FI58403B - ADJUSTMENT OF FUNCTIONS - Google Patents

Description

.... NOTICE OF PUBLICATION Λ Λ, ® ί11) UTLÄCCNINGSSKRIFT 5 84 Ο 3 «Jgft C (45Ραexam issued 12 01 1981

Patent meddelat ^ (51) Kv.ik.Wa3 G 01 Ν '27/22 SUOMI — FINLAND (21) f * K * mmrt * mu * -p «t * n« * »ekninf 791061 (22) Hkkwnlipilvl —Afweknlnpdag 29.03 .79 (23) AJkupUvi — GUtifWsdtg 29.03.79 ¢ 41) Interpreters] ulkMul - BlMt effuntlig 30.09.80

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Patent · och regl * t * r * tyr * l * en v 'An * ök »n utli * d oeh utMkrtftun publicund 30.09. oo (32) (33) (31) Requested «Tuoikm * —Buglrd prtorttet (71) Vaisala Oy, PO Box 26, FI-00421 Helsinki 42, Finland-Finland (FI) (72) Eero Salasmaa, Helsinki, Veijo Antikainen, Vantaa, Jouko Jalava, Klaukkala, Finland-Finland (Fl) (74) Forssen & Salomaa Oy (54) Control unit in humidity sensor - Regleranordning i fuktighetsgivare

The invention relates to a control device for a humidity sensor, the operation of which is based on a change in sensor impedance, in particular a capacitive humidity sensor with a polymer whose dielectric constant changes as a function of humidity and controlled by or to extend its service life, and the control device comprises a bridge circuit or the like comprising temperature-dependent resistance elements for detecting said external temperature and the sensor temperature, the bridge differential voltage being used as a feedback signal for controlling the electrical power heating the sensor.

Applicant's Finnish patent No. 48229 discloses a capacitive humidity sensor in which the dielectric material is a polymer blend, the dielectric constant of which is a function of the amount of water absorbed by the polymer blade. Undesirable phenomena occur in the above-mentioned and other impedance-based humidity sensors, especially when measuring high humidity concentrations. Such phenomena include e.g. slow creep of the sensor, which can be caused by several different factors. In general, however, it is a question of reversible phenomena, which the invention aims to better control, in order to achieve even higher sensor speeds and to improve the measurement accuracy, especially at high relative humidities of e.g. 75-90%.

Applicant's FI patent application No. 773680 discloses a method for reducing the undesired properties of an electronic humidity sensor, which method is mainly characterized in that the moisture-sensitive material of the humidity sensor is heated to a temperature higher than the ambient temperature of the humidity sensor at least higher relative humidity.

In connection with the above, reference is further made to the publication "A stable Thin Film Humidity Sensor, Philip H. Chawner and Cecil A. Cove" presented at the Transducer * 78 conference, which describes a method for maintaining the temperature of a humidity sensor according to the equation T = K · T + C, in which T is the sensor temperature and S3 s T is the ambient temperature. The publication shows values for the constants K and C of 1.041856 and 4.02497 as well as 1.0352 and 4.1000 for different ranges of temperature T, whereby 3 humidity sensors "see" 0.75 times the prevailing ambient humidity.

Said equation T = K · T + C is an approximate equation which, for example, gives an accuracy of approx. ± 0.2 C for the sensor temperature T in the temperature range S -20 C ... + 100 C. The electronic solution of this known device is according to the attached Figure A, in which the resistors R3i> R32, Ra and Rs together with the resistor elements connected in parallel form a bridge connection to which additional components are further connected to the right-hand branch of the bridge for the purposes shown below. By appropriately selecting the values of said resistors and additional components and when Ra is a PTC resistor almost linearly dependent on the ambient temperature T ^ and R likewise a PTC resistor almost s linearly dependent on the sensor temperature T, in the example platinum resistors Pt100, the bridge is in equilibrium when the above equation T = K · T + C has been implemented.

