Article
Temperature Effects on the Wind Direction Measurement of 2D Solid Thermal Wind Sensors Bei Chen, Yan-Qing Zhu, Zhenxiang Yi, Ming Qin and Qing-An Huang * Received: 8 October 2015; Accepted: 24 November 2015; Published: 30 November 2015 Academic Editor: Stefano Mariani Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China; [email protected] (B.C.); [email protected] (Y.-Q.Z.); [email protected] (Z.Y.); [email protected] (M.Q.) * Correspondence: [email protected]; Tel.: +86-25-8379-2632 (ext. 8801)
Abstract: For a two-dimensional solid silicon thermal wind sensor with symmetrical structure, the wind speed and direction information can be derived from the output voltages in two orthogonal directions, i.e., the north-south and east-west. However, the output voltages in these two directions will vary linearly with the ambient temperature. Therefore, in this paper, a temperature model to study the temperature effect on the wind direction measurement has been developed. A theoretical analysis has been presented first, and then Finite Element Method (FEM) simulations have been performed. It is found that due to symmetrical structure of the thermal wind sensor, the temperature effects on the output signals in the north-south and east-west directions are highly similar. As a result, the wind direction measurement of the thermal wind sensor is approximately independent of the ambient temperature. The experimental results fit the theoretical analysis and simulation results very well. Keywords: thermal wind sensors; wind direction; temperature effect
1. Introduction A typical calorimetric two-dimensional silicon thermal wind sensor usually has a symmetrical structure. The sensor consists of a central temperature sensing element, several heaters and some other temperature sensing elements which are distributed symmetrically around the heaters [1,2]. The heaters warm up the chip above the ambient temperature. With the help of the central temperature sensing element and external control circuits, the temperature difference between the sensor chip and the airflow can be a constant (named the Constant Temperature Difference mode) [3–7]. When an airflow passes over the sensor chip, the temperature profile around the heaters will be asymmetric and the temperature sensing elements around the heaters can measure this distorted temperature distribution. Eventually the output signals in two orthogonal directions can be used to determine the wind speed and direction. According to whether thermal insulation is present or not, calorimetric flow sensors can be divided into two categories. One type is those with thermal insulation, which are usually fabricated on silicon dioxide membranes [6,8–12], thin crystalline silicon membranes [13–15], or on porous silicon layers [16,17]. The thermal insulation is so effective that the total power consumption of some thermal flow sensors can be decreased to the milliwatt or even sub-milliwatt range [11,15], which is attractive for some applications, such as biological and medical applications. In these situations, fluids cannot endure a large temperature increase. Though some sensors with thermal insulation could be integrated with the signal-conditioning circuits on a single chip, the fabrication steps to integrate the micromachined-based sensors within the Complementary Metal Oxide Semiconductor (CMOS) process flow must use compatible materials, thermal budget and highly-selective MEMS release techniques [18]. Besides, the suspended membranes are relatively Sensors 2015, 15, 29871–29881; doi:10.3390/s151229771
www.mdpi.com/journal/sensors
Sensors 2015, 15, 29871–29881
fragile and not compatible with further silicon processing. In addition, the fabrication of diaphragms easily induces imperfections, such as misalignment of the diaphragm and the structure on it and non-uniformity of the thin film thickness, which could result in an initial offset of the output signal in the absence of flow. Due to the fragility of the membranes, these sensors are usually packaged in tubes which limits their two-dimensional applications. The other type is the thermal flow sensors without thermal insulation. These sensors are mostly realized on thick silicon substrates by using a standard CMOS process [1,19,20]. The use of standard CMOS technologies allows the combination of the sensor and signal-conditioning electronics on a single chip. Moreover, due to their simple structures and high reliability, the packaging of the sensors is fairly straightforward. The sensors can be directly glued onto a thin ceramic disk [20]. In this way, the sensor can be protected from the contamination or destructive effects of the external environment, thus making these sensors reliable for harsh applications. Because thermal wind sensors often work outdoors, in this paper we focus on the second type, which can be called “solid thermal wind sensors”. In our previous works, the effect of the ambient humidity on solid thermal wind sensors has been investigated [21]. Besides, a semi-empirical model has been established to perform temperature compensation during the wind speed measurement of a solid wind sensor [22]. However, to our knowledge, the influence of the ambient temperature on the wind direction has never been explored. In this paper, a thermal model of the wind direction measurement is presented, which shows that the wind direction is a function of the output voltages in the north-south and east-west directions. Simulations and experiments are performed to study the temperature characteristics of the output signals in these two orthogonal directions. It is found that the thermophysical properties of the airflow have little impact on the output signals, whereas the thermal conductivities of the substrates play an important role. Finally, with the proposed thermal model, the ambient temperature effect on the wind direction measurement of a solid thermal wind sensor is presented. 2. Theoretical Analysis As shown in Figure 1, the two-dimensional thermal wind sensor consists of four heating resistors, four thermopiles and a transistor. The central diode (implemented as a diode-connected substrate Positive-Negative-Positive (PNP) transistor) measures the average temperature of the sensor. The four integrated resistors are symmetrically placed around the center point of the sensor and warm up the sensor to above the ambient temperature. The four thermopiles measure the temperature differences between the upstream and downstream sensor surfaces. The sensor works in the constant temperature difference mode (CTD mode). In our previous work [22], the temperature effect on the output signal of the sensor Vout has been presented:
Vout
1 “ pA ` B ¨ Tamb q ¨ U 2
(1)
It is shown that the output voltage increases linearly with the ambient temperature. Here parameters A and B are functions of the thermophysical properties of the airflow, the thermal conductivities of the silicon and ceramic substrate, and the Seebeck coefficients of the thermocouples. Tamb is the ambient temperature, and U is the wind speed.
29872
difference mode (CTD mode). In our previous work [22], the temperature effect on the output signal of the sensor Vout has been presented: 1
Vout = ( A + B ⋅ Tamb ) ⋅ U 2
(1)
It is shown that the output voltage increases linearly with the ambient temperature. Here parameters A and B are functions of the thermophysical properties of the airflow, the thermal conductivities of the Sensors 2015, 15, 29871–29881
silicon and ceramic substrate, and the Seebeck coefficients of the thermocouples. Tamb is the ambient temperature, and U is the wind speed.
