Staying in the Loop

End-users increasingly are looking for 4-20 mA communications, PID capabilities, and external communication in their loop controller technologies, according to recent Control Engineering research results.Control Engineering polled 1,500 readers to gain more insight to the status of the loop controller market.

By Staff February 1, 2000

Trends in loop controllers

4-20 mA communication

PID capability

External communications capability

End-users increasingly are looking for 4-20 mA communications, PID capabilities, and external communication in their loop controller technologies, according to recent Control Engineering research results.

Control Engineering polled 1,500 readers to gain more insight to the status of the loop controller market.

Electronic/digital single-loop controllers hold a slight edge over pneumatic controllers as the controller of choice for primary control . Respondents use an average of 31 single-loop controllers per application compared to 25 pneumatic controllers.

Not far behind are electronic/digital multi-loop controllers and PC-based controllers. Respondents say they use an average of 21 multi-loop controllers per application, and 20 PC-based controllers.

For back-up control , the controller respondents use most is the multi-loop controller. Respondents use an average of nine multi-loop controllers per application, followed by pneumatic controllers (7), single-loop controllers (5), and PC-based controllers (2).

With loop controllers, the number of features is proportional to the number of I/O channels required; the more features, the more channels. So, PC-based controllers and multi-loop controllers require more channels than a typical single-loop controller.

According to the survey, the average number of analog inputs and outputs respondents’ need for their PC-based controllers is 26 and 21, respectively. The average number of discrete inputs and outputs respondents’ need for PC-based controllers is 29 and 25.

Multi-loop controllers are not far behind. Respondents say they need an average of 18 analog inputs and 12 analog outputs for their multi-loop controllers, and 25 discrete inputs and 20 outputs.

Single loops need relatively few I/O channels, comparatively. Respondents say they only need an average of five analog inputs and three analog outputs, and seven discrete inputs and nine discrete outputs for their single-loop controllers.


Use of communication protocols was separated into three categories: connection to field instrumentation, connection between controllers (peer-to-peer), and connection to enterprise systems. The 4-20 mA analog signal was the most used in two of the three categories. It was the overwhelming choice of respondents when connecting controllers to field instrumentation, where it is used 86.3% of the time. HART was a distant second in the results, used 18.7% of the time. Ethernet was third, with 13.2% (see graph).

Connection between controllers, 4-20 mA is still most used, but its lead shrinks considerably. Twenty-three percent use it for peer-to-peer connectivity, while following closely was Ethernet with 19.8%. WorldFIP came in third, with 13.7%.

When respondents connect their controllers to enterprise systems, they most often use Ethernet. Twenty-one percent in fact use Ethernet in this capacity. Next is 4-20 mA, which nearly 9% use to connect controllers to enterprise systems. Following 4-20 mA is WorldFIP, which is used by 6% of respondents for connecting controllers to enterprise systems.

Communication Protocols Used with Loop Controllers*

To field instrumentation
Between controllers (peer-to-peer)
To enterprise systems

*Results exceed 100% due to multiple responsesSource: 2000 Control Engineering Loop Controller Study

4-20 mA





Profibus PA

Purchasing for the next year

At the very least, purchases of loop controllers for the coming 12 months look to be about the same as they were last year. Forty-six percent of respondents say their company’s spending on loop controllers will remain about the same as last year. Twenty-two percent say loop controller purchases will increase, while 12% say purchases will decrease.

Twenty percent say they are not sure what loop controller purchases will be like for their company in the next year. Even if that entire percentage decides it will decrease spending, the market still looks to be strong.

When buying their next loop controller, respondents say the most important characteristic the controller must have is PID (proportional-integral-derivative control), same as last year’s results. However, respondents have somewhat reprioritized other characteristics. External communications capability replaces provisions for simple interlocking as the second most desired characteristic. User-initiated self tuning fell from third to fifth, and continuous self tuning rose from fifth to fourth (see graph for comparison).

Survey methodology

The objectives of this study were to:

Examine respondents’ primary process applications;

Document the number of loop controllers used in primary control service and back-up service;

Determine how many I/O channels respondents require in electronic/digital loop controllers;

Ascertain the communication protocol respondents use with electronic/digital controller to field instrumentation, between controllers peer-to-peer, and to enterprise systems;

Establish which manufacturers respondents have purchased loop controllers from in the past 12 months, the number of electronic and pneumatic units purchased and the dollar amount purchased; and

Identify respondents’ ratings of electronic/digital controller characteristics.

Two-hundred-thirty-one responded to the survey questionnaire for a 15% response rate. Of the 231, 79% said they specify, recommend, and/or buy loop controllers.

The largest group of respondents says continuous processing is their primary process application, accounting for 41% of the total responses. Thirty percent say they perform a combination of continuous and batch processing, while 14% say they perform batch processing only. Discrete products manufacturing and utility services accounted for 7% and 4%, respectively.

Loop Controllers

For more information on loop controllers, go to .

