The PROFIBUS fieldbus, with its superior technical performance, is widely used worldwide. The electric actuator industry also requires products with PROFIBUS fieldbus interfaces. Shanghai Automation Instrumentation Factory No. 11 highlighted the development, testing, and application of intelligent electric actuators with PROFIBUS fieldbus interfaces. Practical experience has proven their effectiveness, making their promotion significant.
Fieldbus is a serial, bidirectional, multi-point digital communication technology used in production environments to enable serial communication between field devices (instruments) and between automatic control devices (systems) in the field device control room. It is a network and control system that uses individual, distributed, digital, and intelligent measurement and control devices as network nodes, connected by a bus, to exchange information and jointly complete automatic control functions.
There are many types of fieldbuses, with Modbus-RTU, Profibus-DP, FF, and DeviceNet being the main ones used in the electric actuator industry. Among them, Profibus became part of the international standard IEC 61158 in 1999 and was approved as the Chinese industry standard JB/T10308.3—2001 in 2001. Profibus fieldbus technology is an open, digital, multi-point communication-based low-level control network. Using the fieldbus as a link, it transforms individual, distributed field devices into network nodes, replacing the original 4-20mA analog signals and 24VDC switching signals at the device level with digital communication. This enables information sharing between low-level intelligent field devices and higher-level system information, meeting the development requirements of distributed, flattened, and intelligent industrial control systems.
1. Overall Design of Bus Control System
1.1 Hardware Design
Electric actuators, also known as electric devices or electric heads, are generally used in conjunction with valves, dampers, etc. They control the opening of valves, dampers, etc., by receiving control signals (switching, analog, or bus signals) from the control center and feeding back the actuator's status signals to the control center. Based on the data exchange required between the actuator and the control center, a GSD file is written to define the user data field and data exchange format. Then, according to the DP protocol, communication is established with the DP master station via a microcontroller and protocol chip. The control center and actuator typically form a single-master system, with the master and slave stations operating using a cyclic data transmission method. Based on the existing hardware and software structure of the SND intelligent electric actuator, a Profi-bus-DP interface card is developed to easily connect the actuator to a bus communication network. The DP card exchanges data with the master station via an RS-485 bus.
The Profibus-DP bus-type electric actuator adds a DP communication card to the original intelligent electric actuator control system. The core of the DP card consists of a W78E052B40PL microcontroller, an SPC3 protocol chip, and an IL485 interface chip. The SPC3 is a dedicated Profibus-DP protocol chip (requires a matching microcontroller), responsible for unpacking data from the master station and sending it to the microcontroller, while simultaneously packaging data from the microcontroller and sending it to the master station. The W78E052B40PL acts as a bridge between the motherboard and the SPC3, handling initialization and protocol conversion. The advantage of this solution is that the bus card and control system have a modular structure; only the bus card needs to be added to connect to the Profibus network.
The PROFIBUS communication protocol is relatively complex. In principle, it can be implemented in software on any microprocessor, requiring only an asynchronous serial transceiver (UART). However, developers can now completely eliminate the need for complex protocols by using dedicated communication processing chips. A wide range of PROFIBUS communication protocol chips are available, making the implementation of the PROFIBUS protocol simple, convenient, time-saving, and labor-saving, and enabling rapid product delivery. Many ASICs are suitable for intelligent slave stations, such as IAM's PBS, Motorola's 68302 and 68360, Siemens' SPC4 and SPC3, and VIPA's VPC3+ and VPM2L, among which the SPC3 communication protocol chip is particularly representative. Therefore, the application of the SPC3 protocol chip is crucial in bus card design.
