US3737690A

US3737690A - Ultrasonic transducer for intruder alarm system

Info

Publication number: US3737690A
Application number: US00229995A
Authority: US
Inventors: J Antonio
Original assignee: Mosler Safe Co
Current assignee: Mosler Safe Co
Priority date: 1972-02-28
Filing date: 1972-02-28
Publication date: 1973-06-05
Anticipated expiration: 1990-06-05

Abstract

An ultrasonic/electrical transducer for operation at a given frequency in an ultrasonic intruder alarm system is provided with a shallow self-supporting dish-shaped radiator mounted at the center of its concave surface to one face of a piezoelectric ceramic crystal. The opposite face of the crystal is secured to one end of a tuning stub which extends one-half wavelength from the center of the radiator and is connected at its midpoint to a base through an accoustic isolator. The stub is tuned by addition of trimming washes to its free end. The crystal is of the expansion/compression type having its piezoelectric axis parallel to that of the tuning stub and perpendicular to the surface of the radiator. The base is made of two parts joined by means which are the structurally weakest point of the assembly so that, if the transducer is tampered with, this point will move tripping a tamper switch to signal the alarm.

Description

immiwfi @MWS Patent 1 Antonio 5] June 5 973 [54] ULTRASUNHC TRANSDUQER FOR 2,922,999 1 1966 Carlin ..340/15 HNTRUDER ALARM SYSTEM 3,296,586 1/1967 Midlock et al. ..340 15 [75] Inventor: ,llohn Antonio, liairfield, Conn. Primary Examiner l D Miller [73] Assignee: The Mosler Safe Company, Hamil- Assistant ExaminerMark O. Budd 1011,0hi0 tggr ngy- James s. Hight, Herbert C. Brinkman Jr., 22 Filed: 1 61. 2a, 1972 pp N91 229,995 57 ABSTRACT An ultrasonic/electrical transducer for operation at a [52] US. Cl. ..3l0/8.2, 179/110 A, 3 l0/8.3, given f q y in an ultrasonic intruder alarm System 310/8.7, 3l0/9.1, 3l0/8.l, 340/8 MM, is provided with a shallow self-supporting dish-shaped 340/15 radiator mounted at the center of its concave surface to one face of a piezoelectric ceramic crystal. The p 1 8 posite face of the crystal is secured to one end of a 8 9 1 tuning stub which extends one-half wavelength from the center of the radiator and is connected at its midpoint to a base through an accoustic isolator. The stub [56] References Cited is tuned by addition of trimming washes to its free UNITED STATES PATENTS end. The crystal is of the expansion/compression type having its piezoelectric axis parallel to that of the tun- 1,802,781 4 1931 Sawyer et al. ..340 10 mg Stub and perpendicular to the Surface of the radia 3,675,053 7/1972 M1fune ..3l0/8.2 2,800,647 7 1957 Baerwald.... .....310 s.2 x The base 18 ad f two parts jOll'led by means 3,287,693 11/1966 Bango i ..340 15 which are the structurally Weakest P of the 3,5l7,226 6/1970 Jones, Sr. ,.3l0/9.1 Sembly so that, if the transducer is tampered with, this 3,218,488 3 189KB 9 --3l0/8- point will move tripping a tamper switch to signal the 3,210,724 10 1 65 Jones eta ....340/8 MM alarm 2,345,472 3/1944 Goldsmith ..340/15 15 Claims, 4 Drawing Figures PATENTEE M 5 I973 SHEET 2 [1F 2 ULTRASQNTC TRANSDUCER FOR HNTRUDER ALARM SYSTEM The present invention relates to ultrasonic transducers of the type which are used with ultrasonic frequency electrical transmitters and receivers to transmit and receive ultrasonic wave energy through air. More particularly, the present invention relates to such transducers which are specifically useful with ultrasonic space alarm systems which are designed to detect the presence of intruders in certain rooms or enclosures.

Space alarm systems are those which protect an area or enclosure against intruders by detecting the presence or motion of an intruder within the area. This detection is achieved by filling the area with radiated energy and measuring disturbances in the reflected energy caused by the intruders. This energy is usually in one of two forms, either electromagnetic or ultrasonic. Most frequently, detection is achieved by sensing a doppler frequency shift in energy reflected from moving objects within the enclosure. The present invention is primarily intended to improve upon space alarm systems of the ultrasonic type by providing a more effective transmitting and receiving transducer for such applications.

