Information
-
Patent Grant
-
6771006
-
Patent Number
6,771,006
-
Date Filed
Friday, January 18, 200222 years ago
-
Date Issued
Tuesday, August 3, 200420 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Dougherty; Thomas M.
- Aguirrechea; J.
Agents
-
CPC
-
US Classifications
Field of Search
US
- 310 369
- 310 800
- 310 366
- 310 322
-
International Classifications
- H01L4104
- H01L4108
- H02N200
-
Abstract
An ultrasound transducer that includes a piezoelectric film having a first end and a second end, a plurality of electrodes disposed on the piezoelectric film, at least one securing member and a support structure which is generally cylindrical. The first end and the second end of the piezoelectric film are secured to the support structure by at least one securing member.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to ultrasound transducers and, in particular, it concerns cylindrical ultrasound receivers and transceivers formed from piezoelectric films, and their applications in digitizer systems.
It is known to employ cylindrical ultrasound transducers for transmitting ultrasound signals in digitizer systems. The cylindrical form provides all-around signal transmission and simplifies the geometry of time-of-flight calculations by providing an effect similar to a point (or more accurately, line) source. These advantages are detailed in U.S. Pat. No. 4,758,691 to De Bruyne. A further advantage of cylindrical ultrasound transducers is that they can be centered on an element of which the position is to be measured. This is used in a drawing implement digitizer system described in PCT publication WO98/40838.
Structurally, a number of different types of cylindrical transducer have been proposed. The De Bruyne patent proposes a “Sell transducer” which is a capacitive device formed from a complicated arrangement of cylindrical layers intended to produce a cylindrical air gap of about 20 μm. Such a structure is costly to manufacture, and is likely to be unreliable.
A second type of transducer that has been proposed in the field of medical applications is based on piezoelectric elements. An example of a medical transducer of this type may be found in U.S. Pat. No. 4,706,681 to Breyer et al., which discloses an ultrasonic marker. Here, a cylindrical piezoelectric collar is sandwiched between two electrodes. Application of an alternating potential across the electrodes causes vibration of the collar, and hence emits a radially propagating ultrasonic signal.
In principle, any ultrasonic transducer is capable of being operated both as a transmitter and a receiver. In practice, however, many considerations result in many transmitter structures being ineffective as receivers. This is particularly true of cylindrical elements in which almost the entire cylinder contributes to wide angle transmission by actuation with a relatively high power while only a small portion of the cylinder is correctly orientated for receiving an incoming signal from a given direction. Furthermore, the inherent capacitance of the large inactive region of the transducer may absorb a large proportion of the amplitude of a received signal, rendering the transducer insensitive as a receiver.
In the field of transducers in general, much work has been invested in development of devices based on piezoelectric films, such as PVDF. Conductive electrodes are formed on opposite faces of the film, typically by selectively printing conductive ink on regions of the surfaces. These films are cheap to produce, and withstand a wide range of operating conditions including exposure to moisture.
Although a cylindrical ultrasound transducer is relatively simple to implement using piezoelectric film, implementation of a receiver poses additional problems beyond the general complications of cylindrical receivers discussed above. Specifically, referring to
FIGS. 1
,
2
there is shown a schematic plan view of a freely suspended cylinder
10
formed from piezoelectric film.
FIG. 1
shows its relaxed state, while
FIG. 2
shows the response of cylinder
10
to an incoming ultrasound signal wave front
15
. Since the piezoelectric film is flexible, the oscillations of signal
15
generate waves (exaggerated for clarity) traveling around cylinder
10
. The direction and extent of flexing of the piezoelectric film varies along the waveform created around the cylinder, resulting in reversal of the sense of an electrical potential generated between the electrodes. As a result, much of the potential generated by the piezoelectric film may be dissipated in local eddy currents within the electrodes, greatly reducing the overall signal voltage as measured between the electrodes.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is the tendency for the electrode to act as an antenna, picking up unwanted electromagnetic radiation which may result in very low signal to noise ratios.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is to provide mechanical protection for the transducer while minimizing disruption of the ultrasound waves.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is the damage caused through welding the piezoelectric film to form a cylinder.
There is therefore a need for a cylindrical ultrasound receiver structure employing piezoelectric film.
SUMMARY OF THE INVENTION
The present invention is a cylindrical ultrasound receiver structure employing piezoelectric film.
According to the teachings of the present invention there is provided an ultrasound transducer comprising: (a) a piezoelectric film having a first end and a second end; (b) a plurality of electrodes disposed on the piezoelectric film; (c) at least one securing member; and (d) a support structure, which is substantially cylindrical, wherein the first end and the second end are secured to the support structure by the at least one securing member.
According to a further feature of the present invention, there is also provided an electrical contact disposed on the support structure.
According to a further feature of the present invention, the support structure further includes a protrusion and wherein the first end and the second end are secured to the protrusion by the at least one securing member.
According to a further feature of the present invention: (a) the support structure has a central axis; (b) the protrusion is formed as an elongated projecting ridge having a direction of elongation; and (c) the direction of elongation being substantially parallel to the central axis.
According to a further feature of the present invention, there is also provided an electrical contact disposed on the protrusion.
According to a further feature of the present invention, the at least one securing member is a clip.
According to a further feature of the present invention, there is also provided an electrical contact wherein the electrical contact is disposed on the at least one securing member.
According to a further feature of the present invention, the piezoelectric film has a first surface and a second surface and wherein the electrodes include: (a) a first electrode disposed on the first surface; (b) a second electrode disposed on the second surface wherein at least a part of the second electrode is in an opposing relationship with at least a part of the first electrode; (c) a first electrical connecting strip disposed on the first surface wherein the first electrical connecting strip is connected to the first electrode; and (d) a second electrical connecting strip disposed on the second surface in a substantially non-opposing relationship with the first electrical connecting strip wherein the second electrical connecting strip is connected to the second electrode.
