Force sensors

A piezoelectric beam structure for a vibrating beam force sensor in which three coplanar beams of piezoelectric material are spaced side by side and supported between respective common mountings at either end. The two outer counterbalancing beams in operation flex in opposite phase to the center main beam, with each beam flexing in a direction normal to the beam plane. A force sensor which includes this piezoelectric beam structure.

This invention concerns force sensors, and relates in particular to sensors 
in which the variation of a flexing beam's natural resonance frequency 
when the beam is put under tension is used to indicate the amount of that 
tension. 
BACKGROUND OF THE INVENTION 
Vibrating beam force sensors are quite well known--the basic idea was 
described over twenty years ago--and since the early 1960's, these devices 
(in which in essence a beam or strip of a piezoelectric material mounted 
at either end is piezoelectrically driven into flexural vibration while 
under tension, a change in the vibrational frequency indicating a change 
in the tensioning forces), have found a wide range of uses. 
Unfortunately the simplest form of the device, a single strip-like beam 
mounted at either end--tends to have a relatively low Q (the factor used 
to indicate the amount of energy locked in the vibrating structure 
relative to the amount of energy that must be fed in to maintain the 
vibrations), and the energy is lost mainly by transfer to the mountings at 
either end. Much effort has gone into designing beam-like structures that 
do not suffer from the low Q problem--that do not cause a large proportion 
of the input energy to be passed to and absorbed by the mountings--and 
much of this effort has centered on the idea of providing some sort of 
counterbalanced vibrating element such that the vibrations of both this 
and the beam effectively cancel each other at the mountings, so that no 
energy is transferred to the mountings and the whole structure has a high 
Q. 
Although these counterbalanced, or compensated, structures do undoubtedly 
have the desired high Q, they are nevertheless all quite complex, and 
difficult and costly to manufacture. One such structure, put forward in 
the early 1960's, uses the tuning fork principle (two similar members 
vibrating to and from each other, in antiphase) by having two beams 
mounted at their ends in common mountings and disposed one above but 
spaced from the other. Like the arms of a tuning fork, the two beams 
flexurally vibrate in their common plane--that is, towards and away from 
each other. Because they are in antiphase (180.degree. out of phase) the 
vibrations sent to each mounting by one beam are exactly equal but 
opposite to those sent by the other beam, and so they cancel out, and no 
energy is transferred to the mountings. Another structure, suggested in 
the early 1970's, tries to solve the Q problem by securing the beam to 
each mounting via a torsion member at right angles to the beam's long 
axis, and by then providing a counterweight beam section beyond each 
torsion member. Yet another structure, also suggested in the 1970's, 
proposed a variant on the last one, mounting the beam at each end via two 
"isolator springs" spaced above and below the beam plane and then having 
two counterweights extending from these towards the beam center. 
Structures such as these are not only difficult and expensive to 
manufacture from the raw piezoelectric material blank, but in some cases 
the positioning thereon of the necessary electrodes (both by which the 
beams can be driven and by which the vibration's actual frequency can be 
observed) is made particularly irksome because of the complex shapes 
involved. 
It appears that all of the high Q structures suggested so far involve 
balancing beams or counterweights that are in the vibrational plane of the 
"main" beam, and flex in that plane. This seems to have made all these 
structures unnecessarily complex, and it is the hope of the present 
invention that it can provide a mechanically simpler, and cheaper, but no 
less efficient beam structure by placing counterbalancing beams not above 
and below the main beam but on either side thereof. 
SUMMARY OF THE INVENTION 
In one aspect, therefore, this invention provides a piezoelectric beam 
structure for a vibrating beam force sensor, of the type wherein a beam or 
strip of a piezoelectric material mounted at either end is 
piezoelectrically driven into flexural vibration while under stress, a 
change in the vibrational frequency indicating a change in the stressing 
force, wherein the structure has at least three coplanar beams spaced side 
by side and supported between common mountings one at either end for 
flexural vibration in a plane normal to the beams' common plane. 
