OPERATOR INTERFACES
For the automated equipment to be used in a safe and effective manner, however, it is absolutely necessary to have a well engineered human interface system to permit error free interactions between the humans and the automated system. Two distinct groups of plant personnel interact with the control system on a regular basis:
1. Instrumentation and control systems engineers-These people are responsible for setting up the control system initially and adjusting and maintaining it from time to time afterwards.
2. Plant operators-These people are responsible for monitoring, supervising, and running the process through the control system during startup, operation and shut down conditions.
In distributed control systems architecture, a human interface capability can be provided at one or both of two levels:
1. Through a low-level human interface(LLHI) connected directly to the local control unit or data input/output unit (DI/OU) via dedicated cabling.
2. Through a high level human interface (HLHI) connected to an LCU or DI/OU only through the shared communication facility.
The low level human interface equipment used in distributed control systems usually resembles the panel board instrumentation (stations, indicators, and recorders) and tuning devices used in conventional electric analog control systems. The HLHI equipment makes maximum use of the latest display technology (e. g. , CRTs or flat panel displays) and peripheral devices (e. g. , printers and magnetic storage) that are available on the market;it is configured in a console arrangement that allows operator and engineer to be seated during use.
OPERATOR INTERFACE REQUIREMENTS
Despite the continuing trends towards increased automation in process control and less reliance on the operator, the basic responsibilities of the operator have the remained largely the same in the last fifty years. . Most of the changes have come in the relative emphasis on the various operator functions and the means provided to
accomplish them. As a result, the operator interface in the distributed control system must allow the operator to perform tasks in the following traditional areas of responsibility, process monitoring, process control, process diagonostics, and process record keeping. In addition it is important to design the operator interface system using human factors design principles (also called ergonomics) to ensure the operator can perform these tasks in an effective manner with minimum risk of confusion or error. So, the key functional requirements in each of these areas:
-Process monitoring –
1) A basic function of the operator interface system is to allow the operator (whether one or more) to observe and monitor the current state of the process. The current values of all process variable of interest in the system must be available for the operator to view at any time. This includes both continuous process variables (e. g. , flows, temperatures, and pressures) and logical process variables (e. g. , pump on/off status and switch positions). The operator must have rapid access to any variable, and the values displayed must be accurate and current. If the information provided is not valid for some reason( e. g. , a sensor has failed or has been taken out of service for maintenance), this condition should be readily visible to the operator.
2) Another monitoring function of the opertor interface is to detect abnormalities in the state of the process and report them to the operator. In its simpler form, this is familiar function of alarming.
Process control
This process monitoring capabilities just described provide the necessary information for the operator’s primary function - process control. The operator interface must allow the operator to have rapid access to all of the continuous control loops and logic sequences in the process control system. For each continuous control loop, the interface must allow the operator to perform all of the normal control functions:
changing control modes(e. g. , automatic, manual, or cascade), changing control outputs in manual code, changing set points in automatic mode, and monitoring the results of these actions.
Process diagnostics
Monitoring and controlling the process under normal operating conditions are relatively simple functions compared to operations under abnormal or hazardous conditions caused by failures in plant equipment or in the instrumenation and control system. The operator interface system must provide enough information during these unusual condition to allow the operator to identify the equipment causing the problem, take measures to correct it, and move the process back to its normal operating state. The distributed control system should provide the following diagnostic features and make the results of the diagnostic test available to the operator:
1. Ongoing tests and reasonableness checks on the sensors and analyzers that measure the process variable of interest. 7
2. Ongoing self-tests on the components and modules within the distributed control system itself: controllers, communication elements, computing devices, and the human interface equipment itself.
Process record keeping
One of the more tedious duties that operating people in a process plant must perform has been to walk the board;that is, to take a pencil, and clipboard and periodically note and record the current values of all process variables in the plant. Depending on the process, the frequency for doing this may vary from once an hour to once every several hours. This logged information, along with the trend recordings obtained automatically, serves as a useful record of plant operating status during each shift.
The record-keeping burden has increased significantly in recent years due (in part, at least) to governmental reporting requirements related to pollution monitoring, product liability, and worker safety regulations.
LOW-LEVEL OPERATOR INTERFACE
The low level operator interface (LLOI) in a distributed control system is connected directly to the LCU and is dedicated to controlling and monitoring that LCU. This contrasts with the high level operator interface (HLOI), which can be associated with multiple LCUs and is connected to them through the shared communication facility.
LLOIs are used in a variety of applications, in some cases in conjunction with high level operator interfaces (HLOIs) and in others in place of them. In some applications, all operator functions are performed through the HLOI and no low level operator interface is required except during emergency or failure conditions. There are a number of motivations for using an LLOI. These are:
-1. It provides an interface that is familiar to operators trained to use panelboard instrumentaion, since it is usually designed to resemble that type of instrumentation.
2. It is usually less expensive than an HLOI in small applications(say less than 50 control loops).
3. It can provide manual backup in case the automatic control equipment or the HLOI fails.
LLOI instrumentation usually includes the following devices: control stations, indicator stations, alarm annunciators, and trend recorders.
HIGH-LEVEL OPERATOR INTERFACE
In contrast with the low-level operator interface, the high level operaor interface in a distributed control system is a shared interface that is not dedicated to any particular LCU. Rather the HLOI is used to monitor and control the operation of the process through any or all of the LCUs in the distributed system. Information passes between the HLOI and the LCUs by means of the shared communications facility.
While the LLOI hardware resembles conventional panelboard instrumentation, HLOI hardware uses CRT or similar advanced display technology in console configurations often called video display units(VDUs). The HLOI accepts operator inputs through keyboard instead if the switches, push buttons and potentiometers characteristics of coventional operator interface panels. Other digital hardware prints, stores, and manipulated information required in the operator interface system.
In general, the use of microprocessor based digital technology in the design of the HLOI system allows the development of a hardware configuration that departs radically from the design of panelboard-based operator interfaces.