0 Introduction
FCS ( Fieldbus Control System) is a bidirectional serial link connecting control devices with upper-level automation control devices. Due to its simple structure and wiring, accurate and reliable digital transmission, and rich field information, it is increasingly widely used in factory automation control. Its fully digital, bidirectional transmission, and multi-point communication capabilities are gradually replacing the previously widely used DCS (Distributed Control System) in industry. There are many international standards for FCS; this paper adopts the Profibus-DP standard, a fieldbus technology used for workshop-level monitoring and data communication and control of field devices in factory automation. It enables distributed digital control and field communication networks from the field device level to the workshop level, providing a feasible solution for achieving comprehensive factory automation and intelligent field process equipment.
The process equipment discussed in this paper is mainly divided into three categories. The first category consists of equipment requiring only start-stop control, including dust collectors, belt conveyors, and mixing motors. The control objective is to ensure normal sequential start-up and shutdown, as well as interlocking shutdowns under fault or abnormal conditions. The second category comprises equipment requiring speed regulation, including pumps, fans, and feeders. The control objective is to participate in closed-loop control of liquid level, flow rate, and pressure to maintain stable operating conditions. The third category consists of self-contained systems, such as crushers, ball mills, and ceramic filters. This type of equipment is relatively independent, and its information is mainly used for monitoring or with minimal control intervention. For the first two categories of equipment, the directly connected control devices are controllers such as frequency converters, soft starters, and motor protectors. These controllers receive commands from the PLC via the DP bus and simultaneously feed back equipment operation or fault information to the PLC, displaying these statuses on the host computer monitoring screen. The host computer screen contains a wealth of information, including equipment start/stop operation interfaces, operating status information, trend curves, etc. Through statistical analysis and processing of database information, historical curves, machine hours, overall machine efficiency calculations, and electricity and water consumption statistics of the production equipment can also be obtained in the host computer, realizing factory process data visualization and equipment management. It is not difficult to see that the equipment control sequence is host computer → PLC → controller → field equipment.
1 Equipment Control and Management
The procedures are introduced in stages, from low to high, according to the control sequence.
1.1 Controller and Field Devices
Electrical control of field equipment is divided into two modes: local and bus-based. In local control, the start and stop of field equipment rely on the controllers of the power station's frequency converters, soft starters, motor protectors, etc., receiving signals from start/stop buttons or frequency setting devices installed on the local control box near the equipment. In remote control, the start and stop of equipment rely on commands sent to the PLC from the host computer screen received by the controller via the DP bus. Regardless of the control mode, the PLC can read the equipment's operating or fault status stored in the controller via the DP bus. The switching process between local and bus-based control must ensure a smooth transition, maintaining the equipment's original state. This maintenance, besides preventing the stopping or starting of power frequency-operated equipment like soft starters and motor protectors, also requires maintaining the operating frequency of frequency-converter equipment currently operating at a certain frequency during switching—a bumpless switching. Due to the addition of bus control, the switching circuit is carefully considered in terms of external circuitry and parameter settings, making bumpless local/bus-based switching more reliable than using a DCS method.
Before the adoption of FCS (Fluidized Control System), bumpless switching circuit design primarily relied on the time difference between the on/off state of the remote-to-local switching relay and the main circuit contactor to ensure uninterrupted power supply to the equipment's startup or operation circuits during remote-to-local switching. In other words, the time it takes for the main circuit contactor coil to de-energize and open its contacts must be greater than the time it takes for the switching relay coil to energize and close its contacts. For example, using ABB's AL series contactors paired with Phoenix Contact PLC-RSC relays, the former's coil de-energization contact release time is 10-17ms, while the latter's coil energization contact closing response time is 7ms. Therefore, theoretically, the contactor should not be de-energized during relay switching. However, this control method is difficult to guarantee 100% success and places high demands on the brand and performance of the relays and main contactors.