Figure 1. The layout of the thermal wind sensor. Four heaters heat the whole chip. A Figure 1. The layout of the thermal wind sensor. Four heaters heat the whole chip. A central transistor central transistor measures the average temperature of the sensor chip, while four measures the average temperature oftemperature the sensordifference chip, while four measure the temperature thermopiles measure the between the thermopiles upstream and downstream difference between thesurfaces. upstream and downstream sensor surfaces. sensor The two-dimensional direction sensitivity is obtained by measuring the temperature difference in
The two-dimensional direction is obtained by measuring temperature difference the two orthogonal directions, sensitivity the north-south and east-west directions [1]. As shown the in Figure 2, the in the two orthogonal directions, the north-south and east-west directions [1]. As shown in Figure 2, the output signals in the north-south and east-west directions can be accurately represented by sine and cosine functions of the wind direction, ϕ: 1 2 Vns “ pAns ` Bns ¨ Tamb q ¨ U ¨ sin pφ ` ε ns q
(2)
4 1 2 ¨becos Vewnorth-south “ pAewand `east-west Bew ¨ Tdirections pφ ` εrepresented ew q amb q ¨ Ucan output signals in the accurately by sine and
(3)
Sensors 2015, 15
cosine functions of the wind direction, φ:
Here, Vns and Vew are the output voltages in the north-south direction and the east-west (2) V = ( A + B ⋅ T ) ⋅ U ⋅ sin (φ + ε ) direction, Ans , Bns , Aew and Bew are the temperature compensation parameters in the two directions, (3) that the sensor’s V = ( A The + B ⋅ Tconstant φ + ε ) shifts reflect the fact ) ⋅ U ⋅ cos (phase and εns and εew are the constant phase shifts. reference axis is not aligned theoutput reference of the measurement system. According to the fit voltages inaxis the north-south direction and the east-west direction, Here, Vns and Vwith ew are the ˝ and ˝ , εrespectively. and Bew are the temperature compensation the two directions, ns, B ns, Aew ns and εew curves in FigureA2, the constant phase shifts εns andparameters εew arein77.3 76.7and By using are the constant phase shifts. The constant phase shifts reflect the fact that the sensor’s reference axis is Equations (2) and (3), the wind direction can be expressed with V to the and Vew as follows: not aligned with the reference axis of the measurement system. Accordingns fit curves in Figure 2, ns
ns
ew
ew
ns
ew
1 2
amb
ns
1 2
amb
ew
the constant ˆ phase shifts εns and εew are 77.3° and 76.7°, respectively. By using Equations (2) and (3), ˙ pAewcan ` be Bew ¨ Tambwith q¨V ¨ cosε ´ pAns ` Bns ¨ Tamb q ¨ Vew ¨ sinε ns the wind expressed Vnsnsand Vew asew follows: ´1 direction
φ “ tan
pAew ` Bew ¨( ATamb pA ¨T q ¨ Vew ¨ cosε ns + B q⋅¨T Vns − (` ⋅ T `) ⋅B ⋅ sin cos ε ew A + B ns V ns ε amb ) ⋅ V ¨ ⋅sinε φ = tan −1
ew
ew
amb
ns
ew
ns
ns
amb
ew
ns
( Aew + Bew ⋅ Tamb ) ⋅ Vns ⋅ sin ε ew + ( Ans + Bns ⋅ Tamb ) ⋅ Vew ⋅ cos ε ns
(4)
Figure 2. The output voltages in the north-south direction east-westdirection. direction. The Figure 2. The output voltages in the north-south direction and and east-west The wind speed is ˝ wind speed is 7 m/s, and the ambient temperature is 15 °C. According to the fit curves, the 7 m/s, and the ambient temperature is 15 C. According to the fit curves, the constant phase shifts εns constant phase shifts εns and εew are 77.3° and 76.7°, respectively. and εew are 77.3˝ and 76.7˝ , respectively. 3. FEM Simulation of the Wind Sensor A 2D FEM model of the thermal wind sensor is built with COMSOL Multiphysics 4.4 software to 29873 investigate the temperature effects on the performance of the wind sensor. As shown in Figure 3, the sensor model is composed of four parts, including a flow channel, a ceramic substrate, a silicon chip and two heaters. Here, the thermopiles are omitted to shorten the simulation time. The thickness and length of the silicon chip are 0.5 and 4 mm, whereas the thickness and length of ceramic disk are
(4)
Sensors 2015, 15, 29871–29881
3. FEM Simulation of the Wind Sensor A 2D FEM model of the thermal wind sensor is built with COMSOL Multiphysics 4.4 software to investigate the temperature effects on the performance of the wind sensor. As shown in Figure 3, the sensor model is composed of four parts, including a flow channel, a ceramic substrate, a silicon chip and two heaters. Here, the thermopiles are omitted to shorten the simulation time. The thickness and length the silicon chip are 0.5 and 4 mm, whereas the thickness and length of ceramic disk Sensorsof2015, 15 5 are 0.25 and 21 mm. The distance between heaters is 0.6 mm, and the length of the heater is 0.15 mm. At thethefluid ambienttemperature temperature toK300 and the laminar rateAtisthe 2 m/s. fluidinlet, inlet, the the ambient is setistoset 300 andKthe laminar inlet flowinlet rate flow is 2 m/s. At theflow flow outlet, the outlet boundary condition is set to outflow and the flow boundary condition outlet, the outlet boundary condition is set to outflow and the flow boundary condition is set to is set zero to zero relative pressure. top andsurfaces bottom of the flow with slip relative pressure. The top The and bottom of surfaces the flow channel are setchannel with slipare andsetno-slip and no-slip wall conditions, respectively. Besides, the top surface of the flow channel is set to of 300 K. wall conditions, respectively. Besides, the top surface of the flow channel is set to 300 K. The parts The parts of thesurface bottom not with overlapped with thethe ceramic thesurface, ceramic surface, the bottom notsurface overlapped the ceramic disk, ceramicdisk, bottom andbottom the silicon and the silicon bottom surface are set as thermal insulation. The initial temperatures of the silicon bottom surface are set as thermal insulation. The initial temperatures of the silicon substrate and substrate and ceramic disk are set to 300 K. The temperature of the heaters is set to 310 K and ceramic disk are set to 300 K. The temperature of the heaters is set to 310 K and the temperature the temperature difference sensor the ambient temperature kept K. The upstream difference between between the sensortheand the and ambient temperature is kept 10isK. The10upstream and and downstream sensing points are placed symmetrically on the surface of the sensor chip, and the downstream sensing points are placed symmetrically on the surface of the sensor chip, and the distance distance of them is 3 mm. Figure 3 shows the simulation results of temperature distribution and of them is 3 mm. Figure 3 shows the simulation results of temperature distribution and flow velocity flow velocity distribution the model. distribution in thein model.
FigureFigure 3. The 3. simulation result of the thermal wind sensor. The ambient temperature is 300isK, and The simulation result of the thermal wind sensor. The ambient temperature the average temperature of the sensor is 310 K. The wind speed is 2 m/s. 300 K, and the average temperature of the sensor is 310 K. The wind speed is 2 m/s.
Then averagevelocities velocitiesvary vary from from 00 to to 25 25 m/s m/s ininsteps TheThe simulation results are are Then thethe average stepsofof5 5m/s. m/s. simulation results shown in Figure 4. At zero flow, the produced thermal profile is symmetrical with respect to the center shown in Figure 4. At zero flow, the produced thermal profile is symmetrical with respect to the AsAs thethe flow velocity increasing, the symmetrical temperature distribution is distorted. The centerpoint. point. flow velocity increasing, the symmetrical temperature distribution is distorted. temperature difference and downstream sensing points increases with with the the The temperature differencebetween betweenthe theupstream upstream and downstream sensing points increases flow speed. flow speed.
29874
Sensors 2015, 15, 29871–29881 Sensors 2015, 15
6
Figure 4. Simulation results of the temperature distribution on the surface of the chip. The wind speed Figure 4. Simulation results of the temperature distribution on the surface of the chip. The changes from 0 to 25 m/s in a step of 5 m/s. The ambient temperature is 300 K, and the temperature wind speed changes 0 to 25 m/s in a step ambient is 300axis, K, of the heaters is 310 K.from The upstream sensing pointofis5atm/s. the The position of 0.5temperature mm of the lateral and the the temperature ofsensing the heaters 310 The upstream pointaxis. is at the position whereas downstream point is is at theK. position of 3.5 mmsensing of the lateral
of 0.5 mm of the lateral axis, whereas the downstream sensing point is at the position of As reported, the thermal 3.5we mm of the lateral axis. conductivities of the silicon and ceramic substrates are temperature dependent, and the thermophysical properties of the airflow are also functions of the ambient As we reported, the thermal conductivities of the silicon are temperature temperature. The data of thermophysical properties of dryand airceramic and thesubstrates thermal conductivity of silicon and ceramic at different temperatures can be found in [23,24]. First of all, the effect of dependent, and the thermophysical properties of the airflow are also functions of the ambient the thermophysical of the airflow on the difference the sensorofsurface temperature. The dataproperties of thermophysical properties of temperature dry air and the thermal on conductivity silicon was studied. The material properties of silicon and ceramic are set according to the data sheet for and ceramic at different temperatures can be found in [23,24]. First of all, the effect of the 320 K [23]. At the same time, the thermophysical properties of dry air are set according to different thermophysical properties of the airflow on the temperature difference on the sensor surface was temperatures [24]. The inlet flow rate is 15 m/s, the temperature difference between the sensor and studied. properties of difference silicon andon ceramic are set according to the data for 320voltage K [23]. the flowThe is 20material K. The temperature the sensor surface is converted intosheet the output At the same time, the thermophysical properties dry air are set accordingconsists to different [24]. signal by the thermopiles and amplified 500 of times. Each thermopile of 10temperatures polysilicon/Al The inlet flow rate is an 15estimated m/s, the temperature difference between the5sensor flow is 20 K. The thermocouples with sensitivity of 2.095 mV/K. Figure showsand the the simulation results of the output voltages at different temperatures. The temperature difference decreases linearly by less temperature difference on the sensor surface is converted into the output voltage signal by the ˝ C. Next, the effect than 5% when temperature increases 80 thermopile conductivities thermocouples of silicon and thermopiles andthe amplified 500 times. Each consists of the 10 polysilicon/Al ceramic substrates are investigated. As shown in Figure 6, the output voltage has a linear with an estimated sensitivity of 2.095 mV/K. Figure 5 shows the simulation results ofrelationship the output with the ambient temperature. When the temperature increases 80 ˝ C, the temperature difference voltages at different temperatures. The temperature difference decreases linearly by less than 5% when increases 26.4%. It is obvious that the temperature effect of the thermal conductivity of the substrate the temperature increases 80 °C. Next, the effect of the conductivities of silicon and ceramic substrates plays dominant role, which is consistent with our previous research [22]. The output voltage of the are investigated. As shown in Figure 6, the output voltageinhas a linear relationship thermal wind sensor at different temperatures is shown Figure 7. This indicateswith that the the ambient output temperature. When the temperature increases 80 ambient °C, the temperature difference increases 26.4%. It is voltage is approximately a linear function of the temperature.
obvious that the temperature effect of the thermal conductivity of the substrate plays dominant role, which is consistent with our previous research [22]. The output voltage of the thermal wind sensor at different temperatures is shown in Figure 7. This indicates that the output voltage is approximately a linear function of the ambient temperature.