The SPC3 integrates a watchdog timer with three different operating states: baud rate monitoring, baud rate control, and DP control. The SPC3 integrates the FDL layer of the DP protocol, and the entire SPC3 operation is controlled by a microsequencer (MS). It also integrates 1.5KB of dual-port RAM, which uses an 8-byte segment structure divided into 192 segments. All BUF pointers requiring address allocation must point to the beginning of a segment. Within the 1.5KB RAM, the SPC3 itself uses 64 bytes. Most of these 64 bytes need to be set by the microcontroller, such as the interrupt mask, hardware mode registers 0 and 1, various BUF lengths and pointers, and flag words. A portion also consists of various status indicator registers for the SPC3, such as interrupt words, input BUF status, output BUF status, and diagnostic BUF status. Some registers are used as operating mode setting registers when written and as status indicator registers when read, such as the interrupt request register. The remaining 1472 bytes of RAM are allocated to various BUFs. The SPC3 has 3 input buffers, 3 output buffers, 2 diagnostic buffers, 2 auxiliary buffers, 1 configuration buffer, 1 parameter buffer, and 1 address setting buffer. Before the SPC3 can operate, process-specific parameters (such as station address, flag number, and various control bits) must be input into specific register units for the SPC3 to function correctly. The service access point for DP communication is automatically established by the SPC3, and various message information is presented to the user as the internal data of different buffers. The SPC3 is equivalent to an external RAM extension for the W78E052B40PL microcontroller, accessible via the bus interface. The dual-port RAM of the SPC3 should be uniformly allocated addresses in the CPU address space. The CPU treats this RAM as its own external RAM.
The SPC3's internal UART enables the conversion between serial and parallel data streams. The four pins used for serial communication are XCTS, RTS, TxD, and RxD. XCTS, or Clear Transmit, is the SPC3's input signal, indicating that data transmission is permitted. RTS is the SPC3's request to transmit signal. RxD and TxD are the serial receive and transmit ports, respectively. The SPC3 can automatically indicate the bus baud rate (9.6kb/s to 12Mb/s). To improve system immunity to interference, the SPC3's internal circuitry must be electrically isolated from the physical interface. Electrical isolation on the input/output channels uses 6N137 high-speed optocouplers. Electrical isolation for the power supply uses a 5V/5V isolated converter power supply, the DCP0505. The high-speed optocoupler used is 6N137, and the transceiver is SN75ALS176, achieving a communication rate of up to 6Mbaud/s, suitable for most application systems. If users require higher communication rates, they can replace the optocoupler with the HCPL-7720/7721, which can reach up to 25 Mbaud/s.
Therefore, it can be seen that designing a DP interface card using the SPC3 protocol chip does not require much understanding of the DP protocol itself, and there are standard circuits and detailed design data, making the hardware design relatively simple.
1.2 Software Design
The software design primarily involves the initialization of the microcontroller and SPC3, configuring the SPC3 registers, starting the SPC3, and enabling data reception and transmission. Reception and transmission consist of two parts: data transfer between the microcontroller and SPC3, and data transfer between the microcontroller and the motherboard. The entire data transfer process involves the microcontroller reading data from the motherboard and then passing it to the SPC3. The SPC3 processes the data and sends it to the PROFIBUS bus for reception by the master station. Data input and output processing (input/output relative to the master station) for the master station and its slave actuators, as well as user diagnostic data input, are handled within a loop in the application program. Within this application loop, the application refreshes the data in the input buffer to ensure all input data is the latest version. When the SPC3 receives different output data from the PROFIBUS master station, it generates an output flag. The CPU receives master station data by polling this flag within the application loop. For application-specific diagnostic information, real-time transmission to the main station is required. The main application checks for available diagnostic buffers within its application loop; if an idle buffer is available, the application inputs diagnostic information and requests an update. For systems with strict real-time requirements, interrupt-driven output and diagnostic data processing is employed.
2. Definition of User Data Field and Data Exchange Format in the DP Protocol for Electric Actuators
The data format for data read from the master station by the slave station is described below:
The communication data consists of 13 bytes of input and 4 bytes of output.
For example, the control command output data of an electric actuator consists of two words: open, close, and stop operation control commands.
The meanings of each bit in the control command are as follows:
bit0: Disable instruction, 1 = run, 0 = stop
bit1: Enable instruction, 1 = run, 0 = stop
bit2: Stop instruction, 1 = stop, 0 = invalid.
bit3: Emergency shutdown command, 1 = run, 0 = stop.
