Erasable-foil-resistance compensation of strain gage transducers

A strain-gage transducer incorporating a plurality of electrical-resistance strain gages coupled together in a bridge network is caused to remain zero-balanced under varying-temperature conditions by way of dual-resistance foil-type unit interposed at one of the output corners of the bridge with its two like foil-resistance elements occupying adjacent arms. The foil material is selected to exhibit a resistance change with temperature representing a factor which enables a target room-temperature measurement to be calculated once a rate of change of bridge output with temperature has been determined from measurements taken while the unstrained transducer is held at different temperatures. Relatively broad surfaces of the foil elements are left exposed, and the unit of which they are a part is so disposed in relation to the transducer structure that the exposed foil is accessible for controlled erasure-type abrasion which will bring about the target measurement and attendant compensation.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements in precision strain-gage 
transducers which may be readily compensated for temperature effects upon 
zero-balancing, and, in one particular aspect, to high-performance 
transducers, such as miniature load beams, wherein temperature-induced 
instability of zero balance of strain-gage networks is uniquely and 
advantageously counteracted by way of resistance units readily abradable 
to adjust internal network resistances at room temperature and effect 
compensation in accordance with results of predetermined measurements at 
different temperatures and of related computations based upon both the 
measurement data and a factor associated with the resistance material. 
Accurate measurements characterizing such phenomena as force, torque, 
weight and pressure are often performed through the instrumentality of 
so-called strain-gage transducers, wherein electrical-resistance strain 
gages respond to elastic deformations of sensing elements undergoing 
loading. Such transducers have become well known in a variety of sizes and 
forms, and they can be expected to yield measurements with most remarkable 
exactness even under extremely severe operating conditions when 
constructed and compensated with great care. Among the numerous causes of 
possible error which can afflict precision transducers, some of the most 
vexing are associated with adverse effects of temperature changes. 
Virtually every portion of a transducer sensing element and its gages and 
its wiring systems can respond to thermal variations in some way tending 
to degrade resultant measurements, and it has therefore become common 
practice for manufacturers of such assemblies to incorporate 
temperature-compensation provisions into them. The compensation techniques 
and practices may vary, depending upon the nature and extent of thermal 
problems encountered in a particular situation, as well as the time and 
expense which can be justified in achieving desired degrees of 
improvement. 
When properly excited and coupled to impress its output upon a display or 
control stage, a strain-gage transducer in the unloaded condition might 
always be expected to signal a related zero output, and simple adjustable 
balancing resistances in adjacent arms of its strain-gage bridge network 
may in fact suffice transiently to trim the network to that zero-output 
condition when the temperature remains fixed. However, temperature 
excursions no greater than those likely to be encountered in many 
applications will most often have the highly undesirable effect of 
upsetting the pre-adjusted zero-balance, and consequently the accuracy of 
any measurements made without accounting for the imbalance. Repeated 
temperature re-cycling of the transducer, along with repeated bridge 
re-balancings, may well add significant manufacturing costs and 
difficulties without altogether eliminating the possibility of further 
unbalancing due to subsequent temperature changes, and, further, one 
cannot expect that zero-balance resistors built into transducers will be 
accessible for periodic adjustments by the user because such devices are 
commonly both remotely located and permanently hermetically sealed. It has 
been known previously to calculate the amount of temperature-compensating 
resistance which should be introduced in an arm of a transducer bridge 
circuit to achieve a desired balance, and to select and solder into place 
in an appropriate arm a small "charted" or predetermined length of 
temperature-sensitive wire which would provide that resistance, but exact 
lengths were often difficult to maintain effectively, due to such factors 
as the shunting effects of solder, and the attendant manual labor and 
skills involved were negative factors also. 
