Page 359 - Mechatronics with Experiments

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JWST499-c06
JWST499-Cetinkunt
SENSORS 345 254mm×178mm
V sin(w t) sin( )
ref
r
Stator 1
Oscillator
Rotor sin(w t)
r
Stator 2 and
phase
defector
Stator 3

V sin(w t) sin( + 120) V sin(w t) sin( + 240)
ref
ref
r
r
FIGURE 6.16: Syncro and its operating principle.
There are other types of signal processing circuits to extract the angle information
from the resolver phase voltages. For instance, both stator voltages can be sampled at the
same frequency (w ∕(2 ) Hz) where the start of the sampling is synchronized to the V (t)
r
r
by a 90 degree phase angle in order to sample the maximum magnitudes. In other words, by
sampling V , V at the same frequency as the w , we achieve demodulation by sampling.
s1
s2
r
The sampled signals are
V adc = V ⋅ sin( ) (6.60)
s1 mag
V adc = V ⋅ cos( ) (6.61)
s2 mag
where V mag is the sampled value of the k ⋅ V ref ⋅ sin(w t) portion of the signal. Then, we
r
0
can compute the arctan of the two signals to obtain the angle information,
( adc )
V s1
= arctan (6.62)
V adc
s2
This method uses two channels of an A/D converter and a digital computational algorithm
for the arctan(⋅) function. Such a circuit for an RTDC (resolver to digital converter) can be
implemented using ADMC401 chip (AD converter) and a AD2S99 oscillator chip (both by
analog devices).
The operating principle of the syncro is almost identical to that resolved. The only
difference is that there are three stator phases with 120 degrees of mechanical phase angle.
Hence, the induced voltages in the three stator phases will be 120 degrees apart,
V s1 = k ⋅ V ref ⋅ sin(w t) ⋅ sin( ) (6.63)
r
0
V s2 = k ⋅ V ref ⋅ sin(w t) ⋅ sin( + 120) (6.64)
r
0
V s3 = k ⋅ V ref ⋅ sin(w t) ⋅ sin( + 240) (6.65)
0
r
and these signals can be processed to extract the angular position information with circuits
similar to the ones used for resolvers.
Commercial LVDT and resolver sensors are packaged such that the input and output
voltages to the sensor are DC voltages. The input circuit includes a modulator to generate an
AC excitation signal from the DC input. The output circuit includes a demodulator which
generates a DC output voltage from the AC voltage output of the secondary windings.
For instance, LVDT Model -240 (by Trans-Tek Inc.) has a position measurement range in
0.05–3.0 in, 24 VDC input voltage, 300 Hz bandwidth, and an internally generated carrier
frequency of 13 kHz.