OSM for a fob watch

This description uses OSM version 2 (for full paper, see Blandford and Green 1998)

date: Feb 98

a) The shared description

entities:

entity	type	add by	delete by	attribute	set by	change by	notes
GMT/'real' time	user private	-	-	current time	-	-	externally defined
time shown	shared/domain	-	-	minutes angle	-	turn knob (when out)
time shown	shared/domain	-	-	hours angle	-	change minutes angle
energy	device private	-	-	amount	-	decreases as watch runs; increase by winding knob

relationships:

domain constraint:	time shown should be same as GMT
affects:	changing the minutes angle affects hours angle
device constraint:	watch can only work if it has enough energy; only a finite amount of energy can be stored

b) Adding domain knowledge

How users use the device: Fob watches are mainly used for telling the time of day, of course. They are also used to work out elapsed time (how long has this process been going on?), time-remaining (how much longer will it go on?), time-of-completion (what will the time be when this process has continued for X minutes?), and current time in other time zones.

Domain concepts and relationships: From the above, three domain concepts need to be introduced, time-elapsed, time-remaining, and time-of-completion.

Context of usage, and related tools or devices: Because the fob watch has no alarm system, it is frequently used in conjunction with separate devices for countdown timing (e.g. egg-timers) and for reminders at set times (e.g. alarm clocks). Transferring data from one of these to another may require translation between digital and analogue time representations.

c) Expanding the device description

conceptual action	device actions	device object	persistence	disguised?	notes
turn knob (when out)	pull knob out, turn it, press it back in	knob	high	no	moded: knob must be out to change time
wind knob	turn knob	knob	high	no	moded
find elapsed time	read current time and subtract a previous time (possibly recorded on a sub-device such as paper; possibly from memory)	hands (and possibly paper)	high	no	subtraction of times is a reasonably hard mental operation

d) Misfits etc

The device is moded - position of knob.
Position of hour hand cannot be changed independent of minute hand.
User might not be able to see when it has run out of energy.
Otherwise it's pretty usable for the main task, telling the time: no abstractions, viscosity, hidden dependencies, nor premature commitment.
The related tasks (time elapsed, etc) are cognitively difficult, requiring arithmetic on times and durations.

Illustration

diagram of fob watch

the long arrow is the minute hand

the short arrow is the hour hand

the black bit on top is the knob

turn the knob to wind the watch: winder mode

pull the knob out to turn the hands

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last updated: Feb 99