U.S. patent number 3,737,690 [Application Number 05/229,995] was granted by the patent office on 1973-06-05 for ultrasonic transducer for intruder alarm system.
This patent grant is currently assigned to The Mosler Safe Company. Invention is credited to John Antonio.
United States Patent |
3,737,690 |
Antonio |
June 5, 1973 |
ULTRASONIC TRANSDUCER FOR INTRUDER ALARM SYSTEM
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.
Inventors: |
Antonio; John (Fairfield,
CT) |
Assignee: |
The Mosler Safe Company
(Hamilton, OH)
|
Family
ID: |
22863545 |
Appl.
No.: |
05/229,995 |
Filed: |
February 28, 1972 |
Current U.S.
Class: |
310/316.01;
310/322; 381/173; 310/325 |
Current CPC
Class: |
G08B
13/1618 (20130101); B06B 1/06 (20130101) |
Current International
Class: |
B06B
1/06 (20060101); G08B 13/16 (20060101); H04r
017/00 () |
Field of
Search: |
;340/10,15,8L,8MM,8FT
;179/11A ;310/8.2,8.3,8.7,9.1,9.4,8.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Budd; Mark O.
Claims
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;
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.
Description
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 U.S. 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 firmly secured 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. 1 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 3--3 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. 2.
Referring to FIG. 1, 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 10 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 11 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 11 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
three-point 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.degree. 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 38 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 48, in the center thereof, is provided a
mounting hole 55 at which the transducer head assembly 31 is
supported in a manner 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 38. 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 circuit 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 ceiling 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 48 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 dislodging 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 48 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
transducer 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 38 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 relatively 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 61 which will move away from the
mounting pan 38 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 81
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 Corning's 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 81, 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 1 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.
* * * * *