WO1988005606A1

WO1988005606A1 - Non-elastic piezoelectric transducer

Info

Publication number: WO1988005606A1
Application number: PCT/US1988/000078
Authority: WO
Inventors: Robert D. Ricks
Original assignee: Advanced Medical Technologies, Inc.
Priority date: 1987-01-14
Filing date: 1988-01-14
Publication date: 1988-07-28

Abstract

A transducer which can be used on an ambulatory subject, which is extremely responsive, which requires no external power for operation, and which is small in size and calibratable. The transducer is rugged and impervious to incidental movement. A piezoelectric film (15), in the form of two sheets is sandwiched between two lengths of non-elastic material (10a, 10b). Planar electrodes (20) are coupled to the faces of the piezoelectric sheets to provide the output of the transducer. Force is exerted on the lengths of non-elastic material, which bends or flexes, the resultant motion being transduced to the piezoelectric film.

Description

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Descrir-tion Non-Elastic Piezoelectric Transducer This is a continuation-in-part of copending Application Serial No. 824,163, filed January 30, 1986, which was a continuation of Application Serial No. 610,103, filed May 14, 1984, now abandoned.

Technical Field The present invention relates generally to transducers and, more specifically, to a non-elastic piezoelectric transducer.

Background Art In the medical technology area, transducers are necessary to measure blood pressure, heart sounds, fetal movement and pulse rate of subject patients. Of ti es the subject must be kept sedentary, so as to allow the proper and non-disturbed monitoring of any vital signs. Early transducers took the form of microphones which were comprised of box-shaped diaphragms used in conjunction with piezoelectric crystals. The drawbacks to these transducer types were multiple in nature; they were bulky and heavy and required external machinery to create the necessary amplification in order to hear or record the electronic signal.

Sometimes the transducer was fluid-filled; the fluid contained therein being electrically conductive in nature. A slight movement by the subject created oscilliary movement and thus an electronic signal. While this type of transducer was appropriate for use in the measurement of systolic and diastolic rates, the accurate measurement and recordation of greater body movements was impractical by this method. Transducers have also been used to monitor apnea or sudden infant death syndrome in infants. This particular type of transducer, as shown in Manus e . al.. U. S. Patent No. 4,576,179, issued March 18, 1986, is extremely sensitive to respiratory movement, while cancelling out heartbeat signals. Transducers of this type have a particular construction which involves piers and beams, across which the transducer crystal is stretched. The beam and pier construction limits the input to the crystal. The drawbacks to such a transducer are multiple. The subject may not be ambulatory and the specific construction of the transducer makes it more adaptable to an infant's body type than an adult.

Other transducer types are extremely sensitive to atmospheric pressure, which in turn create movement and electronic signals. Still others are so sensitive so as to be unusable to record or measure respiratory movemen .

It can be seen that each of the prior art transducers described above suffers from certain disadvantages. Either the transducer is too sensitive to accurately record body movement or is not sensitive enough and requires a surfeit of amplification attachments. Furthermore many of the prior art apparatuses were bulky, weighty and cumbersome. There was also the additional disadvantage of their relatively fragile construction which required that the subject patient remain inactive while being monitored. Examples of the above described transducers are further described in U.S. Patent Nos. 3,283,181, 3,239,696, 3,786,285, and 4,443,730.

Disclosure Of Invention

The foregoing and other problems and disadvantages of prior art transducer designs are overcome by the present invention of a piezoelectric transducer which converts an applied force into a bending of the transducer and hence into an electrical signal. The bending action is a force transformation which ensures that the forces applied to the transducers are redistributed more evenly across the piezoelectric film. The present invention includes piezoelectric means which provide an output that is proportional to forces applied along its width dimension. Body means are included for receiving the force which is exerted against the planar dimensions and converting that force into a bending force to be transduced along the transducer's width dimension, wherein the body means is defor able by the bending force along its length dimension. The body means and piezoelectric means are affixed to one another along their respective length dimensions so that the body means applies force to the piezoelectric means along its machined direction, which is placed on the longitudinal or width dimension of the body means. As such, the forces applied by the body means to the piezoelectric means have a magnitude that is a function of the bending or flexing of the body means. Also included are means coupled to the piezoelectric means for supplying the output of the piezoelectric means to the user.

