Due to the limitation of early electronic technology, the old ultrasonic reflection thickness gauge is pure hardware structure, and its electrical and physical properties are not satisfactory. With the continuous progress of electronic technology. Especially with the rapid development of single chip microcomputer technology, it has become inevitable to carry out intelligent transformation of ultrasonic thickness gauge. The ultrasonic thickness gauge designed and manufactured by single chip microcomputer can make the thickness calibration, sound speed setting and sound speed adjustment of the thickness gauge intelligent. The whole measurement process is operated by key switch, which realizes the PVC mask structure, simplifies the circuit and reduces the cost and power consumption. This paper will introduce the working principle and intelligent design of ultrasonic reflection thickness gauge to readers.

2 working principle of ultrasonic reflection thickness gauge

The ultrasonic probe is used to send ultrasonic pulse to the measured object, which propagates in the measured object and reflects when it propagates to the bottom surface of the measured object, and the reflected ultrasonic wave is received by the ultrasonic probe. In this way, the time t from the ultrasonic probe sending out the ultrasonic pulse to the ultrasonic probe receiving the reflected pulse can be accurately detected by the circuit, and during the time t, the ultrasonic wave completes a round trip in the measured object. If the thickness of the measured object is represented by d, the total stroke of the ultrasonic wave is 2d. Since the propagation speed c of sound waves in an object is constant (for example, the sound speed of steel is 5950m/s and that of glass is 5570m/s), the thickness of the measured object can be calculated by the following formula:

The actual working process of the thickness gauge is shown in Figure 2- 1. Firstly, the high-voltage narrow pulse T generated by the circuit is transmitted to the ultrasonic probe. The ultrasonic probe converts the high-voltage electric pulse into an ultrasonic pulse with the same frequency, which propagates to the surface of the measured object through the coupling agent. Part of the surface of the measured object is reflected, which is the upper surface wave S, and the rest is injected into the measured object, and then reflected back from the bottom of the measured object, which is the bottom reflected wave B. The reflected wave B is received by the ultrasonic probe, converted into an electrical signal and sent to the circuit. After amplification, it is sent to the gate circuit together with the transmitting pulse to generate a gate pulse. When the measured object is thick, the ultrasonic beam propagates in the object for a long time, resulting in a wide strobe. When the measured object is thin, the propagation time of ultrasonic beam in the measured object is short, and the gating pulse is narrow. The gate pulse and the clock pulse are sent to the AND gate circuit. Because the pulse width of the gate is different, the number of clock pulses output by the gate is different, which reflects the thickness value of the measured object. The number of pulses output by the AND gate is converted and sent to the display circuit to display the actual thickness of the measured object.

Figure 2- 1 Work Flow Chart of Thickness Gauge

The ultrasonic probe adopts an independent transceiver, and its structural schematic diagram is shown in Figure 2-2.

Figure 2-2 Schematic Diagram of Ultrasonic Probe Structure

The transmitting and receiving parts are made in the shell and separated by the isolation layer. The transmitting and receiving parts have the same characteristics and can be interchanged in use. The core element of the probe is piezoelectric ceramic sheet, which has the characteristics of acoustic-electrical conversion. When used as a transmitter, the electric pulse emitted by the circuit is converted into ultrasonic pulse. When used for receiving, it can also convert the received ultrasonic waves into electrical signals and send them back to the circuit. The electrodes on both sides of the piezoelectric ceramic sheet are respectively connected with the inner conductor and the outer conductor of the coaxial cable.

Intelligent design of ultrasonic reflection thickness gauge

See Figure 3- 1 for simplified hardware block diagram of intelligent design of ultrasonic reflection thickness gauge. Due to the limitation of space, the non-intelligent units below the dotted line are only introduced in the block diagram. The intelligent unit above the dotted line will be discussed in detail in the form of schematic diagram.

Figure 3- 1 Hardware Diagram of Thickness Gauge

The non-intelligent unit consists of six parts. Firstly, the transmitting pulse oscillator generates a square wave with a frequency of 2kHz, which is sent to the transmitting pulse shaping and amplifying circuit, making it a narrow pulse with a pulse width of 3μs and an amplitude of 100V to drive the ultrasonic probe. The ultrasonic probe converts the electric pulse into an ultrasonic beam, which is reflected at the bottom of the measured object. The reflected wave is received by the ultrasonic probe and converted into an electric pulse, but at this time, the pulse amplitude has been attenuated to several millivolts in the process of propagation and conversion. Then, the received reflected pulse is transmitted to a reflected pulse amplifier to be amplified to a logic level, and is transmitted to a gate generation circuit together with the transmitted pulse to generate a gate pulse. The leading edge and trailing edge of the gate pulse are determined by the transmitted pulse and the reflected pulse respectively, that is, the gate width is the time taken for the ultrasonic wave to enter the measured object and be reflected. Finally, the gate pulse and the clock pulse of 12.8MHz are sent to the AND gate circuit. When the gate pulse arrives, the clock pulse is output through the AND gate. When the gate pulse does not arrive, the AND gate has no clock pulse output. The number of clock pulses passed by AND gate can reflect the thickness of the measured object and send it to the intelligent unit for relevant processing.

Single chip microcomputer 89C5 1 and its peripheral devices constitute an intelligent unit. 89C5 1 is an 8-bit single-chip microcomputer with low power consumption, including a 4k byte flash EPROM. Its instruction system is fully compatible with 80C5 1. Each function is designed as follows.

