Design and Implementation of Shift Frequency Measurement System for Metal Detector

Transcription

1 Design and Implementation of Shift Frequency Measurement System for Metal Detector Yin Thu Win 1,a*, Aung Lwin Moe 2,b and Aung Ko Ko Thet 1,c 1 Yangon Technological University, Insein, Yangon, Myanmar 2 Malaysia Japan International Institute of Technology, Universiti Teknologi Malaysia Jalan Semarak, 54100, Kuala Lumpur, Malaysia a yinthuwin.ait@gmail.com, b aunglm@ic.utm.my, c aungkokothet@gmail.com Keywords: Shift frequency, Frequency counter, Metal detector, Frequency measurement, Algorithm Abstract. Metal detectors are widely used to find embedded metal within objects which are beyond eye site. This study concerns the design and implementation of frequency counter with 4 digits seven segment LED display for metal detection system using PIC microcontroller. New algorithm for frequency counting is developed. The software is also developed to detect the shift frequency measurement from the VCO output. The corresponding frequency at the VCO output of Induction Balance Metal detector is clarified. The experimental results of this research revealed that using PIC16F628A for frequency measurement system is able to provide very high accuracy for metal detection application. Additionally, the proposed system is the cost effective, less circuitry, high performance control system and feasible for many other metal detection applications. Introduction Metal detectors are all based on the principle of dynamic magnetic field of transmit coil and receive coil while operating the metal is present in a surrounding. Depending on the system performance and distinguish between metals, there are several types for specific applications. The most common types are Induction Balance (IB) system, Beat Frequency Oscillation (BFO). The advantages of this (IB) metal locator are good in penetration in the depth and can distinguish well between ferrous and non-ferrous metals. This Induction Balance metal detector has more sensitivity than the other type of metal detectors. The study of [1] proposed a sensor system to detect and distinguish the two objects example a mind and a stone using 1 GHz antenna Radar. A high precision time interval and frequency counter which can be measured up to 3.5GHz by integrating in a CMOS chip was revealed by [2]. The proposed system has developed a large-range metal detector to find and classify the metallic object by using signal processing method [3].The developed system has established frequency counter by using reciprocal method with two modes of operation and integrated in an FPGA device[4]. A pulse induction Metal Detector using with different size of metal target for measurement purposes was introduced by [5]. The authors [6] presented the system to measure the time-domain frequency stability by using frequency counter.the determination of a digital representation of a value for radio frequency identification sensing systems was explained in [7]. This research aims to design frequency counter with four digit seven-segment LED display for detecting metal object. The PIC microcontroller (16F628A) is used for cost effective, less circuitry and high performance control system in innovative applications. Necessary assembly language program has been developed for this application with programming mode for measurement set up and calibration mode. When the metal detector has been near the metal object, the frequency counter displayed for high sensitivity selection and low sensitivity. The practical experiments were carried out to display the four programming modes of frequency counter and frequency measurements.

2 Proposed method This research work presented a frequency counter displaying the signal from a Induction Balance(IB) metal detector type which could be used for wet or dry ground including beach sand. Fig.1 shows the Block Diagram of frequency counters for metal detector. This type of metal locator uses transmit coil and receive coil in the proposed system. The increased signal at the receive coil which will perform for the develop circuit operations and it means that the metal is detected. Then, the signal passed through the rectifier stage which has been filtered and then passing through the non-inverting inputs of the comparator. The output from the comparator appeared and applied to the VCO of the Induction Balance Metal Locator which is the input signal of the frequency counter. Furthermore, the frequency counter operates by obtaining the signal from the VCO output of metal detector. In order to display the corresponding frequency at the VCO output of Induction Balance Metal Locator, the necessary software has been developed by using PIC16F628A. The proposed system algorithm consists of a microcontroller to implement the frequency counter which has three portions: the input, the output and the process. Input-frequency is at RA4. In the process, firstly to set prescaler and count pulses for automatic range setting at different gate times. Next, frequency calculation was performed and this signal was converted four decimal digits. The four decimal digits were decoded as seven segment display and finally multiplex seven segment display. The output-numerical display of measured frequency can be shown by seven-segment LED display. This proposed system can measure frequency range 1Hz to 50MHz. Coils Oscillator Q1 (transmit) IC1a-DC level ground set Q2 Receive coil (amplifier) Rectifier Filter IC1a comparator VCO IC2 Frequency counter with PIC16F628A Four digits Seven segments display The algorithm of frequency counting Fig.1 Block Diagram of frequency counters for metal detector Flow chart of frequency counter is shown in Fig.2. The developed program for frequency counting has looping and subroutines after giving the input frequency at the programming mode. In the main loop, the measurement of frequency is carried out with step by step procedures. This developed program operates to perform the frequency counting in four programming modes. For the highest frequencies, the pre-scalar is divided by 64 for program execution. The next step is to count the input pulses for 1/16 second, and the performance will work TIMER0 module. Moreover, the counting loop which consists of multiplexer operations takes exactly 50 microseconds. Therefore,