In accordance with teachings and related practices of the present 
invention, zero-balance resistances which are in some respects 
counterparts of prior balance resistances are also utilized, but permanent 
temperature compensation for zero balance purposes is brought about with 
the aid of further adjustable resistances which have known temperature 
dependencies and which are included in both of two adjacent arms of a 
bridge circuit as exposed foil elements accessible for critical adjustment 
by abrasion. By way of calculations based in part upon measurements 
obtained while the unloaded transducer is maintained at different 
temperatures, and by way of control measurements performed automatically 
or by an operator while one of the temperature-sensitive foil elements 
undergoes abrasion, the unsealed strain-gage transducer may be accurately 
temperature-compensated for a zero-balancing which will hold for 
subsequent operations of the finished product within a useful range of 
temperatures. 
Among prior U.S. patents which deal with various aspects of thermal 
compensation in respect of strain gage transducers are U.S. Pat. Nos. 
2,801,388 and 3,178,938. 
SUMMARY OF THE INVENTION 
In one preferred expression of the present invention, a miniature load-beam 
type of transducer, having four strain gages associated two each with 
flexure portions of two parallel beams forming a parallelogram-type 
sensing element, is also provided with a pair of bridge-balance resistance 
elements and a pair of temperature-compensation resistance elements, the 
two pairs of resistance elements being connected into adjacent arms of a 
bridge network formed by the strain gages, respectively at opposite 
"corners" where they can be effective to influence bridge zero-balance 
conditions. Both pairs of resistance elements are fashioned of foil 
materials, one of which has a predetermined temperature dependency to 
promote efficacy of the temperature-compensation elements, and are formed 
as flat units each atop a separate insulating carrier. The foil strands 
which make up the low-resistance elements remain exposed, and the two 
units are mounted adjacent the transducer where they will not interfere 
with the strain-sensing functions and where the exposed foil strands may 
be conveniently engaged and abraded by appropriate material-removal 
tooling. In its wired condition, but before any final sealing can 
interfere with intended abrasive adjustments of its resistance unit, the 
transducer has its bridge network excited by a reliable electrical power 
source and coupled to a suitable output device which will yield accurate 
electrical measurements characterizing even small bridge-unbalance 
conditions. The bridge zero-balance condition is initially measured by the 
output device at room temperature, and the transducer is next raised to a 
relatively high temperature and its voltage unbalance due to all 
temperature effects is then measured. The difference in output 
measurements, divided by the number of degrees of temperature change, 
represents a rate of change in zero balance with temperature, and that 
rate is multiplied by a factor which is equal to the reciprocal of the 
percentage change in resistance of the material of the 
temperature-compensation resistances to achieve the desired zero-balance 
compensation. Depending upon whether the target measurement is to be 
positive or negative, one or the other of the pair of foil 
temperature-compensation resistances is to be increased in resistance by 
abrading it, the sense being determined readily by shorting one or the 
other and thereby establishing which element will cause the measured 
output to change to the target value when increased in resistance. 
Motor-driven erasers afford a convenient and readily-controllable means 
for abrading the elements and changing their resistances. Once the target 
measurement is established, the bridge can be rebalanced following 
standard procedures in which the bridge-balance resistance elements are 
adjusted in value to achieve a zero balance for the transducer. 
Accordingly, it is one of the objects of the present invention to promote 
convenient and effective compensation for effects of temperature changes 
on zero balance of strain-gage transducers, by way of foil-type 
compensation resistance elements which are fashioned and disposed for 
compensating resistance adjustments by mechanical abrasion. 
A further object is to provide foil-type temperature-compensation 
resistance units which lend themselves to fine adjustment by erasure-type 
abrasion and which will correct zero-balance of strain-gage transducers in 
accordance with a unique proccessing wherein abrasive adjustments are 
performed to urge bridge outputs to target values based upon prior 
empirical determinations of rate of change in zero balance for the 
transducers involved and based further upon a factor characterizing 
temperature dependency of the foil material used in the compensating 
units. 
