2. The human-machine system

• Most human-machines are considerably more complex than that shown in Figure 1, but this simple model provides a framework from which we can build models that include factors that are important for specific acitivities, e.g., flying, driving, assembling, inspecting, surfing the web, performing medical procedures, baking a cake, etc.
• The human-machine system concept provides a framework for identifying the information and actions required to complet a given task and is an important job/task analysis tool.
• The human-machine system as analogous to a negative feedback or closed-loop sustem. This is indeed a fair comparison. The human must gather and process information fast enough to exert effective control actions. The time required for necessary control inputs is the time constant for that system and ranges from the limits of human capability to hours or even days. Failure to respond in time or correctly results in an open-loop or out of control system. Open loops occur when there is a "gap" between task demands and human capacities that prevent the necessary human actions to maintain control of the system.
• It has been said that ergonomics is just "common sense." Indeed, it is possible to spot some conspicuous "gaps" that adversly affect system performance From casual observations, e.g., Examples of gaps. A thorough analysis of the all system factors and human machine interaction over time is required to identify and control all factors that affect system performance and operator safety and health.

  • Some words of advice: "We believe that the engineer who deals with human performance should avoid two faults that have made the "efficiency expert" one of the most hated figures in industry: (1) the neglect of personal and of social variables, and (2) the use of people as means to ends they do not share and do not determine. If a person is to design tasks for others he has the responsibility to attempt to see the tasks and their implications in a wider context and to attempt to ensure that they are life-enhancing."
    Sheridan TB, Ferrell WR. Man-machine systems; Information, control, and decision models of human performance. the MIT press, p. 19; 1974.

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Goals should describe:

• the reason(s) that the system exist
• why the task is performed -- not how.
• The expected work/task performance
• Work standard -- product quantity, quality, & rate
• There may be multiple goals
• There may be short-term and long-term goals
• Workers, employers, manufacturers and consumers all may have different goals
• Goals may be explicitly stated or implied
• Examples:

These are good goal statements Correctly enter data from IRS 1040 forms into a computer Assemble 60 widgets per hour with no defects Interview worker to determine if he/she is experiencing symptoms of CTS Send text message to correct recipient without errors

These are NOT so good goal statements typing Work on assembly line talking to worker texting

Work Standards

• Describes the quantity, quality and time for achieving goals
• Considerations:
  • Methods, equipment/tools, workspace, environment, Materials
  • Worker/user qualifications, training, special needs
  • See below
• Standard be set by an employer, equipment manufacturer, worker/user
• Examples:
  • Time for nurse to infomr patient and adminsiter vaccine
  • Time to change oil
  • Time to set up an automated external defibrillators, AED

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4. The people

• The human part of the system gathers and processes information and acts to maintain control of the system and achieve the desired system goals.
• Includes one or more persons
• The ability of a person to achieve desired goals is influenced by a number of personal factors, e.g.,
  • Gender
  • Age
  • Size
  • Physical and mental impairments
  • Education
  • Training
  • Culture/Values
  • Fatigue
  • Injuries & illnesses
• Normal Performance -- should be based on all or most workers/users -- not averages

  

5. Machine Factors

• "Machine" broadly refers to objects that human interact with to achieve a desired goal, e.g.,

  

• Materials or parts
  • Things that become part of the product
  • Hardware, ingredients, information
  • Can be other people, e.g., patients, customers, students, etc.
• Tools
  • Things that are used by the human to perform the job task, but do not become part of the product
  • Hardware or software
  • Other People (clinical, service job/task)
• Physical workspace Where the work is performed
  • E.g., shop, office, laboratory, clinic, park, motor vehicle, etc.
  • spatial relationship between worker/user and job/task
  • Affects location of equipment and materials
  • Affects worker's/user's ability to see, reach and use equipment and materials.
• Machine factors factors affect worker/user performance
  • Size, shape, mass, density, power, action
  • Location, orientation
  • Organization
  • Cooporation

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6. Interactions, methods

• To achieve human-machine system goals, human must constantly:
  • Gather information about the equipment and task (see, hear, touch, smell, taste)
  • Process information (compare information from one or more sources, with stored knowlege)
  • Decide (conscious, unconscious)
  • Act to control the machine (speak, move, reach, touch, grasp, move, hold)

  

• The interactions can be described as a sequence of steps.
• Steps may be
  • large, e.g., load machine, enter data.
  • Large steps can be decomposed into small steps can be aggregated into large steps as needed -- "Hierarchical Task Aanalsyis" or "HTA"
  • small, e.g., individual movements, perceptions, or decisions, e.g., "Therbligs"

  Reach Grasp Preposition Move Use Assemble Disassemble Release Search

  Select Position Hold Inspect Plan Unavoidable delay Avoidable delay Rest to overcome fatigue Look, listen, or feel

• For many jobs the steps follow a consistent pattern or "work method"
• Standard method: An agreed on method for performing a given task the same way under the same conditions.
  • Helps insure that the workers/users do not skip important steps
  • Helps insure that workers/users do not do things that they should not do
  • Facilitates training and retraining
  • Provides a "benchmark" for finding a "better method"
  • Helps workers identify abnormalities that require additional attention

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7. The Environment

The environment may affect individual's ability to gather information, process information and to act as necessary to maintain control of the system

1. Physical environmental factors can interfere with the gathering of necessary information and exerting necessary control actions. Air quality can interfere with mental and physical capacities.
• Lighting
• Visability
• Movement/vibration
• Heat/cold
• Oders
• Air quality



2. Pyscho/social factors can interfere with the the worker/users ability to process information and make objective decisions.
• Co-workers
• Supervisors
• Customers
• Family
• Health
• Danger

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