Aerospace Instrumentation. Proceedings of the Fourth International Aerospace Symposium

281 Pages·1967·9.24 MB·English

by  M. A. Perry (Eds.)

Already published in this series:
FLIGHT TEST INSTRUMENTATION, Volume 1 Edited by M. A. Perry, 1961.
RECENT DEVELOPMENTS IN NETWORK THEORY Edited by S. R. Beards, 1963.
FLIGHT TEST INSTRUMENTATION, Volume 2 Edited by M. A. Perry, 1963.
ADVANCES IN AUTOMOBILE ENGINEERING Part I Edited by G. H. Tidbury, 1963.
ADVANCES IN AUTOMOBILE ENGINEERING Part II Edited by N. A. Carter, 1963.
FLIGHT TEST INSTRUMENTATION, Volume 3 Edited by M. A. Perry, 1965.
ADVANCES IN AUTOMOBILE ENGINEERING Part III Edited by G. H. Tidbury, 1965.
ADVANCES IN AUTOMOBILE ENGINEERING Part IV Edited by D. Hodgetts, 1966.
AEROSPACE
INSTRUMENTATION
VOLUME 4
PROCEEDINGS OF THE FOURTH INTERNATIONAL AEROSPACE SYMPOSIUM
1966
Edited by
M. A. PERRY
JOINTLY SPONSORED BY
THE COLLEGE OF AERONAUTICS
AND
THE INSTRUMENT SOCIETY OF AMERICA
SYMPOSIUM PUBLICATIONS DIVISION
PERGAMON PRESS
OXFORD · LONDON · EDINBURGH · NEW YORK
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Copyright © 1967
Pergamon Press Ltd.
First edition 1967
Library of Congress Catalog Card No. 61-17510
Printed in Great Britain by Bell and Bain Ltd., Glasgow
(3001/67)
INTRODUCTION
THE 4th International Aerospace Instrumentation Symposium marked a change
in the series of symposia held at the College of Aeronautics, Cranfield. In the
first place the name changed from that of Flight Test Instrumentation, demon­
strating the fact that the organizers felt technology must take notice of the
increasing importance of experiments in space, and secondly, the College
jointly sponsored the meeting with the Instrument Society of America.
Although only a selection of papers have been published in these proceedings
they represent a cross section of a programme which covered most aspects of
aerospace instrumentation. Over 350 delegates attended the conference from
some 12 countries and many of these countries were also represented by authors
of papers at the meeting.
The introduction of the tutorial sessions proved very popular and as a result
of many requests these tutorials have been written up by the authors and are
included in the proceedings.
Unfortunately, many excellent papers have not been printed in this book due
to pressure of space and the need to keep the series of books as a continuous and
comprehensive coverage of the present "state of the art".
The full list of papers given at the meeting is published, however, and readers
interested in the additional papers should apply to the author direct or to the
Librarian of the College of Aeronautics.
Cranfield 1966 M. A. PERRY
TRANSDUCERS WITH
SEMICONDUCTOR STRAIN GAUGES
F. E. DUFFIELD
Ether Engineering Limited
PRESENT-DAY flight instrumentation systems are of such size and complexity
that there can be limited space for the primary measuring and sensing transducer.
There exists, therefore, a requirement for small volume lightweight units of
rugged design which are capable of operating satisfactorily in environments
which may involve a high level of mechanical shock and vibration, and possibly
extremes of operating temperature.
High-level voltage output signals are desirable for most of the signal-processing
systems in use, together with an adequate frequency response characteristic. In
the case of pressure transducers a further requirement often exists for minimal
volume of the pressure cavity and for minimal change in volume over the
pressure range.
Techniques based on the use of metallic strain-sensitive materials have been
developed over the past twenty years and applied to many transducers for aero­
space applications. The development of the solid-state semiconductor gauge has
resulted in a useful extension of these techniques, offering transducers with
higher sensitivity and smaller size, with simpler instrumentation requirements.
In 1954, C. S. Smith^ reported on investigations of the piezoresistive effect in
a number of semiconductor materials, which indicated the feasibility of strain-
sensitive devices having gauge factors of approximately 50 to 60 times those
obtainable with the metallic elements in general use. Attention was also drawn
to the temperature dependence of the resistivity and the gauge factor of these
materials. A report by Mason and Thurston^ followed in 1957 on the applica­
tion of semiconductor strain-measuring elements to various transducers. In
subsequent exploitation, semiconductor silicon has emerged as the most
popular material for these piezoresistive devices and most, if not all commercial
strain gauges are manufactured in this material.
