If you are ready to replace the electromechanical thermostats in your electrical heating system with intelligent temperature controls, consider which product programming features will yield the most efficient results for your process.

Figure 1a. DIN controllers can be an alternative to electromechanical controllers.
The buying market for digital temperature controllers is growing steadily, propelled by their decrease in cost and simplicity to implement. These products are being used to replace electromechanical thermostats as processors strive for tighter temperature control to gain higher yields, better product quality and a stronger competitive position. Because of the price drop in digital thermostats, it takes only a modest investment to upgrade temperature control equipment -- and without the need to write control system program code. The key issues you must address are:

  • When to add controls with more advanced intelligence to your application.

  • What type of intelligence is useful.

  • Which product programming features will allow efficient use of embedded intelligence.

The terms "intelligent systems" and "intelligent control" are commonly used, but their interpretation varies widely. In general, these terms refer to controllers with features that go beyond classic open or closed systems. These features allow a controller to complete its mission with or without direct communication from a human supervisor. In temperature controllers, some of the added features might include:

  • Real-time sensing in the process loop.

  • Electrical excitation for transducers that need it.

  • "Predictive" logic that compensates for errors, load changes or thermal shifts.

  • Autotuning of control loop PID coefficients.

  • Diagnostics for internal or external faults.

  • Alarm functions.

  • Data communications with other devices.

  • Recording or transmitting event data such as temperature, date, time, channel number, alarms, etc.

Figure 1b. The latest DIN controllers offer intelligence features such as soft-start/timed-output power limit, which allows a warmup period to protect the process and avoid thermal shock on startup.
Different applications require different levels of intelligence, usually dictated by process economies. For example, packaging applications, extrusion lines, plastic injection presses and environmental chambers probably do not require the same level of control sophistication as fermentation equipment and reactors for chemical or pharmaceutical processes. In between are applications such as petroleum refinery processes, rubber production and polymerization, synthetic fibers plants and food processes. Another important consideration is temperature control resolution and accuracy. Some processes require both heating and cooling control to maintain the setpoint temperature. Multiple loops also might be required for independent zone control in various types of electrical heating systems, including those that use heat trace on process piping. Other controller selection issues include types of sensor inputs, controller size, installation, and interfacing with final power control elements.

Many electromechanical temperature control systems use a bulb-and-capillary sensing element and provide only single-point on/off control. For applications that can tolerate wide temperature swings (frequently, those that involve ambient air sensing), this may be an adequate control method. However, these systems can be upgraded with new DIN controllers designed for universal inputs (thermocouple, RTD, voltage or current), which provide tighter temperature control.

Controllers with little more than temperature control logic are at the lower end of the DIN spectrum. The advantage of these controllers is their setup simplicity: Select the sensor type via DIP switch, connect the sensor to the input, and set the temperature with front panel pushbuttons (figure 1a). These simple DIN controllers are good replacements for aging bulb-and-capillary systems.

Figure 2. Smart self-tuning calculates DIN controller PID coefficients to optimize the rise to setpoint during startup.
More advanced digital systems include DIN controllers that have more general PID control algorithms. These algorithms are field configurable for a range of applications, including control of pressure, flow, level and temperature (figure 1b). In addition to local setup via pushbutton, some of these controllers have a data communications port that allows remote configuration from a host computer. Intelligence features of the microprocessor may include autotuning of the PID coefficients using fuzzy logic to achieve minimum overshoot or minimum elapsed time to a stable setpoint (figure 2).

Other intelligence could include multiple-interval ramp-and-soak with multiple event control outputs. Diagnostic intelligence may be available to troubleshoot internal problems and protect process loops from the harmful effects of external faults such as:

  • Open sensor.

  • Shorted sensor.

  • Sensor reversed.

  • Control output open or shorted.

  • Power control device open or shorted.

  • Load power missing.

This typical integrated controller with local input/output screen shows virtual pushbuttons and process temperature displays.
While these features are pretty impressive in a 1/16 DIN controller, the space available in a physically larger unit allows a more impressive array of intelligent functions. Heat trace systems are one example. When used for temperature control in large process piping systems, heat trace cables and controllers provide multiloop control with low hysteresis and fast response. To achieve intelligent scalable systems, the latest heat trace controllers utilize microprocessors and PLC designs with built-in software.

A key feature of these controllers -- and others with higher levels of intelligence -- is an interface that allows easy connections to an associated power controller and to a plant's data communications network. Heat trace and other customizable "off-the-shelf" systems are available that combine a logic controller with an SCR power controller for a complete, ready-to-run control solution, including power distribution devices. With the continual shrinkage of semiconductors and other electronic components, you can find 100 A, single-phase SCR power controllers in package sizes smaller than 0.5 ft3. Combined logic and power controller packages with similar ratings are not much larger. These controller packages are easily scaled to higher currents and the number of loops required for most processes.