Batch controllers maintain the correct size of a discrete batch of process material, such as the volume of a liquid. They are often interfaced with sensors such as load cells, or with volumetric flow meters. Gravimetric batch controllers control batch size based on media weight. Volumetric batch controllers control batch size based on media volume. Both types of process controllers are available in several different form factors. Some batch controllers are printed circuit boards (PCB) that can be attached to an enclosure or plugged directly into a computer backplane. Others attach to a panel or bolt onto a chassis, wall, cabinet, or enclosure; mount in racks and include hardware such as rail guides, flanges, or tabs; or mount on a standard DIN rail. Benchtop or floor-standing batch controllers with a full casing or cabinet and an integral interface are also available.
Batch 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 include algorithms such as neural networking and adaptive gain.
Specifications for batch 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 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. Batch controllers can also send inputs or receive outputs in serial, parallel, Ethernet or other digital formats which indicate a process variable. Some batch controllers send or receive accept commands and inputs from information converted to an industrial fieldbus protocol such as CANbus, PROFIBUS®, or SERCOS, or other industrial automation protocol. PROFIBUS is a registered trademark of PROFIBUS International.
Batch controllers differ in terms of specific functionality, media, special capabilities, user interface, and compliance. Choices for specific functionality include rate indication or control, totalizing, data logging, and chart recording. Liquid, solids or powders, gases or air, and steam are media parameters. In terms of the user interface, batch controllers can include a digital front panel or analog components such as knobs, switches, and meters. Computer-programmable, web-enabled, and Ethernet or network-ready devices are also available. In terms of compliance, batch controllers that are destined for sale in the European marketplace should meet the requirements of the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronics Equipment (WEEE) directives from the European Union (EU).
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