SENG/INDH 5334: Human Factors Engineering Controls/Displays Relationship Presented By: Magdy Akladios, PhD, PE, CSP, CPE, CSHM Control/Display Applications Three Mile Island: Contributing factors were human error and inadequately designed controls and displays. Control-Response Ratio (C/R Ratio) It is the amount of movement of control that creates a system response. Control movement may be: Linear distance; Angular rotation; or Number or revolutions Presented by: Dr. Magdy Akladios 1
Example of CR Ratio: How much one has to rotate a car steering wheel to turn the car In this case, C/R = movement of control/movement of system response Control Sensitivity Sensitive controls (Low C/R ratio): Need a small control movements to produce large changes in the system. This is best for rapid adjustments. Less sensitive controls (High C/R ratio): Need large control movements to produce small changes in the system. This is best for accuracy Control-response ratio (C/R ratio) No formulas for calculating optimum C/R ratios Recommended ranges: For knobs: 0.2-0.8 For levers: 2.5-4.0 Presented by: Dr. Magdy Akladios 2
Optimum CR Optimum CR for any system will depend on: The type of control; Size of display; Permitted tolerance (allowable over- or underadjustment); System-response lag; Adjustment time; and Travel time trade-off Feedback Change in display: Example: Controls that are activated on a computer screen by mouse or keyboard give feedback on the screen. Change in system response: Example: vehicle moves when gas pedal is depressed, slows when brake is applied), selfilluminating controls indicate activation by "lighting up." Control Order Control order is the hierarchy of control relationships between movements of the control and operations of the output (O/P - display or action). There are 4 levels of control order Presented by: Dr. Magdy Akladios 3
4 Levels of Control Order 1. Zero-order (Position Control): movement of control controls the O/P directly, e.g. moving a flashlight from object to object or tracing or drawing something. 2. First-order (Rate Control): movement of control changes the rate (velocity) at which O/P is being changed, e.g. auto accelerator or computer mouse. 3. Second-order (Acceleration Control): movement of control controls the rate of change of O/P, e.g. auto steering wheel (when wheel is turned, front wheels turn and there is a change in the rate of acceleration towards the turning direction). 4. Higher-order Systems: depend on complexity of linkages, e.g. change in position control creates change in a rate control which leads to a change in acceleration. Control Position At times a control is its own display That is, the position of the control indicates its activation state. Examples of Control Position (being it s own display) A push-button that remains in the "pushedin/depressed" position A lever such as gear-shift or emergency brake that indicates state by its spatial position A light switch that indicates "on" by the upposition of the switch. Presented by: Dr. Magdy Akladios 4
Control Panel Control panels often have indicator lamps that are illuminated or not to show the state of the associated control and/or control mode. Computer screen If controls are activated by means of keyboard, mouse, or other input device to a computer screen, the state of the control is usually indicated by a corresponding change on the computer display. Control-Display Compatibility Control-display compatibility is important in human-to-system interface design. It can be defined as the degree to which the function and use of a control and its associated display are unambiguous and immediately apparent to the user. Highly compatible controls and displays result in decreased reaction time, fewer errors, decreased training time, and higher user satisfaction. Presented by: Dr. Magdy Akladios 5
Control-Display Compatibility... Cont. To achieve high compatibility, it is important to consider user expectations and experience with similar controls and displays. For example, most users associate turning a rotary knob clockwise with increasing a value and expect the associated display to exhibit this increase. Also, the purpose of the information provided by the display and its associated control must be considered. For instance, the precision needed from a display and the precision that can be achieved with its associated control should match. Control-Display Compatibility... Cont. Analog displays may be preferable in some conditions where at-a-glance system status is needed and associated controls do not require extreme precision. By contrast, setting course information into a flight management system or setting the trajectory for a satellite requires digital displays and controls to input precise numeric values. Transfer Effects Presented by: Dr. Magdy Akladios 6
Transfer Effects Care must be taken when designing controls and accompanying displays to avoid negative transfer effects. Negative transfer occurs when a user's prior experience and/or training conflicts with a new or current control or display design Transfer Effects... Cont. Early devices for flight training were simple devices that would bear little resemblance to today's simulators. One such early device was little more than a box with rudimentary rigging. Unfortunately, it was configured so that pushing the stick forward raised the nose of the "aircraft" and pulling the stick back lowered it. This was the reverse of the actual aircraft that trainees would fly. Pilots adjusted to the change initially, but in periods of crisis, some reverted to their initial training and crashed. Transfer Effects... Cont. Human factors psychologists determined the cause of these crashes to be negative Transfer Effects between the trainer controls and the controls in aircraft. Fortunately, Edwin Link developed the Link Trainer for pilot training. In 1934, the U.S. Army Corps purchased six for IFR (instrument flying) training. During WWII, over 10,000 were used to train over 500,000 pilots. Presented by: Dr. Magdy Akladios 7
