must nou be transferred to the negative stage. This is achieved, 
separately for each component, each of uhich has tuo servo control 
systems. 
□ne of these servo systems has photoelectric positional control 
and is primarily far measuring the angle of tilt. Since this system 
excludes the possibility of mechanical stress on the space rod, 
the accuracy with uhich the transfer can be made is relatively 
high. 
The tuo carriages of the simulator cross slide system lie in the 
xz and the yz planes and for the angle measurement each carries 
a Dorm drive uith servo motor and the corresponding control po 
tentiometers D and E for the negative stage movements. The toothed 
segments of these mutually-perpendicular uorm drives together drive 
a projector, consisting of a lamp lined up uith a pinhole aperture, 
about the steering point A; the function of this servo system is 
to co-align the direction from aperture to A uith the longitudinal 
axis of the space rod. 
A lens built into the space rod focuses the image of the pinhole 
aperture sharply as a circle uith 12 mm diameter on a silicon photo 
element at the end C of the space rod. The photo element is divided 
into four sectors. The x and y components correspond each to tuo 
opposite sectors, these being connected to the amplifier of the servo 
circuit concerned. 
As long as the circle of light falls on the center of the photo 
element, the servo system is balanced. Houever, as soon as the direction 
of the space rod changes, the photo element is moved out of the 
center of the light spot. This displacement causes an "error" signal 
uhich activates - by means of a servo amplifier - the corresponding 
motor relay to start the servo motor for the x (or y) component 
turning in the correct sense. This drives the uorm until the light 
spot is centered on the photo element again and the error signal 
is thereby reduced to zero. 
During this corrective movement of a uorm-drive segment, the control 
potentiometers are also turned simultaneously, these being the coarse 
potentiometer D and the ring potentiometer E (for the accurate mea 
suring) uhich is mounted directly on the uorm axis. Mounted on the