An ultrasonic air temperature measurement system with self-correction function for humidity

Transcription

1 INSTITUTE OFPHYSICS PUBLISHING Meas. Si. Tehnol. 16 (25) MEASUREMENTSCIENCE AND TECHNOLOGY doi:1.188/ /16/2/3 An ultrasoni air temperature measurement system with self-orretion funtion for humidity Wen-Yuan Tsai 1,2, Hsin-Chieh Chen 1 andteh-luliao 1 1 Department of Engineering Siene, National Cheng Kung University, Tainan, 71 Taiwan, Republi of China 2 Department of Eletrial Engineering, Kao Yuan Institute of Tehnology, Kaohsiung, 821 Taiwan, Republi of China tlliao@mail.nku.edu.tw Reeived 15 June 24, in final form 2 Otober 24 Published 21 January 25 Online at staks.iop.org/mst/16/548 Abstrat This paper proposes an ultrasoni measurement system for air temperature with high auray and instant response. It an measure the average temperature of the environmental air by deteting the hanges of the speed of the ultrasound in the air. The hanges of speed of sound are omputed from ombining variations of time-of-flight (TOF) from a binary frequeny shift-keyed (BFSK) ultrasoni signal and phase shift from ontinuous waves [11]. In addition, another proposed tehnique for the ultrasoni air temperature measurement is the self-orretion funtionality within a highly humid environment. It utilizes a relative humidity/water vapour sensor and applies the theory of how sound speed hanges in a humid environment. The proposed new ultrasoni air temperature measurement has the apability of self-orretion for the environment variable of humidity. Espeially under the operational environment with high flutuations of various humidity levels, the proposed system an aurately self-orret the errors on the onventional ultrasoni thermometer aused by the hanging density of the vapours in the air. Inluding the high humidity effet, a proof-of-onept experiment demonstrates that in dry air (relative humidity, RH = 1%) without humidity orretion, it is aurate to ±.4 C from Cto8 C, while in highly humid air (relative humidity, RH = 9%) with self-orretion funtionality, it is aurate to ±.3 C from Cto8 C with.5% resolution and temperature hanges are instantly refleted within 1 ms. Keywords: ultrasoni sensors, humidity sensors, temperature measurement 1. Introdution In many industrial proesses, suh as refining, roasting et, the online ontrol of temperature is often very important for saving energy, guaranteeing produt quality and raising the produtivity et. Up to now, thermoouples and radiation thermometers have been the two main methods for measuring temperature in industry. Aurate and sensitive as the thermoouple is, it annot work durably at high temperature and it is hard to realize online ontrol. With the development of photoeletroni tehniques, great improvements have been made in radiation thermometers. However, beause of its suseptibility to fume and mist, aurate measurement is diffiult to obtain in the above hostile environments. Over the past several deades, ultrasoni thermometry has been evolving as a new temperature measurement tehnology for environments where thermoouples, radiation pyrometers and other onventional instruments have failed to operate satisfatorily. Its priniple is that the veloity of sound in any objet is a funtion of temperature, so that, in an ideal gas, /5/2548+8$3. 25 IOP Publishing Ltd Printed in the UK 548

2 the veloity is diretly proportional to the square root of the absolute temperature, in most liquids the dependene is linear, and in solid objets the veloity generally dereases with the inrement of the temperature. Thus, if the veloity of sound is measured, the temperature is measured [1]. Therefore, the high-speed ultrasoni thermometer is designed for measuring the temperature of high-temperature gas streams. The areas of appliation inlude the heat power industry, plasmatrons or other energy equipment, the hemial industry, development and researh of jet engines, et. The major advantages of the ultrasoni thermometer are its high operating speed, reahing 15 measurements per seond, and omplete automation of measuring proess [2]. Others have explored an ultrasoni method of sensing abin air temperature that ould lead to automati limate ontrol (ACC) with improved thermal omfort. Pereived thermal omfort is better orrelated with breath-level temperature