According to a further feature of the present invention, the piezoelectric film has a first surface and a second surface and wherein the electrodes include: (a) a first electrode and a second electrode disposed on the first surface, wherein the first electrode is disposed in a pattern that is non-contiguous with the second electrode; (b) a third electrode and a fourth electrode disposed on the second surface, wherein: (i) at least a part of the third electrode is in an opposing relationship with at least a part of the first electrode; (ii) at least a part of the fourth electrode is in an opposing relationship with at least a part of the second electrode; and (iii) the third electrode is disposed in a pattern that is non-contiguous with the fourth electrode; and (c) an electrical joining strip extending from the first electrode to the fourth electrode, wherein the electrical joining strip includes a first portion of the electrical joining strip on the first surface and a second portion of the electrical joining strip on the second surface, and wherein the first portion and the second portion are electrically connected.
According to a further feature of the present invention, the first portion and the second portion are electrically connected via a hole in the piezoelectric film.
According to a further feature of the present invention, there is also provided a helical metal spring, wherein the helical metal spring is disposed around the piezoelectric film.
According to additional teachings of the present invention there is also provided an ultrasound receiver comprising: (a) a piezoelectric film having a first surface and a second surface; (b) a first electrode disposed on the first surface; (c) a second electrode disposed on the second surface wherein at least a part of the second electrode is in an opposing relationship with at least a part of the first electrode; (d) a first electrical connecting strip disposed on the first surface wherein the first electrical connecting strip is connected to the first electrode; and (e) a second electrical connecting strip disposed on the second surface in a substantially non-opposing relationship with the first electrical connecting strip wherein the second electrical connecting strip is connected to the second electrode.
According to a further feature of the present invention, the first electrical connecting strip is in a substantially non-opposing relationship with the second electrode; and the second electrical connecting strip is in a substantially non-opposing relationship with the first electrode.
According to a further feature of the present invention, there is also provided a substantially cylindrical element, which is hollow, formed primarily from the piezoelectric film, the substantially cylindrical element having a central axis and a height measured parallel to the central axis; and a support structure for supporting the substantially cylindrical element, the support structure being configured to support the substantially cylindrical element in such a manner as to allow propagation of vibration waves circumferentially around a major part of the substantially cylindrical element; wherein the first electrode is formed as a strip extending in an extensional direction substantially parallel to the central axis along at least a part of the height, the strip subtending at the central axis an angle of not more than 90°.
According to a further feature of the present invention, the substantially cylindrical element has an inner surface wherein the first surface forms the inner surface; and the second electrode is grounded.
According to additional teachings of the present invention there is also provided a multi-electrode ultrasound receiver comprising: (a) a piezoelectric film having a first surface and a second surface; (b) a first electrode and a second electrode disposed on the first surface, wherein the first electrode is disposed in a pattern that is non-contiguous with the second electrode; (c) a third electrode and a fourth electrode disposed on the second surface, wherein: (i) at least a part of the third electrode is in an opposing relationship with at least a part of the first electrode; (ii) at least a part of the fourth electrode is in an opposing relationship with at least a part of the second electrode; and (iii) the third electrode is disposed in a pattern that is non-contiguous with the fourth electrode; and (d) an electrical joining strip extending from the first electrode to the fourth electrode wherein the electrical joining strip includes a first portion of the electrical joining strip on the first surface and a second portion of the electrical joining strip on the second surface and the first portion and the second portion being electrically connected.
According to a further feature of the present invention, there is also provided a substantially cylindrical element, which is hollow, formed primarily from the piezoelectric film, the substantially cylindrical element having a central axis and a height measured parallel to the central axis and wherein the first electrode and the second electrode in combination subtend at the central axis an angle of not more than 90°; and a support structure for supporting the substantially cylindrical element, the support structure being configured to support the substantially cylindrical element in such a manner as to allow propagation of vibration waves circumferentially around a major part of the substantially cylindrical element.
According to a further feature of the present invention, the substantially cylindrical element has an inner surface wherein the first surface forms the inner surface; and the third electrode is grounded.
According to a further feature of the present invention, the first portion and the second portion are electrically connected via a hole in the piezoelectric film.
According to a further feature of the present invention, there is also provided a first electrical connecting strip disposed on the first surface, wherein the first electrical connecting strip is connected to the second electrode; and a second electrical connecting strip disposed on the second surface, wherein the second electrical connecting strip is connected to the third electrode and the second electrical connecting strip is in a substantially non-opposing relationship with the first electrical connecting strip.
According to additional teachings of the present invention there is also provided a method for providing shielding for an ultrasound transducer used for a predetermined frequency of ultrasound waves while minimizing disruption to the ultrasound waves, comprising the steps of spacing windings of a helical metal spring at a spatial period of less than about half of a wavelength of the ultrasound waves associated with the ultrasound transducer; and positioning the helical metal spring surrounding the ultrasound transducer.
According to a further feature of the present invention, the step of spacing is performed by spacing the windings at a spatial period of less than about quarter of the wavelength.
According to additional teachings of the present invention there is also provided a digitizer system comprising: (a) an ultrasound transducer associated with a moveable element; (b) two ultrasound transducers; (c) a base unit; wherein the two ultrasound transducers are maintained in fixed geometrical relation by attachment to the base unit; and (d) an acoustic wave-guide; wherein the acoustic wave-guide includes a hollow elongated member and the acoustic wave-guide is disposed between the two ultrasound transducers.
According to a further feature of the present invention, the acoustic wave-guide is substantially straight.
According to a further feature of the present invention, the acoustic wave-guide is curved.
According to additional teachings of the present invention there is also provided a method for operating a system for determining a position of a point on a moveable element, the system including: a moveable group of ultrasound transducers including a first ultrasound transducer and a second ultrasound transducer each mounted on the moveable element where the first ultrasound transducer, the second ultrasound transducer and the point on the moveable element are sequentially spaced along a common axis; and a fixed group of ultrasound transducers including a third ultrasound transducer and a fourth ultrasound transducer spaced apart by a predefined distance, the method for operating comprising the steps of: (a) transmitting a plurality of measurement signals between the first ultrasound transducer and the fixed group and between the second ultrasound transducer and the fixed group; (b) deriving distances between the first ultrasound transducer and each of the third ultrasound transducer and the fourth ultrasound transducer and between the second ultrasound transducer and each of the third ultrasound transducer and the fourth ultrasound transducer from time-of-flight measurements for the measurement signals; and (c) deriving from the distances a position of the point.
