Wireless remote control system for industrial manipulator based on programmable controller

For the proportion and switch control of multi-joint industrial robots, PLC is used as its controller, and data is transmitted through the wireless data transmission module to achieve remote wireless remote control operation.

The handheld operator uses a 16-bit single-chip microcomputer to encode the data of the button and joystick operation detected and send it to the PLC wirelessly. The PLC decodes it and obtains the command to control the movement of each joint of the manipulator. This solution is low-cost and has good versatility and scalability.

Preface

Programmable Logic Controller (PLC) is a highly applicable industrial controller, widely used in various industrial production lines, mobile machinery and equipment control and other industrial occasions with harsh environments. It has stable working performance, strong scalability and easy application. According to different module configurations, it can perform logical and arithmetic operations, control and collect digital switch quantities, analog quantities, etc., and has a rich bus interface form, which can use public protocols to form complex systems with different devices. In the industrial robot we developed, each joint is composed of an open-loop hydraulic control circuit with a proportional valve or a switch valve to achieve proportional speed regulation or switching action. Therefore, we selected Siemens' S7-200 series PLC as its controller. [1>[2>

In some applications with complex environments, especially for mobile mechanical equipment, the wiring of the control system is subject to many restrictions, and the location arrangement of the operator and the control system is inconvenient. Therefore, the application of highly flexible wireless remote control operating systems has been promoted. Currently, wireless remote control operating systems used in industry generally adopt radio digital transmission. 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 and acquisition 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, automatic data collection and other occasions. It has the advantages of high reliability, low power consumption, transparent protocol and easy use. The transmission distance can reach tens to hundreds of meters, and some can even reach thousands of meters. At present, many manufacturers have launched industrialized and serialized products, and you can choose the appropriate product according to different usage occasions and requirements.

At present, some manufacturers have launched dedicated industrial wireless remote controllers based on embedded processors, which have excellent performance and high integration. Such as HBC, JAY, etc., are used in concrete pump trucks, loaders and other occasions. However, since they are generally targeted at special engineering equipment, they do not have good versatility and scalability, and are mainly produced by foreign manufacturers, with high prices, and their application and promotion are limited.

In the multi-joint industrial manipulator we developed, PLC is used as its main controller to complete the switch and proportional control of the hydraulic pump station and the hydraulic valves of each joint, realizing the basic functions of the manipulator. The wireless data transmission module is used to send and receive digital signals through wireless communication to realize the remote wireless remote control function. The input switch quantity and analog quantity of the designed handheld remote control are collected and encoded and output by the 16-bit single-chip microcomputer.

1. Overall plan

The manipulator is used for concrete spraying in the tunnel, and has two modes: automatic and manual. In the automatic mode, the PLC controls the joints of the manipulator to move according to the trajectory preset by the program, adjusts the position and angle of the end of the manipulator, and makes the spraying nozzle move according to the set trajectory and always perpendicular to the tunnel surface to complete the spraying operation on the tunnel surface. In the manual mode, the operator uses buttons and proportional rockers to control the switch valve and proportional valve respectively, drive the movement of each joint, and control the position and angle of the concrete nozzle. The button controls the start and stop of the pump station and the direction switching of the switch valve. The proportional rocker forms an analog quantity within the range of ±10V according to the control of the operator. The proportional valve controls the size and direction of the joint movement speed according to the size and direction of the analog quantity. The functional block diagram of its control system is shown in Figure 1.

Figure 1 Functional block diagram of the manipulator control system [page]

In the case of convenient wiring, the switch and proportional signals of button operation and rocker operation can be directly input into the digital and analog input modules of the PLC through cables. This method has a simple structure, low cost, high reliability, and extremely simplified hardware design and software programming of the control system. However, in actual industrial sites, wiring is subject to many restrictions, especially for mobile equipment, which is not convenient and flexible enough for personnel to operate using wired control. Since the manipulator we developed needs to move long distances on the track and the operation distance is far, wireless remote control operation must be adopted. For this reason, we use wireless data transmission modules as communication modules and design a handheld operator based on a 16-bit single-chip microcomputer. The overall functional block diagram of the improved remote control manipulator control system is shown in Figure 2. Wireless data transmission modules are used in pairs, with one on the handheld operator end and one on the control system end on the manipulator body, respectively completing the wireless transmission and reception functions of data. When the control system needs it, bidirectional transmission and reception can also be performed, that is, the data transmission module at the same end switches between data reception and data transmission functions. At this time, it should be noted that the switching of the transmission and reception functions of the data transmission module takes a certain amount of time.

The handheld operator collects information through the IO port and AD conversion according to the operator's different operations on buttons and joysticks. The collected information is digitally encoded to form instructions, which are sent to the wireless data transmission module in the form of ASCII code through the serial port. The wireless data transmission module then sends the instructions wirelessly; the data transmission module of the control system on the manipulator receives the sent instruction code and transmits it to the PLC through the serial port bus. The PLC decodes the instruction code, decomposes different operation instructions, and then performs logical operations. According to different instructions, different actions to be performed are obtained, and then the corresponding relay or valve action is controlled through digital output and analog output, thereby realizing remote wireless operation of each joint of the manipulator. The button operation input and rocker operation input connected to the PLC through the signal cable are retained on the manipulator body for backup or maintenance. At the same time, remote control operation switching buttons are set on the PLC digital input port and the handheld operator to enter or exit the remote control operation mode. The button operation on the PLC should have a higher priority.

