System for precisely and economically adjusting the resonance frequence of electroacoustic transducers

The resonance frequency of an electroacoustic transducer is precisely and quickly adjusted by the continuous removal of material from the vibratile surface while the resonance frequency is continuously monitored. The material removal rate is electronically controlled and is automatically decreased as the resonance frequency of the transducer approaches close to the desired specified value. The removal of material is automatically stopped at the precise instant when the adjusted resonance frequency becomes equal to the desired specified value.

This invention is concerned with an improved system for adjusting the 
resonance frequency of an electroacoustic transducer, and more 
particularly with the adjustment of either the resonance or anti-resonance 
frequency of the transducer. It is well known that electroacoustic 
transducers designed for use in the ultrasonic or near ultrasonic 
frequency region generally employ resonant vibratile structures if high 
electroacoustic efficiency is desired. It is also well known that the 
mechanical Q of a resonant vibratile transducer structure is generally 
relatively high and, as a result, the frequency response characteristics 
of such transducers are of very narrow band width. When such transducers 
are used to provide an acoustic link in an electroacoustic system such as 
an ultrasonic intrusion alarm, for example, it is essential that the 
transmitting and receiving transducers be precisely tuned to give maximum 
sensitivity at the desired frequency of operation of the system. 
Various methods have been developed for accomplishing the final tuning 
adjustment of resonant transducers, and the adjustment procedure has 
fallen into two broad classes. In one method for adjusting the resonance 
frequency, the mechanical tolerances of the vibrating structure are chosen 
such that the deviation in resonance frequency among the production 
elements falls above the desired operating frequency, and a selected 
weight is added to the vibratile element to increase its effective mass 
and thereby reduce the resonance frequency by the required amount to 
reduce the resonance to the specified value. This procedure has been 
described in U.S. Pat. No. 3,128,532. Another procedure in which the 
surface of the diaphragm is machined in small increments to reduce its 
thickness to bring the resonance to the desired value is also described in 
the referenced patent. Still another procedure is described in the same 
patent in which the resonance frequency of a vibratile element is adjusted 
by removing material from the periphery of the element to raise the 
resonance frequency of the vibratile element to the desired value. 
These prior art methods for adjusting the resonance frequency of 
electroacoustic transducers have accomplished their intended objectives, 
but the added cost for performing the adjustment can not always be 
justified for low-cost mass-production transducers. Also, because of the 
incremental nature of adding selected weights or removing incremental 
amounts of material from the surfaces of the vibratile element, it was not 
possible to perform the frequency adjustment operation continuously while 
the resonance frequency was being simultaneously monitored; therefore, a 
direct adjustment of the resonance frequency to a precise specified value 
could not be quickly achieved. Another limitation to the prior art method 
of removing material from the surface of the diaphragm by grinding or 
machining is caused by the fact that during the material removal 
operation, the temperature of the diaphragm is increased, and the 
frequency measurements must be delayed to allow for cooling between the 
incremental removal of material. 
For low-cost mass-production applications, it has been the general practice 
to accept a manufacturing tolerance of several percent in the resonance 
frequency of transducers to accommodate the average variation in the 
mechanical tolerances of the components which are part of the vibratile 
system assembly. After final assembly, instead of further adjustment of 
the resonance frequency to a uniform precise specified value, the 
resonance frequencies are measured, and the transducers are separated into 
matched lots for use at the average resonance frequency indicated for each 
separately selected lot. Each separate transducer lot is then coded and 
used only at a specified system operating frequency corresponding to the 
designated average resonance frequency of the selected lot. 
This invention overcomes the limitations of the prior art and provides a 
low-cost precise method for quickly and continuously adjusting the 
resonance frequency of an electromechanical vibrating system to an exact 
specified value. The invention permits the continuous removal of material 
from the surface of the vibratile element without raising the temperature 
of the element and without physical contact of machine tool surfaces with 
the surface of the vibratile element. During the material removal 
operation, the resonance frequency of the vibratile element is 
continuously monitored, and at the specified value of resonance frequency, 
the removal of material is stopped and the resonance frequency of the 
transducer is thus automatically adjusted to the precise specified value. 
The primary object of this invention is to provide a system for 
continuously removing material from the surface of a vibratile element 
while the resonant frequency of the vibratile element is being 
continuously monitored and to automatically stop the material removal 
procedure when the resonance frequency reaches the specified value. 
