Single-loop and Pneumatic Technologies Hold Strong

There's something to be said for reliability and staying power when we consider the throngs of fads and faux paradigms common today. From an engineering perspective, who would have predicted 57 years after the first pneumatic loop controller with PID functions was created these same devices would still be a prominent component of process control? Who also could have foreseen single-loop c...


Loop ControllersSingle-Loop primary control


  • Multiple-Loop back-up control

  • PID

  • 4-20 mA communication

Rating Loop Controller Characteristics

There's something to be said for reliability and staying power when we consider the throngs of fads and faux paradigms common today. From an engineering perspective, who would have predicted 57 years after the first pneumatic loop controller with PID functions was created these same devices would still be a prominent component of process control? Who also could have foreseen single-loop controllers would still be the first line of loop control 48 years after the first was created?

These two types of controllers are the kings of the mountain waiting for a newcomer in the industry to come and knock them off, according to a recent survey of loop controller usage mailed to a 1,500 reader sample.

The cross-section of the readers surveyed favored continuous processing, with nearly half of our respondents (47%) stating it's their primary application. Twenty-three percent work with both continuous and batch processing, while batch processing alone accounted for 16% of the responses. Other types of applications mentioned were utility services and discrete products manufacturing.

Specifically, the raw materials processing industry is the most well-represented among survey respondents. It leads in number of respondents (49), controllers used (675), and money spent on loop controllers in the last 12 months $4.2 million. Machine and equipment manufacturing for manufacturing and service industries was second with 13 respondents, 191 controllers used, and $237,800 spent on loop controllers in the last 12 months.

Applying the goods

When applying loop controllers for primary control, respondents were almost evenly split between electronic/digital single-loop controllers (average of 40 units per application) and pneumatic loop controllers (35 units per application). PC-based controllers are quite prevalent (27 units per application), but multiloop controllers are used half as much as their single-loop counterparts. However, when it comes to back-up control, the multiloop controllers are preferred almost two to one over PC-based controllers, and two-and-a-half to one over both electronic single-loop and pneumatic controllers.

Kiku T. O'Shea, loop controller product engineer with Moore Process Automation Solutions (Spring House, Pa.), says the reason for single-loop controllers' dominance is reliability. Ms. O'Shea also says "in the event a single-loop controller does fail, replacing a single loop is an easier solution," meaning fixing one variable is a much more manageable process resulting in less down-time. From a purely physical perspective, Rick Warner, Bailey-Fischer & Porter's (Warminster, Pa.) application manager for control room products, says the trend toward smaller sizes favors the single-loop controllers because they require fewer I/O, thus fewer wiring terminations on the back of the panel. The smaller the controller, the harder it is to fit the terminations required for multiple measurements.

Pneumatic controllers still remain a favorite because of their reliability, durability, and immunity to EMI and RFI.

How we talk to each other

How loop controllers communicate is of utmost importance to CE readers. Not surprisingly, the primary mode of communication between controllers and field instruments is the standardized 4-20 mA signal, which is used in 87% of our surveyed readers' applications. Communication between controllers (peer-to-peer) is much more evenly spread out among the available protocols, though 4-20 mA is still the favorite, gathering 25% of the votes received. It's next closest competitor is Ethernet, which came in with 22% of the responses. DeviceNet is third with 9.5%.

Ethernet is the favored choice when configuring controllers to communicate with enterprise systems, where it is used in one out of every four applications (25.2%). The next closest rival was the 4-20 mA signal, used in 8.2% of applications.

What the future holds

Purchases of loop controllers by survey respondents topped $24 million last year, but the outlook for the next 12 months is unclear. Thirty-seven percent say loop-controller purchases will remain about the same over the next year, while figures for increasing and decreasing sales are nearly equal. Sixteen percent say sales will increase; seventeen percent say sales will decrease. Holding the weight to tip the scale are the 26% who are unsure what loop controller purchases will be like for their companies over the next year.

When buying, engineers have a seemingly endless list of features to choose from, ranging from PID, which is generally regarded as the most important feature, to upward communications capability, adaptive control, downward communication, fuzzy logic, and more. Bailey-Fischer & Porter's Mr. Warner says if engineers look hard enough, they can generally find a controller that will do whatever they require. Besides functionality, other attributes like price, size, and durability are all on engineers' shopping lists. However, he says, "product support is the number one issue to be addressed for sales. Supporting customers in the event a controller fails is what our customers have voiced they want the most."

With more added features and more complex functionality comes the need for more I/O points. As expected, electronic single-loop controllers require the fewest analog I/O points. Electronic multiloop controllers generally require two-and-a-half times more analog I/O than their single-loop kin, while PC-based controllers require three times as many. Ratios were nearly the same for discrete I/O points, with PC-based and electronic multiloop controllers requiring upwards of 40 points.

Though the future of the loop controller market is uncertain, it's not necessarily pessimistic. Both single-loop and pneumatic controllers have maintained a significant place in the industry, rising to the top of the top of the mountain four decades after their beginnings. With that in mind, it isn't likely something will soon come along to knock them off.