Still further, it is an object to provide new and improved adjustable 
foil-resistance compensation units to which soldering of connections may 
be effected without inadvertently altering resistance of sensitive 
portions thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Having reference to the drawings, wherein like reference characters 
designate identical or corresponding components and units throughout the 
several view, and more particularly to FIGS. 1 through 3 thereof, a 
strain-gage transducer, 8, which may be compensated for effects of 
temperature changes upon its zero-balance readings in accordance with 
these teachings, is equipped with a special foil-type resistance unit, 9, 
serving that purpose. The transducer element is fashioned from a flat 
metallic member, with rigid end portions 8A and 8B interconnecting its 
parallel relatively-flexible upper and lower beams, 8C and 8D, 
respectively, to form a parallelogram-type array which has basic design 
features in common with those disclosed in U.S. Pat. No. 2,866,059. In 
addition to the relatively large transverse opening, 8E, which separates 
the mid-portion of the element into the two beams, there are smaller 
openings through the end portions 8A and 8B accommodating mounting bolts 
and a force-applying link, respectively. Arrow 10 (FIG. 3) characterizes 
the intended direction and relative location of the loading forces which a 
linkage would apply to the beam end 8B while its opposite end 8A is held 
relatively fixed, although for purposes of the compensation under 
discussion the element is left unloaded and unstrained insofar as 
possible. In the latter connection, it is also required that the 
compensation resistance unit 9, and a companion bridge-balance resistance 
unit 11, be physically accessible for resistance adjustments by mechanical 
abrasion, and any final sealing which would block such accessibility is 
therefore deferred until the zero-balance temperature compensation is 
established. It is on that account that the transducer assemble of FIGS. 
1-3 is portrayed without the hermetic-sealing bellows which will later 
connect with and span the two spaced circular collars 8F and 8G and 
protectively enclose the transducer strain gages as well as the resistance 
units 9 and 11. Gaging for the parallel beams 8C and 8D is in this 
instance disposed along the top-and bottom-most surfaces of the sensing 
mid portion of the transducer, where the top dual-gage strip 12 includes 
at one end a tension-characterizing foil gage 12A and at the other end a 
compression-characterizing gage 12B, and where the like bottom strip 13 
includes a compression-characterizing foil gage 13A and a 
tension-characterizing gage 13B. 
The pictorial-and block-diagrammed wiring arrangement appearing in FIG. 5 
characterizes a Wheatstone-bridge network in which both the two sets of 
dual strain gages 12A-12B and 13A-13B and the two balance-resistance units 
9 and 11 are disposed within its four arms. With reliably stable 
electrical excitation applied across the input terminals 14 and 15 wired 
respectively with the junctions of strain gages 12A-12B and 13A-13B, the 
bridge should give evidence of electrical unbalance conditions across its 
output terminals 16 and 17 only in response to loading of the transducer. 
However, a sensitive electrical signal detector, 18, which may be in the 
known form of a digital voltmeter in some practices or in the form of an 
input stage to automatic calculating and control equipment in other 
practices, will tend to be falsely supplied with bridge output signals 
which instead reflect the troublesome zero-balance type of variations with 
temperature which have been referred to hereinabove. It is with the object 
of effecting compensation for those variations that resistance unit 9 is 
included within the bridge, in two of the arms adjacent one of the output 
"corners", namely that associated with output terminal 16 in the network 
under consideration. Bridge-balance resistance unit 11, which is not 
temperature-dependent, and which is included only to bring about a 
collateral conventional type of zero-balancing, is similarly disposed at 
the opposite output "corner", near output terminal 17. 
