JPH0553379B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention emits ultrasonic waves to a measuring surface, receives reflected ultrasonic waves from there, and calculates the distance to the measuring surface from the time difference between the emission and reception. The present invention relates to a transducer for an ultrasonic distance meter for measuring, and particularly to a transducer for an ultrasonic distance meter that reduces changes in sensitivity due to temperature of the transducer.

<Prior Art> The configuration of a transducer of a conventional ultrasonic distance meter is shown in FIG. 5, and its outline will first be explained.

Reference numeral 10 denotes a ceramic piezoelectric vibrator, the whole of which is formed into a cylindrical shape, and electrodes are formed on its inner and outer surfaces, from which lead wires 11a and 11b are connected.
is being drawn out.

A damping material 12 surrounds the piezoelectric vibrator 10 and applies damping to the piezoelectric vibrator 12.

Reference numeral 13 denotes a plastic case, which also serves as an acoustic matching layer for acoustic matching with the surrounding air.

Reference numeral 14 denotes a reflector, which changes the direction of the ultrasonic pulse emitted from the piezoelectric vibrator 10 in its radial direction in its axial direction and radiates it onto the measurement surface.

Next, an outline of the operation of the transducer 15 configured as described above will be explained.

When an electric pulse is applied to the piezoelectric vibrator 10 via the lead wires 11a and 11b, the piezoelectric vibrator 10 causes breathing vibration in its radial direction, and generates ultrasonic vibration in the air. The direction of this ultrasonic vibration is changed by the reflector 14 and becomes a donut-shaped beam B, which travels in the axial direction. Conversely, the ultrasonic pulses incident from the axial direction are converged by the reflector 14 and apply stress to the outer circumference of the piezoelectric vibrator 10, and correspondingly, an electric field is generated in the electrodes of the piezoelectric vibrator 10. ,
Charges generated by this electric field are transferred to the lead wires 11a, 1
1b as an electrical signal.

In this case, since the ceramic piezoelectric vibrator 10 generally has a large inertia, the damped residual vibration continues for some time even after the electric pulse has ceased. Therefore, when the measurement distance is short, ultrasonic pulses reflected from the measurement surface are received while this residual vibration exists, and cannot be distinguished from the residual vibration. vice versa,
When the measurement distance is long, the amplitude of the reflected ultrasonic pulse becomes small, so it is necessary to greatly amplify this, but at the same time, the voltage due to residual vibrations is also amplified.
For this reason, in order to separate the residual vibrations and the reflected ultrasonic pulses, a detection prohibited area (dead zone) of the latter is created until the amplitude of the former becomes smaller than that of the latter.
must be extended. That is, when measuring long distances, it becomes impossible to measure short distances, and conversely, if the dead zone is shortened so that short distances can be measured, it becomes difficult to measure long distances with small reflected pulses.

Therefore, in order to avoid this situation, damping is applied to the piezoelectric vibrator 10 using a damping material 12.

However, it is difficult to select a damping material 12 that exhibits an appropriate damping effect over a wide temperature range, and since damping is applied by the damping material 12, the piezoelectric vibrator 10 must be adjusted considerably in order to obtain the necessary vibration amplitude. Large driving voltage, for example 1KV
It is also necessary to give a degree.

Furthermore, since the acoustic impedance (=ρc, ρ: density, c: sound velocity) of the piezoelectric vibrator 10 is about five orders of magnitude larger than the acoustic impedance of air, the energy of the piezoelectric vibrator 10 does not propagate sufficiently into the air. .

Therefore, in order to solve this problem, ultrasonic waves are emitted into the air through a plastic case 13 that is acoustically softer (lower ρc) than the piezoelectric vibrator 10 as an acoustic matching layer. Select the thickness of the plastic case to be 1/4 wavelength thick.

However, when the sound speed of plastic changes due to temperature changes, the equivalent case thickness deviates from 1/4 wavelength, reducing the propagation efficiency of ultrasonic pulses.

Therefore, in order to solve these problems, the present applicant filed a patent application (2) on May 7, 1988 entitled "Name of the invention:
We are proposing a transducer for ultrasonic rangefinders.

The outline of this proposal will be explained below with reference to FIGS. 6 to 9.

Reference numeral 16 denotes a cylindrical holding body made of vinyl chloride, etc., which has a recess 17 formed on its side surface, and a pressure equalization hole 18 penetrating into the recess 17 in its center.
It is drilled in. A reflector 14 is arranged at the upper end of the holder 16.

A polymeric piezoelectric film 19 is placed on the circumferential surface of the holder 16 at a distance from the holder 16 , and is fixed at the upper and lower circumferential surfaces of the membrane 19 .
0 is formed. Electrodes 21a and 21b are formed on the inner and outer surfaces of the piezoelectric film 19, respectively, and lead wires 22a and 22b are drawn out from the electrodes 21a and 21b, respectively.

