Measuring apparatus

Abstract: There is disclosed a measuring apparatus comprising, a first element having its impedance varied depending on a surrounding state, a second element having a variable impedance which is connected to the element, a voltage source for supplying a potential difference across said first element and the second element, a converter for converting a voltage within a certain range into digital value corresponding to the voltage or converting a voltage at a connection point between the first element and the second element into digital value, setting unit for setting an impedance of the second element based on the digital value from the converter, and determining unit for determining the surrounding state based on the digital value from the converter.

Description: The present invention relates to a measuring apparatus for performing measurements using an element having an impedance which varies in accordance with humidity. Conventionally, it has been quite difficult to measure humidity in a wide range from high to low temperatures with high precision. This is because the output of a humidity sensor has a high-order γ characteristic as shown in Fig. 2, which makes linear approximation difficult over a wide range. It is the common practice to make use of the voltage-current characteristics of a diode for the linear conversion, but to compensate for the difference between the actual sensor characteristic and the diode characteristic, it was necessary to adjust individually a respective combination, with insufficient precision to make a precise linear conversion for a wide range of humidity. In addition, a method has been proposed in which a high-order approximate curve is obtained by the least square method, using a CPU, and a sensor output is substituted thereinto to make the linear approximation, and has been already put to practical use, but in order to make the measurement at a high precision over a wide range, higher resolution in the measurement circuit is quite required, thereby bringing about the necessity of higher bits of CPU or A/D converter, or the increase in the processing time of the CPU. However, as shown in Fig. 2, the output of sensor may vary from 1KΩ to 10MΩ or greater with respect to the relative humidity of 20 to 95%. If a measurement is tried at a low humidity of 10% or less, a high resistance of 100MΩ or greater is anticipated. To have a measurement precision for the relative humidity of 5% or less, a precision (resolution) of 0.3KΩ is necessary at 60°C and near 95%, so that a resolution of not more than 1/330000 is required in the detection circuit, A/D converter and CPU· As a result it is impossible to actually measure the humidity over such a wide range with high precision. Also, an apparatus has been proposed in which a humidity sensor having its resistance varied with the humidity and a condenser are connected in series to apply a DC voltage between the humidity sensor and the condenser, whereby the time until the condenser voltage reaches a predetermined value and the sensor characteristic are calculated by using a linear approximate expression. However, in particular, if the range in the low humidity side is expanded, a problem arises that the resistance of humidity sensor and thus the charging time will increase. For example, at 25°C and near 10%, the resistance of humidity sensor is 10GΩ or greater, whereby even if the capacity of condenser to be charged is decreased to about 30PF, the charging time as long as about 300ms is necessary. If the charging time is expanded to such a level, the humidity sensor is effectively equivalent to a state in which the DC voltage is applied, so that the humidity sensor may degrade rapidly due to the deflection of internal ions. It is a concern of the present invention to provide a measuring apparatus which can resolve the aforementioned drawbacks, has high precision over a wide range and is relatively simple and inexpensive. European Patent Specification EP-A-0,108,325 discloses a temperature sensor in which a pair of resistors can be switched to increase the range of the sensor. UK Patent Specification No. 222261 A discloses a humidity bit sensor in which the direction of current flow through the sensor is periodically reversed. In accordance with the present invention there is provided a measurement device as set out in claim 1. Other aspects and features of the present invention will be apparent from the following description and the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit configuration diagram of a humidity measuring apparatus according to an embodiment of the present invention. Fig. 2 is a graph representing the humidityresistance characteristics of a humidity sensor. Figs. 3A and 3B are views showing a substrate with the circuit packaged. Fig. 4 is a graph approximating the characteristic curve of Fig. 2 with a plurality of straight lines. Fig. 5 is a flowchart showing the flow for measuring the humidity with the humidity