Operator interface or human-machine interface (HMI) for SCADA systems provide the functions of status indication, alarm reporting, operator intervention in control action, and data storage and programming.
Several levels or layers of operator interfaces are required to provide a reliable and maintainable system: equipment level, controller level, and supervisory level. At the controller and supervisory level, HMI may also provide the capability to modify the controller program.
Note: The below-discussed points may vary from industry to industry. Consider the below example for understanding the basics. Real-time applications may vary as per specific industry standards.
Equipment Level
Equipment level HMI should consist of a minimum of the control switches and indicators necessary to permit an operator to manually control the equipment in the absence of communications from the controller or for maintenance purposes.
Examples of this level of control are hand-auto-off switches and indicator lights at motor starters; local-remote switches, potentiometers, and meters at variable frequency drives; and circuit breaker control switches, meters and indicator lights at switchgear.
The table lists minimum manual control capabilities to be provided for mechanical and electrical system components.
Manual control substitutes the industrial facility operator for the automatic control system in the feedback loop and leads to the risk of system or equipment misoperation due to human error.
Safety interlocks, such as motor overload, high-high pressure switches, fire detection, etc. should, therefore, be hard-wired into the control circuit such that they are active in both manual and automatic control modes.
Switchgear protective relaying required for fault protection should always be hard-wired in the circuit breaker trip circuit and not dependent upon the controller.
In some cases, hard-wired manual controls for entire industrial facilities have been centrally located in a control panel or bench board at the control room.
Although this simplifies operator intervention upon complete failure of the automatic control system, it is not recommended as the lack of physical segregation compromises reliability. A catastrophic structural or environmental failure at the control room would disable both automatic and manual control capability.
Controller Level HMI
At the controller level, the primary HMI device should be a graphical display/keypad combination providing access to input and output data, timer and register settings, and alarm and status screens.
These devices are commonly panel-mounted in the door of the PLC cabinet and are available in enclosures suitable for hostile environments.
With password-protected access control, the controller level HMI may also provide a means of accessing and modifying the controller program logic. The below table lists the minimum recommended functionality of controller level HMI.
Note (1) Increasing numbers indicate more restricted levels of operator access. Access levels are typically password-protected. Each access level includes the functionality of those below it.
Supervisory Level HMI
Supervisory level HMI devices are typically personal computer workstations located in the central control room and/or management and engineering offices. The quantity and function of these workstations depend on the size and complexity of the industrial facility.
Simple industrial facilities may be provided with a single workstation, which may be located in the main electrical or mechanical space in the absence of a central control room.
Large or complex industrial facilities should be provided with a minimum of two workstations in the control room to permit operators to back one another up, plus the additional workstations required for engineering use, management overview, or data storage and reporting, as determined by the industrial facility
Operator interface or human-machine interface (HMI) for SCADA systems provide the functions of status indication, alarm reporting, operator intervention in control action, and data storage and programming.
Several levels or layers of operator interfaces are required to provide a reliable and maintainable system: equipment level, controller level, and supervisory level. At the controller and supervisory level, HMI may also provide the capability to modify the controller program.
Note: The below-discussed points may vary from industry to industry. Consider the below example for understanding the basics. Real-time applications may vary as per specific industry standards.
Equipment Level
Equipment level HMI should consist of a minimum of the control switches and indicators necessary to permit an operator to manually control the equipment in the absence of communications from the controller or for maintenance purposes.
Examples of this level of control are hand-auto-off switches and indicator lights at motor starters; local-remote switches, potentiometers, and meters at variable frequency drives; and circuit breaker control switches, meters and indicator lights at switchgear.
The table lists minimum manual control capabilities to be provided for mechanical and electrical system components.
Manual control substitutes the industrial facility operator for the automatic control system in the feedback loop and leads to the risk of system or equipment misoperation due to human error.
Safety interlocks, such as motor overload, high-high pressure switches, fire detection, etc. should, therefore, be hard-wired into the control circuit such that they are active in both manual and automatic control modes.
Switchgear protective relaying required for fault protection should always be hard-wired in the circuit breaker trip circuit and not dependent upon the controller.
In some cases, hard-wired manual controls for entire industrial facilities have been centrally located in a control panel or bench board at the control room.
Although this simplifies operator intervention upon complete failure of the automatic control system, it is not recommended as the lack of physical segregation compromises reliability. A catastrophic structural or environmental failure at the control room would disable both automatic and manual control capability.
Controller Level HMI
At the controller level, the primary HMI device should be a graphical display/keypad combination providing access to input and output data, timer and register settings, and alarm and status screens.
These devices are commonly panel-mounted in the door of the PLC cabinet and are available in enclosures suitable for hostile environments.
With password-protected access control, the controller level HMI may also provide a means of accessing and modifying the controller program logic. The below table lists the minimum recommended functionality of controller level HMI.
Note (1) Increasing numbers indicate more restricted levels of operator access. Access levels are typically password-protected. Each access level includes the functionality of those below it.
Supervisory Level HMI
Supervisory level HMI devices are typically personal computer workstations located in the central control room and/or management and engineering offices. The quantity and function of these workstations depend on the size and complexity of the industrial facility.
Simple industrial facilities may be provided with a single workstation, which may be located in the main electrical or mechanical space in the absence of a central control room.
Large or complex industrial facilities should be provided with a minimum of two workstations in the control room to permit operators to back one another up, plus the additional workstations required for engineering use, management overview, or data storage and reporting, as determined by the industrial facility
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