The FCS system, from both circuit and programming perspectives, fully considers smooth switching. Taking the frequency converter circuit as an example, the bus/local switch does not affect the operation of the local start relay. The start signal to the frequency converter is maintained in the state before the switch through the frequency converter's run output relay and the bus/local stop relay. To ensure the frequency of the frequency converter remains unchanged before and after the switch, an intelligent operator is used. This operator can display the frequency setpoint SV and frequency feedback value MV of the frequency converter. Whether on the bus or local, MV corresponds to the actual frequency feedback value of the frequency converter. SV is different. On the local side, SV displays the frequency setpoint given to the frequency converter by the operator; on the bus side, SV displays the value of MV transmitted and output by the operator itself, which is basically consistent with the frequency setpoint given to the frequency converter by the PLC via the bus. At the instant of switching from local to bus, the PLC transmits the frequency real-time data to the inverter via the bus as a frequency setpoint signal; at the instant of switching from bus to local, the operator's own bumpless switching function is utilized. After receiving the conversion signal, the operator instantly outputs the displayed sV value to the inverter as a setpoint frequency, thereby achieving reliable bumpless switching in both directions.
1.2 PLC and Controller
The controller mainly includes frequency converters, soft starters, and motor protectors. To achieve bus control, controller parameters need to be configured. Besides basic settings such as rated voltage, frequency, current, power factor, and bus address, for frequency converters, start/stop modes (e.g., inertia, ramp), acceleration/deceleration times, control signal sources, and frequency sources also need to be configured; for soft starters, start/stop modes (e.g., voltage, torque), voltage ramp-up/deceleration times, current limiting factors, protection categories, and input/output functions need to be configured; and for motor protectors, operating modes, protection settings, and control settings need to be configured. Initial settings are generally completed via the controller's own keyboard. Alternatively, the PLC can set and modify controller parameters via the DP bus and continuously monitor and control the controller's characteristics.
To achieve unified management of motors using different control methods, the PLC is configured with standardized motor control variables, including motor control type, control word, status word, frequency setting, frequency feedback, motor current, motor power, and fault codes. The motor control type displays information such as inverter control, soft starter control, motor protector control, and general motor control. The control word includes motor start/stop and fault reset. The status word includes information such as run/stop, bus/local, fault, emergency stop, and closing/opening. The frequency setting and frequency feedback correspond to the inverter, while motor current, power, and fault codes correspond to all bus control devices. Fault codes are an advantage of FCS over DCS; after reading fault codes via the bus, the PLC can remotely diagnose field devices, quickly determine the cause of the fault, and troubleshoot the problem.
1.3 Host Computer and PLC
Communication between the host computer and the PLC uses DAServer as the interface. DAServer reads and writes data that needs to be exchanged with the PLC according to a set time interval, such as 1000ms. The host computer reads data from the interface in the form of events. This reading and writing of data information requires the host computer to decode and encode it to correspond to specific positions, enabling the display of control words and status words from the PLC on the host computer screen.
Clicking the motor icon on the main screen allows you to point to the corresponding motor's connection variable using the device's pointer. Figure 1 shows the motor control window of a device, displaying the motor's main status and control settings.
Figure 1 Motor control window
For self-contained equipment such as ball mills mentioned above, which have their own comprehensive monitoring systems, parameters that require special attention are read through communication and displayed on the screen. This includes information such as the status and alarms of the ball mill's lubrication station, clutch, slow-drive motor, and main motor; information on bearing and stator temperature, oil pressure and flow, and vibration; and information on the ceramic filter's circulating pump, acid pump, and vacuum pump.
1.4 Host Computer and Server
Communication between the host computer and the PLC allows the screen to display real-time data on equipment operation. If historical production data or key performance indicators are needed, data must be obtained from the server. Each PLC stores production-related equipment data transmitted via the bus to the server. The host computer uses the ActiveFactory analysis and reporting tool to read historical data from the server to track production information and analyze, calculate, and process the information to obtain historical curves, machine hours, overall machine efficiency, power consumption, water consumption, etc., for the production equipment. After the factory process data is visualized, managers can take action to optimize the production process based on detailed data trends and information. Data reports and equipment management reports are generated to improve production performance.
2. Summary
This article introduces the implementation of an intelligent management and control system for electrical equipment using the Profibus-DP bus in FCS. Through seamless switching circuit design, data acquisition from field devices, centralized control, and equipment management, the system's maintainability is enhanced, and the difficulty and intensity of production operations are reduced. The advantages of FCS are demonstrated through practical applications.