This piezoelectric film 19 is made of PVDF (polyvinylidene fluoride), P (VDF-TrFE) <copolymer of vinylidene fluoride and trifluoroethylene>, P (VDF-TrFE),
TeFE) <copolymer of vinylidene fluoride and tetrafluoroethylene>, or P(VDCN-VAc)
A polymeric material exhibiting piezoelectricity such as <alternating copolymer of cyanovinylidene and vinyl acetate> is used.

FIG. 7 is a longitudinal sectional view showing details of the vicinity of the piezoelectric film in FIG. 6.

Electrodes 21a and 21b are fixed to the top and bottom of the piezoelectric film 19 via adhesives 19a and 19b, and are covered with protective films 19c and 19d. Electrode 2
1a, 21b and lead wires 22a, 22b are connected using a conductive adhesive.

Next, the operation of the transducer 23 configured as described above will be explained using FIGS. 8 and 9.

When a voltage is applied between the electrodes 21a and 21b formed on both sides of the stretch-polarized polymeric piezoelectric film 19', the piezoelectric film 19' moves in the stretching direction α as shown by the dotted line.
expands and contracts. Such vibrations in the thickness direction of the plate-shaped piezoelectric film 19' mainly have a frequency in the MHz (megahertz) band.

Here, if the piezoelectric membrane 19' is bent into a cylindrical shape so that the extension direction is the radial direction, this expansion and contraction will be in the radial direction as shown by the dotted line β in FIG. 9, and can be converted into respiratory vibration in this direction.

The resonance frequency ₀ of the piezoelectric film 19 at this time is ₀ = (ε/ρ) ^1/2 / (2πR) where the radius of curvature of the piezoelectric film 19 is R, the elastic modulus is ε, and the density is ρ. Therefore, suppose ε=11.3×10 ⁹ (N/m ² ),
ρ=1.8×10 ³ (Kg/m ³ ), radius of holder 16 is 10 mm
If this is the case, the appropriate frequency for an ultrasonic rangefinder, approximately 40KHz, will be low attenuation in the air.

The transducer of this ultrasonic distance meter using a piezoelectric polymer film has a low acoustic impedance and is easy to match with water, air, etc., and the internal energy attenuation is large, so ultrasonic waves with short residual vibrations It can emit pulses and is flexible, making it easy to manufacture and process.

Furthermore, since the polymer piezoelectric film can be made thinner, the electric field strength E can be increased for the same drive voltage, and it has the advantage of better overall efficiency than ceramic piezoelectric vibrators.

<Problems to be Solved by the Invention> However, although such conventional ultrasonic distance meter transducers have the various advantages described above, they also have the following disadvantages.

(a) The resonance frequency ₀ changes in inverse proportion to the radius of curvature R, which indicates that it changes due to temperature expansion of the outer shape of the holder 16, and the elastic modulus ε is also affected by temperature. For example, the holder 16 is made of hard vinyl chloride, the piezoelectric film is made of PVDF, and the radius of curvature is R.
If R = 10cm, then ₀ = 40KHz at 20℃, but the frequency will vary by about 3KHz/20℃ depending on the temperature. Furthermore, (b) polymer piezoelectric films have a large Q factor indicating the sharpness of resonance, so even if the implantation frequency is kept constant, the sensitivity of transmission and reception will drop significantly due to temperature changes.

Therefore, it is conceivable to perform temperature compensation by detecting the temperature and moving the implantation frequency closer to the resonance frequency, but this has the problem that signal processing becomes complicated. Another idea is to provide a damping material around the polymeric piezoelectric film to lower the Q factor and reduce the effects of temperature fluctuations, but this does not work well in practice.

<Means for solving the problems> In order to solve the above problems, the present invention has the following features:
An ultrasonic rangefinder that emits ultrasonic waves to a measuring surface through a reflective umbrella, receives the reflected ultrasonic waves through the previous reflective umbrella, and measures the distance to the measuring surface from the time difference between the emission and reception. In this transducer, there is a rod-shaped holder fixed to one end of the first reflector, extending in the direction of its central axis and having different curvatures inside the first reflector, and having a space on the circumferential surface of this holder. It has a piezoelectric film oscillator made of a polymer piezoelectric film fixed at the curvature of the tip, and the light is emitted from the piezoelectric film with a different curvature toward the measurement surface through the reflector at the tip. The curvature is selected so that the frequency characteristics of the ultrasonic synthesis are approximately flat over a predetermined range.

<Function> By sending out ultrasonic waves from vibrating surfaces with different curvatures, the synthesized frequency characteristics of the ultrasonic waves emitted from each curvature surface are flattened over a predetermined range before and after the center frequency, and the frequency characteristics are flattened over a predetermined range around the center frequency. Prevent decrease in sensitivity.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. Note that the same reference numerals are given to the parts having the same functions as in the conventional case, and the explanation thereof will be omitted as appropriate.

FIG. 1 is a longitudinal sectional view showing the configuration of one embodiment of the present invention.