Each beam is of a strip-like nature (similar to a rule/ruler), having 
length, breadth (or width) and depth (or thickness); the length is large 
relative to the breadth, and the breadth is large relative to the 
thickness. The plane of the beam may therefore loosely be defined as that 
plane in which the length and breadth dimensions exist. In the beam 
structure of the invention, the planes of all the beams lie in a common 
plane. Each beam is intended to flex (vibrate) in the direction of its 
depth--thus, normal to its plane. In the invention, each beam is intended 
to flex normal to the common plane. The two outer beams (the 
counterbalance beams) are, however, intended in operation to flex in 
antiphase, i.e., opposite in phase, to the center beam (the main beam), 
whereby the energy transferred by the main beam to its mounting is equal 
but opposite to--and thus is cancelled by--the energy fed to the same 
mounting by the two counterbalance beams. 
In use the beam structure will have associated therewith the various 
electrodes necessary for its operation. Their nature and positioning will 
be fairly conventional, and this is discussed further hereinafter. 
The inventive beam structure may be fabricated from any piezoelectric 
material used or suggested for use in the art, and it is one considerable 
advantage of the invention that it allows the use of relatively small, and 
cheap, portions of these materials. Typical piezoelectrics suitable for 
use are single crystal quartz, lithium niobate, lithium tantalate and 
aluminum orthophosphate. 
The inventive structure has three coplanar beams spaced side-by-side. 
Alternatively, there could be any number of beams (provided there are at 
least three)--there could, for example be four (with two inner main beams 
and two outer counterbalance beams), five (with one central main beam, two 
inner counterbalance beams, and two outer counterbalance beams perhaps in 
phase with the central main beam)--but three seem quite satisfactory. 
As has been mentioned hereinbefore, the three beams are coplanar, and 
spaced side-by-side. This means (amongst other things) that the structure 
as a whole can be made simply by taking a piezoelectric strip blank having 
the length of a beam plus its mountings and the breadth of the structure's 
three beam combination and simply removing material therefrom so as to 
form two side-by-side slots therein running parallel with the blank's long 
axis (and suitably spaced either side thereof); these two slots naturally 
define three parallel beams. Methods of so forming the beam structure are 
discussed in more detail hereinafter. 
The three beams are supported between common mountings one at either end. 
These mountings are in fact very conveniently portions of piezeolectric 
material integral with the beams themselves, and are the means by which 
the structure itself is mounted in or on the device in which it is to act 
as the active component of a force sensor. It may be desirable for the 
mountings to be necked--to have an axial portion of less breadth than the 
rest--between where it joins the beam structure and where it is itself 
mounted in or on the device. 
Being supported between common mountings the three beams generally are of 
the same length (which is whatever is suitable for the desired fundamental 
flexural frequency--0.250 inch (6.35 mm) seems quite acceptable). However, 
the three beams are preferably not the same breadth; to ease the problem 
of matching the energy in the main beam to that in the two counterblance 
beams it is preferred that the mass, and thus the breadth, of each of the 
latter two be half that of the former one. With 6.35 mm long, 0.125 mm 
thick beams, a main beam breadth of 0.040 inch (1 mm) and a counterbalance 
beam breadth of 0.020 inch (0.5 mm) were satisfactory. 
Structures with larger length-to-breadth ratios tend to have the higher Q 
values. 
The beam structure of the invention may be manufactured in a number of 
ways. One may employ an air-abrasion technique, in which the piezoelectric 
material blank is held between a backing plate and a slotted mask and a 
jet of abrasive particles is blown through the slots in the mask to remove 
the unwanted material. Air abrasion can cause damage to the material 
surfaces which significantly increases mechanical power losses in the 
vibrating beams, and hence reduces the attainable Q factor, but these 
losses can be greatly reduced by a subsequent chemical polishing. 
Another manufacturing method uses a photolithographic process. This 
technique involves depositing a suitable mask onto the piezoelectric 
material blank, and etching away the unprotected material with an 
appropriate etch solution. When the piezoelectric material is quartz, a 
suitable mask is an evaporated gold-on-nichrome layer electroplated with 
gold to increase the thickness and reduce the penetration of the etch 
through pinholes, and a satisfactory etch is hot aqueous ammonium 
bifluoride. 
