Force sensing ink, method of making same and improved force sensor

An improved carbon-free force sensitive ink for a force sensor. The ink when deposited and dried can function at temperatures of up to 350.degree. F. and pressures of up to 10,000 psi. The preferred ink includes a thermoplastic polyimide binder, conductive particles, intrinsically semi-conductive particles, and dielectric particles, all of an average particle size of 1.0 micron or less. The preferred semi-conductive particles are molybdenum disulfide, ferric and ferrous oxide particles. The preferred conductive particles are conductive metal oxide compounds that deviate from stoichiometry based on an oxygen value of two, such as conductive tin oxide, Fe.sub.3 O.sub.4 iron oxide, and mixtures of them. The preferred dielectric particles are silica. The binder is present in an amount of 20 to 80% by volume. The sensor and a method of making the sensor are also disclosed.

BACKGROUND OF THE INVENTION 
For some time now a variety of techniques have been used to fabricate force 
sensors which provide an indication of the force applied between a pair of 
mating surfaces. These techniques have included the utilization of thin 
layers of semi-conductive materials disposed between the surfaces which 
respond to applied loads and which, when properly provided with conductors 
and associated circuitry, facilitate the display of indications of applied 
loads. 
Early versions of products employing some of those features include those 
disclosed in U.S. Pat. Nos. 3,806,471 and 4,489,302. The common 
characteristic of those products is that they employ a body of 
semi-conductive material which, when stressed by the application of a 
load, will increase in conductivity. That increase in conductivity, which 
tends to increase as a function of the applied load, may then be used to 
provide a measurable output which varies, within limits, as a function of 
the applied load. Force sensing systems employing semi-conductive 
materials and based upon these principles are additionally shown by U.S. 
Pat. No. 4,856,993. 
Typically semi-conductive layers used in force sensors must have certain 
characteristics to be sufficiently electrically conductive to be 
effective. Thus such layers must have electrically conductive areas which 
are close enough together to allow conduction under load. Under load the 
conductive areas must contact each other or the distances between them 
must be so small that electrons can flow from one conductive area to the 
next. The concentration of conductive areas must be large enough to 
provide a conductive path through the layer. The conductivity through the 
layer must be sufficient, under load, to provide a reliable and consistent 
range of different resistances (or conductances) to be able to distinguish 
among a range of applied loads. Typically the application of a load 
increases the capacity of the layer to allow electron transfer. Further, 
the conductivity changes should be reversible to the extent that the layer 
and surfaces on which the layer is applied permit restoration of the 
characteristics of the layer which are altered as load is applied. The 
pressure-sensitive, load responsive characteristics may be at the surface 
of the layer or internally thereof, or both. 
A variety of intrinsically semi-conductive materials have been used to 
provide force sensors of this type. Such materials include particulate 
molybdenum disulfide, and ferrous and ferric oxide, among others. Such 
materials are disclosed in the patents referred to above, as well as in 
U.S. Pat. No. 5,296,837. 
In addition to the use of semi-conductive systems to produce force sensing 
transducers, particulate conductive materials have also been used to 
produce force sensing transducers, as exemplified by the disclosure of 
U.S. Pat. No. 5,302,936. This patent and U.S. Pat. No. 5,296,837 both 
disclose the use of carbon as a conductive material in force sensing inks. 
The latter patent uses stannous oxide as a semi-conductive material in 
combination with carbon. 
In more recent times, as shown by the prior art referred to above, 
semi-conductive, pressure-sensitive transducers have been made by 
depositing semi-conductive material, as in the form of an "ink" deposited 
by spraying or by a silk screening process, to form a thin layer or layers 
between a pair of electrodes. Typically, the electrodes are disposed on 
thin, flexible plastic sheets and have leads to a remote region in which 
the flow of an applied current may be sensed and measured. In such 
sensors, the electrodes and dried ink residue form a sandwich which acts 
as a force transducer, and which will provide a variable resistance (or 
conductance) which is related in a predetermined manner, to applied loads. 
