Abstract: This paper uses a digital inductance converter (LDC) as the detection module and selects the ATmega328 microcontroller as the main controller of the system. The working principle of the precisely controlled stepper motor is explained. The control program was written, and considering the XY axis mechanical structure, this paper, through comparative analysis, selected the digital inductance converter (LDC) as the detection module, completing the design of a simple and low-cost metal detector. This detector has high sensitivity and accurate detection.
introduction
With the rapid development of science and technology, the functions of metal detectors have also increased. The development of metal detectors has undergone several scientific and technological innovations, from the initial analog signal technology to today's digital pulse technology, all of which have brought about qualitative leaps in the sensitivity, resolution, and detection accuracy of metal detectors. The Doppler effect-based metal detector studied in this project belongs to the microwave detection type. It features a simple structure and light weight, and its size, weight, and manufacturing cost do not increase rapidly with the increase in the size of the detected item. This allows for the production of lightweight and easy-to-use metal detectors at a relatively low cost, making it a promising candidate for widespread applications.
1 Metal Detector Design Module
1.1 System Overall Module Introduction
The entire system module, as shown in Figure 1, is mainly composed of five parts: the main control module (MCU), the power supply module, the metal detection module, the motor drive module, and the motor positioning module. To prevent interference between the autonomous motion modules and ensure smooth data transmission, a master-slave control structure is used to process the two master data sources separately, and interrupt control is employed to achieve联动 control. This results in fast speeds and minimal impact.
Figure 1 System Overall Module Diagram
1.2 Introduction to the Main Control Module
The main control module is based on the ATmega328 microcontroller. The core of this microcontroller is the integration of 32 working registers and a rich instruction set. All working registers are directly connected to the ALU (Arithmetic Logic Unit), enabling a single instruction to access (read/write) two independent registers simultaneously within one clock cycle. This architecture improves code efficiency, allowing most instructions to execute in just one clock cycle. Therefore, the ATmega328 can achieve a performance close to 1 MIPS/MHz, operating at speeds up to 10 times faster than ordinary CISC microcontrollers. It features high speed and ease of operation.
1.3 Introduction to Metal Detection Module
This paper presents a metal detection module based on a digital inductance converter (LDC). The LDC utilizes a PCB coil or a self-made coil to achieve non-contact inductance detection. Metal detection can be easily achieved by analyzing the influence of the metal object being tested on the magnetic field of the inductance coil. After analyzing and processing the data detected by the LDC, a motor is driven to move, thereby achieving precise positioning of the metal object being tested. By fully utilizing the advantages of LDC in micro-detection and precise positioning, fast, accurate, and efficient metal object positioning is achieved. This results in low cost and a high level of automation.
1.4 Introduction to the Positioning Module
This paper uses a simple XY axis slide for positioning, and the overall structure is shown in Figure 2.
Figure 2 XY axis slide
During metal detection, the metal detector, driven by the belt along the X-axis, completes horizontal movement.
The scanning is performed. A stepper motor controls the belt along the Y-axis; after the metal detector completes the previous X-axis scan,
Move forward a certain distance, then perform an X-axis scan. This completes the scan of the entire area. In this method, the metal...
The detector performs a step-by-step scan with no blind spots and is not affected by motor vibrations, thus meeting the design accuracy requirements.
Require.
2. Working principle and driving mechanism of stepper motors
Figure 3 Stepper motor microstepping analysis diagram
2.2 Design of Motor Drive
This paper uses the A3967SLB, a dedicated microstepping driver for two-phase stepper motors with PWM constant current control. The A3967SLB is a complete micromotor driver with built-in logic. It is designed to operate bipolar stepper motors in full, half, quarter, and eighth-step modes, with an output drive capability of 30V and ±750mA. Its features include a current regulator with a fixed off-time, capable of slow, fast, or mixed current decay modes. This current decay control scheme reduces audible current noise, increases stepping accuracy, and reduces power consumption. Simultaneously, it can generate one step by inputting a pulse into the "step" input, i.e., producing full, half, quarter, or eighth-step pulses depending on the input. A microstepping circuit breaker based on the A3967 driver chip can provide approximately 750mA of drive per phase for a two-stage stepper motor. It is typically set to 8-step microstepping mode to control the stepper motor rotation, and its drive principle diagram is shown in Figure 4.
Figure 4 Motor drive principle diagram
3 System Software Programming
3.1 Flowchart of the main program for the metal detector
Figure 5 Flowchart of the detection procedure
Figure 5 is a flowchart of the program. When the system is powered on, the program enters the running state. The X-axis conveyor belt begins a lateral scan. If a target object is detected during the scan, the program stops and enters the detection response phase. If no target object is detected, the Y-axis conveyor belt moves forward a small step, and then the X-axis conveyor belt reverses direction to perform the detection again. If a target object is detected, the program enters the detection response phase. If no target object is detected, the Y-axis conveyor belt continues to move forward a small step, and the X-axis conveyor belt begins a new scan, continuing until a target object is detected or the entire area is scanned.
