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method of drive with D.C. servomotors with tachometers which are attached 
directly to the leadscrews. This eliminates annoying gear noise, and 
allows higher performance and more control over the stage movement and 
positioning. 
The leadscrews are precision, one millimeter pitch screws, originally 
having optical rotary encoders attached at the end of the screw opposite 
the drive motor. The encoder fed positioning information back to the 
interface which in turn controlled the motor. This arrangement therefore, 
depended on the accuracy of the leadscrew for stage positioning. Through 
years of use these leadscrews will have undoubtedly lost some of their 
precision. As a result, to preserve the accuracey desired of the completed 
retrofitted system, linear encoders are added. These are one-micrometer 
resolution linear encoders attached directly to the stage carriage for the 
Y axis positioning, and directly mounted to the optics carriage for X axis 
positioning. The addition of these linear encoders insures the accuracy of 
the machine regardless of the wear on the leadscrews or nut assemblies, 
and will also provide correlation for any mechanical wear or deformations 
which may have occurred during the use of the machine prior to this 
retrofit. The retrofitted system will have better positioning accuracy 
than the original viewer had when it was first delivered, and will 
maintain this accuracy due to the inherently better performance of linear 
encoders over the leadscrew and rotary encoder arrangement. 
ELECTRONICS 
In essence, all of the ASII-Bl/s electronics are replaced. This includes 
all of the viewer/s wiring, both internal and external. New rotary 
encoders are installed on the handwheels and the footwheel, the plus/minus 
ninety degree image rotation switch is replaced by a profile switch which 
allows the interchange of handwheel and footwheel functions XY and XZ. 
This is used primarily for profile data collection. The manual control 
potentiometers for the zoom and dove servomotors are checked ‘for 
operation, and replaced if required. The front panel is replaced with a 
new unit giving more capabilities to the system. A new power supply and 
power control unit are added to the viewer to supply necessary power for 
the stage lamps, linear and rotary encoders, and front panel. ‘Lastly, "a 
connector panel is installed for the two cables that interconnect the 
viewer with the interface electronics. 
The retrofit electronics package consists of OMICA's most recently 
designed viewer interface. It is a system using current technology and 
containing all the components necessary to perform one micrometer 
positioning of the stages, as well as computer controlled zoom and dove 
movement. 
The interface is a single board system designed and built by OMICA. It is 
based on a Motorola 68000 microprocessor for motor control, communication 
with the host computer, and communication to the front panel of the 
viewer. Full advantage is taken of the most recent integrated circuit 
17