Application Area : Control Design Challenge : Achieve variable-parameter synchronous motion control of a 4-DOF robotic arm (3 AC servo motors, 1 micro DC motor) within a short period, providing an application solution for the control of wire-driven mechanisms. Application Solution : The solution uses NI's LabVIEW 8.2 as the development platform. The NI PCI-7344 four-axis motion control card and multi-function data acquisition card are used to achieve drive control of the four degrees of freedom of the robotic arm. Simultaneously, the new Project file management function and control design toolkit in LabVIEW 8.2 are utilized to achieve rapid development and release of the control software. Products Used : LabVIEW 8.20 NI PCI-7344 Four-Axis Motion Control Card Introduction : Although the research on robotic arms is not a new topic, how to minimize manufacturing costs and shorten the manufacturing cycle while ensuring high positional accuracy of the robotic arm remains a problem worthy of continuous exploration. Traditional industrial robotic arms are often designed in a series configuration, where drive and transmission components such as motors and reducers are directly mounted near the rotating joint. While simple and direct, this design significantly reduces the effective load of the robotic arm because the drive components themselves become the load. It also introduces vibrations and other adverse effects, reducing the robotic arm's positioning accuracy. This project proposes using a wire mesh transmission mechanism to transmit power from the drive components to the end effector. This design minimizes the impact of the drive components on the robotic arm's load capacity, and the elasticity of the wire itself provides flexibility and self-adaptive capabilities. Due to the position adjustment of the transmission components, the control system design requires accurate feedback control of the robotic arm's final end effector position. This system is based on the PCI-7344 and uses NI's latest LabVIEW 8.2 as the development platform to implement position servoing and encoder feedback for three servo motors. For the DC motors, angle feedback control is achieved using linear potentiometers to provide voltage feedback. Leveraging the powerful capabilities of LabVIEW 8.2, we were able to complete the development of the control system in a short time while ensuring the robotic arm's motion accuracy and load capacity. Four-DOF Robotic Arm System This paper discusses a four-DOF robotic arm designed for small to medium-sized logistics systems. Its basic design requirements are: practicality, relatively large operating space, gripping weight not less than 2.5 kg, repeatability of positioning accuracy not exceeding 10 mm, lightweight, and clean appearance. [align=center]Figure 1 Schematic diagram of the overall structure of the robotic arm[/align] For ease of operation and practicality, four degrees of freedom are designed: waist rotation, upper arm pitch, forearm pitch, and wrist rotation. The overall structure adopts a gravity-folding unfolding type, with upper arm pitch and forearm pitch forming a set of planar degrees of freedom. The robotic arm mainly uses a wire rope transmission mechanism to place the additional loads such as the motor and gear reducer of the forearm pitch joint in the base of the robotic arm, thereby reducing the requirements for other joint drive components and the overall power consumption of the robotic arm, reducing its own weight, and increasing its ability and efficiency in performing external work. [align=center]Figure 2 Schematic diagram of wire rope transmission mechanism[/align] This robotic arm not only meets the performance requirements of being lightweight and having a large external working capacity, but also has the advantages of simple manufacturing and low cost, which is conducive to industrial promotion and popularization. The new embedded wire rope tensioning device allows for easy and convenient on-site adjustment of the tension, solving various problems existing in wire rope transmission, effectively improving the repeatability of the robotic arm, and achieving a weight-to-load capacity ratio of 4:1. It can be widely used in industrial environments represented by small and medium-sized logistics systems, and can also be promoted as a teaching and research demonstration device. [align=center]Figure 3 Four-DOF robotic arm prototype[/align] [align=center]Figure 4 Schematic diagram of wire drive mechanism[/align] Control System Design After comprehensively considering the project's mechanical structure requirements, functional objectives, development cycle, and other factors, we have determined the following scheme for the design of the control system: 1. For the three degrees of freedom of the chassis (waist), upper arm pitch, and lower arm pitch, a closed-loop control system is constructed using servo motor drive and encoder feedback. Due to the positioning accuracy requirements of this project, the servo motor control method is selected as position control (i.e., pulse control). Therefore, we selected the NI PCI-7344 as the motion controller for the servo motor. 