Abstract: This paper addresses the proportional and switching control of multi-joint industrial robots. A PLC is used as the controller, and data transmission is achieved via a wireless data transmission module to realize remote wireless control operation. The handheld operator uses a 16-bit microcontroller to encode detected button and joystick operations and transmit the data wirelessly to the PLC. The PLC decodes the data to obtain instructions for controlling the movement of each joint of the robot. This solution is low-cost and has good versatility and scalability. Keywords: Wireless remote control; Wireless data transmission module; PLC; Robot 0. Introduction Programmable Logic Controller (PLC) is a versatile industrial controller widely used in various industrial production lines and in harsh industrial environments such as the control of mobile machinery. It offers stable performance, strong scalability, and ease of application. Depending on the module configuration, it can perform logic and arithmetic operations, control and acquire digital and analog signals, and has rich bus interface options, allowing it to utilize open protocols to form complex systems with different devices. In the industrial machine we developed, each joint is composed of a proportional valve or a switching valve to form an open-loop hydraulic control circuit to realize proportional speed regulation or switching action. Therefore, we selected Siemens' S7-200 series PLC as its controller. [1][2] In some complex application environments, especially for mobile machinery, the wiring of the control system is subject to many restrictions, and the location arrangement of operators and control systems is inconvenient. Therefore, the application of highly flexible wireless remote control operating systems has been promoted. At present, the wireless remote control operating systems used in industry generally adopt the radio digital transmission method. Some chip manufacturers such as Infineon, Micrel, RF Monolithics, Melexis, CML, ATMEL, etc. have also launched various RF chips suitable for different occasions and requirements. [3] The wireless data transmission module is an integrated module dedicated to wireless data transmission and reception. It can be directly connected to other control, acquisition and other modules through the data bus to complete the wireless data transmission and reception function. It is widely used in industrial remote control, telemetry, wireless meter reading, automated data acquisition and other occasions. It has the advantages of high reliability, low power consumption, transparent protocol and convenient use. Transmission distances can reach tens to hundreds of meters, and some can even reach thousands of meters. Currently, many manufacturers have launched industrialized and serialized products, allowing users to choose the appropriate product based on different application scenarios and requirements. Some manufacturers have launched dedicated industrial wireless remote controls based on embedded processors, which boast excellent performance and high integration. Examples include HBC and JAY, used in concrete pump trucks, loaders, and other applications. However, because they are generally designed for specialized engineering equipment, they lack good versatility and scalability, and are mainly manufactured by foreign companies, resulting in high prices and limited application. In our developed multi-joint industrial robot, a PLC is used as the main controller to control the hydraulic pump station and the hydraulic valves of each joint, achieving the robot's basic functions. A wireless data transmission module is used to transmit and receive digital signals wirelessly, enabling remote wireless control. The input switch and analog signals of the designed handheld remote control are collected and encoded by a 16-bit microcontroller. 1. Overall Scheme This robot is used for concrete spraying operations in tunnels, with both automatic and manual operation modes. In automatic mode, the PLC controls the joints of the robotic arm to move along a pre-programmed trajectory, adjusting the orientation and angle of the robotic arm's end effector to ensure the shotcrete nozzle moves along the set trajectory and remains perpendicular to the roadway surface being sprayed, thus completing the shotcrete operation on the roadway surface. In manual mode, the operator uses buttons and a proportional joystick to control the switching valve and proportional valve, driving the joints to move and controlling the orientation and angle of the concrete nozzle. The buttons control the pump station's start/stop and the switching valve's direction switching, while the proportional joystick generates an analog signal within a ±10V range based on the operator's control. The proportional valve, based on the magnitude and direction of the analog signal, controls the speed and direction of the joint movement. The functional block diagram of the control system is shown in Figure 1. [align=center] Figure 1 Functional block diagram of the control system of the arm[/align] With convenient wiring, the switch and proportional signals from button and joystick operations can be directly input to the PLC's digital and analog input modules via cables. This method is simple in structure, low in cost, and highly reliable, greatly simplifying the hardware design and software programming of the control system. However, in actual industrial settings, wiring is subject to numerous limitations, especially for