Loop Controllers: Dedicated Units Gain Popularity

Process loops can be controlled by a variety of devices: standalone single-loop or multi-loop controllers, PC-based controllers, or pneumatic controllers. Which types users preferred and why were among results in the 2007 survey of user of standalone loop controllers conducted by Control Engineering and Reed Research Group.

By Peter Cleaveland, Control Engineering August 1, 2007

Process loops can be controlled by a variety of devices: standalone single-loop or multi-loop controllers, PC-based controllers, or pneumatic controllers. Which types users preferred and why were among results in the 2007 survey of user of standalone loop controllers conducted by Control Engineering and Reed Research Group.

Fifty-nine percent of those responding to the survey use standalone loop controllers; 57% for in-plant requirements, 23% for both in-plant and OEM requirements, and 20% for OEM (resale) requirements. When asked what percent of all control loops in their plants were run with standalone loop controllers, as opposed to programmable logic controllers (PLCs), programmable automation controllers (PACs), or distributed control systems (DCS), more than half of the respondents (51%) told us they use stand-alone loop controllers on fewer than 20% of control loops, 27% use them for 20% to 59% of loops, and 12% use them for 80% to 100% of loops.

Standalone loop controllers, of course, aren’t favored in every application. One respondent said, “We use single loop controllers only where a DCS or PLC isn’t nearby.”


Forty-two percent of respondents use loop controllers for continuous processing. “Application includes flow sensors with 4-20 mA output connected to process controller for flow control of water or wastewater process,” said one. Another uses them for a “wide variety of temperature, pressure, and flow control on distillation columns and reactors.”

Eleven percent of those responding use loop controllers for batch processing; one, for example, uses them for “batch processing of compounds and extrusion of plastic parts.”

Thirty-six percent of respondents use loop controllers for both batch and continuous processing; one saying he uses them for batch mixing of animal feeds and continuous coating of liquids on pet food products.

Twenty-seven percent use them for utility services. “In the last 6 months,” says one, “I have specified loop controllers for several simple utility applications including fuel oil temperature control, water temperature control, and process batch tank temperature control.”

A fair portion (17%) of respondents use loop controllers in discrete manufacturing applications. One uses them for hydraulic control systems for robotics; another for durability testing of automotive parts or whole structure using servohydraulic testing machines. “We are doing an application to measurement-built gears and take measurements from these,” says another.

Types of controllers in use

When asked about the types of controllers they prefer, 80% of respondents said they use single-loop controllers (up from 40% in 2005), 39% use electronic multiple-loop controllers (up from 21% in 2005), and 30% use PC-based controllers (up from 25% in 2005). Interestingly, pneumatic loop controllers seem to be increasing in popularity, with 21% of respondents using them in 2007 as compared to 13% in 2005. Users gave a variety of reasons for their choices, including retained from legacy systems, operator familiarity, and low cost being mentioned especially frequently. A compelling reason for using stand-alone controllers was for individual display and setting control to provides a level of safety (one failure does not affect other loop control) and for integration of multi-purpose input signal conditioning. Communications options still allow centralized monitoring and control, as necessary, that respondent said.

How many I/O connections?

How many inputs do users want on their controllers? Among users of electronic single-loop controllers, 54% want just one analog input, 25% want two, 8% want three, 6% want four, and 2% want ten or more. Among users of electronic multiple-loop controllers just 10% want only one analog input, while 23% want two, 21% want four, and 11% want ten or more. Among those with pneumatic controllers, 60% want just one analog input, and 14% want two, with higher numbers in the single digits.

PC-based controller users generally want multiple inputs; just 10% want just one, 24% want two, 16% want eight, and 24% want ten or more; others were in the single digits.

Results for desired number of analog outputs paralleled those for analog inputs. Among users of electronic single-loop controllers, 55% want just one output and 27% want two, with no other selection exceeding 8%. For electronic multiple-loop controller users, 7% want just one analog output, 38% want two, 12% want three and 18% want four. As with analog inputs, 65% of those using pneumatic loop controllers want just one analog output, with just a few wanting larger numbers.

Among PC-based users, 15% want one analog output, 19% want two, 19% want four, and 15% want ten or more.