According to a further feature of the present invention, the first ultrasound transducer and the second ultrasound transducer are both cylindrical ultrasound transducers.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
FIG. 1
is a schematic plan view of a freely suspended cylinder formed from piezoelectric film in its relaxed state;
FIG. 2
is a schematic view of the cylinder of
FIG. 1
when exposed to an ultrasonic signal;
FIG. 3
is an isometric view of a cylindrical ultrasound receiver that is constructed and operable in accordance with a preferred embodiment of the invention;
FIG. 4
is a schematic plan view of the film for use in
FIG. 3
;
FIG. 5
is a schematic plan view of a piezoelectric film showing the form of electrode patterns applied to each surface for use in the receiver of
FIG. 3
;
FIG. 6
is a schematic plan view of a piezoelectric film showing the form of multiple electrode patterns applied to each surface for use in the receiver of
FIG. 3
;
FIG. 7
is an exploded isometric view of a support structure for the receiver of
FIG. 3
;
FIG. 8
is an isometric view showing a single electrical contact plate for use in the support structure of
FIG. 7
;
FIG. 9
is a schematic isometric view illustrating a technique for forming electrical contacts with the receiver of
FIG. 3
;
FIG. 10
is a schematic isometric view of a protective helical spring for use in the receiver of
FIG. 3
;
FIG. 11
is a side view of a section of the helical spring of
FIG. 10
;
FIG. 12
is an exploded isometric view of a support structure for a cylindrical ultrasound transceiver that is constructed and operable in accordance with a most preferred embodiment of the invention;
FIG. 13
is a schematic plan view of a piezoelectric film showing the form of electrode patterns applied to each surface for use in the transceiver of
FIG. 12
;
FIG. 14
is a schematic plan view of a piezoelectric film showing the form of multiple electrode patterns applied to each surface for use in the receiver of
FIG. 12
;
FIG. 15
is a schematic plan view of a piezoelectric film showing the form of electrode patterns applied to each surface for use as a transceiver in the receiver of
FIG. 3
;
FIG. 16
is a block diagram illustrating the main components of a transceiver assembly including the transceiver of
FIG. 15
;
FIG. 17
is a schematic representation of the operation of a system for determining the position of a moveable element, constructed and operable in accordance with a preferred embodiment of the invention, operating in a primary mode of operation;
FIG. 18
is a schematic representation of the operation of the system of
FIG. 17
while performing a self-calibration operation;
FIG. 19
is a schematic representation of the operation of a system for determining the position of a moveable element, constructed and operable in accordance with an alternate embodiment of the invention, operating in a primary mode of operation;
FIG. 20
is a schematic representation of the operation of the system of
FIG. 19
while performing a self-calibration operation;
FIG. 21
is a schematic representation of the system of
FIG. 17
while performing a self-calibration mode using an acoustic wave-guide;
FIG. 22
is a schematic representation of the operation of a system for determining the position of a point on a moveable element, constructed and operable in accordance with an alternate embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is a cylindrical ultrasound receiver or transceiver formed from piezoelectric films. The invention also provides applications of such transceivers in digitizer systems.
The principles and operation of receivers and transceivers according to the present invention may be better understood with reference to the drawings and the accompanying description.
Reference is now made to
FIG. 3
, which is an isometric view of a cylindrical ultrasound receiver
18
that is constructed and operable in accordance with a preferred embodiment of the invention. Generally speaking, receiver
18
includes a substantially cylindrical element
20
, which is hollow. Cylindrical element
20
is formed primarily from flexible piezoelectric film, having an outer surface
25
, an inner surface
30
, an upper edge
32
, a lower edge
33
, a central axis
40
and a height h measured parallel to central axis
40
. Cylindrical element
20
is supported by a support structure, represented here by a core element
50
, configured to support cylindrical element
20
in such a manner as to allow propagation of vibration waves circumferentially around a major part of cylindrical element
20
. Cylindrical element
20
is supported from below by a base
55
and above by a cap
60
. As mentioned above, cylindrical element
20
is substantially cylindrical in that cylindrical element
20
approximates to a cylindrical shape over at least a majority of its circumference. This cylindrical portion provides the receiving functionality and therefore it is not critical if the non-functional portion is not cylindrical. Moreover, the cylindrical portion itself does not have to be accurately cylindrical. An application of this is discussed later in reference to FIG.
12
.
Reference is now made to
FIG. 4
, which is a schematic plan view of cylindrical element
20
that is constructed and operable in accordance with a preferred embodiment of the invention. A first electrode
65
is applied to inner surface
30
. A second electrode
70
is applied to outer surface
25
, where at least a part of second electrode
70
is in an opposing relationship with a majority of first electrode
65
. Second electrode
70
is grounded and first electrode
65
acts as a sensing electrode. However, it should be noted that first electrode
65
and second electrode
70
are interchangeable for use in other embodiments of the invention. First electrode
65
is formed as a strip extending in an extensional direction substantially parallel to central axis
40
along a major part of height h (FIG.
3
), and subtending at central axis
40
an angle α of not more than 90°. The dimension of first electrode
65
is preferably chosen such that it corresponds to less than about ¼ wavelength of the vibrations in cylindrical element
20
induced by ultrasound vibrations of the intended working frequency. In most cases, the dimensions are chosen such that cylindrical element
20
supports only about one wavelength of the vibrations (rather than the about four wavelengths illustrated schematically in
FIG. 2
) so as to minimize interference effects and the like. As a result, phase canceling problems can largely be avoided so long as first electrode
65
subtends an angle α of less than about 90° at central axis
40
. Preferably, however, the width of first electrode
65
is typically chosen to subtend an angle α of between about 20 and about 30° at central axis
40
.
