Key words: resistance strain sensor, differential amplifier, liquid crystal.
Digital dynamometer design based on resistance strain gage.
Zhang Chongbin's health core exhibition falls in Jie Jun
Huanghuai college
This paper introduces the design and implementation of electronic dynamometer based on resistance strain sensor. The system is controlled by 5 1 single chip microcomputer, and the measured grip size is automatically displayed by LCD screen. The circuit includes measuring circuit, differential amplifier, A/D conversion and display circuit. The measuring circuit uses the power of the resistance strain device to convert the size signal of an object into a corresponding electrical signal, and the electrical signal will be amplified into the AD chip for identification through the differential amplifier circuit, so that the electrical signal can be converted into a corresponding digital signal, and finally the measurement is carried out on the liuid liquid crystal display.
Keywords: strain sensor, differential amplifier,
I. Design tasks and requirements
1. 1 design task
1. Design a digital grip strength meter, which can display the current strength value.
2. Display the measured force with numbers.
Design idea of 1.2
The measuring process of digital grip strength instrument is to convert the force of the measured object into voltage through resistance strain sensor. Because this is usually very small, it needs to be amplified. The amplified analog is converted into digital quantity by analog/digital chip, and then the intensity is displayed by display.
Working principle of 1.3
When the digital grip strength meter detects the measured object (mainly the arm strength), its force deforms the spring of the grip strength meter and passes through the resistance strain gauge sensor, which then produces the corresponding electrical effect, converting the force on the object into electricity (voltage or current). This is amplified and filtered by an amplifier circuit, then converted by an analog-to-digital (A/D) converter, and the converted digital is judged by a single chip microcomputer, and then sent to a display circuit.
Second, the scheme design and demonstration
2. 1 Overall plan
The resistance strain gauge generates voltage, and the amplified voltage is input to the A/D conversion chip TCL for A/D conversion. Because this chip can be directly used for digital display, the converted digital quantity is directly displayed by the digital display. The advantage of this scheme is that the external circuit is very simple and can achieve high accuracy.
Third, the choice of electronic components.
3. 1 sensor selection
The invention has the advantages of high precision, wide measuring range, long service life, simple structure, good frequency response characteristics, capability of working under harsh conditions, and easy realization of miniaturization, integration and variety diversification.
3.2 Selection of Operational Amplifiers
There are molded integrated operational amplifiers in this field, such as amplifiers for AD instruments, which can be directly used in the amplification part of the design. The integrated chip has more stable performance and smaller error than its own operational amplifier circuit. However, the price of integrated operational amplifier is relatively high, and the error range of its own overlapping operational amplifier circuit has basically met the requirements of this design, so we choose OP07 single operational amplifier to overlap differential operational amplifier, and also exercise the practical ability of analog circuit to some extent.
3.3 Selection of Analog-to-Digital Conversion (A/D) Chip
According to the requirements of this project, in order to meet the resolution of the lowest bit, A/D chip with high bit number must be selected. Serial TLC chip driver is a little more complicated than parallel A/D, but according to the comparison of retail price, this chip is the cheapest chip that meets the requirements. Based on the principle of minimizing the project cost, we finally chose this chip.
3.4 Selection of monitors
Select Ca LCM dot matrix liquid crystal to display the enlarged data.
Fourthly, the concrete implementation scheme of each hardware module.
The system hardware includes power module, data acquisition and amplification module, A/D conversion module, automatic conversion module and digital tube display module.
4. 1 power module
Because the system hardware has OP07, and the operational amplifier circuit needs plus or minus 9 power supply, so the power supply needed by the whole amplifier circuit can be directly used with rechargeable lithium batteries.
4.2 Sensor module
The resistance strain sensor converts the change of measured physical quantity into the change of resistance value, and then displays or records the change of measured value through the corresponding measuring circuit. Here, we use the resistance strain sensor as the core of the measuring circuit. Accuracy and range should be properly selected according to the requirements of the measured object.
