Cornell University Ergonomics Web

DEA 3250/6510 CLASS NOTES

Control Design

Controls

1. Human Control of Systems - There is an enormous variety of control devices for human-machine systems. Examples range from light switches to complex power plants.

2. Function of Controls - The primary function of controls is to transmit information from the operator to the system. Feedback on the consequences of an action is crucial to the maintenance of optimal Human-Machine-System performance.

3. Two Broad Classes of Information the User needs to Provide to the System 
a. Discrete- simple and fixed magnitudes of information provided to the system
Examples: on/off switch, high/medium/low toggles, etc.

b. Continuous - varying level of information provided to the system via the operator including speed, pressure, flow rate, etc. Examples: steering wheels, levers, pedals, etc.

4. Types of Controls - Controls can be classified by: information type (as above), action type (e.g. rotate, push/pull), or by actuating force needed (large or small). New technology also allows controls to act as displays, e.g. touch screens. Multitudinous cursor positioning controls like mice have resulted from the rise in computer usage. Feedback on control impact may be received either from change in display or change in system response e.g. vehicle moves when gas pedal is depressed.

5. Basic Control Design Considerations

a. Identification - Control design can increase speed and accuracy of performance. Identification of controls is a coding problem that depends on various factors such as:

(1) Shape coding - involves tactile sensitivity and impacts on grip strength. The US Air Force has developed 15 knob designs that are not often confused which are grouped into multiple rotation, fractional rotation, and detent positioning. Shape can also be used to strengthen symbolic associations between control and function.
(a) Multiple rotation - Continuous control requiring twirling or spinning. Adjustment range is greater than one turn. Position isn't critical.
(b) Fractional rotation - Continuous control where adjustment range is less than one turn. Position isn't critical.
(c) Detent positioning - Discrete control. Position is critical.

(2) Texture coding - Surface texture of the control can also ease identification. This also allows smaller controls to be used by gloved hands.

(3) Size coding - Size coding aids visual discrimination but isn't as good for tactile discrimination.

(4) Location coding - Controls work better when they are located vertically rather than horizontally.

(5) Operational coding - Each control is activated by a unique movement.

(6) Color coding - A representational display which is useful for visual identification of the control works best when combined with the other types of coding because individuals may have problems with color vision or there may be conditions of low light.

(7) Label coding - Minimum coding requirement of any control, but labels take time to read and if there are many controls discrimination is slow.

b. Control-response ratio (C/R ratio) - amount of movement of control that creates a system response. Control movement may be linear distance, angular rotation, or number or revolutions. For example: How much one has to rotate a car steering wheel to turn the car. C/R = movement of control/movement of system response

(1) Sensitive controls with low control-response ratios need a small control movements to produce large changes in the system. This is best for rapid adjustments.
(2) Less sensitive controls with high control-response ratios need large control movements to produce small changes in the system. This is best for accuracy.

Unfortunately, there are no formulas for calculating optimum control-response ratios though recommended ranges for knobs are 0.2 - 0.8 and for levers are 2.5 - 4.0. Ultimately optimum CR for any system will depend on the type of control, size of display, permitted tolerance (allowable over- or under-adjustment), and system-response lag. Optimum control-response ratios depend on a trade-off between adjustment time and travel time.

c. Control resistance - determines the "feel" of a control and provides most of the feedback for users. Manipulating controls involves both the amount of displacement of the control and the amount of force applied to the control.

(1) Terms -
(a) Deadspace - amount of control movement around the null position that doesn't activate the control. It is the neutral movement of a device being controlled. Deadspace is less important with less sensitive C/R relationships.
(b) Backlash - deadspace of a control at any control position, not just at null position.

(2) Types of resistance -
(a) Elastic resistance - resistance varies in displacement of control, i.e. spring-loaded controls.
(b) Static and coulomb (sliding) friction - Good for reducing accidental activation of control and helping to hold control in place.

(i) Static friction - high resistance to initial movement decreases rapidly.
(ii) Coulomb (sliding) friction - Resistance continues to movement.

(c) Viscous damping - amount of force required to move control to overcome viscous damping is related to velocity of control movement, i.e. the faster the control is moved the more damping must be overcome. This damping helps to resist quick movements and produces smooth control.
(d) Inertia - resistance to movement (or change in direction of movement) caused by mass (weight) of mechanism involved, varies in acceleration. Again, opposes quick movements and aids smooth movement. Especially used in cranks.

d. Control order of systems - control order is the hierarchy of control relationships between movements of the control and operations of the output (O/P - display or action).

(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.

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