Talk to a group of instrument engineers about ''smart'' positioners and it's soon apparent each person has evolved their own definition of what smart positioners are, what they do, and where they are used. Occasionally someone in the group says, ''Really? I didn't know or think about that." Even a casual examination of available smart or intelligent positioners reveals significant differences in d...
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Click here to read the longer version of this article, which includes an expanded list of suppliers .
Talk to a group of instrument engineers about ”smart” positioners and it’s soon apparent each person has evolved their own definition of what smart positioners are, what they do, and where they are used. Occasionally someone in the group says, ”Really? I didn’t know or think about that.”
Even a casual examination of available smart or intelligent positioners reveals significant differences in design philosophies, on-board intelligence, and application options being employed by manufacturers.
At one end of the spectrum, some manufacturers added a microprocessor to an existing positioner, making the positioner ”smart.”
At the other end of the spectrum are positioners designed from the ground-up to take advantage of current and future digital technologies.
No doubt the first group offers benefit. However, it’s the latter group that adds significant intelligence to what has become known as digital valve controllers (DVCs) or digital valve positioners (DVPs).
Unfortunately, DVC and DVP tend to restrict a person’s thinking to valves. IP (intelligent positioner) makes sense as a term, but is likely confused with I/P (current to pneumatic transducers).
Intelligent digital final device controller/positioners is what these devices have evolved into, but IDFDC/P doesn’t exactly roll off the tip of the tongue, so let’s agree, DVC is synonymous with intelligent digital final device positioner/controllers.
Beyond ”smart”
Smart devices tend to improve the ”here and now.” Intelligent devices do that and anticipate and predict what’s about to happen.
The characteristics of DVCs include:
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On-board microprocessor(s);
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Embedded sensors;
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Final device feedback sensors;
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Digital communications;
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Advanced diagnostics; and
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Specialized algorithms.
| Standard discrete inputs change state at a defined control / switch point. If the final control device ”hunts” near the control/switch point, a standard discrete input would repeatedly change state (i.e., switch chatter). Applying a configurable band around a proximity discrete input establishes a trip-point band around the control/switch point. Whenever the final control device enters the configurable band, the discrete input changes state and remains in that state until the final control device travel feedback sensor moves out of the configurable band.. |
Many manufacturers embed a variety of discrete and analog sensors in the DVC. However, each manufacturer attempts to differentiate its DVC design by applying sensors in different ways.
For example, Emerson Process Management’s Fisher Division (Marshalltown, IA) DVCs include one discrete output (DO) and four discrete input (DI) FOUNDATION fieldbus (FF) certified function blocks.
In addition to meeting the certified FF DI and DO functions, Fisher uses a proximity sensor in place of a standard (limit switch like) DI. (See ”Standard Discrete Input Compared to Proximity Discrete Input” diagram.)
Among benefits of using configurable proximity sensor bands is knowing the zero and/or 100% location of the final control device in applications, such as emergency shutdown, damper/louver positioning, and batch fill/dribble.
On-board diagnostics
DVCs use embedded sensors and on-board microprocessors to conduct a plethora of diagnostic tests and manage digital communications to/from host systems.
Depending on the DVC manufacturer, there are variations of tests conducted and the information collected and communicated to the host system in the areas of fieldbus, dynamic scan, and advanced diagnostics.
Some DVC manufacturers, such as ABB (Rochester, NY), have adopted a design philosophy that permits end-users to install the level of feature and diagnostic sophistication appropriate for the application.
Diagnostics available from most DVC manufacturers include:
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Final control device tracking parameters for monitoring total (accumulated) travel and the number of reversals (cycles);
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DVC health parameter tests to provide alerts of memory, microprocessor, and/or sensor problems and permit operator alarms or, for more severe problems, unit or process shutdown actions;
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Final control device alerts. These communicate expected travel deviations, travel to high/low limits, and/or when preset accumulated travel and cycle targets are exceeded;
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Dynamic error band tests to check hysteresis and deadband-plus-slewing in final control devices;
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Drive and output signal tests that vary the transducer setpoint at a controlled rate and plot final control device operation to determine dynamic performance; and
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Pedefined final control device maintenance and signature tests.