29875
Sensors 2015, 15, 29871–29881
Sensors Sensors 2015, 2015, 15 15
77
Figure 5.Figure Simulation results of the effect of the thermophysical properties of the airflow onon the output Figure 5. 5. Simulation Simulation results results of of the the effect effect of of the the thermophysical thermophysical properties properties of of the the airflow airflow on voltage atthe different temperatures. the output output voltage voltage at at different different temperatures. temperatures.
Figure 6.Figure Simulation results ofresults the effect of the silicon andsilicon ceramic on the output 6. of effect of and ceramic on Figure 6. Simulation Simulation results of the the effect of the the silicon andsubstrates ceramic substrates substrates on the thevoltage at Sensors 2015,voltage 15 at 8 different temperatures. output voltage curve a linear function ambient temperature. output different temperatures. The output voltage curve is aa linear function of output voltage atThe different temperatures. Theis output voltage curveof is the linear function of the the
ambient ambient temperature. temperature.
Figure 7.Figure Simulation results ofresults the output of the of wind The output of the 7. Simulation of the voltage output voltage the sensor. wind sensor. The output ofwind the sensor is a linearwind function of temperature. sensor is a linear function of temperature.
4. Experimental Results and Discussion
29876
A fabricated two-dimensional thermal wind sensor consists of four polysilicon heaters, four polysilicon/Al thermopiles, and a central transistor. The sensor chip has been fabricated using a commercial IC fabrication process in a CSMC foundry. Details of the thermal wind sensor can be
Sensors 2015, 15, 29871–29881
4. Experimental Results and Discussion A fabricated two-dimensional thermal wind sensor consists of four polysilicon heaters, four polysilicon/Al thermopiles, and a central transistor. The sensor chip has been fabricated using a commercial IC fabrication process in a CSMC foundry. Details of the thermal wind sensor can be found in [19]. The size of the sensor chip is 4 mm ˆ 4 mm. The sensor is heated by the four polysilicon resistors, each of which has a resistance of 400 Ω. The four polysilicon/Al thermopiles located symmetrically around the central point measure the temperature difference between the upstream and downstream points on the surface of the sensor chip. Each thermopile is composed of 10 p-doped polysilicon/Al thermocouples. Due to the high impurity concentration (1026 m´3 ) the Seebeck coefficient of the thermocouple shows a negligible change with the temperature [22]. Therefore, the sensitivity of the thermopile is nearly constant. The central transistor is connected as a diode which has a temperature coefficient of ´1.5 mV/K. The diode measures the average temperature of the sensor, and the chip temperature is been used in the outer control circuit to maintain a constant temperature difference between the sensor and the ambient temperature. The silicon chip is directly mounted on a ceramic substrate which provides a smooth surface. This structure also helps to shield the sensor from any dust in the airflow [1,19]. The wind sensor is operated in constant temperature difference (CTD) mode which maintains a constant temperature difference between the chip and the airflow. The scheme of the control circuit is shown in Figure 8, where the sensor temperature is measured with the on-chip temperature sensor, and the flow temperature with the off-chip temperature sensor on a similar unheated reference chip. The temperature difference between the sensor and the flow is set as ∆T. For the heating of the sensor, multiple Sensorsheating 2015, 15resistors have been used, indicated by Rh1 to Rh4 . By adding variable resistances9R5 and R6 in the heating circuit of each pair of heating resistors, the sensor heating can be balanced in order temperaturedifference differenceininthe the absence flow. ordertotoremove removethermal thermal offset, offset, i.e., i.e., the the presence presence of a temperature absence of of flow. The whole of the thewind windsensor sensor 400mW. mW. The wholepower powerconsumption consumption of is is 400
Figure 8. Scheme of the circuit used used to maintain a constant temperature difference between Figure 8. Scheme of control the control circuit to maintain a constant temperature difference the between sensor and the airflow. the sensor and the airflow.
Measurements havebeen been performed by installing the sensor in a wind-tunnel wind Measurements have performed by installing the sensor in a wind-tunnel with thewith windthe speed ˝ speed 40and m/sa and a full of range 360 . 9Figure shows the measurement results the wind up toup 40tom/s full range 360°.ofFigure shows 9the measurement results of the windofsensor at ˝ C. It is shown that the output voltages in sensor at different temperatures of 5, 15, 25 and 35 different temperatures of 5, 15, 25 and 35 °C. It is shown that the output voltages in the north-south thedirection north-south direction and east-west increase with the ambient temperature, and the and east-west direction increase direction with the ambient temperature, and the output voltage curves output curves in thealmost north-south direction coincide with theateast-west direction ones in thevoltage north-south direction coincide with thealmost east-west direction ones identical temperatures. at identical temperatures. When the wind speed is 15 m/s, the output signals in the north-south When the wind speed is 15 m/s, the output signals in the north-south and east-west directions are and east-west directions are measured and plotted in Figure 10. Comparing the simulation results measured and plotted in Figure 10. Comparing the simulation results in Figure 7 and the measured in Figure 7 and the measured output in Figure 10, the output voltage curves are seen to be linear output in Figure 10, the output voltage curves are seen to be linear functions of temperature. Some factors contribute to the difference between the simulation results and the measured output. 29877 For instance, some simplification and approximation have been assumed in the COMSOL simulation, and there are some deviations between the theoretical calculation of Seebeck coefficient and actual measurement.
Sensors 2015, 15, 29871–29881
functions of temperature. Some factors contribute to the difference between the simulation results and the measured output. For instance, some simplification and approximation have been assumed in the COMSOL simulation, and there are some deviations between the theoretical calculation of Seebeck coefficient and actual measurement. The output signals in the north-south direction and east-west direction are nearly linear functions of the ambient temperature. From a least-squares fit of the data, the parameters in the north-south and east-west directions can be obtained. Ans = 6.574 ˆ 10´2 V/(m/s)1/2 , Bns = 6.894 ˆ 10´4 V/(m/s)1/2 /˝ C, Aew = 6.537 ˆ 10´2 V/(m/s)1/2 , Bew = 6.842 ˆ 10´4 V/(m/s)1/2 /˝ C. Comparing the values of parameters Ans , Aew , Bns and Bew with the values of parameters in [13], the huge discrepancy primarily derives from the constant temperature difference between the sensor and the airflow. According to the semi-empirical model [13], the constant temperature difference Sensors 15 10 between2015, the sensor and the airflow is incorporated into these fitting parameters A and B. Besides, Sensors 2015, 15 10 some little deviations in the fabrication and packaging process also contribute to these discrepancies.
Figure9.9. The Figure The output output voltages voltages in in north-south north-south and andeast-west east-west directions directions at at different different ambient ambient Figure 9. The output voltages in north-south and east-west directions at different ambient ˝ ˝ temperaturesranging ranging from to 35 temperatures from 5 5 C °C to 35 C. °C. temperatures ranging from 5 °C to 35 °C.
Figure 10. The output voltages in north-west and east-west directions have the ambient temperature Figure The output voltages in north-west and east-west directions have the ambient when the 10. wind speed is 15 m/s.
Figure 10. The voltages and east-west directions have the ambient temperature whenoutput the wind speedinis north-west 15 m/s. temperature when the wind speed is 15 m/s. In Figure 11a, the temperature effect on the29878 output signals has not been compensated. The wind In Figure 11a, the temperature effect on the output signals has not been compensated. The wind direction measurement results are directly calculated by [20]: direction measurement results are directly calculated by [20]: −1 Vns ⋅ cos ( ε ew ) − Vew ⋅ sin ( ε ns )
Sensors 2015, 15, 29871–29881
In Figure 11a, the temperature effect on the output signals has not been compensated. The wind direction measurement results are directly calculated by [20]: ˆ φ=tan
´1
Vns ¨ cos pε ew q ´ Vew ¨ sin pε ns q Vns ¨ sin pε ew q ` Vew ¨ cos pε ns q
˙ (5)
Sensors 2015, 15 11 On the basis of Equation (4), the temperature effect on the output signals has been compensated in Figure 11b. It can be seen that the measurement results of the wind direction have similar
accuracies temperature compensation. wind sensor sensor has has aa accuracies before before and and after after temperature compensation. This This isis because because that that the the wind symmetricalstructure. structure. symmetrical
Figure thethe temperature dependence of the Figure11. 11. (a) (a) The Thewind windspeed speeddirection directionbefore beforeconsidering considering temperature dependence of output signals; (b) The wind speed direction after considering the temperature dependence of the the output signals; (b) The wind speed direction after considering the temperature output signals.
dependence of the output signals.
