A metal detector capable of suppressing product effect and its demodulation circuit

Abstract

The present invention relates to a metal detector capable of suppressing product effect and a demodulation circuit thereof, wherein the demodulation circuit comprises a receiving circuit and a demodulation circuit, the input end of the receiving circuit is connected to a receiving coil, the output end is connected to the demodulation circuit, and the demodulation circuit comprises a multiplier circuit, a differential amplifier circuit, a fifth-order active filter circuit, a first notch filter for suppressing a first frequency band signal, a second notch filter for suppressing a second frequency band signal, and a signal amplification circuit for amplifying a metal response signal, which are connected in sequence. Compared with the prior art, the present invention designs two notch filters in the demodulation circuit, which can suppress noise with a frequency slightly greater than the metal response signal and noise with a frequency slightly less than the metal response signal, respectively, thereby suppressing the product effect and improving the recognition accuracy.

Description

Technical Field

[0001] The invention relates to the field of metal detectors, and in particular to a metal detector capable of suppressing product effects and a demodulation circuit thereof. Background Art

[0002] Metal detectors are widely used in the food and drug inspection industry. Suppressing the product effect can more effectively detect metal foreign matter, greatly improving the efficiency and practicality of metal detection and use is one of the core performance of metal detectors.

[0003] Metal interference or when a product passes through the metal detector probe will generate a response signal, such as Figure 1 As shown, product 1 and metal foreign body 2 pass through the conveyor belt together. Under the excitation of transmitting coil 3, the signal reaches an extreme point (maximum or minimum value) when passing through the first receiving coil 4, and reaches a reverse extreme point when passing through another receiving coil 5. Product response signal 102 and metal response signal 101 are formed. By analyzing metal response signal 101 and product response signal 102, it can be seen that the frequency of metal response signal 101 is higher than that of product response signal 102, but they are generally close.

[0004] Ideally, the subsequent recognition program can correctly identify and distinguish the metal response signal 101 and the product response signal 102, but the product response signal 102 of some products may be too close to the metal response signal 101, so under the interference of some environmental factors, the recognition accuracy may be reduced. Summary of the invention

[0005] According to actual test observations, in general, there is a difference between metal signals and product signals. The waveform of metal signals is sharper, the frequency is higher, and the fundamental wave components are more. The waveform of product signals is smoother, the frequency is lower than that of metal signals, and the high-frequency components are significantly increased. The purpose of the present invention is to provide a metal detector and a demodulation circuit that can suppress product effects. In order to effectively suppress product response signals, two sets of notch filters are designed after low-pass filtering, which can suppress noise with a frequency slightly greater than the metal response signal and noise with a frequency slightly less than the metal response signal, respectively, thereby suppressing product effects and improving recognition accuracy.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A receiving and demodulating circuit for a metal detector comprises a receiving circuit and a demodulating circuit, wherein the input end of the receiving circuit is connected to a receiving coil, and the output end is connected to the demodulating circuit, and the demodulating circuit comprises a multiplier circuit, a differential amplifier circuit, a fifth-order active filter circuit, a first notch filter for suppressing a first frequency band signal, a second notch filter for suppressing a second frequency band signal, and a signal amplifying circuit for amplifying a metal response signal, which are connected in sequence.

[0008] Furthermore, the signal amplifying circuit includes a first integrating amplifying circuit, a second integrating amplifying circuit and a third integrating amplifying circuit connected in sequence, and the second integrating amplifying circuit includes a plurality of gain resistors and a selection switch for selectively connecting one of the plurality of gain resistors.

[0009] Furthermore, the selection switch is a multi-way analog switch.

[0010] Furthermore, the number of gain resistors is 4-15 in total.

[0011] Furthermore, the number of gain resistors is 10 in total.