than it is with the output of the onventional in-ar sensor. To sense bulk air temperature, a method was proposed transparent to the oupant to send a pulse of ultrasound through the air [3, 4, 11]. The most ommon sensors for air temperature are usually sensor-by-ontat. They rely on the sensors to ontat the air and use its radiant energy to ause hanges in their physial harateristis. This type of sensor usually does not respond instantly and their range of measurement is limited. Thus they are not ideal for dynami traking measurements of fast hanging temperature. The speed of sound in gas has been widely explored. It is ommonly understood that the speed of sound is affeted by temperature, pressure, relative humidity and the onstituents of the air [5]. The theoretial expression for the speed of sound in an ideal gas is γp = (1) ρ where P is the ambient pressure, ρ denotes the gas density and γ is the ratio of the speifi heat of gas at onstant pressure to that at onstant volume. In an ideal gas, equation (1) may be rewritten as 1.4RT = (2) M where R is the universal gas onstant, T is the absolute temperature and M is the mean moleular weight of the air. From equation (2), we know is proportional to T. That is why the tehniques of measuring the temperature of air on the propagation path by sound speed are widely adopted [6, 7]. All previous disussion assumed dry air. However, moisture affets the speed of sound by hanging the density of the air and the mean moleular weight, as shown in equations (1) and (2). Moist air is less dense than dry air (not partiularly obvious), so ρ in equation (1) gets smaller. This auses an inrease in the speed of sound. Moisture also auses the speifi-heat ratio to derease, whih would ause the speed of sound to derease. However, the derease in density dominates, so the speed of sound inreases with inreasing moisture. In this paper, we propose an ultrasoni air temperature measurement system with self-orretion for humidity. Furthermore, it is a ontatless sensor and has an instant response. An ultrasoni air temperature measurement system with self-orretion funtion for humidity 2. Corretion for humidity Two terms in equation (2) must be modified to aurately inlude the effets of moisture (water vapour) on the speed of sound. These are the speifi-heat ratio γ (1.4 for dry air) and M, the average moleular weight of the different types of moleules in the air. Development of eah of these terms follows [8 1]. The terms R (universal gas onstant) and T (absolute temperature) remain unhanged. The speifi-heat ratio an be expressed as an exat fration by letting d equal the number of degrees of freedom for the air moleules. This gives γ = d +2 d. (3) Sine the omposition of dry air is mostly two atom moleules, it is said to be a diatomi gas. Diatomi gases have five degrees of freedom, three translational and two rotational; thus d = 5 and γ = 1.4, for dry air. If h is defined to be equal to the fration of moleules that are water, then the presene of water (with six degrees of freedom) auses the average number of degrees of freedom per moleule to inrease to 5 + h. Equation (3) an now be rewritten to inlude the effets of moisture for air as γ w = 7+h 5+h. (4) It is noted that equation (4) is an alternative but equivalent expression to Humphreys equation as used in thermodynamis [8]. The average moleular weight of air dereases with added moisture. To see this, M is alulated first for dry air. Dry air omposition is 78% nitrogen (moleular weight = 28) 21% oxygen (moleular weight = 32) 1% argon (moleular weight = 4) for a total moleular weight equal to M = (.78)(28) + (.21)(32) + (.1)(4) = 29. The presene of water (with a moleular weight of 18) auses the total average moleular weight to derease to 29 (29 18)h,or M w = 29 11h. (5) Equations (4) and (5) modify the two terms from equation (2) affeted by the addition of water vapour to air. Both are a funtion of the introdued water moleule fration h. Relative humidity RH (expressed as a perentage) is defined suh that.1rh e(t ) h = (6) p where p equals ambient pressure ( Pa for 1 atm referene pressure) and e(t ) is the vapour pressure of water at temperature T. For temperature values in degrees Celsius, representative values of e(t ) are e(5) = 872 Pa e(2) = 2338 Pa e(1) = 1228 Pa e(3) = 4243 Pa e(15) = 175 Pa e(4) = 7376 Pa. To express the perentage inrease in the speed of sound due to relative humidity all that remains is to take the ratio of the 549

