More sophisticated DIN controllers offer general PID control algorithms that can be configured for applications beyond temperature, including pressure, flow and level control. In addition to allowing field setup via the front panel, some higher-end controllers include a data communication port for remote configuration from a host computer.

Intelligence features of the microprocessor might include auto-tuning of the PID coefficients and fuzzy logic to achieve minimum overshoot and/or minimum elapsed time to a stable set point. Other intelligence could include multiple interval ramp-and-soak programs for curing or environmental testing ," with multiple event control outputs, as well as timed output-power limits to protect processes.

Diagnostic intelligence might be available to troubleshoot internal problems and protect process loops from the harmful effects of external faults such as the opening or shorting of a control output or power control device. Other faults that might be identified include missing load power, open or shorted sensor and reversed sensor.

Although digital controllers offer a lot of features (even in a 1/16 DIN package size), many process applications might demand a more comprehensive array of intelligent functions. For instance, to achieve intelligent scalable systems, the latest heat-trace controllers use microprocessors and programmable logic controller (PLC) designs with built-in software and virtual control panel graphical user interfaces (GUIs) (Fig. 3).

 

Figure 3. Repeatability might be more important than absolute accuracy.

Heat-trace systems are used extensively in many chemical plants to maintain the temperature and viscosity of fluids being delivered to a process. Sophisticated multiloop controllers provide low hysteresis and a fast response. Besides temperature control, these "high-end" systems monitor cable continuity and heater current.

Multiloop DIN controllers might be sufficient if the heat-trace system does not have too many control zones, and if the engineer is willing to build his or her own system. DIN controllers are available that have been designed specifically for heat-trace applications, and many have input terminals to receive signals from a current transformer that verifies proper operation of the cable.

Communication and installation considerations

Process heating applications often are related to other steps in the manufacturing process. In these cases, the plant should be sure to select a temperature controller capable of digital communication with a central control system. The higher-end DIN controllers will have a serial data communication port that provides the gateway to an overall process system. The status of conditions and alarms can be monitored centrally, and the controller can be reconfigured remotely.

DIN controllers do not have the ability to log data, but they can communicate data to a central software system or to an industrial PC. Datalogging is particularly important for process applications, because trend analysis can allow fine-tuning of heating control parameters and assist in predictive maintenance programs. In addition, data collection might be needed to document compliance with safety regulations or quality assurance requirements.

Regardless of its sophistication, a digital controller is only one component of a complete heat control system. A complete integrated system includes circuit breakers and power switching mounted in an appropriate control panel. Panel builders, integrators and heating system suppliers can combine a digital PID controller, silicon controlled rectifier (SCR) controller and power distribution devices in a complete system.

Today, with the continual shrinkage of semiconductors and innovative heat-sink design, 100-amp, single-phase SCR power controllers are available in package sizes smaller than 0.5 cubic foot. Such developments are helping to shrink overall panel sizes to allow the panels to fit into places not previously possible. Modular designs allow these systems to be scaled easily to higher currents and a larger number of loops for almost any process.

Programming and integration considerations

Often, processes require quick operator decisions and local adjustments to keep production on track. Fortunately, digital controllers are available with touchscreen displays having virtual push-buttons and complete process information such as temperature set points, actual temperatures, loop status, input values, load currents and alarms. A GUI avoids the need for a bulky keyboard and minimizes the time required for operator training. Furthermore, plant personnel do not need to learn cryptic switch labels or write software code.

Pre-packaged temperature control systems cost much less and require less set-up time than PC-based controllers and conventional PLC systems.

If the standard interface is inadequate, it is relatively easy for plants to develop a customized control program using a low-cost Windows-based software development package. These types of packages are auto-configuring supervisory control and data acquisition (SCADA) programs that communicate with control and monitoring equipment such as temperature controllers.

Typical functions include the real-time posting of temperatures in a process diagram, virtual dials and controls, data/event logging, alarms and a recipe system. See Fig. 4.

 

Figure 4. Many temperature control systems offer optional PC software that allows engineers to build custom interfaces.

The goal should be to build an intuitive touchscreen GUI that allows system operators to use the system with minimal training. Software that comes with a controller usually includes a hardware driver that is pre-configured for that controller, simplifying system integration. If third-party software is used, the programmer or integrator must get the proper driver from the control manufacturer. Sometimes a suitable driver is not available, and custom code must be written and tested, adding cost and time to the project.

Hausman is product manager, standard controls and sensors for Chromalox in LaVergne, Tenn. She can be reached at (615) 793-3900, or via e-mail at mary. hausman@chromalox.com.