(1) thickness signal acquisition and output display

The dual-input AND gate 74HC00 outputs a gate pulse under the control of the gate pulse, which is divided by CD4520 and then added to the timing/counting port P3.4 of the single chip microcomputer for acquisition. Frequency division of 256 is necessary to complete the scale conversion between pulse number and thickness value. The measurement of the instrument adopts the measurement signal interruption mode, and no interruption signal is generated when it does not enter the measurement state. When entering the measurement state, the ultrasonic probe receives the reflected pulse, and the strobe generator generates the strobe, which is sent to the interrupt signal generator to generate an interrupt request signal to control the external interrupt port p 3.2 CPU to respond to the interrupt, execute the measurement program, collect the strobe and process the related data. The processed thickness signal is output from the port P 1. 1 of the single chip microcomputer to the input port CI of ICM7224, and then passes through ICM7224. ICM7224 is a single-chip CMOS4-bit counting/decoding //LCD driver. In this design, only four integer bits are used, and the first half is not used, so it is suspended when used. The 7-segment stroke pin of each bit is directly connected with the corresponding stroke segment of the liquid crystal. The BP pin in the figure is the back electrode driving end, which is used to drive the anode electrode of the liquid crystal. The MCU P 1.4 and P 1.5 output control signals to control the latch and reset of ICM7224.

(2) Keyboard interface

In Figure 3- 1, K 1 ~ K4 are four function keys: sound speed setting, thickness calibration, sound speed improvement and sound speed reduction, and the function keys are scanned intermittently. When the key is not pressed, under the action of the pull-up resistor R6 5438+0 ~ R4, the four input terminals of the gate circuit CD4082 and the ports P 1.0, P 1.2 and P 1.6 of the single chip microcomputer are at high level, and the output terminal of CD4082 is also at high level. When the key is pressed, the input terminal of CD4082 is grounded, the output terminal becomes low level, and an interrupt request is sent to CPU. The microcontroller responds to the interrupt, executes the interrupt service program, scans the keyboard, judges the levels of P 1.0, P 1.2, P 1.3, P 1.6, and determines which port to press.

(3) Start and close control

Analog switch CD4066 and key K5 constitute the power switch circuit. When K5 is pressed, the single-chip microcomputer turns on the 5V power supply, and the switch control program makes the P3.0 port output the high-level control terminal of the CD4066 to turn it on, and then supply power to the system. There is a timing circuit in the single chip microcomputer. As soon as the thickness gauge stops measuring, the timing program starts timing. When it reaches 60s, the P3.0 port becomes low level, and the switch of CD4066 is turned off, thus realizing automatic shutdown with delay.

(4) Thickness calibration

Due to the influence of ambient temperature and working voltage, some parameters of the measuring system often drift, so the thickness of the instrument should be calibrated when measuring. The calibration operation is to measure a 5 mm thick standard test block with an instrument. If the instrument display is not equal to 5mm, it is necessary to calibrate the thickness. The old thickness gauge is realized by adjusting the knob of potentiometer. This design is completed by the thickness calibration key. When calibration is needed, press the thickness calibration key, and the single chip microcomputer executes the thickness calibration program, compares the measured results with the standard thickness, and adds and subtracts according to the results to complete the calibration.

(5) Sound speed setting

When measuring the thickness of different materials, the ultrasonic thickness gauge needs to set the sound speed of materials, such as the sound speed of steel 5950 m/s and the sound speed of zinc 4 170 m/s, because the propagation speed of ultrasonic waves in different materials is different. This operation is completed by mechanical 3-bit BCD code disk switch and preset counter to control the number of gates. For materials with high sound velocity, there are more doors, while for materials with low sound velocity, there are fewer doors. In this design, the sound speed of nine typical materials, such as cast steel, steel, aluminum, copper, brass, zinc, timely glass, polyethylene and polyvinyl chloride, is put into the storage unit in the form of a table, and the sound speed setting key is used to select, and then the emission pulse is collected and counted through the P 1.7 port of the single chip microcomputer, so as to control the opening times of the gate and make the sound speed setting operation very simple.

(6) Sound speed adjustment

The sound speed setting of the above nine typical materials is selected by the sound speed setting key. When measuring the thickness of atypical materials, the setting of sound speed is completed by two function keys: sound speed increase and sound speed decrease. Press the button to increase or decrease the sound speed, and the sound speed displayed on the LCD will increase or decrease until the required sound speed is adjusted. At this time, the sound speed adjustment program will store the adjusted value in the sound speed storage unit for later use.

(7) Decimal control and power-on reset circuit

The 4-bit LCD also displays the sound speed and thickness. The unit of sound speed is m/s, for example, the sound speed of steel is 5950 m/s, so the LCD does not need to display decimal points. Display the thickness in mm, and the resolution is 0.1mm. When measuring the material with the thickness of 10mm, the LCD screen displays 0 10.0mm with decimal point display. P3. 1 port of single chip microcomputer is used for decimal point control, which shows high level when sound speed is displayed and low level when thickness is displayed. Under the function of exclusive OR gate CD4077, the presence or absence of decimal point before the unit is controlled.

The function of C 1 and R5 in Figure 3- 1 is to complete the power-on reset of the single chip microcomputer.

4 conclusion

After intelligent transformation, the ultrasonic reflection thickness gauge has received satisfactory results. After being tried by some users, the response is good, and the main technical indicators that can be achieved are as follows:

(1) display mode: 4-bit LCD display.

(2) Thickness measurement range: 001.0 ~ 200.0mm.

(3) Measurement accuracy: 0.65438 0 mm.

(4) Adjustment range of sound speed: 100 ~ 9990 m/s.

(5) Working voltage: DC 5V

(6) Working current: 8mA

(7) Automatic shutdown delay: 60 seconds.