The prior art also teaches the use of blends of semi-conductive particles 
and conductive particles to provide a variably conductive force 
transducer. In particular, the prior art teaches the use of molybdenum 
disulfide as a semi-conductor blended with graphite or finely divided 
conductive carbon (such as acetylene black). The conductivity of inks 
based on these materials may be varied by the concentrations or ratios of 
the conductive and semi-conductive particles, frequently by blending a 
highly conductive ink with a less conductive ink. Polyester is the binder 
frequently used to bind the particles in these inks to a substrate on 
which a dried layer of the deposited materials is disposed. The resistance 
of the dried layer varies with load; hence these inks are referred to as 
being pressure-sensitive or force-sensitive. 
These prior art inks have a number of shortcomings. For example, 
conventional binders, such as polyester binders, limit the useful 
application temperatures to a range of from up to 120.degree. to no more 
than about 150.degree. F. Above that temperature range, binders in 
confronting semi-conductive layers tend to bond to each other. Further, 
conductive carbon black when used as a pigment in resistive inks is very 
difficult to disperse uniformly and tends to agglomerate after dispersion. 
In addition its surface reactivity and adsorption characteristics 
significantly depend on processing variables and heat history. Further, 
graphite platelet orientation in the dried ink film is difficult to 
reproduce from sensor to sensor. These factors add great variability to 
the conductivity of such inks, hence cause unacceptable and undesirable 
variations within a product and from product to product. 
Because molybdenum disulfide becomes more conductive as temperature 
increases, the use of molybdenum disulfide and conductive carbon black to 
provide the conductive paths requires changing their ratios or 
concentrations to adjust the conductivity of the ink for anticipated 
temperature conditions to be encountered. Because of the sensitivity of 
molybdenum disulfide to changes in temperature, compensation for 
temperature is difficult when the concentration of molybdenum disulfide is 
used by itself to adjust conductivity. 
It would be desirable to provide a force transducer having improved force 
sensitivity, reliability, and reproduceability, as well as the additional 
capacity to function effectively not only at current temperatures at which 
force sensors are used, but at elevated temperatures, such as at 
temperatures of from at least 120.degree. F. to 150.degree. F. up to about 
350.degree. F., while providing sufficient sensitivity and 
reproduceability to provide a reliable and consistent indication of 
applied load. 
SUMMARY OF THE INVENTION 
In accordance with the present invention improved high temperature, 
carbon-free force sensing inks, methods of making them and resulting force 
sensors are provided. A high-temperature, carbon-free force sensing ink in 
accordance with this invention is adapted to be deposited in a thin layer 
between a pair of conductors, each conductor being disposed on a support 
surface, the thin layer having a resistance which varies as a function of 
the force applied thereagainst, the thin layer being usable in force 
sensing applications at temperatures of from 150.degree. to 350.degree. F. 
and wherein the ink comprises a high temperature binder, intrinsically 
semi-conductive particles, and conductive particles, the conductive 
particles preferably comprising a conductive metal oxide compound that 
deviates from stoichiometry based on an oxygen value of two. Preferably 
the conductive oxide particles are conductive tin oxide particles, 
Fe.sub.3 O.sub.4 iron oxide particles or mixtures thereof. 
The force sensing ink may include dielectric particles, such as silica 
having a particle size of 10 microns or less. The semi-conductive 
particles are preferably molybdenum disulfide particles. The particles in 
the ink are desirably of a particle size of 10 microns or less (and most 
preferably no more than about 1 micron in average size) and the high 
temperature binder is a thermoplastic polyimide resin. In a preferred 
form, the conductive and semi-conductive particles are present in a 
combined concentration of from at least 20% by volume to 80% by volume of 
the dried ink when deposited in a thin layer, and the binder is present in 
a combined amount of from 20 to 80% by volume. 
In another aspect of the invention, a method of controlling the temperature 
and pressure responsiveness of a carbon-free, pressure sensitive, force 
sensing ink layer is provided. It comprises the steps of providing a first 
mixture of intrinsically semi-conductive particles and conductive 
particles in a ratio of from 15 to 65 parts of semi-conductive particles 
to 55 parts to 5 parts of conductive particles by volume, the remainder 
being a temperature resistant binder, providing a second mixture of 
intrinsically semi-conductive particles and dielectric particles in a 
ratio of from 15 parts to 65 parts of semi-conductive particles to 55 
parts to 5 of dielectric particles by volume, the remainder being a 
temperature resistant binder, mixing quantities of said first and second 
mixtures having the same amounts of semi-conductive particles by volume to 
produce a force sensing particulate in a ratio of from 4 to 96% of the 
first mixture with from 96 to 4% of the second mixture thereby to provide 
an ink for deposit and use in a force sensor. 