3 W-Y Tsai et al CHANGE IN VELOCITY (%) T[deg C] RELATIVE HUMIDITY IN PERCENT (RH%) T=4 T=3 T=2 T=15 T=1 T=5 Figure 1. Relative humidity versus perentage hange in speed of sound as a funtion of temperature. Reeiver Preamplifier and Gain-Controlled Amplifier Thermoouple e-readout Relative Humidity/ Water Vapor Pressure e-readout Temperature/Humidity Control Chamber Transmitted Signal Soure 8951 Single Chip Miroproessor Transmitter Power Amplifier RS232 LCD To PC wet and dry speeds, subtrat 1, and multiply by 1. Sine both wet and dry speed terms involve the same onstant terms (R and T), their ratio will ause these to anel, giving the inrease in sound speed (%) ( ) ( ) w γw RT/M w (%) = 1 1 = 1 1 d γd RT/M d γw = (7) M w Equation (7) is plotted in figure 1 as a funtion of relative humidity for six temperature values. Figure 1 shows the perentage inrease in the sound speed due to relative humidity only; the temperature values are for aurately speifying the relative humidity. 3. Ultrasoni measurement of temperature To aurately measure the temperature of the environmental air, we use the effet of humidity on the speed of sound stated above and ombine the tehnique of transmit/reeipt ultrasoni signals of binary frequeny shift-keyed (BFSK) and ontinuous waves and obtain the average air temperature from the data of time-of-flight (TOF) and phase shift (PS) [11 13]. The ultrasoni sensor system for temperature is shown in figure 2. An ultrasoni transmitter is installed on the righthand side of the temperature/humidity-ontrolled hamber, the reeiver is on the left-hand side. The distane between the transmitter and reeiver is 1 m so that the average of the environmental air temperature an be measured TOF alulation In figure 3, the elapsed time t, whih is the travel time of the signal from the transmitter to the reeiver, an be alulated as t = t 2 t 1 where t 1 is the time when the transmitted signal hanges frequeny from f 1 to f 2,andt 2 is the time when the orresponding reeived signal hanges frequeny from f 1 to f 2. The speed of sound an be expressed by = L/ t, where L is the distane between the transmitter transduer and reeiver transduer. Frequeny Detetor Digital Phase Meter 4/42kHz ontinuous wave PC Detetion & Calibration Figure 2. Blok diagram of the ultrasoni temperature measurement system. Amplitude t 1 t t 2 f 1 f 2 f 1 f 2 Transmitted signals Reeived signals Figure 3. Transmitted signals and reeived signals. T 1 T 2 f 1 θ 1 θ 2 f 2 Transmitted signals Reeived signals f 1 f 2 Figure 4. Illustration of the phase shifts θ 1 and θ Phase shift detetion The detetion of the phase shift is based on the two-frequeny ontinuous wave method of ultrasoni distane measurement [12]. The phase shifts of θ 1 and θ 2 an be deteted by the reeived signals orresponding to the transmitted signals. Figure 4 shows a ontinuous wave with frequeny f 1 and a reeived signal with frequenies f 1 and f 2. Phase shift θ 1 is the differene in phase between the ontinuous wave and the reeived signal at f 1. Phase shift θ 1 an be alulated as θ 1 = 2π(t 2 t 1 )/T 1, where T 1 is the period of the reeived signal with frequeny f 1. Similarly, phase shift θ 2 is the differene in phase between the ontinuous wave and the reeived signal at frequenyf 2. The following will alulate t 55

4 An ultrasoni air temperature measurement system with self-orretion funtion for humidity 2π(6.28) π(3.14) θ λ #n.5 1 n-2 n-1 n n+1 Distane L Figure 5. Relation between L = t and L = (n 1) λ + ( θ/2π) λ. the speed of sound by omparing the two phase shifts. Note that the distane between the transmitter transduer and reeiver transduer remains unhanged. We have ( L = n 1 + θ ) 1 (8) 2π f 1 ( L = n 2 + θ ) 2. (9) 2π f 2 Here Lis the distane between the two transduers, n 1 and n 2 are integers. Due to the differene in frequenies, the phase shift an be dedued from (8) and (9) as follows: θ = 2π L f ( f = f 1 f 2 ). (1) The integers n have only two possible values: n 1 = n 2 and n 1 = n So the differene of the phase shifts an be defined by the following algorithm: 1. if θ 1 >θ 2, θ = θ 1 θ 2, 2. if θ 1 <θ 2, θ = θ 1 +2π θ 2. Hene = L θ 2π 1 f. (11) The variation of