4.3. Differential amplifier circuit
The digital grip strength instrument adopts full-bridge resistance strain sensor, and its core is a bridge circuit composed of resistance strain gauges. This sensor has the characteristics of low cost, high precision and good temperature stability. However, its detection principle determines that the output voltage of this kind of sensor is low, and it must be amplified by differential amplifier circuit for hundreds of times before it can be used for A/D conversion. Generally speaking, the output voltage of the sensor is very small, basically millivolts or even microvolts, and an external amplifier circuit is needed to obtain sufficient gain.
There are many types of instrumentation amplifiers. This paper introduces a widely used instrument amplifier, which is a typical differential amplifier. It only needs high-precision OP07 and several resistors to form an instrument amplifier with superior performance. Widely used in industrial automatic control, instrumentation, electrical measurement and other digital acquisition systems.
OP07 chip is a bipolar operational amplifier integrated circuit, which has low noise and zero stability and does not need chopping. Because of its extremely low input offset voltage, OP07 does not need additional zero adjustment measures in many applications. OP07 has the characteristics of low input bias current and high open-loop gain. This low offset and high open-loop gain make OP07 especially suitable for high-gain measuring equipment and weak amplification sensors.
4.4, analog-to-digital conversion
In the actual measurement and control system, it is often a physical quantity with continuous changes in time and value, which is called analog quantity, and the corresponding electricity is analog electricity. Analog output shows that LCM lattice liquid crystal can be received and processed by analog-to-digital conversion.
Verb (abbreviation of verb) system debugging
5. 1 debugging process
(1). First of all, make sure that the indicator accurately shows zero when the dynamometer is empty.
(2) Then, hold the power arm and apply different forces to observe whether the display accurately shows the force. If there is any deviation, sample 15 times and take the average.
(3) Zero stability is a very important factor affecting the accuracy of the grip dynamometer, and it will be unstable due to temperature or other factors, which is called zero drift. Due to the influence of zero drift, the output may not be zero when the input is zero. In order to eliminate this zero drift value, zero compensation technology is adopted. Zero compensation is to store this zero drift value and subtract this value every time data is collected, and the obtained value is effective for eliminating zero drift.
Systemic function of intransitive verbs
The measuring range of digital grip strength meter is divided into four grades, 0~ 1. Kg, 0~ 19.99 kg, 0 ~ 9 kg, 0 ~ kg. The measured grip strength is displayed by numbers, and the decimal point position corresponds to different ranges, and it has the function of automatic range switching, which can adapt to people of different levels.
Seven. Design summary
At present, there are few small digital grip meters on the market, but they are developing towards miniaturization, high precision and intelligence. TCL adopts small package, small volume and few components, which meets the requirements of miniaturization of grip strength instrument. Various control registers and data registers are built in, which can be easily controlled and read through SPI interface to meet the intelligent requirements of grip strength instrument.
In the course design of electronic technology, we have spent a lot of time and energy on data consulting and scheme demonstration, and combined with our own study, we have seriously solved the problems encountered in each functional module.
We also used the simulation software Proteus 6 Professional to simulate some functional modules, and achieved good results.
Eight. refer to
[1] gang. The application of single chip microcomputer principle machine. Higher education press.
Wang Huaxiang, Zhang Shuying. Principle and application of sensor. Tianjin: Tianjin University Press.
Shi Yan. Fundamentals of digital electronics technology. Higher education press.
[1] children. Basic analog part of electronic technology. Higher education press.
About the author:
Chang (-), male, Minqin, Gansu, undergraduate, network engineer, professional direction: electronic science and technology;
Hua, male, 25, from Xinyang, Henan, undergraduate, Department of Electronic Science and Engineering, Huanghuai University;
Zhan Baobin, male, from Baiyin, Gansu, aged 23, graduated from the Information Engineering Department of Huanghuai University.
SETTING: Department of Electronic Science and Engineering, Huanghuai College, Zhumadian City, Henan Province. It's really hard to make it clear at once. It's best to go to hard city.