Internet does loop controller duties, more

Spring House, Pa.- Procidia leverages the World Wide Web to provide a set of easy-to-use tools throughout the system’s life cycle-from specification, to start-up, through production and maintenance. Said to be the first Internet Control System (ICS), Procidia fills a void in the process unit and area control market, providing the power of a distributed control system and the simplicity of a loop controller at the cost of a programmable logic controller. Because the ICS was designed to address all system needs via web technology, it is conceivable that a customer might never have to use any other mechanism to specify, configure, or maintain a Procidia-based application.

Procidia achieves ease of use with plug-and-play components that feature out-of-the-box functionality, including a hybrid controller that accommodates continuous/ discrete requirements in one device, on-line modules that feature intelligence and auto-configuration, and auto-configuring HMI software that can be viewed over the Internet/Intranet. Also featured is IEC 61131-3 graphical control strategy software that includes both function block and ladder logic languages, rugged operator workstation with pre-installed monitoring and configuration software, and an OPC (OLE for Process Control) server that acts as a global database for the entire system. Moore Process Automation Solutions

Controller features diagnostics

Freeport, Ill.- Honeywell Sensing and Control has announced the availability of the innovative HealthWatch diagnostic software in its UDC3300 Controller. HealthWatch is a low-cost maintenance and diagnostic tool that provides insight into the health of a user’s processes, the process equipment, and the UDC3300 controller itself. Using internal timers and/or counters and associated alarming capabilities, HealthWatch offers users access to information that can help increase productivity and throughput, reduce or eliminate unnecessary downtime and wasted product, and predict process upsets and equipment/controller failure. It can also help establish spare parts and equipment inventory needs, encourage planned versus corrective maintenance practices, and promote health and safety for the plant. The price for the HealthWatch software option on Honeywell’s UDC3300 expanded controller is $50. Upgrade kits for existing UDC3300 controllers are also available. Availability is typically two weeks.

Honeywell’s UDC 3300 is a 1/4 DIN high-function single- or dual-loop controller with universal input/output. Its Accutane II feature is a plug and play algorithm that will accurately identify and tune essentially any process. Honeywell

Compact controller features fuzzy logic, self-tuning

Schaumburg, Ill.- Available in both 1/16 and 1/32 DIN, the E5_N Series digital temperature controllers are said to incorporate communications and advanced self-tuning-capabilities into compact housings. Both devices can communicate to other automation devices and computers via RS-232. Autotuning and self-tuning are also available in both models. The fuzzy logic self-tuning function automatically calculates an optimum PID constant depending on setpoint at startup or when the setpoint is changes, ensuring greater reliability for critical PID tolerances. Temperature inputs available include thermocouple, platinum resistance thermometer, noncontact temperature sensor, and analog inputs.

The 1/16 DIN version also features heating or heating/cooling control, as well as event input that allows multiple setpoint selection and run/stop functions. The 1/32 DIN version provides dual display of process and setpoint value. Omron Electronics Inc.

Loop controllers ‘speak’ DeviceNet

Loves Park, Ill. and Newtown, Pa.- Barber-Colman and Eurotherm Series 2200e controllers support DeviceNet communications and are ideal for connection to PLC- or PC-based systems where high security and accuracy of single-loop control is required. The controllers are available in 1/16-, 1/8-, and 1/4-DIN sizes. The 2200e DeviceNet implementation is a Class 2-only server that supports Polled I/O Messaging (4 input words and 4 output words) for high-speed transfer of primary parameters and Explicit Messaging for configuration of operation and configuration parameters. Data transfer rates are selectable (125, 250, and 500 Kbit/sec. The DeviceNet version of Series 2200e is CE and UL/cUL approved and offers all the features and functionality of the standard unit.


Multifunction controllers

Newnan, Ga.- There are two models of YS100 single-loop controllers: YS170 programmable type which enables customized control by user programs, and YS150 multifunction type with which single-loop control, cascade control, or selector control can be selected. YS100 controllers have a full-dot LCD on their front panel for clear, legible display of information. YS100 controllers have two CPUs; if the main CPU fails, the YS100 can continue to display the process variable. Yokogawa Corp. of America

1/4-DIN process controller

Foxboro, Mass.- The 731C is a general-purpose process controller that accepts thermocouple and RTD temperature inputs directly as well as normal 4-20 mA signals from transmitters. The display has 2-lines with four large, 7-segment characters in each line for continuous indication of both the setpoint and the process variable. Accuracy is better than The Foxboro Co.

Advanced loop control saves energy, lowers costs

Warminster, Pa.- The MOD30 ML and MC5000 advanced controllers have tools and features that are said to enable users to obtain energy savings, reduce material costs, and achieve consistent quality in various applications, including food processing, chemical manufacturing, and water/waste treatment. These loop controllers feature flexible, high visibility front panels that provide application-dependent displays with the information an operator needs to react quickly to changing process conditions.

Other features include on-board, modular, and remote I/O points that allow users to fit the controller to the process. The devices’ control algorithms provide PID, sequence, and logic control, as well as calculation and lookup tables for cost-effective unit control strategies. Communications options include Modbus RTU, peer-to-peer networks, and a HART interface option for direct connection to common PLCs. ABB, Fischer & Porter