mobile equipment, where wired control is inconvenient and inflexible for manual operation. Since our developed robotic arm needs to move long distances along a track and operates over considerable distances, wireless remote control is essential. Therefore, we adopted a wireless data transmission module as the communication module and designed a handheld operator based on a 16-bit microcontroller. The overall functional block diagram of the improved remote-controlled robotic arm control system is shown in Figure 2. The wireless data transmission modules are used in pairs, one on the handheld operator and one on the control system of the robotic arm itself, respectively handling wireless data transmission and reception. When needed by the control system, bidirectional transmission and reception are also possible, meaning the data transmission module on the same end switches between data reception and data transmission functions. It should be noted that switching between transmission and reception functions requires a certain amount of time. The handheld controller collects data via I/O ports and AD converters based on operator inputs to buttons and joysticks. This data is then digitally encoded into instructions and sent to the wireless data transmission module in ASCII format via serial port. The wireless data transmission module then transmits the instructions wirelessly. The data transmission module of the control system on the robotic arm receives the encoded instructions and transmits them to the PLC via serial bus. The PLC decodes the instructions, extracts different operation commands, performs logical operations, and determines the appropriate actions to be executed based on the commands. It then controls the corresponding relays or valves via digital and analog outputs, thus achieving remote wireless operation of each joint of the robotic arm. Button and joystick inputs connected to the PLC via signal cables are retained on the robotic arm itself for backup or maintenance. Remote control operation toggle buttons are provided on both the PLC digital input ports and the handheld controller to enter or exit remote control mode. Button operations on the PLC should have higher priority. Since the joystick operation data acquisition focuses on the proportional position of the joystick, it employs an automatic centering design. Therefore, the control system software uses a cyclic command transmission method. Once in wireless control mode, the handheld operator periodically transmits the collected status data, regardless of whether there is any operator input. [align=center] Fig. 2 Block diagram of the remote control system of the arm[/align] 2. Communication Mode Selection Wireless data transmission modules generally have three interface modes: TTL level UART interface, which can be directly connected to the serial port pin of a microcontroller or other chips; standard RS-232 interface; and standard RS-485 interface. The 232 and 485 interface modes are switched via jumpers. Siemens S7-200 series PLCs have one or two RS-485 standard interfaces, so the latter two methods can be used: directly connecting the PLC to the 485 interface of the wireless data transmission module; or using a PC/PPI cable compatible with the PLC to convert the PLC's 485 interface to a 232 interface before connecting it to the 232 interface of the wireless data transmission module. The communication ports of the S7-200 series PLC support multiple communication protocols, and two of them can be used here. One is the Siemens PPI master-slave protocol. Using this protocol, the master station can directly send commands to the slave station, i.e., the PLC in the control system, to control the various ports and functions of the slave station. In this way, the PLC programming is simple and does not require modification of the original slave station program. However, the PPI protocol is not a public protocol. In the literature [4], a method is mentioned to obtain the PPI protocol through serial port listening and then use the master station to program and control the slave station. Another communication mode is the free port mode, which uses a custom PLC program to control the communication port of the S7-200 CPU and uses the user's own defined communication protocol to realize communication with the outside world. This mode supports ASCII and binary protocols. The free port mode is simple and flexible to use, but it requires special programming of the PLC. Because the amount of data required for wireless remote control is not large, the free port communication mode was selected by comparison, and commands and feedback information are transmitted between the handheld operator and the PLC in the form of ASCII code. A special wireless control program was written in the PLC to realize data communication and control of the robot in the wireless control state. The wireless data transmission module's function is solely to enable wireless communication between the PLC and the handheld operator. For the CPU in both the PLC and the handheld operator, there is no difference in programming between wireless communication via the wireless data transmission module and wired communication via a direct serial port connection. The communication interface between the microcontroller and the wireless data transmission module can choose any of three interface modes: a simple direct connection; or, for improved stability, a RS-232 or RS-485 chip for level conversion before connecting to the data transmission module. For better scalability, we chose the RS-232 interface standard. 