Piezoresistive elevator button

A piezoresistive elevator button assembly comprises a polymer film piezoresistive element sandwiched between a button and a button pressure plate, and circuitry for sensing a change in the resistance of the piezoresistive element. The piezoresistive element is held in place by a high temperature potting compound to create a button less sensitive to fire or heat. A compensating spring is placed between the piezoresistive element and the buttom pressure plate to eliminate the effect on that element due to temperature cycling of the button assembly elements. A frustoconical housing of the button employing catch teeth allows for little movement; the use of the polymer film piezoresistive element allows a signal to be generated though buttom movement is nearly imperceptible.

TECHNICAL FILED 
This invention relates to an improved elevator button assembly using a 
polymer film piezoresistive element to sense activation of the button. The 
button assembly is resistant to heat or flame induced malfunction, 
compact, using a minimum number of parts and readily capable of digital 
processing. Use of a polymer film allows for a button assembly that senses 
change in resistance through button movement which is imperceptible to 
humans. 
BACKGROUND ART 
It is desirable that an elevator button have a number of characteristics 
not required in other buttons. Such a button must be able to withstand a 
greater impact force because the average passenger often does not push an 
elevator button, but rather he/she strikes it. A vandal may strike it even 
harder. Second, elevator buttons must be resistant to heat so that they 
are not damaged in a fire nor register calls in a fire. They must be 
similarly unresponsive to flame. Finally, it is desirable, both for 
aesthetic reasons and functional reasons that the button not have 
perceptible motion to register a call for service. An elevator button with 
no moving mechanical parts will be less likely to be subject to the need 
for repair. 
Three are two types of piezoelements commonly used for pushbuttons: 
piezoresistive and piezoelectric. 
Buttons using a piezoelectric element are based on the piezoelectric 
effect, i.e., the material in the button generates a voltage potential 
when subjected to pressure. Such a button is disclosed in U.S. Pat. No. 
4,805,739, "Elevator Control Switch and Position Indicator Assembly", by 
Lind et al, issued Feb. 21, 1989. Buttons incorporating piezoresistive 
elements use the fact that the resistivity of the piezoelement changes 
with pressure. 
One problem with piezoelectric materials is that they are brittle, a 
characteristic wholly unsuited to elevator buttons because of the daily 
rigors to which they are subjected. Such buttons are defenseless against 
vandals. The brittleness of piezoelectric materials lends them to being 
easily shattered. 
A second problem with piezoelectric buttons is that they produce an output 
signal that is difficult to process with digital circuitry. The output of 
the piezoelectric element, when subjected to a flat load curve such as a 
step function, is a signal similar to the output from a high past filter 
where the time constant is much smaller than the time period of an input 
step function. Third, piezoelectrics are particularly sensitive to damage 
in a fire due to their heat sensitivity. 
Attempts have been made to produce an elevator button with the desirable 
qualities mentioned above. Such an attempt is found in "Pathless 
Piezobuttons For Industrial Applications", components report 14 (1975) No. 
5, which displays a piezoceramic film utilizing the piezoelectric effect. 
In the reference, the travel involved by the finger pressure is less than 
1 micrometer. Hence, the name "pathless". The piezoelectric material in 
this button does not have the inherent temperature resistant 
characteristics needed for an elevator button. It therefore requires a 
flame-protective material for the button housing in order to overcome this 
deficiency. The reference discloses a piezoelectric member located 
perpendicular to the button face and a button backing having support 
members which secure the element vertically. 
Other buttons similarly are unable to meet the needs of elevator systems. 
U.S. Pat. No. 4,267,725 discloses a vertical piezoresistive element braced 
at its center by a set screw such that depression of the button face at 
the upper tip of the element, and transverse to it, causes the element to 
generate a signal proportional to bending of the element. The set screw 
serves to ensure that the bending is only in one direction. This button 
fails to fulfill the need for a button with no observable mechanical 
movement since the button face of the reference must descend in order for 
the piezoelement to bend and thus give a signal. The nature of thin film 
silicon is such that a large amount of deformation is required to change 
the resistance of the element significantly enough that it can be 
measured. Such deformation is achieved, with the least amount of force, by 
bowing the element; it could not be so easily achieved by applying 
pressure transverse to the element on a flat surface. The reference 
requires a large number of complexly interrelated moving parts because 
there is no allowance to choose not to bow. Accordingly, this design is 
undesirable. 
