Electronic weighing apparatus utilizing surface acoustic waves

A weighing apparatus includes a base supporting a cantilevered elastic member bearing a load platform. The interior of the elastic member is hollowed and is provided with first and second piezoelectric transducers mounted on respective opposed posts. The transducers are arranged substantially parallel to each other with a small gap between them and are coupled to an amplifier to form a "delay line" and a positive feedback loop, i.e. a natural oscillator. According to various aspects of the invention, one or both substrates is provided with anti-reflection structure; the transducers are arranged on overlapping substrates to further reduce reflection; the transducers are coupled to a thermal sink to reduce the effects of thermal gradients across the transducers; two pairs of transducers are provided and arranged to provide a pair of oppositely polarized signals which doubles the accuracy of measurements and also compensates for the effects of temperature gradients; a thermal transducer channel is provided on the same substrate to measure the effects of temperature gradients and thereby compensate for temperature effects; a pair of differential transducers is arranged to measure the effects of temperature changes in the same acoustic channel in which displacement measurements are made; and a phase shift is applied to the output of the amplifier(s) to improve gain.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to electronic weighing devices. More particularly, 
the invention relates to an electronic weighing device which employs 
surface acoustic waves to measure weight. 
2. State of the Art 
Precision electronic weighing devices are widely known in the art and there 
are many different technologies utilized in these electronic weighing 
devices. Laboratory scales or "balances" typically have a capacity of 
about 1,200 grams and a resolution of about 0.1 gram, although scales with 
the same resolution and a range of 12,000 grams are available. The 
accuracy of these scales is achieved through the use of a technology known 
as magnetic force restoration. Generally, magnetic force restoration 
involves the use of an electromagnet to oppose the weight on the scale 
platform. The greater the weight on the platform, the greater the 
electrical current needed to maintain the weight. While these scales are 
very accurate (up to one part in 120,000), they are expensive and very 
sensitive to ambient temperature. In addition, their range is relatively 
limited. 
Most all other electronic weighing devices use load cell technology. In 
load cell scales, the applied weight compresses a column which has strain 
gauges bonded to its surface. The strain gauge is a fine wire which 
undergoes a change in electrical resistance when it is either stretched or 
compressed. A measurement of this change in resistance yields a measure of 
the applied weight. Load cell scales are used in non-critical weighing 
operations and usually have a resolution of about one part in 3,000. The 
maximum resolution available in a load cell scale is about one part in 
10,000 which is insufficient for many critical weighing operations. 
However, load cell scales can have a capacity of several thousand pounds. 
While there have been many improvements in electronic weighing apparatus, 
there remains a current need for electronic weighing apparatus which have 
enhanced accuracy, expanded range, and low cost. 
Co-owned application Ser. No. 08/489,365, previously incorporated by 
reference herein, discloses an electronic weighing apparatus having a base 
which supports a cantilevered elastic member upon which a load platform is 
mounted. The free end of-the elastic member is provided with a first 
piezoelectric transducer and a second piezoelectric transducer is 
supported by the base. Each transducer includes a substantially 
rectangular piezoelectric substrate and a pair of electrodes imprinted on 
the substrate at one end thereof, with one pair of electrodes acting as a 
transmitter and the other pair of electrodes acting as a receiver. The 
transducers are arranged with their substrates substantially parallel to 
each other with a small gap between them and with their respective 
electrodes in relatively opposite positions. The receiver electrodes of 
the second transducer are coupled to the input of an amplifier and the 
output of the amplifier is coupled to the transmitter electrodes of the 
first transducer. The transducers form a "delay line" and the resulting 
circuit of the delay line and the amplifier is a positive feedback loop, 
i.e. a natural oscillator. More particularly, the output of the amplifier 
causes the first transducer to emit a surface acoustic wave ("SAW") which 
propagates along the surface of the first transducer substrate away from 
its electrodes. The propagating waves in the first transducer induce an 
oscillating electric field in the substrate which in turn induces similar 
SAW waves on the surface of the second transducer substrate which 
propagate in the same direction along the surface of the second transducer 
substrate toward the electrodes of the second transducer. The induced 
waves in the second transducer cause it to produce an. alternating voltage 
which is supplied by the electrodes of the second transducer to the 
amplifier input. The circuit acts as a natural oscillator, with the output 
of the amplifier having a particular frequency which depends on the 
physical characteristics of the transducers and their distance from each 
other, as well as the distance between the respective electrodes of the 
transducers. 
When a load is applied to the load platform, the free end of the 
cantilevered elastic member moves and causes the first transducer to move 
relative to the second transducer. The movement of the first transducer 
relative to the second transducer causes a change in the frequency at the 
output of the amplifier. The movement of the elastic member is 
proportional to the weight of the applied load and the frequency and/or 
change in frequency at the output of the amplifier can be calibrated to 
the displacement of the elastic member. The frequency response of the 
delay line is represented by a series of lobes. Each mode of oscillation 
is defined as a frequency where the sum of the phases in the oscillator is 
an integer multiple of 2.pi.. Thus, as the frequency of the oscillator 
changes, the modes of oscillation move through the frequency response 
curve and are separated from each other by a phase shift of 2.pi.. The 
mode at which the oscillator will oscillate is the one having the least 
loss. The transducers are arranged such that their displacement over the 
weight range of the weighing apparatus causes the oscillator to oscillate 
in more than one mode. Therefore, the change in frequency of the 
oscillator as plotted against displacement of the transducers is a 
periodic function. There are several different ways of determining the 
cycle of the periodic function so that the exact displacement of the 
elastic member may be determined. In addition, in order to minimize the 
possibility that the oscillator will oscillate in two modes at the same 
time, the frequency response of the delay line is arranged so that no more 
than two modes coexist in the main lobe of the frequency response curve. 
