Simulation Optimization of airflow level posture sensor

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Simulation Optimization of airflow level posture sensor
Duanlei1, a *,Piao linhua2,b ,Dong haoliang3,c
(1Sensor Technique Research Center, Beijing Information Science & Technology University,
Beijing 100101,China;2 Beijing Key Laboratory for sensor, Beijing Information Science &
Technology University, Beijing 100101;3Sensor Technique Research Center, Beijing Information
Science&Technology University, Beijing 100101,China;a duanlei6688@yeah.net; b bjplh@163.com;
)
Keywords: airflow level posture sensor; tilt; temperature of hot source
Abstract. In this paper, using the optimization method of replacing the temperature with power to
calculate the temperature distribution of the airflow level posture sensor in closed cavity. The
temperature distribution of sensor in closed cavity is calculated at different inclination sensor status
and under different environmental temperatures. Calculation results and experimental verification
show that the two thermal resistance difference of airflow level posture sensor has good linear
relationship with the inclination, and the change rate is 2.6183 K/0. It has a good similarity with the
experimental, and provides a theoretical basis for designing airflow level posture sensor.
Airflow level posture sensor is a new type of tilt sensor which is made by the character of
the natural convection gas in the enclosed cavity. In the finite element analysis of airflow level
posture sensor sensitive mechanism, the previous method use the constant temperature which is set
a fixed temperature in the heating resistance device, for making the finite element calculation
simple[1,2] and theoretically explaining the working principle of the sensor. In practice, the circuit of
constant temperature is more complex, difficult to implement, but the method of load a certain
power has been applied widely in practice because of its simple circuit, low cost, easy to implement.
This method is more actual, the error of finite element calculation is smaller, so it has more
theoretical guidance significance. So this paper adopts the method of loading a certain power to
optimize the finite element calculation of airflow level posture sensor, solve the temperature field
and flow field through the FLUENT software, so as to explain the sensitive mechanism of airflow
level posture sensor better, provides theory basis for developing airflow level posture sensor.
The working principle of airflow level posture sensor
Fig.1 sensitive element simplified diagram
The sealed cavity of airflow level posture sensor is axisymmetric, the diameter of the thermal
resistor is far less than the radius of the cylinder. As shown n Fig.1As shown n Fig.1, the two
3rd International Conference on Mechatronics and Industrial Informatics (ICMII 2015)
© 2015. The authors - Published by Atlantis Press
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dimension structure’s heat source is simplified to a circle, acceleration of gravity loads on the Y
axis, sealed cavity radius r is set to 10 mm, four walls keep radiating. The resistance value of heat
resistance wire is the same, two wire structure thermal resistor R1, R2 are symmetrical about the
origin, the spacing of d is 6~10 mm. The heat q of three wire structure is in the origin of the
coordinate axes. By symmetry, the two thermal wires are at the same temperature, the bridge
output is zero at this time. When the sensor tilts, because of the effect of natural convection, the
two thermal wires are in different temperature contours, and thus lead to two different thermal
wire resistance changes, finally the circuit output a certain voltage, through the output voltage
we can get the tilt angle.
As shown in Fig.2, the detection circuit airflow level posture sensor is a whole bridge of
Wheatstone bridge circuit, the two arm of detect bridge consists of two metal thermal resistor
R1 and R2. The working principle is that when the sensor tilts a certain angle relative horizontal,
the two thermistor position change relative in temperature field, make the two thermistor
temperature one drop and another rise, the two metal thermistor resistance change, caused
current change, a bridge becomes out of balance, the output U is corresponding to the angle θ.
V
B
V
o
R1 R0
R2
R0
v
1
v
2
Fig.2 Measure bridge circuit principle diagram
At this point, as shown in Fig.2 the bridge output can be represented as:
0
R/2
B
VV=∆ （1）
10
RR1R2R()
ab
TT
α∆=−=− （2）
Among them, the ΔR is resistance difference caused by the temperature difference on both
thermistor. Put the (2) into (1), the relationship between the output voltage and the temperature
difference can be obtained:
011
(0)()/2(0)T/2
BabB
VVRTTVRαα=−=∆ （3）
After the temperature field of the gas in the sensor sealed cavity is fixed, the performance
of the sensor depends on the thermistor resistance differential output. Seen from (3), and two
thermistor sensor output voltage is proportional to the temperature difference of values, and
setting a fixed temperature on the heat source is very big difference with setting a fixed power.