In the preferred embodiment of the present invention, the piezoelectric means comprise one sheet of piezoelectric film and the body means comprise two lengths of non-elastic material. The sheet of piezoelectric film is sandwiched between the lengths of non-elastic material, and force is exerted by the subject which constitutes a bending force on the planar dimensions of the non-elastic material, causing the non- elastic material to bend or flex. The bending of the non-elastic material causes the sheet of piezoelectric film to produce a electrical signal along its machined direction, which is then transmitted to a monitoring device.

With the above-described device, a transducer is provided which does not require power to be applied to it in order to provide an output signal. Additionally, such a transducer can be worn when the subject is ambulatory. Additionally, the transducer, when coupled with the proper buffering device, can provide a very low frequency response.

A further advantage to the present device is that the electrical output is relatively impervious to any tension on the device. Force must be applied to the device in order to obtain any output. In the prior art transducers, the piezoelectric film was in constant contact, with either the subject, or a diaphragm, or an adjoining sheet of piezoelectric film.

Another advantage to the present invention is that the device is extremely responsive to any change in the circumference of the subject. An electrical signal will only be generated by piezoelectric film contact, which in turn is generated by a circumference change. Therefore a relatively slight change in circumference or diameter of a subject, such as that caused by respiration, will create a measurable electrical output. A further aspect of the invention is the transducer's rugged and impervious construction. By utilizing non-elastic material to surround and protect the piezoelectric means, the present device can withstand shock by ambulatory movement without reducing its output effectiveness. The impervious nature of the non-elastic material also allows the device to be used on active subjects, without monitor and output interference by perspiration. Also included in the present invention is an amplifier circuit which is especially suited for use with the transducer of the present invention. The amplifier includes limiter means positioned at the input thereof to limit the magnitude of the signals to be handled thereby to less than a predetermined level; a gain block which provides first and second gain levels as a function of the magnitude of the signal being amplified; and an automatic nulling circuit for nulling out offsets in the circuit.

These and other objects and advantages of the present invention will be more readily understood upon consideration of the following detailed description of certain embodiments thereof taken in conjunction with the accompanying drawings.

Brief Description Of The Drawings

Figure 1 is a perspective cut-away view of the present invention. Figure 2 is a side view of the present invention.

Figure 3 is a side view of the present invention in its deformable state.

Figure 4 is a schematic of circuitry which can be used in conjunction with piezoelectric film transducers. Figure 5 is a schematic diagram of an improved version of the circuitry of Figure 4, which is especially suited for use in conjunction with the present invention.

Detailed Description

Referring to Fig. 1, the structure of one embodiment of the present invention is shown. A portion of piezoelectric film 15 is affixed to a non-elastic material 10a, 10b such as vinyl. Electrodes 20 are affixed to the top and bottom surfaces of the piezoelectric film 15 and lead wires 23 bring the signal from the piezoelectric film 15 out of the user via a co¬ axial cable, not shown.

As has been well-established in the prior transducer art, piezoelectric material convert mechanical force or movement into electrical signals. The electrical signals are generated without the input of external electrical power.

As shown in Fig. 1, the piezoelectric film 15 is affixed to the non-elastic material 10 so that the machined direction of the piezoelectric film 15 is placed along the longitudinal or width dimension axis of the non-elastic material 10. When force is applied to the planar faces 35 and 40 of the non-elastic material 10, the non-elastic material will bend and flex longitudinally in parallel.

Referring to Fig. 2, a side view of the present invention is shown. As shown, the invention has a very small depth dimension relative to its length and width dimensions.

Electrodes 20 which are planar in shape-, are constructed of some conductive/receptive material, such as silver-plated copper, stainless steel, brass or unplated copper. It is well known in the pertinent art that piezoelectric film has the same electrical output potential along its entire machined or longitudinal dimension. Therefore, while the electrodes 20 shown in Fig. 1 are placed at the midpoint of the piezoelectric film 15, it should be recognized that alternative placement of the electrodes 20 will not effect or hinder the performance of the invention.

Furthermore as piezoelectric film typically displays polarity, pairs of opposing electrodes 20 on the faces of the piezoelectric film 15, provide the positive and negative connections to the piezoelectric film 15. The electrodes 20 are affixed to the piezoelectric film 15 by way of mechanical tension or conductive epoxy 45. Before affixation of the electrodes 20 to the piezoelectric film 15, lead wires 23 are soldered to the electrodes 20.

