Intelligent Control for Electrical Heating
- 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.
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.
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.
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.