Design of Calibration Software for Laser Tracking Measurement System Based on LabWindows/CVI

The independently developed laser tracking measurement system realizes the coordinate measurement of space targets through laser ranging and precise angle measurement. As a precise angle and distance measurement instrument, the laser tracking measurement system needs to be calibrated before leaving the factory to obtain the relevant parameters of the instrument's internal optical system, mechanical structure, and servo system, and to evaluate the instrument's measurement accuracy. In order to make the instrument calibration process simple and effective, it is necessary to develop general calibration software. LabWindows/CVI is a virtual instrument software development tool launched by the American NI company. It provides an ideal software development environment for technical developers familiar with C language to establish computer instrument systems-virtual instruments in the field of measurement and control. The application field of this software is extremely wide, covering various industries such as military industry, telecommunications, industrial production, and aerospace [1-2]. This paper applies virtual instrument technology to develop calibration software based on LabWindows/CVI for the independently developed laser tracking measurement system.

1 System composition and calibration content

The hardware of the laser tracking measurement system mainly includes the tracking measurement unit (including the tracking head, bracket, etc.), the electronic control unit (including the electronic control box, environmental sensors, etc.), the main control computer and the measurement accessories (including the corner reflector, the standard rod, the magnetic base, etc.), as shown in Figure 1. Among them, the tracking head is the core unit of the laser tracking measurement system. It integrates the laser rangefinder (interference ranging and absolute ranging) and the photoelectric theodolite, and can realize the ranging, angle measurement and tracking functions at the same time. The purpose of the laser tracking measurement system calibration is to identify the error terms that affect the ranging and angle measurement accuracy of the system so as to make software corrections. According to the degree of change of the internal parameters of the system within the cycle, it can be divided into laboratory calibration and field self-calibration. Among them, the laboratory calibration mainly calibrates the IFM laser, the atmospheric parameter sensor device, and the angle measurement grating code disk; the field self-calibration mainly calibrates the Home reference distance, geometric error, and ADM ranging. Since some of the above calibration items have high requirements on the experimental environment, the calibration software only involves the calibration of Home reference distance, geometric error, ADM distance measurement, angle measurement grating code disk and other items. In addition, angle measurement accuracy check and system accuracy detection (Bundle test) will also be included in the calibration software.

2 Calibration software design

2.1 Overall design of calibration software

The calibration software and the laser tracking measurement system are connected via Ethernet using TCP/IP. When the software is running, the device is first connected, and then the calibration item is selected. After the calibration sub-interface pops up, follow the prompts to operate. After the calibration is completed, return to the main interface to generate and send the parameter file. After success, exit the main interface to complete the user self-calibration. The operation process of the calibration software is shown in Figure 2.

2.2 Modular design of calibration software

2.2.1 Main interface design

The main calibration interface is designed with simplicity and practicality as its principle. Its main functions are to achieve device connectivity, calibration item selection, parameter file generation, connection status display, and real-time measurement data display, as shown in Figure 3. When the device connection is not achieved, except for the connect and exit buttons, other buttons on the main interface are locked. Only when the connect button is clicked and the status lights of the electric control box and the client turn green, other buttons are available. When the measurement system has data output, the azimuth, pitch angle and distance display in the data display area will change synchronously. When a calibration item button is clicked, the calibration item sub-interface will pop up. The user only needs to follow the prompts on the sub-interface. During the whole process, other calibration item buttons will be locked until the user exits the current calibration item. [page]

(1) TCP/IP connection

The communication between the calibration software and the device is achieved through Ethernet. LabWindow/CVI provides functions for implementing TCP/IP communication between devices. According to the different service objects, they can be roughly divided into three categories: server functions, client functions and support functions [3]. The calibration software plays the role of the client in the laser tracking measurement system. It not only receives data sent from the tracking measurement system data acquisition and processing unit (server side), but also responds and sends data to the server side. The function of the TCP/IP connection button is to start the connection program, connect according to the set server port number and IP address, receive the data sent by the server side, and calculate and display it. After the connection is successful, the indicator lights of the electric control box and the client will turn green. If there is data on the server side, the azimuth, pitch and distance values ​​will be continuously refreshed and displayed, with a refresh frequency of ≤1 000 Hz. The connection process between the TCP/IP calibration software (client) and the data acquisition and processing unit (server) is shown in Figure 4.

