A fieldbus (uncapitalized) is a method for digital communication between control devices.
Since the advent of digital communication technology, several fieldbuses have been developed.
ular segments of the automation industry. The most highly developed fieldbus for process con-trol applications, the FOUNDATION™ Fieldbus,3 was developed by a consortium of manufacturers known as Fieldbus Foundation. It is one of the fieldbuses approved by the IEC 61158 standard.
FOUNDATION™ Fieldbus (hereafter abbreviated as FF) is more than a digital communica-tion technology. The standard also includes the definicommunica-tion of funccommunica-tion blocks that make it pos-sible to distribute the control strategy into field devices (Refs. 5-6, 5-7). For instance, a transmitter can send a signal to a valve positioner, which contains a PID algorithm. Both of these devices communicate over the same two-wire digital network to a host computer, which provides the human interface. Figure 5-17 shows one segment of an FF installation.
Among the cited benefits of this approach are these:
• Lower installation costs, since the amount of wiring, conduit, and marshaling panels required is reduced.
• Lower capital equipment cost, because the system provides single-loop integrity, thus providing high availability without the need to purchase redundant components.
3. FOUNDATION™ is a trademark of the Fieldbus Foundation, Austin, Texas.
Figure 5-17. FOUNDATION™ Fieldbus Architecture
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• Less time spent in commissioning, since there are fewer wiring terminations.
• Reduced maintenance costs, since the field devices provide extensive diagnostic infor-mation regarding their own health, combined with the fact that much of the mainte-nance can be done from the control room, thus avoiding (in many cases) the need to send a technician to the field.
• Interoperability. Since every manufacturer is required to adhere to strict standards, devices from various manufacturers can co-exist on the network.
• Limited interchangeability. Also a benefit of the standardization; the devices of one manufacturer can be replaced with the devices of another. (However, if a manufacturer has added enhanced features, another manufacturer’s product may not provide those same enhancements. At the least, some reconfiguration effort may be required.)
• Fewer spare parts and devices required, due to the interchangeability feature.
• Improved control. Although it is primarily the control strategy itself and the tuning of the controllers that determine the quality of control, not the physical residence of the PID algorithm, there are control improvement benefits that can accrue due to a number of FF features. For example, FF can provide true reset windup protection on every loop, originating from the actual position of the valve. If a DCS were used, obtaining position feedback from the valve would require the positioner to have a 4–20 mA out-put card and the DCS to have an additional AI point, plus the wiring between them.
Although possible in theory, this was rarely done in practice, due to the additional expense. Other features providing control improvement are status determination (e.g., good/bad/uncertain signal validity); increased accuracy due to the elimination of D/A and A/D converters in transmitters, controllers, and positioners; and the availability of secondary measurements from devices, such as process temperature, static pressure, and density.
There are numerous sources of information regarding the communication, engineering, config-uration, and installation aspects of FF. We will briefly cover here only the process control aspects of FF and the basic regulatory control strategies available using FF. Subsequent chap-ters will introduce various types of advanced regulatory control strategies, and each will close with a discussion of the FF application considerations for that particular control strategy.
The Fieldbus Foundation document FF-891, Part 2 (Ref. 5-8) defines ten function blocks:
AI analog input DI discrete input ML manual loader
CS control selector
PD proportional-derivative controller
PID proportional-integral-derivative controller RA ratio
AO analog output DO discrete output
FF-892, Parts 3, 4, and 5 (Ref. 5-9) define additional function blocks. These include the fol-lowing function blocks, which can be used in the advanced regulatory control strategies dis-cussed in subsequent chapters:
IS input selector LL lead-lag DT dead time CA calculate AR arithmetic
The definitions are very explicit about the way function blocks interact with other function blocks, even with function blocks in devices provided by different manufacturers. This includes block linking, status passing, block initialization, and the like. For the PID function block, the standard defines sixty-five parameters, including the mnemonic, the exact data structure for each, and the type of access for each. As one example, the CONTROL_OPTS parameter (parameter #13) is a two-byte bit string, accessible only when the function block is in the out-of-service mode. Each of the bits has a defined purpose. For instance, one of the bits specifies whether the block is direct- or reverse-acting.
All manufacturers who provide a PID function block must use this exact parameter definition list. Manufacturers may, however, enhance their products by adding to the defined parameter list. Since the standard does not specify a mathematical formulation for the PID algorithm itself, manufacturers are free to choose a form or add to the defined parameter list to provide optional forms of the PID algorithm. In their standard PID function block, Smar (Ref. 5-10) implements the “ISA” algorithm (called earlier in this chapter the “noninteractive” form) with derivative on PV. In contrast, Fisher-Rosemount (Ref. 5-11) gives the user a configuration option of the “standard” (called “noninteractive” in this chapter) or “series (“interactive”) form, with choices of PID action on error, PI action on error—D on PV or I action on error—
PD action on PV. This is determined by the setting of parameter #73, MATH-FORM, which is not one of the sixty-five parameters in the standard definition. On the other hand, Smar also offers an advanced PID (APID) function block that provides algorithm configuration options similar to Fisher-Rosemount, determined by the setting of parameter #76, PID_TYPE.