This is achieved by the topology of the electrodes as well as the distance 
between the transmitting electrode and the receiving electrode. The gain 
of the amplifier is also chosen to be at least the absolute value of the 
greatest loss expected to be encountered at an oscillating frequency 
within the main lobe but not great enough to allow oscillation in two 
modes simultaneously. 
According to a disclosed preferred embodiment, the surface acoustic wave 
has a wavelength of approximately 200 microns at 20 MHz. The gap between 
the substrates of the first and second transducers is as small as possible 
and preferably is less than 0.1 wavelength, i.e. 10-20 microns. The 
amplifier preferably has a gain of at least approximately 17 dB in order 
to guarantee natural oscillation, and preferably not more than 
approximately 30 dB so that the oscillator oscillates in only one mode at 
a time. The preferred manner of determining the cycle of the periodic 
output of the amplifier is to provide a second pair of transducers 
adjacent to the first pair and coupled to each other in the same type of 
delay line feedback loop. The second pair of transducers utilize a SAW 
with a different wavelength than the first pair of transducers, e.g. 
approximately 220 microns at 18 MHz. The output of the second amplifier 
is, therefore, a periodic function with a different frequency than the 
periodic function which is the output of the first amplifier. By combining 
the outputs of both amplifiers, a unique value is provided for each 
position of the elastic member. 
Typically, the elastic member is chosen so that it will bend up to 150 
microns under maximum load. Given the wavelength of the SAW, this results 
in about two to three modes of oscillation in the output of the first 
amplifier. 
The provided apparatus can theoretically achieve an accuracy on the order 
of one part in one hundred thousand, e.g. one gram per hundred kilograms. 
In practice, however, a resolution on the order of one part in fifty 
thousand is readily achieved. It has been observed by the inventors herein 
that several factors have varying influence on the accuracy of the SAW 
system. These factors include reflected waves, temperature changes, and 
the frequency of the oscillator. Generally, reflected waves result in 
non-linearity of measurements, and temperature has an effect of about 70 
ppm per degree C. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide an electronic 
weighing apparatus which is accurate. 
It is also an object of the invention to provide an electronic weighing 
apparatus which uses surface acoustic waves and is accurate over a broad 
range of weights. 
It is another object of the invention to provide an electronic weighing 
apparatus which is compact and easy to construct. 
It is a further object of the invention to provide an electronic weighing 
apparatus which is inexpensive to manufacture. 
It is another object of the invention to provide an electronic weighing 
apparatus which utilizes surface acoustic waves and which is provided with 
means for reducing reflected waves. 
It is still another object of the invention to provide an electronic 
weighing apparatus which maintains accuracy despite temperature gradients 
within the system. 
It is yet another object of the invention to provide an electronic weighing 
apparatus which utilizes surface acoustic waves at a relatively high 
frequency. 
In accord with these objects which will be discussed in detail below, the 
improved weighing apparatus of the present invention includes a base which 
supports a cantilevered elastic member upon which a load platform is 
mounted. The interior of the elastic member is hollowed and is provided 
with first and second piezoelectric transducers which are mounted on 
respective opposed posts. Each transducer includes a substantially 
rectangular piezoelectric substrate and a pair of electrodes imprinted on 
the substrate at one end thereof, with one pair of electrodes acting as a 
transmitter and the other pair of electrodes acting as a receiver. The 
transducers are arranged with their substrates substantially parallel to 
each other with a small gap between them and with their respective 
electrodes in relatively opposite positions. The receiver electrodes of 
the second transducer are coupled to the input of an amplifier and the 
output of the amplifier is coupled to the transmitter electrodes of the 
first transducer. The transducers form a "delay line" and the resulting 
circuit of the delay line and the amplifier is a positive feedback loop, 
i.e. a natural oscillator. More particularly, the output of the amplifier 
causes the first transducer to emit a surface acoustic wave ("SAW") which 
propagates along the surface of the first transducer substrate away from 
its electrodes. The propagating waves in the first transducer induce an 
oscillating electric field in the substrate which in turn induces similar 
SAW waves on the surface of the second transducer substrate which 
propagate in the same direction along the surface of the second transducer 
substrate toward the electrodes of the second transducer. The induced 
waves in the second transducer cause it to produce an alternating voltage 
which is supplied by the electrodes of the second transducer to the 
amplifier input. The circuit acts as a natural oscillator, with the output 
of the amplifier having a particular frequency which depends on the 
physical characteristics of the transducers and their distance from each 
other, as well as the distance between the respective electrodes of the 
transducers. 
According to the invention, when a load is applied to the load platform, 
the cantilevered elastic member bends and causes the first transducer to 
move relative to the second transducer. The movement of the first 
transducer relative to the second. transducer causes a change in the 
frequency at the output of the amplifier. The bending movement of the 
elastic member is proportional to the weight of the applied load and the 
frequency and/or change in frequency at the output of the amplifier can be 
calibrated to the displacement of the elastic member. 
