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The plate co-ordinates within the MPS are updated from the ground 
co-ordinates fifty times per second using a rigorous perspective projection 
together with corrections for lens distortion, earth curvature and MPS cali- 
bration constants. 
OFF THE SHELF COMPONENTS 
A major factor in producing a low cost device has been the use of 'off 
the shelf' components. For example, the major part of the optical system 
is a standard binocular microscope which allows the customer to select the 
brand of optics, and a wide range of options such as the zoom range, dual 
viewing heads etc. 
The drive system uses commonly available stepper motors driving recir- 
culating ball screws for simple, reliable positioning. The photo carriages 
are guided by linear races, and are canted to minimize optical components, 
and to make the instrument compact. 
The electronics boards are made up from industry standard components 
common to most PCs. These components are readily available, and most are 
made by more than one manufacturer. 
DIAGNOSTIC SOFTWARE 
  
This is divided into four groups: 
(i) Testing of all MPS functions 
Before any work can be carried out on analysing performance, it is 
essential to verify that all mechanical and electronic components are per- 
forming correctly, and that there is no backlash in the system. A general 
diagnostic program has been written which carries out the following: 
  
MPS80 V1.0 - MPS DIAGNOSTIC 
  
. Drive MPS in Orientation mode 
Drive MPS to User position 
Dynamic test 
Test MPS commands 
Print MPS work file 
Help 
su DXLt 
SOU + D NH 
The first two options are used to check the system after each phase in 
the orientation, or after calibration. The various transformation parameters 
can be set and loaded directly using the operations in option 4 (Test MPS 
commands), and the results observed directly using this mode. They are 
useful in checking the loading of the transformation parameters, and also 
the way that these parameters are used. 
The dynamic test provides confidence in long term stability of the 
system. The user first drives each stage to up to 10 selected points, and 
flags each point by pressing the footswitch. The MPS then drives success- 
ively to each point at maximum speed until requested to stop. The operator 
can then request the system to go to each point in turn, and directly 
measure any accumulated errors in the measuring system. 
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