RE: [CAD_CAM_EDM_DRO] Resolvers vs Encoders

This is all correct. Another way to look at it is two coils which change
their angle to one another. The resolver is sent a sine wave. The signal
goes through the coils and comes back with some phase change. The phase
change is proportional to the angle between the two coils. The resolver
board sends out a sine wave then it reads the difference in phase between
what it sent and what was returned, this provides absolute angle on the
resolver shaft. With a high frequency carrier wave, a resolver is immune to
external electrical fields. This allows the resolver to be as much as 600
feet from the decoding circuits.

15 years ago the support circuits for incremental encoders were pretty poor.
The alternative was an absolute encoder. Those required large glass wheels
in order to get high resolution for angle. In addition, encoders require
the decoding electronics near to the sensor, normally inside the encoder
body itself. Since the electronics were subject to environmental
conditions, it was quite difficult to put an encoder on the back end of a
motor which might get to 190 degrees F or more. A resolver is pretty much
solid state, just a couple coils, and could be put into very difficult
situations such as steel mills and other hot nasty places.

I have used resolvers in many situations. The company I worked for had an
old fashion preference for them. The resolver support boards were certainly
expensive, but that was only because we had a high margin on them. The chip
that does all the work is from Analog Devices and is really not too
expensive.

The only excuse for switching to resolvers is if you have a technical issue
with encoders that cannot be overcome by conventional practices. This is
pretty uncommon these days.

GTJ

-----Original Message-----
From: w.higdon@... [mailto:w.higdon@...]
Sent: Sunday, October 20, 2002 4:40 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Resolvers vs Encoders

I'll try, altough Jon, could probaly do better.
A resolver can be a transformer with a 2 or more phase
primary. The secondary is inside the primary and is free
to move, some are linear, and some are rotary. The
Rotary ones usually use sliprings so the secondary can
rotate through a full circle (360). Being a analog
device the need something to convert them to digital.
The chips are rather expensive now, when I worked at
Entec, a new board cost between $500 to $1000 depending
on accuracy. a encoder can either be a "Absolute
encoder" (works just like a synchro with conversion
built in). These are usually VERY expensive and limited
in resloution. The second kind are "Incremental encoders"
these output a series of pulses as they are moved. If
they put out a pair of pulses streams with a phase
difference between the pulse streams can be used to
determine the direction of movement.
Now that this is as clear as mud, I'll let Jon or
someoone else clarify it.
Bill Higdon
PS you can convert a syncro machine to a eincremental
encoder machine, by replacing the syncro's with suitable
encoders. I did one upgrade (conversion)like that while
at Entec.

> Hi All:
> Can any of you explain to me, the differences between resolvers and
> encoders.
> What is the implication for machine performance?
> Is a retrofit from resolvers to encoders a worthwhile effort?
> When did resolvers go out of favour and why?
> I'm casting amorously about, for small CNC turning centers, and I've heard
> the terms mentioned, especially in regard to Hardinge machines.
> Mid eighties machines seem to be the transition point from the resolver
> system to the encoder system.
> It looks to me like there is a fair amount of very nice old CNC iron from
> that era out there available for very little money...certainly far less

than

> building from scratch or retrofitting a manual machine...especially when

you

> consider that the enclosures, the coolant pumps, the ballscrews, the way
> covers etc, etc are all already there.
> Cheers
>
> Marcus
>
>
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