[0012] A metal detector comprises a host computer, a transmitting circuit, a transmitting coil, a receiving coil, a receiving circuit and a demodulation circuit, wherein the transmitting circuit is connected to the transmitting coil, the receiving coil, the receiving circuit and the demodulation circuit are connected in sequence, the host computer is respectively connected to the transmitting circuit and the demodulation circuit, and the control signal input end of the selection switch in the demodulation circuit is connected to the host computer.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. Designing two notch filters in the demodulation circuit can suppress the noise with a frequency slightly greater than the metal response signal and the noise with a frequency slightly less than the metal response signal, thereby suppressing the product effect and improving the recognition accuracy.

[0015] 2. Through the three-stage integral amplifier circuit, the metal response signal is amplified, and the amplification gain can be adjusted, thereby improving the recognition accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the metal detector principle;

[0017] Figure 2 It is a structural schematic diagram of the demodulation circuit of the present invention;

[0018] Figure 3 is a schematic diagram of a notch filter;

[0019] Figure 4 is a schematic diagram of a signal amplification circuit;

[0020] Figure 5 is a schematic diagram of a fifth-order active filter circuit;

[0021] Figure 6 This is a schematic diagram of the circuit part of the metal detector;

[0022] Figure 7 is a schematic diagram of a transmitting circuit;

[0023] Figure 8 is a schematic diagram of a receiving circuit;

[0024] Among them: 1. product, 2. metal foreign matter, 3. transmitting coil, 4. receiving coil, 5. receiving coil, 6. demodulation circuit, 7. host computer, 8. transmitting circuit, 9. receiving circuit, 61. multiplier circuit, 62. differential amplifier circuit, 63. fifth-order active filter circuit, 64. first notch filter, 65. second notch filter, 66. first integral amplifier circuit, 67. second integral amplifier circuit, 68. third integral amplifier circuit. DETAILED DESCRIPTION

[0025] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0026] A metal detector receiving and demodulating circuit is disclosed. In order to effectively suppress the response signal of product 1, two sets of notch filters are designed after low-pass filtering to suppress high-frequency noise (compared with the metal response signal) with a frequency slightly greater than the metal response signal and low-frequency noise (compared with the metal response signal) with a frequency slightly less than the metal response signal. Such a circuit design can effectively ensure the signal response of metal foreign matter 2 while suppressing noise and product 1 effects. The metal response signal is then amplified by a three-stage integral amplifier circuit. Since the overall order of the signal processing circuit is large and the response time is long, the circuit is suitable for conveyor belt metal detectors with relatively large detection effects of product 1.

[0027] like Figure 2 As shown, it includes a receiving circuit 9 and a demodulation circuit 4, the input end of the receiving circuit 9 is connected to the receiving coil, and the output end is connected to the demodulation circuit 4. The demodulation circuit 4 includes a multiplier circuit 61, a differential amplifier circuit 62, a fifth-order active filter circuit 63, a first notch filter 64 for suppressing the first frequency band signal, a second notch filter 65 for suppressing the second frequency band signal and a signal amplification circuit for amplifying the metal response signal, which are connected in sequence.

[0028] The notch filter circuit can effectively filter out fixed frequency signals. Generally, the Q value of the filter is set relatively high. Two sets of second-order filters are set with different parameters to filter out the frequency of product response signals, which greatly increases the product signal suppression effect and anti-vibration and anti-interference effect of the metal detector. The notch filter is as follows: Figure 3 As shown, in the setting, the first notch filter 64 filters out signals with a higher frequency than the metal response frequency, and the second notch filter filters out signals with a lower frequency than the metal response frequency. Specifically, the suppression frequency of the first notch filter 64 is about 1.2 times the metal response signal, and the suppression frequency of the second notch filter 65 is about 0.8 times the metal response signal. That is, the first filter is a notch filter with a suppression frequency of about 1.2 times the metal response signal, and the second notch filter 65 is a notch filter with a suppression frequency of about 0.8 times the metal response signal.

[0029] Although in actual applications, the suppression frequencies of the first notch filter 64 and the second notch filter 65 can be opposite, since different types of interference signals are mostly concentrated in a range larger than the metal sound signal, processing this part of the noise first is more convenient for the design of the subsequent integral amplifier circuit. Therefore, the suppression frequency of the first notch filter 64 is greater than the suppression frequency of the second notch filter 65.