Some digital fieldbus protocols include pre-defined step maintenance tests for checking and/or establishing a final control device performance ”signature.”
In the case of FF, three pre-defined tests manipulate the DVC transducer-block setpoint and monitor the output pressure and final control device travel. The three FF pre-defined diagnostic tests are ”public,” thus any host system containing FF-defined device descriptions can run the diagnostic tests on any FF-certified DVC.
Some DVC manufacturers include a fourth dynamic step test designed to determine the current performance ”signature” of the final control device. If the device is new, the performance signature becomes the benchmark for future comparisons. If the device has been in service for a while, the present signature can be compared to the benchmark signature to determine when repairs and/or calibration will be required.
| Compared to springs, cams, cam followers, bellows, linkages, etc., found in mechanical positioners, DVC technology appears to be simple. However, appearance is deceiving because DVC technology delivers features and benefits mechanical positioners couldn’t begin to provide. |
Beyond traditional applications
In the beginning, DVC technology was applied as replacements for traditional I/P (current to pneumatic) transducers on rotary and sliding stem control valves. However, as end-users became more familiar with DVC technology, they began to consider other applications where DVCs could be applied.
Working with manufacturers, end-users defined new functions that have allowed DVCs to move from being smart to being highly intelligent and adaptable.
Two examples of where DVC technology has made significant application advances are partial stroke testing of emergency shutdown (ESD) valves and fan/damper actuators.
Emergency shutdown – Many companies are extending the run time of continuous processes to increase profits.
When process run times are extended, better maintenance and safety practices must be developed and applied.
Extending the run times of processes that include safety instrumented systems (SISs) often requires changing safety system testing frequencies and/or using partial-stroke testing. (See, CE , Nov. 2000, ” Partial-Stroke Testing of Safety-Block Valves. ”)
Partial-stroke testing helps operations personnel determine that ESD valves within safety systems are capable of operating on demand.
DVCs with on-board diagnostics and a host system where communicated performance data can be archived are being applied to ESD valves. Such ESD valves can become part of an automated partial stroke testing solution with automated proof of performance designed to meet regulatory requirements.
Patrick Flanders, instrument engineer with Saudi Aramco (Saudi Arabia), says Emerson Process DVCs brought a new level of ESD valve readiness and security to Saudi Aramco SISs at less cost.
Fan/damper actuators – One of the time-consuming activities associated with combustion control applications is conducting load tests and then cutting cams to linearize the action of forced and induced fan/damper actuators.
Mechanical positioners worked well for many years in boiler/combustion applications. However, especially on waste fuel and coal-fired units, mechanical positioners become subject to sticking as components become soiled.
When sticky positioner components are combined with loose or worn actuator linkages the actuator ‘hunts’ for the proper position, thus causing the associated process variable to be always above or below setpoint. The result of hunting manifests itself as poor control, incomplete combustion, additional wear and tear on moving parts, and/or other unnecessary cost and waste.
DVC technology applied to fan/damper actuators includes an independent feedback transducer to ensure the positioner moves to the setpoint demanded by the control system. (See ”Fan/damper Actuator with DVC Technology” diagram.)
Other features and benefits of applying DVC technology to fan/damper positioners include:
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Mechanical lockup on loss of air supply;
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Manual drive adjustment using handwheel or lever arm;
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Proof of position limit switches for purge/lightoff conditions; and
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Digital communications via HART or other fieldbus technology.
Several DVC manufacturers offer retrofit kits that allow replacement of mechanical fan/damper positioners with DVC technology. For example, Emerson Process offers PowerVue field retrofit kits for several models of Hagan torque-type power postioners.
