IEEE SENSORS JOURNAL, VOL. 10, NO. 4, APRIL 2010847
Self Heated Thermo-Resistive Element Hot Wire Anemometer
Richard Jozef Adamec and David Victor Thiel
Abstract—A microelectromechanical systems (MEMS) hot wire
anemometer consisting of thermoresistive elements arranged in a
differential bridge configuration is presented. The excitation of the
elements to the point of self heating allows for dedicated heating
elements to be omitted from the device without compromising op-
eration or accuracy.
Overall power consumption gives air velocity, and the temper-
ature differential of each element pair is used for wind direction
calculation and has demonstrated a sensing resolution better than
1% and a repeatability better than 2%.
Index Terms—Anemometer, hot-wire, microelectromechanical
bridge configuration is presented. Where heating elements are
often used on such devices , the sensor discussed here uses
self heating of the thermoresistive sensing elements to provide
the thermal energy required to generate the thermal plume and
temperature differential across the surface of the device.
The central heating elements used on typical integrated hot-
elements that must be interfaced and frees up silicon real estate
for other purposes, such as other sensing arrays .
The thermoresistive elements reported here are arranged to
serve dual purposes as both the thermal sensing elements and
the heating elements used to create the elevated surface tem-
perature. The arrangement of the elements allows for dedicated
heating elements to be omitted from the device without com-
promising operation or sacrificing accuracy. With the elimina-
count is reduced to only four elements for the dual axis device
compared to typical 5 or 8 element designs , . Power con-
sumption is also reduced along with an improvement in time re-
sponse compared to some, more conventional, designs , .
Thermal and electrical power requirements are minimized by
patterning the Nickel serpentine elements on a 1
from the surrounding Silicon substrate (Fig. 1).
bulk machined dual axis hot wire anemometer consisting
Manuscript received April 30, 2009; revised October 08, 2009; accepted Oc-
tober 08, 2009. Current version published March 10, 2010. The associate ed-
itor coordinating the review of this paper and approving it for publication was
Prof. Evgeny Katz.
The authors are with School of Engineering, Griffith University, Queensland,
4215 Australia (e-mail: email@example.com; firstname.lastname@example.org;
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/JSEN.2009.2035518
Fig. 1. Four element self heating sensor array. (a) FEM simulations showing
isotherms within the ?? ? membrane. (b) Photomicrograph of a fabricated
Velocity information is extracted from the overall electrical
power consumption of the four elements that are heated simul-
taneously from a common supply. Simultaneous solution of the
temperature differential of opposing element pairs was used to
determine the incident airflow direction and has demonstrated a
sensing resolution less than1% and a repeatable accuracybetter
than 2% for correctly dimensioned devices.
a reliable and robust design with exposure of the sensor surface
to the environment including exposure to rain, dust and debris
for periods in excess of 12 months with continuing operation.
II. DATA AND RESULTS
Power consumption for the device was 50 mW at 25 C for
supply to the heating elements achieved an above ambient tem-
perature of 45 C to give an absolute element temperature of
70 C. Temperature differentials seen as the device was rotated
in a wind tunnel reached a peak of 15 C between opposing
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Fig. 2. Excessive element separation distances lead to an approximate ????
and ?????? response as a reduction of sensitivity is introduced at angles per-
pendicular to the particular axis (actual recorded response).
and to distortion of the desired sine/cosine relationship of the
thermal differentials. Even with this inefficient geometry being
used, the relationship of the two differential signals may still
be approximated by
relationship means an analytical solution is possible via the si-
multaneous equation shown in (1)
functions (Fig. 2). This
relative amplitude of axis responses;
rotational offset of the array from 0 .
found (Fig. 3) independent of air speed simultaneously varying
the amplitude of both the responses.
A thin-film membrane hot wire anemometer with self heated
sensing elements was fabricated and proven. This device
omitted the commonly used centrally heated element  or
peripheral heating elements  and instead used self heating
Fig. 3. Calculated solution for the ????????????? characteristic response of
of the sensing elements. Device construction consisted of
thermoresistive serpentine Nickel tracks patterned on a 1
Silicon Nitride layer coating a 600
was reverse bulk etched to produce a membrane for thermal
isolation of the Nickel tracks.
The self heating of the sensing elements concentrated the
highest temperature regions at the sensing elements increasing
the effectiveness of the forced convection heat transfer. The
thermal distribution across the sensing area allowed successful
calculation of the incident airflow angle to typically within 2%
error for air flow velocities up to 20 m/s for a correctly dimen-
sioned device. Temperature differentials between opposing ele-
ments of 15 C were possible at low air flow velocities with a
total heating power of 50 mW in still air at an ambient temper-
ature of 25 C.
silicon substrate that
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