the ultrasoni veloity an be uniquely determined by the differene of the phase shifts ( θ) if the maximum variation does not exeed one period of the frequeny differene ( f ). Otherwise phase ambiguity will our. The minimum resolution and the maximum range of the temperature measurement are determined by the hoie of frequenies (f 1,f 2 ) Temperature alulation The distane L an be expressed as L = t where t is TOF. In figure 5, the distane L is divided into regions [(n 1)λ, nλ] (n = 1, 2, 3,...), λ is the wavelength of f. ] f The distane L an be expressed as L = [ (n 1) + θ 2π where n is an integer. The region defined by [(n 1)λ, nλ] is alled #n region. The n 1 integer an be obtained by an integer operation Int( t f ). The speed of sound an then be expressed as = L [ ]. (12) Int( t f ) + θ 2π 1 f Figure 6. Blok diagram of the transmitted signal soure. From equation (2) we know the temperature in degrees Celsius is ( ( ) ) 2 T = (13) This equation does not onsider the variable of humidity. From equation (7) we know the sound speed inrease due to humidity inrease is. So to inlude the effet of humidity on the sound speed, the sound speed equation would beome = (14) The alulation for temperature would beome [ ( ) ] 2 T = (15) where is the speed of sound with humidity effet orretion and T is the temperature with humidity effet orretion. 4. System implementation Figure 2 is a blok diagram of the omplete system whih onsists of a temperature/humidity-ontrolled hamber, a thermoouple-based thermometer, a relative humidity/water vapour pressure meter, two aousti transduers with mathing exponential horns, a signal generation system, power amplifier, preamplifier and gain-ontrolled system, frequeny detetor and digital phase meter. The thermoouple is used to measure the air temperature and ompare the measurement with the output of the ultrasoni system. A miroproessor ontrols the operation of the entire system and a PC will examine the measurement result and perform alibration Hardware Transmitted signal soure. The transmitted pulse is made up of two sinusoids (4 and 42 khz). Figure 6 shows a rystal osillator iruit used to generate a steady signal with a base frequeny of 8 MHz. The divisors of two dividers are set at 2 and 194 whih is applied to the base frequeny. Two frequenies 4 khz and 42 khz are then produed and sent to the multiplexer (MUX). The MUX is ontrolled by an 8951 miroproessor. 551

5 W-Y Tsai et al Figure 7. Blok diagram of the frequeny detetor. The thermoouple voltage is onverted into a temperature reading with a Testo 946 thermometer. The auray in the speifiation of this instrument is said to be ±.2 C. We use an ie water bath to hek the auray. The differene from the atual temperature at C was +.1 C within the laimed auray. The output of the thermometer is sent to a PC used as the standard temperature. The measurement of the relative humidity/water vapour pressure meter is by Sable System RH-2 meter (auray±1%). Therefore, the PC has t, the elapsed time of the ultrasound, phase shift data and the temperature measured by the thermoouple and the relative humidity/water vapour pressure measured by the gas sensor. From these data, the PC an alulate the errors of the temperature measurement and build up a alibration system Software Figure 8. Blok diagram of the digital phase meter Preamplifier and gain-ontrolled amplifier. The bandwidth of the ultrasoni transduers used in our system is narrow. To redue error from aousti attenuation, the gain of the amplifier must dynamially adjust as the frequeny of the ultrasound hanges. Therefore, the error inurred from aousti attenuation is minimized in the gainontrolled amplifier by keeping the reeived signal amplitude dynamially onstant Frequeny detetor. Figure 7 shows the blok diagram of the frequeny detetor. The frequeny detetor detets the time when the frequeny of the reeived signal hanges from f 1 (4 khz) to f 2 (42 khz). The deteted time is then used by the miroproessor to alulate TOF Digital phase meter. The phase shift is transformed into pulse width by two D-type flip-flops, as shown in figure 8. An 8 MHz signal is used to ount the pulse width. The resolution of the phase metre is.5% for a 4 khz signal. Finally, the ounter is leared by a reset signal generated by the miroproessor for ounting the next phase