67
het beoogde doel onaanvaardbare propagatie-omstandigheden te
vermijden.
De algemene geodetische betekenis van de beschreven grote
nieuwe mogelijkheden is door mij in het laatste deel van deze slot
beschouwing aangegrepen als een excuus om mijn onderdeel van
het thema van deze dag tot op zekere hoogte om te keren tot „de
betekenis van elektronische plaatsbepaling in de landmeetkunde";
gehoopt wordt dat de toehoorders dit excuus zouden willen aan
vaarden.
Angle Transducer
detecting the Direction of Gravity photoelectrically
Up until Keuffel and Esser Engineers attacked the problem, encapsulat
ed liquid in the conventional bubble level vial and the pendulum have been
virtually the only way of detecting the direction of gravity. Unfortunately
the sensitivity of level vials at best is only one second of arc-and then it
must be visually observed. Such visual observation normally precludes the
use of level vials for automatically controlling and recording positional
tilts of structures such as radar axles and launching platforms.
By adding a mirror to a suspended pendulum K E engineers have
designed an instrument that represents a step forward in gravity sensing
equipment. The device consists primarily of two units: a sensing unit and
an indicating unit. The sensing unit reads the angle of tilt against true
horizon in one plane displaying the output on a calibrated dial; an electrical
signal, is available for recording instruments, remote control or for driving
cerval units.
K E engineers selected a photoelectric method for angle measurement
because it utilizes the advantages of the high stability and high magnification
available in optics in conjunction with the convenience of electronic readout.
High magnification results from the fact that a reflected beam is deviated
by twice the angular displacement of a mirror, inherently containing twice
as sensitive a measurement method as any other available. High stability
results from the application of the null principlesimilar to comparing the
weight of two blocks on a balance rathei than weighing each block separately
and then comparing the results.
In the photo detection of an angle K E engineers considered three
methods: the interferometer method, the phase shift method and the inten
sity comparison method.
In the interferometer method, light waves, reflected back and forth
between two mirrors, interfere with each other causing fringes to appear.
Commonly called Newton Rings, each fringe for a given mirror diameter
corresponds to a certain angular value. Counting the fringes permits the angle
to be determined. Since the measuring of the wave length of light is a well
established procedure this method is probably one of the most accurate
standards available to engineers and can be considered a primary method
of measurement when applied to angles. Unfortunately, the interferometer
is still an extremely delicate laboratory instrument, not easy to handle.
In addition the electronics involved in counting numerous fringes puts the
cost of an instrument in the 20,000 range.
The phase shift method is based on comparing the phase shift of two waves
generated by the same light source but out of phase originally by 180 degrees.
One wave is directed at a photocell and the other at a target. The mechanical
arrangement is such that phase changes from the original 180 degree phasing
is a mesure of target shift or angle of tilt. Its advantage is that the method
requires only one photo cell and thus is insensitive to changes in photo cell