In a force sensor device using an inventive beam structure the latter is 
mounted (at either end) so that the force applied to the device, and to be 
measured, is transmitted to the beams. One such device has the beam 
structure mounted across a shallow slot in the surface of a cantilever 
beam; application of force to the free end of the cantilever produces a 
strain in the beam structure which can be calculated with reasonable 
accuracy from the device geometry. The material of the cantilever should 
have a thermal coefficient of linear expansion in the strain direction 
that is closely matched to that of the beam structure in order to minimize 
the temperature coefficient of vibration frequency, and to guard against 
the possibility of cracking the piezoelectric material at extremes of 
temperature. Indeed, the cantilever could be of the same material and 
crystal orientation as the beam structure, in which case the stress 
induced by temperature change would be zero. Even then, however, there 
could be a temperature-dependent frequency change, so ideally the 
cantilever is made from a material with a thermal expansion coefficient 
such that the actual differential expansion produces a stress which in 
turn produces a frequency shift that effectively cancels the temperature 
coefficient of the unstrained device. 
A quite different type of mounting is one wherein the beam structure is 
fixed across the free ends of two rigid levers pivoted together, the force 
to be measured being applied to the levers to separate them (and thus 
produce a strain in the beam structure). A somewhat similar type of 
mounting particularly suitable for measuring pressures is one wherein the 
beam structure is attached to a diaphragm via a short pillar at either end 
of the beam structure. The pressure to be measured is applied to the 
diaphragm, and acts to rotate the pillars about their points of attachment 
to the diaphragm, thus producing a strain in the beam structure. 
Examples of these two mounting systems are discussed hereinafter with 
reference to the accompanying drawings. 
The inventive beam structure is fabricated from a piezoelectric material, 
and is driven into flexure by electrical signals delivered to electrodes 
mounted on the structure. The mechanism by which flexural vibrations 
occurs is now well known. Briefly, however, it involves producing an 
electric field across the depth of the beam between an electrode on one 
face and a matching electrode on the other face so causing the volume of 
the piezoelectric material between the electrodes to distort sideways in 
shear, and the forces arising from this distortion then cause the beam to 
move bodily up (or down). The beam will thus flex at the frequency of the 
applied signals, and this flexing will have maximum amplitude when the 
signal frequency is a resonance frequency of the beam. 
It is conventional to place a pair of driving electrodes near but to one 
side of the beam center, to place a pair of pick-up electrodes in the 
equivalent position on the other side of the beam center, and to use the 
signal obtainable at the pick-up electrodes in a feedback loop to direct 
the driving signal's frequency to, and maintain it at, a chosen beam 
reasonance frequency. Any force applied to the beam changes its reasonance 
frequency, and this can be used as a measure of that force. In the 
inventive beam structure, of course, each of the three (or more) beams may 
have its own driver and pick-up electrode pairs, which may be formed in 
position by any of the usual techniques, and the signal circuit used (an 
example of which is discussed further hereinafter with reference to the 
accompanying drawings) applies the counterbalance beam signals in 
antiphase to the main beam signals. Moreover, it is possible--and, indeed, 
desirable--to have all the electrodes on one surface of the beam structure 
combined into a common electrode (advantageously maintained at earth 
potential). Furthermore, it is possible--surprising though this may seem 
at first--to drive only either the main beam or the counterbalance beams 
rather than all three, for the energy fed into the flexing of one of these 
is transferred laterally across the end mountings into the other, and 
provided the driving frequency is correctly chosen this transferred energy 
will itself drive the other into anti-phase flexure. One interesting 
corrollary of this is that the driving electrodes can be decoupled from 
the pick-up electrodes by placing one on the main beam and the other on 
one of the counterbalance beams. 
The advantages of the beam structure of the invention over those presently 
used in the prior art may be summarized as follows: 
1. Because of the relatively small size of the beam structure it can be 
made from cheaper starting materials, it uses less quartz to produce a 
smaller device, and several devices can be made from each blank, so 
spreading the processing costs per device. 
2. Because of the mechanical simplicity of the beam structure it can be 
made using photolithographic processes, and the electrode pattern 
deposited very simply. 
3. Because the beam structure is inherently thin, it can be mounted on 
assemblies themselves cheap to construct. 
The invention exends, of course, to a force sensor when employing a beam 
structure as described and claimed herein.

DETAILED DESCRIPTION 
Two shapes for the beam structure of the invention are shown in outline in 
FIGS. 1A and B. That of FIG. 1A is a strip 10 of piezoelectric material 
that has had two centrally located narrow slots 11T,B cut into it parallel 
to but spaced either side of the strip long axis; the strip material 
between and outside the slots are the beams--the main beam 12 in the 
center and the two counterbalance beams 13T,B on either side. The three 
beams have at each end a common mounting 14L,R which is contiguous with 
the strip end portions 15L,R by which the strip is mounted in or on the 
device in which it is used. 