When the ambient temperature varies, the ambient temperature effects on the output voltages When the ambient temperature varies, the ambient temperature effects on the output voltages in in these two directions are similar. Furthermore, the parameters Aew , Bew , Ans and Bns only depend these two directions are similar. Furthermore, the parameters Aew, Bew, Ans and Bns only depend on the on the thermophysical properties of the airflow, the thermal conductivities of the silicon and ceramic thermophysical properties of the airflow, the thermal and conductivities of thedifference silicon and ceramic substrates, the Seebeck coefficients of the thermocouples the temperature between the substrates, the Seebeck coefficients of the thermocouples and the temperature difference between the sensor and the airflow. In the same sensor, these physical parameters and the temperature difference sensor andthe thesensor airflow. theairflow same sensor, these andand the north-south temperaturedirections. difference between andInthe are almost thephysical same inparameters the east-west This canthe be proved by the of the Ans , Bin Aeweast-west and Bew for output signal of the between sensor and thevalue airflow are parameters almost the same andthe north-south directions. ns , the wind sensor when the wind speed 15 m/s in Figure 10. This can be proved by the value of the parameters Ans, Bns, Aew and Bew for the output signal of the
wind sensor when the wind speed 15 m/s in Figure 10. 5. Conclusions Solid thermal wind sensors play an important role in the agriculture, industry and meteorology 5. Conclusions fields. However, the performance of thermal wind sensors is affected by the ambient temperature. InSolid previous works, thesensors effect ofplay the ambient temperature wind speed measurement has been thermal wind an important role in on thethe agriculture, industry and meteorology studied and a new temperature compensation method has been presented. However, how the fields. However, the performance of thermal wind sensors is affected by the ambient temperature. ambient temperature affects the wind direction measurement has not been investigated. In this In previous works, the effect of the ambient temperature on the wind speed measurement haspaper, been a temperature model for the wind direction measurement is proposed. It is found that the output
studied and a new temperature compensation method has been presented. However, how the ambient temperature affects the wind direction measurement has not been investigated. In this paper, a temperature model for the wind direction measurement 29879 is proposed. It is found that the output signals in two orthogonal directions can determine the wind direction, and each output signal is a linear
Sensors 2015, 15, 29871–29881
signals in two orthogonal directions can determine the wind direction, and each output signal is a linear function of the temperature. For the wind sensor with a symmetrical structure, the temperature drift characteristics in the two orthogonal directions are quite similar, therefore, the wind direction of the wind sensor is approximately impendent of the ambient temperature. Acknowledgments: The authors would like to acknowledge the support offered by the National High-Tech Development Program of China under Contact 2013AA041106 and the Innovation Project for Graduate Student of Jiangsu Province under Grant No. KYLX15_0099. Author Contributions: Bei Chen developed the model, Bei Chen, Yan-Qing Zhu, Zhenxiang Yi and Ming Qin jointly performed the experiments and the data analysis. Qing-An Huang proposed the idea for the model and edited this manuscript. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18.
Van Oudheusden, B.W. Silicon thermal flow sensor with a two-dimensional direction sensitivity. Meas. Sci. Technol. 1990, 1, 565–575. [CrossRef] Zhu, Y.Q.; Chen, B.; Qin, M.; Huang, Q.A. 2-D micromachined thermal wind sensors—A review. IEEE Internet Things J. 2014, 1, 216–232. [CrossRef] Nam, T.; Kim, S.; Park, S. The temperature compensation of a thermal flow sensor by changing the slope and the ratio of resistances. Sens. Actuators A Phys. 2004, 114, 212–218. [CrossRef] Sosna, C.; Buchner, R.; Lang, W. A temperature compensation circuit for thermal flow sensors operated in constant-temperature-difference mode. IEEE Trans. Instrum. Meas. 2010, 59, 1715–1721. [CrossRef] Que, R.Y.; Zhu, R.; Wei, Q.Z.; Cao, Z. Temperature compensation for thermal anemometers using temperature sensors independent of flow sensors. Meas. Sci. Technol. 2011, 22, 085404. [CrossRef] Dijkstra, M.; Lammerink, T.S.J.; de Boer, M.J.; Berenschot, E.J.W.; Wiegerink, R.J.; Elwenspoek, M. Thermal flow-sensor drift reduction by thermopile voltage cancellation via power feedback control. J. Microelectromech. Syst. 2014, 23, 908–917. [CrossRef] Bruschi, P.; Dei, M.; Piotto, M. An offset compensation method with low residual drift for integrated thermal flow sensors. IEEE Sens. J. 2011, 11, 1162–1168. [CrossRef] Ashauer, M.; Glosch, H.; Hedrich, F.; Hey, N.; Sandmaier, H.; Lang, W. Thermal flow sensor for liquids and gases based on combinations of two principles. Sens. Actuators A Phys. 1999, 73, 7–13. [CrossRef] Glaninger, A.; Jachimowicz, A.; Kohl, F.; Chabicovsky, R.; Urban, G. Wide range semiconductor flow sensors. Sens. Actuators A Phys. 2000, 85, 139–146. [CrossRef] Sabate, N.; Santander, J.; Fonseca, L.; Gracia, I.; Cane, C. Multi-range silicon micromachined flow sensor. Sens. Actuators A Phys. 2004, 110, 282–288. [CrossRef] Dalola, S.; Cerimovic, S.; Kohl, F.; Beigelbeck, B.; Schalko, J. MEMS thermal flow sensor with smart electronic interface circuit. IEEE Sens. J. 2012, 12, 3318–3328. [CrossRef] Sturm, H.; Lang, W. Membrane-based thermal flow sensors on flexible substrates. Sens. Actuators A Phys. 2013, 195, 113–122. [CrossRef] Van Oudheusden, B.W. High-sensivity 2-D flow sensor with an etched thermal isolation structure. Sens. Actuators A Phys. 1990, 21–23, 425–430. [CrossRef] Kim, S.; Kim, S.; Kim, Y.; Park, S. A circular-type thermal flow direction sensor free from temperature compensation. Sens. Actuators A Phys. 2003, 108, 64–68. [CrossRef] Bruschi, P.; Dei, M.; Piotto, M. A low-power 2-D wind sensor based on integrated flow meters. IEEE Sens. J. 2009, 9, 1688–1696. [CrossRef] Kaltsas, G.; Katsikogiannis, P.; Asimakopoulos, P.; Nassiopoulou, A.G. A smart flow measurement system for flow evaluation with multiple signals in different operation modes. Meas. Sci. Technol. 2007, 18, 3617–3624. [CrossRef] Hourdakis, E.; Sarafis, P.; Nassiopoulou, A.G. Novel air flow meter for an automobile engine using a Si sensor with porous Si thermal isolation. Sensors 2012, 12, 14838–14850. [CrossRef] [PubMed] Andre, N.; Rue, B.; Scheen, G.; Flandre, D.; Francis, L.A.; Raskin, J.P. Out-of-plane MEMS-based mechanical airflow sensor co-integrated in SOI CMOS technology. Sens. Actuators A Phys. 2014, 206, 67–74. [CrossRef]
29880
Sensors 2015, 15, 29871–29881
19. 20. 21. 22. 23. 24.