3. Profibus-DP communication testing and field use
3.1 Communication Test
Since its development, the intelligent electric actuator with a Profibus-DP bus interface has undergone a series of tests and field applications. The tests employed a single master station and three slave stations. The master station hardware consisted of a laptop computer and a Siemens CP5512 card, while the software comprised SOFTNETDPV6.2SP1, STEP7V5.3SP3, and OPC Search. The laptop computer was configured as a Profibus master station and simultaneously set up as an OPC server using STEP7. The three SND series electric actuators were configured as slave stations and the configurations were downloaded to the PC (CP5512) master station, thus forming a Profibus-DP master-slave system.
Using OPC client software (OPCScout) on a PC, the data of the DP slave station can be read and written through the OPC server, thereby enabling the monitoring and control of the operating status of the electric actuator.
The bus baud rate was set to 1.5 Mbaud/s, the master actuator address was set to 2, and the slave actuator addresses were set to 3, 4, and 5. The master station sent open, close, stop, and run control words, as well as a set opening degree run control word. The actuators performed actions such as fully open, fully close, immediate stop, and run to the designated position, achieving the expected results. The master station read the slave station's content; in the status feedback, the actuator status word, remote control mode status word, actuator opening degree, torque percentage, etc., were all accurately reflected in the experiment.
3.2 On-site use
This product utilizes 28 SND series intelligent electric actuators with Profibus-DP bus interfaces in the seawater desulfurization system of the Shantou Haimen 2×1000MW power plant. The control system adopts the OVATION 3.1.2 system from Westinghouse Control Systems Co., Ltd., with Changzhou SND-Z type electric actuators, UK ROTORKIQ10F10A, and Suzhou intelligent MCC control device ST500 within the Profibus-DP bus segment. The transmission mode is Profibus-DP/V0, the data transmission rate is 185.7kb/s, the GSD file is VALVEFPS.GSD, and the communication data consists of 13 inputs (8 bytes), corresponding to INPUT1-INPUT13 of the OVATION system; and 4 outputs (8 bytes), corresponding to OUTPUT1-OUTPUT4 of the OVATION system. The bus topology is shown in Figure 9.
The PROFIBUS communication protocol ensures high reliability, but this is based on hardware and software design. When designing the communication interface, certain specifications must be followed, such as signal isolation, avoiding excessively long lines between the bus interface and the transceiver, power supply filtering, and ensuring that the current-limiting resistors and load resistors of the transceiver and optocoupler are properly matched. When signals are transmitted on the bus, impedance discontinuities can cause signal reflections, leading to signal distortion. Therefore, resistors must be added at the end of the transmission line to eliminate impedance discontinuities. The value of the added resistor should be as close as possible to the characteristic impedance of the transmission line. The characteristic impedance value is independent of the wire length and is generally 100–165Ω. In practical applications, attention must be paid to signal isolation, avoiding excessively long lines between the bus interface and the transceiver, power supply filtering, ensuring that the current-limiting resistors and load resistors of the transceiver and optocoupler are properly matched, and addressing issues such as line return reflections and terminating resistors.
After more than a year of use, the bus system has been operating normally and stably, requiring very little maintenance, and has met the owner's requirements.
The SND series intelligent electric actuators with Profibus-DP bus interface have also been used in some export projects, such as the Ramu project in Papua New Guinea by China ENFI and the Indian project by China CMEC, as well as some domestic projects, such as Sinopec Pipeline Company and Dalian Qingshan Water Plant. The results have been excellent and have been well received by users.
4. Conclusion
This paper introduces the design, testing, and application of a smart electric actuator with a PROFIBUS-DP bus interface. The actuator successfully connected to the PROFIBUS bus network via the PROFIBUS-DP bus card, achieving the expected control and feedback objectives. The development process also demonstrated that using the SPC3 protocol chip in slave development simplifies development and saves manpower and time. Due to the continuous nature of factory operations, it is necessary to develop redundant DP cards to ensure communication reliability and facilitate maintenance.
Currently, Distributed Control Systems (DCS) are relatively mature, while Fieldbus Control Systems (FCS) are gradually developing. With the development of fieldbus technology and the standardization of standards, field devices with fieldbus capabilities will inevitably become mainstream. In the long run, fieldbus systems offer significant advantages in terms of economy, accuracy, openness, flexibility, and maintainability. Since there are not many manufacturers in the actuator industry in China with successful experience in applying the PROFIBUS-DP bus, fieldbus-based products have a broad market prospect.
