JPH0414757B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field This invention temporally modulates the frequency of microwaves and transmits them, detects the beat signal by mixing it with the reflected wave from the object to be measured, and detects its phase. This invention relates to a device that measures the level of an object to be measured based on changes.

Prior Art Generally, FM (frequency modulation) radar using microwaves is used as a method of measuring the level of a measurement target with high precision in a high temperature, dusty environment. In measurements by FM radar, when the microwave reflectance of the object to be measured is very small or when the measurement distance is long, the reflected wave amplitude becomes small, and as a result, the beat signal amplitude becomes small. Under such conditions, unnecessary reflected waves generated inside the microwave circuit become large noise, which affects measurement accuracy.

This type of level measuring device using an FM radar has the configuration shown in Fig. 2, and the antenna 1 is directed from above the hot metal in a converter to the top surface of the hot metal whose level is to be measured. Microwaves are emitted from this antenna 1 toward the hot metal. The microwave output from the microwave oscillator 2 is sent to the antenna 1 via the microwave magic T circuit 3, and the microwave radiated from the antenna 1 hits the measurement target and is reflected, and the reflected wave is received by the antenna 1. The transmitted wave and the reflected wave are mixed into the magic T circuit 3 and detected by the detector 4, and the beat of the transmitted wave and the received wave is obtained. This beat is appropriately amplified by an amplifier 5, and a distance measuring circuit 6 calculates the distance between the object to be measured and the antenna 1 from the beat wave.

FIG. 3 is a detailed circuit diagram of FIG. 2, in which the FM-CW microwave from the microwave oscillator 2 whose frequency is modulated by the modulation oscillator 2' passes through the circulator 12 in the microwave circuit 30. A reflected wave transmitted from the antenna 1 and reflected from the measurement object A is guided to the mixer 13 via the antenna 1 and the circulator 12. On the other hand, a part of the transmitted wave passes through the lower circuit in the figure, is guided to the mixer 13 via the circulator 12 and the variable attenuator 14, and is mixed and detected with the reflected wave. Usually, a balanced mixer is used as the mixer 13, and the AM of the microwave oscillator is
The structure is such that noise can be suppressed and the amplitude and phase of the signal entering the mixer can be adjusted using a variable attenuator 14 and a short circuit. However, with the above circuit configuration, it is difficult to suppress unnecessary reflected waves generated inside the microwave circuit for the following reasons.

In conventional microwave circuits, the cause of unnecessary reflection is signal leakage b in the circulator 12, which is an element that controls the direction of transmitted waves and received waves.
and the reflected wave a in the mismatch at antenna 1. These are input to the mixer from the same input end as the reflected wave from the measurement target, so when mixed detection is performed with a part of the transmitted wave that enters from the other end, the unnecessary signals generated in the two detectors in the balanced mixer are: It has the same opposite phase relationship as the beat signal due to the reflected wave from the object to be measured, and as a result, it remains without being canceled.

As shown in FIG. 4, the balanced mixer consists of a magic T circuit, which is a branching circuit element, and two detectors. The Magic T circuit has four terminals, two of which are the RF signal (measurement signal; in Figure 3, it includes the reflected wave from the object to be measured) and the LO signal (reference signal;
(corresponding to a part of the transmitted wave field in Figure 3),
A detector is provided at the remaining two terminals. If configured as shown in the figure, the RF signal and LO signal will be input into one detector in the same phase, and the other will be input in opposite phase. By subtracting both signals, the oscillator included in the LO signal will be input. Effective in removing AM noise.

In the prior art, the unnecessary signals shown in FIGS. 3a and 3b are included in the same two RF signals as the reflected wave from the object to be measured, so they are not removed and remain.

Problems to be solved by the invention
In FM radar type level meters, there are reflected waves due to mismatch in the antenna and signal waves due to leakage in the circuit element that separates the transmitted and received waves, so unnecessary beat signals are generated and the object to be measured is The reflected waves interfere with the separation of the beat signals, deteriorating level measurement accuracy.

This invention eliminates the influence of the above-mentioned unnecessary beat signal, and even when the amplitude of the beat signal between the transmitted wave and the received wave is small, the signal representing the level of the measurement target can be obtained with high accuracy without being affected by the above-mentioned unnecessary beat signal. The purpose of the present invention is to provide a measuring device that can be obtained.

Means for Solving the Problems The microwave radar level measuring device of the present invention is a microwave radar level measuring device using a transmitting/receiving antenna. A correction circuit that generates a correction signal having the same phase as the noise signal mixed in the received signal, and a circuit that removes unnecessary signals by subtracting the correction signal from the received signal. Features.