Another object of this invention is to provide an economical method for 
adjusting the resonance frequency of an electroacoustic transducer by the 
continuous removal of material from the vibratile surface of the 
electroacoustic transducer while the motional impedance of the transducer 
is being monitored, and to stop the removal of material from the vibratile 
surface when the motional impedance measurement indicates that the 
specified resonance frequency has been reached for the transducer. 
Still another object of this invention is to provide an economical method 
for adjusting the resonance frequency of an electromechanical vibrating 
system by the continuous removal of material from the vibrating surface of 
the electromechanical vibrating system while the resonance frequency of 
the vibrating system is being monitored and automatically stopping the 
removal of material from the vibrating surface when the specified 
resonance frequency has been reached. 
An additional object of this invention is to rapidly and automatically 
adjust the resonance frequency of an electroacoustic transducer by 
continuously measuring and electronically tracking either the minimum or 
maximum value of the motional impedance of the transducer while material 
is being continuously removed from the vibratile surface of the 
transducer, and to electronically control the relative rate of material 
removal as a function of the difference in the measured value of the 
resonance frequency and the desired specified value of the resonance 
frequency so that the rate of removal of material is taking place at a 
relatively lower rate as the actual measured resonance frequency of the 
transducer approaches closer to the specified resonance frequency desired.

Referring specifically to FIG. 1, the output of a sweep oscillator 1 is 
connected to the terminals 2--2 of the transducer 3. A resistor 4, whose 
resistance value is preferably at least ten times the maximum value of the 
motional impedance of the transducer over the sweep frequency range, is 
connected in series with the output of the sweep oscillator, as 
illustrated. The use of the series resistor in combination with the 
constant voltage oscillator output will maintain constant current through 
the transducer 3 as the frequency sweeps through the transducer's 
resonance frequency region. If an oscilloscope 5 is connected across the 
transducer terminals with the vertical axis displacement adjusted to 
indicate the magnitude of the voltage appearing across the transducer and 
the horizontal axis adjusted to indicate the frequency during the sweep, 
the trace illustrated by curve 6 will represent the motional impedance 
magnitude of the transducer as a function of frequency. In this 
illustrative example, the electroacoustic transducer 3 includes a 
vibratile diaphragm 7 which is driven by a polarized ceramic disc attached 
to the inner surface of the diaphragm (not shown) as is well known to 
anyone skilled in the art of transducer design. 
As the frequency applied to the transducer terminals is swept, the motional 
impedance of the transducer will become a minimum, Z.sub.MIN, at its 
resonant frequency f.sub.R, and its impedance will become a maximum, 
Z.sub.MAX, at its anti-resonant frequency f.sub.A. If two transducers are 
to be used as a transmitter and receiver pair at an operating frequency 
f.sub.0, it is desirable that the motional impedance Z.sub.MIN for the 
transmitter be made to occur at the specified operating frequency f.sub.0, 
and also that the motional impedance Z.sub.MAX for the receiver be made to 
occur at the operating frequency f.sub.0. Under such conditions, the 
maximum acoustic output per volt will be generated by the transmitter at 
the specified operating frequency, and the maximum receiver sensitivity 
will also be achieved at the specified operating frequency. If the 
manufacturing tolerances for the transducer illustrated in FIG. 1 are so 
chosen that the variations in Z.sub.MIN for the transmitters and the 
variations in Z.sub.MAX for the receivers all lie above the specified 
operating frequency f.sub.0, then the resonance frequency of each 
transducer may be automatically adjusted by the system illustrated in FIG. 
1 so that all the transmitters will have their motional impedances 
Z.sub.MIN set to occur at precisely the specified operating frequency 
f.sub.0, and, similarly, the receivers will have their motional impedances 
Z.sub.MAX set to occur at the same specified operating frequency f.sub.0. 
In order to accomplish the continuous resonance frequency adjustment of the 
transducer 3, an air-jet abrasive spray 8 is discharged from the nozzle 9 
of an air abrasive machine 10. Such machines are well known in industry 
such as, for example, the Airbrasive machines manufactured by S. S. White 
Industrial Products, 151 Old New Brunswick Rd., Piscataway, N.J. The 
nozzle pressure may be adjusted to permit the removal of material at any 
desired rate. For example, the adjustment may be made such that the 
material removal rate may be sufficiently low to cause a resonance 
frequency change at a rate as slow as 1 Hertz per second, or the material 
removal rate may be increased to cause a resonance frequency change at a 
rate as fast as several hundred Hertz per second. 