Sun, J.B.; Qin, M.; Huang, Q.A. Flip-chip packaging for a two-dimensional thermal flow sensor using a copper pillar bump technology. IEEE Sens. J. 2007, 7, 990–995. [CrossRef] Makinwa, K.A.A.; Huijsing, J.H. A smart wind sensor using thermal sigma-delta modulation techniques. Sens. Actuators A Phys. 2002, 97–98, 15–20. [CrossRef] Chen, B.; Zhu, Y.Q.; Qin, M.; Huang, Q.A. Effects of ambi-ent humidity on a micromachined silicon thermal wind sensor. J. Microelectromech. Syst. 2014, 23, 253–255. [CrossRef] Huang, Q.A.; Chen, B.; Zhu, Y.Q.; Qin, M. Modeling of temperature effects on micromachined silicon thermal wind sensors. J. Microelectromech. Syst. 2015. in press. [CrossRef] Harper, C.A. Electronic Materials and Processes Handbook; McGraw-Hill Companies: New York, NY, USA, 2004; pp. 393–396. Tsilingiris, P.T. Thermophysical and transport properties of humid air at temperature range between 0 and 100 ˝ C. Energy Convers. Manag. 2007, 49, 1098–1110. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
29881
Temperature Effects on the Wind Direction Measurement of 2D Solid Thermal Wind Sensors Bei Chen, Yan-Qing Zhu, Zhenxiang Yi, Ming Qin and Qing-An Huang * Received: 8 October 2015; Accepted: 24 November 2015; Published: 30 November 2015 Academic Editor: Stefano Mariani Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China; [email protected] (B.C.); [email protected] (Y.-Q.Z.); [email protected] (Z.Y.); [email protected] (M.Q.) * Correspondence: [email protected]; Tel.: +86-25-8379-2632 (ext. 8801)
Abstract: For a two-dimensional solid silicon thermal wind sensor with symmetrical structure, the wind speed and direction information can be derived from the output voltages in two orthogonal directions, i.e., the north-south and east-west. However, the output voltages in these two directions will vary linearly with the ambient temperature. Therefore, in this paper, a temperature model to study the temperature effect on the wind direction measurement has been developed. A theoretical analysis has been presented first, and then Finite Element Method (FEM) simulations have been performed. It is found that due to symmetrical structure of the thermal wind sensor, the temperature effects on the output signals in the north-south and east-west directions are highly similar. As a result, the wind direction measurement of the thermal wind sensor is approximately independent of the ambient temperature. The experimental results fit the theoretical analysis and simulation results very well. Keywords: thermal wind sensors; wind direction; temperature effect
1. Introduction A typical calorimetric two-dimensional silicon thermal wind sensor usually has a symmetrical structure. The sensor consists of a central temperature sensing element, several heaters and some other temperature sensing elements which are distributed symmetrically around the heaters [1,2]. The heaters warm up the chip above the ambient temperature. With the help of the central temperature sensing element and external control circuits, the temperature difference between the sensor chip and the airflow can be a constant (named the Constant Temperature Difference mode) [3–7]. When an airflow passes over the sensor chip, the temperature profile around the heaters will be asymmetric and the temperature sensing elements around the heaters can measure this distorted temperature distribution. Eventually the output signals in two orthogonal directions can be used to determine the wind speed and direction. According to whether thermal insulation is present or not, calorimetric flow sensors can be divided into two categories. One type is those with thermal insulation, which are usually fabricated on silicon dioxide membranes [6,8–12], thin crystalline silicon membranes [13–15], or on porous silicon layers [16,17]. The thermal insulation is so effective that the total power consumption of some thermal flow sensors can be decreased to the milliwatt or even sub-milliwatt range [11,15], which is attractive for some applications, such as biological and medical applications. In these situations, fluids cannot endure a large temperature increase. Though some sensors with thermal insulation could be integrated with the signal-conditioning circuits on a single chip, the fabrication steps to integrate the micromachined-based sensors within the Complementary Metal Oxide Semiconductor (CMOS) process flow must use compatible materials, thermal budget and highly-selective MEMS release techniques [18]. Besides, the suspended membranes are relatively Sensors 2015, 15, 29871–29881; doi:10.3390/s151229771
www.mdpi.com/journal/sensors
Sensors 2015, 15, 29871–29881
fragile and not compatible with further silicon processing. In addition, the fabrication of diaphragms easily induces imperfections, such as misalignment of the diaphragm and the structure on it and non-uniformity of the thin film thickness, which could result in an initial offset of the output signal in the absence of flow. Due to the fragility of the membranes, these sensors are usually packaged in tubes which limits their two-dimensional applications. The other type is the thermal flow sensors without thermal insulation. These sensors are mostly realized on thick silicon substrates by using a standard CMOS process [1,19,20]. The use of standard CMOS technologies allows the combination of the sensor and signal-conditioning electronics on a single chip. Moreover, due to their simple structures and high reliability, the packaging of the sensors is fairly straightforward. The sensors can be directly glued onto a thin ceramic disk [20]. In this way, the sensor can be protected from the contamination or destructive effects of the external environment, thus making these sensors reliable for harsh applications. Because thermal wind sensors often work outdoors, in this paper we focus on the second type, which can be called “solid thermal wind sensors”. In our previous works, the effect of the ambient humidity on solid thermal wind sensors has been investigated [21]. Besides, a semi-empirical model has been established to perform temperature compensation during the wind speed measurement of a solid wind sensor [22]. However, to our knowledge, the influence of the ambient temperature on the wind direction has never been explored. In this paper, a thermal model of the wind direction measurement is presented, which shows that the wind direction is a function of the output voltages in the north-south and east-west directions. Simulations and experiments are performed to study the temperature characteristics of the output signals in these two orthogonal directions. It is found that the thermophysical properties of the airflow have little impact on the output signals, whereas the thermal conductivities of the substrates play an important role. Finally, with the proposed thermal model, the ambient temperature effect on the wind direction measurement of a solid thermal wind sensor is presented. 2. Theoretical Analysis As shown in Figure 1, the two-dimensional thermal wind sensor consists of four heating resistors, four thermopiles and a transistor. The central diode (implemented as a diode-connected substrate Positive-Negative-Positive (PNP) transistor) measures the average temperature of the sensor. The four integrated resistors are symmetrically placed around the center point of the sensor and warm up the sensor to above the ambient temperature. The four thermopiles measure the temperature differences between the upstream and downstream sensor surfaces. The sensor works in the constant temperature difference mode (CTD mode). In our previous work [22], the temperature effect on the output signal of the sensor Vout has been presented:
Vout
1 “ pA ` B ¨ Tamb q ¨ U 2
(1)
It is shown that the output voltage increases linearly with the ambient temperature. Here parameters A and B are functions of the thermophysical properties of the airflow, the thermal conductivities of the silicon and ceramic substrate, and the Seebeck coefficients of the thermocouples. Tamb is the ambient temperature, and U is the wind speed.
29872
difference mode (CTD mode). In our previous work [22], the temperature effect on the output signal of the sensor Vout has been presented: 1
Vout = ( A + B ⋅ Tamb ) ⋅ U 2
(1)
It is shown that the output voltage increases linearly with the ambient temperature. Here parameters A and B are functions of the thermophysical properties of the airflow, the thermal conductivities of the Sensors 2015, 15, 29871–29881
silicon and ceramic substrate, and the Seebeck coefficients of the thermocouples. Tamb is the ambient temperature, and U is the wind speed.
