4 
CARLO TROMBETTI 
A drawing table may be linked to an analogical plotter also by electronic 
means. 
Such a system offers many advantages as compared with the classical mecha 
nical links. First of all the optical model can be enlarged much more without 
affecting accuracy, and its operation does not cause the operator any additional 
effort except that required for moving the plotter carriage. Furthermore this 
control allows quickly to pass from the plotting of a planimetry to that of a verti 
cal section, and finally allows speedily to reverse the direction of one or both 
coordinates. 
An additional feature of such a type of control is to allow to record the coor 
dinates by using the same members employed for transmitting the data to the 
drawing table. 
i. Working principles of an electronic servo-system with synchronic 
TRANSMISSION. 
The quantity transmitted and repeated at the suitable scale, by means of an 
electronic control, by the plotter to the drawing table, is an angular displacement. 
Suitable mechanical transmission systems must : 
— before the electronic control : transform the linear displacement of the 
plotter carriage into an angular displacements (for instance using a rack and pinion) ; 
— after electronic control : change the angular displacement into a linear 
displacement at the drawing table. 
The member picking up the first angular displacement (the plotter one) 
is a synchro-transmitter (described later on) ; the member which, being 
electronically linked to the former, repeats the angular shift to the table mem 
bers, is a synchroreceiver (see later). 
The transmission of angular displacement takes place through a type i repeater 
with synchronic transmission : i. e. the control takes place with a zero static error, 
as it repeats the position transmitted without any error, in rest condition. Such 
a type of control operates upon the following conditions. 
The synchro-transmitter (fig. i) is mainly an electro-magnetic device, con 
sisting of a rotor with a winding (which can be reached from the outside through 
two isolated rings) and a stator, formed by three windings with their axes symmetri 
cally set at 120° in the rotor cavity. 
The winding of the rotor is fed by an alternate current (115 volts, 400 cycle/sec.) 
and it therefore creates in the rotor cavity an alternate magnetic field of the same 
frequency, whose direction depends on the rotor position. This magnetic field 
induces into each of the three stator windings an alternate current of the same 
frequency, but whose size depends on the angle position of the rotor with regard 
to the stator. 
The synchro-receiver (fig. 2) is equal to a synchro-transmitter, except for the 
winding resistance and the transformation ratio between stator and rotor windings : 
by this ratio we bring down the maximum tension to about one half (57 volts)