According to one aspect of the invention, one or both substrates are 
provided with anti-reflection structure which may be an angled cut, a 
rounded end, or a surface damper. 
According to a second aspect of the invention, the transducers are arranged 
on overlapping substrates which allows more room for a damping material to 
further reduce reflection and allows more room for additional transducers. 
According to a third aspect of the invention, the transducers are coupled 
to a thermal sink to reduce the effects of thermal gradients across the 
transducers. 
According to a fourth aspect of the invention, two pairs of transducers are 
provided and arranged to move in opposite directions which doubles the 
readability of measurements and also compensates for the effects of 
temperature gradients. 
According to a fifth aspect of the invention, a thermal transducer channel 
is provided on the same substrate to measure the effects of temperature 
and thereby compensate for temperature effects. 
According to a sixth aspect of the invention, a pair of differential 
transducers is arranged to measure the effects of temperature changes in 
the same acoustic channel in which displacement measurements are made. 
According to a seventh aspect of the invention, a phase shift (preferably 
180.degree.) is introduced in the oscillator of the delay line, when 
required, in order for the oscillator to oscillate in the most optimal 
section of the frequency response curve (near the center) where 
temperature effects are minimized. 
Additional objects and advantages of the invention will become apparent to 
those skilled in the art upon reference to the detailed description taken 
in conjunction with the provided figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1, 1a, and 1b, an electronic weighing apparatus 10 
according to the invention includes a base 12 which supports a 
cantilevered elastic member 14 having a cut-out 15, and upon which a load 
platform 16 is mounted. The cut-out 15 is provided with two opposed posts 
17, 19 upon which are respectively mounted a first piezoelectric 
transducer 20 and a second piezoelectric transducer 22. The posts 17, 19 
serve to locate the transducers 20, 22 at the center of the elastic member 
14 and to mechanically couple the transducers to opposite ends of the 
elastic member 14. 
The first transducer 20 includes a substantially rectangular piezoelectric 
substrate 20a and a pair of electrodes 20b imprinted on the substrate at 
the upper end thereof. The second transducer 22 includes a substantially 
rectangular piezoelectric substrate 22a and a pair of electrodes 22b 
imprinted on the substrate at the lower end thereof. The substrates are 
preferably made of Lithium Niobate. The transducers are arranged with 
their substrates substantially parallel to each other with a small gap 
fight between them. The electrodes 22b of the second transducer 22 are 
coupled to the input of an amplifier (not shown) powered by a power source 
(not shown) and the output of the amplifier is coupled to the electrodes 
20b of the first transducer 20. The circuit arrangement is the same as 
shown in the parent application Ser. No. 08/489,365, previously 
incorporated herein by reference. The resulting circuit is a positive 
feedback loop natural oscillator, a "delay line". The output of the 
amplifier generates an alternating voltage in the electrodes 20b of the 
first transducer 20 which generates a surface acoustic wave ("SAW") 26 
which propagates along the surface of the first transducer substrate 20a 
away from its electrodes 20b. Since the substrate 20a of the first 
transducer 20 is relatively close to the substrate 22a of the second 
transducer 22, an oscillating electric field which is induced as a result 
of the SAW waves 26 in the piezoelectric substrate 20a is able to in turn 
induce similar SAW waves 28 on the surface of the second transducer 
substrate 22a which propagate in the same direction along the surface of 
the second transducer substrate toward the electrodes 22b of the second 
transducer 22. The induced waves 28 in the second transducer 22 cause the 
electrode 22b of the second transducer 22 to produce an alternating 
voltage which is provided to the input of the amplifier. As long as the 
gain of the amplifier 24 is larger than the loss of the system, the 
circuit acts as a natural oscillator with the output of the amplifier 
having a particular frequency which depends on the physical 
characteristics of the transducers and their distance from each other, as 
well as the distance between the respective electrodes of the transducers. 
In particular, the frequency of the oscillator is directly related to the 
time it takes for the SAW to propagate from the electrodes 20b to the 
electrodes 22b. 
According to presently preferred embodiments of the invention, described in 
more detail below, the SAW 26 has a wavelength of approximately 100-200 
microns at 20-50 MHz. In order to limit loss in the system, the gap "g" 
between the substrates of the first and second transducers is as small as 
possible and preferably no more than 0.1 wavelength. In one preferred 
embodiment described below, the gap is 5-10 microns. With such a gap, an 
oscillating system can typically be generated if the amplifier 24 has a 
gain of at least approximately 17 dB. It will be appreciated that when a 
load (not shown) is applied to the load platform 16, the free end of the 
cantilevered elastic member 14 moves down and causes the second transducer 
22 to move relative to the first transducer 20. In particular, it causes 
the electrodes 22b of the second transducer 22 to move away from the 
electrodes 20b of the first transducer 20. This results in a lengthening 
of the "delay line". The lengthening of the delay line causes an decrease 
in the frequency at the output of the amplifier. The displacement of the 
elastic member is proportional to the weight of the applied load and the 
frequency or decrease in frequency at the output of the amplifier can be 
calibrated to the distance moved by the elastic member. 
It will be appreciated that locating the transducers at the center of the 
elastic member compensates for any torque on the member which would 
exhibit itself at the free end of the member. This results in an improved 
accuracy as compared to the weighing instrument of the parent application. 
Depending on the application (e.g. maximum load to be weighed), the 
elastic member is made of aluminum or steel. The presently preferred 
elastic member exhibits a maximum displacement of 0.1 to 0.2 mm at maximum 
load. 