The principle of operation of receiver
18
may be appreciated by referring back to
FIGS. 1 and 2
. As described above, the incident pressure waves
15
tend to induce vibration waves, which propagate around the periphery of cylinder
10
. As a result, an arbitrarily positioned localized sensor on the surface of cylinder
10
experiences substantially the same vibrations substantially independent of the direction from which pressure waves
15
are incident. At the same time, since the circumferential extent of first electrode
65
is small relative to the wavelength of the vibrations propagating through the film, the aforementioned problems of phase canceling and large capacitance are avoided. The result is a highly effective, wide-angle ultrasound receiver. These and other advantages of the configuration of the present invention will become clearer from the following more detailed description.
With regard to materials, it should be noted that the present invention might be implemented using any piezoelectric film material and suitable conductive electrode material. A particularly preferred example for the film itself is Polyvinyl Diflouride (PVDF). The direction of polarization should be oriented circumferentially around the cylindrical element. The use of such films provides particular advantages due to its wide frequency-band response. Specifically, it has been found that conventional narrow frequency-band receivers based on piezo-ceramics tend to shift signal noise into the frequency range of measurement, drastically reducing the signal-to-noise ratio. In contrast, the wide frequency-band receivers of the present invention, used in combination with subsequent filtering to identify the signal of interest, have been found to provide a greatly enhanced signal-to-noise ratio.
Suitable conductive materials for the electrodes include, but are not limited to, compositions containing carbon, silver and gold. In applications in which a transparent structure is required, a transparent conductive material is used. The conductive materials have been described as being “applied” to the piezoelectric film, as application of the conductive material is the typical production process. However, it should be notes that the conductive materials could be “disposed” on to the piezoelectric film using other methods known in the art.
Reference is now made to
FIG. 5
, which is a semi-transparent plan view of a piezoelectric film sheet forming cylindrical element
20
showing the form of electrode patterns applied to each surface for use in receiver
18
that is constructed and operable in accordance with a preferred embodiment of the invention. A first electrical connecting strip
75
is applied to inner surface
25
and first electrical connecting strip
75
is connected to first electrode
65
. The application of first electrical connecting strip
75
is in a substantially non-opposing relationship with second electrode
70
to reduce problems associated with capacitance. A second electrical connecting strip
80
is applied to outer surface
30
and second electrical connecting strip
80
is connected to second electrode
70
. The application of second electrical connecting strip
80
is in a substantially non-opposing relationship with first electrical connecting strip
75
to reduce problems associated with capacitance. It is also advantageous to apply first electrical connecting strip
75
in a substantially non-opposing relationship with second electrode
70
and second electrical connecting strip
80
in a substantially non-opposing relationship with first electrode
65
to avoid possible problems associated with capacitance. It should be noted that the terminology “substantially non-opposing” implies that it is preferable that a total non-opposing relationship exists so as to eliminate problems associated with capacitance. However, some opposition of electrical contact strips, although possibly increasing problems due to capacitance does not negate the essence of the invention, which is aimed at minimizing problems due to capacitance. First electrical connecting strip
75
and second electrical connecting strip
80
extend from first electrode
65
and second electrode
70
respectively to tabs
85
at lower edge
33
(
FIG. 3
) of cylindrical element
20
.
Reference is now made to
FIG. 6
, which is a semi-transparent plan view of the piezoelectric film sheet forming cylindrical element
20
showing the form of multiple electrode patterns applied to each surface for use in receiver
18
that is constructed and operable in accordance with a preferred embodiment of the invention. Increasing the cross-sectional area between the sensing and grounded electrode can increase the electrical current produced by an ultrasound transceiver. However, it is generally more advantageous to increase the electrical voltage produced by an ultrasound receiver. This can be achieved by having multiple electrode patterns set up in series. For receiver
18
this is achieved by applying a first electrode
90
and a second electrode
95
to inner surface
25
of cylindrical element
20
. The application of first electrode
90
is in a pattern that is non-contiguous with second electrode
95
. As discussed previously, in reference to the case of the single sensing electrode, first electrode
65
(FIG.
4
), first and second electrodes
90
,
95
are each formed as a strip. First and second electrodes
90
,
95
extend in an extensional direction substantially parallel to central axis
40
along at least part of height h (FIG.
3
). First electrode
90
and second electrode
95
in combination subtend an angle of not more than 90° at central axis
40
. A third electrode
100
and a fourth electrode
105
are applied to outer surface
30
of cylindrical element
20
, such that at least a part of third electrode
100
is in an opposing relationship with the majority of first electrode
90
and at least a part of fourth electrode
105
is in an opposing relationship with the majority of second electrode
95
. The application of third electrode
100
is in a pattern that is non-contiguous with fourth electrode
105
. Fourth electrode
105
is grounded. An electrical joining strip
110
,
115
includes a first portion of electrical joining strip
110
on inner surface
25
and a second portion of electrical joining strip
115
on outer surface
30
. First portion of electrical joining strip
110
extends from first electrode
90
to a hole Q in cylindrical element
20
and second portion of electrical joining strip
115
extends from hole Q to fourth electrode
105
. First portion of electrical joining strip
110
and second portion of electrical joining strip
115
are joined at hole Q using conductive material. First electrical connecting strip
75
and second electrical connecting strip
80
discussed in reference to
FIG. 5
can be used here for this embodiment of the invention. First electrical connecting strip
75
is applied to inner surface
25
and first electrical connecting strip
75
is connected to second electrode
95
. Second electrical connecting strip
80
is applied to outer surface
30
and second electrical connecting strip
80
is connected to third electrode
100
. The application of second electrical connecting strip
80
is also in a substantially non-opposing relationship with first electrical connecting strip
75
, to reduce problems associated with capacitance across surfaces
25
,
30
of cylindrical element
20
. First electrical connecting strip
75
and second electrical connecting strip
80
extend from second electrode
95
and third electrode
100
respectively to tabs
85
at lower edge
33
(
FIG. 3
) of cylindrical element
20
. It should be noted that first electrode
90
and second electrode
95
can be applied to outer surface
30
and third electrode
100
and fourth electrode
105
can be applied to inner surface
25
in an alternative embodiment of the invention. It should also be noted that more electrodes could be applied to cylindrical element
20
and connected in series to increase voltage output of receiver
18
.