measuring apparatus of Fig. 1. Fig. 6 is a circuit configuration diagram of a humidity measuring apparatus according to another embodiment of the present invention. Fig. 7 are voltage waveform of various portions in the humidity measuring apparatus of Fig. 6. Fig. 8 is a graph showing the humidity-output voltage characteristic of the humidity sensor. Fig. 9 is a flowchart showing the flow for measuring the humidity with the humidity measuring apparatus of Fig. 6. Fig. 10 is a circuit configuration diagram of a humidity measuring apparatus according to another embodiment of the present invention. Fig. 11 is a circuit configuration diagram of a humidity measuring apparatus according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) Fig. 1 is a circuit configuration diagram of a humidity measuring apparatus according to an embodiment of the present invention. 1 is an oscillation circuit for providing a signal of a stable amplitude at an oscillation frequency selected as a specific frequency from 100 Hz to 10 kHz in accordance with the characteristics of a humidity sensor 2 the resistance of which varies with temperature and humidity as indicated in the output characteristic curves of Fig. 2 and is,for example, a high-molecular type ceramic sensor. R1 to R5 are reference resistors for use in making a linear approximation of the output characteristic curve of the humidity sensor 2 by dividing it into five regions as shown in Fig. 4, the resistors being connected to respective electronic switches SW1 to SW5 for switching the conductive state. C1 is a condenser for the AC coupling having such a value that the impedance of the oscillation circuit 1 at an oscillation frequency can be ignored with respect to the humidity sensor 2 or reference resistors R1 to R5. 3 is an AC amplifier for amplifying the output from the humidity sensor 2 and the reference resistors R1 to R5, 4 is a wave detection circuit for converting the AC signal amplified by the AC amplifier 3 to the DC signal, and 5 is an A/D converter for converting the DC signal converted into the DC form in the wave detection circuit 4 into digital value. 6 is a CPU for use in measuring the humidity by making the linear approximation for the output characteristic curve of the humidity sensor 2 based on the digital value input from the A/D converter 5. The CPU 6 controls the switching of the electronic switches SW1 to SW5. 7 is a thermistor (temperature sensor) the resistance of which varies with temperature, which is disposed near the humidity sensor 2. 8 is a reference power source for applying a reference voltage VR via a resistor 10 to the thermistor 7, and 9 is a DC amplifier for amplifying the output of the thermistor the DC signal amplified by the DC amplifier 9 being output to the A/D converter 5. Fig. 3A is a plan view of a substrate having the circuits including the oscillation circuit 1, the humidity sensor 2 and the AC amplifier 3 as shown in Fig. 1 packaged therein, and Fig. 3B is a crosssectional view of the substrate. On the ceramic substrate 13, there are printed the humidity sensor 2, the reference resistors R1 to R5, and the circuit electrodes 15, 17, onto which the electronic switches SW1 to SW5, the AC coupling condenser C1, and the AC amplifier 3 are soldered by the reflow method. An upper portion of the humidity sensor 2 is covered with a dust protective filter 12, and further a front surface of the ceramic substrate 13 is entirely covered with a shield plate 11 of metallic mesh. A back surface of the ceramic substrate 13 is printed with a shield electrode 14 via an insulation layer 16. With the above configuration, the voltage of the junction between the reference resistors R1 to R5 and the humidity sensor 2 which depends on the ratio of the resistance on either side of the junction is input via the AC amplifier 3, the detection circuit 4 and the A/D converter 5 into the CPU 6, which reads out the resistance corresponding to the ratio between the reference resistors R1 to R5 and the humidity sensor 2 from a table stored in the CPU 6 so as to calculate the resistance of the humidity sensor 2. On the other hand, the ratio (output of the thermistor 7) between the resistor 10 and the thermistor 7 is input via the A/D converter 5 into the CPU 6, which reads out the temperature corresponding to the resistance ratio between the resistor 10 and the thermistor 7 so as to calculate the ambient temperature of the humidity sensor 2. The CPU 6 calculates the humidity from the resistance of the humidity sensor 2 and the ambient temperature of the humidity sensor 2. Fig. 4 is a graph representing the linear approximation for the output characteristic curve of the humidity sensor 2 as shown in Fig. 2 by dividing it into five regions. D is the resistance of the humidity sensor 2, and H is the humidity corresponding to the resistance X of the humidity sensor 2. L41 is an approximate line when only the reference resistor R1 is in conduction by turning on the electronic