Ultrasonic space alarm systems usually employ ultrasonic continuous wave energy at a frequency just above the audible frequency range at approximately 20 kilohertz. These transducers are either driven by transmitters which generate ultrasonic frequency electrical signals or drive receivers to produce electrical signals. Normally, one transducer is used for transmitting and another is used for receiving, although in some applications a single transducer can be used for both purposes. Some transducers employ magnetostrictive devices, while others employ piezoelectric ceramic crystals. The present invention relates to transducers of the piezoelectric type. Such transducers are disclosed in the prior art in Bango US. Pat. No. 3,287,693. The transducer of Bagno employs a deep parabolic bell-shaped diaphragm supported by elastic mounts at its edges to a base. The center of the diaphragm has affixed to it a piezoelectric ceramic crystal which bends when excited by electrical energy to induce standing wave undulations in the diaphragm. This transducer is not particularly efficient and tends generally to have a broad band characteristic.

In space alarm system applications, it is desirable that the transducers employed have a generally omnidirectional pattern so that the room or enclosure may be filled with relatively uniform strength radiation. These transducers are normally mounted in the ceiling of the room so that the radiation will propagate in more or less uninterrupted paths throughout the room. It is a further desirable feature of such transducers that they have a generally high efficiency so that, in transmitting applications, less power is required to drive them, and secondly, in receiver applications, so that a stronger signal can be derived to be supplied to the receiver for detection.

Most space alarm systems operate on a doppler principle. For such doppler systems it is desirable to radiate only a single frequency of energy, and for this purpose, the most desirable transducer is one which has a narrow radiation band. It is further desirable that such transducers be provided with a rugged and sturdy mounting, one which will allow a long reliable life of the transducer, which will enable continued high efficiency operation, and which will allow the addition of simple means to detect tampering with the transducer. It is generally an objective of the present invention to provide a transducer which has such features and results in generally greatly improved operation over devices of the prior art provided for this purpose.

Accordingly, the present invention is predicated in part upon the concept of providing an ultrasonic transducer having a generally self-supporting shallow dish radiator which is mounted at its center, through a piezoelectric crystal, to a base. The present invention is further predicated in part upon the concept of providing such a transducer with a tuning stub, and more particularly, with a stub which operates to provide high efficiency narrow band operation of the transducer and, further, which operates as a mounting means for the crystal which supports the radiator.

More specifically, the radiator is generally of a shallow dish shape having one fae of a piezoelectric crystal tirmlysecured to the center of the concave surface of the radiator. The radiator is self-supporting when mounted at its center, rather than of the diaphragm type which is supported at its edge as are common loudspeaker cones and transducers of the Bagno type. The crystal has its piezoelectric axis perpendicular to the surface of the radiator at the point to which it is attached. The face of the crystal opposite the radiator is secured to one end of a one-half wavelength rod which serves as the tuning stub. Means are provided for securing washers to the free end of the stub to facilitate in trimming the stub to the proper resonant frequency. The transducer, when driven by electrical energy, will expand and contract in the direction of its axis to displace the radiator at its center in relation to the tuning rod. These oscillations will propagate along the rod and reinforce the motion of the crystal at the proper resonant frequency. The mounting of the resonator and crystal is achieved by securing the midpoint of the tuning stub through an accoustic isolator to a rigid base. This midpoint will be essentially an accoustic null point on the tuning stub, and mounting at this point will least affect the efficiency of the transducer and the narrowness of the band at which it is designed to operate.

Furthermore, the base is designed in two parts which are joined in a manner which is less structurally strong than the mounting between the tuning rod and the base so that if there is any tampering with the transducer the base will separate into two parts. At the two parts, a tamper switch is provided which will signal any parting of this connection. In this manner, tampering can be detected without the need to additionally connect any components to the head portion of the transducer which comprises the radiator, crystal and tuning stub. Such connections will interfere with the general narrow band operation of the transducer and generally adversely affect the efficiency of the transducer unless extensive design compromises are made to accommodate them.