Reference is now made to
FIG. 7
, which is an exploded isometric view of support structure
117
for receiver
18
that is constructed and operable in accordance with a preferred embodiment of the invention. As mentioned earlier, one major problem associated with implementation of a cylindrical ultrasound transducer using piezoelectric film is the tendency of the electrodes to function as an antenna for electromagnetic radiation. To minimize or eliminate this problem preferred implementations of the present invention include one or more features, which help to shield the sensing electrode from electromagnetic radiation. Firstly, second electrode
70
, which is grounded, provides some shielding for first electrode
65
. This, incidentally, is the reason it is preferred to position first electrode
65
on the inner surface of the film rather than externally thereto. A further or alternative contribution to electromagnetic shielding is preferably provided by employing an electrically grounded conductive core element
50
disposed within cylindrical element
20
in such a manner as to avoid electrical contact with first electrode
65
. Core element
50
is typically, although not necessarily, part of support structure
117
for cylindrical element
20
. One preferred implementation of core element
50
is a metal core element, which may be solid or hollow. In order to ensure that the film of cylindrical element
20
is free to vibrate, core element
50
is here formed with a reduced diameter portion
120
over a major part of its height. In certain cases, the non-contact regions defined by reduced diameter portion
120
may be sufficient to avoid electrical contact with first electrode
65
. Alternatively, an additional insulating layer may be interposed between core element
50
and first electrode
65
. An alternative implementation of core element
50
can be formed from a cylinder of conductive foam (not shown). In this case, contact between core element
50
and cylindrical element
20
typically does not significantly interfere with propagation of vibrations within cylindrical element
20
. In this case, an additional insulating layer is generally required between core element
50
and first electrode
65
. As mentioned above, cylindrical element
20
is supported from below by a base
55
and above by a cap
60
. Base
55
includes electrical contact springs
140
. Base
55
and cap
60
are secured to core element
50
by a bolt
145
.
Reference is now made to
FIG. 8
, which is an isometric view showing a single electrical contact plate for use in support structure
117
that is constructed and operable in accordance with a preferred embodiment of the invention. Base
55
has one electrical contact spring
140
. This can be used where electrical connecting strip
75
,
80
are combined onto a single tab
85
, or electrical connecting strips
75
,
85
extend to different edges
32
,
33
(
FIG. 3
) of cylindrical element
20
.
Reference is now made to
FIG. 9
, which is a schematic isometric view illustrating a technique for forming electrical contacts with receiver
18
that is constructed and operable in accordance with a preferred embodiment of the invention. A tab
85
containing an electrical connecting strip
75
,
80
of receiver
18
, is pushed into electrical contact spring
140
. Tab
85
is held in place by the pressure of electrical contact spring
140
.
Reference is now made to
FIG. 10
, which is a schematic isometric view of a protective helical spring
150
for use in receiver
18
, constructed and operational according to an embodiment of the present invention. Helical spring
150
is placed surrounding receiver
18
. Helical spring
150
provides mechanical and electromagnetic shielding for receiver
18
, while minimizing interference with the incident ultrasound waves, as will be explained below in reference to FIG.
11
. Helical spring
150
is formed from a conductive material and is grounded to provide electromagnetic shielding.
Reference is now made to
FIG. 11
, which is a side view of a section of helical spring
150
. Helical spring
150
has windings
155
of thickness t and a spatial period S. Mechanical protection must often be provided for transducers, particularly those using piezoelectric films that are easily damaged. Many existing transducer structures suffer from significant signal distortion alone or in combination with “blind spots” (i.e., directions in which transmitted intensity or sensitivity of reception are significantly impaired) due to the presence of various protective structures in front of the transducer. To minimize or eliminate such problems, the present invention uses helical spring
150
with windings
155
having a spatial period S of no more than λ/2, and preferably no more than λ/4, where λ is the wavelength of the ultrasound working frequency in air. By using helical spring
150
with a spatial period S significantly smaller than existing systems, little or no directional disruption is caused to the ultrasound signals. By way of a practical example, for a working frequency of 90 kHz, corresponding to a wavelength in air of about 4 mm, a value for S of 1.9 mm has been found to offer minimal disruption to the transmission and reception of signals.
Reference is now made to
FIGS. 12 and 13
.
FIG. 12
is an exploded isometric view of a support structure for receiver
18
that is constructed and operable in accordance with a most preferred embodiment of the invention.
FIG. 13
is a semi-transparent plan view of a piezoelectric film
175
showing the form of electrode patterns applied to each surface for use in receiver
18
of FIG.
