Construction of a Ultrasonic Gas Flowmeter

Feng Wan

, Renjie Liu

School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300130 China

Keywords: Ultrasonic; Gas flow meter; Time chip.

Abstract: In this paper, a new method on how to design ultrasonic gas flow meter is presented. Ultrasonic sensor with

large area and center frequency 120kHz is selected and ultrasonic sensor driving circuit based on IRF7389

is designed to drive the sensor, the received ultrasonic signal can reach 1000mv in the distance of 30cm,

thus can make the amplifying circuit easy to design. In the ultrasonic receiving circuit, the no inverting

amplifier is used to realize preliminary amplification; followed by the second-order band pass filter with the

center frequency for 120kHz to filter out the noise, gain adjustable amplifier circuit is designed based on

digital potentiometer, ultrasonic signal after comparison circuit outputs square wave to control chip time

TDC-GP2 to achieve time interval measurements. Finally, the system is calibrated and the calibration

results show that the measurement accuracy is 1%.

1 INTRODUCTION

Ultrasonic gas flowmeter is a new type of gas flow

meter. It has the advantages of wide diameter

measurement range, without changing the fluid flow

state for no measuring element in the fluid, simple

structure, easy installation and maintenance and

other advantages. But it also has some technical

difficulties, such as the measurement precision

sensitive to noise for small output of the ultrasonic

transducer. In this paper, the author will put forward

a new method on how to design an ultrasonic gas

flowmeter, solve the problems mentioned above and

realize the high precision measurement[1] [2].

2 INTRODUCTION OF THE

WORKING PRINCIPLE OF

ULTRASONIC FLOWMETER

Flow measurement based on ultrasonic has several

types: phase difference method, speed difference

method, frequency difference method, ultrasonic

Doppler method and so on [3] [4] [5]. The design in

this paper uses the time difference method, the

system structure is shown in figure 1.

Fig. 1 principle diagram of time difference method.

P1, P2 are two ultrasonic transducers, v is the

flow velocity of fluid, and θ is the angle between

P1P2 and the fluid flow direction; d is the linear

distance between P1 and P2, L is the vertical length

between P1 and P2, c is the sonic velocity. First, P1

is the transmitter and P2 as the receiver, propagation

time is downstream time t1:

)

sin

()

(

cos2











In the formula above, the influence of the

temperature change to the measuring result is

avoided, and the accuracy of the flow measurement

is ensured.

3 THE OVERALL DESIGN OF

ULTRASONIC GAS FLOW

METER

The overall structure of ultrasonic gas flow meter is

shown in Figure 2, including ultrasonic transducer

drive circuit, analog switch, signal processing circuit,

single-chip microcomputer, timing circuit, display

circuit.[6]

Fig. 2 the overall block diagram of ultrasonic gas flow meter.

4 THE CHOICE OF THE

ULTRASONIC TRANSDUCER

AND DESIGN OF THE

DRIVING CIRCUIT

4.1 Selection of Ultrasonic Transducer

Because of measuring gas flow, the center frequency

of the transducer can only be in the range of tens

KHz to hundreds KHz. In order to transducer are: (1)

the center frequency: 120 kHz; get large output

signal, the area of transducer chip must be as large

as possible, with large driving power, strong

ultrasonic signal is output, which will alleviate the

burden of the sequential circuit and improve the

stability and accuracy of the overall system signal.

In this design, the diameter of the transducer

wafer is about 15mm. The performance parameters

of the selected ultrasonic.

(2) the driving pulse amplitude: 300V; (3) band

width: 2 KHz; (4) the static capacitance: 500p (2600

ohm impedance equivalent in 120 kHz).

4.2 Ultrasonic Transducer Drive Circuit

Design

It is not easy to produce amplitude 300V pulse

directly. In this design, the basic ideas is to produce

small amplitude square wave with same frequency

first, then a transformer is used to realise 300V pulse

after amplitude amplification and power

amplification. The diagram of driving circuit is

shown in Figure 3.

Fig. 3 Integral block diagram of driving circuit .

