Most industrial HMI panels come equipped with touch-sensitive screens, allowing operators to press their fingertips on displayed objects to change screens, view details on portions of the process, etc. These HMI panels are really nothing more than “hardened” personal computers built ruggedly and in a compact format to facilitate their use in industrial environments. This next photograph shows an example of a special-purpose HMI panel designed and built expressly to be used in industrial operating environments:
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The personal computer running this HMI software connects to the PLC(s) via digital network cables such as Ethernet. Somewhere, a PLC (or collection of PLCs) is monitoring and controlling this VSA process, with the HMI software acting as a “window” into the PLC’s memory to display pertinent variables in an easy-to-interpret form for operations personnel. The display shown here happens to be for monitoring a vacuum swing adsorption (VSA) process for purifying oxygen extracted from ambient air. This first photograph shows an example of an ordinary personal computer (PC) with HMI software running on it: HMIs may take the form of general-purpose (“personal”) computers running special graphic software to interface with a PLC, or as special-purpose computers designed to be mounted in sheet metal panel fronts to perform no task but the operator-PLC interface. One solution to this problem is a dedicated computer display programmed to provide selective access to certain variable’s in the PLC’s memory, generally referred to as Human-Machine Interface, or HMI. In order for operators to monitor and adjust parameters inside the PLC’s memory, we need a different sort of interface allowing certain variables to be read and written without compromising the integrity of the PLC by exposing too much information or allowing any unqualified person to alter the program itself.
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A technician or engineer with access to a personal computer and the requisite software for editing the PLC’s program may connect to the PLC and view the program’s status “online” to monitor signal values and variable states, but this is not a practical way for operations personnel to monitor what the PLC is doing on a regular basis.
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By itself, a PLC generally lacks the capability of displaying those signal values and algorithm variables to human operators. Programmable logic controllers are built to input various signal types (discrete, analog), execute control algorithms on those signals, and then output signals in response to control processes.