S 3

The humidity sensor, not shown in Fig. A, the resistor Rg and the separate heating resistor R 1 are mechanically connected by gluing, so that the resistor R 1 is a thermal connection TC to the resistor R and the humidity sensor attached thereto. The gain of the Oletta racket operational amplifier IC1 is very high, which is realized in practice, the operation of the circuit is physically easy to understand. A power is continuously applied to the resistor R 1, which keeps the bridge in equilibrium by adjusting the temperature and thus the resistance of the temperature-sensitive resistor Rg. If, for example, the ambient temperature T rises, the resistance of the resistor R increases. In this case, the bridge becomes 3 3 3 58403 unbalanced, causing an increase in the current flowing through the transistor TRO after IC1, increases the power to the heating resistor and thus the temperature of the resistor Rg rises until the equilibrium of the bridge is reached. Substantially constant current through temperature-sensitive resistors is obtained by stabilizing the bridge supply voltage and selecting resistors R 1 and R 2 as highly resistive, e.g. 10 kO.

The constant K is obtained by adjusting the resistance RV so that the flow ratio I / 1 1 as has a numerical value equal to the desired K value. The value of the constant C is adjusted to the desired value by changing the resistance RV 1, which affects the difference voltage between the points X, Y.

Em. as presented in the publication, the controlled temperature control of the humidity sensor has been implemented almost satisfactorily. However, certain drawbacks have been found in this known method, which will be discussed in detail below.

The device requires two separate additional elements connected to the humidity sensor, namely a temperature-sensitive resistor Rg and a heating resistor R1, which makes the sensor-resistor combination difficult to manufacture. Resistors R and R, whose resistance S 3 si is temperature dependent, are, for practical reasons, industrially manufactured resistive metal temperature sensors with a relatively low impedance, e.g. Pt100 resistors. In particular, in order to avoid self-heating of R, which must have a low thermal inertia, care must be taken to ensure that the voltage acting on R is small, of the order of 150 mV. Thus, very high requirements must be placed on the 3 electronics, e.g. the operational amplifier IC1, and the electronics will also become difficult to calibrate and the operation will require special care, e.g. to avoid contact potentials in the terminals. In addition, the temperature control of the known device, when it takes place according to the approximate equation T = K · T + C, causes significant errors in the results of the measurement S 3. The disadvantage is also that the device requires many precision components and a stabilized power supply.

The object of the invention is to provide a control device in which the above-mentioned disadvantages are avoided. It is also an object to provide a control device with which, if necessary, the temperature dependence of the humidity sensor can also be corrected.

In order to achieve the above and later objects, the invention is mainly characterized by the combination that a positive heating coefficient resistance element is used as the heating element of the sensor, e.g. a platinum resistor known per se as a resistor-thermistor arrangement placed opposite one of the branches of the bridge connection such that a certain function T = f (T) specific to each sensor type is realized by means of a control device.

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As a further advantage, the invention simply provides the possibility to provide a humidity meter that displays not only relative humidity (RH) but also absolute humidity and / or dew point. An additional advantage of the control device of the invention are the simplicity of implementation and low power consumption.

The invention will now be described in detail with reference to some embodiments of the invention schematically shown in the figures of the accompanying drawing, based on a capacitive humidity sensor in which the moisture-sensitive substance is an organic polymer.

Fig. A shows the previously known control device discussed above.

Figure 1 shows schematically a humidity sensor and a control device connected to it.

Figure 2 shows section II-II in Figure 1.

Figure 3 shows the bridge connection used in the control devices of the invention and the associated current control devices.

Figure 4 shows a schematic implementation of the measuring head of a humidity measuring device provided with a control device according to the invention.

Figure 5 shows a more detailed implementation of the bridge connection used in the control device according to the invention.

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Fig. 6 shows the differential temperature ΔΤ realized by the control device according to the invention as a function of the outdoor temperature T.

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Fig. 7 shows a detailed connection diagram of a measuring device provided with a control device according to the invention.

The humidity sensor shown in Figures 1 and 2 is known per se from the applicant's Finnish patent No. 48229. The sensor 10 is based on a support base 11, such as a glass plate, which is passive with respect to water absorption. The substrate 11 is made of a thin film technique by metallizing the bottom contact 12 in a known manner, to which contact wires 16 are attached by soldering, the capacitance of which is measured and indicated by the device schematically shown in Fig. 1 by blocks 20 and 22. The sensor 10 has a thin thickness of approx. A polymer film 13 of the order of 13 is vacuum-vaporized, sputtered or chemically formed on the polymer film 13 to form a thin water vapor permeable surface contact 14 which is not in galvanic contact with either bottom contact 12.

Thus, the capacitance to be measured in the region d and e (Fig. 2) is formed by a series connection of the capacitances generated between the bottom contacts 12 and the surface contact 14.

