Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention relates to a thin film strain gage
apparatus.
Force transducers embodying thin film strain gaga
resistance bridges have been in use for many years. Typical-
ly, the gages are provided on a flexure element which deforms
in response to an applied force. In such cases, temperature
effects may cause unequal expansion of the legs of the
bridge even when no actual force is being applied. This cau-
ses a shift in the zero point of the bridge since an output
will be produced even when no force is applied. Similarly,
temperature effects may result in differential changes in the
elasticity or spring constant of various parts of the trans-
ducer, so that a given deflection of the flexure element will
cause different bridge outputs as the temperature varies.
This causes a shift in the span of the bridge, also known as
the gage factor or sensitivity.
Various approaches to compensation for temperature
effects have been followed in the past. Bodner et al dis-
closed in U.S. Patent 2,930,224 a type of temperature compen-
sating strain gage in which a strain-insensitive thermocouple
is used to generate a current flow opposite to that flowing
in the gage resistance in order to cancel out temperature ef-
fects. The temperature compensating elements, however, are lo-
cated on the strained portion of the flexure element and there-
fore in fact are subject to resistanoe variations due to applied strain.
Starr also disclosed in U.S. Patent 3,034,346 a technique for
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compensation of strain gage nonlinearity in which the compen-
sating resistances are placed on the strained portion of the
flexure element. Billette et al show in U.S. Patent No.
3,886,799 a type of semiconductor pressure transducer in which
compensating elements are provided on the flexure element with
the strain gage bridge.
While these prior art devices have achieved a mea-
sure of success in compensating for temperature effects, the
location of the compensating elements on the strained portion
of the flexure element causes resistance variations due to
strain which tend to interfere with the desired function of
the compensating elements: the minimization of temperature
effects. Moreover, due to the complicated procedures by which
prior art thin film strain gage transducers have been made,
manufacturing time has been rather long and cost high.
OBJECTS OF THE INVENTION
An object of the invention is to provide an improved
thin film strain gage transducer havin~ provision for tempera-
ture compensation.
Another object of the invention is to provide such a
transducer in which the compensating elements are not subject
ta applied strain which would influence their performance.
Still another object of the invention is to provide
such a transducer in which the structure of the strain gages
and compensating elements is quite simple, thereby facilitat-
ing quick and less expensive manufacture.
These objects are given only by way of example;
thus, other desirable objectives and advantages inherently
achieved by the disclosed invention may occur to those skilled
in the art. Nonetheless, the scope of protection is to be
limited only by the appended claims.
SUMMARY OF THE INVENTION
The above objects and other advantages are achieved
with the invention which comprises, in one embodiment, a
flexure element having at least one thin film strain gage
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3~
resistance element deposited thereon in a position to be
strained upon deformation of the flexure element. Leads of
a material having a temperature coefficient of resistance
opposite to that of the strain gage resistances are attached
to the gages. Temperature compensation resistors are f~rmed
in the leads and deposited at a location on the flexure ele-
ment which is unstrained during operation. A bridge of the
strain gages is usually used. Due to the simplified process
used to make the transducer, the leads are superposed on an
underlying thin layer of the same material as the strain
gage resistances.
As used in this application, the term "thin film"
refers to elements of minute thickness which are deposited
using sputtering or vacuum deposition techniques. The thick-
ness of such films is typically measured in Angstrom unitsor microns so that several layers of such "thin films" may
have a thickness of only 4 to 30 microns and an individual
layer may have a thickness of about 200 Angstrom units to 1
micron. Such thin film elements are used in integrated cir-
cuits and are readily distinguishable from discrete elementsor, as in the case of strain gages, from bonded gages or
wire gages.
BRIEF DESCRIPTION OF THE DRAWING
-
Figure 1 shows a greatly enlarged, perspective view
of a flexure element having deposited thereon a temperature
compensated strain gage bridge according to the present in-
vention.
Figure 2 shows a schematic diagram of the bridge
illustrated in Figure 1.
Figure 3 shows a greatly enlarged cross-section
taken along line 3-3 in Figure 1, indicating portions of the
individual thin films deposited to form the bridge strain
gage resistances and electrical leads.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The following is a detailed description of the
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invention, reference being made to the drawing in which
like reference numerals identify like elements of structure
in each of the several Figures.
Referring to Figures 1 to 3, a force transducer
embodying the invention is seen to comprise a flexure beam
or element 10 having an immovable portion 12 and a movable
portion 14 joined by a flexible portion 16. Flexure element
10 typically is made from a resilient material such as steel
in a rectangular parallelepiped configuration, as illus-
trated; however, any suitably resilient~material may beused. Flexible portion 16 is formed by drilling or other-
wise forming two holes 18,20 laterally through element 10,
joining the holes with a slot 22, and opening hole 20 to
the bottom of element 10 with a slot 24. Thus, when im-
movable portion 12 is fixed and a force is applied to mov-
able portion 14 as indicated by the arrow in Figure 1, the
upper surface 26 of flexible portion 16 deforms into a
curved configuration so that the thin section 28 above
hole 18 is placed in tension; and the thin section 30
a~ove hole 20 is placed in compression.