In another button, disclosed in U.S. Pat. No. 4,644,315, a cantilevered 
piezoresistive element is used. In one embodiment the cantilevered element 
is vertical while in another the cantilevered element is horizontal. This 
reference discloses a donut-shaped magnet connected to a plunger on a 
button face such that the magnet descends with the button face to deflect 
an oppositely-poled magnet mounted on the tip of a cantilever having a 
piezoresistor hinge. This button, however, does not meet the need for an 
elevator button having no mechanical movement. Further, the use of the 
magnet/hinge apparatus and the need to position the magnet/hinge assembly 
with great precision make for a button that is costly. The same factors 
make it likely to be difficult to manufacture insofar as setting the 
magnet/hinge arrangement such that it interferes with the donut-shaped 
magnet so much as to sense its movement, but not so much as to be directly 
in the path of the donut-shaped magnet to as it descent. Also, this 
reference does not achieve a signal output readily susceptible of 
processing through digital circuitry. The second embodiment shows the 
cantilever mounted in horizontal fashion. This tends to produce more equal 
forces of attraction and repulsion leading to more symmetrical output 
pulse shapes. However, again, the fine tuning of the positioning of the 
hinge/magnet arrangement as well as the need for moving parts make this an 
undesirable button. 
Finally, U.S. Pat. No. 2,632,062, "Semiconductor Transducer", is predicated 
upon the discovery that a PN junction, when subjected to pressure, changes 
resistance. The patent shows in several embodiments, various elements, 
shaped like pencil tips, pressing upon a piezoresistive PN junction. It 
also shows one or more axles, in other embodiments, passing through one or 
more PN junctions. Pressure upon the axles subjects the PN junction to 
pressure, yielding a change in resistance. This reference does not satisfy 
the needs of elevator button. The silicon material is not resilient. The 
pencil tip used to stress the PN junction would not lead to a button with 
a long life, while the axle embodiment is quite inappropriate for use in 
an elevator--it does not disclose a button. 
DISCLOSURE OF THE INVENTION 
This invention relates to an improved elevator button assembly using a 
polymer film piezoresistive element to sense activation of the button. The 
button assembly is resistant to heat or flame induced malfunction, 
compact, using a minimum number of parts and readily capable of digital 
processing. Use of a polymer film allows for a button assembly that senses 
change in resistance through button movement which is nearly imperceptible 
to humans. 
According to the preferred embodiment of the invention, an elevator button, 
when subjected to a force perpendicular to it, translates the resultant 
pressure to a piezoresistive film material which is mounted between the 
button and a button pressure plate such that no mechanical movement is 
required to register a signal indicative of the depression of the button. 
A potting compound with a high temperature rating is used to secure the 
piezoresistive material to the button and to the button pressure plate. 
This type of adhesive allows for a substantially fire resistant button. A 
compensating spring, to reduce the effects of temperature cycling of the 
button assembly components upon the piezoresistive element, is interposed 
between the polymer film piezoresistive element and the button pressure 
plate. 
In a second embodiment, a plunger having a plunger plate remote from the 
button, descends down from the button toward the piezoresistive element. 
Perceptible motion springs, connected to the button, pressure plate and a 
collar, bias the plunger plate away from the piezoresistive element. The 
button is placed in a frustoconical housing that has catch teeth. The 
result is that movement of the button is perceptible to a human but the 
catch teeth and the housing prevent excessive force, caused by striking 
the button, from damaging the button assembly. 
Objects of the invention include providing an elevator button which is 
relatively fire and heat resistant, uses no moving or mechanical contacts, 
and is resilient and not susceptible to being shattered easily. 
These and other objects of the present invention will become more apparent 
in light of the detailed description of the best mode embodiment thereof, 
as illustrated in the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
A polymer film that exhibits a decreasing resistance with increasing force 
has been developed by Interlink Electronics, of Santa Barbara, Calif., 
United States of America. The material, marketed under the trademarks 
"FORCE SENSING RESISTOR" and "FSR", is described in an Interlink 
Electronics publication entitled, "Electrical Design Considerations for 
Force Sensing Resistor Technology", Applications Note IL-01, by Stuart I. 
Yaniger. It is known to apply this film to the exterior of robotic hands 
by wrapping the material around the robotic hands, DOE Technology 
Application Announcement 23, No. 3, "Tiny Sensor may Help Robots Handle 
Materials Accurately". 
