Allocation of Function

This form of implanned manual operation is unsatisfactory on a number of counts. The fact that the operator may normally be insulated from the process by the automatic control systems means that he or she will probably not be able to develop the knowledge of process dynamics ("process feel") necessary to control the system manually, particularly in extreme conditions. Also, the fact that manual control was not "designed into" the systems at the outset may mean that the display of process information and the facilities for direct control are inadequate. A number of techniques are available to assist designers in the allocation of function process. Some of these are described in Meister (1985). In a paper entitled "Ironies of Automation" Bainbridge (1987) notes four areas where the changed role of the human in relation to an automated system can lead to potential problems. These will be discussed below. 

Allocation of function problem (personnel responsible believe other personnel will activate blowdown). 

Man-machine interface consists of such aspects as allocation of functions (man vs. machine), automation, accessibility, human tasks, stress characteristics, information presented to the human, and the reliability of interfaces coupled with the decisions on the basis of such information. Both human and machine elements of a system can fail, and their failures have varying effects on the system s performance. Some human errors cause total system failute or increase the risk of such failure. Human factors exert a strong influence on the design and ultimate reliability of a system (Kirwan 1994). 

In Figure 33.1, both the human and product functionally contribute to and work together in performing the application task. The functions and lower-level tasks needed to perform the application task are distributed between them. This distribution, known as allocation, is based on the capabilities and limitations each possesses. The allocation of function is a major factor in determining the interactions between the human and product that need to be performed. 

Human Factors means those biomedical, psychosocial, work place environment, and engineering considerations pertaining to people in a human-machine system. Some of these considerations are allocation of functions, task analysis, human reliability, training requirements. Job performance aiding, personnel qualification and selection, staffing requirements, procedures, organizational effectiveness, and workplace environmental conditions. 

Top-down , i.e. the FHA is often first carried out for the whole aircraft - working from a description of aircraft functions. Then, following allocation of functions to aircraft systems, the FHA is then performed again for each subsystem (Wilkinson and Kelly, 2005). A bottom-up approach would typically be in the format of an FMECA. 

Function binding There is a fixed allocation of functions to modules. 

The paper presents a pragmatic methodology to fully integrate human ftictors analyses with safety engineering analyses to take account of both human and technology capabilities and limitations, thereby addressing the major risks to systems safety. The approach presented here addresses the specification of both Operational-level and System-level safety requirements down to the allocation of functions and safety requirements to subsystems comprising equipment, people and procedures. 

A discussion on the sctfe initial allocation of function between human and machine will be given later in the paper. The hazards and risks associated with failure of each subsystem may be assessed, using the broad Bow Tie approach described above, any mitigations are identified and allocated (as domain knowledge or additional safety functions, as appropriate), and the safety integrity requirements for each subsystem determined. 

The human success case must be built upon two main activities relating to the system safety requirements specification which are the initial Allocation of Function between human and machine and an initial CTA of the functions (or tasks) allocated to the human subsystems to determine what constitutes successful human task performance requirements. Both of these activities are examined here in more detail. 

The allocation of functions between humans and machines, and defining the extent of operator involvement in the control of the system is a critical activity in safety-related systems. Figure 7 shows a general process for deriving the subsystem safety requirements from a high-level architectural design. 

The production of a high-level architectural design requires initial decisions to be made on the allocation of flmctions to human or equipment sub-systems, in full knowledge of the safety risks involved. Functional allocation decisions need to be informed by good human factors principles and yet the allocation of function is still considered exclusively an ergonomics problem by many systems developers. 

This paper has examined problems associated with the specification and realisation of functional safety requirements for the human elements of a system for which a target level of safety is specified at the service level. It was shown that the high-level allocation of functions to hardware, software or humans must be done by taking human performance and limitations into account and a generic approach was presented for the specification of both service-level and system-level safety requirements down to the allocation of functions and safety requirements to subsystems. 

Figure 23.2 A schematic representation of the shift in the man-machine interface and change in allocation of function in different types of vehicle production (see text for details)...

Nam-Sung Woo and Hyunchul Shin, A Technology-Adaptive Allocation of Functional Units and Connections , Proc. of the 2 h DAC, pages 602-605, Jvme 1989. 

The book is divided into chapters covering these 14 topic areas, each of which addresses the main areas of analysis— namely, competencies for control room work, training needs analysis, communication processes within and without the control room, manning of the control room, automation and allocation of function, supervision of staff, shift patterns, control room layout, SCADA interface, alarms, environment, human error identification, and safety culture. Within each of the subject areas... 

Automation is impacting staffing levels, as it allows system designers to automate various system control tasks and thus change the role of human operators from frontline controllers to systan monitors or supervisors. An important consideration here is appropriate allocation of functions—that is, identifying the tasks that should be automated and which tasks should continue to be the responsibility of human operators. 

Designers should recognize that operators are not inherently flawed and subsequently should not simply remove or automate as many tasks as possible. This approach is likely to lead to mental underload and could leave operators with incoherent sets of tasks. The goal instead should be to design the system and procedures to exploit the skills of human operators. Physical workload should be kept to a minimum, whereas mental workload should be optimized. Ways to optimize mental workload include the use of adaptive interfaces (systems that can detect periods of elevated workload and then assist the operator accordingly) and dynamic allocation of function (systems that relieve the operator of task elements during high workload portions of the task). 

Allocation of functions—Functions should be allocated appropriately between the operators and the system (automation). 

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