Although the teachings of this invention may be carried on with a fixed 
material removal rate setting of nozzle pressure throughout the resonance 
frequency adjustment cycle, a preferred embodiment of the invention is to 
use a variable electronically-controlled material removal rate that 
automatically removes material at a relatively lower rate as the adjusted 
resonance frequency of the transducer approaches closer to the desired 
specified value. In this manner, the frequency adjustment process may be 
accomplished in a very short time and with very great precision. It is 
also advantageous in specific instances to electronically control the rate 
of the sweep of the oscillator as well as the frequency range of the 
sweep, as will be described, to achieve further reduction in the time 
required to complete the automatic adjustment of the transducer resonance 
within a few seconds, and to achieve a further increase in the precision 
of the frequency adjustment to a tolerance as low as about 0.01% as 
compared with a tolerance in the order of 1% which is the best that can be 
economically realized by previous state-of-the-art mass-production 
techniques. 
In the schematic representation of a preferred embodiment of the inventive 
system illustrated in FIG. 1, an impedance-monitoring electronic circuit 
11 is connected across the transducer terminals 2 for sensing the 
variation of the voltage across the transducer terminals during each 
frequency sweep which represents the motional impedance variation of the 
transducer during each sweep. The electronic system also includes control 
logic circuits illustrated by the block diagram 12, which are well known 
in the art of digital electronics and microprocessors, to perform the 
necessary recognition and control functions for the system, including the 
continuous measurement of the exact frequency at which either Z.sub.MIN or 
Z.sub.MAX occurs during each sweep, and also to control both the rate and 
band width of the frequency sweep to accomplish the desired objectives of 
the invention. The magnitude of the motional impedance, which corresponds 
to the magnitude of the voltage appearing across the transducer terminals, 
is monitored by the impedance monitor electronics 11 as the oscillator 
frequency is varied. The frequency measurement at the occurrence of either 
Z.sub.MIN or Z.sub.MAX during the sweep is made in the conventional 
well-known manner of counting the number of pulses from a high-frequency 
crystal-controlled clock during one or more periods of the sweep 
oscillator frequency. 
The circuit 12 includes logic for detecting the sharp reversals in the 
rate-of-change of the voltage across the transducer terminals which 
corresponds to Z.sub.MIN or Z.sub.MAX, as illustrated in the oscilloscope 
trace 6. Upon the detection of a sharp reversal in the rate-of-change of 
motional impedance versus frequency from an increasing to a decreasing 
rate, which occurs when the frequency is changing in the vicinity of 
Z.sub.MAX, or alternately, upon the detection of an opposite reversal in 
the rate-of-change of motional impedance versus frequency from a 
decreasing rate, which occurs when the frequency is changing in the 
vicinity of Z.sub.MIN, the logic circuit will generate a logic signal to 
control the sweep rate of the oscillator 1 to cause the sweep to be 
reversed in direction immediately after each recognition of the reversal 
in the rate-of-change of the motional impedance which takes place as the 
frequency is sweeping selectively either in the vicinity of Z.sub.MIN or 
Z.sub.MAX. Thus the oscillator sweep is being automatically controlled to 
selectively track either the resonance or anti-resonance frequency of the 
transducer Z.sub.MIN or Z.sub.MAX, as desired, while material is being 
removed from the vibratile surface of the transducer to selectively adjust 
either the resonance or anti-resonance frequency to a specified value. 
Additional logic can be provided in the jet spray control circuit 13 to 
reduce the intensity of the jet spray from the machine 10 as the measured 
resonance frequency of the transducer approaches close to the desired 
specified operating frequency. This additional control is particularly 
advantageous where a very high degree of precision is desired for 
adjusting the transducer resonance frequency. The logic circuit 12 also 
includes logic to perform the control function for turning off the 
abrasive jet spray machine 10 when the resonance frequency of the 
transducer has reached the specified value. 
Transducers being mass-produced for ultrasonic control systems generally 
operate in the frequency region above 25 kHz. This means that an 
adjustment of the resonance frequency of the transducer within a few Hertz 
of a specified value, which can be accomplished by the inventive system, 
represents a variation in the order of 0.01% in the frequency adjustment, 
which is completely negligible for most applications. A variation in 
frequency as much as 100 times greater is considered an excellent 
achievement in production uniformity when using prior art methods for 
adjusting the resonance frequency of transducers. Details of the 
electronic circuits to perform the functions described have not been shown 
because they are well known in the art of digital electronics and computer 
science, and the electronic circuit details are not, in themselves, a part 
of this invention. 