These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings illustrating one preferred form of an ultrasonic transmitting and/or receiving transducer suitable for use in ultrasonic space alarm systems and generally embodying the principles of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. l is a diagrammatic perspective view illustrating an ultrasonic alarm system for protection of a room or enclosure, the alarm system utilizing ultrasonic transducers according to principles of the present invention.

FIG. 2 is a cross-sectional view of the ultrasonic transducers of FIG. 1.

FIG. 3 is a horizontal cross-sectional view of an ultrasonic transducer of the present invention taken along line 33 of FIG. 2.

FIG. 4 is a horizontal cross-sectional view similar to FIG. 3, but of reduced scale, of an ultrasonic transducer of the present invention taken along line 4--4 of FIG.

Referring to FIG. I, a simplified diagrammatic representation of an ultrasonic alarm system is illustrated. Such a system is disclosed in my co-pending application Ser. No. 110,735 filed Jan. 28, 1971.

This alarm system is illustrated in FIG. 1 as protecting a room or enclosure by radiating ultrasonic energy at a given frequency into the enclosure through a transmitting transducer 11. This energy is reflected from the enclosure 10 and received by a receiving transducer 12. If an object such as an intruder is moving within the enclosure 10, a doppler frequency shift will occur in the received signal which is detected to generate an alarm signal. Both transducers l1 and 12 are preferably mounted in the ceiling of the enclosure.

The alarm system generally includes an ultrasonic transmitter 21 which has an output connected to the transmitting transducer 11. The system is further provided with an ultrasonic receiver 22 which has its input connected from the receiving ultrasonic transducer 12. A detector 23 is provided having a pair of inputs connected one to the output of the transmitter 21 and another from the output of the receiver 22. The detector detects the doppler frequency shift by comparing the received signal with the transmitted signal to generate a detected signal representative of the doppler frequency component of the received signal which is supplied through the logic circuit 24 to trigger an alarm 25.

The transducers l1 and 12, according to the present invention, are substantially identical and can be better understood by reference to FIG. 2.

The preferred embodiment of a transducer according to the present invention includes two basic parts, the ultrasonic head assembly 31, and the transducer base 32 which mounts the head assembly 31 to the closure structure as, for example, the ceiling of a room.

The base 32 includes a cylindrical plastic shell tube 34. The shell tube 34 is provided with a pair of

annular grooves

35 and 36 on the interior surface thereof and near the upper end of the tube 34. The shell tube 34 is connected to a mounting pan assembly 38 by a threepoint spider-like support arrangement at the uppermost groove 36. This spider-like support arrangement includes a pair of pins 39 and a set screw 40 each extending radially outward from the mounting pan side wall and spaced thereabout at approxamately 120 intervals. This mounting arrangement can be better understood by reference to FIG. 3 which illustrates the vertical side wall portion of the mounting pan 38, the pins 39 extending radially therefrom and riding in the groove 36 in the inner wall of the tube 34, and the set screw 40 extending radially from the side wall of the pan 33 and through an opening in the wall of the tube 34 at the groove 36 therein. The set screw is provided with a pair of elastic stop nuts 41 and 42 which tighten screw 40 with respect to the walls of the mounting pan 38 and the shell tube 34, respectively.

A dust cover plate 45 underlies the mounting pan assembly 38 (FIG. 2) at a point immediately above a groove 35. The lower surface of this plate 45 is covered with a pad damper 46. The dust cover 45 is mounted to a chassis 48 at upper flange portions 49 thereof by a pair of screws 50. The flange portion 49 of the chassis 48 is square in shape (FIG. 4), and is mounted to the tube 34 by its corners which are positioned in the lower groove 35. At the lower end of the chassis 411, in the center thereof, is provided a mounting hole 55 at which the transducer head assembly 31 is supported in a man ner which will be more completely described below. A tamper switch 61 is mounted to the dust cover 45 by way of a tamper switch bracket 62. A switch actuator 63 projects through the bracket 62 to contact the undersurface of the mounting pan assembly 33. The switch circuit, illustrated diagrammatically in FIG. 2 is made to be in a normally open or closed condition when the actuator 63 is depressed as indicated in the Figure, and to change to the opposite condition in the event that the actuator is allowed to extend, which is the case which will occur if someone attempts to tamper with the transducer to disable it. The circuit connectors 67 connect the tamper switch to an alarm cir cuit which will sound an alarm if tampering occurs.