3. Handheld Operator Design The handheld operator's function is to detect the input from buttons and joysticks using the processor's digital I/O and AD functions, encode the converted digital signals to form control commands, and then send them through the wireless data transmission module. The XC166 series microcontroller is a 16-bit microcontroller product from Infineon Technologies. Its excellent core architecture, efficient instruction set, and continuously expanding product line make it widely used in automotive electronics, industrial control, and information technology. This series of microcontrollers has rich interface modes, such as 14-channel 10-bit AD converter, synchronous/asynchronous serial channel USART, high-speed synchronous serial channel SPI, CAN module, 79 IO pins, etc., and can form a communication network with various devices. At the same time, this series of microcontrollers is suitable for harsh industrial environments, and the working temperature can be between -40 and 125°C. [6] For the functions of the handheld operator we designed, we selected the XC164CS microcontroller in the XC166 series. In this way, only a few peripheral hardware is needed to realize the required functions, and at the same time, the remote control has the advantages of stable working performance and easy function expansion. The functional modules of the handheld operator mainly include AD sampling function, that is, to collect the proportional control voltage generated by the joystick operation; digital IO function, that is, to collect the button operation status and perform some function status display; communication function, that is, to periodically encode the collected control commands in a specified format and send them wirelessly through the serial port. The handheld operator is powered by battery, and the XC166 series microcontroller and wireless data transmission module can meet the requirements of low power consumption. 4. Data Reliability The wireless remote control operating system must ensure high reliability of data transmission and security of control to avoid loss of control and erroneous control commands. This system mainly ensures this through the following aspects: High anti-interference capability and low bit error rate of the wireless data transmission module; efficient forward error correction channel coding technology; serial communication protocol verification, generally using parity checking; software protocol verification; through software programming, the transmitted data is verified, which can be done using CRC check, interactive confirmation, or multiple transmission comparison. Our program uses a method of repeatedly sending the same action command; the PLC only accepts the command if the command and confirmation commands are exactly the same; otherwise, the command is ignored. This completely avoids erroneous commands generated by communication. The PLC's "software watchdog"; a timer program is set in the PLC program, and when no wireless command is received within a timeout period, the robot arm's movement is stopped to prevent loss of control due to communication interruption. Through these four measures, the reliability of wireless remote control can be effectively guaranteed, preventing erroneous operation commands or robot arm loss of control. 5. Conclusion The experiments demonstrate that this remote control method using a PLC and wireless data transmission module is simple and feasible. Since both PLCs and data transmission modules are readily available industrial products with high reliability, low cost, and good scalability, this solution has high practical value and simplifies the design process. However, the microcontroller module in the handheld operator requires further industrial design to meet higher industrial reliability requirements and realize more functions such as bus communication and self-testing. The author's innovation lies in designing a new architecture that combines PLC control with wireless remote control using an industrial wireless data transmission module. This allows the robotic arm control system to combine the advantages of both, reducing costs and meeting the requirements for control system stability, compatibility, and wireless control. References [1] SIMATIC S7-200 System Manual [M]. Beijing: Siemens (China) Co., Ltd. 2004.6 [2] Xiong Xingming et al. PLC control of an industrial robot [J]. Microcomputer Information. 2006 (11-1): P.120-122 [3] Li Chaoqing et al. Selected compilation of wireless transmitting/receiving IC chips and their data communication technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press. 2003.5 [4] Peng Weizhen. PPI protocol analysis [J]. Chemical Automation and Instrumentation. 2006 (4): P.47-50 [5] Guo Shigang. PLC human-machine interface and programming [J]. Microcomputer Information. 2006 (7-1): P.42-44 [6] Cheng Jun. Yiheng (Siemens) C166 series 16-bit single-chip microcomputer principle and development [M]. Beijing: Beijing University of Aeronautics and Astronautics Press. Author Biography: Gao Bo (June 1975 -), male, Han nationality, Shandong Province, holds a PhD in Engineering, specializing in Mechatronics Engineering. He studied at Beijing University of Aeronautics and Astronautics from July 1995 to December 2005. Since December 2005, he has been working at the Robotics Center of Shandong University of Science and Technology, where he is currently engaged in research on underground engineering robots.