Choosing the right equipment can improve operations, save money and provide more secure systems.
In many industries and applications, measuring and controlling temperature is critical to ensuring quality and safe operation. Temperature controllers are used in laboratories, product development centers, process plants, and other industrial environments.
In a clean, temperature-controlled lab, a cheap standard controller might be the right product. But the same controls often cannot withstand the harsh conditions common in heavy industrial processes and remote areas.
Maintaining temperature control is essential, but it is also one of the most difficult parameters to control. Cheap controls are best suited for simple applications, but there are other important factors to consider besides the initial cost.
Determining which controller to use can be confusing because at the base level, all controllers work similarly. The controller scans the value sent by the temperature sensor multiple times per second and compares the process variable to the set value.
Whenever a process variable deviates from its set value, the controller sends an output signal to activate other devices (such as heating and cooling mechanisms) and return the temperature to the set value. Although very similar on initial inspection, the functionality of different types of temperature controllers can provide significant advantages depending on the type of application.
On-OFF Temperature Controller
The on-off temperature controller is cheap, but only determines if the output needs to be turned on or off. For example, if the boiler is set to 245 degrees and the process temperature drops to 244 degrees, the controller will send an ON signal. This signal can turn on the heater, open the steam valve, or take other measures to raise the temperature of the boiler. When the temperature reaches the set value, the controller output returns to the closed state.
Similar to a home thermostat, the controller works well in some applications, but with some significant limitations. In the above case, set the frequency band in which the controller works to the desired value at one time. Therefore, the controller only changes the initial state if the process variable is changed at least once.
Process variables usually take some time when the initial state changes. This means that the actual temperature may deviate from the set value by more than 1 degree. This may be acceptable for some applications, but not for all.
Another problem is that on-off controls are often inefficient because they must be fully turned on or off. If the device to be controlled is a valve, the ON/OFF controller must always open and close the valve, which can lead to excessive wear.
In addition to limited control options, these devices often have no display and only limited communication options. These basic on-off controls can only be used for non-critical thermal systems where accuracy is not strictly required.
PID Temperature Controller
The advanced digital temperature controller has multiple output and programmable functions. It is usually located in the front and has a display screen for easy access by the operator. These advanced controllers automatically calculate proportional integral differential (PID) parameters to determine the precise output values required to maintain the desired temperature, thus achieving more accurate and stable control.
For example, if you set the cycle time to 8 seconds, the output will be on for 4 seconds and off for 4 seconds for a system that requires 50% power. If the output power reaches 25% in the same 8-second cycle time, the output will remain on for 2 seconds, then off for 6 seconds (figure 2). This type of cyclic power control is commonly used to control semiconductor devices, such as thyristors.
If the controlled device can continuously change its state, the output of the PID controller can be set to continuously change to control the device. For example, you can use 4-20 mA PID output to continuously change the position of the control valve. This type of continuous control allows for very precise temperature control.
These advanced digital temperature controllers can often be programmed for many different types of alarms. For example, you can set a high alarm to turn off the heat source when the temperature exceeds the preset value to prevent heat from damaging the device. The deviation alarm can be set to a positive or negative value based on the set value to notify the operator when the temperature exceeds the range.
Another practical function is to sound an alarm when the output signal is 100 percent, but after a while, the input sensor does not detect any temperature change, indicating a failure in the temperature control loop.
Flexible Temperature Controller
Typically, single-loop controllers have inputs and outputs. Controls with multiple control loops have multiple inputs and outputs that can be used to control multiple control loops at the same time, allowing more functionality to be monitored for the process system.
In addition, multi-circuit controllers are compact and modular, and can operate in stand-alone mode or as part of advanced automation systems, such as programmable logic controllers or programmable automation controllers in distributed control systems.
When used instead of temperature control in one of these advanced automation systems, the multi-loop controller provides fast PID control and offloads many memory swap calculations from the automation system processor.
As an alternative to DIN controls, multi-loop controls provide a central software access point for all control loops. These controls also provide functionality that regular panel controls do not. High loop density, small space requirements and reduced cabling are achieved by providing common connection points for power and digital communication interfaces.
Compared to simple controllers, temperature controllers with multiple control loops typically have extended security features that prevent unauthorized access to important Settings. These capabilities give you complete control over what the controller reads or writes, limiting what the operator can read or change.
Advanced controls also provide excellent communication capabilities that allow them to communicate with advanced automation systems via digital communication links. It can be quickly and easily configured using pc-based software, making it easy to save for future use. When connected to the Internet or Intranet, you can remotely access these controllers and perform complete remote viewing, configuration, and control from any location that has access to the Internet or Intranet.
This article introduces the functions and types of temperature controller. In addition to the original cost, there are many factors to maintain safe and effective operations, from sensor types to precision requirements to remote access. Cheap controls can be very expensive if the components need to be repaired frequently, the required precision cannot be maintained, or accidents occur due to inadequate safety features. You need to provide the appropriate controller based on the process requirements and look at each application in detail.
Figure 1: the controller's IP and NEMA values are important indicators of the protection level of installed equipment.
Figure 2: PID control improves efficiency by providing the exact baseline required to maintain the set value.