[0030] The signal amplification circuit includes a first integral amplification circuit 66, a second integral amplification circuit 67 and a third integral amplification circuit 68 connected in sequence, and the second integral amplification circuit 67 includes a plurality of gain resistors and a selection switch for connecting one of the plurality of gain resistors. Figure 4 As shown, the upper computer 7 of the metal detector is also connected to the selection switch. The selection switch can be a multi-way analog switch. There are 4 to 14 gain resistors in total. In this embodiment, there are 10 gain resistors in total.

[0031] like Figure 5 As shown, the low-pass filter unit adopts a fifth-order active filter circuit 63, which is designed as a Butterworth fifth-order low-pass filter circuit when the parameters are selected. The Q value of the circuit parameters is relatively low, which effectively eliminates the peaks in the filter frequency characteristic curve, so that the circuit has improved anti-interference ability to a certain extent.

[0032] like Figure 6 As shown, the metal detector using the above-mentioned demodulation circuit 4 includes a host computer 7, a transmitting circuit 8, a transmitting coil 3, a receiving coil, a receiving circuit 9 and a demodulation circuit 4, the transmitting circuit 8 is connected to the transmitting coil 3, the receiving coil 4, the receiving coil 5, the receiving circuit 9 and the demodulation circuit 4 are connected in sequence, the host computer 7 is connected to the transmitting circuit 8 and the demodulation circuit 4 respectively, and the control signal input end of the selection switch in the demodulation circuit 4 is connected to the host computer 7.

[0033] Transmitting circuit 8 Figure 7As shown, the use of the AB class cascode circuit can effectively increase the amplified signal frequency of the circuit, reduce the Miller effect and eliminate self-excitation, and the power amplifier circuit structure diagram. Figure 8 As shown in the figure: the transformer selection of the receiving circuit 9 can use a transformer with strong common-mode suppression capability to optimize the circuit.

Claims

1. A receiving and demodulating circuit for a metal detector, comprising a receiving circuit and a demodulating circuit, wherein the input end of the receiving circuit is connected to a receiving coil, and the output end is connected to the demodulating circuit, characterized in that: The demodulation circuit includes a multiplier circuit, a differential amplifier circuit, a fifth-order active filter circuit, a first notch filter for suppressing a first frequency band signal, a second notch filter for suppressing a second frequency band signal, and a signal amplification circuit for amplifying a metal response signal, the suppression frequency of the first notch filter being greater than the suppression frequency of the second notch filter. The signal amplifying circuit comprises a first integrating amplifying circuit, a second integrating amplifying circuit and a third integrating amplifying circuit connected in sequence, wherein the second integrating amplifying circuit comprises a plurality of gain resistors and a selection switch for selectively connecting one of the plurality of gain resistors; The selection switch is a multi-way analog switch; There are 10 gain resistors in total.

2. A metal detector, comprising a host computer, a transmitting circuit, a transmitting coil, a receiving coil, a receiving circuit and a demodulation circuit, wherein the transmitting circuit is connected to the transmitting coil, the receiving coil, the receiving circuit and the demodulation circuit are connected in sequence, and the host computer is respectively connected to the transmitting circuit and the demodulation circuit, characterized in that: The demodulation circuit includes a multiplier circuit, a differential amplifier circuit, a fifth-order active filter circuit, a first notch filter for suppressing a first frequency band signal, a second notch filter for suppressing a second frequency band signal, and a signal amplification circuit for amplifying a metal response signal, the suppression frequency of the first notch filter is greater than the suppression frequency of the second notch filter, and the output end of the signal amplification circuit is connected to a host computer; The signal amplification circuit comprises a first integral amplification circuit, a second integral amplification circuit and a third integral amplification circuit connected in sequence, the second integral amplification circuit comprises a plurality of gain resistors and a selection switch for selectively connecting one of the plurality of gain resistors, and the control signal input end of the selection switch is connected to the host computer; The selection switch is a multi-way analog switch; There are 10 gain resistors in total.

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