Figure 1. The layout of the thermal wind sensor. Four heaters heat the whole chip. A Figure 1. The layout of the thermal wind sensor. Four heaters heat the whole chip. A central transistor central transistor measures the average temperature of the sensor chip, while four measures the average temperature oftemperature the sensordifference chip, while four measure the temperature thermopiles measure the between the thermopiles upstream and downstream difference between thesurfaces. upstream and downstream sensor surfaces. sensor The two-dimensional direction sensitivity is obtained by measuring the temperature difference in
The two-dimensional direction is obtained by measuring temperature difference the two orthogonal directions, sensitivity the north-south and east-west directions [1]. As shown the in Figure 2, the in the two orthogonal directions, the north-south and east-west directions [1]. As shown in Figure 2, the output signals in the north-south and east-west directions can be accurately represented by sine and cosine functions of the wind direction, ϕ: 1 2 Vns “ pAns ` Bns ¨ Tamb q ¨ U ¨ sin pφ ` ε ns q
(2)
4 1 2 ¨becos Vewnorth-south “ pAewand `east-west Bew ¨ Tdirections pφ ` εrepresented ew q amb q ¨ Ucan output signals in the accurately by sine and
(3)
Sensors 2015, 15
cosine functions of the wind direction, φ:
Here, Vns and Vew are the output voltages in the north-south direction and the east-west (2) V = ( A + B ⋅ T ) ⋅ U ⋅ sin (φ + ε ) direction, Ans , Bns , Aew and Bew are the temperature compensation parameters in the two directions, (3) that the sensor’s V = ( A The + B ⋅ Tconstant φ + ε ) shifts reflect the fact ) ⋅ U ⋅ cos (phase and εns and εew are the constant phase shifts. reference axis is not aligned theoutput reference of the measurement system. According to the fit voltages inaxis the north-south direction and the east-west direction, Here, Vns and Vwith ew are the ˝ and ˝ , εrespectively. and Bew are the temperature compensation the two directions, ns, B ns, Aew ns and εew curves in FigureA2, the constant phase shifts εns andparameters εew arein77.3 76.7and By using are the constant phase shifts. The constant phase shifts reflect the fact that the sensor’s reference axis is Equations (2) and (3), the wind direction can be expressed with V to the and Vew as follows: not aligned with the reference axis of the measurement system. Accordingns fit curves in Figure 2, ns
ns
ew
ew
ns
ew
1 2
amb
ns
1 2
amb
ew
the constant ˆ phase shifts εns and εew are 77.3° and 76.7°, respectively. By using Equations (2) and (3), ˙ pAewcan ` be Bew ¨ Tambwith q¨V ¨ cosε ´ pAns ` Bns ¨ Tamb q ¨ Vew ¨ sinε ns the wind expressed Vnsnsand Vew asew follows: ´1 direction
φ “ tan
pAew ` Bew ¨( ATamb pA ¨T q ¨ Vew ¨ cosε ns + B q⋅¨T Vns − (` ⋅ T `) ⋅B ⋅ sin cos ε ew A + B ns V ns ε amb ) ⋅ V ¨ ⋅sinε φ = tan −1
ew
ew
amb
ns
ew
ns
ns
amb
ew
ns
( Aew + Bew ⋅ Tamb ) ⋅ Vns ⋅ sin ε ew + ( Ans + Bns ⋅ Tamb ) ⋅ Vew ⋅ cos ε ns
(4)
Figure 2. The output voltages in the north-south direction east-westdirection. direction. The Figure 2. The output voltages in the north-south direction and and east-west The wind speed is ˝ wind speed is 7 m/s, and the ambient temperature is 15 °C. According to the fit curves, the 7 m/s, and the ambient temperature is 15 C. According to the fit curves, the constant phase shifts εns constant phase shifts εns and εew are 77.3° and 76.7°, respectively. and εew are 77.3˝ and 76.7˝ , respectively. 3. FEM Simulation of the Wind Sensor A 2D FEM model of the thermal wind sensor is built with COMSOL Multiphysics 4.4 software to 29873 investigate the temperature effects on the performance of the wind sensor. As shown in Figure 3, the sensor model is composed of four parts, including a flow channel, a ceramic substrate, a silicon chip and two heaters. Here, the thermopiles are omitted to shorten the simulation time. The thickness and length of the silicon chip are 0.5 and 4 mm, whereas the thickness and length of ceramic disk are
(4)
Sensors 2015, 15, 29871–29881
3. FEM Simulation of the Wind Sensor A 2D FEM model of the thermal wind sensor is built with COMSOL Multiphysics 4.4 software to investigate the temperature effects on the performance of the wind sensor. As shown in Figure 3, the sensor model is composed of four parts, including a flow channel, a ceramic substrate, a silicon chip and two heaters. Here, the thermopiles are omitted to shorten the simulation time. The thickness and length the silicon chip are 0.5 and 4 mm, whereas the thickness and length of ceramic disk Sensorsof2015, 15 5 are 0.25 and 21 mm. The distance between heaters is 0.6 mm, and the length of the heater is 0.15 mm. At thethefluid ambienttemperature temperature toK300 and the laminar rateAtisthe 2 m/s. fluidinlet, inlet, the the ambient is setistoset 300 andKthe laminar inlet flowinlet rate flow is 2 m/s. At theflow flow outlet, the outlet boundary condition is set to outflow and the flow boundary condition outlet, the outlet boundary condition is set to outflow and the flow boundary condition is set to is set zero to zero relative pressure. top andsurfaces bottom of the flow with slip relative pressure. The top The and bottom of surfaces the flow channel are setchannel with slipare andsetno-slip and no-slip wall conditions, respectively. Besides, the top surface of the flow channel is set to of 300 K. wall conditions, respectively. Besides, the top surface of the flow channel is set to 300 K. The parts The parts of thesurface bottom not with overlapped with thethe ceramic thesurface, ceramic surface, the bottom notsurface overlapped the ceramic disk, ceramicdisk, bottom andbottom the silicon and the silicon bottom surface are set as thermal insulation. The initial temperatures of the silicon bottom surface are set as thermal insulation. The initial temperatures of the silicon substrate and substrate and ceramic disk are set to 300 K. The temperature of the heaters is set to 310 K and ceramic disk are set to 300 K. The temperature of the heaters is set to 310 K and the temperature the temperature difference sensor the ambient temperature kept K. The upstream difference between between the sensortheand the and ambient temperature is kept 10isK. The10upstream and and downstream sensing points are placed symmetrically on the surface of the sensor chip, and the downstream sensing points are placed symmetrically on the surface of the sensor chip, and the distance distance of them is 3 mm. Figure 3 shows the simulation results of temperature distribution and of them is 3 mm. Figure 3 shows the simulation results of temperature distribution and flow velocity flow velocity distribution the model. distribution in thein model.
FigureFigure 3. The 3. simulation result of the thermal wind sensor. The ambient temperature is 300isK, and The simulation result of the thermal wind sensor. The ambient temperature the average temperature of the sensor is 310 K. The wind speed is 2 m/s. 300 K, and the average temperature of the sensor is 310 K. The wind speed is 2 m/s.
Then averagevelocities velocitiesvary vary from from 00 to to 25 25 m/s m/s ininsteps TheThe simulation results are are Then thethe average stepsofof5 5m/s. m/s. simulation results shown in Figure 4. At zero flow, the produced thermal profile is symmetrical with respect to the center shown in Figure 4. At zero flow, the produced thermal profile is symmetrical with respect to the AsAs thethe flow velocity increasing, the symmetrical temperature distribution is distorted. The centerpoint. point. flow velocity increasing, the symmetrical temperature distribution is distorted. temperature difference and downstream sensing points increases with with the the The temperature differencebetween betweenthe theupstream upstream and downstream sensing points increases flow speed. flow speed.
29874
Sensors 2015, 15, 29871–29881 Sensors 2015, 15
6
Figure 4. Simulation results of the temperature distribution on the surface of the chip. The wind speed Figure 4. Simulation results of the temperature distribution on the surface of the chip. The changes from 0 to 25 m/s in a step of 5 m/s. The ambient temperature is 300 K, and the temperature wind speed changes 0 to 25 m/s in a step ambient is 300axis, K, of the heaters is 310 K.from The upstream sensing pointofis5atm/s. the The position of 0.5temperature mm of the lateral and the the temperature ofsensing the heaters 310 The upstream pointaxis. is at the position whereas downstream point is is at theK. position of 3.5 mmsensing of the lateral
of 0.5 mm of the lateral axis, whereas the downstream sensing point is at the position of As reported, the thermal 3.5we mm of the lateral axis. conductivities of the silicon and ceramic substrates are temperature dependent, and the thermophysical properties of the airflow are also functions of the ambient As we reported, the thermal conductivities of the silicon are temperature temperature. The data of thermophysical properties of dryand airceramic and thesubstrates thermal conductivity of silicon and ceramic at different temperatures can be found in [23,24]. First of all, the effect of dependent, and the thermophysical properties of the airflow are also functions of the ambient the thermophysical of the airflow on the difference the sensorofsurface temperature. The dataproperties of thermophysical properties of temperature dry air and the thermal on conductivity silicon was studied. The material properties of silicon and ceramic are set according to the data sheet for and ceramic at different temperatures can be found in [23,24]. First of all, the effect of the 320 K [23]. At the same time, the thermophysical properties of dry air are set according to different thermophysical properties of the airflow on the temperature difference on the sensor surface was temperatures [24]. The inlet flow rate is 15 m/s, the temperature difference between the sensor and studied. properties of difference silicon andon ceramic are set according to the data for 320voltage K [23]. the flowThe is 20material K. The temperature the sensor surface is converted intosheet the output At the same time, the thermophysical properties dry air are set accordingconsists to different [24]. signal by the thermopiles and amplified 500 of times. Each thermopile of 10temperatures polysilicon/Al The inlet flow rate is an 15estimated m/s, the temperature difference between the5sensor flow is 20 K. The thermocouples with sensitivity of 2.095 mV/K. Figure showsand the the simulation results of the output voltages at different temperatures. The temperature difference decreases linearly by less temperature difference on the sensor surface is converted into the output voltage signal by the ˝ C. Next, the effect than 5% when temperature increases 80 thermopile conductivities thermocouples of silicon and thermopiles andthe amplified 500 times. Each consists of the 10 polysilicon/Al ceramic substrates are investigated. As shown in Figure 6, the output voltage has a linear with an estimated sensitivity of 2.095 mV/K. Figure 5 shows the simulation results ofrelationship the output with the ambient temperature. When the temperature increases 80 ˝ C, the temperature difference voltages at different temperatures. The temperature difference decreases linearly by less than 5% when increases 26.4%. It is obvious that the temperature effect of the thermal conductivity of the substrate the temperature increases 80 °C. Next, the effect of the conductivities of silicon and ceramic substrates plays dominant role, which is consistent with our previous research [22]. The output voltage of the are investigated. As shown in Figure 6, the output voltageinhas a linear relationship thermal wind sensor at different temperatures is shown Figure 7. This indicateswith that the the ambient output temperature. When the temperature increases 80 ambient °C, the temperature difference increases 26.4%. It is voltage is approximately a linear function of the temperature.
obvious that the temperature effect of the thermal conductivity of the substrate plays dominant role, which is consistent with our previous research [22]. The output voltage of the thermal wind sensor at different temperatures is shown in Figure 7. This indicates that the output voltage is approximately a linear function of the ambient temperature.