The transducer may be mounted either against the ceiling through screws 69 extending through the mounting pan assembly 38 into the ceiling as indicated by the reference line 71 in FIG. 2. Alternatively, the transducer may be flush mounted against the ceiling by use of a bezel 72 having an adhesive backing 73. In such a case, the tube 34 is inserted into the hole in the ceiling from approximately the same or slightly larger diameter as the tube 34 so that it rests against the celling when the position indicated by the line 74 in FIG. 2. The transducer is secured tightly to the ceiling by a mounting strap 75 which tightens about the tube 34.

As will be apparent from the description below, the transducer head 31 is mounted more securely to the chassis 43 than is the chassis 48 to the shell tube 34, or for that matter than is the shell tube 34 to the mounting pan 38. Thus, any attempt to disable the transducer by removing the head 31 will cause a clislodging of the chassis 48 with respect to the mounting structure which is stationary with respect to the ceiling. This will cause the dust cover to move with the bracket chassis to move the tamper switch 61 with its actuator 63 away from the mounting pan 38 causing the switch 61 to actuate giving a tamper alarm. For example, if the trans ducer assembly is mounted against the ceiling represented by line 71, the spider mounting. legs 39 will yield first causing the entire tube 34 and head assembly portion 31 to move away from the mounting plate 33 actuating the tamper switch which will be carried with it. Normally, when this mounting only method is contemplated, the chassis 48 is more securely constrained to the groove 35. On the other hand, if the transducer head assembly is mounted flush with the ceiling represented by line 74 in FIG. 2, the tube 34 will be rela tively securely constrained to the ceiling. If an attempt is made to remove the chassis 48, it will dislodge from the groove 35 carrying with it the tamper switch at which will move away from the mounting pan 33 which is connected to the tube 34. This will also actuate the tamper switch alarm. For this mounting method, the chassis 48 may be less strongly constrained in the groove 48.

The transducer head assembly 31 includes a piezoelectric crystal 81 mounted with its axis oriented vertically. Electrode faces 82 and 83 at the upper and lower ends of the crystal 81 connect through leads 84 either to the output of a transmitter or to the input of a receiver. The crystal 81 will expand and contract in the direction of its vertical axis when signals are applied to the leads 84 when used with a transmitter, or alternatively, will generate electrical signals on the leads 84 when caused to expand or contract in the vertical direction when used with a receiver. A suitable crystal 8] is the Gulton Model M-3 D8 HDT 31.

An upwardly concave shallow aluminum dish 86 having an outside diameter somewhat less than the inside diameter of the tube 34 is situated at the lower end of the crystal 81. Through the center of the dish 86 is connected a circular, thin aluminum slug 87 which is secured to the dish 86 by nonconductive epoxy cement. The electrode face 83 of the crystal 81 is secured to the center of the slug 87 also by epoxy cement. Suitable epoxy cement for this purpose is the Emerson & Cummings Resin No. 2850 FT used with their No. 9 catalyst hardener. The electrode face 82 at the upper end of the crystal 81 is similarly secured by the same epoxy cement to a cylindrical aluminum resonator rod 88. The resonator rod extends upwardly through the hole 55 in the bracket 48. The transducer head assembly 31 is secured to the base 52 at this hole in the bracket 48 by means of a rubber grommet 89 which electrically and accoustically isolates the rod 88 from the bracket 48. The grommet is connected to the respective rod 88 and chassis 48 by a synthetic rubber adhesive sealant 90, such as Dow Cornings SILASTIC RTV 891. The rod 88 is connected to the chassis 48 at an accoustic node or null point which is approximately half the distance from the lowermost surface of the dish 86 to the uppermost tip of the rod 88. This midpoint is approximately onequarter of an accoustic wavelength (speed of sound within the rod 88) from either end of the transducer head assembly 31. The rod 88 engaged in this manner serves as an accoustic band pass filter tuned to the midban frequency at which the transducer is to be operated. The rod 88 is manufactured slightly less than the desired length and trimming washers 92 are added to the upper end of the rod 88 during tuning of the transducer head. The washers 92 are secured to the upper end of the rod by a stainless steel screw 93.