Useful properties of this material for strain-gauge manufacture include:
(i) High strain-sensitivity.
(ii) Chemical inertness.
(iii) Freedom from hysteresis and creep effects.
2 F. Ε. DUFFIELD
(iv) Good fatigue life,
(ν) Low cross-sensitivity.
(vi) Strength and flexibility are maintained at temperatures up to SOO^'C
but characteristics of the electrical connections to the strain element, and
of any bonding medium employed, will usually impose a lower tempera­
ture limit.
(vii) P-type or N-type material available.
The piezoresistive eff"ect in silicon is highly anistropic and strain-gauge
elements must be fabricated from single-crystal material of appropriate
orientation with respect to the crystallographic axes. Bulk silicon is a somewhat
brittle material with a strain limit of 1600 microstrain;^ however, when fabri­
cated in the form of thin-section filaments, flexibility is considerably increased
such that it may be formed with considerable curvature. In the thin-filament
form its strength is also considerably increased and working strains of up to
3000 microstrain are readily achieved. Care is essential during processing to
avoid surface damage to the silicon, which could severely limit the working
strain.
The concentration of impurities introduced into the material, or doping level,
governs such properties as the magnitude and sense of its piezoresistive effect,
its electrical resistivity and thermal coefficient of resistivity, and thermal
coefficient of strain sensitivity.
P-type material with positive gauge factor may be formed by doping with
boron.
N-type material with negative gauge factor may be formed by doping with
phosphorus.
A number of useful techniques are possible involving the application of semi­
conductor strain-sensitive elements to transducers—these include:
(a) Bonded strain-gauge techniques, involving flexible silicon strain gauges
cemented to metal flexures, which operate as either strain-responsive or
deflection-responsive elements.
(b) Techniques involving rigid or semi-rigid elements mounted in mechanical
flexures and responding to deflection.
(c) Diffusion techniques involving the formation of P-N strain-sensitive
junctions on silicon substrates which may themselves be in the form of
flexural members.
Bonded strain gauge transducers with metallic wire or foil strain gauges have
been developed over the past twenty years or so. Their well-known advantages
include infinite resolution, good accuracy and stability, and high mechanical
natural frequencies. Strain gauges are in the form of thin-section flexible
filaments fabricated from single-crystal silicon of orientation P(lll) or N(IOO)
material with resistivities in the range of 0Ό1 to 0-2 ohm-cm.
TRANSDUCERS WITH SEMICONDUCTOR STRAIN GAUGES 3
These gauges respond to tensile, compressive or flexural strains and are suit­
ably shaped for cementing to a variety of stress-members. The elements are
fabricated from an ingot of single-crystal silicon having the appropriate orienta­
tion. They are sawn and lapped to an initial cross-section of0-003 χ 0-009 — 0-018
in. according to final gauge resistance, followed by a chemical etch to further
reduce the section and to remove any surface damage from the previous pro­
cesses. Ohmic electrical contacts are then made, using suitably doped gold wire.
A eutectic bond is formed between the silicon and gold at a temperature of
approximately 400°C; a reducing atnlosphere is necessary to prevent oxidation
of the silicon. Further lead-out wires may be attached to the contacts if required.
The gauge elements are then cemented on to the substrate and electrolytically
o φ 20
II "'Ν" type
σ ω
10 Κ
2 Ε
Ο
Η
I I I I I I I I
*" -10 -40 -20 20 40 60 80 100 120
Temp, °C
FIG. la. Variation of gauge resistance with temperature. Gauges bonded to steel.
''N"type >w ^^P"iype
Compression >^ Tension
0
Compression ^^^^.^.^^^^ Tension
1 1 1 1 1 1 1 1 1 1
5000 4000 3000 2000 1000 0 1000 2000 3000 4000 5000
Micro strain
FIG. lb. Strain sensitivity of P- and N-type gauges. Gauges bonded to steel.
etched to finally size them and adjust their resistance value. The units are then
individually tested to confirm their upper strain limit and their resistance, and
gauge factor is also measured.
In many transducer applications un-backed gauge elements are used, cemented
directly to the stress-member, to improve strain-transfer. However, these units
are more fragile and require care in handling; pre-coating of the stress-member
is necessary before cementing to maintain adequate electrical insulation.