shift single-hip miroproessor. The measurement system is ontrolled by an 8951 single-hip miroproessor (Atmel, made in USA). The funtions of the miroproessor inlude ontrolling the BFSK signals of the ultrasound, obtaining the digital phase shift and humidity/vapour pressure data, alulating the TOF and the air temperature and displaying it Calibration system. As shown in figure 2, a hamber with onstant temperature/humidity inside has an internal fan to maintain the inside air temperature/humidity uniform. A thermoouple measures the air temperature inside the hamber. The algorithm of the software program in the miroproessor an be expliated by the flowhart shown in figure 9. First, the 8951 miroproessor will feth the atual temperature T 1 measured by the thermoouple and the relative humidity/water vapour pressure meter measured by the sensor from the PC. Next, it will alulate the inrease in sound speed due to the inreased humidity, assign the transmitted signal, adjust the gain-ontrolled amplifier, wait for the interrupt from either the frequeny detetor or the digital phase metre to alulate the TOF, obtain θ 1 and θ 2 and alulate the orreted speed of sound and the temperature T 2. Then, it will ompare T 1 with T 2.If T 2 T 1 < 1 C, it will display T 2 on the LCD. Otherwise, the PC will realulate aording to the temperature data and humidity variables from the alibration system. If the waiting time is longer than 5 ms, 8951 will reassign the transmitted signal. TOF, θ 1, θ 2, and are all sent to the PC via the RS232 interfae of the Experimental results and disussions 5.1. Ultrasoni experiment To ollet data, we first measure the air temperature and humidity/vapour pressure in the hamber with the thermoouple and gas sensor. Seondly, three times we reord the TOF of the BFSK ultrasoni pulse in the hamber, the θ 1 and θ 2 from ontinuous waves. Then, we measure the air temperature and humidity/vapour pressure with the thermoouple and gas sensor for a seond time. Finally, we ompare the average of the three TOF, θ 1 and θ 2 measurements with the average of the two thermoouple measurements. Both an represent the air temperature at the same point of time, i.e. halfway through the measurement yle. From Cto8 C with 1 C as the interval, we will repeat the measurement and reord the data at different temperatures. Using this measurement system and alulating the speed of sound with equation (12), we an obtain the average temperature of the air on the propagation path Experimental results Figure 1 shows, from Cto8 C, the data diagram of the temperature measured by the ultrasoni thermometer without humidity orretion and the atual temperature measured 552

6 An ultrasoni air temperature measurement system with self-orretion funtion for humidity Figure 9. The flowhart of the software. Ultrasoni Temperature [deg C] (a) Error Temperature [deg C] (b) Figure 1. (a) At relative humidity = 1%, without humidity orretion, a logged data graph of the atual thermoouple temperature versus alulated ultrasoni temperature with humidity orretion. (b) The plot of temperature error. by thermoouple, when the relative humidity RH of the environment is 1%. The maximum error is ±.4 C. Figure 11(a) shows, from Cto8 C, the data diagram of the temperature measured by the ultrasoni thermometer without humidity orretion and the atual temperature measured by thermoouple, when the relative humidity RH of the environment is 9%. From the diagram, we an observe that the temperature measured by the ultrasoni 553

7 W-Y Tsai et al Ultrasoni Temperature [deg C] (a) Ultrasoni Temperature [deg C] (a) Error Temperature [deg C] Figure 11. (a) At relative humidity = 9%, without humidity orretion, a logged data graph of the atual thermoouple temperature versus alulated ultrasoni temperature without humidity orretion. (b) The plot of temperature error. thermometer is far higher than the atual temperature beause of the humidity effet. The errors are shown in figure 11(b). As the temperature rises, the error inreases. When the atual temperature is 8 C, the temperature measured by the ultrasoni thermometer without humidity orretion is an astounding C. The reason for this large error is that the relative humidity in the environment aused an inrease in the speed of sound. Figure 12(a) shows the data diagram of the