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Pressure controllers are used to regulate positive or negative (vacuum) pressure. They receive pressure sensor inputs, provide control functions, and output control signals. Pressure controllers use several control types. Limit controls protect personnel and equipment by interrupting power through a load circuit when pressure exceeds or falls below a set point.
Advanced controls use non-linear control strategies such as adaptive gain, dead-time compensation, and feed-forward control. Linear controls use proportional, integral and derivative (PID) control; proportional and integral (PI) control; proportional and derivative (PD) control; or proportional (P) control. PID control uses an intelligent input/output (I/O) module or program instruction for automatic closed-loop operation. PI control integrates error signaling for steady-state or offset errors. By contrast, PD control differentiates error signals to derive the rate of change. PD control increases the speed of controller response, but can be noisy and decrease system stability.
Pressure controllers differ in terms of performance specifications, control channels, control signal outputs, and sensor excitation supply. Performance specifications include adjustable dead-band or hysteresis, minimum and maximum set points, update rate or bandwidth, and percentage accuracy. Hysteresis or switching differential is the range through which an input can be changed without causing an observable response. Hysteresis is usually set around the minimum and maximum end points. Control channel specifications for pressure controllers include the number of inputs, outputs, and feedback loops. Multi-function controllers and devices with multiple, linked looped are commonly available. Control signal outputs include analog voltages, current loops, and switched outputs. Some controllers power sensors with voltage levels such as 0 5 V or 0 10 mV. Others power sensors with current loops such as 0 20 mA, 4 20 mA, or 10 50 mA.
Selecting pressure controllers requires an analysis of discrete I/O specifications, user interface options, and special features. Devices differ in terms of the total number of inputs, total number of outputs, and total number of discrete or digital channels. Some pressure controllers provide alarm outputs or are designed to handle high power. Others are compatible with transistor-transistor logic (TTL). Analog user interfaces provide inputs such as potentiometers, dials and switches. Digital user interfaces are set up or programmed with a digital keypad or menus. Pressure controllers with a graphical or video display are commonly available. Devices that include an integral chart recorder can plot data on a strip chart, in a circular pattern, or on a video display. Special features for pressure controllers include self-tuning, programmable set points, signal computation or filters, and built-in alarms or indicators.