Measuring and testing – Gas content of a liquid or a solid – By vibration
Reexamination Certificate
1998-09-01
2001-04-10
Williams, Hezron (Department: 2856)
Measuring and testing
Gas content of a liquid or a solid
By vibration
C076S018000, C076S018000
Reexamination Certificate
active
06212936
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an ultrasonic transmitter, and in particular to an ultrasonic transmitter for an air bubble detector, having a transmitting stage and an ultrasonic crystal.
RELATED TECHNOLOGY
An ultrasonic crystal customarily comprises a piezoelectric element that is driven at one of its resonance frequencies. A problem is providing a transmitting stage that excites the ultrasonic crystal at precisely that resonance frequency whereby an output signal which is as strong as possible is generated in the ultrasonic crystal.
If the ultrasonic transmitter is put to use in an air bubble detector, then particular requirements pertaining to reliability must also be imposed on the ultrasonic transmitter. In an air bubble detector there is, between the ultrasonic transmitter and an appropriately arranged ultrasonic receiver, a tube that is to be monitored for air bubbles. When infusions are being administered to a patient, or when transfusions are being performed on a patient, the admission of air into the tubes used for these purposes must be detected, or the patient may face a life-threatening situation. In order to detect air bubbles in these cases, use is made of the fact that the attenuation of the ultrasonic transmission line changes as soon as an air bubble enters the tube located between the ultrasonic transmitter and the ultrasonic receiver.
U.S. Pat. No. 5,583,280 discloses an air bubble detector having a transmitting stage, comprising a frequency synthesizer that sweeps linearly within a frequency range, wherein a resonance frequency of the ultrasonic crystal also lies in this frequency range. As soon as the sweeping frequency synthesizer finds the resonance frequency of the ultrasonic crystal, the ultrasonic crystal is excited and the frequency synthesizer's sweep may start anew.
An air bubble detector having a transmitting stage that has a variable frequency synthesizer is described in European Patent Application No. 0 416 911 A2. A test circuit is also provided for identification of malfunctions.
European Patent Application No. 0 340 470 A1 describes an atomizer having an ultrasonic crystal and a voltage-controlled oscillator for excitation of the ultrasonic crystal. The oscillator is regulated by a delta generator so that its own frequency is periodically swept in a range that includes the series resonance of the ultrasonic crystal. By means of a superposed closed loop it is possible to lock in place the voltage-controlled oscillator onto the series resonance frequency of the ultrasonic crystal.
An atomizer having an ultrasonic crystal is also known from European Patent Application No. 0 084 485 A2. For excitation of the ultrasonic crystal a multivibrator and a pulse generator are interconnected in such a way that the pulse generator emits pulses to the ultrasonic crystal in beat with the multivibrator's natural frequency. The pulse has a system-excitation effect on the ultrasonic crystal, so that the ultrasonic crystal reacts to the pulse with a damped resonance vibration. Of course, with pulse operation there is the disadvantage that a pulse generator having an appropriate energy storage element is always required in order to provide the pulse energy, resulting in comparatively complex circuitry.
U.S. Pat. No. 5,583,280 describes a liquid-level indicator utilizing the ultrasonic principle and a feedback band-pass filter for excitation of the ultrasonic crystal. The filter locks onto the resonance frequency of the ultrasonic crystal, similar to the phase-locked loop device described above.
Prior ultrasonic transmitters have the disadvantage that the transmitting stages have relatively highly complex circuitry.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an ultrasonic transmitter having a simply designed transmitting stage for excitation of a transmitting-end ultrasonic crystal which at the same time generates a strong output signal in the ultrasonic crystal.
The present invention provides an ultrasonic transmitter (
20
), particularly for an air bubble detector, having a transmitting stage (
23
) and having an ultrasonic crystal (
5
), the transmitting stage (
23
) having a multivibrator (
22
). The multivibrator (
22
) includes a time-base circuit (
1
) having two stable output states, and a timing element (
2
,
3
) interconnected in the time-base circuit's feedback loop. The ultrasonic crystal (
5
) is interconnected with the time-base circuit's feedback loop in such a way that the transmitting stage (
23
) oscillates at or near a resonance frequency of said ultrasonic crystal, while, when said ultrasonic crystal is disconnected, the multivibrator (
22
) oscillates automatically at its own natural frequency.
According to the present invention, the transmitting stage has a multivibrator including a time-base circuit and a timing element interconnected in a feedback loop of the time-base circuit. The multivibrator starts oscillating almost automatically at its own natural frequency, which frequency is influenced essentially by the time-base circuit. According to the present invention, the ultrasonic crystal is interconnected with the feedback loop of the time-base circuit in such a way that the transmitting stage oscillates at or near a resonance frequency of the ultrasonic crystal. In contrast to prior excitation circuits, the ultrasonic crystal itself thereby functions as a frequency-determining component for a continuously generated transmission frequency. This makes the circuitry considerably less complex, and at the same time particularly insensitive to electromagnetic interference, which enhances reliability. In addition, the ultrasonic transmitter according to the present invention generates a very strong output signal, which in turn makes possible a simple design for the receiving circuit.
According to a preferred embodiment of the present invention, the timing element of the multivibrator comprises at least one RC element, and the ultrasonic crystal is connected in parallel to the resistance of an RC element. This ensures reliable buildup of oscillations in the circuit because the multivibrator will first start to oscillate at its own natural frequency and then will excite the ultrasonic crystal at a resonance frequency due to the abrupt level changes at the output of the time-base circuit.
According to another preferred embodiment of the present invention, a low-pass filter is provided at the output of the time-base circuit, in order to suppress buildup of oscillations of the ultrasonic crystal at the crystal's harmonics. In addition, selective buildup of oscillations can also be attained by having the transmitting stage oscillate below the appropriate resonance frequency of the ultrasonic crystal.
The ultrasonic crystal is driven at its lowest series resonance frequency. When dealing with a piezoelectric element for the ultrasonic crystal, the series resonance frequency will be largely independent of external influences, while the poorly defined electrode capacitance of the piezoelectric element will form the parallel resonance frequency.
According to another preferred embodiment of the present invention, the transmitting stage is interconnected logically with a test input, in order to activate or deactivate the ultrasonic transmitter for testing purposes. In this case the test input is interconnected with the input of the time-base circuit by means of an AND gate. In this way the ultrasonic transmitter can be tested for specific purposes for safety-engineering applications.
In another preferred embodiment of the present invention, the time-base circuit is a Schmitt trigger. A conventional Schmitt trigger is readily available as an integrated component and is barely loaded by a timing element in the feedback loop, permitting great leeway in choosing the components of the timing element.
An air bubble detector of the present invention includes an ultrasonic transmitter according to the present invention, an ultrasonic receiver and a tube inserted between
Fresenius AG
Kenyon & Kenyon
Miller Rose M.
Williams Hezron
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