Control Automation Control Automation Advantages Enabled the development and implementation of many systems that would be impossible to operate with manual controls alone. Tremendous gains in productivity and precision with the use of systems employing sensors and automated controls Control Automation Disadvantages With some highly complex systems, the humanto-system interfaces have been more "automation-centered" than "human-centered." The functions chosen for automation were those easiest to automate instead of those most appropriate to automate. Some industries such as the nuclear power industry are now faced with the need to upgrade components which will in turn require changes to human-to-system interfaces Presented by: Dr. Magdy Akladios 8
Applications of Control Automation Computer controlled manufacturing equipment Computer controlled industrial processes Modern jet aircraft Air traffic control systems Nuclear power plants Control Automation Charles E. Billings and other aviation researchers have presented the concept that cockpit automation systems should be designed to function as a member of the crew Hence, the "electronic crew member" and the human crew should monitor and maintain awareness of each other's status and actions The automation should help, not hinder, situational awareness and assist in maintaining crew performance at optimum levels Billings has also developed "first principles" for the design of human-centered, automated systems. Billings First Principles for Human- Centered Systems 1. Humans are responsible for outcomes in humanmachine systems 2. Humans must be in command of human-machine systems 3. Humans must be actively involved in the processes undertaken by these systems 4. Humans must be adequately informed of humanmachine processes 5. Humans must be able to monitor the machine components of the system 6. The activities of the machines must therefore be predictable 7. The machines must also be able to monitor the performance of the humans 8. Each intelligent agent in a human-machine system must have knowledge of the intent of the other agents Presented by: Dr. Magdy Akladios 9
Control Automation & Human Factors With respect to how most automated systems are currently designed, Sarter and Woods astutely observed, "What is needed is a better understanding of how the machine operates, not just how to operate the machine." Indeed, this understanding is essential to forming a correct mental model of the system. This also holds true for human factors in system design. The Mental Model Mental Model The user's mental model is his/her concept and mental representation of how the system operates. With complex automated systems, the user's Mental Model directly impacts operating decisions and emergency response. A correct understanding of the system is needed in response to infrequent or unusual system events. Presented by: Dr. Magdy Akladios 10
Mode Errors In safety critical systems, lack of an accurate and complete mental model can lead to disastrous results (Mode Errors). These occur when the user is either unsure of the mode that the system is in or does not understand how the mode effects the system Mode Errors Examples The meltdown at Chernobyl was attributed to a lack of understanding of how the reactor operated on the part of the operators, technicians, and specifically the engineer who developed the test that required by-passing safety controls. Mode Errors Examples Mode error was attributed as the cause of the China Air Lines (CAL), Airbus A300 crash in 1994. The pilot flying, new to the Airbus A300, activated the "touch and go" mode, perhaps inadvertently. In this mode, the aircraft was programmed to pull up during landing, precedent to a "go around" aborted landing. The Captain (non-flying pilot) noticed the autopilot was engaged and repeatedly instructed the pilot to disengage it, but to no avail. The pilot continued trying to force the aircraft nose down. The autopilot performed as it was supposed to in this mode and forced the nose up. Finally the pilot flying stopped pushing the yoke forward, and the autopilot disengaged. The Captain took control, but it was too late, the nose angle was too high, the aircraft stalled, fell 1,000 feet, striking the ground tail first. Presented by: Dr. Magdy Akladios 11
Mode Errors/Interface Other mode errors are due to poor interface design. To reduce the "clutter" on display and control panels and economize on space requirements, designers may resort to using the same displays and controls for several "modes" of operation. Typically, a push-button or switch is used to change modes. Mode Errors/Interface Example An Air Inter, Airbus A320, crash on approach to the Strasbourg Airport in France was attributed to a mode error. The flight crew intended to set a 3.3 degree flight path angle of descent. Instead, they commanded a 3,300 foot per minute descent. Both the mode for descent in degrees and the mode for descent in vertical speed were set using the same knob. Mode selection was accomplished by a push-button. The same display was also used for both modes. The only difference was that degrees were displayed as "3.3" and feet per minute as "33". Subsequently, the display was changed so that degrees were displayed in decimal form (3.3) and vertical speed was displayed as a four-digit number (3,300). Benefits of Mental Model Accurate mental models produce positive outcomes Captain Robert Pearson and First Officer Maurice Quintal were at 41,000 ft in a Boeing 767 when they began to run out of fuel While approaching 28,000 ft, first one and then the other engine flamed out for lack of fuel With no computer system, displays, and minimal hydraulics, Captain Pearson drew upon his experience as a glider pilot He flew the 767 as a glider for thirteen minutes, successfully reached the former Gimli airfield, and safely landed the aircraft He accomplished this by combining his knowledge of the 767 with his mental model of piloting a glider, including a side slip maneuver to quickly decrease altitude--inconceivable in a 132-ton, unpowered 767 Presented by: Dr. Magdy Akladios 12