Process Control Goes 'Off to See the Wizard'

This is the second installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Others include temperature sensing (June), flow measurement (September), and level sensing (November).If you really want to stir up some controversy in the control room or up in the engineering department, get on the subject of smart sensors.

05/01/1998


KEY WORDS

 

  • Process control & instrumentation

  • Smart Sensors

  • Pressure sensing

  • Temperature sensing

Sidebars:
Top 10 commercial applications for smart sensors
Fiber optics 'get into the act'
No fish with these chips!

This is the second installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Others include temperature sensing (June), flow measurement (September), and level sensing (November).

If you really want to stir up some controversy in the control room or up in the engineering department, get on the subject of smart sensors. Even though these devices have been around in one form or another almost since the development of solid-state electronics, one of the hottest topics of conversation still is—just what is a smart sensor? Certainly developing a consensus on sensors with a "brain" would be difficult to obtain.

Simply put, whether so called smart sensors do something as simple as communicating digitally or as advanced as handling on-board algorithmic calculations, silicon is where the well-known rubber meets the (Yellow Brick) road. Silicon-based intelligence at the point of measurement provides not only the ability to react to and manipulate input but also to tailor outputs, identify and describe themselves to the control networks, respond to network software instruction, and interact with data they collect.

Besides their ability to manipulate and send data—on command or of their own volition—to PCs or controllers that require it, smart sensors often address more basic problems. The small size and high power of stand-alone micromachined sensors allow intelligent monitoring of process variables in areas where conventional smart instrumentation cannot be adapted. The "single lump" construction of the "sensor on a chip" package is resistant to overpressure, shock, and vibration. Smart sensors are commonly available in traditional instrumentation packaging. These devices share the "look" of standard devices but have the onboard computing power to handle sophisticated control situations.

Quick communication

The most basic capability of smart instruments is the ability to communicate in nonanalog fashion. Digital communication of the highway addressable remote transducer (HART) type is one of the most common protocols used with smart sensors. This form of communication is one of several—RS-485 and Profibus PA are others—used in Endress+Hauser's (E+H, Greenwood, Ind.) Promag electronic flowmeter. Evolved from E+H's original silicon-based magmeter, Promag's features include permanent calibration, onboard memory/storage capabilities, and onboard computing that can calculate instantaneous flow rate and totalized volume.

Although digital communication was required by the installation, its other onboard capabilities allowed a Promag 33A, installed in a BASF Corp. chemical plant near Baton Rouge, La., to accurately measure a very small but critical flow (0-0.5 gph). Responsiveness afforded by onboard computing and the unit's calibration stability helped provide the user with an accuracy of less than 0.1% of reading. The instrument's responsiveness also helped identify a line surge that had previously caused total flow at this point to be suspect.

"Keeping a sensor signal digital from the very beginning allows for more complex and precise signal processing," according to Smar International's (Houston, Tex.) Jonas Berge. "Smart sensor signals also remain free of the noise pickup and signal degradation associated with analog sensing. The benefit of digital sensors is higher performance, better flexibility, less maintenance, and more diagnositic capability."

Standing true to this technology, Smar International manufactures the LD302 Fieldbus pressure transmitter with uses a digital sensor and digital processing for onboard computations and control. The device features self-diagnostics with alerts and remote interrogation ability. It uses FOUNDATION Fieldbus protocol.

Self manipulation of data is really what smart sensors are all about. Once a manufacturer has found a way to store information on the sensor, adding calibration data can be the next step. Once a device is calibrated and the calibration data are stored, a smart sensor can use that information to correct for nonlinearity and other anomalies.

Quicker changes

When properly leveraged, onboard memory and storage for configuration and calibration data enables sensors to be easily field calibrated. Drexelbrook Engineering Co.'s (Horsham, Pa.) SLT RF/Admittance level transmitters have this capacity. They are available with either Highway Addressable Remote Transducer (HART) art or Honeywell Digitally Enhanced protocols and can be calibrated through a hand-held communicator or a laptop computer using proprietary software for the appropriate protocol.

However, when level needs to be measured in undedicated tanks, liquids that change volume due to temperature, batch reactors, and mixing tanks, ease of configuration is clearly not enough. Sufficient onboard computing power allows the Drexelbrook True Level III model to gather secondary variable inputs, make necessary computations, and adjust its calibration, based on changes in the material's electrical or physical properties. These devices can also be used in liquids, slurries, granular and hazardous materials, and liquid interfaces.

