Many process plant units have temperature loops that must be monitored and controlled. If these units are self-contained to some degree, as with a process skid or a remote unit, they often have just a few discrete and analog I/O points that must be monitored and controlled in addition to temperature. These units also often need some limited operator interface.
In the past, these remote units were often controlled by basic temperature controllers, a programmable logic controller (PLC), and a human-machine interface (HMI) terminal. But now, a single hybrid temperature controller can perform all these functions by controlling and monitoring multiple temperature loops, by acting as a mini-PLC, and by providing limited but often sufficient operator-interface functionality.
These hybrid temperature controllers excel at loop control, often employing self-tuning and basic artificial intelligence algorithms. This makes them suitable for the most demanding temperature control applications.
Two forces that continue to drive the development of hybrid temperature controllers are the consolidation of instrumentation, and the migration from discrete controls to PLCs (table 1). User demands typically guide changes in products as they try to force as much capability as possible into a controller in an effort to squeeze every dollar out of costs.
In many applications, temperature control, logic control, and operator interface are all required at the process unit. In cases where a lot of computation or loops are required, a full-blown PLC-based system with a sophisticated separate HMI system makes sense. But in situations with just a few control loops and 10 to 20 other I/O points, this type of solution is cost prohibitive. In the past, many users would turn to one or more temperature controllers, a mini-PLC, and an operator-interface panel to get the needed functionality, but there is a better alternative.
To help bring costs down, some instrument manufacturers are combining temperature control, PLC logic control, and operator-interface functionality into a single hybrid temperature controller package (figure 1). This approach makes sense for a couple of reasons. First, powerful semiconductors allow complex operations to be performed at higher speeds than traditional controllers. Second, the existing operator interface on the temperature controller can also provide an interface to the logic control, and provide additional operator interface as required.
There are many benefits of combining these functions into a single instrument. Besides the reduced cost of the unit itself, there are other cost savings that emerge in the larger implementation.
- The size of the enclosure can be reduced, because less space is needed for housing one control device versus three.
- The amount of wiring and the wiring time can be minimized, as wiring does not need to be split between two controllers and the operator-interface terminal in the panel.
- Many wired connections that are typically required are no longer needed.
- Programming time and interface complexity can be reduced or eliminated.
- Operators need to learn and maintain only one PC-based programming software package, as opposed to three.
Many of these functions, such as timers, counters, and relay latches, are intuitively tied together inside the controller. This simplifies programming and speeds implementation.
Reduced need for complex HMIs
Organizations reduce costs, because they may no longer require a separate HMI on some applications, or may employ a smaller, cheaper one. Some controller features that merge well with internal ladder logic functions can work with programmable function keys, customizable displays with scrolling messages, and data entry capability. Function keys on the front of the controller can replace panel push buttons. These can be used as digital inputs to a ladder program, allowing operator interaction without the need for external hardware (figure 2).
A process alarm or a door switch can trigger the display to scroll a text message such as “Low Flow Rate” or “Door Open.” This tight coupling of proportional, integral, derivative control and a ladder logic program gives more benefit at a lower cost than using a separate temperature controller with a PLC, especially where an HMI is needed.
The displays on temperature controllers typically provide custom parameters. These parameters are twofold: data display and data entry, similar to a traditional HMI. Program variables, such as values for timers or ratio factors, can be entered for use by the ladder program. Custom calculated values from the ladder program, such as BTU or flow totals, can be displayed for the operator. This type of information typically is entered via a costly HMI, but these controller hybrids can do an adequate job of replacing touch panels in cases where costs are an issue. The capabilities of hybrid temperature controllers can open up other applications, such as replacing remote distributed control system (DCS) or PLC I/O.
Avoiding costly remote I/O
A hybrid temperature controller can often be used as remote I/O for a larger PLC or DCS that is managing the entire process. A smaller, task-specific controller can be placed near the part of the process it is controlling, providing local operator interface while being monitored and directed from the host. Organizations can achieve significant savings by avoiding the need to buy I/O hardware manufactured by the PLC/DCS provider, which is typically quite expensive. In addition, the local hybrid temperature controller can offload processing tasks from the host control system, and can provide an additional level of safety by allowing remote process units to shut down safely if the host fails.
This approach is extremely reliable for collecting process information and controlling remote I/O. Many PLC-based communication protocols are available in these hybrid controllers for this type of situation: Modbus TCP/IP, EtherNet/IP, CC-Link, Profinet, and many other protocols.
The selection of a protocol for your situation will be based on many factors, including the hardware you have selected, the types of data and diagnostics you require, and the technical expertise available to engineer and implement a communications system.
Hybrid temperature controllers have certainly added significant functionality over the past few years, but there are limitations to their use.
I/O count: Hybrid temperature controllers are typically limited to about 20 discrete points and a handful of analog points. This is fine for small process units where a micro- or mini-PLC is needed, but replacing a large-scale PLC is impractical.
Execution speed of the ladder program: Logic execution in hybrid temperature controllers is going to be limited to the scan rate of the controller, which is typically 50-200 ms, so high-speed switching or sequence-of-events applications are out of the question.
Programming capacity: All functions need to work within about a 1,000 ladder step limit. This is usually plenty of space given the types of programs that work with small I/O counts, but applications that must crunch a lot of numbers or perform floating-point math will chew through ladder steps pretty quickly, so caution is in order.
That said, building a group of totalizers or calculating temperature/pressure compensation for process variables does not require much in the way of ladder programming, particularly because hybrid temperature controllers are optimized for these types of functions. This leaves a wide range of applications open to hybrid temperature controllers, including the one described below.
A good application example for a hybrid temperature controller is a batch furnace where the operation calls for a guaranteed soak time for a load, but where the complexity of a profile controller is not desirable. The controller would employ a short ladder program that compares the process temperature to the set point. When the temperature reaches its set point, a timer with a user-defined time variable would start. When the time expires, the controller mode would be changed to stop, and a digital output would close to signal that the process was finished.
If a flame detector is needed for a gas-fired furnace, it could be brought into one of the I/O points. When a flame out is detected, the controller would go to manual mode and turn the output off. Another input would cause the output to go to 100 percent for a purge.
Temperature controller capabilities have increased substantially over the past few years. Some of the products that users have become accustomed to have been morphed by market forces into some truly interesting variants, such as hybrid temperature controllers. These controllers are carving out a niche with significant benefits for applications needing good temperature control capability, modest I/O logic control, decent computational capability, and limited operator interface.