Not all users need discrete I/O connectivity, but most want at least one. Asked about it, 74% of users of PC-based controllers gave no response, perhaps indicating that they had no need for such I/O connections. Of those who did respond, 9% wanted one, 21% wanted two, 14% wanted eight, while 37% wanted ten or more. For discrete inputs, 20% of users of electronic single-loop controllers who responded wanted a single discrete output and 40% wanted two; very few wanted more than that. Among users of electronic multiple-loop controllers, over 65% gave no answer about discrete I/O needs; of those who did respond, 91% wanted a single discrete input and only 7% wanted a single discrete output, while 25% wanted two inputs and 34% wanted two outputs.

In the 2007 Control Engineering subscriber survey on loop controllers, 42% of respondents use loop controllers for continuous processing, 36% for both batch and continuous processing, 27% for utility services, and 11% for batch processing.

Only about 26% of PC-based controllers answered the questions about discrete I/O requirements; of those who did, just 7% wanted a single discrete input and 9% wanted a single discrete output, while 21% wanted two discrete inputs and 18% wanted two discrete outputs. When they wanted them (and answered the question), PC-based controller users want lots: 37% wanted ten or more inputs and 32% wanted ten or more outputs.

Only 13% of users of pneumatic loop controllers answered when asked about discrete I/O connections; of those who did, 45% wanted no discrete inputs and 32% wanted no discrete outputs. Of those who did want them, 55% wanted a single discrete input and 32% wanted a single discrete output; 18% wanted two inputs and 14% wanted two outputs.

Communication protocols

Loop controllers are available with a wide range of communication protocols. The survey asked about use of 4-20 mA, ARCnet, AS-Interface, ControlNet, DeviceNet, Ethernet protocol, FOUNDATION fieldbus, HART, Profibus (PA), Profibus (DP), and Modbus/Modbus RTU. The most popular was 4-20 mA, with 79% using it. Of those, 98% used it to connect to field instrumentation, 30% between controllers (peer-peer) and 17% to connect enterprise systems.

Next most popular was Ethernet, at 60%. Of those, 38% used it to connect to field instrumentation, 65% between controllers (peer-peer) and 71% to enterprise systems. Third most popular was DeviceNet, with 28%. Of those, 63% used it to connect to field instrumentation, 63% between controllers (peer-peer), and 37% to connect to enterprise systems.

Grouped close together were HART, 19%; ControlNet and Profibus (PA) and others tied at 14%; while Foundation fieldbus and Profibus (DP) got 13% and 12.5%, respectively.

User preferences for single-loop control have shifted since 2005, with 80% of respondents using single-loop controllers in 2007 (up from 40% in 2005), 39% using electronic multiple-loop controllers (up from 21% in 2005), and 30% using PC-based controllers (up from 25% in 2005). Interestingly, pneumatic loop controllers seem to be increasing in popularity, with 21% of respondents using them as compared to 13% in 2005.

When asked what manufacturer’s loop controllers they use from a list provided, 41% reported that they use Honeywell, 22% use ABB Instrumentation (Fisher & Porter, Bailey, or Commander), 21% use Rockwell Automation (Allen-Bradley), 20% checked Red Lion Controls, while Emerson, Omega Engineering, and Omron products reached 17%. Other suppliers were Siemens (Moore Process), AutomationDirect, Moore Industries, GE Fanuc, Johnson Controls, Chromalox, Pepperl+Fuchs, Schneider Electric, Fuji Electric, Mitsubishi Electric Automation, Burkert, Bristol Babcock, and Intelligent Instrumentation.

Asked their plans for purchases of stand-alone loop controllers in the next 12 months, 15% of respondents said they planned an increase, 55% said they would purchase the same number, and 30% said fewer.

What users want

Users were asked to rank capabilities they want as very important, important, or not important. PID capability was demanded by the vast majority, with 92% ranking it as either very important (60%) or important (32%).

External communications capability came in second, with 38% ranking it very important and 42% as important, for a total of 80%. Seventy-nine percent felt that user-initiated self-tuning was very important (23%) or important (56%). Next was provision for simple interlocking, with 53% considering it important and 21% very important. Continuous tuning capability was desired by a solid 72%, with 47% finding it important and 26% very important.