12
. Welding piezoelectric film
175
to form cylindrical element
20
is an expensive process and welding can lead to damage to piezoelectric film
175
. Piezoelectric film
175
used in cylindrical element
20
can be formed into cylindrical element
20
without welding piezoelectric film
175
, while allowing propagation of vibration waves circumferentially around a major part of receiver
18
. This is achieved by wrapping piezoelectric film
175
around a support structure
160
, which is substantially cylindrical, with ends of film
192
,
193
resting on a protrusion
165
on support structure
160
. Protrusion
165
is typically an elongated projecting ridge that has its direction of elongation substantially parallel to the central axis of support structure
160
. Protrusion
165
has substantially parallel clamping surfaces
166
. A securing member, typically a clip
170
secures ends of film
192
,
193
to protrusion
165
to form piezoelectric film
175
into a substantially cylindrical shape. Typically one securing member is used to secure ends of film
192
,
193
to protrusion
165
, however more than one securing member could be used to perform the same function. Clip
170
performs a clamping function that can be performed with other clip designs performing the same clamping function. Prior to wrapping piezoelectric film
175
on support structure
160
, piezoelectric film is applied with the necessary electrodes and electrical contacts needed. A sensing electrode
180
is applied to a first side
182
of piezoelectric film
175
and a grounded electrode
190
is applied to a second side
183
of piezoelectric film
175
. When piezoelectric film
175
is wrapped around support structure
160
, first side
182
of piezoelectric film
175
will typically face towards support structure
160
, thereby grounded electrode
190
is on the outside providing electromagnetic shielding for sensing electrode
180
. Grounded electrode
190
substantially extends to one of ends
192
of piezoelectric film
175
. Extended grounded electrode
190
provides additional electromagnetic shielding for sensing electrode
180
and also enables grounded electrode
190
to be connected directly to an electrical contact
172
on the inside of clip
170
. An electrical connecting strip
185
is applied to first side
182
of piezoelectric film
175
. Electrical connecting strip
185
extends from sensing electrode
180
to substantially the other one of ends
193
of piezoelectric film
175
. This enables sensing electrode
190
to be connected directly to an electrical contact
167
on protrusion
165
. It should be noted that many other electrode designs are possible such as adding an additional electrode to use piezoelectric film
175
in an ultrasound transceiver.
Reference is now made to
FIG. 14
, which is a schematic plan view of a piezoelectric film showing the form of multiple electrode patterns applied to each surface for use in the support structure of FIG.
12
. In a most preferred embodiment of the invention, the multiple electrode patterns discussed in
FIG. 6
can be adjusted for use with the support structure of FIG.
12
. First electrode
90
and second electrode
95
are applied to first side
182
of piezoelectric film
175
. Third electrode
100
and fourth electrode
105
are applied to second side
183
of piezoelectric film
175
. Electrical joining strip
110
,
115
extends from first electrode
90
via a hole Q in piezoelectric film
175
to fourth electrode
105
. First electrical connecting strip
75
is connected to second electrode
95
. Second electrical connecting strip
80
is connected to third electrode
100
. First electrical connecting strip
75
and second electrical connecting strip
80
extend from second electrode
95
and third electrode
100
respectively to ends
192
,
193
of piezoelectric film
175
. The relative positions and non-overlapping of electrodes and electrical connecting strips has already been explained in reference to FIG.
6
.
Reference is again made to FIG.
12
. In a most preferred embodiment of the invention, mechanical protection and additional electromagnetic shielding can be provided for receiver
18
by placing helical spring
150
described in
FIG. 10
,
11
around receiver
18
.
Reference is now made to
FIG. 15
, which is a semi-transparent plan view of a piezoelectric film showing the form of electrode patterns applied to each surface for use as a transceiver that is constructed and operable in accordance with a preferred embodiment of the invention. Although device
18
has been described thus far as an ultrasound receiver, the same structure is highly suited for use in a transceiver system, i.e. for both receiving and transmitting signals, as will now be described. In addition to the application of first electrode
65
, first electrical connecting strip
75
, second electrode
70
and second electrical connecting strip
80
(all described in reference to
FIG. 5
above), an additional electrode
195
is applied to inner surface
25
of cylindrical element
20
. Additional electrode
195
is connected to an electrical connecting strip
200
that extends to tab
85
. Second electrode
70
is enlarged to cover a larger area of cylindrical element
20
. The application of additional electrode
195
is in a pattern that is non-contiguous with first electrode
65
and in a substantially opposing relationship with second electrode
70
. When not in use as a transmitter additional electrode
195
can be grounded to provide additional electromagnetic shielding. When in use as a transmitter, a driving potential can be applied between additional electrode
195
, together with first electrode
65
, if required and second electrode
70
to generate an ultrasound signal, similar to the operation of a conventional cylindrical ultrasound transmitter.
Reference is now made to
FIG. 16
, which is a block diagram illustrating the main components of a transceiver assembly employing device
18
. As mentioned earlier, it is advantageous that both second electrode
70
and additional electrode(s)
195
are grounded for shielding purposes during reception of ultrasound signals. In order to maintain this advantage, a switching system
225
may be used to selectively switch connection of second electrode
70
or additional electrode
195
to transmitter circuitry when transmission is required. Thus, there is shown a representation of a transceiver assembly, employing device
18
. The transceiver assembly further includes a control module
205
having receiver circuitry
210
electrically connected to first electrode
65
, typically via an amplifier
215
. Control module
205
also includes transmitter circuitry
220
, and switching system
225
. Switching system
225
is associated with either second electrode
70
or additional electrode
195
which serves as an actuating electrode, alternately connecting it to the transmitter circuitry for transmission and to ground during reception. The entire assembly is typically operated under control of a processor
230
, details of which are not essential to the present invention.
In operation, when the assembly is being used for reception, both additional electrode
195
and second electrode
70
are connected to ground, thereby offering the maximum available electromagnetic shielding. When transmission is required, a driving voltage is applied to either second electrode
70
or additional electrode
195
to generate the desired signal.
It should be noted at this point that many variations and refinements might be made within the scope of the principles of the present invention. By way of example, it should be noted that receiver
18
may employ more than one sensing electrode spaced around cylindrical element
20
. This may be useful for a number of reasons. Firstly, by analyzing the detected signals separately and identifying phase differences between the signals, it is possible to derive approximate direction information from measurements at a single receiver. Alternatively, in an example in which the wavelength is short compared to the size of cylindrical element
20
, it may be possible to choose the spacing of a number of commonly connected sensing electrodes to achieve inherent tuning of the receiver to frequencies of interest. In other words, if the spacing corresponds to in-phase spacing around cylindrical element
20
for a given frequency, the signals from each sensing electrode will have the same sign and will add up to an increased amplitude. At many other frequencies, some degree of cancellation will occur as was described in the context of
FIG. 2
above.
