Instrumentation & Measurement Magazine 26-3 - 21

Energy Autonomous
Wake-Up Detectors
Marko Gazivoda and Vedran Bilas
W
ith the growing need to monitor, understand
and manage our surroundings, the demand for
wireless sensor networks has been constantly
increasing. To make practical application of wireless sensor
networks possible, their sensor nodes utilize specialized lowpower
always-on wake-up detectors to increase their energy
efficiency. Advances in energy conversion and harvesting
techniques and utilization of micro- and nano-electromechanical
systems allow for development of autonomous wake-up
detectors, powered by ambient or event energy.
The Wake-up Concept
To allow continuous monitoring and detection of events of interest
during long periods of time, the sensor networks and
nodes must achieve high energy efficiency [1],[2]. In order for a
sensor node to achieve the levels of energy efficiency required
for energy autonomy [3], autarky [4], or neutrality [5], it must
be kept in a low-power ( " sleep " ) mode for most of the time and
only activated to perform its tasks in limited time spans [3].
While this activation can be performed synchronously, utilizing
duty cycling, this can lead to missed events or unnecessary
activations leading to significant waste of power.
A more responsive and energy-efficient way of detecting
events is to utilize asynchronous,
event-driven
activation and the wakeup
concept [6]. A general
review of the wake-up concept
and detectors can be
found in [7].
With advances in ene
rgy convers i on and
harvesting technology and
techniques and utilization
of micro- and nano-electromechanical
systems
(MEMS and NEMS) a
novel subgroup of autonomous
wake-up detectors
May 2023
is being developed. These devices consist of passive or ultralow-power
(under 10 nW) detectors which also function as
energy harvesters.
Recent Developments and State-of-the-art
The state-of-the-art energy autonomous wake-up detectors
can be divided into subgroups, depending on their method of
operation. This paper presents different techniques and principles
used to develop energy autonomous wake-up detectors,
while a more in-depth look into each device's physical implementation
and technology can be found in the original papers.
Mechanical Power-gating
In [4] a binary wake-up detector design is proposed (Fig. 1) to
mechanically gate the power supply for the system's processing
unit, when a potential event or state of interest is detected.
The wake-up detector is passive and consists of a hydrogel
transducer whose volume is changed by ambient pressure,
humidity, temperature, pH value, or concentrations of chemicals
in its surroundings. A conductive plate is positioned next
to the hydrogel, and upon the hydrogel's volume change,
the plate bends into position so that it forms an electrically
conductive path between the power supply and processing
Fig. 1. The autonomous wake-up detector presented in [4]. Ambient pressure, humidity, temperature, pH value, or
concentration of chemicals change the hydrogel's volume, which causes the conductive plate to bend into open or closed
switch position and power-gate the processing circuitry. (a) System prior to change of relevant physical value (processing
circuitry inactive); (b) System after the change of relevant physical value (processing circuitry activated).
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
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