Loop Controllers: Lone Logic is More Connected

Stand-alone loop controllers, desired because of cost and simplicity, still need to collect and report data, even if they operate outside of a larger control system. External and peer-to-peer communications are the most important traits in such controllers, after ability to do PID (proportional-integral-derivative) control.

By Mark T. Hoske July 1, 2005

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Product Focus research, Oct. 2003: Stand-alone Loop Controllers Make Connections

Stand-alone loop controllers, desired because of cost and simplicity, still need to collect and report data, even if they operate outside of a larger control system. External and peer-to-peer communications are the most important traits in such controllers, after ability to do PID (proportional-integral-derivative) control. Also, more users are checking daily to ensure controllers are operating closed-loop.

Those were among key research findings this month from Control Engineering subscribers, and from related discussions with providers of stand-alone loop controller manufacturers.

Responses to the 2005 survey from Control Engineering and Reed Research Group were based on 104 subscribers who buy or specify loop controllers. Of these, 62% were for in-plant requirements; 18% for OEM (resale) requirements; and 20% for both. Responses in 2003 numbered 127; 62% for in-plant requirements; 25% for OEM (resale requirements); and 10% for both.

In 2005, 52% of respondents listed continuous processing as the primary application; 28% both batch and continuous processing; 20% discrete manufacturing; 19% utilities services; 15% batch processing; and 2% other. In 2003, the mix was 40% continuous processing, 12% both batch and continuous processing, 42% discrete manufacturing, 23% utilities services, and 10% batch processing.

More electronic, less pneumatic

More electronic single-loop controllers and fewer pneumatic loop controllers are being used for primary control. In 2005, type of loop controllers for primary control, in percentage of total responses, were (more than one answer allowed): 40% electronic single-loop controller; 25% PC-based controller; 21% electronic multiple-loop controller; 13% pneumatic loop controller; and 1% other. In 2003, responses were 33% electronic single-loop controller; 26% PC-based controller; 22% electronic multiple-loop controller; 18% pneumatic loop controller; and 2% other.

Dennis Schneider, product manager for Bulletin 900 Single Loop Temperature/Process Controllers at Rockwell Automation, was impressed with the increase in the use of single-loop and multi-loop controllers, ‘especially in the use of single-loop controllers (7%) and a decrease in usage of all other controller types.’

The change makes sense, Schneider says, given functional enhancements and general improvements made in single-loop controller technology even before 2003. ‘I believe that the control engineers are just now beginning to use the functions and also beginning to appreciate the flexibility and distributed control possibilities presented to them by this small, yet powerful and cost-effective device,’ he adds.

In communications with field instruments, 4-20 mA is less dominant, respondents said. In 2005, the most common communications from stand-alone loop controllers to field instrumentation was 4-20 mA at 79%, down from 89% in 2003. Most common communications between controllers was Ethernet at 52%, about the same as 2003.

Have it both ways

Fabian Monino, product manager for controllers, Siemens Energy & Automation Inc., process instrumentation, says the most significant trend in loop controllers is communications. ‘Traditional loop controller users still want the stand-alone controller, out on the floor and easily accessible to the operator. However, they also want to collect and record process data. There are a variety of reasons. Most often it is to satisfy the emissions reporting requirements for the EPA. In boiler control applications, energy use and steam generation data are used to monitor boiler and plant efficiencies. In heat treatment applications, the record of the heating cycle serves as part of the product documentation.’

In the next 12 months, 24% plan to increase purchases of stand-alone loop controllers compared to the past year, 52% remain the same, and 24% plan to purchase fewer.

About half of respondents said that stand-alone loop controllers handle less than 20% of their control loops; 13% said 20-39% of control loops, 12% said 40-59% of control loops, 7% said 60-79% of control loops, 9% said 80-89% of control loops, 5% said 90-100% of control loops, and 7% didn’t know.

More daily attention is given to keeping control loops running closed loop in 2005 than in 2003. Asked how often respondents check if stand-alone loop controllers are running in open or closed loop, 24% said daily, 13% weekly, 7% monthly, 16% several times a year, and 41% didn’t know. In 2003, 20% said daily, 17% weekly, 8% monthly, 12% several times a year, and 42% didn’t know.

Respondents in 2005 and 2003 were nearly identical on how they program and configure most loop controllers: 52% do so on the device; 37% connect locally; and 11% connect remotely.

Looking at I/O channels in 2005 (descending order for each):

For PC-based controllers, the most numerous categories selected were: 2 or 3 analog inputs; 4 or 5 analog outputs; 21+ discrete inputs; and 21+ discrete outputs.

For electronic single-loop controllers, the most numerous categories were: 1 analog input; 1 analog output; 2 or 3 discrete inputs; and 2 or 3 discrete outputs.

For electronic multiple-loop controllers, the most numerous categories were: 2 or 3 analog inputs; 2 or 3 analog outputs; 4 or 5 discrete inputs; and 3-way tie between 2 or 3, 4 or 5, and 11-20 discrete outputs.