4.3 Primary Pulse Generation Circuit

Design

The initial source of the driving pulse is the pin of

the single chip microcomputer Msp430f149 which

outputs the +3.3V square wave pulse with 120 KHz

frequency. Each transducer of the driving pulse is

divided into two parts, for a transducer using the

microcontroller pin P4.3 and P4.4, and the other one

using pin P4.5 and P4.6. The drive pulse is shown in

figure 4:

Fig. 4 Initial driving pulse

4.4 Power Amplifier Circuit Design

The voltage of transformer primary coil is +30V

while the transform ation ratio is 10:100. MCU

sends out +3.3V pulse amplitude needing amplitude

amplification and power amplification before

reaching +30V. IRF7389 power field effect tube is

used to realise power amplifier, whose parameters

are VDSS = + 30V, on the resistance RDS (ON) =

0.029 ohm (N channel) and 0.058 ohm (P channel),

ID = 4A. Due to its large junction capacitance

(1000PF), IRF7389 needs 15mA to work. A single

pin of Msp430F149 can only output the maximum

current 3mA, unable to meet the requirements.

Therefore additional driver chips must be used. In

this paper, the TC4426EOA field effect transistor

driver is selected, and the driving circuit schematic

diagram is shown in Figure 5.

Fig. 5 Schematic diagram of IRF7389.

As shown in Figure 5, driving circuit based on

TC4426EOA converts the +3.3 V signal to +15V

pulse with 100mA current output. It can drive the

IRF7389 power circuit, the power application circuit

with the transformer is shown in Figure 6.

Fig. 6 Power amplifier and transformer circuit diagram.

Two pieces of IRF7389 is used in the circuit.

Each chip comprises two power field effect tube, a

p-channel and an n-channel tube. When it outputs

high level signal, n-channel tube works. When the

low level signal is outputs, p-channel tube works.

Therefore, it can guarantee the value of the voltage

between the M1 and M9 is 30V; 300V amplitude

can be gotten after 10:100 transformer. At same time,

the current can meet the requirements to drive

ultrasonic transducer. The output signal of the

ultrasonic transducer is shown in Figure 7. The

amplitude of the signal has reached the peak-peak

value of 1V, which provides convenience for the

design of the subsequent processing circuit.

Fig. 7 Ultrasonic signals received by the receiving

transducer.

5 SIGNAL PROCESSING CIRCUIT

DESIGN

Signal processing circuit is the ultrasonic signal

receiving and processing circuit, including:

switching circuit, no-inverting amplifier circuit,

filter circuit, variable gain amplifier and comparison

circuit, timing circuit, LCD display circuit, and

several other parts. The overall structure of the

receiving circuit is shown in Figure 8.

Fig. 8 Overall block diagram of signal receiving circuit.

5.1 Signal Amplification Circuit Design

Midpoint voltage single supply noninverting

amplifying circuit is used in the design who can

realize the signal extraction and amplification

because of its large input impedance and can avoid

the use of negative power supply to reduce the noise

of the system and save the use of components.

Operational amplifier in this design is MAX4331,

circuit as shown in Figure 9.

The relationship between the input signal and the

output signal of the circuit can be calculated by the

following formula:

Figure 9 Midpoint voltage no inverting amplifier circuit

diagram.

Through the above formula, we can know that

the output signal is superimposed on the half power

supply, so in single power circuit positive and

negative signal can be amplified at the same time,

the magnification times are 2. Amplification factor

is small because non inverting amplifier input

impedance is too large to lead to large noise that will

cause difficulty to design the later filter circuit.

6 COMPARISON CIRCUIT DESIGN

In order to accurately measure the time interval, it is

necessary to have a zero crossing comparison circuit,

after which the ultrosnic signal is converted into

square wave to trigger time chip to record the time

of transmission. The comparator in the comparison

circuit is MAXIM Company’s MAX998, the

circuit as shown in fig.

Fig. 10 Schematic diagram of zero crossing.

)

()

(

DDDD

INOUT







)1(

2 DD

INOUT

VV 

7 TIMING MODULE CIRCUIT

DESIGN

Time chip TG-2 is used in this design to measure

time difference. TG-2 has two kinds of modes. In

this design, mode 2 is choiced: 50ps time resolution,

500ns - 4ms measurement range, rising edge or

falling edge trigger. The interface circuit between

microcomputer MSP430f149 and time chip TG-2

circuit is shown in figure .

Figure 11 TDC-GP2 and MSP430 MCU SPI interface

circuit.

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