It has been recognized by the inventors that reflected waves may occur on 
the substrate of the transmitting transducer which interfere with SAW wave 
generation and result in non-linearity of measurements. More particularly, 
when the wave 26 propagates along the substrate 20a, it reaches the end 
20c of the substrate and a portion of the wave is reflected back 
180.degree. toward the electrodes 20b. The reflected wave interferes with 
the propagated wave 26. In fact, a portion of the reflected wave is again 
reflected off the other end 20d of the substrate 20a causing additional 
interference. Reflected waves can also be a problem in the receiving 
transducer. FIGS. 2-4 show several anti-reflection structures according to 
the invention. 
Turning now to FIG. 2, a transducer 120 according to the invention includes 
a piezoelectric substrate 120a and a pair of electrodes 120b for 
generating a SAW wave 126. According to the invention, the end 120c of the 
substrate 120a is cut at an angle relative to the propagation path of the 
SAW wave 126. Thus, when the wave 126 reaches the end 120c of the 
substrate, any reflection of the wave is at an angle relative to the line 
of propagation so that the reflected wave does not interfere with the 
propagated wave. 
Turning now to FIG. 3, a transducer 220 according to the invention includes 
a piezoelectric substrate 220a and a pair of electrodes 220b for 
generating a SAW wave 226. According to the invention, the end 220c of the 
substrate 220a is rounded (e.g., by sandblasting) relative to the 
propagation path of the SAW wave 226. Thus, when the wave 226 reaches the 
end 220c of the substrate it is scattered rather than reflected back. 
Turning now to FIG. 4, a transducer 320 according to the invention includes 
a piezoelectric substrate 320a and a pair of electrodes 320b for 
generating a SAW wave 326. According to the invention, a damper such as a 
soft elastomeric 320d is placed on the surface of the substrate adjacent 
the end 320c. Thus, when the wave 326 reaches the damper 320d, it is 
absorbed by the damper rather than reflected back. 
Of the different anti-reflection structure described above, the damper 
material shown in FIG. 4 appears to be the presently preferred structure. 
Accordingly, as shown in FIG. 5, a pair of transducers 320, 322 are 
provided with damper material 320d, 322d adjacent ends 320c, 322c opposite 
electrodes 320b, 322b on the respective substrates 320a, 322a. The 
transducers 320, 322 are advantageously arranged in an overlapping manner 
as shown in FIG. 5. 
As mentioned above, in addition to problems associated with reflected 
waves, changes in temperature adversely affect the accuracy the weighing 
apparatus. It is known from the parent application that overall changes in 
ambient temperature may be compensated for by including a temperature 
sensor in the weighing apparatus and using a look-up table for appropriate 
temperature corrections. However, in addition to overall changes in 
ambient temperature, it has been discovered that temperature gradients can 
occur across the substrates of the transducers. More particularly, it has 
been discovered that the lithium niobate substrates have a temperature 
effect of about 70 ppm per degree C. In a 20 Mhz system, this results in a 
change of 1.4 Khz per degree. In order to obtain the desired accuracy, the 
temperature difference between the transducers should be less than 0.01 
degrees C. At 20 Mhz, the full scale output for one mode is about 200 Khz. 
Since the substrate shows a temperature effect of 1.4 Khz per degree C., 
this results in a 0.7% variation of the full scale per degree C. In order 
to maintain an accuracy of within 0.007% of the full scale, therefore, the 
temperature difference should be less than 0.01 degree C. 
Several aspects of the present invention combine to help overcome the 
effects of temperature on the accuracy of the weighing apparatus. Some of 
these aspects also enhance the overall accuracy of the apparatus 
regardless of temperature effects. 
FIGS. 6-8 show a pair of transducers 420, 422 according to the invention 
which embody several aspects of the invention. Referring now to FIGS. 6-8, 
the transducer system includes a transmitting transducer 420 and a 
receiving transducer 422. The transmitting transducer 420 includes two 
piezoelectric substrates 420a, 420a' which are mounted on one side a 
thermal sink 421, and a thermal insulator 423 is mounted on the other side 
of the thermal sink 421. The thermal sink 421 helps to maintain a constant 
temperature across both substrates 420a, 420a' and the thermal insulator 
423 helps to prevent ambient temperature changes from affecting the 
temperature of the thermal sink 421 and thus the substrates 420a, 420a'. 
Each substrate 420a, 420a' has a pair of electrodes 420b, 420b' at one end 
thereof and a surface damper 420d, 420d' at the other end thereof. The 
substrates are arranged on the thermal sink so that their respective 
electrode pairs are close to each other and adjacent to the center of the 
transducer as seen best in FIGS. 6 and 7. The transmitting transducer 420, 
therefore generates two SAW waves 426, 426' which propagate in opposite 
directions from the approximate center of the transducer 420. The dampers 
420d, 420d, serve to inhibit reflections of the SAW waves 426, 426'. 
The receiving transducer 422 includes a piezoelectric substrate 422a which 
is mounted on one side of a thermal sink 425, and a thermal insulator 427 
is mounted on the other side of the thermal sink 425. The thermal sink 425 
helps to maintain a constant temperature across the substrate 422a and the 
thermal insulator 427 helps to prevent ambient temperature changes from 
affecting the temperature of the thermal sink 425 and thus the substrate 
422a. The substrate 422a has two pairs of electrodes 422b, 422b', each 
pair being located approximately at opposite rounded ends 422c, 422c' of 
the substrate 422a, and a surface damper 422d is located approximately at 
the center of the substrate 422a. The receiving transducer 422, therefore 
receives two SAW waves 428, 428' which are induced respectively by the 
transmitted SAW waves 426, 426', and which propagate in opposite 
directions. 