29875
Sensors 2015, 15, 29871–29881
Sensors Sensors 2015, 2015, 15 15
77
Figure 5.Figure Simulation results of the effect of the thermophysical properties of the airflow onon the output Figure 5. 5. Simulation Simulation results results of of the the effect effect of of the the thermophysical thermophysical properties properties of of the the airflow airflow on voltage atthe different temperatures. the output output voltage voltage at at different different temperatures. temperatures.
Figure 6.Figure Simulation results ofresults the effect of the silicon andsilicon ceramic on the output 6. of effect of and ceramic on Figure 6. Simulation Simulation results of the the effect of the the silicon andsubstrates ceramic substrates substrates on the thevoltage at Sensors 2015,voltage 15 at 8 different temperatures. output voltage curve a linear function ambient temperature. output different temperatures. The output voltage curve is aa linear function of output voltage atThe different temperatures. Theis output voltage curveof is the linear function of the the
ambient ambient temperature. temperature.
Figure 7.Figure Simulation results ofresults the output of the of wind The output of the 7. Simulation of the voltage output voltage the sensor. wind sensor. The output ofwind the sensor is a linearwind function of temperature. sensor is a linear function of temperature.
4. Experimental Results and Discussion
29876
A fabricated two-dimensional thermal wind sensor consists of four polysilicon heaters, four polysilicon/Al thermopiles, and a central transistor. The sensor chip has been fabricated using a commercial IC fabrication process in a CSMC foundry. Details of the thermal wind sensor can be
Sensors 2015, 15, 29871–29881
4. Experimental Results and Discussion A fabricated two-dimensional thermal wind sensor consists of four polysilicon heaters, four polysilicon/Al thermopiles, and a central transistor. The sensor chip has been fabricated using a commercial IC fabrication process in a CSMC foundry. Details of the thermal wind sensor can be found in [19]. The size of the sensor chip is 4 mm ˆ 4 mm. The sensor is heated by the four polysilicon resistors, each of which has a resistance of 400 Ω. The four polysilicon/Al thermopiles located symmetrically around the central point measure the temperature difference between the upstream and downstream points on the surface of the sensor chip. Each thermopile is composed of 10 p-doped polysilicon/Al thermocouples. Due to the high impurity concentration (1026 m´3 ) the Seebeck coefficient of the thermocouple shows a negligible change with the temperature [22]. Therefore, the sensitivity of the thermopile is nearly constant. The central transistor is connected as a diode which has a temperature coefficient of ´1.5 mV/K. The diode measures the average temperature of the sensor, and the chip temperature is been used in the outer control circuit to maintain a constant temperature difference between the sensor and the ambient temperature. The silicon chip is directly mounted on a ceramic substrate which provides a smooth surface. This structure also helps to shield the sensor from any dust in the airflow [1,19]. The wind sensor is operated in constant temperature difference (CTD) mode which maintains a constant temperature difference between the chip and the airflow. The scheme of the control circuit is shown in Figure 8, where the sensor temperature is measured with the on-chip temperature sensor, and the flow temperature with the off-chip temperature sensor on a similar unheated reference chip. The temperature difference between the sensor and the flow is set as ∆T. For the heating of the sensor, multiple Sensorsheating 2015, 15resistors have been used, indicated by Rh1 to Rh4 . By adding variable resistances9R5 and R6 in the heating circuit of each pair of heating resistors, the sensor heating can be balanced in order temperaturedifference differenceininthe the absence flow. ordertotoremove removethermal thermal offset, offset, i.e., i.e., the the presence presence of a temperature absence of of flow. The whole of the thewind windsensor sensor 400mW. mW. The wholepower powerconsumption consumption of is is 400
Figure 8. Scheme of the circuit used used to maintain a constant temperature difference between Figure 8. Scheme of control the control circuit to maintain a constant temperature difference the between sensor and the airflow. the sensor and the airflow.
Measurements havebeen been performed by installing the sensor in a wind-tunnel wind Measurements have performed by installing the sensor in a wind-tunnel with thewith windthe speed ˝ speed 40and m/sa and a full of range 360 . 9Figure shows the measurement results the wind up toup 40tom/s full range 360°.ofFigure shows 9the measurement results of the windofsensor at ˝ C. It is shown that the output voltages in sensor at different temperatures of 5, 15, 25 and 35 different temperatures of 5, 15, 25 and 35 °C. It is shown that the output voltages in the north-south thedirection north-south direction and east-west increase with the ambient temperature, and the and east-west direction increase direction with the ambient temperature, and the output voltage curves output curves in thealmost north-south direction coincide with theateast-west direction ones in thevoltage north-south direction coincide with thealmost east-west direction ones identical temperatures. at identical temperatures. When the wind speed is 15 m/s, the output signals in the north-south When the wind speed is 15 m/s, the output signals in the north-south and east-west directions are and east-west directions are measured and plotted in Figure 10. Comparing the simulation results measured and plotted in Figure 10. Comparing the simulation results in Figure 7 and the measured in Figure 7 and the measured output in Figure 10, the output voltage curves are seen to be linear output in Figure 10, the output voltage curves are seen to be linear functions of temperature. Some factors contribute to the difference between the simulation results and the measured output. 29877 For instance, some simplification and approximation have been assumed in the COMSOL simulation, and there are some deviations between the theoretical calculation of Seebeck coefficient and actual measurement.
Sensors 2015, 15, 29871–29881
functions of temperature. Some factors contribute to the difference between the simulation results and the measured output. For instance, some simplification and approximation have been assumed in the COMSOL simulation, and there are some deviations between the theoretical calculation of Seebeck coefficient and actual measurement. The output signals in the north-south direction and east-west direction are nearly linear functions of the ambient temperature. From a least-squares fit of the data, the parameters in the north-south and east-west directions can be obtained. Ans = 6.574 ˆ 10´2 V/(m/s)1/2 , Bns = 6.894 ˆ 10´4 V/(m/s)1/2 /˝ C, Aew = 6.537 ˆ 10´2 V/(m/s)1/2 , Bew = 6.842 ˆ 10´4 V/(m/s)1/2 /˝ C. Comparing the values of parameters Ans , Aew , Bns and Bew with the values of parameters in [13], the huge discrepancy primarily derives from the constant temperature difference between the sensor and the airflow. According to the semi-empirical model [13], the constant temperature difference Sensors 15 10 between2015, the sensor and the airflow is incorporated into these fitting parameters A and B. Besides, Sensors 2015, 15 10 some little deviations in the fabrication and packaging process also contribute to these discrepancies.
Figure9.9. The Figure The output output voltages voltages in in north-south north-south and andeast-west east-west directions directions at at different different ambient ambient Figure 9. The output voltages in north-south and east-west directions at different ambient ˝ ˝ temperaturesranging ranging from to 35 temperatures from 5 5 C °C to 35 C. °C. temperatures ranging from 5 °C to 35 °C.
Figure 10. The output voltages in north-west and east-west directions have the ambient temperature Figure The output voltages in north-west and east-west directions have the ambient when the 10. wind speed is 15 m/s.
Figure 10. The voltages and east-west directions have the ambient temperature whenoutput the wind speedinis north-west 15 m/s. temperature when the wind speed is 15 m/s. In Figure 11a, the temperature effect on the29878 output signals has not been compensated. The wind In Figure 11a, the temperature effect on the output signals has not been compensated. The wind direction measurement results are directly calculated by [20]: direction measurement results are directly calculated by [20]: −1 Vns ⋅ cos ( ε ew ) − Vew ⋅ sin ( ε ns )
Sensors 2015, 15, 29871–29881
In Figure 11a, the temperature effect on the output signals has not been compensated. The wind direction measurement results are directly calculated by [20]: ˆ φ=tan
´1
Vns ¨ cos pε ew q ´ Vew ¨ sin pε ns q Vns ¨ sin pε ew q ` Vew ¨ cos pε ns q
˙ (5)
Sensors 2015, 15 11 On the basis of Equation (4), the temperature effect on the output signals has been compensated in Figure 11b. It can be seen that the measurement results of the wind direction have similar
accuracies temperature compensation. wind sensor sensor has has aa accuracies before before and and after after temperature compensation. This This isis because because that that the the wind symmetricalstructure. structure. symmetrical
Figure thethe temperature dependence of the Figure11. 11. (a) (a) The Thewind windspeed speeddirection directionbefore beforeconsidering considering temperature dependence of output signals; (b) The wind speed direction after considering the temperature dependence of the the output signals; (b) The wind speed direction after considering the temperature output signals.
dependence of the output signals.