Four thin film strain gage resistance elements
Rl, R2, R3 and R4 are deposited on upper surface 26 in a
manner to be described below, so that Rl and R3 are above
thin section 28 and R2 and R4 are above thin section 30.
Figure 2 indicates schematically which strain gage resis-
tance elements are in tension ~T) and compression (C~,
and also shows their interconnection into a Wheatstone bridge
pattern. Resistance elements Rl and R4 are connected at
node 32 by thin film metal leads 34,36. A long thin film
lead 38 runs from node 32 off movable portion 14, onto im-
movable portion 12 and to a serpentine thin film temperature
compensation resistance element RSl which is of the same
metal as lead 38. The other end of resistance element Rsl
joins a connector pad 40. A thin film lead 42 runs from re-
sistance element R4 off movable portion 14, onto immovableportion 12 and to a serpentine thin film temperature compen-
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sation resistance element Rzl which also is of the same
metal as lead 42. The other end of resistance element Rzl
joins a second of connector pads 44. Resistance elements Rl
and R2 are connected at node 46 by thin film metal leads 48,50.
A long thin film lead 52 runs from node 46 to a connector pad
54 deposited on immovable portion lZ. A thin film lead 56
runs from resistance element R2 to node 58 which is connec-
ted~to resistance element R3 by thin film lead 60. A long
thin film lead 62 runs from node 58 to a further serpentine
thin film temperature compensation resistance element RS2
deposited on immovable portion 12. The other end of resis-
tance element Rs2 joins a connector pad 64. Finally, a long
thin film lead 66 runs from resistance element R3 to a fur-
ther serpentine thin film temperAture compensation resistance
element R 2 deposited on immovable portion 12 and formed of
the same metal as lead 66. Resistance element Rz2 terminates
at a second of connector pads 68.
In Figure 3, a schematic sectional view is shown,
taken along line 3-3 of Figure 1, next to resistance element
Rl. Resistance elements Rl to R4 and elements 32 to 68
preferably are deposited on flexure element 12 using a unique
four layer structure and conventional photolithographic tech-
niques to define resistor and lead geometries. Following suit-
~ble cleaning of flexure element 12, an electrically insula-
tive layer 70, a resistive layer 72 and a conductive layer
74 are deposited seriatim on surface 26, so that the entire
surface 26 is covered by three congruent layers. Then, using
a suitable photomask, layer 74 is etched away to leave behind
only those portions of layer 74 re~uired for the lead pattern
and temperature compensation resistance geometries discussed
above. After th~t, using another suitable photomask, layer
72 is etched away to leave behind only resistance elements
Rl, R2, R3 and R4 joined to their respective leads. As shown
in Figure 3, each lead and temperature compensation resistance
element actually is made up of two superposed thin films of
congruent geometry, an upper metal film remaining from layer 74
and beneath it a lower resistive film remainlng from layer 72.
A passivatio~ layer 76 preferably is applied over the entire
- gage assembly, followqng which through holes or vias (not shown),
S are etched through to connector pads 40, 44 (2) 54, 64 and 68 (2).
Insulative layer 70 may be formed of ~ 05; resistive
layer 72, of oonventional cermet material; and conductive layer 74,
of gold. Other suitable materials may also e used such as alumina
or Fosterite (a trade mark for a magnesium-silicon-oxygen
- 10 insulating mater;~l) for insulative layer 70; Nichr e,
(a trade mark for a nickel-iron-chromium alloy) MOSI or CRSI, for
resistive layer 72; and nickel, for conductive layer 74. The
temperature ccefficient of resistance of the strain gage resistive
material 72 is chosen to be of opposite polarity to that of the lead
material 74.
In operation, as movable portion 14 is deflecte~ upwardly
due to applied force, the resistance of elements Rl to Rg will
change due to the applied strain. Bridge power is applied across
connector pads 40,64 and the bridge output is taken across
connector pads 54 and 44-68, in the well known manner. Should
the temperature of the various resistances change from the level
at which the transducer was calibrated, the resistance of elements
Rl to R4 will chan~e in one direction; and that of elements Rsl
and Rs2 and the elements of R 1 and/or Rz2 (left in the circuit)
will change in the opposite direction. The determuning factor of
whether Rzl and Rz2 are left in the circuit or shorted out of the
circuit during calibration depends on the zero setting cali-
bration requirements. The changes in resistance Rsl and Rs2
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tend to maintain a relatively constant span or gaye factor;
whereas, the changes in Rzl and/or Rz2 tend to maintain a
relatively constant zero setting when no load is applied,
even as temperature varies.
Resistances Rsl and RS2 are shown in the input cir-
cuit to the bridge; however, placing them in the output cir-
cuit is also within the scope of the invention. Similarly,
resis'cances Rzl and Rz~ are shown in series with the strain
gage resistances in the legs of the bridgei but they could
13 also be placed in parallel with the strain gage resistances
and still be within the scope of the invention. Also, while
serpentine geometries are shown for the temperature compensa-
tion resistances, this geometry is not critical, other ar-
rangements being encompassed by the invention. For example,
variation of the thickness of the gold layer to affect the
compensation resistances is an alternate approach.
Having described my invention in sufficient detail
to enable those skilled in the art to make and use it, I
claim:
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