The strip of FIG. 1B is a wider, longer version of that of FIG. 1A with the 
addition of a neck 16L,R of material separating each beam mounting 14L,R 
from the relevant strip end portion 15L,R. 
FIGS. 2A, B and C show (in side elevation, part section and part plan 
respectively) a beam structure like that of FIG. 1A mounted over a slot in 
a cantilever. The cantilever 21 is rigidly mounted at one end on a support 
22, and moves up and down (as viewed) under the influence of force F. 
Along the cantilever a slot or notch 23 is cut in the surface, and 
bridging that slot (and affixed to the cantilever surface portions on 
either side) is an inventive beam structure 24 in accordance with the 
present invention. The details of this are shown more clearly in FIGS. 2B 
and C (the former shows how the beam structure 24 is free to flex). 
An alternative type of mounting arrangement employs a flexible frame such 
as that shown in FIG. 3A. Forces applied at the ends of the frame 31 are 
coupled more-or-less directly into the beam structure 32 but no large 
forces are generated by differential thermal expansion. A flimsy structure 
of this kind would probably be most appropriate in an atmospheric pressure 
transducer, where one end of the frame is attached to a rigid mount 33 and 
the other is attached to a pressure diaphragm 34. The force produced by 
the pressure diaphragm is coupled into the beam structure by the 
magnification ratio given by the relative lengths of the lever arms 35, 
36, and provided that the cross piece 37 is relatively thin no large 
forces will be generated in the beam structure by thermal expansion. 
A simpler structure suitable for measuring pressures is shown in FIG. 3B. A 
beam structure 32 (like those in FIGS. 1A and 1B, shown in side 
elevation), is attached via pillars 38 to a diaphragm 34 itself mounted on 
a support 39. The pillars 38 are preferably formed integrally with the 
diaphragm and/or with the beam structure 32. Applied pressure P acts to 
rotate the pillars, and therefore to extend the beam structure. 
Two layouts for the beam structure electrodes are shown in FIGS. 4A and B. 
In FIG. 4A each beam is driven and carries a pick-up electrode; the main 
beam 12 has a drive electrode 41 just to the left (as viewed) of its 
center line and a pick-up electrode 42 just to the right, while each 
counterbalance beam 13T,B has its own drive 43T,B and pick-up 44T,B 
electrode. Each of the drive and pick-up electrodes is connected via a 
thin conducting track (as 45) to a pad (as 46) to which in use a wire to 
the relevant circuitry is attached. 
The electrode layout of FIG. 4B has a single drive electrode 41 driving the 
main beam and a single pick-up electrode 44T on the upper right (as 
viewed) counterbalance beam. By correctly choosing the driving frequency 
for the main beam the two outer beams automatically flex in antiphase--and 
having the pick-up electrode on one of these electrically decouples it 
from the drive electrode. 
In both FIGS. 4A and B, the opposite bottom side of the beam structure not 
seen carries a single common electrode extending over the whole surface, 
illustrated as electrode 48 in the fragmented corner of FIG. 4B. 
When in use in a force sensor device, the beam structure is maintained in 
vibration by means of a tracking oscillator circuit which follows the 
changes in resonance frequency of the vibrating beams produced by the 
applied strain, so that the drive frequency is always identical to the 
mechanical resonance frequency. The well-known circuit shown schematically 
in FIG. 5 can be used for this. The circuit consists of a charge amplifier 
51 followed by an amplifier 52 with a band-pass characteristic chosen to 
reject frequencies outside the operating range and with a gain sufficient 
to ensure operation of the driving phase-locked loop integrated circuit 
chip 54--which may be a CD 4046. The band-pass characteristic of the 
amplifier 52 is necessary to ensure that the device does not oscillate 
either at higher harmonics or at some resonance frequency of the whole 
structure. 
The voltage-controlled oscillator in the phase-locked loop is centered on 
the middle of the operating frequency range, and locked to the beam 
mechanical resonance frequency by the amplifier's output signal. The 
square wave output from this oscillator is filtered by active filter 55 to 
remove the harmonics, and re-applied to the beam structure's drive 
electrode. 
The loop phase shift of the circuit is arranged so that the oscillator 
frequency is set at the resonance frequency of the beam structure and 
tracks the changes in resonance frequency produced by the applied strain. 
The oscillator output provides the strain-dependent output signal of the 
sensor.