When the ambient temperature varies, the ambient temperature effects on the output voltages When the ambient temperature varies, the ambient temperature effects on the output voltages in in these two directions are similar. Furthermore, the parameters Aew , Bew , Ans and Bns only depend these two directions are similar. Furthermore, the parameters Aew, Bew, Ans and Bns only depend on the on the thermophysical properties of the airflow, the thermal conductivities of the silicon and ceramic thermophysical properties of the airflow, the thermal and conductivities of thedifference silicon and ceramic substrates, the Seebeck coefficients of the thermocouples the temperature between the substrates, the Seebeck coefficients of the thermocouples and the temperature difference between the sensor and the airflow. In the same sensor, these physical parameters and the temperature difference sensor andthe thesensor airflow. theairflow same sensor, these andand the north-south temperaturedirections. difference between andInthe are almost thephysical same inparameters the east-west This canthe be proved by the of the Ans , Bin Aeweast-west and Bew for output signal of the between sensor and thevalue airflow are parameters almost the same andthe north-south directions. ns , the wind sensor when the wind speed 15 m/s in Figure 10. This can be proved by the value of the parameters Ans, Bns, Aew and Bew for the output signal of the
wind sensor when the wind speed 15 m/s in Figure 10. 5. Conclusions Solid thermal wind sensors play an important role in the agriculture, industry and meteorology 5. Conclusions fields. However, the performance of thermal wind sensors is affected by the ambient temperature. InSolid previous works, thesensors effect ofplay the ambient temperature wind speed measurement has been thermal wind an important role in on thethe agriculture, industry and meteorology studied and a new temperature compensation method has been presented. However, how the fields. However, the performance of thermal wind sensors is affected by the ambient temperature. ambient temperature affects the wind direction measurement has not been investigated. In this In previous works, the effect of the ambient temperature on the wind speed measurement haspaper, been a temperature model for the wind direction measurement is proposed. It is found that the output
studied and a new temperature compensation method has been presented. However, how the ambient temperature affects the wind direction measurement has not been investigated. In this paper, a temperature model for the wind direction measurement 29879 is proposed. It is found that the output signals in two orthogonal directions can determine the wind direction, and each output signal is a linear
Sensors 2015, 15, 29871–29881
signals in two orthogonal directions can determine the wind direction, and each output signal is a linear function of the temperature. For the wind sensor with a symmetrical structure, the temperature drift characteristics in the two orthogonal directions are quite similar, therefore, the wind direction of the wind sensor is approximately impendent of the ambient temperature. Acknowledgments: The authors would like to acknowledge the support offered by the National High-Tech Development Program of China under Contact 2013AA041106 and the Innovation Project for Graduate Student of Jiangsu Province under Grant No. KYLX15_0099. Author Contributions: Bei Chen developed the model, Bei Chen, Yan-Qing Zhu, Zhenxiang Yi and Ming Qin jointly performed the experiments and the data analysis. Qing-An Huang proposed the idea for the model and edited this manuscript. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18.
Van Oudheusden, B.W. Silicon thermal flow sensor with a two-dimensional direction sensitivity. Meas. Sci. Technol. 1990, 1, 565–575. [CrossRef] Zhu, Y.Q.; Chen, B.; Qin, M.; Huang, Q.A. 2-D micromachined thermal wind sensors—A review. IEEE Internet Things J. 2014, 1, 216–232. [CrossRef] Nam, T.; Kim, S.; Park, S. The temperature compensation of a thermal flow sensor by changing the slope and the ratio of resistances. Sens. Actuators A Phys. 2004, 114, 212–218. [CrossRef] Sosna, C.; Buchner, R.; Lang, W. A temperature compensation circuit for thermal flow sensors operated in constant-temperature-difference mode. IEEE Trans. Instrum. Meas. 2010, 59, 1715–1721. [CrossRef] Que, R.Y.; Zhu, R.; Wei, Q.Z.; Cao, Z. Temperature compensation for thermal anemometers using temperature sensors independent of flow sensors. Meas. Sci. Technol. 2011, 22, 085404. [CrossRef] Dijkstra, M.; Lammerink, T.S.J.; de Boer, M.J.; Berenschot, E.J.W.; Wiegerink, R.J.; Elwenspoek, M. Thermal flow-sensor drift reduction by thermopile voltage cancellation via power feedback control. J. Microelectromech. Syst. 2014, 23, 908–917. [CrossRef] Bruschi, P.; Dei, M.; Piotto, M. An offset compensation method with low residual drift for integrated thermal flow sensors. IEEE Sens. J. 2011, 11, 1162–1168. [CrossRef] Ashauer, M.; Glosch, H.; Hedrich, F.; Hey, N.; Sandmaier, H.; Lang, W. Thermal flow sensor for liquids and gases based on combinations of two principles. Sens. Actuators A Phys. 1999, 73, 7–13. [CrossRef] Glaninger, A.; Jachimowicz, A.; Kohl, F.; Chabicovsky, R.; Urban, G. Wide range semiconductor flow sensors. Sens. Actuators A Phys. 2000, 85, 139–146. [CrossRef] Sabate, N.; Santander, J.; Fonseca, L.; Gracia, I.; Cane, C. Multi-range silicon micromachined flow sensor. Sens. Actuators A Phys. 2004, 110, 282–288. [CrossRef] Dalola, S.; Cerimovic, S.; Kohl, F.; Beigelbeck, B.; Schalko, J. MEMS thermal flow sensor with smart electronic interface circuit. IEEE Sens. J. 2012, 12, 3318–3328. [CrossRef] Sturm, H.; Lang, W. Membrane-based thermal flow sensors on flexible substrates. Sens. Actuators A Phys. 2013, 195, 113–122. [CrossRef] Van Oudheusden, B.W. High-sensivity 2-D flow sensor with an etched thermal isolation structure. Sens. Actuators A Phys. 1990, 21–23, 425–430. [CrossRef] Kim, S.; Kim, S.; Kim, Y.; Park, S. A circular-type thermal flow direction sensor free from temperature compensation. Sens. Actuators A Phys. 2003, 108, 64–68. [CrossRef] Bruschi, P.; Dei, M.; Piotto, M. A low-power 2-D wind sensor based on integrated flow meters. IEEE Sens. J. 2009, 9, 1688–1696. [CrossRef] Kaltsas, G.; Katsikogiannis, P.; Asimakopoulos, P.; Nassiopoulou, A.G. A smart flow measurement system for flow evaluation with multiple signals in different operation modes. Meas. Sci. Technol. 2007, 18, 3617–3624. [CrossRef] Hourdakis, E.; Sarafis, P.; Nassiopoulou, A.G. Novel air flow meter for an automobile engine using a Si sensor with porous Si thermal isolation. Sensors 2012, 12, 14838–14850. [CrossRef] [PubMed] Andre, N.; Rue, B.; Scheen, G.; Flandre, D.; Francis, L.A.; Raskin, J.P. Out-of-plane MEMS-based mechanical airflow sensor co-integrated in SOI CMOS technology. Sens. Actuators A Phys. 2014, 206, 67–74. [CrossRef]
29880
Sensors 2015, 15, 29871–29881
19. 20. 21. 22. 23. 24.
Sun, J.B.; Qin, M.; Huang, Q.A. Flip-chip packaging for a two-dimensional thermal flow sensor using a copper pillar bump technology. IEEE Sens. J. 2007, 7, 990–995. [CrossRef] Makinwa, K.A.A.; Huijsing, J.H. A smart wind sensor using thermal sigma-delta modulation techniques. Sens. Actuators A Phys. 2002, 97–98, 15–20. [CrossRef] Chen, B.; Zhu, Y.Q.; Qin, M.; Huang, Q.A. Effects of ambi-ent humidity on a micromachined silicon thermal wind sensor. J. Microelectromech. Syst. 2014, 23, 253–255. [CrossRef] Huang, Q.A.; Chen, B.; Zhu, Y.Q.; Qin, M. Modeling of temperature effects on micromachined silicon thermal wind sensors. J. Microelectromech. Syst. 2015. in press. [CrossRef] Harper, C.A. Electronic Materials and Processes Handbook; McGraw-Hill Companies: New York, NY, USA, 2004; pp. 393–396. Tsilingiris, P.T. Thermophysical and transport properties of humid air at temperature range between 0 and 100 ˝ C. Energy Convers. Manag. 2007, 49, 1098–1110. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
29881