Flat part 2 of the reflector 14 having a flat part 24 at the top
One end of a cylindrical polymer piezoelectric membrane vibrator 25 having a predetermined radius of curvature R ₁ is fixed to 4, and the other end has a radius of curvature R ₂ different from the radius of curvature R ₁ of the piezoelectric membrane vibrator 25. A piezoelectric membrane vibrator 26 is fixed. The piezoelectric membrane vibrator 25 has a resonant frequency ₁ , the piezoelectric membrane vibrator 26 has a resonant frequency ₂ , and these are combined to form a piezoelectric membrane vibrator 2 with a central resonant frequency _f3 .
7.

The piezoelectric film vibrator 27 and the reflector 14 constitute a transducer 28.

As shown in FIG. 2, each of the piezoelectric membrane vibrators 25 and 26 has a holder 29 with a diameter of 2R ₁ and 2R ₂ .
30, recesses 31, 3 provided on the peripheral surfaces of these
2. Pressure equalizing holes 33 that open these recesses to the outside;
34, the piezoelectric film vibrator 25 has piezoelectric films 35, 36 disposed on the outer circumferential surface of these recesses, and electrodes 37a, 37b, 38a, 38b, etc. provided on the circumferential surface of these piezoelectric films. and 26 have the same structure as shown in FIG.

Next, the operation of the transducer shown in FIG. 1 constructed as above will be explained using FIG. 3.

FIG. 3 is a characteristic diagram showing the frequency characteristics of the transducer in FIG. 1. The horizontal axis shows the resonant frequency, and the vertical axis shows the sensitivity S. The piezoelectric membrane vibrator 25 has a frequency characteristic with resonance frequency ₁ as its apex, and the piezoelectric membrane vibrator 26 has a frequency characteristic with resonance frequency ₂ as its apex. The curve shown is a curve showing a nearly flat frequency characteristic during the frequency change Δ due to temperature with respect to the central resonant frequency ₀ .

On the other hand, the curve shown by the transducer 23, which is a conventional transducer, shows the maximum sensitivity at the resonance frequency _{of 0} , but the sensitivity decreases significantly at the end of the frequency change Δ.

This shows that the transducer shown in FIG. 1 is resistant to temperature changes.

FIG. 4 shows various modifications of the present invention. In FIG. 4A, two holders are semicircular, each having a different radius of curvature. The radius of curvature of the semi-cylindrical holder 39 is R ₄ , and the radius of curvature of the semi-cylindrical holder 40 is R ₅ (R ₄ >R ₅ ), and these are arranged back to back to face each other. This gives the radius of curvature
Ultrasonic waves containing resonance frequencies corresponding to R ₄ and R ₅ are emitted.

FIG. 4B shows a modification in which the holder 41 has an elliptical cross-sectional shape so as to have different resonance frequencies.

FIG. 4C shows a modification in which the shape of the holding body 42 is a truncated cone so as to have different resonance frequencies.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, ultrasonic waves are transmitted from vibrating surfaces having different curvatures, and the synthesis of ultrasonic waves emitted from each curvature surface is achieved. Since the frequency characteristics are made flat over a predetermined range around the center frequency, sensitivity can be prevented from decreasing over a wide range of temperature changes.As a result, the configuration of the signal processing circuit can be simplified, and operation stability can be improved. will improve.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing the configuration of one embodiment of the present invention, FIG. 2 is a longitudinal sectional view showing details of the configuration of the piezoelectric membrane vibrator in FIG. 1, and FIG. 4 is a perspective view showing various modified embodiments of the present invention, FIG. 5 is a longitudinal sectional view showing the configuration of a conventional transducer, and FIG. FIG. 7 is a vertical cross-sectional view showing the configuration of a second conventional transducer that is an improved version of the transducer shown in FIG. , FIG. 9 is a perspective view showing the operation of the piezoelectric film in FIG. 6. 10... piezoelectric vibrator, 12... damping material,
13... Case, 14... Reflective umbrella, 15, 23,
28... Transducer/receiver, 16, 29, 30, 39, 4
0,41,42...Holding body, 19...Piezoelectric film, 2
5, 26, 27...Piezoelectric membrane vibrator.

Claims (1)

[Claims] 1. An ultrasonic distance meter that emits ultrasonic waves to a measuring surface through a reflective umbrella, receives the reflected ultrasonic waves from there through the reflective umbrella, and measures the distance to the measuring surface from the time difference between the emission and reception. In the transducer, a rod-shaped holder fixed to one end of the reflector and extending in the direction of the central axis inside the reflector and having different curvatures; It has a piezoelectric membrane vibrator made of a polymer piezoelectric membrane with a fixed curvature, and the synthesized frequency of ultrasonic waves emitted from the piezoelectric membrane having different curvatures toward the measurement surface via the reflective umbrella. 1. A transducer for an ultrasonic distance meter, characterized in that the curvature is selected so that the characteristics are substantially flat over a predetermined range.