temperature measured by ultrasoni thermometer with humidity orretion and the atual temperature measured by thermoouple, when the relative humidity RH of the environment remains 9%. Figure 12(b) shows the errors between these two measured temperatures. The standard error of measurement is alulated as follows: SE = n [RP(i) PP] 2 (16) n i where RP is the temperature of the ultrasoni measurement, PP is the temperature measured by thermoouple, n is the number of measurements. The average error is.19 Cand the standard error is.24 C. Through repeated experiments, if the temperature is under 8 C, the differene between the ultrasoni measurement and the atual temperature onsistently remains within ±.3 C. (b) Error Temperature [deg C] Figure 12. (a) At relative humidity = 9%, with humidity orretion, a logged data graph of the atual thermoouple temperature versus alulated ultrasoni temperature with humidity orretion. (b) The plot of temperature error Disussions We have established a new ultrasoni air thermometer with the funtion of humidity orretion. Our system suessfully ombines the tehniques of TOF, PS and humidity sensor. With the transmission of BFSK signals, upon reeiving the ultrasoni pulse, the TOF is readily alulated by the time the hange between eah disrete frequeny ours. To ahieve higher auray, ontinuous wave transmission is used to alulate the phase shift between the transmitting and reeiving signals. Phase shift operation offers a speial advantage by eliminating a lass of attenuation problems that often aompany short-burst transmissions whih go through nonlinear signal distortion during start up as a result of transmitting transduer mehanial spring oeffiients produing audio signals with slow-onset envelopes. The slow onset makes the exat signal start time unlear to the reeiver. Continuous wave transmission has similar start/stop envelope problems. But during ontinuous operation these problems are gone. With the help of a humidity sensor, the error aused by environmental humidity is effetively orreted in our ultrasoni thermometer system. From the experiments, the error between the atual temperature measured by thermoouple and the temperature measured by our system is only ±.3 C. This is the result (b) 554

8 An ultrasoni air temperature measurement system with self-orretion funtion for humidity when the system repeats the measurement every.1 s. This level of auray with the speed of ultrasoni system detetion is more than adequate for average temperature ontrol systems. Referenes [1] Mi X B, Zhang S Y, Zhang J J and Yang Y T 23 Automati ultrasoni thermometry Paper presented at 15th Symp. on Thermophysial Properties (Boulder, CO, June 22 27) [2] Butkus J and Jakeviius L 21 Investigation of stability of ultrasoni system for gas flow veloity measurement Ultragarsas 4 no 3 36 [3] Han T, Lambert D, Oberdier L, Partin D and Sultan M 23 Ultrasoni air temperature sensing for automati limate ontrol-sensor development SAE Paper (SAE World Congress, Detroit) [4] Han T, Lambert D, Oberdier L, Partin D and Sultan M 24 Ultrasoni air temperature sensing for automati limate ontrol-vehile test SAE Paper (SAE World Congress, Detroit) [5] Wong G S K 1986 Speed of sound in standard air J. Aoust. So. Am [6] Bramanti M, Salerno E A, Tonazzini A, Pasini S and Gray A 1996 An aousti pyrometer system for tomographi thermal imaging in power plant boilers IEEE Trans. Instrum. Meas [7] Huang K N, Huang C F and Li Y C 22 High preision, fast ultrasoni thermometer based on measurement of the speed of sound in air Rev. Si. Instrum [8] CRC Handbook of Chemistry and Physis th edn (Boa Raton, FL: CRC Press) [9] Piere A D 1981 Aoustis: An Introdution to its Physial Priniples and Appliations (New York: MGraw-Hill) [1] Dennis A B 1988 Environmental effets on the speed of sound J. Audio Eng. So [11] Liao T L, Tsai W Y and Huang C F 24 A new ultrasoni temperature measurement system for air onditioners in automobiles Meas. Si. Tehnol [12] Gueuning F E, Varlan M, Eugene C E and Dupuis P 1997 Aurate distane measurement by an autonomous ultrasoni system ombining time-of-flight and phase-shift methods IEEE Trans. Instrum. Meas [13] Huang S S, Huang C F and Huang K N 22 A high auray ultrasoni distane measurement system using binary frequeny shift-keyed signal and phase detetion Rev. Si. Instrum