The electrode pairs 420b and 422b are coupled to one amplifier to form one 
delay line oscillator and the electrode pairs 420b' and 422b' are coupled 
to a second amplifier to form a second delay line oscillator. From the 
foregoing, it will be appreciated that when the transducers 420, 422 move 
relative to each other as described above with reference to FIG. 1, one of 
the delay line oscillators will increase in frequency and the other delay 
line oscillator will decrease in frequency by an equal amount. This 
"differential" transducer system provides several benefits. The effects of 
ambient temperature are automatically accounted for and the resolution of 
the displacement measurement is doubled. 
For example, if the frequency of a delay line oscillator is f.sub.0 at a 
predefined temperature, the frequency after a change in ambient 
temperature can be expressed as f=f.sub.0 (1+k.DELTA.t), where k is a 
constant and .DELTA.t is the change in ambient temperature. Similarly, if 
the frequency of a delay line oscillator is f.sub.0 when no load is placed 
on the weighing apparatus, the frequency after the transmitter and 
receiver have been displaced by a load can be expressed as f=f.sub.0 
(1.+-.g.DELTA.x), where g is a constant and .DELTA.x is the displacement 
(positive or negative) due to a particular load. The combined change in 
frequency .DELTA.f due to temperature and displacement, therefore equals 
f.sub.0 (k.DELTA.t.+-.g.DELTA.x). 
Given the foregoing, it will be appreciated that the combined effects of 
temperature and displacement on the frequency of the delay line 
oscillators of FIGS. 6-8 can be expressed by the equations indicated below 
at (1) and (2) where f1 is the frequency of the oscillator which includes 
electrodes 420b, 422b and f2 is the frequency of the oscillator which 
includes electrodes 420b', 422b'. 
EQU .DELTA.f1=f1.sub.0 k.DELTA.t+f1.sub.0 g.DELTA.x (1) 
EQU .DELTA.f2=f2.sub.0 k.DELTA.t-f2.sub.0 g.DELTA.x (2) 
The quantities .DELTA.f1, .DELTA.f2, f1.sub.0, and f2.sub.0, will therefore 
be known during any weight measurement and may be subjected to the cross 
product expressed below at (3). 
EQU (.DELTA.f1.times.f2.sub.0)-(.DELTA.f2.times.f1.sub.0) (3) 
By substituting equations (1) and (2) in expression (3), the expression 
listed below at (4) is obtained. 
EQU f2.sub.0 f1.sub.0 k.DELTA.t+f2.sub.0 f1.sub.0 g.DELTA.x-f1.sub.0 f2.sub.0 
k.DELTA.t+f1.sub.0 f2.sub.0 g.DELTA.x (4) 
By combining terms, it will be appreciated that the effects of temperature 
will be canceled out of expression (4) and that expressions (3) and (4) 
can be expressed as the simplified equation listed below at (5). 
EQU (.DELTA.f1.times.f2.sub.0)-(.DELTA.f2.times.f1.sub.0)=2f1.sub.0 f2.sub.0 
g.DELTA.x (5) 
It will therefore be appreciated that the differential transducer system 
described above, not only eliminates the affects of ambient temperature 
change from the weighing measurement, but also provides double the 
resolution scale of a single transducer system. Nevertheless, the system 
described with reference to FIGS. 6-8 will not automatically compensate 
for temperature gradients across the substrates of the transducer and that 
is why the transducer system is provided with the thermal sinks described 
above. It will also be appreciated that the overall length (height) of the 
transducer system is substantially doubled as compared with the 
non-differential system. In order to reduce the overall size of the 
transducer system and to increase the sensitivity and resolution of the 
system, the present invention preferably uses a frequency of approximately 
50 mhz as compared to the 20 mhz frequency of the parent application. At 
this frequency, in order to achieve the desired accuracy (0.5 grams per 10 
kg), the temperature difference between the substrates must be kept below 
0.01 degrees C. 
FIGS. 9-11 show a schematic illustration of a differential transducer 
system similar to the one described above, but with both transducer 
channels operating on the same substrate and with one of the transducer 
channels being split. Referring now to FIGS. 9-11, the transducer system 
includes a transmitting transducer 520 and a receiving transducer 522. The 
transmitting transducer 520 includes a piezoelectric substrate 520a which 
is mounted on one side a thermal sink 521, and a thermal insulator 523 is 
mounted on the other side of the thermal sink 521. The thermal sink 521 
helps to maintain a constant temperature across the substrate 520a and the 
thermal insulator 523 helps to prevent ambient temperature changes from 
affecting the temperature of the thermal sink 521 and thus the substrate 
520a. A first transmitting electrode 520b is located at one end of the 
substrate 520a on a central axis thereof. A second transmitting electrode 
pair 520c, 520d is located at the other end of the substrate on opposite 
sides of the central axis. Each of the electrodes 520c, 520d is 
approximately half the wavelength of the electrode 520b and the electrodes 
520c, 520d are coupled in parallel to form the electrode pair. The 
transmitting transducer 520, therefore generates three SAW waves 526, 
526', and 526". The first SAW wave 526 propagates along a central channel 
on the substrate and in a first direction. The second and third SAW waves 
526' and 526" propagate along two side channels on the substrate an in a 
direction opposite to the first SAW wave. 
