Miniature quartz resonator force transducer

The invention relates to a piezoelectric quartz force transducer having the shape of a double-ended tuning fork.

BACKGROUND OF THE INVENTION 
Measurement of force with high accuracy and digital output has potential 
for application in widely varying fields such as pressure measurement, 
well logging, electronic engine control, oceanography, meteorology, tilt 
sensors, intrusion detectors, seismology, weighing, accelerometers, and 
industrial process control. 
A widely-used technique for force measurement utilizes a vibrating quartz 
resonator with frequency of vibration proportional to the force applied. 
These resonators are capable of high resolution and result in a digital 
output which make them attractive for use with digital microprocessors. 
However, existing quartz resonators require precision machining of complex 
shapes which may be prohibitively expensive to manufacture or which may 
result in costly, unreliable units. 
A desirable property of a vibrating quartz resonator force transducer is to 
have a high mechanical Q. Q is proportional to the ratio of energy stored 
to energy lost per cycle in the vibration system. A lower Q means that a 
larger source of external energy must be supplied to maintain the 
oscillations and the oscillator will possess a less stable resonant 
frequency. The present invention generally possesses a higher mechanical Q 
than that available with existing quartz resonator force transducers. 
Some quartz resonator force transducers are exemplified by U.S. Pat. Nos. 
3,399,572; 3,470,400; 3,479,536; 3,505,866; 4,020,448; 4,067,241; 
4,091,679; 4,104,920, and 4,126,801. As can be seen, these resonators 
either require complex and therefore expensive crystal shapes or complex 
and therefore expensive metal parts. 
SUMMARY OF THE INVENTION 
In view of the difficulties and disadvantages as noted above, it is an 
object of this invention to provide a novel force transducer. 
It is a further object of this invention to provide a relatively simple and 
inexpensive quartz resonator force transducer. 
It is a still further object of this invention to provide a digital force 
transducer with high resolution and accuracy. 
It is another object of this invention to provide a piezoelectric quartz 
resonating force transducer with high Q. 
The invention comprises a thin, rectangular piezoelectric quartz crystal 
divided into two end portions and two wide bar portions by a narrow slot. 
Each bar is excited into vibration by electrical contacts carried thereon 
and an appropriate oscillator circuit. The frequency of vibration is 
dependent on the magnitude of forces transmitted from the end portions to 
the bar portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is now made to FIG. 1 which illustrates in perspective the quartz 
resonator force transducer 10 of the present invention. This transducer 
may be likened to two tuning forks secured or glued end-to-end (as indeed 
were some early developmental examples of the invention). 
The transducer may be very small; the embodiment illustrated is generally 
rectangular and only 0.4 inches long and only 4 mils (1mil=0.001 inch) 
thick. The device may be fabricated by photolithographic etching 
techniques from a thin quartz sheet. These techniques are well-adapted to 
the mass production of this invention with final actual costs estimated to 
be less than one dollar per transducer. The quartz sheet may be made by 
cutting a thin layer from a larger quartz crystal in a desired 
crystalographic orientation so as to minimize the frequency dependence on 
temperature. With the present invention, a sheet cut about 4 mils thick 
and at an orientation of +13/4 degrees from the XY crystal plane and 
rotated about the X-axis has proven satisfactory. 
The preferred embodiment is relatively wide in comparison to its thickness; 
its width (W) of 35 mils is about 10 times its thickness (T). Its length 
(L) of about 400 mils is about 100 times its thickness (T). The term 
"about" is intended to include values .+-.50% from the given figure. 
A narrow slot 16 is photolithographically etched through the transducer 10 
so as to provide the transducer into two non-vibratory end portions 12 and 
two vibratory bar portions 14. The slot is extremely narrow being about 3 
mils wide or having a width (W.sub.s) less than thickness (T). The slot 
and therefore each bar portion has a length (L.sub.s or L.sub.b) about 240 
mils or 60% of the overall length (L). 
Each bar portion 14 therefore has a width (W.sub.b) of about 16 mils which 
is about 4 times their thickness (T). In an alternate embodiment, this 
width W.sub.b is trimmed to about 10 mils or about 2.5 times thickness 
(T). 
Each end portion is at least as long from end of slot 16 to the outer 
extremity as it is wide and preferably has a length (L.sub.e) of about 80 
mils or about 2 times width (W). 
Along the top surface (and/or bottom surface) of each bar portion 14 is a 
photolithographically defined metallic electrode. In operation, these 
electrodes are connected to an oscillator circuit which provides the 
necessary energy to cause the bars of the piezoelectric quartz transducer 
to vibrate at a characteristic frequency. 
Because the end portions 12 serve to couple together the vibrations of the 
bar portions 14 in an efficient manner isolating them from the extremities 
of the transducer, the transducer has a Q of about 100,000 as measured in 
vacuum. When forces are applied to the extremities of the transducer, the 
vibrations of each bar are changed by an equal amount and are detected as 
a change in frequency of the oscillator circuit. The narrowness of the 
slot aids in insuring that the force applied to each bar is equal, so that 
the change in vibration is equal, and so that the vibration of each bar 
does not fall out of phase with the other. 
The narrowness of the slot further aids in insuring a high Q in that during 
vibration, the flexing of one bar portion causes a slight flexing of the 
end portion which, in turn, is efficiently transmitted to the second 
flexing bar. This process effectively couples the vibrations of the two 
bars and helps correct any tendency to vibrate at differing frequencies. 
A quartz force transducer made as described hereinabove would be capable of 
sensing forces as small as 1 dyne (0.00003 ounce) and as large as 
1,000,000 dynes (2 pounds) with a resolution of 1 part per 1,000,000. Such 
a transducer would have a Q of 100,000 and require only 10 microwatts of 
electrical power to maintain the oscillation. It has been found that the 
use of a pair of the above transducers in a double-cantilever geometry 
will further increase sensitivity by a factor of 40. 
A previous double-ended tuning fork quartz resonator force transducer 20 
and that of the present invention 10 are shown to scale in FIG. 2. The 
force transducer of FIG. 2a is described in U.S. Pat. No. 3,238,789 and in 
a publication entitled "Technical Report on the Quartz Resonator Digital 
Accelerometer", Norman R. Serra, 43rd AGARD Conference Proceedings 1967. 
The actual dimensions of the earlier device are taken from the Serra paper 
but are also supported by FIGS. 2 and 6 of the patent. That work does not 
teach the critical nature of the dimensions discovered by the present 
inventors. Transducer 20, although of roughly equivalent size, does not 
teach certain critical dimensions of transducer 10. For example, the 
central hole of the earlier device is 60 mils wide as opposed to the 3 
mils narrow slot of the present invention. Also, the cross section of the 
arms of the earlier device is 40 mils deep by 10 mils wide as opposed to 
the 4 mils deep by 16 mils wide of the present invention. 
The Q reported for the earlier device is approximately 32,000 as opposed to 
the 100,000 of the present invention. It is felt that the critical 
features discovered by the present inventor account for this suprising 
difference. 
The various features and advantages of the invention are thought to be 
clear from the foregoing description. However, various other features and 
advantages not specifically enumerated will undoubtedly occur to those 
versed in the art, as likewise will many variations and modifications of 
the preferred embodiment illustrated, all of which may be achieved without 
departing from the spirit and scope of the invention as defined by the 
following claims.