As mentioned earlier, cylindrical element
20
is preferably configured so that is supports only about a single wavelength of the vibration waves within the piezoelectric film induced by ultrasound signals at the working frequency. More specifically, half of the circumference (πD/2, D being the diameter of the cylindrical element) is preferably equal to the wavelength of the vibration waves within the film. For this reason, the diameter of cylindrical element
20
is generally chosen to be inversely proportional to the intended working frequency. By way of example, for a working frequency of 90 kHz, a cylindrical element of diameter about 5 mm is generally preferred.
Reference is now made to
FIG. 17
, which is a schematic representation of the operation of a system for determining the position of a moveable element
240
, constructed and operable in accordance with a preferred embodiment of the invention, operating in a primary mode of operation. It should be noted that the transceiver functionality of transducers
18
of the present invention are particularly useful for implementing a self-calibration mode according to another aspect of the present invention which offers increased precision and reliability in a system for determining the position of moveable element
240
. The system includes a moveable ultrasound transducer
235
associated with moveable element
240
and at least two ultrasound transducers
245
,
250
maintained in fixed geometrical relation by attachment to a base unit
255
. In the case illustrated here, the normal measurement mode of the system includes transmitting at least one measurement signal from moveable ultrasound transducer
235
which is received by fixed ultrasound transducers
245
,
250
. A position of moveable element
240
is then derived using time-of-flight measurements for the ultrasound measurement signal.
Reference is now made to
FIG. 18
, which is a schematic representation of the operation of the above system while performing a self-calibration operation. By way of introduction, it should be noted that ultrasound time-of-flight based digitizer systems suffer from problems of accuracy due to significant variations in the speed of sound through air which result from changes in temperature, pressure or humidity. In order to compensate for such variations, the present aspect of the present invention provides a self-calibration facility whereby, the system is also intermittently operated in a calibration mode. In this mode transducer
245
switches from its normal receiving function to transmitting, sending out a calibration signal which is received by transducer
250
. Since the distance between transducers
245
,
250
is a fixed value defined by the structure of base unit
255
, time-of-flight measurements for the calibration signal can be used to derive calibration information indicative of variations in the speed of sound in the environment within which the system is currently operating. This calibration information is then used to correct the derivation of the position of moveable element
240
.
Reference is now briefly made to
FIGS. 19 and 20
. These illustrate an implementation of this aspect of the present invention for a system where the moveable transducer
235
functions as a receiver for receiving signals transmitted by fixed transducers
245
and
250
. In this case, the calibration mode is implemented by momentarily employing transducer
250
as a receiver to receive a calibration signal transmitted by transducer
245
. In all other respects, the principles of the invention remain as before.
Reference is now made to
FIG. 21
, which is a schematic representation of the system while performing a self-calibration mode using an acoustic wave-guide
260
. By way of introduction, it should be noted that a physical obstruction
265
could block the path of the calibration signal. Physical obstruction
265
may be due to the inherent design of the system or an external obstruction. Acoustic wave-guide
260
is placed between fixed transducers
245
,
250
. Acoustic wave-guide
260
ensures that the calibration signal transmitted by one fixed transducer
245
is received by the other fixed transducer
250
. Acoustic wave-guide
260
is an elongated tube which can either be straight or curved depending on physical obstruction
265
.
Reference is now made to
FIG. 22
, which is a schematic representation of the operation of a system for determining the position of a point P on a moveable element
270
, constructed and operable in accordance with a preferred embodiment of the invention. By way of introduction, ultrasound time-of-flight based digitizer systems suffer from problems of accuracy due to the fact that the transducers cannot normally be placed exactly at the position to be determined. For example, in the case of ultrasound time-of-flight based digitizer systems involving electronic pens, the transducer will be above the nib of the pen. If the pen is tilted, as is commonly the case, the nib and the ultrasound transducer will be at different horizontal positions in the plane of measurement. In order to compensate for such variations, the present aspect of the present invention provides a system to correct for the tilt error. The system includes maintaining two ultrasound transducers
275
,
280
and point P in fixed geometric relation along a common axis W, by attaching two ultrasound transducers
275
,
280
to moveable element
270
. The cylindrical form of the ultrasound transducers provides all-around signal transmission and simplifies the geometry of time-of-flight calculations by providing an effect similar to a point (or more accurately, line) source. Therefore, ultrasound transducers
275
,
280
are centered on common axis W. It should be noted that ultrasound transducer
280
is typically positioned as close to point P as possible and ultrasound transducer
275
is typically positioned as distant from ultrasound transducer
280
as possible to give better correction for the tilt error. It is also possible to use more than two transducers in the moveable element to allow for problems resulting from temporary blocking of ultrasound signals to one of the transducers. The system also includes another two ultrasound transducers
285
,
290
maintained in fixed geometrical relation by attachment to a base unit
295
. In the case illustrated here, the normal measurement mode of the system includes transmitting a first measurement signal from ultrasound transducer
275
to be received by ultrasound transducers
285
,
290
. A second measurement signal is transmitted from ultrasound transducer
280
to be received by ultrasound transducers
285
,
290
. The first and second measurement signals are sequential. Distances between ultrasound transducer
275
and each of ultrasound transducers
285
,
290
are derived from time-of-flight measurements for the first measurement signal. Distances between ultrasound transducer
280
and each of ultrasound transducers
285
,
290
are derived from time-of-flight measurements for the second measurement signal. A position of point P is derived from geometrical calculations for the above-calculated distances.
The system also intermittently operates in a calibration mode by sending a calibration signal between fixed ultrasound transducers
285
,
290
. This calibration information is then used to correct the derivation of the position of point P.
It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art which would occur to persons skilled in the art upon reading the foregoing description.
Claims
- 1. An ultrasound transducer comprising:(a) a piezoelectric film having a first end and a second end; (b) a plurality of electrodes disposed on said piezoelectric film; (c) at least one securing member; and (d) a support structure, which is substantially cylindrical, said support structure including a protrusion formed as an elongated projecting ridge having a direction of elongation, said support structure having a central axis, said direction of elongation being substantially parallel to said central axis, said first end and said second end being secured to said protrusion of said support structure by said at least one securing member.