The lengths of non-elastic material 10a and b are bonded to one another, by the application of an adhesive 76 such as vinyl glue, about the perimeter of the lengths of non-elastic material 10a and b. The lengths of non-elastic material 10a and 10b, with the piezoelectric film 15 and electrodes 20 form a sandwich 12. Elastic cord and buckles 30 are fastened to the lateral sides 55 and 60 of the transducer to enable the coupling of the invention to a subject.

Figure 3 illustrates a side view of the present invention in a deformed state. As can be seen, force, such as that exerted by the expulsion and inhalation of air in the chest cavity of a subject, focused upon any point located on the planar faces 35 and 40 of the transducer causes a bending and flexing of the lengths of non-elastic material 10a and b. The subsequent deformation of the non-elastic material 10a and b causes a flexure of piezoelectric film 15, which in turn is converted to measurable electrical potential.

The embodiments of the present invention shown in the figures is uniquely well-suited to provide unobtrusive monitoring of subjects, either active or not. The present invention is light-weight and close fitting, so that it may be used on subjects undergoing laboratory tests which require free movement. Thus, the present invention may be incorporated under or over clothing, without hindering movement. The transducer structure may include connectors such as snaps 75, which are incorporated at the ends of the lengths of non- elastic material 10.

In the preferred embodiment of the present invention, the piezoelectric film 15 can be KYNAR piezoelectric film manufactured by Penwalt Company of King of Prussia, Pennsylvania. In the structure shown in Fig. 1, the piezoelectric film is cut to be approximately 12 inches long and one-half (1/2") inch wide. Other combinations of length and width, selected to preserve the total area of piezoelectric film used, can be employed, such as 25" long by 1/4" wide, and 40"long-by 5/32" wide. The longer length piezoelectric transducers are especially useful with 'or in' large circumference applications. In the preferred embodiment of the present invention, the non-elastic material 10 can be polyvinylidene fluoride or PVF2 manufactured by Dow Chemical Company of 2020 Dow Center, Midland, Missouri 48640. This material is inherently extremely rugged, non-elastic and impervious to moisture. For the structure shown in Fig. 1, length 10a is preferably one and a half (1-1/2") inches wide and fourteen (14") inches long with a width dimension of less than one- eighth inch (1/8") . Finally, in the preferred embodiment of the present invention, the adhesive 76 used to bond the non-elastic material 10 around its perimeter 50 may be a vinyl adhesive cement no. 634, manufactured by Bond Adhesives Company, Jersey City, New Jersey 07303. in practice, as the movement or deformation being sampled is converted to a bending force and is applied to the transducer, the non-elastic material 10 bends in proportion thereto. The portion of non-elastic material 10 which is bonded to the piezoelectric film 15 transmits the proportionate force or strain to the piezoelectric film 15. This strain is converted into electric potential between the faces of the piezoelectric film 15. This electrical potential is then provided to the user via electrodes 20 and lead wires 23.

The transducer of the present invention is more resilient and is non-elastic compared to the prior art and therefore more deliberate force must be applied in order to create electrical output. From the above description, it should be apparent that the present invention provides superior performance over prior transducers. Because the transducer is responsive to bending force and is flexible, rather than elastic, tensional variations in general will create minimal output. Therefore the present invention may be used in ambulatory subjects as well as on subjects who are torsially active.

A further advantage to the present invention is the extreme responsiveness to any change in a subject's circumference. It has been experimentally found that the present invention when used to transduce very low frequency motion such as respiration, increases the accuracy of the frequency measurement, constituting a one hundred to two hundred time improvement in the voltage level reading. Further, the performance of the transducer of the present invention is such that a usable output signal can be obtained when placed flat on a bed and the subject lies on top of the transducer-.

Another advantage to the present invention is that it is inherently extremely rugged due to the impervious and resilient nature of the non-electric material. While vinyl may be used as the non-elastic material, it is contemplated that other materials with similar properties may be used in place of vinyl. Yet another advantage to the present invention is the negligible depth dimension. As seen in Fig. 2 of the present invention, this feature in addition to the small over-all dimensions allows the transducer to be used unobtrusively.