The preferred frequency for space alarm applications has been selected at 22 kilohertz. At such frequency, certain dimensions of the transducer head have been found desirable. The preferred length for the rod 88, including the crystal 8], is about 3.838 inches measured from the lower surface of the dish 86 to the upper end of the rod 88 beneath the trimming washer. Washers are provided of aluminum in thicknesses of .010, .020, .030, and .050 inches. This has been found quite suitable for trimming the rod 88 to resonate at the desired frequency in that production of the rod can be easily held to .015 inches tolerance and some allowance is still present for other variations in an assembly. The diameter of the rod is selected at one-half inch. The distance from the lower surface of the dish 86 to the center line of the chassis 48 at the mounting hole 55 for the rod 88 is suitable 1.98 inches. The diameter of the dish 86 is suitably 3.91 inches clearing the inside diameter of the tube 34 by roughly one-eighth of an inch on all sides.

Certain electrical circuit components are provided in the transducer assembly. These are mounted by a printed circuit board 94 to the chassis 48 through screws 95 and spacer washers 96. A transformer 97 is also secured to the chassis 48. Transformer 97 serves as an impedance matching transformer for the crystal 81 and the printed circuit board 94 carries appropriate terminals and, when the transducer is used with a receiver, carries a preamplifier circuit. A schematic of this circuit appears in the lower right-hand corner of FIG. 2. When the transducer is used as a transmitter, the transformer 97 schematically represented by windings 97A is merely connected between the leads 84 and the output of the transmitter 21. The transmitter has approximately 35 to l step-up ratio with this purpose. The impedance levels at the input is 50 ohms and at the output 62.5 K ohms. The transformer 97 is used in the same orientation in the receiving application, however, a preamplifier 98 is connected between the crystal 81 and the transformer winding 97A with the high impedance winding of the transformer 97 connected to the amplifier output and the preamplifier inputs connected across leads 84.

The transducer described above'in detail thus provides a rugged, efficient device for use with an ultrasonic intruder alarm system for both transmitting and receiving. The transducer head, when constructed in accordance with the dimensions described in relation to the wavelength selected, and the specific relation to the base constitute only a preferred form of the general concepts of the present invention, but this preferred form has considerable improved performance over devices of the prior art particularly when used in alarm systems of this type. It will be appreciated that this and other forms of the invention can also greatly improve performance of not only alarm systems but other types of systems in which transducers are generally employed.

What is claimed is:

1. An ultrasonic transducer comprising:

a piezoelectric crystal having first and second electrode faces on opposite sides thereof;

a self-supporting shallow dish radiator mounted at its center to the first face of said crystal to be solely supported thereby; 1

a support member mounted at one end to the second face of said crystal and having its other end structurally unconnected;

a base; and

a connecting element secured between said support member at an acoustical node in said support member and said base.

2. The transducer of claim 1 wherein:

said connecting element includes an ultrasonic acoustic isolator.

3. The transducer of claim 1 additionally including:

an electrical switch having alternately conducting and nonconducting states mounted on one of said base or said support member and having an actuator contacting the other of said base or support member for changing between said states when said support member and said base move relative to each other, thereby providing a tamper detection means.

4. A transducer according to calim 1 for operation at a given ultrasonic frequency wherein:

said support member comprises a tuning stub con- 5 nected at one end to said second face and aligned perpendicular to said second face, said stub being approximately one half the wavelength in said stub of an acoustical wave at said given frequency, and said stub being connected to said base at the midpoint thereof.

5. The transducer of claim 1 wherein:

said radiator has a curved shape presenting one concave and one convex surface; and

said radiator is mounted to said crystal at the center of said concave surface.

6. The transducer of claim 1 further comprising:

an impedance transformer mounted to said base having first and second windings, the leads of said first winding being connected to the electrode faces of said crystal and the leads of said second winding being connectable across the outputs of an ultrasonic frequency transmitter.

7. The transducer of claim 1 further comprising:

a preamplifier having inputs connected across the electrode faces of said crystal; and

an impedance transformer mounted to said base having first and second windings, the leads of said first winding being connected to the output of said preamplifier and the leads of said second winding being connectable across the inputs of an ultrasonic frequency receiver.