2. For the wrist rotation degree of freedom and the control of the gripper's opening and closing, considering that this part of the mechanism is mainly located near the end load and requires a small size, we chose to use a DC motor with a gear reducer and indirectly measure the angle value through the voltage value of a linear potentiometer. 3. LabVIEW itself has a large number of digital signal processing VIs, which can effectively solve the signal interference and filtering problems often encountered in the control system. Using LabVIEW can significantly shorten the project development cycle. Within just three months, we rapidly completed the progress from mechanical design, material processing, and control system hardware and software design. This is also a key reason why we prioritized LabVIEW as the system development platform. Figure 5 shows the overall block diagram of the control system. After considering the technical requirements of the robotic arm, its main functions can be roughly divided into the following categories: system hardware information feedback, motion parameter setting, manual and automatic motion control, capture and reproduction of the robotic arm's spatial position, and file operations. The interrelationships between these can be represented by the following software flowchart. Software Design The control system software design, considering the overall requirements, should note that the detection of the servo motor position is mainly achieved by reading the corresponding servo motor encoder to obtain feedback on the actual position. At extreme positions, we use a Hall sensor to transmit a trigger signal to the PCI-7344 to realize the detection of extreme positions, and periodically read the values ​​of the IO registers to achieve feedback on the robotic arm's motion state. The position detection of the DC motor is achieved by indirectly measuring the DC motor's rotation angle through the voltage of a linear potentiometer fixed to the gear reducer. The main interface of the software is shown in the following figure: Development Process In versions of LabVIEW prior to 8.0, many functions that needed to be reused multiple times in the main program were called through subvi wrappers. When there were many such subvi, managing these files became a significant workload. Furthermore, poor management of some subvi (such as file loss) could severely impact the operation of the entire software system. In LabVIEW versions 8.0 and 8.2, the introduction of the Project development method offered hope for solving this problem. Through Project Explorer, we can not only effectively develop each function (such as servo drive status feedback, DC motor position measurement, DC motor position control, etc.) independently during the development process, but also ensure the consistency between the functions called in the main VI and the subvi, unlike in earlier versions where the main VI needed to be manually updated one by one if a subvi changed. Another advantage of using Project Explorer is the convenience of program packaging and distribution. As shown in Figure 8, different choices in the build specification can package the source program into an EXE file, an Installer file with LabVIEW Runtime Engine, a dynamic link library (DLL), and a VI with the source code hidden (Block Diagram is invisible). Furthermore, when using a board to acquire the input voltage of a DC motor potentiometer, we encountered the influence of AC interference signals. Using LabVIEW's built-in signal processing functions, the impact of interference signals on the program's judgment logic can be effectively suppressed (when a physical filter cannot be quickly obtained on-site, LabVIEW's software filtering can be considered). File Operations During program development, we often encounter situations where we need to add, save, delete, and read data. In development platforms such as VC or VB, file operations are relatively complex due to the design of document template structures, file pointer operations, and message mapping. However, in LabVIEW, using file operation VIs such as `write to spreadsheet` and `read from spreadsheet`, as well as table controls, array operation VIs, and custom controls, we can easily implement functions such as reading and writing common data file formats like txt and xls (as shown in Figure 9). Conclusion With the help of LabVIEW, we were able to quickly combine the development and debugging process in the project. We used LabVIEW to develop a control system design and corresponding software that can effectively integrate the hardware resources required by the robotic arm, such as motor driver, position encoder, limit position sensor and digital IO port, and ensured the accuracy of the mechanism and the progress requirements of the development time. References [1] Lei Zhenshan. LabVIEW 7 EXPRESS Practical Technology Tutorial. China Railway Publishing House. 2004 [2] Yang Leping. LabVIEW Advanced Programming. Tsinghua University Press. 2003 [3] Jianjun Yuan, Weijun Zhang, "Research on Novel Wire Driving Robot Manipulator for Local Industrial Production Line", Proc. of the IEEE Int. Conf. on Mechatronics and Automation, 2007 [4] Gong Jinliang, Zhao Xianchao, Gao Feng. Design of Seismic Simulation Vibration Table Control System Based on LabVIEW and PXI-7538 Multi-Axis Motion Control Card. National Instruments China Co., Ltd. 2006 Excellent Paper Collection