These are merely examples of manufacturer-to-manufacturer differences in areas beyond the standard FF definition. They do, however, highlight the fact that if you are using enhancements to a function block beyond the FF standard definition, and you replace the device with another manufacturer’s device, the enhanced features on the original device may not be present on the replacement device. Or you may have to make some changes to the configuration to utilize
turers may not be truly interchangeable.
FF Function Block Classes
There are four subclasses of function blocks (Ref. 5-6):
• Input class
• Control class
• Calculate class
• Output class
Input class blocks connect to sensor hardware via an input transducer block over a hardware channel.4 Control class blocks perform closed-loop control and have back-calculation func-tionality to provide bumpless mode transfers and reset windup protection, among other fea-tures. Calculate class blocks perform auxiliary functions required for control or monitoring, but do not support the back-calculation mechanism. Output class blocks connect to actuator hardware via output transducer blocks over a hardware channel and support the back-calcula-tion mechanism.
FF Basic Control Strategy
A basic control strategy configured from standard FF function blocks is shown in Figure 5-18.
This does not appear to be all that different from Figure 5-14, except that the AI function block must be in a transmitter and the AO function block must be in a valve positioner. The PID function block can be in the transmitter, the valve positioner, or in some other device. Its loca-tion will have an implicaloca-tion on loading of the communicaloca-tions network, however.)
The AI block receives its input from a transducer block. It filters and scales the input; performs any required calculations, such as square-root extraction; and passes the value to its OUT parameter in engineering units (e.g., 307ºF). Since every function block has a number of modes, including out-of-service (O/S), automatic (AUTO), manual (MAN), and others, the AI block can be placed in the MAN mode and a simulated measurement value entered at the host.
In the basic regulatory control strategy, the PID block receives an operator-entered set point.
Its IN parameter, the process variable, is linked to the OUT parameter of the AI block. The input is filtered, scaled, and passed to the PID algorithm and also to alarm detection. The out-put of the PID algorithm is scaled, passed to outout-put selection (manual/automatic switch), then to output limiting, and then to the block’s OUT parameter. The BKCAL_OUT parameter will be used in cascade control schemes, which are presented in chapter 9.
All parameters that are passed from one block to another are also passed with appended status bits. The configuration of the receiving block determines the action to be taken in the event of abnormal status. For example, the block may revert to an initialization mode (IMAN) or hold its output at the last value.
The AO block receives its cascaded set point (CAS_IN) from the OUT parameter of the PID.
(Note that the input to the AO block is considered as its “set point”; since this is originating from another block, this is considered as “cascade.” (This is slightly different from conven-tional instrumentation terminology. The AO block can be considered as a “servo positioning PID controller” where the desired valve position is the SP and the actual valve position is the PV.) This signal is scaled, rate-limited (if configured), and passed to the transducer block for valve actuation.
Normally, the scaled and limited set point is passed to the AO block BKCAL_OUT parameter.
This is linked to the BKCAL-IN parameter of the PID block. Suppose the loop is opened and Figure 5-18. Feedback Control Strategy Using FOUNDATION™ Fieldbus Function Blocks
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to set the AO block to Auto, rather than its usual Cas mode. This is reported to the PID through the BKCAL_OUT – BKCAL_IN link. If the I/O option “Use PV for BKCAL_OUT” is set in the AO block, the process variable (actual stem position) is used for the BKCAL-OUT param-eter, and the status bits of BKCAL_OUT force the PID block into an initialization mode (IMAN). Not only does this assure bumpless transfer when the valve is taken out of “hand”
operation, but it also prevents windup in the event that the valve stem is limited, either physi-cally or in software.
There are many additional features of the FF function blocks, but they are beyond the scope and space limitations of this book.
REFERENCES
5-1. F. G. Shinskey. Process Control System – Application, Design and Tuning. 4th ed., McGraw-Hill. 1996.
5-2. C. L. Smith. Digital Computer Process Control. Intex Publishers, 1972.
5-3. Samuel M. Herb. Understanding Distributed Processor Systems for Control.
ISA,1999.
5-4. Honeywell Automation and Control Solutions. HPM Control Functions and Algo-rithms.
5-5. Siemens Energy and Automation. Model 354N Universal Loop Controller User’s Manual.
5-6. J. Berge. Fieldbuses for Process Control: Engineering, Operation and Mainte-nance. ISA – The Instrumentation, Systems, and Automation Society, 2002.
5-7. Fieldbus Foundation, FOUNDATION Fieldbus System Engineering Guidelines.
AG-181, Rev. 1.0.
5-8. Fieldbus Foundation, Foundation Specification: Function Block Application Pro-cess, Document FF-891, Part 2.
5-9. Fieldbus Foundation, Foundation Specification: Function Block Application Pro-cess, Document FF-892, Parts 3, 4, and 5.
5-10. Smar, FOUNDATION Fieldbus Function Block Manual, February 2003.
5-11. Fisher-Rosemount, Foundation Fieldbus Blocks, 00809-0100-4783, Rev. BA.