue, 27 Apr 2010 10:23:50 -0700, Jim Stewart <jstewart(a)jkmicro.com>
wrote:

>-jg wrote:
>> On Apr 27, 6:19 am, Spehro Pefhany<speffS...(a)interlogDOTyou.knowwhat>
>> wrote:
>>>
>>> If you only care about speed and not direction, one edge on one channel may be sufficient.
>>
>> A purist could argue, that for position, all edges should be used,
>> (you can easily discard precision later) but for speed readings, only
>> the same edges should be considered, as quadrature encoders are NOT
>> guaranteed to have 50.00% duty cycles (or even 90.00' alignment).
>
>Machine-tool grade encoders had damn well better be
>guaranteed to have accurate angular alignment.

What do you reckon the accuracy of quadrature is?

Here's a typical Renishaw read head datasheet:

http://resources.renishaw.com/download/%28a76a7e13ca474c538f1ff1f9e301418b%29?lang=en&inline=true

The only hint I see is a 4:1 "safety factor" on the counter clock
frequency max.

From: Jim Stewart on 27 Apr 2010 14:44
Spehro Pefhany wrote:
> On Tue, 27 Apr 2010 10:23:50 -0700, Jim Stewart<jstewart(a)jkmicro.com>
> wrote:
>
>> -jg wrote:
>>> On Apr 27, 6:19 am, Spehro Pefhany<speffS...(a)interlogDOTyou.knowwhat>
>>> wrote:
>>>>
>>>> If you only care about speed and not direction, one edge on one channel may be sufficient.
>>>
>>> A purist could argue, that for position, all edges should be used,
>>> (you can easily discard precision later) but for speed readings, only
>>> the same edges should be considered, as quadrature encoders are NOT
>>> guaranteed to have 50.00% duty cycles (or even 90.00' alignment).
>>
>> Machine-tool grade encoders had damn well better be
>> guaranteed to have accurate angular alignment.
>
> What do you reckon the accuracy of quadrature is?
>
> Here's a typical Renishaw read head datasheet:
>
> http://resources.renishaw.com/download/%28a76a7e13ca474c538f1ff1f9e301418b%29?lang=en&inline=true
>
> The only hint I see is a 4:1 "safety factor" on the counter clock
> frequency max.

In this case I would imagine the accuracy would be
determined more by the scale than the readhead.

But that's just a guess...
First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4
Prev: Running MIPS on Altera board
Next: Finally, Death of the 3.5 inch floppy disk