The receiving transducer 522 includes a piezoelectric substrate 522a which 
is mounted on one side of a thermal sink 525, and a thermal insulator 527 
is mounted on the other side of the thermal sink 525. The thermal sink 525 
helps to maintain a constant temperature across the substrate 522a and the 
thermal insulator 527 helps to prevent ambient temperature changes from 
affecting the temperature of the thermal sink 525 and thus the substrate 
522a. The substrate 522a has a first receiving electrode 522b and a second 
receiving electrode pair 522c, 522d. The receiving electrode 522b is at 
one end of the substrate on a central axis thereof and the receiving 
electrodes 522c, 522d are located at the other end of the substrate on 
opposite sides of the central axis. The receiving electrodes 522c, 522d 
are half the wavelength of the electrode 522b and are coupled in parallel 
to each other. The receiving transducer 522, therefore receives three SAW 
waves 528, 528', and 528" which are induced respectively by the 
transmitted SAW waves 526, 526', 526". 
The arrangement shown in FIGS. 9-11 has several advantages. The SAW wave 
propagation channels are closer together on the same substrate and 
therefore the temperature gradient between them will be smaller. The 
overall size of the transducer system is smaller. In addition, the 
transducers are axially symmetrical which enhances mechanical performance. 
FIGS. 12-14 show a schematic illustration of a differential transducer 
system similar to the one described above, but with additional electronic 
means for measuring the temperature of the, substrates. Referring now to 
FIGS. 12-14, a transmitting transducer 620 includes a first piezoelectric 
substrate 620a and a second piezoelectric substrate 620a', both of which 
are mounted on a base 623 which is preferably constructed as a sandwich of 
insulating and conductive materials as described above with reference to 
FIG. 6. The first piezoelectric substrate 620a is provided with a first 
pair of transmitting electrodes 620b at a lower central portion thereof 
and the second piezoelectric substrate 620a' is provided with a second 
pair of transmitting electrodes 620b' at an upper central portion thereof. 
In addition, first piezoelectric substrate 620a is provided with two pair 
of electrodes 630a, 630b which are spaced apart from each other and 
arranged off center from the first pair of transmitting electrodes 620b. 
The electrodes 630a, 630b are respectively a transmitter and receiver 
which are used to measure the temperature of the substrate 620a as 
described below. Accordingly the substrate 620a' is also provided with two 
pair of temperature measuring electrodes 630a', 630b'. 
A receiving transducer 622 includes a first piezoelectric substrate 622a 
and a second piezoelectric substrate 622a', both of which are mounted on a 
base 627 which is preferably constructed as a sandwich of insulating and 
conductive materials as described above with reference to FIG. 6. The 
first piezoelectric substrate 622a is provided with a first pair of 
receiving electrodes 622b at an upper central portion thereof and the 
second piezoelectric substrate 622a' is provided with a second pair of 
transmitting electrodes 622b' at a lower central portion thereof. In 
addition, the first piezoelectric substrate 622a is provided with two pair 
of temperature measuring electrodes 632a, 632b which are spaced apart from 
each other and arranged off center from the first pair of transmitting 
electrodes 522b. The substrate 522a' is also provided with two pair of 
temperature measuring electrodes 632a', 632b'. 
The transducer arrangement shown in FIGS. 12-14 incorporates several of the 
features of the transducer arrangement described above with reference to 
FIGS. 6-8. The electrode pairs 620b, 620b', 622b, 622b' are arranged to 
provide a differential displacement measurement system as described above. 
In this regard, and with reference to FIG. 14, it will be appreciated that 
the differential system utilizes two acoustic channels which are centrally 
located, one on the upper pair of substrates and the other on the lower 
pair of substrates. In addition, it will be appreciated that the 
transmitting substrates are slightly larger than the receiving substrates 
and therefore overlap the receiving substrates with the same advantages as 
described above. According to a presently preferred embodiment, the 
transmitting transducer 620 is approximately 45 mm long and the space 
between the first and second substrates is approximately 5 .mu.m. The 
receiving transducer 622 is approximately 10 mm shorter than the 
transmitting transducer and the space between the first and second 
receiving substrates is approximately 5 .mu.m. 
As mentioned above, each of the four piezoelectric substrates is provided 
with a two pair of temperature measuring electrodes which are arranged as 
a fixed position delay line on each substrate. Each of the four sets of 
temperature measuring electrodes is coupled to a respective amplifier and 
thereby forms a natural oscillator which preferably oscillates at a 
frequency which is different from the frequency at which the displacement 
measuring oscillators oscillate. Since the temperature measuring electrode 
sets are stationary on their respective substrates, the frequency of their 
respective oscillators will vary only due to changes in temperature. With 
this provided arrangement, the temperature of each of the four substrates 
can be determined and accounted for when making displacement and weight 
measurements. 
As seen in FIG. 14, each of the two measuring electrode sets operates in a 
separate acoustic channel. It is possible, however, to provide two 
measuring electrode sets which operate in almost the same channel. This 
minimizes the thermal gradient between the two channels. FIGS. 15-17 
illustrate one embodiment of such an arrangement. 