For pneumatic loop controllers, the most numerous categories were: 1 analog input; 1 analog output; 21+ discrete inputs; and (tie) 2 or 3, 4 or 5 discrete outputs.

Loop controller products

For more manufacturers, see www.cesuppliersearch.com , for integrators with expertise with controllers, go to www.controleng.com/ integrators. For more on products, visit the Web sites below.

Single-loop controllers or more

Users can select a traditional Bulletin 900 Single Loop Temperature/Process Controller or I/O device with integrated logic to do PID or other process-related tasks, or various PLC or a programmable automation controller, such as Logix, according to Allen-Bradley offerings from Rockwell Automation. The Bulletin 900 line, in 1/32, 1/16, and 1/8 DIN form factors, offers multi-function control, easy LED displays, auto-tuning, and self-tuning for higher efficiencies and faster startups. Software is available; upgrades are free within 6 months of purchase. Other options include the 1790/P CompactBlock LDX I/O device with temperature control module. www.rockwellautomation.com Rockwell Automation

Low-cost temperature controller

The UDC100 is a low-cost 1/4 DIN, microprocessor-based controller designed for basic temperature applications. A timer is optionally available. A large dial allows easy setpoint adjustment Front face is 96 mm x 96 mm (3.78 in. x 3.78 in.) This controller is used in applications such as processing, bakery ovens, small industrial furnaces, and ceramics kilns. www.honeywell.com/imc Honeywell

Digital indicating controller

Model UT551-xA to xD is an intelligent, micro-processor based digital indicating controller with Ethernet communication functions. It has a large numerical display with an ‘Active Color PV Display’ color-changing function. Product features many standard functions for various control applications; all functions, such as control, control computation, and signal computation functions, can be configured by using keys on the front panel. The controller has eight types of control strategies, the ‘Super’ overshoot suppressing function, and ‘Super 2’ hunting suppressing function, as well as an auto-tuning function. A position-proportional control model is also available. www.yokogawa.com/cis Yokogawa

Temperature control

Series SD family of PID temperature controllers is said to provide an exceptional value to processors through their accuracy and performance in a broad range of applications in temperature and process control. They include an improved user interface display, faster update rates, and expanded I/O capability. Advanced features include the implementation of EIA 485 Modbus serial communications, Watlow’s Infosenset sensor technology, infrared remote communications operation, Watlow’s patented User Definable Menu System, and a ‘Save and Restore’ feature that allows the restoration of either factory or user-defined settings. With pre-loaded instant HMI (human-machine interface) software, a Palm-brand handheld PDA device can be used to remotely program and control the Watlow Series SD controller. The wireless Infrared Data Communications option can support configuration and operation to facilitate recipe management and data logging and allows easier controller setup, operation, and monitoring. Devices are available in 1/32nd, 1/16th, 1/8th and 1/4 DIN panel-mount sizes. www.watlow.com Watlow

Control for harsh use

Commander line of single loop process controllers from ABB Instrumentation offers simple-to-highly-advanced control strategies and withstands harsh environments. Panel configurations include 1/16, 1/8, and 1/4 DIN; 3 x 6-in. units, and a wall/pipe mount. Most models can be configured using ABB Commander PC software, providing a fast and simple way method to set up the controllers. All can be linked to a central PLC or SCADA system using the Modbus protocol. Many standard features and options can match virtually any application. The Commander 100 1/8 DIN process controller, for example, comes with universal inputs for such process variables as temperature, pressure, flow, and level. Analog, relay and logic outputs, alarms, ramp/soak profiles, and PID control are standard. Addition of a remote setpoint option permits two or more units to operate in a master/slave (cascade) configuration. Other variants offer direct control of motorized valves and reduced case depth for installation and panel savings. www.abb.us/instrumentation ABB Instrumentation

Single, dual-loop vs. system

Single- or dual-loop microprocessor-based controllers can offer PID, velocity limiting, and lead/lag, among other calculations. A system like DeltaV can offer advantages over such controllers, such as greater reliability and functionality during abnormal events, redundancy, better event and history recording, better interfaces with smart devices, easier configuration, and diagnostics, according to Emerson Process Management. Programming is within a Microsoft Windows environment. www.easydeltav.com Emerson Process Management

Control multiple zones, factors

Omron E5AR 1/4 DIN and E5ER 1/8 DIN temperature/process controllers are multi-loop models that allow for integrated control of multiple zones or environmental factors, such as temperature, pressure and humidity. A fast sampling response time of 50 ms gives precise control for quick-changing temperatures in critical applications. Fast sampling response time allows the controller to manage signals more precisely. Selectable inputs on all models include: thermocouples, Pt100 RTDs, analog current, or analog voltage. All E5R Series controllers display PV, SP, and MV data simultaneously in a 3-row backlit LCD display for high visibility. E5AR is available in 1-, 2- and 4-loop models, and the E5ER in 1- and 2-loop models. 2-loop models are pre-programmed for cascade control for easy set-up and programming. E5R controllers provide critical control of quick temperature changes, for ceramic heaters, environmental chambers, high-temperature kilns, and furnaces. They can also be used for semiconductor production lines and food and beverage sterilization processes. www.omron.com/oei Omron Electronics

