Displacement Measurement System Based on Digital Displacement Sensor

0 Introduction
In many computer application systems, parallel interface and RS 232 serial universal interface are the two most commonly used standard interfaces. The standard parallel interface adopts 8-bit parallel forward transmission, which is characterized by fast transmission speed and short transmission distance. RS 232 serial interface is often used for the connection between data terminal equipment and data communication equipment, which is characterized by relatively slow transmission speed but long transmission distance.
The primary mirror focusing of the Yuntai One-meter Infrared Solar Telescope (hereinafter referred to as YNST) will adopt a displacement measurement system based on digital displacement sensor. Among the proposed scientific goals, the focusing range of M3 mirror in the primary mirror focusing is required to be 0~20 mm, and the displacement measurement value is displayed in real time on the digital display or control software. The transmission rate requirement is not high, but the transmission distance must be 10~15 m. The high-precision digital displacement sensor selected by the system only provides a standard parallel port (SPP). The SPP data transmission line connecting it to the host computer is short. Therefore, under the premise of ensuring that the data transmission rate of the system feedback link meets the system requirements, an SPP to RS 232 interface circuit is connected between the host computer and the digital displacement sensor to solve the problem of short transmission distance.

1 System structure and working principle of displacement sensor
1.1 System structure
The overall structure of the system is shown in Figure 1. The system mainly consists of a host computer, motion control link and displacement feedback link. The host computer is an industrial control computer with at least two COM ports. The motion control link is mainly composed of a motion controller, a driver, a hybrid stepper motor and a horizontal mechanical motion device; the displacement feedback link is mainly composed of a digital displacement sensor with a digital display and an SPP to RS 232 interface circuit. The digital displacement sensor model is 5CB~10C, the displacement measurement range is 0~20 mm, the digital display reading range is 0~19.999 mm, the resolution is 1μm, and the linearity reaches 0.05% of the full scale, that is, the accuracy reaches 10μm.

1.2 Working Principle of Digital Displacement Sensor
The digital displacement sensor consists of a differential transformer (LVDT) and an electrical measuring instrument. LVDT is a sensor that converts the measured displacement into an electrical signal. It has the characteristics of simple structure, easy use, and long service life. It can be directly used to measure the relative displacement between objects and the length change of objects. It can measure not only static displacement but also dynamic displacement. The electrical measuring instrument consists of an electronic measurement circuit, a digital panel meter, and an A/D conversion board. The A/D conversion board has a single-ended 16-channel analog input, the A/D conversion bit is 14 bits, and the conversion speed reaches 10μs. The most basic structure of LVDT is composed of a coil consisting of a spiral primary and two secondary windings wound on a cylindrical frame and a movable iron core. Its principle is shown in Figure 2.

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When an oscillating voltage is supplied to the primary side, due to electromagnetic induction, the two secondary sides generate induced voltages V1 and V2 respectively. If the iron core is at the center of the coil, the mutual inductance of the two secondary sides to the primary side is exactly equal. At the same time, the AC voltages of the two secondary sides are detected by the detection circuit respectively, and the difference between the two DC voltages is taken, then the output differential DC voltage V is zero. When the iron core moves upward, the mutual inductance between the upper secondary side and the primary side increases, while that of the lower side decreases, that is, V1>V2 appears, then the differential DC output voltage V>0, and vice versa. The qualitative relationship between the induced voltages V1, V2 of the two secondary sides of the LVDT and the differential DC output voltage V and the position of the iron core in the coil is shown in Figure 3. It can be seen that the differential DC voltage V is linearly related to the size of the iron core displacement within a certain range. When measuring, the LVDT housing is clamped on the reference object, and the measuring rod connected to the iron core is fastened to the measured point. When the measured object is displaced, the iron core is driven to move relative to the coil, thereby linearly changing the output voltage of the LVDT. In this way, the displacement of the object being measured can be measured by measuring the output voltage of the LVDT with an electrical measuring instrument.

2 Implementation of data acquisition interface hardware and software
The motion controller in the system is independently developed by the Yuntai Optoelectronics Laboratory. It communicates with the host computer through RS 232 or RS 485 serial interface. The serial port signal format is: COM1 port, baud rate 4 800 b/s, no parity check, 8 data bits, 1 stop bit. The digital displacement sensor in the displacement feedback link only provides a standard parallel interface to connect with peripheral devices. Its A/D card occupies pins 2 to 9 as input ports and pins 10 to 13 as output ports. The input and output levels of its ports are both TTL levels. The SPP to RS 232 interface circuit with AT89C51 microcontroller as the core sends the operation status to the A/D by writing the input port, and reads the output port to obtain the A/D conversion data and other information. The A/D conversion result is a 14-bit binary number, which is read in two groups. The first group reads the high 8 bits first, and then the second group reads the low 6 bits. Each group is read in two small times, 4 bits each time, and the last time is 2 bits.
2.1 Hardware design of data acquisition interface circuit of the system
Figure 4 is the schematic diagram of the SPP to RS 232 interface circuit with AT89C51 as the core. AT89C51 is a low-power, low-price, high-performance CMOS 8-bit microcontroller with 4 KB flash programmable read-only memory. The P1 port of the microcontroller is connected to the input port of the digital displacement sensor to input the operating status to the A/D in the sensor. The high four bits of the P2 port are connected to the output port of the sensor to read the 14-bit data information converted by the A/D.

2.2 System software design
Figure 5 is a program flow chart, in which Figure 5(a) is a program flow chart of the upper computer mainly controlling the rotation of the stepper motor through the motion controller, and Figure 5(b) is a program flow chart of the displacement feedback of the system feedback link. Figure 6 is the upper computer control software interface written in VC. The upper computer control software calls the MSComm control to realize the serial communication between the upper computer and the motion control link and the position feedback link. The upper computer receives the 14-bit data sent by the interface circuit, shifts and processes the data, and finally displays it in real time on the control software interface. [page]

3 Displacement measurement experiment
First, use Protel and KeiI software to simulate the SPP to RS 232 interface, design and make the PCB board of the interface circuit, select the devices for welding after processing, write the assembly program, and download the program to the single-chip computer through the download line after debugging. Assemble the entire system and conduct the displacement measurement experiment. The stepper motor is connected to a horizontal moving mechanical device with high precision and a reading scale. The upper computer controls the rotation of the stepper motor through the motion controller, thereby driving the movement of the mechanical device.
It is the displacement measurement experimental data. It can be seen from Table 1 that the displacement measurement results are stable, the error is small compared with the actual displacement value of the mechanical device, and the measurement accuracy is relatively high. In addition, during the experiment, the system response time is short and the real-time performance is high, which meets the requirements of the M3 mirror focusing in the primary mirror focusing of the infrared solar telescope scientific target;

4 Conclusion
The displacement measurement system based on digital displacement sensor is a closed-loop control system consisting of motion control link and displacement feedback link. The main part of the system is developed by the laboratory, with low cost, stable operation and strong anti-interference ability. Experiments show that the displacement measurement accuracy and real-time performance of the system are high, which can meet the requirements of scientific research. The system will be put into trial use at the end of 2009.