Fig. 1 shows in block 17 devices for measuring the external temperature T, the signal of which is passed together via 19 to the measuring and control device 20.

3

Figure 2 shows a resistance element R made in connection with the substrate 11, e.g. by evaporation, with which the sensor 10 is held using a control device according to the invention at a temperature T higher than the ambient temperature T. In Fig. 1, the connection 18 denotes the functional connection through which the heating current 1 ^ is conducted to the resistor RgT and through which the temperature T ^ of the sensor 10 is measured.

In the bridge connection of Figure 3, RgT is a low-impedance temperature-sensitive resistor element, e.g. a Pt100 platinum resistor, which measures and raises the temperature of the humidity sensor 10 or the surrounding medium, usually air, by means of the heating itself. The resistor element Rg ^ is attached to the humidity sensor 10, formed on the surface of the sensor 10 or is a separate element which heats the medium with which the sensor 10 comes into contact. In the bridge connection of Fig. 3, R 1 T is an ambient temperature-dependent high-impedance NTC element, for example a thermistor-resistor circuit arrangement, made so that the impedance of the R T is a function of the sensor and the external temperature T in a certain selected way. The bridging resistors R .. and R ~ have a resistance of 3 I. Z

are constant in value and have a low temperature dependence, so that they can be placed outside the vicinity of the sensor 10, at a temperature different from the temperature of the medium to be measured.

According to Fig. 3, the equilibrium of the bridge R 1 is measured by detecting the differential voltage AU, which controls the operational amplifier 1C1, which in turn controls the transistor TR 1, through which a current I * 1 + + 1 is applied to the bridge.

According to Fig. 4, the resistance elements R 1, and R 8, and the humidity sensor 10 are placed at the same probe close to each other.

6 58403

According to the invention, when R is correctly a function of the outdoor temperature T and A1 the gain of the operational amplifier IC1 shown in Fig. 3 is large, the device can adjust the temperature T = f (T) of the sensor 10 so that f (T) is a desired function. According to the invention, TB f (T) is determined in such a way that the humidity sensor S1 "sees" a value which is suitably lower than the prevailing relative humidity, e.g. 0.75 x (relative humidity), as in previously known devices, but in addition the temperature dependence of the sensor can be corrected using a specific, appropriate second dependence T = f (T).

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In the invention, the currents of the branches of the bridge of Fig. 3 are arranged so that the current I2 heats the resistor Rg ^, which at the same time acts as a resistive temperature sensor measuring its own temperature, I2 always adjusting its magnitude so that the bridge equilibrium equation R ^ · R2 = R ^ 'Rg ^ . Condition 1 ^ << I2 is obtained by ensuring that the impedance (R ^, + R ^) >> - (R „+ R__).

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It is also within the scope of this invention to achieve the desired temperature dependence of R 1. It can first be stated that f (T) is most preferably a conductance and a linear function of temperature, in contrast to the previously isolated solution (Figure A), where Tfl is an almost resistance-linear function of temperature.

In order to carry out the invention, the element R 1 can be constructed from commercially available, so-called thermlinear thermistor components and appropriately selected resistors. The circuit of Figure 5 operates over a wide temperature range, with Ti and T2 being a commercially available thermistor combination. Resistors R 1 and R 2 are resistors supplied by the manufacturer of the thermlinear thermistor or the like.

R 1 and R 2 are chosen to have a resistance such that the required T = f (T) is obtained, taking into account, if desired, the temperature coefficient of the humidity sensor 10, whereby T = f (T).

The invention also includes the replacement of the temperature-dependent R 1 with a temperature-independent resistor. It is obvious that if R 1 T is constant in resistance, the bridge of Fig. 3 is in equilibrium with one of the constant values T 1 of the sensor temperature Tg, and T remains constant in T if T. > T, so that the temperature of the humidity sensor 10 in question is also constant. In this case, the sensor 10 displaying the R 1 value can be calibrated to display either the dew point or the absolute humidity within certain limits.

According to Figure 4, the probe comprises a cylindrical elongate housing 100 inside which the electronic components of the device are connected to the circuit board 102. Attached to the insulating piece 101 is a radiation shield 103, which is e.g. a plate piece with a mirror surface. On one side of the cover 103 there is a humidity sensor 10 provided with a heating resistor R as described above, and on the other side there is a thermistor arrangement T detecting the external temperature T, which includes a resistor-thermistor arrangement R._

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thermistor and T2. From the probe 100, a line 104 leaves the pointing device.

