Measuring and controlling temperature is vital for ensuring quality and safe manufacturing operations. Temperature controllers are used in process plants, research laboratories, product development centers and other industrial settings. In a clean temperature-controlled laboratory setting, an inexpensive off-the-shelf controller may be the right product. However, these same controllers typically cannot survive the harsh conditions common to heavy industry processes and remote areas.
While maintaining temperature control is imperative, it is one of the most difficult parameters to successfully manage. Several important factors should be considered in addition to initial cost. Determining which controller to use can be confusing because, at a basic level, all controllers work in a similar manner: The controller samples a value transmitted from a temperature sensor many times per second, and it compares this process variable against the setpoint. Whenever the process variable deviates from the setpoint, the controller sends an output signal to engage other devices such as heating and cooling mechanisms to bring the temperature back to the setpoint.
Despite being fairly similar on initial inspection, different controller types have features and functions that offer important advantages, depending on the type of application.
Braving the Elements
A review of the input sensors is the best place to start when selecting a controller for deployment in areas subject to dust, extreme temperatures and noise. Depending on the application, input sensors may include thermocouples, RTDs and linear inputs such as millivolt (mV) and milliamp (mA).
For harsh environments, thermocouple or RTD sensors usually are the best choice. Thermocouple sensors are economical, rugged and provide accurate measurements for a range of temperature values. Available in multiple types and configurations, they work well in many different types of industrial installations.
RTDs provide greater temperature accuracy than thermocouples but they are more expensive, have narrower temperature range and are less rugged. For example, RTDs have an upper temperature limit of approximately 1,200°F (649°C) compared to 4,200°F (2,316°C) for thermocouples.
Whatever temperature sensor type is selected, the controller should contain a sensor break detection feature. This alerts the controller when a sensor is faulty or absent, enabling it to adjust the output to a preset value that will prevent harm to equipment and personnel.
An on/off controller is inexpensive, but it can only determine if an output needs to be turned on or off. For example, if the setpoint on a boiler is 245°F (118°C) and the process value temperature falls to 244°F (117°C), the controller will send an on signal. This signal might turn a heater on, open a steam valve or take other action to increase the boiler temperature. When the temperature reaches the setpoint, the controller output reverts to the off state.
Similar to a home thermostat, this type of controller works well in some applications but has some limitations. The band in which the controller operates is set to the desired value. (In the above case, it would be set to one degree.) The controller does not change its output state unless the process variable changes by at least one degree.
Once the output state is changed, it typically takes some time for the process variable to change, meaning the actual temperature can deviate from the setpoint by more than one degree. This may be acceptable in some applications but not all.
Another issue is that on/off control often is highly inefficient because the controlling device must either be full on or full off. If the controlled device is a valve, an on/off controller might require the valve to slam open and shut frequently, which can lead to excessive wear and tear.
In addition to their limited control capabilities, these devices usually lack a display and have limited communication capabilities. Therefore, these basic on/off controllers should only be used for noncritical thermal systems without stringent accuracy requirements.
When PID Control Is Preferable
More advanced digital temperature controllers have multiple outputs and programmable functions. They also usually are placed in the front panel with a display to ease operator accessibility. These advanced controllers deliver accurate and stable control by automatically calculating proportional-integral-derivative (PID) parameters to determine the exact output value needed to maintain the desired temperature.
For example, if the cycle time is set to 8 seconds, a system calling for 50 percent power will have the output on for 4 seconds and off for 4 seconds. When output power should be 25 percent for the same 8 second cycle time, the output will be on for 2 seconds and off for 6 seconds (figure 1). This type of cycling output control often is used to control a solid-state device such as a thyristor.
If the controlled device has the capability to continuously vary its state, then the PID controller output can be set to vary continuously to control the device. For example, a 4 to 20mA PID output could be used to continuously vary the position of a control valve. This type of continuous control can result in highly accurate temperature control.
These advanced digital temperature controllers typically offer the ability to program many different types of alarms. For instance, a high limit alarm could be set to prevent a heat source from damaging equipment by de-energizing the source if the temperature exceeded a preset value. Deviation alarms can be set at a determined plus-or-minus value from the setpoint to notify an operator if the temperature goes out of range.
Another useful feature provides an alarm when the output signal is 100 percent but the input sensor does not detect any change in temperature after a certain time period, indicating a malfunction in the temperature control loop.
Highly Flexible Controllers
Single-loop controllers typically have one input and one output. Multiloop controllers have several inputs and outputs and can be used to simultaneously control numerous loops, enabling the supervision of more process system functions. Moreover, multiloop controllers are compact and modular. They can operate either in a stand-alone mode or as part of an advanced automation system such as a programmable logic controller. When used a replacement for temperature controls in any of these advanced automation systems, a multiloop controller can provide fast PID control, and it can off-load much of the memory-zapping calculations from the automation system processors.
As a replacement for multiple DIN controllers, multiloop controllers provide a single point of software access to all control loops. These controllers also provide features not available on traditional panel-mounted controllers. They have higher loop density and a smaller footprint, and wiring is reduced via a common connection point for power supply and digital communications interfaces.
As compared to simpler controllers, multiloop temperature controllers usually have enhanced security features to prevent unauthorized access to critical settings. These features offer complete control over the information being read from or written to the controller, thus limiting the information an operator can read or change.
Advanced controllers also offer better communication capabilities, allowing them to communicate with advanced automation systems via digital communication links. They can be configured quickly and easily using PC-based software, allowing configurations to be saved for future use. When connected to the Internet or to an intranet, these controllers can be accessed remotely, allowing full remote viewing, configuration and control from any location with Internet or intranet access.
This article serves as an introduction to the various features and types of temperature controllers. A cheaper controller can become expensive if frequent repairs to associated components are required, if required accuracy cannot be maintained, or if an accident occurs due to inadequate safety features. Each application should be examined in detail and the right controller deployed depending on the process requirements.