Example This invention will be described below with reference to one embodiment.

In FIG. 1, 20 is a directional coupler provided in a waveguide 21 between the microwave oscillator 2 and the magic T circuit 3 in the measuring device, and this directional coupler 20 allows the transmission microwave to be Take out a part of the magic T circuit 2 in the correction circuit 24.
Supply to 3. The magic T circuit 23 has a waveguide 2
A movable shorting plate 26 is connected to the waveguide 25 via a movable shorting plate 26.
The length is equal to the sum of the length between the receiving wave branching circuit element) and the received wave branching circuit element. The output of the magic T circuit 23 is
The signal is applied to the FM detector 27, and the detector 27 is configured to obtain a beat signal having the same period as the above-mentioned unnecessary signal.

The output of the detector 27 is connected to a subtracter 29 via an amplifier 28.

On the other hand, the output of the amplifier 5 of the measuring device is connected to a subtracter 29. The output of the subtracter 29 is connected to the level calculation circuit 6.

In the above configuration, when the level of the measurement target is measured, the same operation as that of the circuit shown in FIGS. 2 and 3 causes the detector 4 to receive a signal representing the level height of the measurement target and the above-mentioned unnecessary signal. A beat signal containing the following is obtained.

On the other hand, a portion of the transmitted wave oscillated by the microwave oscillator 2 is guided to the correction circuit 24 by the directional coupler 20 .

A beat signal having the same period as the unnecessary signal is obtained by the magic T circuit 23 and the detector 27. Note that the phase of this correction signal is controlled by the movable shorting plate 26.
The signal is adjusted to have the same phase as the noise signal. The signals generated in both detectors 4 and 27 are input to amplifiers 5 and 28. Here, the amplitude of the correction signal is adjusted by an amplifier so that it has the same amplitude as the signal. The outputs of both amplifiers are input to a subtracter 29, and unnecessary signals are removed from the received signal, thereby improving the ratio of the beat signal to the unnecessary signal due to the reflected wave from the object to be measured. The output of the subtracter 29 is input to the distance calculating circuit 6, and is processed by, for example, (i) a circuit that measures the period of the beat signal, or (ii) a circuit that counts the number of zero crossing points of the beat signal from a phase modulation circuit and a counter circuit. Measured distance data is output.

The magic T circuit 23 is the same as the magic T circuit shown in FIG. Furthermore, a circulator may be used as the magic T circuit.

In a level meter using a microwave FM radar, etc., it is important to improve the ratio of the beat signal due to the reflected wave from the object to be measured to the unnecessary signal in the radar circuit in order to improve the measurement accuracy. The beat signal amplitude depends on the characteristics of the radar such as transmission power and antenna gain, the measurement distance, the reflectance of the measurement target, etc., but it is limited by equipment and measurement, and is especially applicable to level meters for hot metal etc. in converters. In this case the signal will be very small. Therefore, as shown in the above-described embodiment, the above-mentioned ratio is improved by creating a beat signal of unnecessary signals by the correction circuit 24 and removing unnecessary signals due to unnecessary reflections occurring in the microwave circuit of the FM radar. Improve ranging accuracy.

Effects of the Invention As detailed above, this invention provides a correction circuit that creates a beat signal of unnecessary signals and removes the beat signal of unnecessary signals from the beat signal caused by reflected waves from the measurement object. The ratio of the beat signal to the unnecessary signal in the radar circuit can be improved, and the level of the object to be measured can be accurately measured.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing an example of a conventional measuring device, Fig. 3 is a detailed circuit diagram of the circuit shown in Fig. 2, and Fig. 4 is a block diagram showing an example of a conventional measuring device. 4 is a drawing showing the operation of the magic T circuit of the circuit shown in FIG. 3; FIG. 1...Antenna, 2...Microwave oscillator, 3
... Magic T circuit, 4 ... Detector, 6 ... Level calculation circuit, 20 ... Directional coupler, 23 ... Magic T circuit, 24 ... Correction circuit, 29 ... Arithmetic circuit.

Claims (1)

[Claims] 1. FM-CW using a transmitting/receiving antenna
In a microwave radar level measuring device, a waveguide circuit with a length equal to the length between the transmitting/receiving wave branching circuit element and the antenna plus the antenna length, with one end short-circuited, and a direction in which a part of the transmitted microwave is taken out are used. a correction circuit that generates a correction signal having the same phase as a noise signal mixed in the received signal by coupling the waveguide circuit to a microwave circuit obtained from the directional coupler; A microwave radar level measuring device comprising: a circuit for removing unnecessary signals by subtracting a correction signal obtained by the correction circuit from .