Single-Loop Controllers Dominate Marketplace
In 1942, John G. Ziegler and Nathaniel B. Nichols devised the Ziegler-Nichols Closed-Loop Method and Equation (aka PID). At that time, loop control was, at best, "cut-and-try." The hardware that Mr. Ziegler developed to make the Z-N method successful included the first pneumatic controller with reset (integral) added to the gain (proportional) control, as well as the first controller with...
In 1942, John G. Ziegler and Nathaniel B. Nichols devised the Ziegler-Nichols Closed-Loop Method and Equation (aka PID). At that time, loop control was, at best, "cut-and-try." The hardware that Mr. Ziegler developed to make the Z-N method successful included the first pneumatic controller with reset (integral) added to the gain (proportional) control, as well as the first controller with rate (derivative) function on the same device.
Pneumatic controllers have been with us ever since. Over 35 controller manufacturers make and sell pneumatic controllers. One prominent controller manufacturer says almost 40% of last year's controller sales were pneumatic models.
In 1951, the Swarthout brothers took loop control to the next step when they introduced the first electronic loop controller. The rest as they say, is history.
64% are single loop
To find out the status of the loop controller market today, CE conducted a survey of 1,500 readers. We received 273 replies (18.2%). Eighty-six percent of the respondents claimed their primary application involved continuous processing (36.0%), batch processing (14.5%), or both (35.5%).
Loop controllers come in either single- or multiple-loop versions. Single-loop models account for 64% of the controllers, while multiloop, 36%. Figure 1 shows how the survey respondents categorized their use of loop controllers.
Feedback control (85%) is a method in which a process variable's measurement is compared to the desired value. Any deviation produces an error signal. It's the task of the controller to act upon this error signal in such a way as to reduce its amplitude to zero, if possible.
Feedforward control (6%) is where information concerning one or more conditions that can disturb a controlled variable is converted into corrective action that minimizes predicted deviations of the controlled variable. Using feedforward control doesn't change the stability of the system because it isn't part of the feedback loop.
Cascade control (9%) is where the output of one controller is used as the setpoint for a second controller.
Automatic features desired
Today, many loop controllers supplement PID functions by offering adaptive control and fuzzy logic algorithms to enhance controller response and operation. PID and startup self-tuning are among the most important features.
Bob Heilman, control products marketing, Honeywell IAC (Phoenix, Ariz.) reports the four trends seen by Honeywell for the loop controller marketplace are: migration of PID loops to PLC-based systems (where the PID is in the PLC), increased competitive pressures (everyone has a controller), trend from 1/4-DIN to smaller sizes (1/8- and 1/16-DIN controllers), and single-loop controllers becoming a commodity item.
William T. Wright, manager of control products product management at Moore Products Co. (Spring House, Pa.) says "We find that many customers still want a controller to handle only one or two loops so as to provide a more manageable process in case of failure. Other customers demand higher capability, including multiloop capability, reusable function blocks, advanced math, AGA (American Gas Association) calculations, built-in trending, logic design with Boolean or ladder, etc." Mr. Wright goes on, "We are also requested to provide users different packaging options in addition to the conventional panel-mounted version." These configurations are wall mounted in shallow cabinets in the field, face-plates to be mounted directly on the controller for infrequent supervision, or mounted on the cabinet door with a NEMA 4 rating for day-to-day operations and periodic washdowns.
The survey shows users want loop controllers to communicate. Most important is upward communications, with peer-to-peer networking also important. The capability for manual control is the next requested characteristic. Users like automated control and its ability to move data but still want to maintain the process in the event of a failure in the automation.
At Yokogawa Corp. of America (Newnan, Ga.) Bill McPeters, marketing manger, reports, "communication options allowing use of PC/Windows-based GUI packages and configuration/programming software," are demanded by customers.
Ed Kulawiak, supervisory control product marketing manager, Omron Electronics Inc. (Schaumburg, Ill.) says, "customers demand for more 'openness' is driving development for device-level networks such as DeviceNet. In fact, Omron will soon release a single-loop temperature/process controller with DeviceNet."
Everybody wants everything
The survey asked respondents to rank eight controller characteristics with number one meaning the most desired and number five, the least desired. Theresults show that everybody wants all features—but, some features more than others. Even the lowest ranking feature is desired by almost all respondents. It's the old Cadillac or Lincoln Town Car debate. Neither is terrible; having either would be wonderful.
Honeywell IAC's Mr. Heilman reports that his company is seeing more demand for user-friendly configuration via front panel or PC, higher accuracy, standardized communication options, and more sophisticated autotune software.
Software is an important factor in loop controllers. Omron's Mr. Kulawiak says, "As controllers incorporate more complex programming menus than in the past, the availability of set-up and monitoring software for loop controllers is now a common request from many of our customers."
According to Alex Cheng at Anafaze (Watsonville, Calif.), "The concept of 'robustness' has become a critical issue when online processes depend on computers that might 'crash.' One solution proposed by Anafaze is to use PCs for programming and noncritical monitoring, but not for actual operation of the process. The hard-core operation should be controlled by industrial-grade embedded processors."
