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time of at most
(
tram detected
). In the subsequent 5 seconds the
lter should ignore any further stuttering of inputs
"
) output
T
no tr
or
tr
from the sensor
and stay with output
. We stipulate here that successive trams are at least 6
seconds apart and that after 5 seconds any stuttering of the sensor has ceased
so that the input is
T
no tr
and the lter (after a reaction time of at most
"
)can
N
tr
return to output
will be treated as signalling
that a new tram approaches, thus causing again output
. Afterwards any further input
T
.
in
tr
no_tr
e
e
e
e
5s
5s
out
T
N
Fig. 4.
Timing diagrams for the lter
There is one input though which the lter
FES
must not ignore: the input
Error
indicating an erroneous sensor value. Then the lter should proceed as fast as
possible (i.e. after a reaction time of at most
.
These informal requirements can be formalised using Constraint Diagrams.
In the sequel we show CDs for the most important aspects of the desired lter
behaviour.
"
) to (state and) output
X
Initial state.
The commitment is that the initial output is
N
.
N
out
Filtering inputs for 5 seconds.
Assuming that input
tr
is present for
"
seconds
while output is
N
, the lter is committed to change its output to
T
and keep this
output for the next 5 seconds provided no
Error
(i.e. only
no tr
or
tr
) occurs
as input. Note that if the input
tr
is present for less than
"
seconds, nothing is
required from the lter. The assumption of
being present for some duration
of time anticipates that hardware cannot react arbitrarily fast.
tr
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