switch SW1 only. H41 indicates the approximating range for the approximate line L41. XM41 is a resistance of the humidity sensor 2 at a humidity HM41 intermediate of the range H41. In the approximate line L41, when the temperature is T[°C] and the resistance of the humidity sensor 2 is X[Ω], the humidity H[%] can be expressed as, H = A1(T)·X + B1(T) A1(T) = C1·T + D1 and B1(T) = E1·T + F1 Where Cl, Dl, El, and Fl are line parameters for the approximate line L41 which are prestored in the CPU 6. That is, the CPU can calculate the humidity H from the resistance ratio between the resistor 10 and the thermistor 7, i.e., the ambient temperature T of the humidity sensor 2, the ratio between the reference resistor 1 and the humidity sensor 2, i.e., the resistance X of the humidity sensor 2, and the line parameters Cl, Dl, El, Fl of the approximate line L41. For the other approximate lines L42 to L45, the CPU 6 can also calculate the humidity H from similar line parameters stored therein. Referring now to a flowchart of Fig. 5, the method of calculating the humidity with the CPU 6 will be described. First, a variable n is initialized (n=0) (step S1), the variable n is added by 1 (n=1) (step S2), and the electronic switch SWn (SW1) is turned on to allow the reference resistor R1 to conduct (step S3). And the resistance Xn (Xl) of the humidity sensor 2 is calculated from the ratio between the reference resistor Rn (R1) and the humidity sensor 2 input via the A/D converter 5, and then stored (step S4). The absolute value Gn (Gl) of the difference between the resistance XMn (XM1) of the humidity sensor 2 at a humidity HMn (HM1) intermediate of the line approximate range Hn (Yl) prestored in the CPU 6 and Xn (Xl) is calculated and stored (step S5). The above-described steps S2 to S5 are repeated five times (n=2 to 5) (step S6). Thereafter, the temperature of the thermistor 7, i.e., the ambient temperature T of the humidity sensor 2 is calculated from the ratio between the resistor 10 and the thermistor 7 input from the A/D converter 5 (step S7). And the minimum n among G1 to G5 calculated at step S5 is calculated (step S8). The line parameters Cn, Dn, En, Fn corresponding to the value of n are read out from the CPU 6 to calculate An(T) and Bn(T) from the temperature T, and then the humidity H from An(T), Bn(T) and the resistance Xn (step S9), and thus the routine is terminated. In this way, the output of the measuring element having the curve characteristic can be approximated by a plurality of approximate lines, so that the humidity measurement is allowed at high precision in a wide range. (Embodiment 2) Fig. 6 is a circuit configuration diagram of a humidity measuring apparatus according to a second embodiment of the present invention. This embodiment is the circuit configuration according to the embodiment 1 as shown in Fig. 1 is simplified. Reference resistors R1 to R5, a coupling condenser Cl, and a humidity sensor 2 are identical to those as shown in Fig. 1, and the explanation thereof will be omitted. 20 is a reference power source for applying a reference voltage VR2 to the humidity sensor 2. SW6 and SW7 are electronic switches to supply a bias voltage to the humidity sensor 2 by switching alternately between the reference voltage VR2 and the ground. C2 is a coupling condenser, and SW8 is an analog switch of the CMOS type, constituting a clamp circuit for the DC restoration to input the AC output V2 of the humidity sensor 2 to an A/D converter 5. 5 is the A/D converter of the CMOS type, which outputs a digital value converted at that time to digital form to the CPU 6 if an A/D strobe 21 is input from the CPU 6. Referring now to Fig. 7 illustrating the voltage waveforms, the operation will be described. The CPU 6 generates a timing signal having a frequency close to an optimal drive frequency (e.g., 1 KHz) of the humidity sensor 2 with a hardware timer or a software timer provided within the CPU 6, and switches alternately SW6 and SW7 to conduct in accordance with the timing signal. Thereby, the reference voltage VR2 and the ground are alternately applied to the reference resistors Rl to R5 as a bias voltage. The pulse signal having an amplitude VR is applied to the reference resistors R1 to R5 as indicated by V1 in Fig. 7. The pulse signal is applied via the reference resistors R1 to R5 and the AC coupling condenser C1 to the humidity sensor 2, to which an AC signal is then applied having a maximum of +α·VR2/2, a minimum of -α·VR2/2, and an amplitude of α·VR2, as indicated by V2 in Fig. 7. It is noted that α can be expressed as follows. α = Xn/(R1 + Xn) The output of the humidity sensor 2 is DC restored by the coupling condenser C2 and the electronic switch SW8 That is, the CPU 6 turns on the SW 8 when the humidity measurement is not made, while the CPU 6 turns it on at predetermined timings during the low level period of V1 and V2 and-turns it off during other period, when the humidity measurement is made, as indicated by SW8 