Making use of secondary variable inputs and on board computing power allows smart sensors to lend added value to some applications. According to Larry Rice, vp of field measurement and control at The Foxboro Co. (Foxboro, Mass.), "Smart features built into I/A Series intelligent pressure transmitters have solved some interesting real-world problems. Case in point; a control engineer at a Canadian processing plant uses the pressure sensor's secondary temperature measurement as a means of monitoring the transmitter body temperature. The output is used to trigger alarms if the heat tracing on the transmitter fails to keep the transmitter temperature within acceptable limits."

This particular application uses a FoxCom two-wire I/A Series device digitally integrated into the I/A Series system with the pressure and temperature readings transmitted simultaneously as a digital signal. This approach eliminates the need to purchase, install, and field wire two separate transmitters (pressure and temperature) to gain peace of mind from knowing that potential transmitter damage and the resulting process downtime are being avoided. "The customer applied a value of $300,000 to this feature," Mr. Rice continues.

Cheap and powerful

As today's microprocessors get even less costly and more powerful, sensor manufacturers "shoehorn" them into the package. With all this on board capacity, can algorithmic data manipulation be far away?

Parti-Mag II manufactured by Bailey-Fischer & Porter Co. (BF&P, Warminster, Pa.) is an example of an intelligent sensors used in an application-specific flow measuring device. The Parti-Mag is a magnetic flowmeter that measures flow in partially filled pipes, making full use of algorithmic manipulation.

According to its developer, before the release of this device only flumes, weirs, and some ultrasonic devices could measure flow under partial-fill conditions. Unlike flumes and weirs, however, the BF&P device can be used on pipe lines up to 5° slope.

Parti-Mag uses two interactive variable readings to determine flow rate. First, fill level and average fluid velocity are determined. From these data, the cross-sectional area is calculated and then volumetric flow rate. Because average fluid velocity is affected by fill height, a correctional algorithm must be included to offset computational errors. Additional complexity is added to the computations by electrodes that must alternate taking fill and velocity measurements to make the meter viable.

Collecting data

Ability to collect process data for both control and quality-related functions, such as statistical quality control tracking or ISO 9000 certification, is a function of onboard digital signal processing that can be found in smart instruments. Raytek's (Santa Cruz, Calif.) Marathon MA2S infrared (IR) process temperature sensors have been used to do both. According to a Alan Young, Marathon's product line manager, many manufacturers use the device precisely that way.

Dr. Young continued, "Raytech is currently supplying a titanium rivet manufacturer with a Marathon Series device that it uses to control forming temperature (900 8F) of rivets at 400 per min. Besides being able to 'see' each rivet at this speed with its 1.0 millisec response time, the MA2S also has optics that allow a measurement spot as small as 0.06 in. required for these small fasteners.

"Onboard circuitry allows the IR device to act as both transmitter and receiver, allowing bidirectional communication between the sensor on the factory floor and control room computers. In this case, smart communication allows process engineers to use it as part of a closed-loop control system and for data archiving to a hard disk over time. In addition, engineers can receive temperature data in real time for process tracking and modification."

The Honeywell-Measurex (Cupertino, Calif.) Pulpstar sensor is an in-line device that provides continuous measurement of various pulping quality variables. Raw signals are processed by this optical device's microprocessor to generate real-time measurements. The continuously generated signal provides reliable measurements for closed-loop control of the pulp's bleaching process, ensuring correct application of bleaching chemicals and minimizing environmental impact of the process.

The ability to handle store and use data on board puts the Pulpstar into the smart category. The device is equipped with a dynamic integral data logger that records all internal measurements as well as external analog inputs used in the measurement and control algorithms. This feature is said to provide an easy-to-access, accurate record of the process for use in commissioning, tuning, and for tracking sensor maintenance.

In the Pulpstar's case, the precision and accuracy that microprocessor-based tuning and tracking functions impart to device's operation have allowed it to satisfy the U.S. Environmental Protection Agency's requirements for mill control while reducing use of chlorine-containing chemicals and maintaining final quality specs.

More power

Because two pressure transmitters are traditionally required to accurately measure the mass flow of saturated steam, keeping track of this process variable adds extra hardware, setup, and maintenance expense. Additionally, converting from steam flow to mass flow requires steam table calculations.

