tv; o rue a sur 
tes!; apparatus as well (2), The linear encoder has 
ing stages, the coarse one has a ruler onto which a code mask 
is described and the fine one which is made as a raster scann 
ing arrangement* Coordinate information is produced by taking 
in parallel the readings of the code tracks of the coarse stage 
and the readings of a compensation device which continuously 
tends to set the indicating raster to zero* For matching the 
coarse and fine stages, use is made of a logical correction 
circuit which adds to or subtracts unity from the low-order po 
sition of the coarse stage* This ensures the goon matching of 
the stages even if the code mask is not sufficiently accurate. 
Figure 2a shows the ruler with the mack using the Grey code 
and a precision xaster* In Fig’ire 2b, a block diagram of the 
fine stage is illustrated* The encoder relies on the compensa 
tion principle, thus eliminating the phase shift between the 
line marks of the indicating and measuring rasters. The relata- 
ve shift ui the- indicating raster within its spacing is accomp- 
lished by an electromagnetic driver. Applied to the windings 
of the latter are a d.c* component proportional to the raster 
shift and a.c. component necessary for measuring an error sign 
al due to line mark misalignment* The error signal of null de 
tector (IF)) controls the operation of key (K) sc that an error 
signal of one sign causes the pulses from interpolating gene- 
rat ox* (TG) to pass to the addition input of reversible counter 
(RC), and to the subtraction input of the latter in the case 
of an error signal of other sign. Ccde-to-analog converter (CA) 
controls electromagnetic driver (ED) and provides the follcw-up 
operation, of the latter. The error of the system does not ex 
ceed 1 um at a raster spacing of 100 urn. Speed of operation of 
the encoder mainly depends on the operating delay of the elect 
romagnetic driverj in fast recorders, use is made for the pur 
pose of practically inertialess dissector* 
Linear—tc-code encoders dxscussed above are advantageous 
for the use in precision photogr amine trie instruments* In the 
Soviet Union, some instruments, however, u^aanalog-to code en 
coders that feature increased reliability, small size and com-