NAGAI Koji1 HASHIZUMI Kazuhio1 MURAKAMI Tatsuhiro1 SAITO Yoji1
Yokogawa released the 5502 analog current to pneumatic converter and 5503 valve positioner in 1961, and the PK200 analog current to pneumatic converter and VP200 valve positioner in 1992. These models have been chosen for many plant control applications. Furthermore, it released the YVP110 FOUNDATION Fieldbus valve positioner in 1999, endeavoring to improve the efficiency and usability of positioners and to reduce lifecycle cost through online diagnostics. Meanwhile, demand for FOUNDATION Fieldbus to pneumatic converters has grown as 1) pneumatic signals are now used for final control elements even in fieldbus instrumentation, 2) there are requests to convert plants instrumented with pneumatic control devices into fieldbus-networked plants, and 3) many instruments need to be driven directly by pneumatic pressure. We therefore developed the YPK110 fieldbus to pneumatic converter. This paper introduces the structure, control method, and online diagnostics of the YPK110.
|Figure 1 YPK110 Fieldbus
to pneumatic Converter
Recent improvements in digital signal processing and network technologies have enabled advanced functions to be built into field devices, such as diagnostics and local controls, which could not be incorporated in traditional 4-20 mA analog field devices. Also, due to international standardization initiatives for field networks such as FOUNDATION Fieldbus, it has become possible to freely choose field devices from many different manufacturers in order to build the optimum field network system.
As a result of such technological progress and standardization, there is a growing trend toward replacing the pneumatic or analog instrumentation of plants with fieldbus-based instrumentation, and many customers are revamping by replacing their pneumatic instrumentation with the latest FOUNDATION Fieldbus-based instrumentation. Some of these projects require system conversion to FOUNDATION Fieldbus instrumentation while leaving the existing final control elements such as control valves and pneumatic positioners in order to minimize the switching cost. At the same time, the so-called "hot cut-over," the method of replacing equipment without shutting down the plant, is required.
To meet these requirements, we have developed the YPK110 fieldbus to pneumatic converter that can start controlling various valves and pneumatic positioners without requiring individual adjustments after installation on site. Its unique control algorithm, structure, and functions are introduced below. Figure 1 shows the appearance of the YPK110, and Table 1 the general specifications.
Table 1 General Specifications of YPK110
|Power supply||9 to 32 V DC|
|Communication||Digital two-way communication (FOUNDATION Fieldbus)|
|Supply air pressure||130 to 150 kPa (for 20-100 kPa standard output)
230 to 260 kPa (for 40-200 kPa doubled output)
|Output pressure||20 to 100 kPa standard output 40 to 200 kPa doubled output|
|Output signal||P a, bar, psi|
|Power consumption||Max. 17 mA (16 mA typical)|
|Function blocks||1 Analog Output
2 Discrete Inputs
1 PID control function (optional)
1 Output Splitter
|Linearity||± 0.2% of full span|
|Repeatability||0.1% of full span|
|Hysteresis||0.2% of full span|
|Air consumption||0.32 Nm3/h or less (@140 kPa)|
|Air delivery||6.6 Nm 3/h or more (@140 kPa)|
|Mounting||2" pipe or wall mounting|
|Ambient temperature||–40 to 85°C|
Figure 2 Block Diagram of YPK110
Figure 2 shows the block diagram of the YPK110. The digital signal input via the fieldbus is transferred to the media access unit (MAU), fieldbus modem, then microprocessor (CPU) for computation and conversion into an analog signal, which is input to the current to pneumatic (I/P) converter. There, the current is converted into the pneumatic pressure (nozzle back pressure) Pn, and amplified by the control relay, and transferred to the valve, pneumatic positioner, or the like as the output pressure Po. This output pressure Po is also returned to a pressure sensor in the YPK110 for feedback control.
In the YPK110, the air exhausted from the I/P converter and control relay is utilized to maintain the inside of the YPK110 casing at a positive pressure, thereby preventing the external humidity and ambient gases from intruding into the casing. For this, two pressure sensors, one for measuring the output pressure Po and the other for the ambient pressure Pa, are incorporated and the correction by ambient air pressure is performed at all times based on the differential pressure between them. At the same time, a built-in temperature sensor is used for temperature compensation to eliminate the errors on pressure measurement from changes in temperature.
Figure 3 Control Block Diagram of YPK110
The devices to be driven by the YPK110 vary widely in load capacity. Pneumatic positioners require tens of cubic centimeters but direct drive of a valve may require hundreds or sometimes thousands of cubic centimeters (a booster relay is often used for capacities of 3000 cc or more). Besides, the length of the pneumatic signal piping between YPK110 and valve may also vary widely from less than 1 m to 5 m or more since the YPK110 will be installed on a panel or mounted on a stanchion pipe for ease of maintenance. Even where the load conditions differ greatly as above, stable control of valves and valve positioners without adjustments is required.
Furthermore, the control relay, which is an essential component for amplifying the output pressure, is structurally associated with a dead band within which the output pressure does not respond to changes in the input pressure, and this makes control difficult. To solve these challenges, the YPK110 adopts a new control approach as described below.
|Figure 4 Load Condition Estimation|
|Figure 5 Control Parameter Selection|
|Figure 6 Control Characteristics|
The YPK110 implements self-diagnostics (see the next paragraph for details) and integration functions (the air pressure rise/fall time, total air pressure, air pressure output time, and air pressure low-cutoff time) for online diagnostics of the valve and control device. The device temperature and measured output pressure can also be represented as parameters. These enable not merely diagnostics of the YPK110 itself but also detection of a drop in the supply air pressure and leakage in piping. Moreover, by comparing the current working statuses with previously collected data values, the service life of the valve can be predicted.
The major items monitored by the YPK110's self-diagnostics include:
The features of the new YPK110 fieldbus to pneumatic converter were outlined above.
The YPK110 is a low-cost optimum solution for revamping of the existing pneumatic instrumentation of plants into fieldbus- based instrumentation while leaving the existing final control elements and plant utility, and can also be used as a leading-edge signal converter to convert fieldbus signals into pneumatic signals at new plants. It can control actuating cylinders, long- stroke valves such as gate valves, and small-capacity valves, that cannot be handled by a valve positioner.
These merits will reduce the total cost not only of renewing existing plants but also building new ones. We will continue to expand the lineup of the fieldbus devices and develop functions for reducing lifecycle cost in response to customer and market needs.
The PK200 current-to-pneumatic (I/P) converter offers a low-cost method to convert a milliamp input (4-20 or 10-50mA analog signals) to a proportional pneumatic (air pressure) output. The unit is designed for a broad range of process control applications, including those that feature pneumatic actuating equipment. The PK200 features built-in stainless steel input/output gauges.
The VP200 current-to-pneumatic valve positioner converts a 4 to 20mA DC or 10-50mA DC signal to a pneumatic signal for control valve actuation.