Turning now to FIGS. 15-17, a transmitting transducer 720 includes a 
piezoelectric substrate 720a with a pair of transmitting electrodes 720b 
located at a lower end thereof and a pair of receiving electrodes 730 
located at an upper end thereof. A receiving transducer 722 includes a 
piezoelectric substrate 722a with a pair of receiving electrodes 722b at 
an upper end thereof. It will be appreciated that the transmitting and 
receiving electrodes 720b, 722b are arranged with amplifier 750 to form a 
delay line for measuring displacement and weight as described above. In 
addition, the receiving electrodes 730 on the transmitting substrate form 
a stationary delay line with amplifier 760 and the transmitting electrodes 
720b for measuring the temperature of the transmitting substrate 720a. The 
amplifiers 750 and 760 may be operated simultaneously if the delay lines 
have significantly different frequencies. Alternatively, the amplifiers 
750, 760 may be switched on and off alternatingly. It will be appreciated 
that all of the electrodes operate in the same acoustic channel. The 
transducer system shown in FIGS. 15-17 is a non-differential system 
wherein the temperature of the transmitting substrate is assumed to be 
close to that of the receiving substrate. However, it is possible to apply 
the technology of this system to a differential system wherein separate 
measurements are made for the two channels. Such a system is shown in 
FIGS. 18-20. 
Turning now to FIGS. 18-20, a transmitting transducer 820 includes a 
piezoelectric substrate 820a with a pair of transmitting electrodes 820b 
located at a lower end thereof and a pair of transceiving electrodes 830 
located at an upper end thereof. A receiving transducer 822 includes a 
piezoelectric substrate 822a with a pair of receiving electrodes 822b at 
an upper end thereof and a pair of transceiving electrodes 832 at a lower 
end thereof. It will be appreciated that the transmitting and receiving 
electrodes 820b, 822b are arranged to form a delay line for measuring 
displacement and weight as described above. In addition, the transceiving 
electrodes 830 and 832 can be used to form a stationary delay line with 
respective electrodes 820b, 822b or may be used with each other to form a 
displacement measuring delay line which is differential to the delay line 
formed by electrodes 820b, 822b. In operation, the four electrode pairs 
may be multiplexed such that at one moment, two differential displacement 
measuring delay lines are activated and at another moment two stationary 
temperature measuring delay lines are activated. In this manner, the 
temperature of each substrate can be ascertained prior to or even during 
temperature measurement. It will be appreciated that all of the electrodes 
operate in the same acoustic channel. 
As mentioned above and in the parent application, the delay lines according 
to the invention may oscillate in more than one mode and within each mode, 
the gain will vary as the frequency changes. According to the invention, 
the phase of the oscillator may be shifted .+-.180.degree. in order to 
increase gain (decrease loss). FIGS. 21-26 illustrate how the modes of 
oscillation change during weighing and how phase shifting can be used to 
increase gain. 
Referring now to FIG. 21, in the idle state, with no weight applied to the 
scale, the delay line will oscillate at a frequency "f.sub.0 " which is 
shown in FIG. 21 as the point having the most gain (least loss). The 
optimal gain area of the graph of FIG. 21 is shown in the shaded area 
surrounding f.sub.0 and represents a range of .+-.100 Khz, for example. 
This area is considered optimal-not only because it is the area of least 
loss, but because it is the area wherein the curve exhibits the least 
"non-linearity" and is least influenced by temperature. As described in 
detail in the parent application, the delay line may oscillate in any of 
several modes and the modes are separated from each other by a phase 
difference of 2.pi.. In the example shown in FIG. 21, the frequency 
f.sub.0 has a lower mode f.sub.0 -2.pi..omega. and a higher mode f.sub.0 
+2.pi..omega., where .omega. is the interger one, the phase difference 
between the modes representing approximately 340 Khz in this example. As 
weight is added to the scale, the delay line will oscillate at a higher 
frequency "f.sub.0 +n". For example, after adding a relatively small 
weight, the frequency of oscillation will rise to f.sub.0 +70 Khz which is 
shown in FIG. 22 as f.sub.1. 
Referring now to FIG. 22, it will be seen that the new frequency of 
oscillation f.sub.1 is still within the optimal gain area and the higher 
and lower modes of oscillation have greater loss than the mode at f.sub.1. 
It will be appreciated, however, that with the addition of additional 
weight, the frequency f will soon pass out of the optimal gain area. For 
example, an additional weight could shift the frequency an additional 50 
Khz to the position shown in FIG. 23. 
Referring now to FIG. 23, an oscillation frequency f.sub.2, which is 
approximately 120 Khz higher than f.sub.0, would move the frequency out of 
the optimal gain area. Nevertheless, as shown in FIG. 23, the higher and 
lower modes of oscillation would still show greater loss than the central 
mode at f.sub.2. Those skilled in the art will therefore appreciate that 
from this point onward, additional weight will raise the frequency of 
oscillation though an increasingly high loss area until the lower mode 
achieves greater gain than the central mode which is shown in FIG. 24. 
