Fieldbus control system is a factory field network control system. It is a new technology combining computer network and control. It is a new generation of control system after distributed control system DCS. In the 800 thousand ton / year methanol project of CNOOC, field bus control technology is used in all control. FF bus valve positioner, as an important device in FCS, greatly improves the positioning accuracy and responds to process interference more quickly, which plays a major role in the stable operation of the device.

1, CENTUMCS3000 field bus system structure
The 800 thousand ton / year methanol plant adopts the field bus control system of YOKOGAWA. In the system, CPU, I/O card, power, bus communication card ALF111 and bus power are all equipped with redundant configuration. Each segment is equipped with 7~9 bus instruments. All control valves in the system use the FF bus locator. The number of locators with a locator is generally not more than 2 units, as much as possible to reduce the impact of the segment failure on the regulating valve.

2, FF bus valve positioner brief introduction
The internal structure of FF locator is different from the ordinary HART intelligent locator or conventional (4~20mA.DC) locator. It is a digital valve controller designed for FF fieldbus system. It has many prominent features: interoperability, process control, communication function, and microprocessor (CPU) technology between instrument and equipment. The FF bus locator is powered by 9-32V bus, and is controlled by digital signal. Its transmission conforms to the FOUNDATIONTM field bus regulation, and transfers data at the speed of 31.25kbit/s.

3, FOUNDATIONTM field bus block model
The FOUNDATION field bus assigns all the device functions and data to 3 different blocks. Each module has different task scope in the module model.

3.1, resource block (ResourceBlock)
The resource block contains all the data used to identify the device, which is similar to an electronic device label. The resource block parameters include device type, device name, factory ID, serial number and parameters that affect the performance of all other device blocks. Each bus device has only one resource block.

3.2, conversion block (TransducerBlock)
The conversion block contains all data and specific device parameters to connect process parameters (sensors or executor) and devices. The AO conversion block of the FF bus locator can directly affect the output signal of the locator.

3.3, functional block (FunctionBlock)
The function block is responsible for the control state of the FOUNDATIONTM field bus device. Including AI/AO/PID and so on. The FOUNDATIONTM field bus device can be configured by the input and output of the connection function block. The AO function block transforms the output value from the upstream function block to the control value of the control valve. The PID function block has a flexible proportion - integral - differential control algorithm, which can be configured according to the needs of the application.

4, CS3000 system configuration
The FF bus locator is very different from the traditional locator in the configuration. Each bus locator must assign the corresponding address, define the bit number, get the ID and so on.

4.1, Fieldbusbuilder
The bus locator is added to the FieldbusBuilder of the YOKOGAWACENTUM software Systemview and the bit number, address, and access to the device ID are defined.

4.2. Definition of module parameters in a locator
The AOPID module parameters are configured in ParameterEdit, including module mode, measurement range, output range, unit, control options, status options, etc.

Advantages of 5, FF bus locator relative to conventional locator

Compared with the conventional locator, the FF bus locator is more advantageous than the conventional locator through the experience of construction, debugging and daily maintenance.

5.1, save construction commissioning time
The use of traditional valve positioner through a process of calibration and correction circuit when installing and debugging, but also in the process of the loop will spend more time. As long as the FF bus valve can be automatically checked on the PRM, all the work can be done in the main control room, including the actual feedback of the valve.

5.2, decentralization of risk control
In 800 thousand tons of methanol project, make full use of the FF bus PID modules, the simple control all locator, direct control of process variables in the field, greatly reduced on a layer of the automatic control system to control the number of tasks, to realize the risk of distributed control system.

5.3. Powerful self diagnostic function
In the Henghe CS3000 bus system, the greatest advantage of the bus locator is a powerful self diagnostic function. The PRM of fieldbus provides various information for locator operation, such as locator setting value, output value and actual valve position, and also includes other information such as gas pressure, output pressure and device temperature. According to the opening of the valve, we can judge whether the valve has a hysteresis phenomenon, is the packing too tight or the stem has friction and other reasons.

When this special situation occurs, we can use the functional diagnostic software (DeviceViewer) of the FF bus system PRM. The related instrument operators will use this intelligent software to quickly and accurately discover the abnormal problems of the valve and prompt the corresponding solutions. The parameters of the valve positioner in normal operation are all normal (green). When the parameter is abnormal, it generates the corresponding parameters of the alarm (red), and shows the detailed description of the abnormal parameters in the drop-down menu bar. It provides a clear reference and basis for the instrument operators to solve the abnormal problems of the regulating valve faster and more accurately.

6. Conclusion
With the development of science and technology, the factory on the control system of the increasingly high demand, the FF bus device in the bus control system is greatly improved as the actuator control valve of the precision control circuit terminal, with self diagnostic function strong, greatly improved the control loop regulating quality, stable operation of chemical equipment plays an important effect. With the technical reform and continuous improvement of the FF bus valve positioner, the control system will be more and more accurate and efficient.