JPS6333682A - Ultrasonic sensor - Google Patents

Abstract

PURPOSE:To stabilize the detecting capacity of an ultrasonic sensor at its highest point by comparing the phases of transmission signals from an oscillator and the signals of both ends of a piezoelectric type ultrasonic vibrator with each other and controlling the oscillation frequency of the oscillator so that phase differences have a prescribed set value. CONSTITUTION:Transmission signals generated from an oscillator 1 are amplified by a power amplifier 2 and transmitted as an ultrasonic wave from an ultrasonic vibrator 4 via a transmission and reception separating circuit 3. The phases of the signals applied across the vibrator 4 and the output signals of the power amplifier 2 are compared with each other in a phase comparator 5. The oscillator 1 is controlled by a frequency control circuit 6 and phase differences are kept always constant. Thus, the transmitting and receiving sensitiveness of the vibrator 4 becomes always best and the detecting capacity of an ultrasonic sensor is stabilized at its highest point.

Description

[Detailed description of the invention] 【Technical field】

The present invention relates to an ultrasonic sensor that transmits ultrasonic waves and receives reflected waves from an object to be detected to detect the presence or movement of an object within a monitoring space.

[Background technology]

A piezoelectric ultrasonic transducer is used to transmit ultrasonic waves into the monitoring space, and the reflected waves of the ultrasonic waves from the object to be detected within the monitoring space are received, and the presence or absence of the object is detected from the received signal. Various types of ultrasonic sensors for detecting the movement of objects have been proposed and implemented in the past. FIG. 4 shows a block circuit diagram of a conventional ultrasonic sensor with integrated transmission and reception. In FIG. 4, a transmission signal oscillated by an oscillator 21 is amplified by a power amplifier 22 and applied to an ultrasonic transducer 24 via a transmission/reception separation circuit 23. The ultrasonic transducer 24 transmits an ultrasonic wave corresponding to the applied signal toward the monitoring space, and the transmitted ultrasonic wave is reflected by the object to be detected in the monitoring space, and is transmitted to the ultrasonic transducer 24. The waves are received and converted into electrical signals. The transfer signal converted into an electric signal is amplified by an amplifier 25 and then mixed with a transmission signal by a mixer 26. At this time, if the receiving signal frequency is different from the transmitting signal frequency due to the Doppler effect, the amplitude of the output signal of the mixer 26 changes at a frequency corresponding to the moving speed of the object to be detected. A detection circuit 27 detects the amount of change in amplitude of the output signal, and an amplifier 28 amplifies the output signal. A signal processing circuit 29 determines the presence of a moving object and drives a display 30. However, the resonance characteristics of piezoelectric ultrasonic transducers are very sharp as shown in Figure 5, and moreover, the resonance characteristics are very sharp with respect to temperature changes.
．． It is sensitive to ambient temperature changes such as high temperature A, room temperature B, and low temperature C, and the resonance point changes depending on the temperature, and there are many variations between individual elements, so if the oscillation frequency is fixed as in the past, However, there was a problem in that the ultrasonic transducer 24 could not be used at its optimum transmitting and receiving sensitivity, and the characteristics of the entire ultrasonic sensor were not stable.

[Purpose of the invention]

The present invention has been made in view of the above points, and its purpose is to always maximize the transmitting and receiving sensitivity of the piezoelectric ultrasonic transducer, and to improve the detection ability of the ultrasonic sensor to the highest point. The purpose is to stabilize it.

[Disclosure of the invention]

