1 Human-Machine Systems A system could be defined with mathematical symbols and set theory. A Human-Machine System could be defined verbally as an arrangement of people and machines (or physical components) interacting within an environment in order to achieve a set of system goals. Examples of Human-Machine Systems include a person with a hammer, an office machine with its operator, oil refineries, hospitals, a rescue operation of a crashed airplane. The task of a human factors specialist is to optimize the interaction between people and machine elements of the system while taking the environment into account An example of an interaction between a person and a machine is shown in figure 1.1(p15)

2 Systems are generally classified into three main classes: 1. Manual Systems: Consisting of hand tools and other aids coupled by a human operator who controls the operation. The source of power is human physical energy. 2.Mechanical Systems (Semiautomatic): Consisting of integrated physical parts (such as powered machine tools). The function is performed with little variation and the power is provided by the machine. The user usually manipulates the control devices. 3.Automated Systems: All operational functions are performed with little or no human intervention (such as robots). But are they Human-Machine Systems? All automated systems require humans to install, program, reprogram, and maintain. Hence, Automated systems must be designed with the same attention paid to manual or mechanical systems.

3  Characteristics of Systems: 1.Systems Are Purposive: A system has a purpose. The purpose of the system is the system goal or objective. A system could have more than one objective. 2.Systems Can Be Hierarchical: A system may be composed of a number of subsystems. System boundary and resolution limit must be set. 3.Systems Operate in an Environment: The system environment is every thing outside its boundaries. It could range from immediate environment (such as a chair) through intermediate (such as an office) to general (such as a city). 4.Components Serve Functions: Each component serves at least one function that is related to achieving the system goal. There are four basic functions:

4 A.Sensing (information receiving). B.Information Storage. C.Information Processing and Decision. D.Action Functions (Physical control action or process and communication action). Figure1.2 page17 5.Components Interact: components work together to achieve a system goal. Each component has an effect on other components. 6.Systems, Subsystems, and Components Have Inputs and Outputs: The outputs of one system or component are the inputs to another. A system receives inputs from the environment and makes outputs to the environment. Inputs and outputs could be physical entities (such as materials), electric impulses, mechanical forces, or information.

5 Closed-loop or open-loop systems?  A closed-loop system is continuous performing a process requiring continuous control and a continuous feedback (providing information about errors) for its successful operation.  An open-loop system, when activated, needs no further control or cannot be further controlled. Feedback can improve future operations of such a system. System Reliability: Reliability is usually expressed as the probability of successful performance. It could be also measured by the mean time to failure (MTF) which relates to the amount of time a system or individual performs successfully either until or between failures. Components in series? The successful performance of the system depends on the successful performance of each component. Components in parallel? The system will fail if all components fail