Preferably the semi-conductive particles are molybdenum disulfide particles 
and the semi-conductive and conductive particles are of an average size of 
1.0 micron or less. Desirably the binder is a thermoplastic polyimide 
binder and the conductive and semi-conductive particles are present in an 
amount of at least 20% by volume and less than 80% by volume of the dried 
ink when deposited in a thin layer. In a most preferred form, the binder 
in present in a combined amount of from 20 to 80% by volume and the 
conductive and semi-conductive particles are present in a combined amount 
of from 80 to 20% by volume. 
The resulting pressure-sensitive force sensor of the present invention 
comprises a thin, flexible film, a first electrode on the film, a 
carbon-free, pressure sensitive, resistive material deposited on the 
electrode, the material comprising a high temperature resistant binder, 
intrinsically semi-conductive particles and conductive particles 
comprising in the most preferred form, a conductive tin oxide, Fe.sub.3 
O.sub.4 ferric oxide or mixtures thereof, the conductive and 
semi-conductive particles being present in an amount of from 20 to 80% by 
volume of the material, and a second electrode spaced from the first 
electrode by the pressure sensitive, resistive material so that the 
material may be squeezed between the electrodes to vary the flow of 
current therethrough as a function of the force applied. 
Desirably the material further comprises dielectric particles, the 
semi-conductive particles are molybdenum disulfide particles, and the 
semi-conductive and conductive particles are of an average size of 1.0 
micron or less. Preferably the binder is a thermoplastic polyimide binder. 
In a most preferred form, the binder in present in a combined amount of 
from 20 to 80% by volume and the conductive and semi-conductive particles 
are present in a combined amount of from 80 to 20% by volume when 
deposited in a thin layer. 
Further objects, features and advantages of the present invention will 
become apparent from the following description and drawings.

DETAILED DESCRIPTION 
In accordance with the present invention, inks are prepared which, when 
deposited, produce intrinsically semi-conductive layers which are stable 
and usable at customary temperatures as well as at temperatures of from 
about 120.degree. F. to 150.degree. F. up to 350.degree. F. and which 
reliably reproduce responses to forces of as much as 10,000 psi at 
350.degree. F., even after repeated loading or prolonged exposure to 
elevated temperatures and loads. 
A high-temperature resistant force sensor employing inks of the present 
invention is illustrated in FIGS. 1 and 2. As there is shown, a button 
sensor 10 comprises a pair of thin, flexible films 20, 40 which may be 
transparent. Films 20, 40 may be separate or may be the same sheet which 
is adapted to be folded into a sandwich array to produce the sensor 10. 
Polyester or polyimide films are preferred. Such films may be ICI 
polyester film and DuPont Kapton polyimide film. ICI polyester film is 
available from ICI Americas Inc., Concord Pike, New Murphy Road, 
Wilmington, Del. 19897. Films 20, 40 are provided with electrodes 22, 42, 
respectively, which are electrically connected to conductors 24, 44, 
respectively, and contacts 28, 48. The electrodes, conductors and contacts 
may be deposited, as by silk-screening a conductive silver ink, in a known 
manner, or by sputter coating a layer of copper with an overcoat of 
nickel, such as to a total thickness of 2400 angstroms. The conductors are 
adapted to be connected in an electrical circuit in a manner known to the 
art so that current flow through the sensor 10 may determined in use. The 
electrodes may be of any desired shape. In this case they are shown as 
being round. Each has a diameter of 0.5 inch. 
