Furnace controllers are used to control temperature, heat delivery, and other variables in industrial furnaces. They control the burner flame, air mixture and trim to maintain and optimize furnace performance. Furnace controllers receive inputs from thermocouples and sensors, and can be integrated with other industrial control systems. Some products are designed to calculate levels of carbon dioxide or methane. Others are used to monitor ammonia additions. Choices for furnace controller functionality include: rate indication and control, data logging, and totalizing. The user interface may consist of a digital front panel or analog components such as knobs and switches. Computer-programmable, web-enabled, and network-ready furnace controllers are commonly available. A furnace controller with digital controls may include human machine interface (HMI) software or provide supervisory data acquisition and control (SCADA) functionality.
Specifications for furnace controllers include form factor or mounting style, number of inputs, number of outputs, input types, and output types. Some furnace controls have a printed circuit board (PCB) form factor. Others are designed for mounting in a rack, on a wall, or with a DIN rail. Stand-alone furnace controllers are benchtop or floor-standing units with a full-casing or cabinet and an integral interface. Furnace control suppliers specify the number of inputs as the total number of signals sent to the furnace controller. The number of outputs is the total number of signals used to control, compensate or correct the process. Input types include: direct current (DC) voltages, current loops, analog signals from resistors or potentiometers, frequency inputs, and switch or relay inputs. Output types include analog voltages, current loops, switch or relay outputs, and pulses or frequencies. Furnace controllers can also send inputs or receive outputs in serial, parallel, Ethernet or other digital formats which indicate a process variable. Products that send and receive commands via industrial fieldbus protocols are commonly available. Furnace control protocol types include CANbus, SERCOS, and PROFIBUS® (PROFIBUS International).
Furnace controllers differ in terms of control techniques. Limit control establishes set points or limits that, when reached, sends a signal to stop or start a process variable. Linear control matches a variable input signal with a correspondingly variable control signal. Proportional, integral, and derivative (PID) control requires real-time system feedback. Feedforward control provides direct-control compensation from the reference signal. Fuzzy logic is a type of furnace control in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Advanced or nonlinear controls for furnace controllers use algorithms such as neural networking and adaptive gain.