Environmental controllers are used to control environmental factors such as temperature, humidity, and air pressure. The environment to control can be an office or workspace, or inside a chamber for testing or process purposes. Environmental controllers for buildings receive inputs from thermostats and space sensors, and can be integrated with air handling units (AHU), variable air volume units (VAV) roof top units (FTU), and fan coil units (FCU). Some devices can also be used to adjust lighting, heating and cooling systems. Choices for functionality include rate indication or 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 environmental controllers are commonly available. Products with digital controls may include integral software. Some environmental controllers 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 environmental controllers are benchtop or floor-standing units with a full-casing or cabinet and an integral interface.
Specifications for environmental controllers include number of inputs, number of outputs, input types, output types, and number of zones (if applicable). The number of inputs equals the total number of signals sent to the environmental controller. The number of outputs equals the total number of outputs 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. Environmental controllers can also send inputs or receive outputs in serial, parallel, Ethernet or other digital formats which indicate a process variable. Some environmental controllers send or receive commands via industrial fieldbus protocols such as CANbus, SERCOS, or PROFIBUS® (PROFIBUS International).
Environmental controllers differ in terms of control techniques. Limit control establishes set points or limits that, when reached send a signal to stop or start a process variable. This off-on or bang-bang control is a fairly simple type of control. Linear control matches a variable input signal with a correspondingly variable control signal. Signal conditioning, filtering, and amplification can be used to produce the proper output control signal. Proportional, integral, and derivative (PID) control requires real-time system feedback. Feedforward control provides direct-control compensation from the reference signal. It may be open-loop or used in conjunction with more advanced PID control. Fuzzy logic is a type of 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 environmental controllers use algorithms such as neural networking and adaptive gain.