This material has a number of desirable characteristics which are 
advantageous in an elevator button assembly, namely, this material 
provides an output that is less sensitive to heat than piezofilm; and less 
sensitive to fire than piezoelectric materials; this material is both 
resilient and nearly paper-thin, bending like a soft plastic. The amount 
of deformation needed to cause a change in resistance is imperceptible to 
a human. The material is less susceptible to damage when compared to 
piezoelectric materials; its rigidity is nearer to that of a very thin, 
flexible plastic than to any metal. 
The signal produced by the application of pressure to an "FSR" film follows 
the curve of the load, unlike piezoelectrics. For example, for a step 
function load curve, the response of a force sensing resistor is a step 
function. The "FSR" polymer film output is more readily converted into a 
signal capable of being processed through digital circuitry. 
Referring to FIG. 1, the preferred embodiment, a button 3 is depressible 
against an adhesive 5 which in turn exerts a force on an exemplary 
disc-shaped piezoresistive element 7 which element is a polymer film 
described in the aforesaid "Electrical Design Considerations for Force 
Sensing Resistor ("FSR") Technology". The piezoresistiye element 7 is 
sandwiched between first and second adhesives 5,9 so that the 
piezoresistive element 7 is brought to bear on a button pressure plate 11. 
A potting compound with a high temperature rating is preferably used as 
adhesives 5,9 to render the button more heat and fire resistant. 
Application of pressure to the piezoresistor 7 yields a signal directly 
proportional to force, which signal is detected by external circuitry 13 
through wiring 15. Between the adhesive 9 and the button pressure plate 
11, a compensating spring 17, that is a spring with a negative coefficient 
of expansion, may be used to compensate for thermal expansion of the 
button assembly elements. The spring 17 may be attached to the button 
pressure plate 11 and piezoresistor 7 by means of a third adhesive or 
potting compound 19, also having a high temperature rating. 
The button requires almost no perceptible motion to register a signal and 
therefore is not subject to wear for that reason. The button may have a 
frustoconical housing 21 so that a sharp blow to the button face will be 
transmitted to the housing rather than the force sensing resistive 
material 7 although, due to the extreme resiliency of the paper-thin 
polymer film used as the piezoresistor 7, such a blow would do less damage 
to said material than to other materials. Catch teeth 23 prevent the 
button 3 from being pulled from its housing 21. 
Referring now to FIG. 2, there is shown a second embodiment of the 
invention wherein the button face does not bear directly upon the 
piezoresistor 7. Rather, a plunger 25 is connected to the underside of the 
button 3 such that movement of the button 3 causes movement of the plunger 
25 which in turn causes a plunger plate 27 to bear against the 
piezoresistor 7. As in the prior embodiment, an adhesive or potting 
compound 5 may be applied to the piezoresistive material 7 in order to 
attach the latter to button pressure plate 11. 
The embodiment shown in FIG. 2 uses perceptible motion springs 29. In some 
cases, it may be preferable that the button 3 exhibit perceptible motion. 
This would be especially true in an elevator used by blind persons who may 
not otherwise know that they have depressed the elevator button. A range 
of motion 31 is set so as to allow for human perception of the depression 
of the button 3. To accommodate this need, one or more perceptible motion 
springs 29 are employed so that depression of the button 3 does not engage 
the piezoresistor 7 until the button 3 has descended a predetermined 
distance. The perceptible motion springs 29 are connected to a collar 33 
and the plunger plate 27. Compression of the piezoresistive material 7 
does not occur until the plunger plate 27 has descended the range of 
motion 31. The plunger plate 25 prevents the button 3 from being pulled 
from its housing 21. If desired, the length of plunger 25 may be large 
enough to provide perceptible motion and yet small enough that the button 
3 may not be grasped with the fingers of, for example, a vandal. 
The spring or springs 29 may be coil springs, leaf springs or any type of 
spring desired, including a single resilient material having the 
characteristics of a spring. Another example of such spring means might be 
a closed, but compressible gas pressure chamber or pneumatic spring. Nor 
is it necessary to the present invention that the perceptible motion 
spring be connected to a collar 33. The same result could be achieved by 
placing a spring means between the sides of the button face and the 
housing. 
Although the invention has been shown and described with the respect to a 
best mode embodiment thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention as defined by the 
appended claims.