Analog Devices

In many applications, the status of the most critical parameters are handled by analog devices. Even systems like computerized flightdecks (a.k.a. the "glass cockpit") usually emulate analog devices in the images that they display [Mac03, Nas00]. Pure digital displays (i.e. ones that show numbers and letters only) tend to be difficult to use.

The model of an analog device fits well with the physiology of human perception. The indicator needles of such devices display a "gesture" effect: their position can quickly tell the operator the state of the system. Most importantly, analog devices are exactly that — an analog model of the underlying variables they monitor.

The analog device gives the operator many opportunities to notice if something is going wrong with the system. If the dial face is marked with familiar green (everything is fine), yellow (caution) and red (critical) regions, it is possible to see when the needle is approaching the yellow range, or quickly gauge how far into the red the needle has moved, etc. Also, the pattern of motion of the indicator needles conveys a great deal of information.

As the operator becomes familiar with a particular system, the analog instruments provide a great opportunity for the use of human capacity for pattern recognition. The experienced operator knows the pattern that the indicator needles make during normal operation. A deviation from that pattern is likely to cause the operator to respond even if no internal watchdog detects the fault.

An analog instrumentation system is also much more resilient to misconfiguration errors. A speedometer that is labeled in the wrong units, or one whose reading deviates from the correct value, can still be used provided that the nature of the error is known. In contrast, a misconfigured annunciator is unlikely to be useful at all, because there is no mental correction (except state reversal) that the operator can apply to such a display in order to learn the condition of the monitored system.

Of course, analog instruments have the disadvantage of being much less compact than annunciators, and their transition from the normal to the error range is less visibly indicated than an annunciator's transition. This is why annunciators are used to supplement such systems. A brief look a the dashboard of a car, the cockpit of an airplane, or even the common kitchen range will tell, however, that the most critical variables are displayed via analog devices, with annunciators playing a supporting role. Only in the user interfaces of general purpose computers do these ideas remain underused.