(2) Calibration item selection

The calibration software has 6 items, namely ADM distance calibration, HOME distance calibration, angle error check, GEC geometric error calibration, angle code disk calibration and Bundle test system accuracy detection. The buttons of the calibration items are interlocked, that is, if you click any button, the buttons of other items will be unavailable until you exit the current calibration item.

(3) Parameter file generation

After executing certain calibration items, click the parameter file generation button to enter the new parameter file generation process: load the original parameter file - calculate the corresponding calibration parameters according to the execution of the calibration item - generate a new parameter file - send the new parameter file to the data acquisition and processing unit - save the original measurement data - exit the parameter file generation program. [page]

2.2.2 Calibration project sub-interface design

Since there are many calibration items, only the ADM distance measurement calibration is used as an example to illustrate the design of the sub-interface. At the user's measurement site, after the laser tracking measurement system is started, the target is placed in a suitable position, and the ADM and IFM distance measurement values ​​are observed. If the difference between the two is large and the change is unstable, the ADM distance measurement calibration is required. The method adopted is to compare with the IFM distance measurement that comes with the laser tracking measurement system, and use the cubic spline function to fit the error curve. To ensure the measurement accuracy, the number of measurement points is required to be no less than 6 points, the total length is up to 36 points, and the measurement point interval is preferably 1 m. When the angle and distance data meet the requirements of the set measurement point, the measurement data turns green, prompting the operator that the point can be collected. Click the collection button, the current data is saved, and the next operation prompt pops up. Figure 5 is the ADM distance measurement calibration sub-interface and the pop-up operation prompt. The design of the sub-interfaces of other calibration items is similar to the ADM calibration sub-interface.

2.2.3 Calibration software parameter file design

The parameter file is a file used to save servo control parameters, error correction and calibration parameters and other important parameters (such as laser calibration wavelength, etc.) in the laser tracking measurement system. The system will provide a default parameter file for error correction before leaving the factory. During the use of the system, in order to ensure the stability and reliability of the system to adapt to the environment when working on site, some data in the parameter file allows the user to calibrate and refresh through the calibration software as needed. The parameter file is divided into three parts according to the content: parameter area, parameter description area and calibration history area. After the user calibrates, the parts of the parameter file that are modified are the parameter area and calibration history area. The parameter file is saved in the text file (*.txt). Figure 6 is a schematic diagram of the parameter file.

3 Calibration software joint debugging experiment

Based on the existing equipment of the laser tracking measurement system, the joint debugging experiment of the calibration software and the data acquisition and processing unit is carried out. The required equipment includes: two computers, a data acquisition and processing unit and several connecting cables, one of which is used as the host computer (calibration software) and connected to the data acquisition and processing unit through a network cable, and the other is used as the program refresh device of the data acquisition and processing unit. The experimental content includes data connectivity experiment, instruction transmission experiment and parameter file refresh experiment. The data connectivity experiment refers to the TCP/IP connectivity and measurement data solution and display experiment between devices; the instruction transmission experiment refers to the control instruction transmission and successful execution return experiment in the calibration software; the parameter file refresh experiment refers to the calibration software refresh operation experiment on the parameter file area of ​​the data acquisition and processing unit.

After the equipment is connected, the status lights of the electric control box and the client turn green. Except for the connect button on the main interface of the calibration software, all other buttons can be operated, as shown in Figure 7. When one of the calibration items is selected, the operation sub-interface of the item pops up. At this time, other buttons on the main interface are unavailable (except the exit button) until the current sub-interface is exited, as shown in Figure 8. After the user executes the required calibration item, he can click the parameter file generation button to generate, store and send a new parameter file. Figure 9 shows the software operation process after clicking the parameter file generation button.

In response to the calibration requirements of the self-developed laser tracking measurement system, a calibration software based on LabWindows/CVI was designed for measurement users. After the laser tracking measurement system is started, the measurement user can automatically check the distance and angle measurement errors through the calibration software to determine whether calibration of related items is required. This software is based on the principle of humanized design. During the calibration process, the software automatically guides the user to complete the calibration of the corresponding items, and can display and store the original measurement data and calibration parameters, realizing the automation and virtualization of the entire calibration process.