FLUENT Solving
Read the .mesh file exported by the GAMBIT, before the start of the FLUENT software
calculating, need to inspection the grid, and set up a series of parameters. Inspection on the grid,
click the check, and check whether the minimum is negative, whether occurs any instructions of
warnings or errors, this means that the mesh quality is not good enough, because grid’s good and
bad has great influence on computing hands no convergence, time, and the result. Then size
change. Solver Settings: Solver select Pressure-based, Gravity is 9.81m/s2 set in the Y direction.
Space is two dimension. Model choice: open the energy equation and laminar flow. Material
Settings: set the Fluid basin is air, air properties is boussinesq, and thermal expansion coefficient is
0.003125. The Fluid work temperature is 300K. Boundary condition setting: the external boundary
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type is wall, conditions is convection type, and temperature is the environment temperature of 300K.
The internal boundary type is couple, the other keeps default. Watershed conditions set: FLUID
zone keeps the default, open the SOLID setting options, select the Source Term, and set the
corresponding numerical values. Solving control: Scheme of the Solution Methods set as SIMPLE,
the Pressure in the Spatial Discretization set as PRESTO, other keep the default. Convergence
Settings: just open the Energy and keep the default. Convergence count set as 100. At last start
calculating, and display the temperature contours graphically, then read and save the temperature of
the corresponding position.
Calculating results and discussion
As shown in Fig.3 is nephogram of temperature in airflow level posture sensor sensitive
element under horizontal. Due to the heat source at the center of the origin, two thermal wire is
symmetrical about the origin, no matter how much is the angle, the temperature field distribution is
vertical upward, cloud is always the same, what change is the location of the thermal wire, the
position of the thermal wire is point a and point b.
Fig.3 Nephogram of temperature in airflow level posture sensor sensitive element under
horizontal
Fig.4 The relationship curve between the temperature difference and angle of the two thermal
wire
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Fig.5The relationship curve between the measured output voltage and angle
As shown in Fig.4 the environmental temperature of airflow level posture sensor is 300K, heat
source keeps a certain power, with the increase of angle, the temperature difference of two thermal
wire also increase. When angle increases, the airflow keeps vertical upward, the nephogram
unchanged relatively under horizontal, but the location of the thermal wire changed, one moves to
the direction of the higher isothermal temperature value, another moves to the direction of the
lower temperature isotherm value. Within the scope of a certain Angle, the greater the angle is, the
bigger the temperature difference is, and the temperature difference has good linear relationship
with angle, using matlab to calculate the linear relationship between them it is concluded that the
slope is 2.6183 K/0.
According to the formula 01
(0)T/2
B
VVRα=∆, Fig.5 is the use of such sensor experimental
measurement results, is similar with the simulation results, and have a good linear relationship.
Conclusions
By using FLUENT software, this paper calculates the temperature distribution in the sealed
cavity, respectively calculated the temperature field distribution in the condition of different angle
and different environmental temperature, the calculation results and experimental verification show
that airflow level posture sensor’s two thermal resistance difference has a good linear relationship
with angle, the change of the slope is 2.6183K/0.
Acknowledgement
Beijing Natural Science Fund Project&Beijing City Board of Education Science and technology key
project（KZ201511232034); Key Laboratory of Beijing open projects funded project; Modern
Control Technology funded by Ministry of Education Key Laboratory;
Reference
[1] Sm Shen, The method of the finite element analysis on the problem of heat
conduction-convention, calculating math, No.2, 1994: 170-182
[2] Y Lin, Lh Piao, Fx Zhang. The structure principle of omnibearing gas pendulum inclination
sensor [J]. Electronic components and materials. May 2006, 25(5):19–22
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