3 1250 counting loops will become in a gate time of 1/16 seconds. According to the coarse frequency measurement, the pre-scalar and measuring intervals are selected respectively. START Set Prescalar (1/64) A F< 10kHz? NO YES Count pulses TIMER0 module Gate Time = 1/2s Gate Time = 1s NO F<1MHz? YES F = Count Pulses x 2 F = Count Pulses x 1 Gate Time = 1/4s Prescalar OFF B YES F<100kHz? Convert Four Decimal Digits F = x B NO Gate Time = 1/4s A Decode Seven Segment Display Multiplex F = x 4 END B (a) (b) Fig. 2 (a) Flowchart of Frequency Counter (b) Flowchart of Frequency Counter (Contd.) Next, the developed counter s pre-scalar is reprogramed then the divided input frequency is less than 1MHz which is the highest value if the clock is 4 MHz. If the input frequency is less than 1MHz, the pre-scalar is turned off in this measurement step. While the program is operating, the gate time of measuring interval will be 0.25 second, 0.5second, or 1 second. During this operation time, the display is working continuously. Then gate time completed and counting pulses are stopped. If the pre-scalar was active while operating for the counts, multiply the counted pulse with the pre-scalar ratio. If the gate time is 0.5 seconds, the counted pulse will be multiplied by 2; if the gate time is 0.25 seconds, the counted pulse will be multiplied by 4. The result of the input frequency is in Hz, any pre-scalar ratio or gate time can be used. The display multiplex routine while executing will work these registers to the LED display. Experimental detail Fig.3 shows the frequency measurement in various khz range. It has high accuracy for the measured frequency and fast processing speed. The metal locator is capable to perform sensitivity selection for object detection. While system operation is carried out, the frequency counter will

4 Fig.3 Frequency measurement in khz range display the detected frequency from VCO of Induction balance metal locator. When no metal present frequency of 955Hz is designated as high sensitivity selection and frequency 625Hz is denoted as low frequency 955Hz is designated as high sensitivity selection and frequency 625Hz is denoted as low sensitivity selection. When the metal locator is brought near the metal object and low sensitivity is chosen, 745Hz is displayed on LED. Otherwise, high sensitivity is chosen, 1.149kHz is displayed on seven-segment LED. Results and discussions The measurement accuracy for the constructed frequency counter has been performed using frequency generator with the frequency range of 1Hz to 2MHz capability. Table 1 shows the measurement result of and error in wide frequency ranges. It revealed the result of generated frequency, measured frequency from the constructed frequency counter and the error. The signal frequency produced from the frequency generator (1Hz to 2MHz) is measured by the frequency counter which is implemented with four digits seven-segment display. The first experimental performance was done with the range 1Hz to 10Hz. In a similar performance, the measurements were done to display in frequency counter by reading the signal from frequency generator with the set of frequency range (100Hz to 200Hz), (1Hz to 10kHz), (10kHz to 20kHz). According to the experimental results, it could be seen that the measurement of frequency range between 1kHz and 10kHz can give the lowest error percentage of this system operation. According to the practical measurement results, it can be found that the measured frequency values displayed on the frequency counter are very closed to the frequency from the frequency generator. Moreover, the practical measurement results proved that, higher the frequency, lower the percentage of error in frequency. Therefore, the constructed frequency counter has high accuracy and very useful for many applications. Conclusions The frequency counter and 4 digits seven segment LED display with the latest technology PIC microcontrollers (16F628A) has been designed and implemented in this research. Necessary assembly language program has been developed for this application for measurement set up and calibration mode. The frequency counter displayed khz for high sensitivity selection and khz for low sensitivity when the metal is detected. The practical experiments were carried out to display the four programming modes of frequency counter and frequency measurement for 66Hz. The results obtained in this experiment indicate that the developed frequency counter using

5 PIC16F628A is able to provide major benefits such as high accuracy and excellent performance feature in wider frequency measurements. Hence, developed frequency counter for the metal locator is efficient, cost effective, less circuitry and useful for many other applications. For further research, this system could extend to distinguish the type of metals and locations by using digital signal processing method and GPS signal while monitoring from personal computer. Table 1. Measurement results and error of frequency (Frequency range 1Hz to 2MHz) 10 to 100 Hz True value [Hz] Measured value [Hz] Error [Hz] to 200 Hz True value [Hz] Measured value [Hz] Error [Hz] kHz to 10kHz True value [khz] Measured value [khz] Error [khz] kHz to 20kHz True value [khz] Measured value [khz] Error [khz] kHz to 2 MHz True value [khz] Measured value [khz] Error [khz] References [1] C. Bruschini, et al., Ground penetrating radar and imaging metal detector for antipersonnel mine detection, Journal of Applied Grophysics. 40 (1998) [2] R.Szplet, et al., High precision time and frequency counter for mobile applications, WSEAS Transactions on Circuits and Systems 9. 6 (2010)

6 [3] Feng Qiu, et al., A differential probe design for large-range metal detector, Sensor Review. 34/1 (2014) [4] R. Szplet, et al., A 45PS time-interval counter board with a PCI interface, Miltary University of Technology Warsaw (Poland), [5] A.Rerkratn,et al, Pulse induction metal detector using sample and hold method, Control, Automation and Systems (ICCAS), 11 th International Conference on. IEEE, [6] S.T. Dawkins, et al., Considerations on the measurement of the stability of oscillators with frequency counters, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, IEEE Transactions on 54.5 (2007) [7] M.McCoy, et al., A frequency counter based analog-to-digital converter for a RFID telemetry system, Circuits and Systems, MWSCAS th Midwest Symposium on. IEEE, 2007.