Bringing things into perspective
"It's important to understand," says Larry Rice, vice president of field measurement and control at The Foxboro Co. (Foxboro, Mass.), "not all customers want the latest and greatest in technology. While today's fractional-DIN microprocessor-based loop controllers are very appealing in many stand-alone control applications, some customers—particularly those in less developed countries—feel much more comfortable using mature control technologies, such as pneumatics. As an example, Foxboro's Model 43A pneumatic controller still being sold has controlled processes reliably and safely since its introduction in 1955. Pneumatic controllers such as these are easy to use, easy to maintain, and virtually indestructible."
While PID, adaptive control, fuzzy logic, and other attempts to bring loops under control have been developed, too often, when one tours a plant, many loop controllers are found set on "M," (for manual). Until control room personnel learn to trust controllers, no amount of automation of loop control will really help.
Multifunction process controller
Sepulveda, Calif.— The I/O Equation Station is a multifunction instrument that can be configured by the user to act as a sensor-level I/O module, signal conditioner, converter and isolator, temperature transmitter, alarm trip, math/computation module, or process controller. It handles up to four analog or direct sensor inputs, up to four digital I/O, and one isolated 0-20 mA output.
Moore Industries-International Inc.
Autotune PID controller
Thompson Ridge, N.Y.— Shimaden SR60 Series of autotune PID controllers comes in three cutout sizes, 72 mm, 1/4 DIN, and 1/8 DIN. Accuracy is rated
Vespo Marketng Assoc. Inc.
Remote setpoint capability available
Schaumburg, Ill.— NEMA 4X and IP66 certified "K-Series" digital process controllers are available in 1/16, 1/8, and 1/4 DIN sizes, and offer 22 types of standard field-selectable inputs including two linear current, three linear voltage, 15 thermocouple, and two RTD options. Normally found only on more costly and complex controllers, remote setpoint capability is available on the 1/4 and 1/8 DIN models.
Omron Electronics Inc.
Loop controllers for OEM market
Foxboro, Mass.— 716 (1/16 DIN) and 718 (1/8 DIN) Series controllers are designed specifically for the OEM marketplace and offer Smart Auto-Tune and Adaptive Tuning features. An Anti-Windup feature provides safe operation for batch changes, and a standard ramp-and-soak function lets the user control the rate at which the process changes. Both controllers come with logic inputs for remote operation and an IP65 panel for washdown protection.
The Foxboro Co.
9 inputs in 1/16-DIN body
Stamford, Conn.— CN77000 Series temperature and process controller accepts up to nine inputs in a single, square 1/16-DIN unit. Choice of inputs includes one of nine types of thermocouples, RTDs, voltage to 10 V dc, or current to 20 mA. Models are available with an choice of NEMA 4 or NEMA 12 case.
Omega Engineering Inc.
Spring House, Pa.— APACS 353 combines functionality of a small distributed control system and a programmable logic controller. It has the ability to use function blocks and ladder logic in the same controller configuration. Up to 25 control and/or ladder logic loops can be accommodated. Communication is via LonWorks.
Moore Products Co.
Controller has built-in PLC functions
Watsonville, Calif.— PPC-2000 is a multiloop, hybrid temperature controller with built-in PLC logic functions. The unit combines rugged PLC logic elements with robust multiloop PID control. The basic module handles 64 analog inputs and 48 PID control loops.
Plymouth Meeting, Pa.— Series 16c is a microprocessor-based, autotuning instrument that can be used for closed-loop control of temperature or virtually any other process variable to a full scale accuracy of
Athena Controls Inc.
Digital indicating controller
Newnan, Ga.— Model UT350 is a 1/4 DIN digital indicating controller designed for level measurement and control applications. It offers software programmability for input and output types, three alarms, four local setpoints and PID sets, two digital inputs, and RS-485 communications (optional). Accuracy is rated 0.1%.
Yokogawa Corp. of America
Universal multiloop controller
Phoenix, Ariz.— UMC 800 universal multiloop controller is a modular control system designed to address the analog and digital control requirements of small unit processes. UMC 800 uses a split architecture that separates control functions from operator panel functions to provide installation flexibility. Features include eight analog control loops, four setpoint programs, up to 24 analog inputs, eight analog outputs, 66 digital inputs or outputs, and an assortment of analog and digital control algorithms.
Williston, Vt.— Fuji Electric PVX programmer/controller comes with free loader software, allowing the unit to be configured from the user's desktop computer. It can store up to nine patterns of up to 20 segments each, making it useful for manufacturers who use the same capital equipment to make a variety of products—each with its own "recipe."
Total Temperature Instrumentation Inc.
Loop Controller Characteristics
External (upward) communications capabilities.
Multiple loop control capability.
Multiple-level alarm priorities.
Multiple configuration memory storage and recall.
Removable configuration modules.
Manual control station.
Source: Control Engineering
Loop Controller Features
Source: Control Engineering