# Old School Resolvers

B

#### Bart

Jan 1, 1970
0
I've surfed much and am about "googled" out. I run into older technology
that use resolvers instead of encoders for positioning feedback from DC
motor driven movement. The resolver reads revolutions (or parts of) of the
motor shaft. Is there a way to check to see if the resolver is working with
a DMM? I've read they use AC voltage (some up to 28 vac) and measure a phase
shift, using terminology like "a rotating transformer", and I'm still
confused. I'm up to speed on quadrature encoders (square waves) but would
like to grasp how to check a resolver, even if just to see if its dead.
Normally I place my scope on one of the four wires of an encoder, turn the
motor shaft any amount and watch for a clean square wave. How can I check a
resolver?
Any help is appreciated,
Bart

C

#### Chris

Jan 1, 1970
0
Bart said:
I've surfed much and am about "googled" out. I run into older technology
that use resolvers instead of encoders for positioning feedback from DC
motor driven movement. The resolver reads revolutions (or parts of) of the
motor shaft. Is there a way to check to see if the resolver is working with
a DMM? I've read they use AC voltage (some up to 28 vac) and measure a phase
shift, using terminology like "a rotating transformer", and I'm still
confused. I'm up to speed on quadrature encoders (square waves) but would
like to grasp how to check a resolver, even if just to see if its dead.
Normally I place my scope on one of the four wires of an encoder, turn the
motor shaft any amount and watch for a clean square wave. How can I check a
resolver?
Any help is appreciated,
Bart

Hi, Bart. Before digital logic, resolvers were the best way to get
continuous angular data on a rotating object. In fact, because of
their speed of response and high resistance to electrical noise,
they're still frequently used in many applications.

The simplest resolvers are six wire -- they have an AC power input (2
wires), and a sine (2) and cosine (2) output. If your meter is true
RMS up to the recommended frequency, you can just input the required
voltage/frequency (they're relatively high impedance, you can probably
drive it with a function generator output), and measure the magnitude
of the sine and cosine outputs. A quick check would be to 1) ohm out
each pair to check for continuity, 2) ohm out between pairs to check
for shorts, and then 3) apply a voltage and see if there's an output.

But sadly, it usually isn't that simple. There are many different
types of resolvers, and figuring out a way to do a quick check (like
powering up the encoder and looking for square wave outputs when you
spin the shaft) kind of depends on knowing what you've got.

Start with the data sheet on the resolver -- it's usually available
from the manufacturer if they still make it (and sometimes, even if
they don't). Either that, or you can get the schematic of the
interface and try to muscle it out from there.

Possibly if you could mention what kind of resolver you have, someone
could provide more help.

Good luck
Chris

T

#### Tim Williams

Jan 1, 1970
0
Chris said:
Hi, Bart. Before digital logic, resolvers were the best way to get
continuous angular data on a rotating object. In fact, because of
their speed of response and high resistance to electrical noise,
they're still frequently used in many applications.

So let me guess, do these have an AC dipole on the shaft (or the inverse),
and respective biphase pickup windings?

Makes good sense, kind of AM-ey though. Low impedance would bring good
noise immunity, I suppose.

I wonder if there's a rotational 'doppler effect' going on when RPM ~
carrier F? Just wandering (not necessarily wondering)...

Tim

C

#### Chris

Jan 1, 1970
0
Tim said:
So let me guess, do these have an AC dipole on the shaft (or the inverse),
and respective biphase pickup windings?

Makes good sense, kind of AM-ey though. Low impedance would bring good
noise immunity, I suppose.

I wonder if there's a rotational 'doppler effect' going on when RPM ~
carrier F? Just wandering (not necessarily wondering)...

Tim

Hi, Tim. Most modern resolver signals are in the KHz range -- a lot
faster than rotation will ever be. But some older ones do work at 60Hz
-- I'd guess you might get that effect.. ;-)

Cheers
Chris

T

#### Tom Del Rosso

Jan 1, 1970
0
Chris said:
Hi, Tim. Most modern resolver signals are in the KHz range -- a lot
faster than rotation will ever be. But some older ones do work at
60Hz -- I'd guess you might get that effect.. ;-)

These "resolvers" are the same as "selsyns" or "syncros" aren't they? You
can tie two together and if you turn one, the other turns?

I remember when "selsyns" came up in SED once, somebody said he played with
a pair coupled together and found that he could spin one at a certain rate
and produce feedback that made them both accelerate out of control.

D

#### Don Bruder

Jan 1, 1970
0
Tom Del Rosso said:
These "resolvers" are the same as "selsyns" or "syncros" aren't they? You
can tie two together and if you turn one, the other turns?

I remember when "selsyns" came up in SED once, somebody said he played with
a pair coupled together and found that he could spin one at a certain rate
and produce feedback that made them both accelerate out of control.

I'd believe *THAT* just about as much as I'd believe a newborn baby can
pick up and throw a full-grown elephant the length of a football field.

(Reference "perpetual motion machines"...)

T

#### Tom Del Rosso

Jan 1, 1970
0
Don Bruder said:
I'd believe *THAT* just about as much as I'd believe a newborn baby
can pick up and throw a full-grown elephant the length of a football
field.

(Reference "perpetual motion machines"...)

Wrong reference. These devices have external power applied.

D

#### Don Bruder

Jan 1, 1970
0
Tom Del Rosso said:
Wrong reference. These devices have external power applied.

Ahhh... In that case, I retract the previous.

"selsyn" things (which I'm not familiar with, nor really interested
enough in to bother trying to read up on at the moment) were similiar to
stepper motors - take two same type/size steppers and wire them
together, then spin one of the shafts and watch what happens.

Yes, Virginia, I'm aware that there are losses in that system, but I'm
not ambitious enough to bother listing them. For illustrative purposes,
however, saying "twist one shaft and the other shaft twists the same
amount/ at the same speed" is an accurate enough statement.

H

#### Homer J Simpson

Jan 1, 1970
0
"selsyn" things (which I'm not familiar with, nor really interested
enough in to bother trying to read up on at the moment) were similiar to
stepper motors - take two same type/size steppers and wire them
together, then spin one of the shafts and watch what happens.

Nope.

http://www.navweaps.com/index_tech/tech-051.htm

J

#### jasen

Jan 1, 1970
0
"selsyn" things (which I'm not familiar with, nor really interested
enough in to bother trying to read up on at the moment) were similiar to
stepper motors - take two same type/size steppers and wire them
together, then spin one of the shafts and watch what happens.

they're like steppers but with AC magnetic field instead of permanment
magnets. also sesigned not to "cog"

Bye.
Jasen

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