To simplify making this measurement and control both setup and hardware costs, Moore Products Co. (Spring House, Pa.) has introduced a smart instrumentation line with a purpose-built device for steam applications. The XTC line's Model 340S uses its multivariable silicon sensor to measure both absolute and differential pressure across an orifice plate or similar flow element. The unit's microprocessor uses these measurements and steam table data stored in its memory to directly compute flow, energy transfer (BTU/hr), and mass flow (lb/hr).

So, will smart sensor, like the scarecrow, have to prove themselves to discover the value of intelligence? If integration into a fully digital control system is required to get the most out of them, then the process industries still have some time to wait out migration from analog to full digital. However, for many benefits that are compatible with digital over analog technologies such as HART, their time is closer at hand.




Top 10 commercial applications for smart sensors

According to a Battelle Research Institute (Columbus, O.) focus group, smart sensors have potential in many sectors, even nonindustrial applications have definite control overtones. Possible commercial applications include the following:

Adaptive optimizing plant controls;

Adaptive machine diagnostics and maintenance;

Personal health treatment and monitoring;

Intelligent highways and vehicles;

Voice recognition security and control systems;

Automated home systems;

True system plug and play connectivity;

Autonomous service robots;

Automated microclimate farming; and

Automated stores.

Fiber optics 'get into the act'

Using fiber-optic technology as the platform for smart sensors has been the focus of development efforts at the Centre for Advanced Materials (CAMT) at Monash University (Melbourne, Australia). According to a spokesperson for this national center for teaching and research, CAMT developed both extensive expertise and practical experience in fiber-optic-based sensors.

Researchers at CAMT have discovered that optical fibers can be used to sense changes in chemistry and in physical properties such as pressure, temperature, strain, and magnetic field strength. And because they use light instead of electricity, they are safe to use in explosive or hazardous environments. Fiber-optic sensors can theoretically be built to different degrees of "smartness" by internally processing and screening input data to reduce erroneous output and resulting false alarms.

Acting as sensors

The capacity of optical fibers to act as sensors is based on the property of light to bend or refract at the boundary between two different materials. Optical fibers are made of two different materials—an inner glass (or plastic) core and the glass cladding that surrounds it. Functionally, light is trapped inside the inner core and reflected back at the boundary where the core and cladding meet, allowing it to travel down a "pipe" with a mirrored inner surface.

There are only a limited number of paths—down the center or bouncing off the mirrored interface in set patterns, or nodes—that a beam of light can take in a fiber. CAMT's research has shown that changes in pressure and temperature acting on a fiber affect the number of paths the light takes as well as the amount of light traversing each path. Therefore, sensing pressure and temperature changes depends on monitoring how the light travels down a fiber.

Monitoring chemistry

Another property of fiber-optic transmission—some of the light traveling down the fiber escapes from the glass core into the glass cladding before it is reflected back—enables fiber optics to be used in monitoring chemical properties. If the cladding is sufficiently thin, it is possible for light to travel through whatever material is surrounding it before reflecting back to the core. And determining the components of light absorbed by the surrounding material can give an indication of what that material is. This principle can allow a length of optical fiber with thin cladding to be used to detect certain chemicals within surrounding materials.

Although still in the research stage, these applications have shown that pure fiber-optic-based devices are a "possibility" in process applications. As breakthroughs in fiber-optic technologies continue, users of smart processing sensing may just see the light.

No fish with these chips!

Although it sounds like a menu option, an all chip system for data acquisition could be in your future. ADuC812 developed by Analog Devices Inc. (Wilmington, Mass.), is a single-chip device with microcontroller, memory functions, and data conversion circuits. According to the company, it will be the first IC to support the IEEE 1451.2 common transducer interface standard. This means that smart transducers, when used with ADuC812, become network-independent, interchangeable devices.

The device features dual 12-bit D/A conversions and a 12-bit A/D conversion. Included are low-voltage flash memory and industry-standard 8052 microcontroller cores with several standard serial port configurations. ADuC812 is intended for data acquisition applications that require precision, data retention without power (nonvolitale memory), remote management, or comounting of standard transducer interface module and the sensor.

According to Stan Woods, chairperson of the IEEE 1451.2 working group, "This device will simplify the design of 1451.2-based smart transducer interface modules. Its single chip provides the functional blocks to implement IEEE 1451.2 interface logic, memory to store the transducer electronic data sheets, and circuitry to convert sensor signals or drive actuators." The device's small size allows a 5:1 reduction in footprint size over current data acquisition system.



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