in Fig. 7. The A/D converter 5 is a CMOS type having a very high input impedance, so that the electric charge of C2 will not change. Therefore, to the input of the A/D converter 5, the positive pulse V3 is DC restored with the reference to 0 volt while the amplitude α·VR2 of V2 is held, as indicated by V3 in Fig. 7. The CPU 6 supplies an A/D strobe 21 to the A/D converter 5 in synchronism with a positive peak period of the positive pulse V3 output from the coupling condenser C2 to convert a peak value of this positive pulse V3 into digital value which is then read by the CPU 6. In this way, the resistance X of the humidity sensor 2 can be calculated based on the digital value of the output of the humidity sensor 2. And the CPU 6 calculates the ambient temperature T of the humidity sensor 2 based on the output of the thermistor 7, and calculates the humidity H from the resistance X, the temperature T and the line parameters as described in the embodiment 1. As above described, it is possible to provide a simplified humidity measuring apparatus by eliminating the oscillation circuit 1, the AC amplifier 3, the wave detection circuit 4 which were required in the embodiment 1. Since the reference voltage VR2 and the ground are switched to be applied to the reference resistors R1 to R5, instead of using the oscillation circuit 1 with the output amplitude stabilized, to raise the measurement precision, it is possible to apply the signal having the stable amplitude. Also, there is no necessity of adjusting the amplitude at the input portion of the A/D converter 5, and thus suppressing the dispersion factor by making each circuit highly precise. Also, the signal processing at high precision such as the oscillation circuit 1 is eliminated, so that the cost reduction is realized. (Embodiment 3) While the aforementioned embodiment 1 and embodiment 2 were such that the CPU 6 calculates the resistance of humidity sensor 2 from the output voltage of humidity sensor 2 and calculates the humidity based on the relation curve between the resistance and the humidity of humidity sensor 2, this embodiment is such that the CPU 6 calculates the humidity based on the relation curve between the output voltage and the humidity of humidity sensor 2 by inputting the output voltage V3 of humidity sensor 2 in the same circuit configuration (Fig. 6) as that of the humidity measuring apparatus in the embodiment 2. Fig. 8 is a graph representing the relation between the output voltage V3 and the humidity of humidity sensor 2. Dotted lines C81 to C85 in Fig. 8 are curves when reference resistors R1 to R5 are allowed to conduct by turning on electronic switches SW1 to SW5, respectively. Solid lines L81 to L85 superposed on the curves C81 to C85 are approximate lines. An approximate line L81 can be expressed by V3 = I1·H + J1 using parameters I1, J1. The approximate lines L82 to L85 can be expressed likewise using parameters I2 to I5 and J2 to J5. The CPU 6 prestores the parameters I1 to I5 and J1 to J5, and can calculate the humidity H by inputting the output voltage V3 of humidity sensor 2. As indicated within the dashed lines in Fig. 8, it is possible to calculate the humidity ranging from 15% to 100% by switching the reference resistors R1 to R5. Fig. 9 is a flowchart for calculating the humidity with the CPU 6. First, a variable n is 1 (step S11), a switch Sn is turned on to allow a reference resistor Rn to conduct, and the output voltage V3 of the humidity sensor 2 is input (step S12). And when the output voltage V3 is in a range from 0.25 to 0.8 (step S13), the parameters In, Jn of the approximate line for the reference resistor Rn are read out from the CPU 6 (step S14), and the humidity H is calculated (step S15). Also, at step S13, when the output voltage V3 is greater than or equal to 0.8, the content of the variable n is incremented by 1 (step S16), and if the content of variable n is smaller than 6, the routine proceeds to step S12 (step S17). At step S17, if the content of variable n is greater than or equal to 6, the humidity is judged to be 15% or less (step S18), and the humidity calculation process is terminated. It should be noted that if the number of reference resistors as shown in Fig. 6 is increased, more precise humidity measurements for the humidities of 15% or less is permitted as shown in Fig. 8. (Embodiment 4) Fig. 10 is a circuit configuration diagram of a humidity measuring apparatus according to a fourth embodiment of the present invention. In the previous third embodiment, if the value of the humidity sensor 2 is 10 MΩ or greater, a measurement error may occur due to floating capacities present in the switches SW1 to SW5, a floating capacity C5 present in the humidity sensor 2, or floating capacities present in other circuits. This embodiment is to reduce such a measurement error caused by those floating capacities. A humidity sensor 2, an A/D converter 5, a CPU 6, a reference power source 20, an A/D strobe 21, switches SW1 to