Re: [CAD_CAM_EDM_DRO] Resolvers & Encoders

Mina Aboul Saad wrote:

>Hi group,
>
>Could someone explain the difference between encoders and resolvers (basic
>info.) for a newbie who wishes to learn?
>
>

The classic encoder that most of us use on small machines has a disc
with slots
of some kind in it. Some are made with glass discs with patterns
photographically
reproduced on it. There is a light source, and two photosensors arranged in
such a way that there is a 90 degree phase relationship between the signals
the two photosensors receive. If an encoder was being turned at a
steady rate,
the electrical signals would look like this (in constant spaced font)

_______________ _____
____________| |______________|

_____ _______________
|_____________| |_____________

This is called a quadrature signal, and its advantage is that the sequence
of the rising and falling edges always tell you which direction the encoder
is being moved in.

These quadrature signals can be directly decoded and counted by standard
circuits, such as the US Digital LS7166 encoder counter chip.

Many encoders also have an index channel, which generates one pulse for
every revolution. This can be used for more accurate determination of the
home position of the machine.

These can be either linear or rotary, which is also true of resolvers.

A resolver is a magnetic device, with coils of wire in both the moving
and stationary sections of the unit. One coil is excited by a high
frequency
sine wave, and a pair of coils, arranged so they pick up a maximum signal
as the mechanism is moved by 90 degrees, pick up this signal. A fairly
complex set of amplifiers, phase comparators, analog to digital converters,
phase locked loops and counters convert this signal to a position value.
This is a lot more circuitry than is required for the encoder, and thus
ends up costing quite a bit more. In the "old" days, encoders with
incandescent
light bulbs and leaky seals were quite unreliable, and resolvers were
thought
to be the only acceptable choice for high quality machines. I think this
advantage has now turned the other way. Encoder failures are a rarity,
while the extra circuitry required for the resolver is a burden, and an
increased possibility of failure.

Jon