A further advantage of the present invention is that no external source of electrical energy be supplied. The transducer produces its own electrical discharge. In the cross referenced copending application, an amplifier circuit was disclosed which was suitable for use with piezoelectric film transducers of the type in which the film is sandwiched between and bonded to two stretchable layers of material, and in which the force being transduced causes tension to be applied to the stretchable layers of material. A tensioning of the piezoelectric film was therefore employed. This amplifier setup has been modified in accordance with the present invention to be especially suitable for use with signals from the transducer structure of the present invention.

The circuitry of Fig. 4 operates as follows. The transducers of interest provide an output signal that looks like a current source with low capacitance. Because of the low capacity, the low impedances of conventional amplifiers will quickly discharge the capacitance, which results in very low signal levels at the inputs to^' the conventional amplifiers. Conversely, where amplifying circuits used have high input impedances, the current source characteristic of the transducer causes the range of transducer output swing to vary between large extremes.

The above difficulties are overcome by the circuit of Fig. 4 which includes a current-to-voltage converting circuit, and which provides an amplifier having a predetermined high level input impedance.

In Fig. 4, the piezoelectric film transducer 12 of the present invention is connected in parallel with the current-to-voltage converting element 112. In the preferred embodiment of the present invention, the converting means comprise a capacitor 114. Preferably, this capacitor is a low loss capacitor and has a capacitance value which is related to the piezoelectric transducer film area. Thus, the output voltage produced across capacitor 114 is proportional to the ratio of the piezoelectric transducer film area to the capacitance value. The ratio can be selected to limit the maximum output voltage of the combination. In Fig. 4, the buffering block 116 is shown coupled to the piezoelectric film transducer 12 and capacitor 114. The load impedance presented to capacitor 114 by buffering means 116 is selected so that the discharge rate of capacitor 114 is compatible with the frequency of the motion which is to be monitored. Thus, where very low frequency motions or movements are sought to be monitored, the discharge rate will be selected to be hundreds of times lower than the rate being monitored. Differential amplifier 118 has an inverting input 120, a non-inverting input 122, and an output 124. The output 124 is connected to the inverting input 120. The non-inverting input 122 is connected to the junction of capacitor 114 and piezoelectric film transducer 12.

In order to form the bootstrap feature of buffering circuitry 116, a first resistance 126 is connected at one end to the inverting input 120. One end of a second resistor 128 is connected to the other end of resistor 126. The other end of resistor 128 is connected to ground or the reference point for the circuit. A third resistor is connected at one end to a non-inverting input 122 of the differential amplifier 118 and at the other end to the junction of first and second resistor 126, and 128, respectively.

It can be shown that where differential amplifier 118 is an ideal amplifier, the load impedance presented by buffering means 116 is substantially equal to the ratio of the value of the second resistor 128 to the value of first resistor 126 multiplied by the value of third resistor 130. Where the input bias current of inverting input 120 and non-inverting input 122 are negligible, the current flowing through third resistor 130 is determined by the voltage across first resistor 126. This is because, for an ideal amplifier, the voltage difference between its inverting and non-inverting inputs is zero.

The voltage across first resistor 126 is determined by the voltage divider relationship of first resistor 126 and second resistor 128 applied to the output voltage level. Because the differential amplifier 118 is connected in a voltage follower mode, the output voltage level at output 124 is substantially equal to the input voltage level present at non-inverting input 122.

Given that impedance is defined by the voltage divided by the current, the load impedance presented by buffering circuitry 116 is defined by the voltage across capacitor 114 divided by the current into buffering circuitry 116. When differential amplifier 118 is an ideal amplifier, i.e., the current into non-inverting input 122 is substantially 0, substantially all of the current flowing into buffering device 116 from capacitor 114 flows through resistor 130.