Universal PID control

Universal 535 1/4 DIN single-loop PID process controller can monitor and control temperature, pressure, level, flow, and other analog variables for batch and continuous processes. Available models provide an isolated analog (4-20 mA or 0-20 mA) control signal output that is typically used to regulate a control valve or similar final control element. Models with alarm trip outputs can be used for on/off control or to warn of unwanted high/low conditions. Optional RS-485 communications allows interface with digital-communicating DCS, PLC, and PC-based SCADA systems. Quick and easy setup is achieved using the processor’s large, backlit keys and user-friendly menu prompts. Extra-bright front panel display meets NEMA 4X requirements. Patented PowerTune feature delivers pretuning, adaptive tuning, and powerback anti-overshoot. www.miinet.com Moore Industries-International Inc.

Ethernet, card loop control

353 Loop Controller is available with an Ethernet communication card. Enhancement enables network connectivity with multiple 353 loop controllers, HMIs, and other devices that communicate Modbus over Ethernet. With the Ethernet communication capability, remote monitoring and control allow the user to view the process from the control room or any workstation on the network. Such design reduces plant costs and learning time, and provides easier access to plant data for timely decision-making. Ethernet communications provide: peer-to-peer communication with other controllers; host device (HMI) communication with multiple controllers; integration with Modbus RTU I/O devices; simple connection to multiple operator workstations; and the option to upgrade panel-board controllers to an integrated network. As part of the Ethernet communications release, the new V2.40 controller firmware has new function blocks to enable controllers to request data from other controllers on a periodic basis; data can be requested from Modbus RTU devices using a Ethernet/Modbus RTU gateway. www.sea.siemens.com/ia Siemens Energy & Automation Process Industries Div.

Online Extra

Loop controllers’ communications expand

Need for communications in loop controllers isn’t new, says Fabian Monino, product manager for controllers, Siemens Energy & Automation Inc. “Loop controllers have supported communications for many years, most notably Modbus, because it has been an open standard. The communications trend is toward a universal network that allows the user to integrate multiple products from a variety of vendors. Ethernet is drawing a lot interest. If your device supports TCP/IP protocol, then you can get your data across the network. The user does not have to install multiple buses. Ethernet also permits multiple masters to co-exist.”

Monino continues, “A parallel trend is with OPC servers. OPC is a standard that permits software utilities from different manufacturers to exchange data. The OPC server is the link that is connecting the loop controller to an HMI and the trend historian. With an OPC compliant HMI, the user can bring his stand-alone loop controllers into the control room.

“This is not to say,” Monino says, “that loop controllers with Ethernet communication can compete against a DCS or PLC. There are good reasons why loop controllers are here to stay. They are very popular in ancillary operations, especially boilers. Ancillary operations are independent of the main production operation. If the process goes down, you still want steam to the equipment. However, in modern operations, you still want that data available in the control room. To have both loop controller on the floor and an HMI in the control room seems redundant. However, this gives the user the best of both worlds. When everything is running well, the operators can monitor their boilers from the control room. When things are not doing so well, the operators can put the controllers in manual and stabilize the system from the floor.”

Monino points out, “Another use of stand-alone controllers with DCS or PLC controllers is for supervisory/regulatory control. The DCS or PLC controls the process, but it delegates control of a pressure/flow/temperature to a stand-alone controller. The sending of the setpoint and reading of actual process data are handled via a communication line. Again, the ability to put a controller into manual mode in emergencies is a recurring benefit.”

Stand-alone loop controller features

Ranking of those seen as very important or important

In 2005

PID capability

External communications capability

Peer-to-peer communications

User-initiated self-tuning

Multiple-level alarm priorities

Continuous self-tuning capability

Multiple-loop control capability

Provisions for simple interlocking

User-defined algorithms

Adaptive control

Hard-Manual control station

Cascade control

Multiple-configuration memory storage and recall

Feed forward capability

Removable configuration modules

Fieldbus communications capability

Fuzzy logic capability

In 2003

PID capability

External communications capability

Adaptive control

User-initiated self-tuning

Provisions for simple interlocking Source: 2005 and 2003 research from Control Engineering and Reed Research Group

Author Bio: Mark Hoske has been Control Engineering editor/content manager since 1994 and in a leadership role since 1999, covering all major areas: control systems, networking and information systems, control equipment and energy, and system integration, everything that comprises or facilitates the control loop. He has been writing about technology since 1987, writing professionally since 1982, and has a Bachelor of Science in Journalism degree from UW-Madison.