Turning now to FIG. 24, an oscillation frequency f.sub.3, which is 
approximately 170 Khz higher than f.sub.0 would cause a shift to the lower 
mode of oscillation f.sub.3 -2.pi..omega.. It will be appreciated, 
however, that even after the mode shift, the frequency of oscillation must 
shift through approximately 70 Khz more before oscillation of the lower 
mode will occur in the optimal gain area. According to the invention, 
therefore, it is possible to apply a phase shift of m to the oscillator in 
order to force early oscillation in the optimal gain area. For example, as 
shown in FIGS. 23 and 24 as soon as the oscillation frequency f.sub.2 
exhibits loss which indicates it is no longer in the optimal gain area, a 
phase shift of -.pi..omega. is applied. This causes the oscillator (delay 
line) to oscillate at f.sub.2 -.pi..omega. which, as can be seen in FIG. 
23, is within the optimal gain area of the frequency response curve. This 
phase shift will maintain the frequency of oscillation f.sub.3 
-.pi..omega. within the optimal area even when the frequency rises to 
f.sub.3 as shown in FIG. 24. Eventually, however, additional weight on the 
scale will raise the frequency of oscillation to a point where the phase 
shifted frequency is outside the optimal gain area. 
For example, as shown in FIG. 25, when the frequency of oscillation f.sub.4 
is increased to approximately f.sub.0 +255 Khz, the phase shifted central 
mode of oscillation f.sub.4 -.pi..omega. will exit the optimal gain area. 
At this point, according to the invention, the -.pi..omega. phase shift is 
discontinued and the oscillator will oscillate in its lower mode f.sub.4 
-2.pi..omega. which is within the optimal gain area. If additional weight 
is added to the scale, the frequency of oscillation will continue to rise 
until the lower mode of oscillation passes beyond the optimal gain area as 
shown in FIG. 26. 
Referring now to FIG. 26, if the frequency f.sub.5 is raised to 
approximately f.sub.0 +400 Hhz, the lower mode oscillation f.sub.5 
-2.pi..omega. will pass beyond the optimal gain area. According to the 
invention, at this point, a -.pi..omega. (phase shift will be applied to 
the oscillator. This will cause the lower mode of oscillation to reside at 
f.sub.5 -3.pi..omega. which is within the optimal gain area. 
FIG. 27 shows a simplified schematic diagram of a positive feedback loop 
with phase shifting according to the invention. Referring now to FIG. 27, 
a simplified delay loop according to the invention includes a first 
transducer 920, a second transducer 922, a first differential amplifier 
950, a second differential amplifier 952, a pair of matching transformers 
954, 956, a frequency counter and amplifier controller 958, and an output 
processor and weight display 960. The first transducer 920 includes a 
piezoelectric substrate 920a and electrodes 920b. The second transducer 
922 includes a piezoelectric substrate 922a and electrodes 922b. The 
electrodes 920b are coupled via the matching transformer 954 to the inputs 
of the differential amplifiers 950, 952 in a parallel manner. The 
electrodes 922b are coupled to the outputs of the amplifiers 950, 952 via 
the matching transformer 956. As shown in FIG. 27, the polarity of the 
outputs of the amplifier 950 is opposite to the polarity of the outputs of 
the amplifier 952. In addition, the enable input of each amplifier is 
coupled to the frequency counter and amplifier controller 958 which is 
also coupled to the outputs of the amplifiers. According to the invention, 
the amplifiers 950, 952 are turned on at one time by the frequency counter 
and amplifier controller 958. It will be appreciated that the phase of the 
outputs of the amplifiers differs by 180.degree. or .pi.. Thus, in order 
to apply or remove a phase shift, one of the amplifiers is turned off and 
the other is turned on. Those skilled in the art will appreciate that 
other circuits can be utilized to produce substantially the same type of 
phase shifting and that the circuit of FIG. 27 is merely one example. 
According to the example shown in FIG. 27, the frequency counter and 
amplifier controller 958 monitors the output of the amplifier 950 and 
detects when the frequency passes beyond the optimal gain area as 
described above, e.g., increases by 100 Khz. When the frequency increases 
by a preselected amount, the frequency counter and amplifier controller 
958 turns off amplifier 950 and turns on amplifier 952. The frequency 
counter and amplifier controller 958 then monitors the output of amplifier 
952. After the frequency increases by an additional preselected amount, 
e.g. 100 Khz, the frequency counter and amplifier controller 958 turns off 
amplifier 952 and turns on amplifier 950. While the frequency counter and 
amplifier controller 958 is monitoring frequencies, the frequencies are 
passed to the output processor and weight display 960 which analyzes the 
frequency of oscillation, correlates the frequency with a particular 
weight according to the methods described in the parent application, and 
displays the weight. 
There have been described and illustrated herein several embodiments of an 
electronic weighing apparatus utilizing surface acoustic waves. While 
particular embodiments of the invention have been described, it is not 
intended that the invention be limited thereto, as it is intended that the 
invention be as broad in scope as the art will allow and that the 
specification be read likewise. Thus, while particular geometries of the 
base, elastic member, and load platform have been disclosed, it will be 
appreciated that other geometries could be utilized. Also, while 
particular wavelengths have been disclosed, it will be recognized that 
other wavelengths could be used with similar results obtained. Moreover, 
while particular configurations have been disclosed in reference to the 
location of transmitting and receiving electrodes, it will be appreciated 
that the respective locations of transmitters and receivers could be 
reversed. Furthermore, while several different aspects of the invention 
have been disclosed as solving various problems, it will be understood 
that the different aspects of the invention may be used in combination 
with each other in configurations other than those shown. It will 
therefore be appreciated by those skilled in the art that yet other 
modifications could be made to the provided invention without deviating 
from its spirit and scope as so claimed.