The use of the air-jet abrasive material removal system develops no heat, 
such as occurs with grinding wheels or sanding discs. Also, because there 
is no physical contact by machine tools with the surface of the vibratile 
diaphragm during the material removal operation, the motional impedance 
measurement can be made continuously during the material removal procedure 
while the oscillator frequency is swept at a rate greater than one sweep 
per second, and the precise adjustment of the resonance frequency is 
completed automatically within a few seconds, as compared with as much as 
several minutes which may be required with the resonance frequency 
adjustment procedures used prior to this invention. 
FIG. 2 illustrates a half-wavelength magnetostriction vibrator 20, well 
known in the art, which includes a surrounding coil 21 with terminals 22 
and 23. If the magnetostriction resonator 20 is substituted for the 
transducer 3 in FIG. 1 and the terminals 22, 23 are connected in place of 
the transducer terminals 2, the same frequency adjustment procedure 
described above for the transducer 3 can be used to adjust the frequency 
of the resonator 20. 
FIG. 3 illustrates another application of the inventive system for the 
adjustment of the planar resonance frequency of a polarized ceramic disc. 
The ceramic disc 30 is shown in an edge-wise view with its two opposite 
plane surfaces held between electrically conducting foam rubber pads 31 
and 32 which serve to establish electrical connection from the ceramic 
disc electrode surfaces to the metal discs 33 and 34. The bottom metal 
disc 34 is connected to a motorized shaft 35 which provides rotary motion 
for the disc 34. Gravity maintains contact of the electrically conducting 
pads 31 and 32 to the electrode surfaces of the ceramic disc 30. The outer 
edges of the metal discs 33 and 34 act as slip rings, and spring contact 
members 36, 37 make sliding electrical contact from the rotating slip ring 
surfaces to the terminal conductors 38 and 39, as illustrated 
schematically in FIG. 3. The top metal disc 33 includes guide means (not 
shown) to hold its center in axial alignment with the bottom disc 34. 
Means are also provided (not shown) for separating the two disc members 33 
and 34 for removing the ceramic 30 after completing the adjustment of its 
resonance frequency. The details of the mechanical structure are not shown 
in the schematic illustration of FIG. 3 because they are obvious to any 
mechanical engineer, and their details are not part of this invention. 
If the terminals 38 and 39 are connected in place of terminals 2--2 in FIG. 
1 and the nozzle 9 is mounted to direct the jet spray 8, as illustrated in 
FIG. 3, then the system of FIG. 1 can be used for automatically adjusting 
the planar resonant frequency of the ceramic disc in the same manner as 
described above for adjusting the resonance frequency for the other 
transducer structures. In the example of FIG. 3, the ceramic disc is 
rotated when the motorized shaft 35 is set in motion and the air-jet 
abrasive spray 8 removes material from the outer periphery of the ceramic 
disc 30, as illustrated. As the material is removed, the resonance 
frequency of the ceramic disc increases until it reaches the specified 
value at which instant the abrasive jet-spray is turned off by the control 
logic circuits, as previously described, and the ceramic is released from 
the fixture. The conducting foam rubber pads 31 and 32 are selected in 
softness to have no effect on the resonance frequency of the ceramic when 
the rubber pads are held in contact with the ceramic surfaces, as 
illustrated in FIG. 3. 
In the example illustrated in FIG. 1, the resonance frequency of the 
vibratile diaphragm 7 is lowered as material is removed from its surface; 
therefore, the manufacturing tolerances for the transducer 3 are so chosen 
that the resonance frequency variation of the production transducers fall 
above the specified operating value. For the examples illustrated in FIGS. 
2 and 3, the resonance frequencies of the elements will increase as 
material is removed from the surfaces; therefore, the manufacturing 
tolerances for the elements 20 and 30 are chosen to make the resonance 
frequency variations among the production elements fall below the 
specified operating values. 
Several examples have been given to illustrate some of the various uses 
that can be made of the disclosed invention. The use of the inventive 
system for automatically and precisely adjusting the resonance or 
anti-resonance frequency of large quantities of production electroacoustic 
transducers has made it possible to manufacture ultrasonic transducers 
with accurately controlled frequency tolerances at low cost and with 
greatly improved sensitivity and uniformity of the operating 
characteristics for the transducer system. 
Other embodiments of my invention will readily occur to those who are 
skilled in the art. Hence, the appended claims are to be construed broadly 
enough to cover all equivalents falling within their true spirit and 
scope.