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

#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.

#The novel features which are characteristic of this invention are set forth with particularity in the appended claims. However, the invention itself both as to its organization and method of operation will best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

#FIG. 1 is a schematic representation of one embodiment of my invention which illustrates a widely used type of ultrasonic ceramic-driven diaphragm-type electroacoustic transducer whose motional impedance is being monitored while material is being removed from the diaphragm surface to adjust the resonance frequency. A control logic circuit responsive to the motional impedance characteristic of the transducer stops the material removal when the specified resonance frequency is reached.

#FIG. 2 illustrates a half wavelength magnetostriction resonator element whose resonant frequency can be precisely adjusted by connecting the magnetostriction resonator in place of the electroacoustic transducer in FIG. 1.

#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.