3. 
(4) 
The above analysis gives an oversimplified but useful view of 
the problem of driving the mass M , It serves to establish the fact 
that if the plotting velocity is made a function of track shape, average 
tracking velocities far in excess of manual tracking speeds are attain- 
able with electric servomotors. Hydraulic servomotors provide figures 
of merit that may be an order of magnitude better than that of electric 
servomotors, 
At this stage in the development of automatic stereo-plotting 
machines, intrinsic limitations imposed by the characteristics of avail- 
able servomotors are not in my opinion serious. Equations (5), (6) and 
(7) show to what extent forms of machines that require the moving of 
small masses are to be preferred. 
It should be observed here that every practical servomotor has 
a maximum attainable or allowable rotational speed. This physical limit- 
ation places an upper bound on the attainable plotting velocity v . 
This limitation is in addition to the constraint imposed by acceleration 
limits as discussed above. Until completely electronic tracking becomes 
practicable, this additional limitation is likely to be secondary to the 
acceleration limits that affect resolution of track detail. 
Performance Limitations Determined By Feedback System Characteristics: 
At the present stage in th» development of automatic stereo- 
plotting machines, the most serious problems lie in the topographic 
track-sensing equipment necessary to control the mass positioning servo- 
motors. Any form of machine such as that of the Helava analytical 
plotter that improves the available light transmission from the scanning 
spot is likely to permit far more significant improvements in the auto- 
mation of plotting than will, for example, the design of light weight 
projection plotters. 
The simple analysis given in Section 2 of this paper indicates 
how servomotor characteristics limit plotting speed 1f the torque of 
these motors can be controlled in an optimum manner. How the torque 
varies in automatic machines of the Hobrough type is determined by the 
static and dynamic characteristics of the complete feedback control 
system that includes the track-sensing equipment, the amplifiers to 
drive the servomotors, the masses to be driven, the characteristics 
of the topography being mapped. This system characteristic of the 
&utomatic plotter is indicated by the block diagram in Figure 2. 
This diagram may be considered to be a control system representation 
for the contouring mode of a macnine such as Hobrough's. Since 
Hobrough? has described the functioning of his system the discussion 
here will attempt only to outline certain basic feedback control system 
problems that are encountered in such systems. A very simple mathemat- 
ical analysis is sufficient to establish the character of these problems. 
Before considering the system of Figure 2 let us consider 
Figure 3(a). Here a section of a contour at height Z_ is shown. The 
effective tracking point (the centre of symmetry of the local scan in 
the Hobrough machine) is to be driven along the contour. At any point 
on the contour the direction of steepest slope on the terrain has some