Page 79 - Instrumentation Reference Book 3E

64 Measurement of length
Guard-ring around superior to inductive arrangements if properly
moving electrode Fixed elecrroder
designed. As with inductive systems they also
need a.c. excitation to obtain a length change
response for slowly moving events.
Forces exerted by the magnetic and electric
fields of the two alternatives can be designed to
be virtually non-existent. Use in sensitive force
length
,"put balance systems allows the same plates to be used
to apply a d.c. voltage to the central plate of a
differential system. The plate can thus be forced
back to the central null position at the same time
Electrical as a higher frequency excitation signal is applied
O"tD"1
for position detection.
Although the plate-separation capacitance
method fundamentally provides an inverse rela-
tionship to length change, the signal can be made
t directly proportional by placing the sensing cap-
Electrical acitance in the feedback path of an operational
0"tD"t amplifier.
! c2
3.5.4 Use of electromagnetic and acoustic
Figure 3.14 Some differential capacitance length- radiation
sensing structures.
Radiation ranging from the relatively long radio
wavelengths to the short-wavelength X-rays in
Potential sensitivity of the alternatives (induct-
ive and capacitive) is virtually the same in practice: the electromagnetic (EM) radiation spectrum,
and from audio to megahertz frequencies in the
each having its own particular signal-to-noise pro- acoustic spectrum, has been used in various ways
blems. For low-sensitivity use, capacitance devices to measure length.
are more easily manufactured than the inductive In the industrial range the main methods
alternatives. adopted have been those based upon optical and
Noise occurs in capacitance systems from
charge pick-up produced by stray voltage poten- near-optical radiation, microwave EM, and
acoustic radiation. These are now discussed in
tials in the high impedance of the capacitance turn.
assembly. It is reduced by appropriate shielding
and use of earthed guard plates that surround the
active plates to collect and dump the unwanted 3.5.4.1 Position-sensitive photocells
charge. Capacitance structures lend themselves
more for original equipment design rather than An optical beam, here to be interpreted as ran-
as ready-made sensor units applied after the basic ging in wavelength from infrared ( N 10 pm) to
plant is designed. This is because the layout of the the short visible ( =4pm), can be used in two
working plant to be sensed can often provide basically different ways to measure length. The
directly one or more plates of the sensor as a beam can be used to sense movements occurring
non-contacting arrangement. For example, to transverse to it or longitudinally with it.
monitor position of a pendulum in a tilt sensor Various position-sensitive optical detectors
it is straightforward to use the pendulum bob as have been devised to sense transverse motion.
the central plate that moves inside two plates Their range is relatively small, being of the order
added one to each side. of only millimeters. They are simple to devise and
In general, therefore, it will be found that com- apply and can yield discrimination and stability
mercial sensor makers offer more inductive sys- of the order of a micrometer.
tems than capacitive alternatives, whereas in Figure 3.15 outlines the features of the struc-
scientific and research work the tendency is to ture of the three basic kinds that have been
use capacitance systems as they are easier to designed. Consider that of Figure 3.15(a). A
implement at the prototype stage. Commercial beam having uniform radiation intensity across
suppliers also wish to offer a product that is its cross-section falls upon two equal characteris-
self-contained and ready to apply and, therefore, tic photocells. When the beam straddles the two
a unit that can be verified before delivery. cells, thereby providing equal illumination of
At extreme limits of discrimination (sub-nano- each, the differentially connected output will be
meters) capacitance sensing can be shown to be zero. At that point any common-mode noise