Specifications

Resolver Encoders
Resolvers are essentially a rotary transformer, having one rotor
winding and two stator windings. The stator windings are locat-
ed 90 degrees apart. Either rotor or stator winding can be used
as primary. Typically, the rotor winding is driven by a reference
voltage at a frequency ranging from 400 Hz to several KHz.
As the shaft rotates, the output voltages of the sta-
tor windings vary as the sine and cosine of the
shaft angle. See figure 2.
The two induced stator voltages are a measure of
the shaft angle and are converted to a digital sig-
nal in resolver-to-digital decoder. Among various
R/D decoding methods available, the two most
commonly used are:
❐ Ratiometric Tracking Converter
❐ Phase Method Converter.
Ratiometric Tracking Converter
(A typical block diagram for a Ratiometric Tracking
Converter is shown in figure 3.)
The circuit features a Type II servo-loop that comprises of
sine/cosine multiplier and an error amplifier together with
phase sensitive demodulator, error processor, voltage controlled
oscillator (VCO) and an up/down counter. Since the VCO is
controlled by an error integrator, greater the lag between the
actual shaft angle and the digital angle in the counter, faster
will the counter be called upon to "catch-up" or "track" and
eliminate the error.
The information produced by this type of converter is always
"fresh", being continually updated and always available at the
output. As an added bonus, additional outputs, such as, an
analog output proportional to the shaft RPM to eliminate
external tachometers and a busy signal pulse for incremental
pulse applications, are also available. The basis of determining
the shaft angle in a ratiometric converter is the ratio between
the two stator signals:
From this relationship it can be noted that the angle is no longer
a function of the induced rotor voltage Vr, but rather the ratio of
V
S1
and V
S2
. Therefore, variations in the rotor voltage Vr, fre-
quency and temperature are no longer factors in a ratiometric
converter. This results in a highly accurate and repeatable resolver-
to-digital converter.
800-TEC-ENGR
www.avg.net
A-4
Position Transducers/Sensors
(Figure 1)
(Figure 2)