Each of the electrodes is overlaid with a layer 26, 46 of carbon-free, 
pressure-sensitive resistive material of a diameter of 9/16 inch which is 
the dried residue of an ink which was deposited thereon. Such an ink may 
be deposited via silk screening, spraying or other known application 
techniques. In a preferred form, that material comprises a 
high-temperature resistant binder, semi-conductive particles, such as 
molybdenum disulfide or ferric or ferrous oxide particles, and conductive 
particles comprising a conductive metal oxide compound that deviates from 
stoichiometry, such as the reaction product of stannic oxide and antimony 
oxide, Fe.sub.3 O.sub.4 iron oxide, or mixtures thereof. A layer is 
preferably formed over each of the electrodes 22, 42 in a diameter 
slightly greater than the area of the electrode, so that when a sensor 
sandwich is formed from films 20, 40 there are two thin layers of 
pressure-sensitive resistive material in contact with each other, and 
which layers entirely overlay the electrodes, thereby to assure that the 
desired contact area is uniform from sensor to sensor. 
In a preferred form, the thin film sensor 10 is from about 2.5 to about 3.5 
mils thick in the sensing area. The films 20, 40 are each about 1 mil 
thick, the electrodes 22, 42 are each about 0.2 to 0.3 mil thick, and each 
dried resistive ink layer is about 0.3 to about 0.5 mil thick. Other 
thicknesses of the elements of the sensor 10 can be used depending upon 
the application and other factors relevant to a particular application, 
all as is well understood by those working in the art. 
In one preferred form of the practice of the present invention a 
high-temperature, carbon-free force sensing ink adapted to be deposited in 
a thin layer between a pair of conductors was prepared as follows. 
EXAMPLE I 
First, a 20 percent solution of thermoplastic polyimide resin was prepared 
by dissolving the polyimide in acetophenone. The particular polyimide used 
was Matrimide 5218, available from Ciby-Geigy Corporation, Three Skyline 
Drive, Hawthorne, N.Y. 10532. Matrimide 5218 is a fully imidized soluble 
thermoplastic resin based on 5(6)-amino-1-(4' 
aminophenyl)-1,3,-trimethylindane. To 30 grams of this solution, 10.6 
grams of molybdenum disulfide (technical fine grade) and 2.6 grams of the 
reaction product of stannic oxide and antimony oxide (sometimes referred 
to as a conductive tin oxide) were added. The reaction product used had an 
average particle size of 0.4 micron and is available from Magnesium 
Elektron, Inc., 500 Point Breeze Road, Flemington N.J. 08822 under the 
trade name CP40W. The reacting material are primarily tin oxide (as 
SnO.sub.2), namely 90 to 99%, with a minor amount of antimony oxide (as 
Sb.sub.2 O.sub.3), namely 1 to 10%. The semi-conductive molybdenum 
disulfide and the conductive tin oxide reaction product particles had an 
average particle size of 0.7 and 0.4 micron, respectively. 
The polyimide solution and added particles were mixed in a high speed 
laboratory mixer for ten minutes. The resulting ink was then silk screened 
in a conventional manner onto each of two circular conductors 
(approximately one-half inch diameter) and dried for 15 minutes at 
275.degree. F., at which time the acetophenone was completely driven off. 
The two layers of pressure-sensitive resistive material were placed in 
confronting contact in a conventional manner and the sensor thus formed 
was positioned between a pair of mating surfaces and placed under load. 
The results of testing under load are shown in FIG. 3 which illustrates, 
for temperatures of 250.degree. F. and 350.degree. F., the resistances in 
Kohms at the loads indicated. 
EXAMPLE II 
As another example of the practice of the present invention, a 20% solution 
of Matrimide polyimide resin was prepared as described above. To 30 grams 
of this solution was added 10.6 grams of molybdenum disulfide and 2.6 
grams of conductive iron oxide (as Fe.sub.3 O.sub.4). After mixing, 
depositing and drying in the manner described in Example I, and 
juxtaposing the semi-conductive layers, the sensor thus formed was 
positioned between a pair of surfaces and placed under load. The results 
of the testing under load are shown in FIG. 4 which illustrates, for 
temperatures of 250.degree. F. and 350.degree. F., the resistance in Kohms 
at the loads indicated. 
As may be seen from each of FIGS. 3 and 4 at both temperatures of 
250.degree. F. and 350.degree. F., and at loads of from 200 to 3000 psi, 
the sensors produced will satisfactorily discriminate the loading to which 
such sensors are exposed. 