As discussed above, the current through third resistor 130 is determined by the voltage across first resistor 126. The voltage across first resistor 126 is substantially equal to the voltage at output 124 multiplied by the value of first resistor 126 divided by the sum of the value of first resistor 126 and second resistor 128. The current then flowing through the third resistor 130 is determined by the voltage across first resistor 126. divided by the value of third resistor 130. The load impedance presented by buffering circuitry 116 is then the voltage across capacitor 114 divided by the current through third resistor 130. For example, if the output voltage of differential amplifier 118 is one volt and the ratio of second resistor 128 to first resistor 126 is 100, then only on 100th of a volt, or 10 millivolts, will be applied across third resistor 130. If only one looth of the voltage is applied, then the effective value of third resistor 130 is multiplied by the ratio of the second resistor 128 to the first resistor 126. Using the above example and a 10 megohm value for resistor 130, then the effective value of resistor 130 equals 1000 megohms. For the values discussed in the example above, buffering circuitry 116 has been found to be satisfactory when used with a capacitance for capacitor 114 of 0.47 microfarads and a piezoelectric film transducer film area of 6 square inches. Preferably, the input bias current of differential amplifier 118 is negligible. That is, the maximum bias current of the differential amplifier used should be low enough to allow the bootstrapped high impedance described above to control the discharge rate and the low frequency response characteristics of the transducer system. In the example described above, amplifiers with picoampere bias currents, such as the ICL 7611 series manufactured by Maxium Inc. of Milpitas, California, have been found to be satisfactory. Fig. 5 shows an improved version of the above described amplifier circuitry which is especially suited for operation with the transducer structure of the present invention. One of the advantages of the transducer structure of the present invention is the large dynamic range of the output levels available from it. Another advantage is a very low frequency response. However, these two advantages also represent disadvantages from the standpoint of buffering or amplifying the signal for use with recording or processing equipment.

First of all, with a large dynamic range of possible signal levels there can be large signal level di ferences between those which represent the physiological function being monitored and those which represent occasional movement by the subject. It is often the case that the signal levels corresponding to 'the physiological event being monitored are very small in comparison to the movement artifact. The signal levels caused by movement of the subject can be large enough to overdrive and saturate the buffer stage. Because there is a certain recovery time required by the buffer stage before it can operate upon signals of more nominal levels, it is possible that a substantial number of cycles of the physiological function being monitored will occur during the recovery time, and therefore be lost. Thus, it is desirable to minimize the recovery time of the buffer stage.

As to the low frequency response characteristics of the transducer, the closer it is to a DC response, the greater the offset effects which can be caused by very slow variations in the forces being applied to the transducer. Thus, because the transducer structure of the present invention can respond to forces which are varying in the tenths of hertz range, it is desirable to null out signals in such a range. Otherwise, the output signal can include large DC offsets.

Finally, because of the wide dynamic range provided by the transducer structure of the present invention, there can be large differences in the magnitudes even in the signals sought to be processed. Thus, it is desirable to provide different gain levels through the circuit which change as a function of the signal levels being amplified at the time. Referring to Fig. 5, input block 132 corresponds generally to the structure of Fig. 4. Common reference numerals are used to refer to elements common to both figures. From Fig. 5 it can be seen that two pairs 134 and 136 of series connected diodes are connected in shunt across transducer 12. Each pair is positioned to be at an opposite polarity to the other. It has been found that when the diodes are operated at a point where their square law characteristic begins, their on/off characteristics are compatible with the high impedance requirements for operation with piezoelectric film transducers.

More specifically, as discussed earlier, the size of the piezoelectric film and that of capacitor 114 can be selected to provide a scaling of the signal level from the transducer 12 down to a desired level. Thus, the two variables can be sized so that the starting point of the square law characteristic of the diode pairs 134 and 136 is at a point where signal levels above the point correspond to the high end of the signal range sought to be processed.

^~ The square law characteristic of the diodes provides that the diodes have a very high impedance below the square law starting point, and an impedance of a few hundred ohms above the starting point. As such, the signal levels applied to capacitor 114 never get above a predetermined level, which in turn permits the buffer stage 132 to return more rapidly to its nominal condition. It has been found that when the component values shown in Fig. 5 are used with the transducer structure of the present invention, and when breath sounds are being monitored, the time for recovery of buffer 132 is typically no longer than one breath of the subject, even when very heavy breathing is present.

Preferably, diode pairs 134 and 136 are constructed from planar, silicon, small signal diodes. Industry standard part number 1N4148 small signal diodes are suitable for such use. These diodes exhibit the low leakage and very high impedance which is desirable for the present application. Resistor 140, shown as a 100 Kohm resistor which connects node 142 to the non- inverting input 122 of differential amplifier 118, is effectively a short circuit when compared to the bootstrapping of resistors 126, 128 and 130. Thus, capacitor 114B is effectively in parallel with capacitor 114A such that 114A and 114B collectively correspond to capacitor 114 of Fig. 4.

