Development Of The Intelligent Strain Sensor

At present, the traditional strain sensors are used in large quantities, and in a longer period of time, they will still hold a considerable share in the market. As the digital information age comes, the intelligent measuring instruments will be used more and more, therefore, the development of a common intelligent sensor module, which can improve the analog measurement system to a digital & intelligent one, will be a shortcut to develop the intelligent measuring instruments. In this background, based the on the mechanical signal detection part of the project "the development of MWD system for geophysical parameters ", China Geological Survey Bureau, a subject about intelligent strain sensors is raised, which is to develop a common intelligent software & hardware module, and to lay a foundation for the study of intelligent instruments based on the strain sensors.Now, the technology of traditional strain sensors has been improved greatly, by an increasingly improved design and manufacturing technology. However, there are still a lot of deficiencies about these traditional sensors technically. Due to the progress of modern electronic technology and integrated circuit technology, not only the design of measurement & control system is more convenient, but also it is much easier to intelligentize the measurement & control instruments. The intelligent strain sensor has many functional advantages that the traditional sensor does not have. There are two structures of intelligent sensors: overall and separate. The separate intelligent sensor not only retains the analog sensor performance, but has all the advantages and functions of the overall intelligent sensor, the work environment of the A/D converter circuit is also changed greatly. Therefore, the separate structure is proposed in the design of intelligent strain sensor system.The hardware of this sensor system includes the circuit to acquire the sensor output signal and the interface circuits between the microcontroller and the external modules. In order to achieve high-precision and miniaturization of the sensor system, the sensor signal is directly interfaced to the AD converter. In this article, the AD7705 is a 16-Bit, Sigma-Delta ADC, and the ratio method of measuring is used to overcome system errors introduced from the variations of excitation source and reference source. The AD7705 is compatible with SPI interface protocols, and the main synchronous serial port (MSSP) module of the PIC16F877A is configured in the SPI interface mode. Taking into account of the capacity constraint of MCU internal EEPROM, I choose 25LC640 to store the result data. The 25LC640 is also compatible with SPI bus interface. The LCD module (LCM) JM1602C is used to display not only the measuring results, but also the suggesting information. To achieve the serial communication function between the intelligent sensor and the computer (PC, industrial, etc.), the transceivers MAX232A/MAX488 are interfaced between TTL signals and RS-232/RS-485 standards, which are selectable based on the distance between the sensor working site and the computer. PORTB of the MCU can be programmed for internal weak pull-ups on all inputs, which make the design of keyboard circuit simple. The sound & light alarm circuit based on one piezoelectric buzzer and one highlighted LED, is used to discover emergencies and exceptional states.The system software includes MCU software and PC software. With the powerful MPLAB IDE (Integrated Development Environment) and the MPLAB ICD2 (In-Circuit Debugger and Programmer) that is suitable for laboratory use, the development of the MCU software is completed. The MCU software includes a main program, an interrupt procedure and some subroutine modules, These subroutines includes a system initialization subroutine, a sensor calibration subroutine, a node configuration subroutine, a filtering software subroutine, a linear interpolation operation subroutine, an EEPROM read/write subroutines, a LCD module display subroutine, and so on. Using Visual C++ 6.0 development environment, an operating platform for intelligent strain sensor system is developed, and it includes one main interface and two pop-up interfaces. The relevant serial communication protocols are designed in accordance with the general rule of designing the protocols. Make sure the serial port is opened in the main interface when the procedure begins. If the sensor calibration is needed, the calibration interface will pop up, and if the automatic mode of operating is selected, the interface of setting automatic mode parameters will pop up.To evaluate the performance of the intelligent strain sensor system, interfacing a load cell to the intelligent sensor module, an intelligent load cell system is formed. After a large number of experiments, a lot of experimental data are collected, and the test results are acquired. According to the experimental data, the relative error of the intelligent load cell system is less than 0.05%, while the error of the load cell is 0.02%, so the accuracy of the load cell system has reached a high level. In the same circumstances for repeated experiments, the three coordinate curves of data error have the similar trends, therefore it can be inferred that the repeatability of the intelligent load cell is good.