8. The transducer of claim 1 wherein:

said crystal has its piezoelectric axis oriented perpendicular to said radiator.

9. The transducer of claim ll wherein:

said transducer is of the expansion-compression type whereby said radiator moves with respect to said base in accordance with the electrical potential across the faces of said crystal.

10. The transducer of claim 4 wherein:

said connection between said stub and said base includes an ultrasonic accoustic isolator.

11. An ultrasonic transducer for operation at a given ultrasonic frequency comprising:

a centrally supported curved shallow dish radiator;

a tuning rod having a length of approximately, one half the wavelength, in said rod, of an accoustic wave at said given frequency;

a piezoelectric crystal connected between the center of said radiator and one end of said rod.

12. The transducer of claim 11 further comprising:

a base for supporting said transducer on an external structure, said base being connected to said rod at an accoustic null point along the length thereof.

13. The transducer of claim 11 further comprising:

14. The transducer of claim 11 wherein:

the connection between said base and said rod includes an ultrasonic accoustic isolator.

15. The transducer of claim 11 wherein:

said crystal has its piezoelectric axis aligned with the axis of said rod and perpendicular to the surface of said radiator.

Claims

1. An ultrasonic transducer comprising: a piezoelectric crystal having first and second electrode faces on opposite sides thereof; a self-supporting shallow dish radiator mounted at its center to the first face of said crystal to be solely supported thereby; a support member mounted at one end to the second face of said crystal and having its other end structurally unconnected; a base; and a connecting element secured between said support member at an acoustical node in said support member and said base.

2. The transducer of claim 1 wherein: said connecting element includes an ultrasonic acoustic isolator.

3. The transducer of claim 1 additionally including: an electrical switch having alternately conducting and nonconducting states mounted on one of said base or said support member and having an actuator contacting the other of said base or support member for changing between said states when said support member and said base move relative to each other, thereby providing a tamper detection means.

4. A transducer according to calim 1 for operation at a given ultrasonic frequency wherein: said support member comprises a tuning stub connected at one end to said second face and aligned perpendicular to said second face, said stub being approximately one half the wavelength in said stub of an acoustical wave at said given frequency, and said stub being connected to said base at the midpoint thereof.

5. The transducer of claim 1 wherein: said radiator has a curved shape presenting one concave and one convex surface; and said radiator is mounted to said crystal at the center of said concave surface.

6. The transducer of claim 1 further comprising: an impedance transformer mounted to said base having first and second windings, the leads of said first winding being connected to the electrode faces of said crystal and the leads of said second winding being connectable across the outputs of an ultrasonic frequency transmitter.

7. The transducer of claim 1 further comprising: a preamplifier having inputs connected across the electrode faces of said crystal; and an impedance transformer mounted to said base having first and second windings, the leads of said first winding being connected to the output of said preamplifier and the leads of said second winding being connectable across the inputs of an ultrasonic frequency receiver.

8. The transducer of claim 1 wherein: said crystal has its piezoelectric axis oriented perpendicular to said radiator.

9. The transducer of claim 1 wherein: said transducer is of the expansion-compression type whereby said radiator moves with respect to said base in accordance with the electrical potential across the faces of said crystal.

10. The transducer of claim 4 wherein: said connection between said stub and said base includes an ultrasonic accoustic isolator.

11. An ultrasonic transducer for operation at a given ultrasonic frequency comprising: a centrally supported curved shallow dish radiator; a tuning rod having a length of approximately, one-half the wavelength, in said rod, of an accoustic wave at said given frequency; a piezoelectric crystal connected between the center of said radiator and one end of said rod.

12. The transducer of claim 11 further comprising: a base for supporting said transducer on an external structure, said base being connected to said rod at an accoustic null point along the length thereof.

13. The transducer of claim 11 further comprising: a base for supporting said transducer on an external structure, said base being connected to said rod at an accoustic null point along the length thereof.

14. The transducer of claim 11 wherein: the connection between said base and said rod includes an ultrasonic accoustic isolator.

15. The transducer of claim 11 wherein: said crystal has its piezoelectric axis aligned with the axis of said rod and perpendicular to the surface of said radiator.