Since the object of the joystick operation is the proportional position state of the joystick, it adopts an automatic centering design. Therefore, in the control system software structure, the method of sending instructions cyclically is adopted. That is, after entering the wireless control state, the handheld operator will send out the collected state regularly, regardless of whether there is any operator input at this time.

Figure 2 Functional block diagram of remote control manipulator control system

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 pins of a single-chip microcomputer or other chips; standard RS-232 interface; standard RS-485 interface. The 232 interface mode and the 485 interface mode are switched by jumpers. Siemens' S7-200 series PLC has one or two RS-485 standard interfaces, so the latter two methods can be used to connect the PLC directly to the 485 interface of the wireless data transmission module; or use a PC/PPI cable that matches the PLC to convert the PLC's 485 interface to a 232 interface, and then connect it to the 232 interface of the wireless data transmission module.

The communication port of the S7-200 series PLC supports multiple communication protocols, of which two can be used here. One is the Siemens PPI master-slave protocol, which allows the master station to directly issue instructions to the slave station, that is, the PLC in the control system, to control the ports and functions of the slave station. This method is simple to program the PLC and does not require modification of the original slave station program. However, the PPI protocol is not an open protocol. Reference [4> mentions a method of obtaining the PPI protocol through serial port monitoring and using the master station program to 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-defined communication protocol to achieve communication with the outside world. This mode supports ASCII and binary protocols. The free port mode is simple and flexible to use, but requires special programming for the PLC. Because the amount of data required for wireless remote control is not large, the free port communication mode was selected through comparison to transmit commands and feedback information between the handheld operator and the PLC in the form of ASCII code. A special wireless control program was written in the PLC to achieve data communication and control of the robot under wireless control.

The function of the wireless data transmission module is only to realize the wireless communication function between the PLC and the handheld operator. For the CPU in the PLC and the handheld operator, there is no difference in programming between the wireless communication through the wireless data transmission module and the wired communication directly connected through the serial port.

The communication interface between the single-chip microcomputer and the wireless data transmission module can choose any of the three interface modes. It can be a simple direct connection; to improve stability, it can also use 232 or 485 chips to convert the level before connecting to the data transmission module. In order to maintain better scalability, we chose the RS-232 interface standard. [page]

3. Design of handheld operator

The function of the handheld operator is to detect the operation input on the buttons and joystick through the digital IO and AD functions of the processor, and to encode the converted digital signals to form control instructions, which are sent out through the wireless data transmission module.

The XC166 series of microcontrollers are 16-bit microcontroller products from Infineon Technologies. With its excellent core structure, efficient instruction set, and continuously expanding and updating product lines, it is widely used in automotive electronics, industrial control and information technology. This series of microcontrollers has a rich interface mode, 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 can operate at temperatures between -40 and 125°C. [6> For the functions of the handheld operator we designed, we chose the XC164CS microcontroller in the XC166 series. In this way, only a few peripheral hardware are required to achieve the required functions. 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, collecting the proportional control voltage generated by the joystick operation; digital IO function, that is, collecting the button operation status and performing some functional status displays; communication function, that is, regularly encoding the collected control instructions in the prescribed format and sending them wirelessly through the serial port.

The handheld operator is powered by a 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 safety of control to avoid loss of control and erroneous control instructions. This system mainly ensures this through the following aspects:

The wireless data transmission module has high anti-interference capability and low bit error rate, and efficient forward error correction channel coding technology;

Serial communication protocol check, generally using parity check;

Software protocol verification: Through software programming, the sent data can be verified by CRC verification, interactive confirmation or multiple sending comparison. In our program, the same action instruction is sent repeatedly. Only when the command instruction is exactly the same as the confirmation instruction, the PLC will accept the instruction, otherwise it will ignore the instruction. In this way, the wrong instructions generated by communication are completely avoided;

PLC "software watchdog"; set a timing program in the PLC program. When the wireless command is not received within the time limit, the robot will stop moving to prevent the robot from losing control due to communication interruption;

Through the above four measures, the reliability of wireless remote control can be effectively guaranteed to prevent erroneous operating instructions or loss of control of the robot.

5. Conclusion

The experiment proved that this remote control method using PLC and wireless data transmission module is simple and feasible. Since PLC and data transmission module are mature industrial products with high reliability, low cost and good scalability, this solution has high practical value and simplifies the design process. However, the single-chip microcomputer module in the handheld operator still needs further industrial design to meet higher industrial reliability requirements and realize more functions such as bus communication and self-testing.

The author's innovation: By designing a new architecture system and utilizing industrial wireless data transmission modules, PLC control and wireless remote control are combined, so that the robot control system can have the advantages of both, reducing costs and meeting the requirements of control system stability, compatibility and wireless control.