- 2. The ultrasound transducer of claim 1 further comprising an electrical contact disposed on said support structure.
- 3. The ultrasound transducer of claim 1 further comprising an electrical contact disposed on said protrusion.
- 4. The ultrasound transducer of claim 1 wherein said at least one securing member is a clip.
- 5. The ultrasound transducer of claim 1 further comprising an electrical contact wherein said electrical contact is disposed on said at least one securing member.
- 6. The ultrasound transducer of claim 1 wherein said piezoelectric film has a first surface and a second surface and wherein said electrodes include:(a) a first electrode disposed on said first surface; (b) a second electrode disposed on said second surface wherein at least a part of said second electrode is in an opposing relationship with at least a part of said first electrode; (c) a first electrical connecting strip disposed on said first surface wherein said first electrical connecting strip is connected to said first electrode; and (d) a second electrical connecting strip disposed on said second surface in a substantially non-opposing relationship with said first electrical connecting strip wherein said second electrical connecting strip is connected to said second electrode.
- 7. The ultrasound transducer of claim 1 wherein said piezoelectric film has a first surface and a second surface and wherein said electrodes include:(a) a first electrode and a second electrode disposed on said first surface, wherein said first electrode is disposed in a pattern that is non-contiguous with said second electrode; (b) a third electrode and a fourth electrode disposed on said second surface, wherein: (i) at least a part of said third electrode is in an opposing relationship with at least a part of said first electrode; (ii) at least a part of said fourth electrode is in an opposing relationship with at least a part of said second electrode; and (iii) said third electrode is disposed in a pattern that is non-contiguous with said fourth electrode; and (c) an electrical joining strip extending from said first electrode to said fourth electrode, wherein said electrical joining strip includes a first portion of said electrical joining strip on said first surface and a second portion of said electrical joining strip on said second surface, and wherein said first portion and said second portion are electrically connected.
- 8. The ultrasound transducer of claim 7 wherein said first portion and said second portion are electrically connected via a hole in said piezoelectric film.
- 9. The ultrasound transducer of claim 1 further comprising a helical metal spring, wherein said helical metal spring is disposed around said piezoelectric film.
- 10. An ultrasound receiver comprising:(a) a piezoelectric film having a first surface and a second surface; (b) a first electrode disposed on said first surface; (c) a second electrode disposed on said second surface wherein at least a part of said second electrode is in an opposing relationship with at least a part of said first electrode; (d) a first electrical connecting strip disposed on said first surface wherein said first electrical connecting strip is connected to said first electrode; (e) a second electrical connecting strip disposed on said second surface in a substantially non-opposing relationship with said first electrical connecting strip wherein said second electrical connecting strip is connected to said second electrode; (f) a substantially cylindrical element, which is hollow, formed primarily from said piezoelectric film, said substantially cylindrical element having a central axis and a height measured parallel to said central axis; and (g) a support structure for supporting said substantially cylindrical element, said support structure being configured to support said substantially cylindrical clement in such a manner as to allow propagation of vibration waves circumferentially around a major part of said substantially cylindrical element; wherein said first electrode is formed as a strip extending in an extensional direction substantially parallel to said central axis along at least a part of said height, said strip subtending at said central axis an angle of not more than 90°.
- 11. The ultrasound receiver according to claim 10 wherein:(a) said first electrical connecting strip is in a substantially non-opposing relationship with said second electrode; and (b) said second electrical connecting strip is in a substantially non-opposing relationship with said first electrode.
- 12. The ultrasound receiver according to claim 10 wherein:(a) said substantially cylindrical element has an inner surface wherein said first surface forms said inner surface; and (b) said second electrode is grounded.
- 13. A multi-electrode ultrasound receiver comprising:(a) a piezoelectric film having a first surface and a second surface; (b) a first electrode and a second electrode disposed on said first surface, wherein said first electrode is disposed in a pattern that is non-contiguous with said second electrode; (c) a third electrode and a fourth electrode disposed on said second surface, wherein: (i) at leapt a part of said third electrode is in an opposing relationship with at least a part of said first electrode; (ii) at least a part of said fourth electrode is in an opposing relationship with at least a part of said second electrode; and (iii) said third electrode is disposed in a pattern that is non-contiguous with said fourth electrode; (d) an electrical joining strip extending from said first electrode to said fourth electrode wherein said electrical joining strip includes a first portion of said electrical joining strip on said first surface and a second portion of said electrical joining strip on said second surface and said first portion and said second portion being electrically connected; (e) a substantially cylindrical element, which is hollow, formed primarily from said piezoelectric film, said substantially cylindrical element having a central axis and a height measured parallel to said central axis, said first electrode and said second electrode in combination subtending at said central axis an angle of not more than 90°; and (f) a support structure for supporting said substantially cylindrical element, said support structure being configured to support said substantially cylindrical element in such a manner as to allow propagation of vibration waves circumferentially around a major part of said substantially cylindrical element.
- 14. The multi-electrode ultrasound receiver according to claim 13 wherein:(a) said substantially cylindrical element has an inner surface wherein said first surface forms said inner surface; and (b) said third electrode is grounded.
- 15. The multi-electrode ultrasound receiver according to claim 13 wherein said first portion and said second portion are electrically connected via a hole in said piezoelectric film.
- 16. The multi-electrode ultrasound receiver according to claim 13 further comprising:(a) a first electrical connecting strip disposed on said first surface, wherein said first electrical connecting strip is connected to said second electrode; and (b) a second electrical connecting strip disposed on said second surface, wherein said second electrical connecting strip is connected to said third electrode and said second electrical connecting strip is in a substantially non-opposing relationship with said first electrical connecting strip.
US Referenced Citations (15)
Foreign Referenced Citations (1)
Number |
Date |
Country |
WO9840838 |
Sep 1998 |
WO |