The physical construction of compensation unit 9 is important to the 
success not only of the temperature compensation itself but also of a 
precision transducer-manufacturing operation which can be implemented 
economically and either without unusual operator skills or by way of 
automation. It is a foil unit (FIG. 4) having three relatively large- or 
broad-area wiring tabs, 9A, 9B and 9C, and two relatively low-resistance 
strands or half-loops, 9D and 9E, all of which are part of one layer and 
lie in one plane atop an insulating carrier, 9F, and are integral with one 
another. The rectangular carrier need only be of a paper-like thickness, 
9G, and may in fact comprise a plastic-impregnated paper material, or the 
like, and may typically have a length, 9H, of about 7/16 inch and a width, 
9I, of about 0.187 inch. Metals preferred for the foil layer are those 
with a relatively high temperature coefficient of resistance, such as 
nickel, copper, and a stable nickel-iron alloy known commercially as 
Balco. For example, the percentage change of resistance (increase), over a 
100.degree. F. range, is about 30% in the case of nickel, and about 20% 
for copper, and about 27% for Balco. Conventional printing and acid-etch 
and like foil-gage and printed-circuit manufacturing techniques may be 
used to produce the desired pattern wherein the thin but relatively wide 
strands 9D and 9E are elongated and looped broadly outwardly from the 
clustered wiring tabs 9A, 9B and 9C, on opposite sides, as shown in FIG. 
4. Both half-loops or strands are joined directly with a common tab, 9B, 
and, in each instance, the integral connections between a strand and it 
wiring tabs is narrowed or necked-down to form a constriction beyond which 
molten solder from the wiring tabs will not tend to flow. The four 
constrictions, 9J, are narrow enough to hold the molten solder at those 
sites, by what evidence themselves as surface-tension effects akin to 
those of capillary action, and yet the constrictions do not themselves 
unduly increase the as-formed low resistance (example: one-half ohm) which 
the half-loops 9D and 9E offer between the tabs which they span. Unless 
the molten solder is blocked in that way, it may spread from the tabs 
while wiring is being connected, with the result that one or both 
resistance loops may become partially shunted lengthwise by the solder of 
very low resistance, and the desired predetermined resistance and 
temperature-dependency may be altered enough to impair the intended 
compensation adjustments. 
When a transducer like that of FIGS. 1-3 is fully wired into a network like 
that of FIG. 5, its output device 18 is used to characterize its 
zero-balance signalling at a first relatively low temperature, i.e., while 
no loads are being experienced. A room-temperature zero-balance signalling 
is obtained, if the first temperature is not room temperature. The 
unstrained transducer is then raised to a second relatively high 
temperature, at which another zero-balance signalling is characterized by 
the output device, and a rate-of-change of zero-balance is established by 
the ratio of the difference in the two characterizations to the difference 
between the first and second temperatures. In the case of 
unbalance-voltage indications by device 18, for example, its two 
characterizations of voltage yield a simple difference voltage value, 
which leads to a ratio which reduces to a voltage change per degree, 
specifically voltage per degree Fahrenheit in a preferred context. From 
that ratio, a product is obtained with a factor which is the reciprocal of 
the aforementioned percentage change of resistance which is characteristic 
of the material of which the foil is made, namely 3.33 for nickel 
(1/0.30), 5 for copper (1/0.20), and 3.8 for Balco (1/0.27). That product 
represents a signalling value, as sensed by device 18, which is next 
subtracted from a zero-balance signalling which had been realized at room 
temperature, to result in a "target" zero-balance signalling which should 
be achieved by adjustment of resistance in unit 9 in order to effect the 
desired temperature compensation. Numerical readings related to such 
signalling, such as voltage readings, may be observed by an operator, for 
example, and the adjustment of resistance is then made without 
specifically determining what the resistance values in unit 9 are, but, 
instead, by observing the numerical output readings and making the 
adjustment progressively until the "target" reading is registered by 
device 18. Although direct numerical indications, and simple calculations, 
may be observed and made readily, the processing may be accelerated and 
rendered less costly and demanding of skill by having the measurements and 
computations performed automatically, either in whole or in part. 