Block 144 of Fig. 5 is a gain block which provides a high gain for small input signal levels and low gain for large input signal levels. This is achieved by providing a resistor 146 in series with a diode 148 in the negative feedback path of the amplifier. As such, any signal greater than the turn-on voltage of the diode will cause diode 148 to turn on and place resistor 146 into the negative feedback loop, thus reducing the gain. Preferably, gains of 100 and 20 are used. This permits signals corresponding to shallow breathing to be handled with the high gain, and signals corresponding to unusually heavy breathing to be handled with the lower gain. Finally, referring to block 150 of Fig. 5, the auto null circuit will now be explained. A differential amplifier 152 is employed which receives an offset voltage at its non-inverting input 154 and the output signal from gain block at its inverting input 156, via resistor 160. A large feedback capacitor 162 provides a negative feedback path for the amplifier 152. The output of differential amplifier 152 is coupled to the non-inverting input of the gain block amplifier 145. This structure operates as an integrator, which provides a signal at amplifier 145 that cancels out any DC-like offsets due to the operation of transducer 12.

The offset voltage is derived using resistive divider 164 and a negative voltage to provide a voltage which is less than zero. In this manner, the sum of the signals applied to gain block amplifier 145 are centered in the middle of the high gain range to enhance the processing of shallow breathing signals. The time constant for capacitor 162 and resistor 160 is set so that for a DC level at the high end of the range sought to be processed, the output of differential amplifier 152 will be at a stable level within approximately a one breath time period.

The autonull circuit 150 tracks the low frequency offsets that occur during the operation of transducer 12 and operates to remove such offsets at gain block 144.

Amplifier 145 can be part number ICL7642 manufactured by Maxium Inc. of Milpitas, California. Differential amplifier 152 can be the same as that used for amplifier 118.

The terms and expressions which have been employed here are used as terms of description and not of limitations, and there is no intention, in the use of such terms and expressions of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

Claims

ClaimsWhat is Claimed is:

1. A transducer of the type for converting a subject force into an electrical signal comprising i) piezoelectric means having a length dimension for providing an output which is proportional to forces applied along its length dimension; ii) body means having top and bottom surfaces and a length dimension for connecting the subject force incident at an obtuse angle to the top and bottom surfaces into flexure forces, wherein the body means is flexibly deformable along the entire transverse axis length dimension and the piezoelectric means is affixed to the width of the body means so that the body means applies the flexure forces along a length dimension of the piezoelectric means, which flexure forces have a magnitude that is a function of the subject force; and iii) means coupled to the piezoelectric means for supplying the output of the piezoelectric means to the user.

2. The transducer of claim 1 wherein the piezoelectric means has substantially the form of a sheet with the length dimension of the piezoelectric means lying parallel to the plane of the body means.

3. The transducer of claim 2 wherein the piezoelectric means comprises piezoelectric film.

4. The transducer of claim 1 wherein the body means comprises two lengths of non-elastic material.

5. The transducer of claim 4 wherein the two lengths of non-elastic material further comprises two lengths of vinyl.

6. The transducer of claim 3 wherein the piezoelectric film comprises a sheet of Kynar film.

7. The transducer of claim 5 wherein the body means include means for bonding the piezoelectric means to the two lengths of vinyl comprising a layer of vinyl adhesive which is distributed around the perimeter of the two lengths of vinyl.

8. The transducer of claim 2 wherein the output supplying means comprises a first and second electrodes having a planar shape.

9. The transducer of claim 8 wherein the piezoelectric means has top and bottom planar faces and wherein the first electrode is coupled to the top planar face and further wherein a second electrode is coupled to the bottom planar face.

10. The transducer of claim 4 wherein the two lengths of vinyl are polyvinylidene fluoride.

11. The transducer of claim 3 wherein the body means comprise a first length of vinyl.

12. The transducer of claim 11 further including a second length of vinyl wherein the piezoelectric means is sandwiched between the first and second lengths of vinyl.

13. A transducer for converting force exerted on the transducer's planar face into bending force which generates an electrical signal comprising a strip of piezoelectric film; two lengths of vinyl; and means coupled to the piezoelectric film for providing the signal generated by the piezoelectric film to the user; wherein the piezoelectric film is sandwiched between the two lengths of vinyl and force is applied to the piezoelectric film through the two lengths of vinyl.

14. A transducer for converting force into an electric signal comprising a sheet of piezoelectric film; means coupled to the sheet of piezoelectric film for coupling the force to the piezoelectric film along its length dimension; and means coupled to the piezoelectric film for providing the electrical signal from the piezoelectric film to the user.