Finally, we discuss two more fine-grained power management techniques
without analyzing their e?®ects on the lifetime of ExScal.
??? Duty Cycling the Wakeup Sensor: In some sensors such as the acoustic,
it is possible to reduce the energy consumption of this sensor by letting
the sensor sleep in between its samplings. In ExScal, an acoustic sensor
collected samples at the rate of 4 kHz for 30 ms, after every 300 ms. Since
the startup time of the acoustic sensor is less than 1 ms, it can be put to
sleep in between its samplings to save energy. After it collects one set of
samples, it can be put to sleep for the next 269 ms, at the end of which
it will wake up, collect another set of samples for 30 ms and go back to
sleep. If its sampling frequency is reduced (so that it sleeps for more than
269 ms in every cycle), in order to conserve even more energy, its sensing
range may get reduced. It may still be possible to meet the monitoring
requirements of the application with this reduced sensing range. A careful
analysis needs to be performed before reducing the sensing range of a sensor
in order to ensure that the application requirements with respect to
coverage can still be met with the reduced sensing range. If the acoustic
sensor was used as a wakeup sensor, significant energy savings could have
been achieved with this duty cycling. Unfortunately, duty cycling could not
be used to reduce the energy consumption of the PIR sensor (which was
the wakeup sensor in ExScal that remained continuously active) because
of its high startup time (more than 1 second).
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