FIG. 6 illustrates the use of an eraser-type abrasive member 19, rotated in 
the chuck of a small motor 20, to adjust resistance in the 
temperature-compensation unit 9 in furtherance of the aforesaid 
compensation procedures. While the member 19 is being rotated, as 
suggested by arrow 21, it is brought into abrading engagement separately 
with the foil loop 9D and/or 9E, as required, to reduce the thickness of 
the foil there and thereby raise the effective resistance and change the 
reading of device 18 to the "target" reading, at room temperature. Before 
abrading is commenced, the one of half-loops 9D and 9E which should be 
raised in resistance to drive the reading to the "target" value is 
conveniently established by shunting or shorting tabs 9A and 9B, or 9B and 
9C, and determining whether the change in reading is in direction toward 
or away from the "target" value. If the change is in a direction which 
would enable the "target" value to be reached, then the half-loop opposite 
that which was shorted should be abraded and raised in resistance until 
the "target" reading obtains; otherwise, that same half-loop should be so 
abraded. Reasonable amounts of "overshoot" of the "target" reading, caused 
by excessive abrasion of the foil in one half-loop, can be readily 
corrected by abrading the other until the desired reading returns. The 
abrasive eraser 19 is preferably large enough to engage a large area of 
any half-loop such as 9D or 9E, rather than just a small spot which might 
then be completely worn through by too-vigorous abrading. Any surface film 
remaining on the exposed foil half-loops is removed by the initial 
abrading, and the rate of change of zero-balance readings tends to 
increase sharply once such film is removed. Heat generated by the abrading 
is to be avoided, and it is therefore advisable to abrade in steps, 
stopping every so often to permit such heat to dissipate and/or to apply a 
fluid cleaner or coolant. 
Once the "target" output is reached, signifying completion of the 
zero-balance temperature-compensation measures, the transducer bridge 
network is preferably rebalanced so there is no zero-loading output. That 
may be accomplished with the aid of bridge-balance resistance elements 11D 
and 11E of unit 11 (FIG. 5), those elements preferably being of the same 
foil configuration as the elements of compensation unit 9 but being made 
of a foil material, such as constantan (about 60% copper and 40% nickel), 
which has negligible resistance change with temperature over a 
commonly-expected range of temperatures. They may also have somewhat 
greater resistance, such as about three times the resistance of elements 
9D and 9E. Erasure-type abrasion, like that described hereinabove, may be 
empolyed to increase resistance of one of the elements 11D and 11E in 
relation to that of the other, and thereby cause the output of the bridge 
network to become balanced to substantially zero while the transducer 
remains unstrained. 
An alternative useful configuration of foil unit, 9', is represented in 
FIG. 7, together with a fragment of the rigid end portion 8B of the same 
transducer, 8. The unit construction is generally like that of 
temperature-compensation unit 9, and like reference characters, 
distinguished by single-prime accents, are therefore used to designate the 
same or like parts. As in the case of unit 9, the carrier, 9F', is mounted 
in relation to transducer rigid end portion 8B, as by direct bonding 
thereto, such that it maintains the same temperature as the nearby strain 
gages and deformable sensing portions of the transducer body. However, the 
two half-loop elements 9D' and 9E' are substantially semicircular and lack 
corners, which is more conducive to trouble-free abrasion by cylindrical 
eraser-type tools having substantially flat circular ends conveniently 
moved into and out of abutting abrading engagements along paths of 
movement perpendicular to the flat carrier 9F' and the planar foil 
elements 9D' and 9E'. Moreover, the problem of unwanted molten solder flow 
along the half-loop elements from wiring end tabs 9A' and 9B' is resolved 
by providing small openings, 22 and 23, within them at locations where the 
half-loops merge or join integrally with those tabs, the openings serving 
to separate the junctures into two spaced parallel paths each so narrow as 
to have surface-tension effects which will block the flow of solder 
significantly beyond them. The openings 22 and 23 are of course large 
enough and sufficiently off-centered to make bridgings by solder unlikely 
when reasonable care is being exercised during the wiring. Mid tab 9B' may 
have a like opening also, but a single necked-down constriction 24 
connecting it with the junction of the two half-loops is instead shown to 
offer the same type of blockage. 
It should be understood that the specific embodiments and practices 
described in connection with this specification have been presented by way 
of disclosure rather than limitation, and that various modifications, 
combinations and substitutions may be effected by those skilled in the art 
without departure either in spirit or scope from this invention in its 
broader aspects and as set forth in the appended claims.