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'[ot]: what's the difference between absolute and '
2002\12\19@224744 by rad0

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hello,

I'm looking over enclosed linear encoders, and I see there
are two types, absolute and incremental,

what does this mean?

Of couse I want a 'raw' sensor that I can hook up
my pic to, but I don't understand this term


thanks for any help

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2002\12\19@231332 by Josh Koffman

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This is just a guess, but I'd assume that absolute encoders give you an
output that is referenced to one of the ends, ie 5cm from the end. An
incremental encoder would give you speed and direction, ie moving 5cm/s
CCW.

Again, just a guess.

Josh
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rad0 wrote:
> I'm looking over enclosed linear encoders, and I see there
> are two types, absolute and incremental,
>
> what does this mean?
>
> Of couse I want a 'raw' sensor that I can hook up
> my pic to, but I don't understand this term

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2002\12\19@235314 by Spehro Pefhany

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At 09:47 PM 12/19/02 -0600, you wrote:
>hello,
>
>I'm looking over enclosed linear encoders, and I see there
>are two types, absolute and incremental,
>
>what does this mean?
>
>Of couse I want a 'raw' sensor that I can hook up
>my pic to, but I don't understand this term

Absolute encoder gives you position, typically in something like
a Gray code (a binary code which, like the usual, has 2^N states
for N bits, but only one bit changes at a time, which makes
things a lot easier- they can easily be converted back and
forth).

Incremental encoder gives you (usually) quadrature pulses that
you can count up/down to measure changes in position. You need to
set the reference point with a "Home" microswitch or whatever.
Some microcontrollers have hardware to handle quadrature inputs,
you can also use a (C)PLD etc. or the micro directly if the
speed requirements are modest.

There are also hybrid types- incremental with index tracks etc.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\12\20@004825 by rad0

picon face
Absolute encoder gives you position, typically in something like
a Gray code (a binary code which, like the usual, has 2^N states
for N bits, but only one bit changes at a time, which makes
things a lot easier- they can easily be converted back and
forth).

does this mean there is a unique number/output for each spot
on the encoder?

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2002\12\20@053304 by Alan B. Pearce

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>Absolute encoder gives you position, typically in something like
>a Gray code (a binary code which, like the usual, has 2^N states
>for N bits, but only one bit changes at a time, which makes
>things a lot easier- they can easily be converted back and
>forth).
>
>does this mean there is a unique number/output for each spot
>on the encoder?

Yes, otherwise it cannot give you an absolute position.

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2002\12\20@095220 by Hazelwood Lyle

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>>Absolute encoder gives you position, typically in something like
>>a Gray code (a binary code which, like the usual, has 2^N states
>>for N bits, but only one bit changes at a time, which makes
>>things a lot easier- they can easily be converted back and
>>forth).
>>
>>does this mean there is a unique number/output for each spot
>>on the encoder?
>
>Yes, otherwise it cannot give you an absolute position.
>

The methods can vary between encoder types. Here at work we use an
absolute encoder that connects just like an incremental one. The signals are A,B,Z, and their complements. During operation, you get quadrature pulses on A and B, 90 degrees out of phase, Z gives a single ref pulse per revolution.
The "absolute" part comes on every reset. The encoder then bit bangs
a serial port on the A line, giving total revolutions from ref point,
then position within current revolution, all as ASCII

The result is an encoder that has the lean wiring of a relative unit,
but with the advantage of not needing to home on power up.

The units I describe are on five of our robots, and are probably
proprietary, but the info may get you thinking. It would be fairly
simple to implement one of these with a small PIC and the internal
EEPROM..

Lyle

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2002\12\20@133357 by Spehro Pefhany

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At 11:08 PM 12/19/02 -0600, you wrote:
>Absolute encoder gives you position, typically in something like
>a Gray code (a binary code which, like the usual, has 2^N states
>for N bits, but only one bit changes at a time, which makes
>things a lot easier- they can easily be converted back and
>forth).
>
>does this mean there is a unique number/output for each spot
>on the encoder?

Exactly. So you might have a 10-bit encoder, which would require
10 outputs and would have 1024 unique ranges of position from the POV
of the encoder.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\12\20@135502 by Harold Hallikainen

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I'd say that an absolute encoder gives you a binary (or Gray code) output indicating the absolute position at that point in time. An incremental encoder just indicates how far it moved and the direction, but does not indicate what the current position is. This is typically done with quadrature encoding (two streams of pulses. Which pulse has the edge first indicates direction, while you count the pulses to indicate how far you've moved).

Years and years ago I designed a Z80 system to steer a "Big Ugly Dish" for TV stations. This was written in Turbo Pascal and burned to ROM. Our software would let the user (through a dumb terminal) schedule the recording of different programs off different satellites. After recording a program, the system would steer the dish and tune the receiver to pick up the next program. The dish had a pretty big motor that would move the dish. There was a "flag" on the back of the motor with an opto interrupter. We'd count pulses out of the opto interrupter to see how far the dish moved. We assumed the direction was the direction we had TOLD the dish to move. We also had to account for overrun of the dish (it didn't stop immediately when you turned the motor off). So, we had to turn off the motor a bit before the proper position, considering which direction we were coming from. Originally, we thought we'd run the dish all the way to one end where it hit the limit switch to sync our counts with the dish position. Turned
out, though, that this was not very accurate. So, instead, after moving the dish, we'd store the current count in battery backed RAM (a TimeKeeper RAM that also had our real time clock/calendar). After a power interruption, we'd assume the dish had not moved while the power was out.

    So, summarizing, THAT was an incremental encoder...

Harold



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