Loop Controller Products

For more information about process variable transmitters, visit

Controller for single or multiple loops

Warminster, Pa.— The 53MC5000 Process Control Station incorporates a microprocessor Application Specific Integrated Circuit and Surface Mount Technology, and features a high-resolution dot matrix faceplate display. The controller is available in a one-, two-, or four-loop versions. In addition to its basic I/O configuration, signal capacity can be increased by adding plug-in I/O modules. Users may start with the standard function block oriented Flexible Control Strategy and its library of preconfigured control strategies. Capabilities may be expanded by using the f-CIM function block tool or the high-level programming language, F-TRAN. Bailey-Fischer & Porter

Preconfigured control strategies are built in

Newnan, Ga.— Yokogawa Corp. of America's new US1000 series of process controllers are positioned between the company's YS100 SLC process controllers and its Green Series industrial controllers. The US1000s offer universal inputs and outputs, auto-tune and "Super Control" functions, as well as up to 13 preconfigured control strategies to provide a best-fit solution to a wide range of automation needs. Yokogawa Industrial Automation

Controller triples its processing power

Charlottesville, Va.— GE Fanuc's DSM300 uses dual processors to triple the processing power over its predecessor, the Power Mate J controller, and is said to have position capturing capabilities of 250ltiaxis positioning and control. The main CPU is a 386EX and possesses the control programs and interfaces to the backplane. GE Fanuc

Easy-to-read, versatile controllers

Loves Park, Ill.— Series 20 1/4 and 1/8 DIN process controllers incorporate easy- to-read instrument keys with two vertical bar graphs. These instruments comprise universal inputs, including all thermocouples, RTD, mA, mV dc, and V dc. Up to six outputs are available in a variety of functions: control outputs, actuator drive, alarms, and event outputs and analog retransmission. SMART autotuning ensures process control is reached quickly without process overshoot or droop. A 24 V dc nonisolated auxiliary power supply is available to power 2-, 3-, or 4-wire transmitters. RS-485 Modbus protocol is available. Barber-Colman

Module for plastics applications

Mayfield Heights, O.— The Allen-Bradley 1746-BLM specialty control I/O module for blow molding applications is based on the company's Small Logic Controller platform. The module is capable of simultaneous servo control of four machine axes, such as two accumulators and two machine heads, or four heads of a continuous extrusion machine. Sharing the same chassis as the SLC controller, the BLM is installed by just sliding it into the rack. The BLM module performs its servo control task independently, but receives configuration and runtime information from the SLC processor. Software configuration is simplified because the BLM is a "smart" module that only requires configuration and calibration. Rockwell Automation/Allen-Bradley

Hybrid multiloop temperature controller

Watsonville, Calif.— The PPC-2000 is a multiloop PID controller with built-in programmable logic functions for users who need multiloop temperature control. This modular controller allows various analog and digital I/O points and control loops to be centralized into one unit and programmed as an integral system. The unit handles up to 48 control loops and 48 digital I/O points. Process data, alarms, and digital states can be shared among process and logic controls. All available analog and digital I/O modules snap together and mount on a DIN rail. Watlow Anafaze

Rugged pneumatic controllers

Foxboro, Mass.— The 43AP Series Pneumatic Indicating Controllers continuously indicate and control pressure, vacuum, temperature, or flow. They offer a broad choice of control modes including on/off, proportional, proportional plus derivative, proportional plus integral (reset), proportional plus integral (reset) plus derivative, differential gap, and automatic shutdown. These instruments are weatherproof, with a glass-fiber reinforced case, gasketed door, and are certified to provide NEMA Type 3 protection. Accuracy is unaffected by mounting since the measurement element and control unit are mounted on a rigid steel plate. Foxboro

Small-scale DCS/PLC for local control

Spring House, Pa.— The 353 Process Automation Controller combines the functionality of a small distributed control system (DCS) and programmable logic controller (PLC), offering such features as filter control and backwash sequencing for water systems, compressor surge control and interlocks, and combustion and burner management for packaged boilers. The 353 uses function blocks and ladder logic in the same controller configuration, and uses LonWorks fieldbus communication technology to connect high speed remote I/O to the controller through a single twisted pair. Both built-in and remote I/O capabilities allow it to solve applications of over 100 points. Combined with the support of up to 25 control and/or ladder logic loops, the 353 is said to have the power to solve complex control problems. Moore Process Automation Solutions

Controller with Profibus interface

Reston, Va.— Models 2408 f ( f (ramping, Self and Adaptive tuning, Gain Scheduling, and Feedforward Instant Accuracy. Eurotherm

Control by microprocessor

Phoenix, Ariz.— The UDC 2300 1/4 DIN microprocessor-based controller is capable of increased functionality and comes with standard features including Accutune II auto tuning with fuzzy logic overshoot suppression, input/output isolation, and a universal AC power supply. The unit controls temperature, pressure, flow, level, and rotation variables in various applications. Other features available are universal switching power, dual display, dual setpoints, and setpoint ramp. A two-year warranty is standard. Honeywell IAC

Rating Loop Controller Characteristics


Provisions for simple interlocking

User-initiated self-tuning

Multiple-level alarm priorities

External (upward) communications capability

Continuous self tuning

Hard-manual control station

Adaptive control

User-defined algorithms

Multiple loop-control capability

Source: 1999 Control Engineering Loop Controllers Study

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