DRAWING TABLES DRIVEN BY ELECTRONIC SERVOS, ETC. 
27 
is brought down by switch 32 on the pencil-holding carriage), the plug for the 
end pin of the pedal electric cable. 
Out of the box comes the cable, whose connector must be inserted into the 
corresponding connector of the branching box. 
6. Tests effected. 
Several tests have been effected on table 108 to check the transmission pre 
cision of the X and Y coordinates from the plotting instrument to the table. 
To do this, a standard glass rule was set on the table, parallel to movement 
X ; the rule was graded with marks at 1 mm distance from one another and the 
enlargement ratio between instrument and table was fixed at 5:1. 
The X carriage was moved every 10 mm, reading the corresponding va 
lues on the plotter X scale (engraved on glass). Correspondingly the X beam 
on the table should have moved by 50 mm every time : the actual position of the 
beam could be read by means of a vernier collimating microscope, put into the 
pencil place. 
Measurements effected backwards and forwards, are shown on table 26. 
The same operations have been performed with reference to the movement 
of the Y carriage of the plotter, using the same sample rule. The results are shown 
on table 27. 
The two tables show immediately that the electronic transmission error for 
each centimetre of the stereocartograph scale (enlarged by five times) has always 
been less than 0,05 mm. Furthermore the graphs of fig.s 28 and 29, referring to the 
progressive errors, show that the five time enlargement brings about a small scale 
variation, of about 0,1 mm every 1000 mm, corresponding to a 0,02 mm error 
every 200 mm in the plotting instrument’s scales : this error may be ascribed to 
the different temperature existing in the factory where the rules have been engra 
ved, considering that the control scale is made of glass whereas the punched tapes, 
which finally correspond to the plotting grading on the table, are made of steel. 
The tables further show the differences between the forward and the return 
movement, which are small and random. This is a characteristic feature of any 
pantograph-operated equipment, different from what occurs where the move 
ment is screw-operated, so that when checks are effected with instruments of the 
latter type, one must be careful to turn always in the same direction. 
The errors shown in the most unfavourable conditions, are nevertheless always 
within the tolerance allowed in drawing any map or chart, bearing in mind that 
one generally considers a graphic error to be 0,2 mm. It would be difficult for a 
different type of servo to achieve, for so large a table, a greater accuracy than that 
achieved by electronic means.