4 F. Ε. DUFFIELD
Under temperature variation, N-type gauges exhibit lower apparent strain
values than P-type gauges, as shown by the curves in Fig. 1(a). However,
P-type gauges have a more linear strain-sensitivity characteristic as shown in
Fig. 1(b) which makes them generally more suitable for transducer work.
Despite their higher apparent strain zero-compensation is easier to achieve.
The choice of stress-member for a semiconductor strain gauge transducer is
important; generally better linearity and temperature performance are obtained
with installations where all gauges are under the same initial strain and undergo
the same strain-excursion with applied stimulus.
Beams, ring flexures and diaphragms may form the basic elements of many
transducers for the measurement of displacement, load, pressure and accelera­
tion.
FIG. 2. Bonded semiconductor strain gauge pressure transducer.
If working strains are limited to 500-1000 microstrain, a significant output is
obtained, together with a high degree of overload capacity, without the use of
mechanical overload stops. In addition the limited deformation of the stress-
member ensures a high mechanical natural frequency and gives a linear trans­
ducer characteristic.
The bonded semiconductor strain gauge transducer shown in Fig. 2 is designed
to measure gauge pressures. Precipitation-hardening stainless steel is used and
the pressure cavity is fusion-welded to ensure maximum compatibility with
corrosive fluids by the elimination of any organic seals. The pressure-sensing
element is a planar diaphragm which is machined integral with the body
housing. This is a strain-responsive element with the gauges cemented to the
diaphragm on the side remote from the applied pressure. They are positioned to
sense the induced tensile and compressive strains and are connected as a fully-
active bridge. By selection and matching of the gauges, units can be produced
with consistent performance characteristics.
TRANSDUCERS WITH SEMICONDUCTOR STRAIN GAUGES
Considering the diaphragm as a flat plate with edge clamping under uniform
pressure p, it can be shown that the mean radial strain on the diaphragm distant
X from the centre is given by:
^2
(1)
1 i Μ .
^ ^ ^ ^^
Comp/
'VVT//////Í m
//YJ
FIG. 3a. Strain distribution across planar diaphragm.
FIG. 3b. Gauging arrangement of planar diaphragm.
where ρ = applied pressure,
a = radius of plate to clamped edge,
Ε = Young's modulus of elasticity for plate material,
Η = plate thickness,
μ = Poisson's ratio,
Κ = x/a.
6 F. Ε. DUFFIELD
Figure 3(a) shows the strain distribution across a planar diaphragm; referring
to equation (1) it is apparent that the strain at the clamped edge is twice the strain
at the centre and is of opposite sign. In this instance we have tensile strain at the
centre and compressive strain at the edge. Zero strain occurs at a radius of
0·577α. Figure 3(b) also shows a suitable gauging arrangement which will give
substantially equal mean working strains.
Pressure ranges covered by this design extend from 0-10,000 psi to 0-50 psi
with diaphragm natural frequencies of 47 kc/s to 5kc/s according to range.
Variations of the design are possible to cover absolute and differential pressure
instruments. For lower pressure ranges the deflection-responsive design shown
in Fig. 4 is attractive. The centre of the diaphragm is coupled by a push rod
to a mechanical flexure to which the gauges are bonded. Diaphragm deflection
Stimulus
P~elements-
P-elemenfs-
FiG. 4. Coupled constant stress beam element to diaphragm stress-member.
actuates the flexure to generate strains in the gauges. For low pressure/deflection
sensitivity the diaphragm may be of thin metal sheet and convoluted to reduce
stiffness.
Figure 6 shows details of a miniature linear accelerometer which was originally
designed for physiological applications but which has since been employed in a
number of other ñelds. A cantilever spring/mass seismic system is used, two
active strain arms being provided with semiconductor gauges cemented one on
either side of the spring. In view of the small size of the instrument and its
intended field of application, a nominal degree of damping was provided by a
fillet of silicon grease placed at the root of the cantilever. This system provides a
typical damping factor of 0-4 critical. Acceleration range is ±20^^ with a
natural frequency of 140 c/s.
The semi-rigid "unbonded" type of silicon strain-sensitive element provides a
unique application in transducer design. These elements are shaped for cement
ing across suitable mechanical flexures to act as displacement-responsive
elements. Figure 5 shows a number of possible configurations involving these
gauges mounted in position across the stress-member. Electrical contacts are