1 Introduction

The antenna switch station is an important part of the low-frequency transmission system. It acts as a bridge between the high-power transmitter and the antenna. In the antenna switch station, the closing and opening of a series of isolating switches controlled by AC motors can realize the functions of radiating radio signals through antennas in different directions and grounding the antenna. Therefore, timely and accurate monitoring of multiple motors in the switch station is the key to switch station control. At present, the distance between most switch stations and high-power transmitters is relatively far, and the field equipment and cables are in a strong electromagnetic interference environment. Traditional control methods are prone to complex field wiring and poor control signal stability. In response to the above problems, this article introduces an antenna switch station control based on the Q series programmable logic controller (PLC) CC-Link fieldbus.

2 CC-Link fieldbus

2.1 cc-link Overview

CC-Link (Control & Communication Link) is an open fieldbus developed and announced by Mitsubishi Electric in 1996 with the concept of "multi-vendor equipment environment, high performance, and wiring saving". It is a complex, open, and highly adaptable network system that can adapt to different ranges from higher management level networks to lower sensor networks. CC-Link fieldbus is a network based on the device layer. The entire layer of the network consists of a master station and a maximum of 64 slave stations. The master station in the network is a PLC, and the slave station can be a remote I/O module, a special function module, a local station with a CPU and PLC, a human-machine interface, a frequency converter, and various measuring instruments, valves and other field instrument equipment [1].

The underlying communication bus protocol of cc-link follows the RS485 interface standard. Generally speaking, cc-link mainly uses broadcast polling for communication. The specific method is: the master station sends refresh data (ry/rww) to all slave stations, and at the same time polls slave station 1, slave station 1 responds to the master station's polling (rx/rwr), and informs the other slave stations of the response; then the master station polls slave station 2 (no refresh data is sent at this time), slave station 2 responds, and informs the other slave stations of the response, and so on, in a cycle. The data transmission rate of this method is very high. In addition to the broadcast polling method, cc-link also supports instantaneous communication between the master station and the local station and the intelligent device station. Instantaneous transmission will not affect the cyclic scanning time of broadcast polling [2][3].

2.2 cc-link advantages and features

(1)减少配线，提高效率。cc-link显著减少了当今复杂的生产线上的控制线和电源线。减少配线和安装设备的费用；减少配线时间；更有利于维护。

(2) High-speed input and output response. CC-Link achieves a high-speed communication speed of up to 100 Mbps, reliable input and output response, and fast response time, reliability and determinism, and can easily cope with applications that require fast I/O response and large-capacity data transmission.

(3) Convenient to extend the distance. The total distance covered by the CC-Link network can be extended to 1200m (at 156kb/s), and by using repeaters and optical repeater modules, the transmission distance can be further extended to 13.2km, which can flexibly support the required equipment.

(4) Rich RAS functions. RAS (reliability, availability, serviceability) is the abbreviation of reliability, effectiveness, and maintainability. CC-Link relies on RAS functions to achieve high reliability. This function includes backup master station, slave station separation, automatic recovery, testing and monitoring, which provides a highly reliable network system and minimizes the time of network paralysis.

3 System composition

The antenna switch station is composed of 18 isolating switches. The control of the antenna switch station mainly completes the start and stop, interlocking control and status detection of the isolating switch control motor remotely. Based on the above requirements, the cc-link control system structure diagram is shown in Figure 1.

The CC-Link control system of the antenna switch uses Mitsubishi Melsec-Q series PLC as the main control. Nine remote I/O input modules, nine remote I/O output modules and one analog input module are connected via CC-Link bus cables. The base expansion connector on the Melsec-Q series PLC main base is connected to a Mitsubishi GOT touch screen. The slots on the Melsec-Q series PLC main base are used to install Q series input modules, output modules, CC-Link master station communication modules and H network modules [4][5].

The CC-Link field bus transmits the field data to the master station at high speed for management. A Mitsubishi Q series PLC is used to manage the CC-Link network, send the data to the monitoring computer through the upper network for analysis, and send the instructions from the central control room to the field slave station. The touch screen displays the working status information of all controlled equipment in the switchgear for operators to monitor.

Figure 1 Structural block diagram of the cc-link antenna switch control system

Figure 2 Schematic diagram of communication between master station and remote station

4 System Communication Implementation

The communication process between the master station and the remote station is shown in Figure 2. When communicating with a remote station, the signals exchanged with the remote station (initial data request flag, error reset request flag, etc.) are communicated using remote input rx and remote output ry; while digital data (average processing specifications, digital output values, etc.) are communicated using remote registers rww and rwr [6].

Before communication, the master station must be initialized with the initialization program to achieve the mapping of soft components and addresses. The parameters that need to be set for the master station are: station number; transmission rate; number of connected modules; number of retries; number of automatic return modules; standby master station designation; specified operation when the CPU is down; scan mode designation; delay time setting; reserved station designation; specification of invalid stations with errors; number of stations; allocation of communication buffer and automatic update buffer. Record these parameters in the master station module in advance, so that there is no need to write parameters every time the master station module is started. Each slave station must also set the corresponding communication parameters (station number, number of occupied stations, transmission rate) and compile the corresponding communication program.

Finally, the system operation status is confirmed through the LED display or sequential control program.

5 System Software Design

The control system uses Mitsubishi's GX Developer Ver.8 software to program the Q02H PLC. The switch control system software design mainly completes the frequency selection, switch access or grounding, system self-checking and other controls. In order to ensure the reliability and readability of the program, the main program of the control system is designed as a subroutine module structure, and different subroutine modules complete different functions. The main program flow of the switch control system is shown in Figure 3.


Figure 3 Main program flow of the switch control system

6 Conclusion

The application of cc-link bus technology in antenna switch station simplifies the system structure. It not only reduces costs, simplifies the configuration of the control system, saves a lot of control cables, but also improves the reliability of system operation, especially its flexible and comprehensive fault diagnosis ability and network communication ability, which facilitates construction, commissioning and maintenance, and greatly improves system performance. It takes the control system to a higher level and realizes full-process automatic monitoring of equipment operation.