DESIGN AND IMPLEMENTATION OF SHIFT

FREQUENCY MEASUREMENT SYSTEMS FOR

METAL DETECTOR

Yin Thu Win1,a*, Aung Lwin Moe2,b and Aung Ko Ko Thet1,c

1Yangon Technological University, Myanmar2Malaysia Japan International Institute of Technology,

University Technology Malaysia, Malaysia

World Virtual Conference on Advanced Research in Materials and Engineering Applications

September 22-26, 2014

3. The algorithm of frequency counting

Contents

1. Introduction

4. Experimental Details

5. Measurement results and accuracy

2. Proposed Method

6. Conclusions

�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.

�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 distinguishes well between

ferrous and non-ferrous metals.

�This Induction Balance metal detector has more sensitivity

than the other type of metal detectors.

Introduction

�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.

Objectives

Benefits of Proposed systems

• Additionally, the proposed system is the cost effective,

less circuitry, high performance control system and

feasible for many other metal detection applications.

� 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.

System Description

�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.

�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.

Proposed Method

Figure 1. Block Diagram of frequency counters for metal detector

Oscillator

Q1 (transmit)

Q2 Receive

coil

(amplifier)

IC1a-DC level ground set

Rectifier Filter IC1a

comparato

r

VCO

IC2

Frequency counter

with PIC16F628AFour digits

Seven segments

display

Coils

�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.

Convert Four

Decimal Digits

NO

YES

Decode Seven

Segment Display

Multiplex

Seven

Segment

DisplayEND

A

Gate Time = 1/2s

Count Pulses

F = Count

Pulses x 2

F = Count

Pulses x 1

B

Gate Time =

1s

F< 10kHz?

Count Pulses

YES

NO

YES

NO

Prescalar OFFGate Time = 1/4s

START

Set Prescalar

(1/64)

Count pulses

TIMER0 module

F<1MHz?

F<100kHz?

Gate Time = 1/4sA

F = Count Pulses x

4

B

Count Pulses

Count Pulses

F = Count Pulses

x 4 x 64

B

Figure 2.(a) Flowchart of Frequency Counter (b) Flowchart of Frequency Counter (Contd.)

The algorithm of frequency counting

�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.

� 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,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.

�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 Details

Figure 3 Frequency measurement in kHz range

�When no metal is present, 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.

� The measurement accuracy for the constructed frequency counter has been performed

using frequency generator with the frequency range of 1Hz to 2MHz capability.

�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.

Measurement results and accuracy

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)10.7 11 0.310.3 10 0.332.6 33 0.447.2 47 0.293.9 94 0.1

100 to 200 HzTrue value (Hz) Measured value (Hz) Error (Hz)

102.47 102 0.47139.49 139 0.49159.10 159 0.10181.81 182 0.19189.10 189 0.10

1kHz to 10kHzTrue value (kHz) Measured value (kHz) Error (kHz)

1.972 1.97 0.00023.853 3.85 0.00036.824 6.82 0.00048.432 8.43 0.00029.712 9.71 0.0002

10kHz to 20kHzTrue value (kHz) Measured value (kHz) Error (kHz)

10.702 10.70 0.00213.570 13.57 0.00014.262 14.26 0.00216.904 16.90 0.00418.364 18.36 0.004

100kHz to 2 MHzTrue value (kHz) Measured value (kHz) Error (kHz)

106.04 106.0 0.04130.44 130.4 0.04189.05 189.1 0.04280.49 280.4 0.091850.9 1850.0 0.9

� The frequency counter and 4 digits seven segment LED display with the PIC

microcontrollers (16F628A) has been designed and implemented

� The frequency counter displayed 1.149 kHz for high sensitivity selection and

0.955 kHz for low sensitivity when the metal is detected.

� The practical experiments were done to display the four programming modes

of frequency counter and frequency measurement for 66Hz.

� The developed frequency counter using PIC16F628A is able to provide

major benefits such as high accuracy and excellent performance feature in

wider frequency measurements.

� It is efficient, cost effective, less circuitry and useful for many other

applications.

� 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.

Conclusions

Thank you