Future trends
ARC Advisory Group’s (Dedham, MA), senior analyst David Clayton has been tracking the control valve and associated accessories market space for several years. During an interview with Control Engineering, Mr. Clayton indicated he’s surprised and encouraged how quickly the DVC market has grown.
Year 2000 sales of DVCs was more than $128 million, according to ARC’s ”Control Valve Worldwide Outlook” data. ”That means end-users are abandoning the ‘if it ain’t broke, don’t fix it’ attitude and coming to terms with the reality that poor valve [final control device] performance is capable of sabotaging the performance of the entire manufacturing system and negatively affecting a company’s bottom line,” Mr. Clayton says.
That jives with what Michael Jost, vp of measurement and instrumentation for Invensys Production Management (Foxboro, MA) says: ”In the past few years end-users and engineering contractors have significantly increased how often they specify DVCs. They’ve come to realize the contribution DVCs deliver in the way of improved quality and lower costs of ownership.”
Technology user/specifiers often experience nervousness about when to ”jump in.” Jumping in while the technology is very new exposes users to technology bugs and growing pains. Waiting till the technology is deemed ”proven in use” often means buying at the end of the technology cycle and losing years of benefits.
DVC technology appears to be solid, and because of the modular design most manufacturers have adopted, users should encounter little pain updating and upgrading to obtain ”future” features.
Manufacturers have a myriad of future DVC features planned, such as described by Invensys’ Mr. Jost. ”Invensys is applying the German electrical and electronic manufacturers’ association (Zentralverband der Elektrotechnik und Elektronikindustrie [Frankfurt, Germany] ZVEI) field device tool (FDT) standard as part of the ArchestrA software platform.
The ZVEI FDT standard allows devices from a variety of manufacturers to be transparently integrated into a common host automation system, independent of fieldbus communication technologies.” ( Read the expanded version of this article to learn more about the ZVEI FDT/DTM standard .)
Other technology enhancements, such as non-contact position sensing, autotuning, more sophisticated diagnostics, and expert systems are, or soon will be, available.
For example, Masonelian (Addison, TX) recently introduced the use of solid-state, magnetic, non-contact position sensor for detecting valve stem movement through its SV II DVC positioner wall.
At Emerson Process, DVC product development manager Mike Rauber, says, ”We are working to make DVC devices easier to use by moving diagnostics that currently run in a host system into the DVC. We also are developing more sophisticated, predictive diagnostics served up as easily accessible, easily understood alerts that include specific instructions on how to solve developing problems. Simultaneously, we are tightening DVC integration with computerized maintenance management systems to allow automatic generation of work orders based on diagnostics reported by the DVC.”
Like workhorses, final control devices come in a variety of sizes, shapes, and descriptions. When working together in a production operation, each workhorse must pull its weight, or production goals are jeopardized. Making the team stronger also means making the team smarter. DVC technology is all about making those workhorses smarter.
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Intelligent electro-hydraulic actuators offer another option
Most digital valve controllers require pneumatics (air, inert gas…) to move the valve actuator and thus change the flow through the valve body. Compressibility of the pneumatic controlling media can influence final device positioning.
For several years, servo-valve electro-hydraulic actuators have been able to overcome pneumatic compressibility issues. Unfortunately, such solutions have been complex, unreliable, and costly.
One company claiming to have overcome the shortcomings of servo-valve electro-hydraulic actuators is Rexa Koso (West Bridgewater, MA).
Rexa Koso’s Electraulic technology uses a unique hydraulic circuit to control hydraulic pressure independent of the magnitude or direction of the load. The company claims positioning performance as low as 0.05% deadband without instability is achievable.
Other features/benefits of electro-hydraulic actuators are:
100% modulating duty cycle;
Menu-driven configuration and calibration;
User-selectable deadband from 0.1% to 5%;
Positioning accuracy to 0.15%;
No operational effects of static or dynamic friction;
Fail-in-place or spring-activated fail action; and
Declutchable handwheel.
Electro-hydraulic actuator manufacturers offer a wide assortment of control valve mounting options.
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