Figure 7 shows, in addition to the electronics of the control device according to the invention, electronics measuring the capacitance CM of the humidity sensor 10.

The following shows the resistance and capacitance values of the resistors and capacitors used in the circuit according to Figure 7, as well as the type designations of the other components.

Rx - 390 Ω 0χ = 10 nF

R2 = 10 ΚΩ C2 = 10 nF

R3 = 10 kfi C3 = 10 nF

R. = 390 Ω C, = 10 nF

4 4

R5 = 390 Ω C5 = 39 pF

R6 = 390 Ω Cfi = 0.8 ... 10 pF

R f = 2 k Ω, triram = 22 nF

Rg = 5700 Ω, 0.1% Cg = 10 PF, tantalum

Rq = 12000 Ω, 0.1% CQ = 1 pF, tantalum 4 - - R = 100 Ω TR3 = 2N 3227 R12 = 4698 Ω TR2 = 2N 3227 R,. = 4110 Ω TR, = 2N 3904 14 3 R15 - 47 k Ω

RgT Pt100, Heraus IC1 * LM 358 T1 x YS1 thermistor 44202 T2 =

In Figures 5 and 7, the following resistors correspond to each other R- = R_, R. = R, _, J 394 12 R5 = R8 R6 = R10 · „58403

In the implementation of Figure 7, the operating temperature range is chosen to be -5 ° C <T ^ <45 ° C. The temperature Tg of the sensor 10 is adjusted so that the humidity "seen" by the sensor is 0.75 times the humidity of the medium to be measured. In addition, the temperature dependence of the humidity sensor 10 is corrected, which is assumed to be +0.07% Rf / 1 ° C. The YSI (Yellow Springs Instrument Co., Ohio) thermilinear thermistor package 44202, which includes thermistors and T2 and the resistors R 1 = 5700 Ω and R 1 = 12,000 Ω, has been selected as the temperature-dependent part of R 1. Rg ^, is a platinum temperature sensor Pt100 (DIN 43760).

The target temperature dependence T f (T) is initially tabulated using S 8l

Tables published in the Smithsonian Meteorological Table's. Thereafter, a new corrected table T f. (T) is formed, which takes into account the temperature dependence correction of the humidity sensor S 1 3. Using the bridge equilibrium equations and the thermistor T 1 and resistance values, values are realized for R 1 and R 2 that realize the required temperature dependence T = f. (T).

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When the values 4698 ohms and 2 ohms are selected for the resistors R 1 and R 2 and when the product of R 1 and R 2 is 411037 ohms, the theoretical deviation ΔΤ according to the graph of Fig. 6 from the desired temperature T = * f (T) is obtained. SSX 3 with state T values in the range -5 ... 45 ° C. a

The following are claims within the scope of which the various details of the invention may vary and differ from those set forth above.

Claims (6)

1. A humidity sensor control device (10) whose function is based on impedance change in the sensor (10), in particular in such a capacitive humidity sensor whose insulating material is a polymer whose dielectric constant changes as a function of the humidity and under the control of which the sensor device (10) ) is heated with electrical power indirectly or directly to a temperature (T) higher than the external temperature (T) of the sensor (10) to improve the measurement accuracy of the S 3 sensor (10) and / or extend its service life and which control device comprises a bridge coupling (R Or correspondingly comprising temperature-dependent resistor elements with the aid of which said external temperature (T) and sensor temperature (T) are observed and which 3 S bridge coupling differential voltage (AU) is used as a feedback signal which regulates the electrical power that heats the sensor (10), characterized by the combination that such a heating element (R) with positive temperature coefficient (PCT) is used, e.g. a ϋ X platinum resistor, which in a known manner simultaneously acts as a sensor for the sensor temperature (T) and that as a sensor for the external temperature (T) ss, a position relative to the latter resistor element (Rg placed on the second branch of the bridge coupling such resistance-thermistor arrangement (R) that a certain function T = f (T) for each sensor type characteristic is realized by means of the control device. S 3 Control device according to claim 1, characterized in that said resistor-thermistor arrangement (R) is dimensioned to realize AI a certain function T = f (T), by using the sensor temperature S J.