Fluidic Sensors and some Large Scale Devices: Course held at the Department of Hydro- and Gas-Dynamics October 1970

41 Pages·1973·2.249 MB·English

by  Barry E. A. Jacobs (auth.)

INTERNATIONAL CENTRE FOR MECHANICAL SCIENCES
CO U R S E S 1<\ N D L E C Ten E S - No. 52
BARRY E. A. JACOBS
BRITISH HYDROMECHANICS HESEARCH ASSOCIATION, BEDFORD
FLUIDIC SENSORS AND SOME LARGE
SCALE DEVICES
COURSE HEiLD AT THE DEPARTMENT
OF HYDRO-lAND GAS-DYNAMICS
OCTOBER 1970
UDINE 1973
SPRINGER-VERLAG WIENGMBH
This work is suqect to copyright
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specifically those of translation, reprinting, re-use of illustrations,
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or similar means, and storage in data banks.
© 1972 by Springer-Verlag Wien
Originally published by Springer-Verlag Wien-New York in 1972
ISBN 978-3-211-81228-0 ISBN 978-3-7091-2906-7 (eBook)
DOI 10.1007/978-3-7091-2906-7
PRE F ACE
As is well known fluidics has developed as
a new technology during the last ten years. The initial
work was performed in the U.S.A. but since then, and
particularly during the last five years, a considerable
world wide interest has developed. As a result of this
recent development many students will be unaware of all
types of element, particularly the large scale elements.
Thus the lectures are mainly descriptive rather than
analytical.
These lectures are entitled "Sensors and
large size fluidic elements". The author's own expe
rience is concerned mainly with large long-range sensors,
diodes, and the special element for the fluidic chimney.
All of which have been investigated with a view to
practical application. Those students who are interest
ed in other forms of large scale element should refer
to the lectures presented during this session of CISM
entitled "Heavy Current Fluidics" by D.C. Bain.
The author is most grateful for the invita
tion by CISM to present the lectures, and is indebted
to Mr. L.E. Prosser director of the British Hydromechanics
Research Association, for permission to deliver them.
Udine,October 1970
Barry Edward Adrian Jacobs
1. INTRODUCTION
It has often been claimed that fluidic control
systems show advantages over other systems on the grounds of re
liability, and resistance to adverse environments. These claims
may well be substantiated with respect to the main fluidic cir
cuit, however, another area in which fluidics is advantageous
is that in which sensing of parameters such as object position,
velocity, temperature etc., can be accomplished by a fluid me
chanic process. Such a process, known as fluidic sensing, is ca
pable of supplying a signal compatible with the fluidic circuit.
This capability obviates the use of components which convert an
electrical signal, for example, into a pneumatic signal. Such
components known as interface devices, add complexity to the
system, and can also introduce reliability problems.
Considering the fluid mechanic processes which
enable various sensors to work it is found that many are depen
dent primarily on an interaction between viscous and momentum
forces. This is particularly so of the back pressure and prox
imity sensors which are the first to be described. The vortex
rate sensor, and one type of temperature sensor employ phenom
ena mainly dependent on momentum forces, while another type of
temperature sensor employs a predominately viscous effect. Since
sensors cannot be categorised easily by their flow processes it
6 1. Introduction
has been thought best to describe first the types most frequent
ly met, and then to progress to the less common forms.
The description of small scale sensors leads na
turally into a discussion of large scale sensors. After these
have been considered some further large scale fluidic elements
are discussed.
2. BASIC TYPES OF SENSOR
2.l. Back-Pressure, and Proximity Sensors
Back-pressure sensors work by a process which is
analogous to that illustrated by the electrical circuit shown
in Fig. 1. A current flows
from a source via parallel
~-------- v ----------.~~I
paths to the earth. If one
of the resistors is varied,
a variable voltage is gen
erated across it. In the
case of the fluidic sensor,
Fig. 2, the presence of an VARIABE VO LTAGE
object close to the sensor
Fig. 1. Analogue circuit for fluidic sensor
mouth raises the resistance
SENSOR
~~~:~~ o
-
.(.. ./.. . .../. .. .../.". ""k
SUPPLY OBJECT
Ps
Fig.2. Back-pressure sensor
8 2. Basic Types of Sensor
to the flow of air (or other fluid) and thus causes a rise in
pressure at the signal output port. Complete closure of the port
causes the signal to rise to the supply pressure. The analogy
cannot be taken too far since for the fluidic system, unlike
that for the electrical circuit, the variation of the output sig-
nal with flow rate is non linear.
The variation of output pressure with object po
sition has long been used, before the advent of fluidics, as the
method of air gauging. Fig. 3 shows the general form of the curve
obtained when, for a given gauge, ~ and b are kept constant, and
P2 and a are varied. The graph shows that, for part of the curve,
the variation of output signal with the area ratio b/a is linear. In
order to obtain linearity between the output signal and the di-
mens ion being measured it is necessary that the area available
for the escape of air should vary linearly with the measured di
mension. Air gauging is normally regarded as a short range tech-
nique.
o·s ---
~
AREA'-"b~--I[3::]- 0'6 - --
AREAa '
o ____________ _
~
Fig.3. Air gauge
Back Pressure and Proximity Sensors 9
Typical figures for a 0.02 in. dia. air gauge are:
Distance from object 0.02 in.
Supply pressure 10 psig.
Air consumption 0.09 ft3/min.
Proximity sensors when compared with backpressure sensors are
regarded as those having a range greater than 0.02 in. Their ge
ometry is different in that they do not work on a direct back
pressure principle. Fig. 4 illustrates the main features of a
proximity sensor. Two tubes are arranged concentrically with one
another. Air is supplied to the annular region formed by the two
tubes. On leaving the mouth of the sensor the air forms a sepa
rated region of low pressure, and this low pressure forms the
signal output transmitted to any external equipment by the cen
tral tube. An object approaching the sensor mouth disturbes the
separated-flow region, thus changing the signal output. A small
sensor, operating at 3 psig. might have a range of approximately
0.1 in. The output at 0.08 in. range could be 5 in. water for an
3 .
air consumption of 0.3 ft /mln., and the frequency response
_ SUPPLY
~r~::U';;:~UTPUT
-,.----- 2 ? ? u n t
SEPARATED
FLOW REGION
Fig.4. Proximity sensor
10 2. Basic Types of Sensor
could be up to 500 Hz. As a result of the unusual shape of the
signal output curve with range, the repeatability of dimensional
positioning with such a sensor depends on the range. A typical
figure would be 0.005 in.
Some fluidic sensors have been constructed in
which the sensing performance has been improved by the use of a
mechanical device. For instance, the thickness of a porous mate-
rial may be gauged by allowing a light impervious follower to
rest on the material and to sense the movements of this follower
by means of a back-pressure sensor. Similarly limit switches may
be constructed in which the object contacts a lever which in turn
closes the mouth of a back-pressure sensor. Such mechanical ad-
ditions can be designed to safeguard the sensor in case of over
travelling of the object.
Very similar to the last type of
PUSH
BUTTON sensor described is the input sen-
sor, Fig. 5. This sensor is re-
t
quired, for example, when a se-
VENT
quencing operation is to be ini-
h=::=:J-OUTPUT tiated. Here the movement of a
push button closes a vent thus di
recting the input to the output
~
INPUT
port. Under some circumstances it
is possible to eliminate the push
Fig. 5. Input sensor
button and to close the vent by