Web controllers are used to regulate tension, rate or alignment; or to detect breakage in a web process. They receive sensor inputs, provide control functions, and output control signals. Web controllers use several control types. Limit controls protect personnel and equipment by interrupting power through a load circuit when a monitored value exceeds or falls below a set point. Advanced controls use non-linear control strategies such as adaptive gain, dead-time compensation, fuzzy logic, neural networks, 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. P control is proportional to the error between reference and feedback signals.

Differences in Web Controllers

Web 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 web process 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 web controllers power sensors with voltage levels such as 0 – 5 V or 0 – 10 V. Others power sensors with current loops such as 0 – 20 mA, 4 – 20 mA, or 10 – 50 mA.

Selecting Web Controllers

Selecting web process 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 web process 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. Web 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 web process controllers include self-tuning, programmable set points, signal computation or filters, and built-in alarms or indicators.

Communications and Networking

Web controllers vary in terms of communications and networking. Both serial and parallel interfaces are available. Common protocols include attached resource computer network (ARCNET), the AS-interface (AS-i), Beckhoff I/O, controller area network bus (CANbus), DeviceNet, Ethernet, FOUNDATION Fieldbus, general-purpose interface bus (GPIB), Seriplex, smart distributed system (SDS), small computer system interface (SCSI), INTERBUS-S®, process fieldbus (PROFIBUS®), and Sensoplex®. INTERBUS-S is a registered trademark of Phoenix Contact GmbH & Co. PROFIBUS is a registered trademark of PROFIBUS International. Sensoplex is a registered trademark of Hans Turck GmbH & Co.


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