A total of 70% of respondents considered multiple-level alarm priorities either important (51%) or very important (19%), while 30% found the feature unimportant.

Adaptive control was desired by 68% of respondents, with 41% finding it important and 26% very important. Peer-to-peer communications was important to 51% and very important to 14%, while 63% felt that multiple-configuration memory storage and recall was important (53%) or very important (10%).

While multiple-loop controllers are readily available from multiple sources, 63% found this an important (46%) or very important (17%) feature.

A total of 61% of those responding felt that feed-forward capability was important (48%) or very important (13%), while 37% found it unimportant. Cascade control was desired by 54% of respondents, 38% finding it important and 16% very important.

On the other side, fully 64% of respondents considered fuzzy logic capability to be unimportant, while 29% found it important and just 7% felt it was very important. Fieldbus capability was evenly split, with 50% considering it unimportant and 50% finding it either important (37%) or very important (13%).

Software, control capabilities

Respondents were pretty uniformly (91%) satisfied with the software used to program or configure single-loop controllers: 70% were somewhat satisfied and 21% were very satisfied; while 9% were somewhat dissatisfied or very dissatisfied (1%).

Asked how they programmed and configure their loop controllers, 62% said they do it right on the device, 27% by connecting to the device locally, and 11% remotely.

Sometimes when a process is difficult to stabilize, or during certain maintenance procedures, it seems easiest just to run the controller open-loop (on manual) instead of closed loop. This discards the benefits that closed-loop control is supposed to provide, and is, of course, to be avoided if possible, yet sometimes it’s unavoidable. Most (71%) respondents reported that fewer than 20% of their loop controllers were running open loop at any one time, and 12% reported that between 20% and 39% were open loop. Six percent said 40% to 59% of their loops ran open. Just 1% or 2% said that 60% to 79%, 80% to 89%, or 90% to 100% open loop. Eight percent of respondents didn’t know how many of their controllers were in manual. Of the respondents, 24% said that they check daily to see if their standalone controllers are running open loop; 14% check weekly; 11% check monthly; 12% check “several times a year,” and 39% don’t know how often they check.

Loop controller products

As mentioned, Control Engineering subscribers identified the following vendors as leading suppliers of loop controllers; representative products are shown. To find other suppliers, search www.cesuppliersearch.com . To find system integrators with related expertise, go to www.controleng.com/integrators .

Software tools for controller selection

The Honeywell Website includes a programmers product selector and a table to compare available controller products. The selection tool allows the user to select number of programs, segments per program, data storage, loops of control, control output, analog inputs, digital inputs, input signal type, and digital and auxiliary outputs. Other specifications to select include accuracy, autotune, communications, size, front-face format, prompts, universal power, and approvals. www.honeywell.com/ps Honeywell Process Solutions

Universal process controller

ABB C310 Wall/pipe-mount Universal Process Controller is a highly versatile single-loop controller packaged in a robust field-mounting housing with IP66 and NEMA4X water and dust protection. There’s no need to fit an expensive instrument panel when installing or upgrading process equipment. The C310 can be rapidly commissioned by fixing it to any flat surface or pipe and making the electrical connections via the cable entry glands on the underside of the unit. The instrument has extensive control and I/O capabilities fitted as standard, allowing it to be rapidly configured for most applications. www.abb.com ABB

Single-loop controllers

Allen-Bradley Bulletin 900 single-loop electronic temperature/process controllers combine thermocouple and RTD sensing capability with either on/off or analog outputs into a global temperature controller, allowing one unit to satisfy multiple temperature applications. In addition, 900-TC16 and 900-TC8 controllers with analog input and on/off or analog output capability are also available. Features include auto-tuning PID control, tactile-feedback keys and an 11-segment LED display. Keypad and display allow users to switch from manual to automatic control without system disruption (bumpless transfer). The units can connect to a PC using free configuration software; RS-485 connectivity is optional, with Modbus RTU slave protocol available. Sizes are 1/8 DIN (900-TC8), 1/16 DIN (900-TC16), and 1/32 DIN (900-TC32). www.rockwellautomation.com Rockwell Automation

