introduction
Studies have shown that different frequencies and intensities of ultrasound have different effects on the body. Secondly, temperature also has a certain effect on transdermal administration. For example, a certain temperature can increase the penetration rate, but too high a temperature can cause skin burns because of ultrasound There is also a thermal effect; therefore, how to comprehensively consider various factors in order to achieve a higher penetration rate while avoiding burns to the human body and damage to physiological functions is one of the difficulties in research.
As an intelligent control chip, the single chip microcomputer plays an extremely important role in various fields of electronic information and automatic control. The design and development of medical instruments also use more and more single-chip technology to make them develop towards intelligence and miniaturization. Comprehensive consideration of various factors, here introduced a high-frequency single-chip microcomputer C8051F340 as the core control chip, the output frequency, intensity and other adjustable, controllable constant temperature low-frequency ultrasonic transdermal system.
1 Hardware design
The system includes: temperature measurement and control module, power ultrasonic generator, constant temperature control module, overcurrent protection module, display module and power-down storage module. The overall structure is shown in Figure 1.
The work of each part of the system is controlled and coordinated by the high-performance MCU C8051F340. It is the core of the entire system. C8051F340 single chip microcomputer is a mixed system level single chip microcomputer integrated on one chip. It has a CIP-51 core that is fully compatible with the 8051 instruction set and uses a pipelined instruction structure. Its running speed can be up to 48 MIPS. Compared with the standard 8051, the single-cycle instruction running speed is 12 times the same under the same clock. And, it integrates the functional components required to form a single chip data acquisition or control system in a chip; analog multiplexer, programmable gain amplifier, ADC, voltage comparator, voltage reference, temperature sensor, SMBus / I2C, Almost all analog and digital peripherals and other functional components such as UART, SPI, USB programmable counter / timer array (PCA), timers, I / O ports, internal oscillators, watchdog timers, and power monitors. Through programming the C8051F340, a variety of functions can be achieved.
The system uses the keyboard input circuit to obtain the operator's requirements for the working conditions of each part. According to these requirements, the heating wire and the ultrasonic emission are set. At the same time, the temperature value of each part is sent to the LCD for display during each measurement and control, and the Limit judgment, when the temperature exceeds the limit, the alarm circuit is activated to issue an alarm. The temperature control adopts the power adjustment method to control the heating of the electric furnace wire to keep the temperature constant during the experiment, including reading the temperature, processing the temperature value, and outputting the signal to drive the load.
1.1 Temperature measurement and control module
The temperature sensor of this system adopts PT100 platinum resistance temperature sensor, which has the advantages of good linearity, wide measurement range, high sensitivity and no need for reference point. Using the temperature resistance characteristics of platinum resistance, the temperature signal is directly converted into an electrical signal. Then, after pre-amplification, it is sent to the 12-bit switched capacitor successive approximation type A / D converter in the single-chip microcomputer to complete the analog-to-digital conversion. In the electronic measurement system, it is necessary to detect a large amount of electricity or non-electricity information, and the detected electrical signal is very weak, and it is very vulnerable to interference. Therefore, the following requirements are required for the amplification of the pre-stage acquisition signal: high input impedance, high common mode rejection ratio, low drift, low noise, and low output resistance. Based on the above principles, the preamplifier selects the low-voltage universal dual-channel instrumentation amplifier INA2128 produced by the US BURR-BROWN company. In addition to meeting the above requirements, it can easily set the gain from 1 to 100,000 with an external resistor, so that the INA2128 can Widely used in signal acquisition and amplification. Many fields, such as medical instruments and multi-channel systems, can work at a power supply voltage as low as +2.25 V, and the static working current is very small. Its temperature detection and pre-amplification with Pt100 are shown in Figure 2.
INA2128 is a high-performance measuring amplifier composed of overvoltage protection circuit and three op amps; A1 and A2 form a dual-end input / dual-end output differential amplifier. Because the signal is input from the two non-inverting ends, the input impedance is high 10 MΩ or more. The second stage uses a differential input. When the op amp parameters and R5 are strictly symmetrical, the circuit has high common-mode rejection and low temperature drift.
The above formula shows that the circuit can control gain by adjusting RG.
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