EXAMPLE III 
Other carbon-free formulations of force sensing inks were made in 
accordance with the present invention. Each was found to have superior 
pressure-sensitive sensing characteristics at elevated temperatures. These 
formulations resulted from mixing moieties of Mixtures A and B. The 
solvent used in each moiety was acetophenone which completely evaporates 
after the ink is deposited. Thus the formulations are based on the 
compositions of the dried layer. 
Mixture A consisted of: 
______________________________________ 
% % 
Amount By Weight By Volume 
______________________________________ 
Molybdenum Disulfide 
85 grams 53.08 27.71 
(technical fine) 
Conductive Tin Oxide 
25 grams 15.64 5.71 
Matrimide 5218 
50 grams 31.28 66.58 
100.00 100.00 
______________________________________ 
A typical Mixture A would use 260 grams acetophenone as a solvent. 
Mixture B consisted of: 
______________________________________ 
% % 
Amount By Weight By Volume 
______________________________________ 
Molybdenum disulfide 
60 grams 45.12 19.56 
(technical fine) 
Minusil 5 25 grams 17.29 13.86 
Matrimide 5218 
50 grams 37.59 66.58 
100.00 100.00 
______________________________________ 
A typical Mixture B would use 260 grams of acetophenone as a solvent. 
Minusil 5 is a crystalline silica (SiO.sub.2) available from U.S. Silica, 
P.O. Box 187, Berksley Springs, W. Va. 25111. 
Carbon-free formulations comprising mixtures of moieties of Mixture A and 
Mixture B were prepared as set forth in Table I. Each was found to have 
superior pressure-sensitive sensing characteristics. 
TABLE I 
______________________________________ 
Amounts By Volume* 
Mixture A 
20ml 30ml 40ml 50ml 55ml 57.5ml 
Mixture B 
40ml 30ml 20ml 10ml 5ml 2.5ml 
Total 60ml 60ml 60ml 60ml 60ml 60.0ml 
______________________________________ 
*All formulations in Table I have identical ratios of particulate materia 
to Matrimide 5218 by volume. 
It is also to be understood that as the ratio of Mixture A to Mixture B 
increases, the ink layer becomes more conductive because the layer 
contains more conductive and semi-conductive particulates. 
The force sensing ink system of the present invention is capable of sensing 
forces of up to 10,000 psi or more at temperatures of up to 350.degree. F. 
The basic formulation of high temperature binder, semi-conductive 
particles and conductive particles may be supplemented or modified by 
changes in ratios and, as indicated, by incorporation of a dielectric 
particulate material, such as silica, thereby to optimize the 
responsiveness and sensitivity of the sensor for a given range of 
anticipated loads at anticipated operational temperatures for a particular 
load sensing application. Although the dielectric particulate tends to 
reduce the conductivity of the ink somewhat, it tends also to improve 
uniformity and repeatability of the ink layer resistance. 
Preferred compositions in accordance with the present invention usually 
fall within the following ratios of components by volume. The sum of all 
components will equal one. 
______________________________________ 
% of Volume 
______________________________________ 
High temperature binder 
20 to 80 
Semi-Conductive particles 
15 to 50 
Conductive particles 
5 to 50 
Dielectric particles 
4 to 50 
______________________________________ 
In preferred compositions Mixture A contains a ratio of 15 to 65 parts of 
semi-conductive particles and 55 to 5 parts of conductive particles by 
volume and Mixture B contains a ratio of 15 to 65 parts of semi-conductive 
particles and 55 to 5 parts of dielectric particles by volume, the 
remainder being the high temperature resistant binder. The admixture of 
Mixtures A and B is usually in a ratio of from 4 to 96 parts to 96 to 4 
parts of contained particulate by volume. 
The total concentration of conductive and semi-conductive particles should 
equal at least 20% by volume of the dried ink layer. That is because for 
the dried ink films to be conductive, there must be sufficient 
semi-conductive or conductive (or both) particles and they must be close 
enough together to allow electrical conduction and to obtain a conducting 
pathway through the layer. For a given particle size or distribution, the 
number of particles per unit volume is directly related to the number of 
conducting pathways in the ink. The upper limit of the particulate is 
approximately 80% by volume, and will depend upon adhesion and flexibility 
requirements of the dried ink layer. The thickness of the dried ink layer 
will be dictated in part by the environment in which the sensor is to be 
used, and the required flexibility and adhesion parameters. 