3.2 Motor drive module programming
Due to mechanical limitations, the XY-axis slide can only slide within a 50cm x 50cm range. However, the stepper motor...
During the control process, the number of pulses is recorded. Tests show that 32,000 pulses are recorded to complete the control of one axis.
A stepper motor generates 1600 pulses per step, therefore it takes 20 steps to complete one axis. Since the detector coil is 2.5 cm long...
The area can be divided into 20 equal parts, and each stepper motor can move forward one step to complete the scanning of the entire area.
3.3 Metal Detection Module Programming
Figure 6 shows the pulse peak value obtained by the coil when detecting metal in the experiment. A metal detection threshold is set for the detector.
The detected detection signals are compared and contrasted. The objects to be detected include a one-cent coin, a one-yuan coin, and an iron coin with a diameter of 4cm.
Wire loops. One-yuan and one-jiao coins are easier to position compared to wire loops.
For one-yuan and one-jiao coins, the metal detection module collects a high-amplitude pulse signal. The pulse amplitude is largest when the detection coil passes through the center of the coin, as shown in Figure 5.4. The first image shows the pulse of a one-jiao coin, the second the pulse of a one-yuan coin, and the third the pulse of a metal ring. The program determines whether the coin is metal by comparing the detected peak data with the threshold. Further testing determines the threshold for different metals, thus identifying the differences between them.
Figure 6 Metal detection pulse diagram
4. Metal Detector Test Results and Analysis
4.1 Time Test Results and Analysis
For this design, two aspects need to be tested: the detection time and accuracy of the metal detector. First, the detection time is the time taken from the start of the metal detector's operation to the detection of the target object. The most extreme case, where the target object and the detector's starting point are at opposite corners of the diagonal, must be considered, as this would take the longest. Regarding the completeness of the test results, since the test considers the worst-case scenario, it is only necessary to test the time it takes for the detection module to scan the entire XY-axis slide. Therefore, a metal object can be placed under the glass plate. Table 1 shows the time required for the metal detection module to scan the entire XY-axis slide starting from each of the four corners.
Table 1. Scanning time of the XY axis slide table by the metal detection module.
Because the self-made mechanical structure is made of a relatively soft material, it cannot guarantee perfect perpendicularity. Therefore, the two ends cannot move synchronously during the scanning process, resulting in slight time errors. Different diagonal scans also have minor errors, but these do not affect the measurement of the detection time. Analysis of the data in the table above proves that the scanning time of the system solution meets the design requirements.
4.1 Accuracy Test Results and Analysis
In terms of detection accuracy, for one-yuan and one-jiao coins, it is sufficient to detect the area within the edge of the coin; therefore, visual inspection is sufficient.
Determine if the positioning accuracy meets the requirements. Since the iron ring only needs to correspond to the marked position, visual inspection is also used.
This allows us to determine whether the requirements have been met. Table 5.2 shows the accuracy detection results for different objects.
Table 2 Accuracy Detection Test Results
Based on the above data analysis, the system's detection time and accuracy both meet the design requirements. The error mainly comes from...
Despite mechanical friction within the mechanical structure, the system completion time remained within the design range, and the accuracy of several data set tests also met the design requirements. Therefore, the design meets the requirements.
5. Design physical drawings and their descriptions
Figure 7. Actual image of a metal detector.
This design employs inductive sensing technology, one of the most commonly used remote-controlled, short-range sensing technologies in metal detection. This technology enables low-cost, high-resolution sensing of conductive targets, making it extremely reliable in harsh environments with dust, dirt, grease, and moisture. It can be created using coils of sensing elements on printed circuit boards to achieve an ultra-low-cost system solution. Inductive sensing technology enables high-precision measurements of linear/angular position, displacement, motion, compression, vibration, metal composition, and many other applications, including automotive, consumer, computer, industrial, medical, and communications applications.
Conclusion
This article discusses the current state of inductance sensing technology, inductor-to-digital converters, the development of stepper motors, and the principles of stepper motor subdivision.
The article also introduced specific applications of the technologies.
The main contributions of this thesis are:
(1) Provide an overview and analysis of the development and current status of inductive sensing technology;
(2) Analyze current digital inductor converters (LDCs) and compare them with other sensors and electromagnetic sensors.
This paper compares the advantages and disadvantages of a product, contrasts its stability and flexibility, and analyzes its principles, internal structure, and module design principles.
(3) Provide an overview and analysis of the development and current status of stepper motors, as well as the principle of stepper control microstepping.
(4) Finally, the design of the metal detection device is completed by connecting the microcontroller with the stepper motor and the LDC module.
