ISSN 10637397, Russian Microelectronics, 2013, Vol. 42, No. 7, pp. 389–395. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © V.B. Topil’skii, 2011, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Elektronika, 2011, No. 4, pp. 51–58.
In many science and technology fields such as
micro and nanoelectronics, the instrumentmaking
industry, mechanical engineering, and robotics, as
well as in various systems of controlling the process
equipment, an accurate measurement of linear and
angular displacements at a high speed is required.
Photoelectric digital displacement–code transducers
or optical encoders are widely used industrially [1–4].
Encoders must have the system of fixed axes for
increasing the reliability of control systems and photo
electric raster interpolators (PRIs) [1, 4], in which the
modulation of the light flux is performed using the
multiphase raster coupling [1, 2], for increasing the
resolution. Moreover, rasters with a long step
25 to 100
m, which are easily fabricated and repli
cated by photolithographic methods, are used and
constitute an advantage of the PRI. In this case, the
is from 2
and the resolution reaches
fractions of a micron given the moderate interpolation
) . The interpolation
coefficient in each case is determined by the compro
mise between the resolution and the speed.
Given the reasonable parameters of the raster cou
pling [1–3], the modulation in the raster coupling is of
harmonic character, and the current signal of photo
detectors (photodiodes) is described by the following
is the phase number;
are the direct
and alternating component of the current signal;
the depth of modulating the raster coupling;
is the displacement; and
is the number
of phases of the raster modulator.
In the subsequent analysis we will consider the four
phase raster coupling (
= 4); this is predetermined by
existing optoelectronic hardware components (com
mercial integrated fourarea photodiodes) and the abil
ity of the fourphase PRI to suppress the even parasite
harmonics of the raster coupling. In the fourphase ras
ter coupling, alternating components of photodetec
tors’ current signals will be in the quadrature.
As follows from expression (1), the key feature of
the raster modulator is the presence of the direct com
in the output signals of the photodiodes. The
direct component gives no useful information, distorts
quadrature ratios between channels, and can be
regarded as interference. The interference suppression
(Signal/Noise) is determined by the fol
lowing wellknown ratio [2, 3]:
are the alternating and direct
components of the signals.
Ratio (2) must hold with a safety margin
for all operating conditions of the PRI and with allow
ance for other sources of interferences and distortions.
As follows from (2), the
ratio must be no less than
38 dB given
= 4 and
In order to suppress (eliminate) the direct compo
nent or increase the
ratio, it is required to operate
it at the variable frequency (introduce the carrier fre
quency), as in the case of the phaseshifting units of the
goniometric type . Goniometric PRIs possess a high
resolution, with the interpolation coefficient
. However, the introduction of the carrier fre
quency and the operation at the alternating current
limit the PRI speed (sometimes unacceptably). In some
cases, extrapolators are used to increase the speed [4, 5].
Track interpolators operating on the direct and
alternating current possess a higher speed but are more
sophisticated [1, 3, 4]. However, there exist problems
of stability and dynamic errors inherent in any track
ing system in one degree or another. The speed of such
PRIs does not exceed 0.1 m/s.
Signal Conditioning Circuitry in UltrahighSpeed
Photoelectric Raster Interpolators
V. B. Topil’skii
National Research University of Electronic Technology MIET, Moscow, Russia
Received December 8, 2010
—A circuitry of signal conditioners of ultrahighspeed photoelectric raster interpolators used in
absolute highresolution optical encoders is described.