The median particle size of the conductive, semi-conductive and dielectric 
particles should be less than 10 microns, and preferably no more than 1.0 
micron in average size. Where possible, as is apparent from the foregoing, 
the particle size of the constituents should average no more than 1.0 
micron in size. 
As is known, most conductive and semi-conductive materials become more 
conductive as temperature increases. Changes are not linear. Neither is 
the coefficient of resistance change a constant with temperature. Indeed, 
the curve of resistance versus temperature or pressure is parabolic. All 
of these make clear why as temperatures increase, pressure-sensitive force 
sensing layers tend to become less discriminating and less resistive. 
By blending, mixing, and balancing in accordance with the present 
invention, greater sensitivity and reproduceability, especially at higher 
temperatures and pressures, can be obtained over both broad and narrow 
ranges as compared to those available with presently available systems and 
inks. 
Tests were conducted to ascertain the reliability of inks prepared in 
accordance with the present invention. To that end a 16% solution of 
Matrimide 5218 in acetophenone was prepared and was mixed with 23.5 grams 
of technical fine grade molybdenum disulfide (0.7 micron), 4 grams of 
conductive tin oxide (0.4 micron) and 4 grams of ground silica (1.0 
micron) in a laboratory mixer at high speed to produce inks. Button 
sensors as described above were prepared by silk-screen deposition of the 
inks using a 280 mesh screen. 
Using the mixing protocols indicated, resistances in (Kohms) at 3000 psi 
(at 350.degree. F.) were obtained, all as indicated in Table II. 
TABLE II 
______________________________________ 
Mixing Protocol Sensor 1 Sensor 2 Sensor3 
______________________________________ 
High Speed Mixing-15 Min. 
3.37 3.80 3.55 
High Speed Mixing-15 Min., 
4.05 3.78 3.90 
then aged 24 hours and 
mixed by hand with a 
spatula 
High Speed Mixing-15 Min., 
3.78 3.65 -- 
then aged 6 months and 
mixed by hand with a 
spatula 
______________________________________ 
Tests were then conducted with carbon black as a conductive pigment. The 
results of these tests showed that the inks of the present invention 
produced sensors which were superior in quality and reliability to those 
produced using conductive carbon black. The carbon black tests also 
confirm that carbon black is very difficult to mix into a liquid carrier 
and to separate and disperse into its ultimate particle size. 
To that end 20% solutions employing Matrimide 5218 in acetophenone were 
mixed with 13.2 grams of technical fine grade semi-conductive molybdenum 
disulfide (0.7 micron maximum particle size) and 4.32 grams of conductive 
carbon black (Shawingen acetylene black which is available from Chevron 
Chemical Co., P.O. Box 3788, Houston Tex. 77253). Button sensors as 
described above were prepared by silk-screen deposition of the inks using 
a 280 mesh screen. The inks were dried for 15 minutes at 275.degree. F. 
Using the mixing protocols indicated, resistances (in Kohms) at 3000 psi 
(at 350.degree. F.) were obtained, all as indicated in Table III. 
TABLE III 
______________________________________ 
Mixing Protocol Sensor 1 Sensor 2 Sensor 3 
______________________________________ 
High Speed Mixing-15 Min. 
0.41 4.2 20.9 
High Speed Mixing-30 Min. 
3.75 5.1 5.23 
High Speed Mixing-60 Min. 
3.92 4.15 3.75 
High Speed Mixing-60 
1.09 4.08 12.0 
Min., then aged one week 
and mixed with wide 
wooden stick 
______________________________________ 
The last test of well mixed material aged one week demonstrated that 
pigments had settled and agglomerated, which is typical of conductive 
carbon black based inks. This data, as well as the results of testing of 
well-mixed, promptly applied inks incorporating carbon black, shows that 
reliability and reproduceability of results of applied carbon black based 
inks are so variable and erratic that such inks are not acceptable for use 
in force sensors. 
From the foregoing it will be apparent to those skilled in the art that 
modifications may be made without departing from the spirit and scope of 
the invention. As such it is intended that the invention is to be limited 
only as may be made necessary by the claims appended hereto.