Abstract: This article focuses on the working principle of a straightness measuring instrument, the overall design, hardware design, and software design of a straightness measuring instrument for large machine tool guideways, as well as the algorithm for evaluating the straightness error of the straightness measuring instrument for large machine tool guideways.
Foreword
In recent years, with the increasingly widespread and mature application of automation technology, computer technology, and communication technology in the industrial field, and the gradual increase in the production of large machine tools, it is imperative to design a scientific and advanced large machine tool guideway straightness measuring instrument using modern information technology in order to improve the linear motion accuracy of machine tool guideways and enhance the working performance of large machine tools.
1. Working principle of a straightness measuring instrument
First, place the semiconductor laser on one side of the straight section to be measured, so that the light emitted by it is parallel to the guide rail of the machine tool under test. Then, place the wireless optical target on the guide rail of the machine tool under test, so that the light emitted by the laser falls exactly at the center of the two photocells. At the same time, using the light emitted by the laser as the straightness measurement reference, move the wireless optical target along the straight section of the guide rail under test. When the laser shines on the photocell in the wireless optical target, the two photodiodes will simultaneously output a weak current signal proportional to the illuminated area [1]. After these two weak current signals are amplified by the preamplifier of the amplifier circuit and converted into voltage signals, the range of movement of the wireless optical target can be predicted based on the voltage difference generated by the two photocells, thereby calculating the straightness of the guide rail under test.
Finally, the processed signal is input into the microcontroller, converted into a digital signal, and then divided to obtain a signal reflecting the position of the incident light spot. This signal is then transmitted to the data acquisition module via the wireless data transmission module in the wireless light target. After receiving the signal, the data acquisition module transmits it to the PC, where the PC performs the final data processing.
2. Design of a straightness measuring instrument for large machine tool guideways
2.1 Overall Design
According to the specific purpose of the large machine tool guide rail straightness measuring instrument, the article divides its system design into four major functional modules, including data acquisition module, semiconductor laser, wireless optical target and PC. Among them, the data acquisition module is mainly composed of two parts: microcontroller and wireless data transmission, the wireless optical target is mainly composed of four parts: amplifier circuit, microcontroller, photocell and wireless data transmission, and the PC is mainly composed of four parts: database, serial communication, straightness calculation and drawing [2]. While each module completes its assigned responsibilities according to the established procedure, they also work together to complete the straightness measurement of large machine tool guide rails well.
2.2 Hardware Design
Amplifier Circuit Design. The design of the amplifier circuit requires the use of photoelectric sensors. Common photoelectric sensors include photodiodes and four-quadrant photovoltaic cells. The illuminance of a photoelectric sensor is proportional to the reverse current, offering advantages such as fast response, low noise, and convenience, and is often used in laser detection. Four-quadrant photovoltaic cells are commonly used in photoelectric switches and optocouplers. Different photoelectric sensors have different characteristics and applicable ranges. For the amplifier circuit of a large machine tool guideway straightness measurement system, this article selects a photodiode. Since the photoelectric output is a weak analog signal, easily affected by external environmental interference, the preamplifier needs to preprocess the analog signal, i.e., it should integrate multiple capabilities such as high input impedance and low output impedance. This article selects the ICL7650 series integrated operational amplifier as the preamplifier.
Microcontroller selection. In the design of a large machine tool guide rail straightness measuring instrument, the microcontroller is the most core component of the entire system. It controls the operation of each part of the system and completes data acquisition, processing and other operations together with other components [3]. For the selection of microcontroller, low power consumption and high cost performance should be selected as much as possible to reduce energy consumption and extend the service life of the equipment. This article selects the MSP430 microcontroller with ultra-low power consumption, multiple memory forms, powerful data processing capabilities and efficient development environment as the control center of the entire straightness measurement system. This microcontroller can provide a reliable guarantee for the low power operation of the system and the full utilization of the equipment.
2.3 Software Design
CRC check protocol writing. Due to the interference of the external environment or other factors during the actual operation of the system, data transmission errors are very likely to occur. Therefore, in order to ensure the integrity, correctness and reliability of data transmission, it is necessary to use corresponding check methods to check the data communication. At present, the commonly used check methods are mainly Cyclic Redundancy Check (CRC) and Parity Check. Cyclic Redundancy Check codes can be implemented in hardware or software. This article adopts the software method to implement cyclic redundancy check for data communication. First, set all 1 bits of a 16-bit register, perform an XOR operation on the 8 bytes in the data packet with its current value, shift the low bits to the high bits, and fill the high bits with 0. Then, judge the value of the shifted-out lowest bit. If the shifted-out value is 0, no operation is performed; if the shifted-out value is 1, the register is XORed with a preset fixed value once, and the above operation is repeated until all 8 bits are shifted [4]. When the last bit shift is completed, the next 8 bytes are XORed with the current value of the register. After all the data in the data packet is processed according to the same operation method, the generated data sequence is the CRC check code. The flowchart of this step is shown in Figure 1.
Figure 1 Flowchart of the CRC check subroutine
Implementation of the communication function. The communication function needs to be implemented according to the following steps: In the VB programming software, use the MSComm control to initialize the serial port, set the verification method and baud rate using port 232, load the CRC checksum, load and initialize the uplink and downlink timers, and test whether the communication connection is normal. At this time, the serial port sends commands to the microcontroller according to the preset time interval, and records the number of transmissions. If the data function bit is 0 and a data signal is received, it indicates that the communication connection is successful. If more than 100 commands are sent without receiving data, it indicates a serial communication error, and the data receiving terminal needs to be checked.
3. Algorithm for evaluating the straightness measurement error of large machine tool guideways
The spatial straightness error assessment algorithm, as one of the commonly used algorithms for measuring the straightness error of large machine tool guideways, involves projecting a spatial straight line onto the XY plane of a given coordinate system and calculating the straightness error after projection. Based on the calculation results, the measurement result of a spatial line is plotted, and this line is projected onto the XY plane to calculate the corresponding coordinates, thus transforming the spatial line into a planar line. The straightness error of the first projected planar line is then calculated using the planar line straightness rotation method.
Rotate the spatial line around the X-axis as the center of rotation. While the X-coordinate of the line remains unchanged, calculate the straightness error of its projection onto the XY plane. Repeat the above steps until the spatial line returns to its initial position. During this process, many planar straightness errors will be obtained; find the maximum value, which is the spatial straightness error.
4. Conclusion
As the above analysis shows, in order to design a powerful, high-performance, highly scalable, and widely applicable large machine tool guideway straightness measuring instrument, it is necessary to conduct scientific and reasonable design in all aspects, including microcontroller selection, wireless data transmission, amplification circuit, communication function implementation, data verification, and straightness error evaluation algorithm, and to carefully select the required components in order to further improve the market share of the system and enhance the performance of the product.
