Computer control operator interface

With the rapidly increasing processing power of computers, improved operator interfaces and distributed control systems improved overall controls available to processors. Real-time process and product data and SPC tools became much more accessible and better utilized. However, in this period, perhaps the two most important factors affecting the adoption of process monitoring and control strategies have been... 

Although digital control technology was first apphed to process control in 1959, the total dependence of the early centralized architectures on a single computer for all control and operator interface functions resulted in complex systems with dubious rehability. Adding a second processor increased both the complexity and the cost. Consequently, many installations provided analog backup systems to protect against a computer malfunction. 

Host computers. These are the most powerful computers in the system, capable of performing func tions not normally available in other units. They act as the arbitrator unit to route internodal communications. An operator interface is supported and various peripheral devices are coordinated. Computationally intensive tasks, such as optimization or advanced control strategies, are processed here. 

Single-loop controllers provide both the process control functions and the operator interface function. This makes them ideally suited to very small applications, where only two or three loops are required. However, it is possible to couple single-loop controllers to a personal computer (PC) to provide the operator interface function. Su(m installations are extremely cost effec tive, and with the keen competition in PC-based produc ts, the capabilities are comparable and sometimes even better than that provided by a DCS. However, this approach makes sense only up to about 25 loops. 

Figure 4.8 shows a specific example of this type of diagram which includes some symbols. The diagram shows the tasks that the operator and the computer must perform in a computer controlled reactor. The central column is used to show any functional requirements of the human-computer interface. 

Products in Group 3 seem to us to represent the future of practical batch process control. In such systems, modern workstations perform the single-user functions (e.g control system design, set-up, and maintenance operator interface data collection historical reporting) for which they were designed, while powerful multitasking controllers perform actual control. As computer hardware and software standards continue to evolve toward distributed networks of processors optimized for specific kinds of tasks, such systems will, we feel, proliferate rapidly. 

The central computer is called the master terminal unit, or MTU. The MTU has two main functions to periodically obtain data from RTUs/PLCs and to control remote devices through the operator station. The operator interfaces with the MTU using software called human machine interface (HMI). The remote computer is called the program logic controller (PLC) or remote terminal unit (RTU). The RTU activates a relay (or switch) that turns mechanical equipment on and off. The RTU also collects data from sensors. Sensors perform measurement, and actuators perform control. 

MS Carey, "Safety Management of Process Faults A Position Paper on Human Factors Approaches for the Design of Operator Interfaces to Computer Based Control Systems", HSE Contract Research Report No 60/1993, HSE Books, 1993... 

High transmission rates can be achieved, if necessary, over the relatively short distances required in a process plant. The PCM equivalent of the 4-20 mA analog transmission system shown in Fig. 6.1 can operate at up to 9,600 baud for distances up to 3000 m. The standard RS-232C transmission link is limited to about 15 m at rates up to 20,000 baud. Higher speed interfaces (such as versions of the IEEE-488 connection) used for computer control systems can handle up to 20,000 bytes/s (which for an 8-bit system is about 1.6 x 105 baud). However, in this case, the distance between devices is limited to about 2 m(4). The more recent RS-422A standard allows the transmission of data rates of 107 baud over distances not exceeding 16.4 m and 105 baud over distances not exceeding 1220 m(9S). 

The order of testing should be considered to ensure retesting is minimized. Operator interface and screen displays are best tested before the system is used for other tests. Input/outputs need to be satisfactorily tested before other tests that are dependent on proven I/O signals, and trend display testing may be needed to support loop testing. For interfaces to other computer systems the main consideration is which system controls the access, selection, transfer, and use of validated data. 

In practice, this process ensures that the system (computer hardware, computer software, controlled equipment, interfaces, operating environment, operators, equipment, procedures, and so on), in its normal operating environment, produces acceptable quality product and that sufficient documentary evidence exists to demonstrate this assessment. 

Nearly all mass spectrometers today are interfaced with a computer. Typically, the computer controls the operation of the instrument including any chromatography, collects and stores the data, and provides either graphical output (essentially a bar graph) or tabular lists of the spectra. 

An operator interface is used to record data gathered by the PLC.1 The operator interface is usually another computer (sometime called the human-machine interface or HMI). The HMI uses process displays with real-time sensor readings so that the operator can quickly assess the status of the system (see Figure 6.17). The operator uses the control panel to adjust alarm settings and to turn on and off process equipment. Once running, however, the PLC controls and runs the system automatically, without further input from the operator. Common HMI status indicators are listed below ... 

An automatic control system consists of various pressure, temperature, and flow sensors, valves and components controlled by a programmable logic controller (PLC), and an operator interface device. The operator interface control device can be as simple as a stop-go for totally dedicated systems, or a personal computer. A personal computer connected to the PLC to provide operator interface, data collection and analysis capability is recommended especially if the system parameter and/or the parts being processed will be changing. More importantly, a computer control system assures consistent and repeatable operation. 

For some ports, the computer s operating system—the ROM which controls the machine s functions—provides routines that handle much or all of the "dirty work" of communicating with peripheral devices. To use other ports, you must program all the necessary support routines yourself. That task can range from very easy (for example, reading a joystick) to quite complex (interfacing with a parallel printer through the user port, for example). 

The depressurization system acts as an interface between the supercritical conditions in the extraction cell and the atmospheric conditions to which the extract is eventually subjected when the extractor is not connected on-line to a chromatographic system for the individual separation of extracted species with a view to their subsequent detection. A wide variety of commercially available devices for this purpose exists that range from straightforward glass capillaries — the end of which can be readily cut off in the event of clogging — to hand-operated restrictors to computer-controlled units. This is one of the characteristic components of commercial SF extractors (one that can differ markedly among manufacturers). 

A distributed control system is a system that is functionally integrated but consists of subsystems that may be physically separate and remotely located from one another. A shared display is an operator interface device such as a computer screen or video screen that is used to display process control information from a number of sources at the command of the operator. Most plants built since 1990 (and many older plants) use shared displays instead of instrument panels. 

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