The impedance and phase frequency characteristics of the piezoelectric ultrasonic transducer exhibit steep resonance characteristics as shown in FIG. 2, and the phase also changes rapidly from the resonance point fr to the anti-resonance point f&. In addition, the piezoelectric ultrasonic transducer has a maximum transmission output near the resonance point, and a maximum reception sensitivity near the anti-resonance point, so when one ultrasonic transducer is used for both transmission and reception,
The overall sensitivity of the transmission and reception is maximized between the resonance point and the anti-resonance point. Focusing on the fact that the phase of the ultrasonic transducer changes only at the leap between the resonance point and anti-resonance point where the transmitting and receiving sensitivity is maximum, the transmitting oscillation frequency is controlled so that the phase is at a certain fixed value. Accordingly, the present invention is such that the transmitting and receiving sensitivity is always maximized even if the impedance of the ultrasonic transducer changes with temperature or there is individual variation, and will be explained below with reference to Examples. FIG. 1 is a block circuit diagram of a moving object detection type ultrasonic sensor according to an embodiment of the present invention, in which a transmission signal oscillated by an oscillator 1 is amplified by a power amplifier 2, and then sent to a transmission/reception separation circuit 3. The ultrasonic wave is applied to the ultrasonic transducer 4 through the ultrasonic transducer 4, and the ultrasonic wave is transmitted from the ultrasonic transducer 4. Here, the phase of the signal applied to both ends of the ultrasonic transducer 4 and the output signal of the power amplifier 2 or the output signal of the oscillator 1 is compared by the phase comparator 5. For example, the signal of the ultrasonic transducer 4 is If the phase is ahead of the signal by θ+α, the oscillation frequency will be lower.
Also, if the oscillator 13 is advanced by θ-β, the frequency control circuit 6 controls the oscillator 13 so that the oscillation frequency becomes high, and the transmitting and receiving sensitivity of the ultrasonic transducer 4 is maintained at the maximum by making the phase difference always θ. be able to. Further, the transmitted ultrasonic waves are reflected by objects to be detected in the monitoring space, received by the ultrasonic transducer 4, and converted into electrical signals. The transfer signal is amplified by an amplifier 7, and then mixed with a transmission signal by a mixer 8. The amount of change in amplitude of the output signal of the mixer 8 is detected by a detection circuit 9 and amplified by an amplifier 10, and a signal processing circuit 11 determines the presence of a moving object and drives a display 12. Next, FIG. 3 is a block circuit diagram of a pulse type presence detection type ultrasonic sensor according to another embodiment of the present invention. The pulse type ultrasonic sensor transmits ultrasonic waves by applying a transmission signal generated by an oscillator 1 to an ultrasonic transducer 4 via a transmission/reception separation circuit 3 only when a transmission gate 13 is ON. , the ultrasonic wave reflected by the object to be detected is received by the ultrasonic transducer 4, the transfer signal is amplified by the amplifier 7, the waveform is shaped by the waveform shaping circuit 14, and the transfer signal is sent to a preset monitoring space. If present when the corresponding transfer gate circuit 15 is ON, it is determined that there is an object to be detected in the monitoring space and drives the display 12 via the AND gate 16. A phase comparator 5 detects the phase difference between the signal transmitted by the ultrasonic transducer 4 and the output signal of the oscillator 1, and the frequency control circuit 6 detects the phase difference so that the phase difference becomes a preset constant value. By controlling the oscillation frequency of the oscillator 1, the ultrasonic transducer 4 can always be used with the highest sensitivity.

Effects of the Invention As described above, the present invention compares the phases of the transmitted signal from the oscillator and the signals at both ends of the piezoelectric ultrasonic transducer, and adjusts the oscillator so that the phase difference becomes a preset constant value. Since the oscillation frequency of the ultrasonic transducer is controlled, ultrasonic waves can be transmitted and received between the point where the ultrasonic transducer's transmitting and receiving sensitivity is maximum, that is, the resonance point and the anti-resonant point. This has the effect that the detection ability of the ultrasonic sensor can be kept stable at the highest point even if the frequency characteristics change due to changes in ambient temperature or the like, or even if individual variations occur.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram of one embodiment of the present invention, FIG. 2 is a frequency characteristic diagram of the same piezoelectric ultrasonic transducer as described above, FIG. 3 is a block circuit diagram of another embodiment of the present invention, and FIG. The figure is a block circuit diagram of a conventional example, and FIG. 5 is a frequency characteristic diagram of the same piezoelectric ultrasonic transducer as above. DESCRIPTION OF SYMBOLS 1... Oscillator, 2... Power amplifier, 3... Transmission/reception separation circuit, 4... Ultrasonic vibrator, 5... Phase comparator,
6... Frequency control circuit, 7... Amplifier. Agent Patent Attorney Stone 1) Chief 7 Figure 1 Figure 2 Darkness Figure 3 Figure 5 n Cunning

Claims (1)

[Claims] (1) In an ultrasonic sensor that drives a piezoelectric ultrasonic transducer with a transmission signal from an oscillator to transmit ultrasonic waves, and detects an object by receiving reflected waves from an object to be detected, The phase difference between the transmitted signal from the oscillator and the signal at both ends of the piezoelectric ultrasonic transducer is compared, and the oscillation frequency of the oscillator is controlled so that the phase difference becomes a set constant value. Features of ultrasonic sensor.