Incremental encoders
have output signals which repeat over
the range of motion and thus each mechanical
position is not uniquely defined. The
current position sensed is only incremental
from the last position sensed. Thus
at power up, the position of an incremental
encoder is not known since the output
signals are not unique to any singular
position. They are made up of 2 major
parts, the disk and the sensor. The
disk of an incremental encoder is patterned
with a single track of lines near the
outside edge of the disk. The disk count
is defined as the number of dark/light
linepairs that occur per revolution
(CPR). As a rule, one or more tracks
are added to generate a signal that
occurs once per revolution (index signal),
which can be used to indicate zero or
home on the encoder.
Count and direction information can
be obtained from both absolute and incremental
encoders.
LVDT/RVDT
|
| LVDT
|
LVDT or Linear Variable Differential
Transformer operates with two
small transformers sharing the same magnetic
core. As the core moves the output of
one increases while the other decreases.
The 'out of balance' current is a measure
of the core position with the best linearity
occurring at the mid way point when the
transformers are almost in balance.
The LVDT is sensitive, reliable and
repeatable.
The accepted mode of operation is
to measure a perfect sample (a setting
master) and to then measure the unknown
sample. Thus the accuracy is derived
from the setting master and the LVDT
is used as a comparative tool. The master
is measured once a day, or whenever
the temperature changes to effectively
calibrate out all other variables relating
to support fixtures etc. Angular displacement
or rotation using an RVDT - Rotational
Variable Differential Transformer.