Modular controller

Red Lion’s DIN rail-mounted Modular Controller series integrates multi-zone PID control, data acquisition and I/O connections into PC, DCS, or PLC controls. Controller data can dragged and dropped to PLC registers using point-and-click data mapping from a menu of built-in serial and Ethernet drivers, with no need to write PL C code. All modules can be hot-swapped—the master automatically programs a replaced module. The newest addition is the CSOUT Analog Output Module, which offers four isolated analog outputs. Each can be individually configured and scaled to generate output ranges of 0-5 V, 0-10 V, +/-10 V, 0-20 mA, or 4-20 mA. www.redlioncontrols.com Red Lion

Loop controllers work alone or in a system

Foxboro Ah unit can have up to two process controllers, which can operate in a simplex mode or in an optional redundant mode. No additional engineering is required to make the system redundant. The standard T2550 control and I/O baseplate can also house up to 16 I/O cards that can be replaced live, under power, without disturbing field wiring or other I/O connections. Redundant processors with automatic switchover, redundant power supplies, and redundant network connections can be specified on a controller-by-controller basis. Control strategies are configured graphically using IEC-61131 tools including function block, sequential function charts, structured text, and PID and advanced regulatory control. A library of pre-constructed function blocks is also available to speed application development. www.foxboro.com Invensys Process Systems

Integrated control loop function

EZ-Zone PM controller from Watlow combines PID and over/under limit functionality with high amperage no-arc power switching output in a single panel mount package to save space and simplify integration. The unit can be used as a discrete PID controller with adaptive tuning; as a discrete over/under limit controller, or as an integrated control loop solution. It can communicate via EIA-485 (commonly called RS-485), -232, EtherNet/IP or Modbus TCP for remotely configuration, management, and system performance monitoring. Other options include load current measuring capability, a remote set point input, ramp soak programming capability and control variable retransmission. www.watlow.com Watlow

Single-loop controllers, better operator interface

Yokogawa Electric Corp.’s YS1000 single-loop controllers are successors to and are compatible with the company’s YS80/YS100 series. Enhancements include a configurable color LCD with custom display messages for improved alarm visualization; small size (25 cm deep and 1.6 kg), communications via Ethernet, Modbus RTU, Modbus TCP and peer-to-peer; improved connectivity with supervisory control systems; and dual CPU and hard manual control. www.yokogawa.com/us Yokogawa

Dual-zone controller with independent fuzzy logic indication

The 1/32 DIN CN79000 dual-zone controller from Omega Engineering offers a single setpoint on each zone with independent fuzzy logic, autotune, and peak/valley indication. Other features include a large dual display with illuminated keypad, compatibility with five thermocouple input types, and free software. RS-485 serial communication is optional. Suggested applications are in food processing, automotive-engine testing, HVAC, lab use, storm water, water or chemical tanks, plastics molding, air ovens, or any temperature control application. www.omega.com Omega Engineering

Scalable multi-loop controller

Omron’s EJ1 modular temperature controller fits in the same space as the company’s CJ-series PLCs and can be hosted by a PLC, programmable terminal or PC. It can communicate with PLCs without programming. Each module is a system can have two or four loops, and systems with up to 256 loops can be assembled, with thermocouple, RTD, analog voltage and analog current input selectable for each loop. RS-485 serial communications help reduce wiring. www.ia.omron.com Omron Electronics

Multivariable level controller

Rosemount Model 3095 Multivariable Level Controller is a multivariable, microprocessor-based, analog and digital output device for use in single loop, level process applications. It directly measures differential pressure (DP) and computes a separate process variable that represents level in a tank above a reference point. The Level Controller uses the level variable in a control function to compute a control output value. The control function is a PID algorithm whose output is a 4–20 mA analog signal. A digital representation of the 4–20 mA output may be obtained via HART digital communications. Optional process variables can be measured and obtained as a secondary HART process variable. The process variables available via HART are level, DP, control output (CO), and process temperature (PT). www.rosemount.com Emerson Process Management

Author Information
Peter Cleaveland is contributing editor for Control Engineering.