SW8, and a coupling condenser C1 are the same as those of Fig. 6, and the explanation will be omitted. A resistor R10 is a reference resistor for enabling the measurement for a humidity of 10% to 15%, and SW10 is a switch for switching for conductive state of the reference resistor R10. C3 is a coupling condenser having a smaller capacity than the coupling condenser C1, and connected in series to the reference resistor R10. One end of the series circuit or one end of the condenser C3 is connected to a contact point of the condenser C1, the condenser C2, and the humidity sensor 2, and the other end of the series circuit or one end of the reference resistor R10 is connected to a contact point of SW6, SW7, and the reference resistors R1 to R5. A switch SW9 and a condenser C4 are connected in parallel, with its parallel circuit inserted in a connection of the condenser C2 and the switch SW3. A connection point between the switch SW3, the switch SW4 and the condenser C4 is connected to the A/D converter 5. It is noted that the values of C1, C2, C3, and C4 satisfy the relation, C2/C1 ≃ (1/(1/C2 + 1/C4))/C3 The flow of calculating the humidity with the CPU 6 will be described below. The CPU 6 turns on the switch SW9, and make the same control as that of the embodiment 2 shown in Fig. 9. If it is found at step 8 that the voltage for permitting the linear approximation can not be obtained by the selection of reference resistors R1 to R5, the CPU 6 turns off the switches SW1 to SW5, turns on the switch SW10, and turns off the switch SW8 And it makes smaller the frequency of switching the switch SW6 and SW7. For example, the frequency is switched from 1 kHz to 50 Hz. And the CPU starts the clamp operation to read the value of V2 with the A/D conversion in the same way as in the embodiment 3. The frequency is reduced in inverse proportion to the time constant in the product of C1 and a resistor of humidity sensor. For example, when C1 is 0.3 µF, and the impedance of the humidity sensor 2 is 60 MΩ, the frequency is 50 Hz, and when C1 is 0.3 µF and the impedance of the humidity sensor 2 is 6 kΩ, the frequency is 500 Hz. The clamp timing for use is a timing as shown in Fig. 7, and when the drive frequency of VR2 is changed, the timing is adjusted in proportion to it, so that the same clamp operation as in Fig. 7 is conducted. If the value of the humidity sensor 2 is 10 MΩ or greater, the approximation calculation is not allowed by a method of the embodiment 3 due to the existence of floating capacities present in the humidity sensor 2 and other circuits, but in this embodiment, when the resistance of the humidity sensor 2 is larger, the values of coupling condensers C3, C2, C4 and the drive frequency of a reference power source are changed in consideration of such floating capacities, so that a measurement error caused by such floating capacities can be reduced to make a correct measurement. Because the value of the condenser C3 is decreased, the time during which the condenser C3 gets to a stationary state in the clamp operation can be shortened, resulting in faster measurement. (Embodiment 5) Fig. 11 is a circuit configuration diagram of a humidity measuring apparatus according to a fifth embodiment of the present invention. A humidity sensor 2, an A/D converter 5, a CPU 6, a reference power source 20, an A/D strobe 21, switches SW1 to SW8, and a coupling condenser C1 are the same as those of Fig. 6, and the explanation will be omitted. SW10 is a switch connected in parallel to a reference resistor R1 and a switch SW1. It is considered that this circuit is one in which a reference resistor having a resistance of 0Ω is connected in parallel to the reference resistors R1 to R5. The flow of calculating the humidity with the CPU 6 will be described below. The CPU 6 makes the same control as that of the embodiment 2 shown in Fig. 9. If it is found at step 8 that the voltage for permitting the linear approximation can not be obtained by the selection of the reference resistors R1 to R5, the CPU 6 turns off the switches SW1 to SW5, and turns on the switch SW10. And the CPU 6 calculates the humidity by inputting the output voltage V3 of the humidity sensor 2 via the A/D converter 5. In this way, when the humidity is low, or when the resistance of the humidity sensor 2 is high, the reference resistance is set at 0Ω by the switch SW10, so that the influence of the floating capacity present in the condenser C1 on the side of the reference resistors R1 to R5 can be reduced. Thereby, the output V2 of the humidity sensor 2 is unlikely to contain the noise, and the clamp output voltage V3 can be stabilized. Therefore, a stabler humidity measurement is allowed. The present invention is not limited to a humidity sensor, but is applicable to a thermistor having its resistance varied with the temperature, or a photoconductive cell having its resistance varied with the light quantity. It is also applicable to an element having its capacitance or inductance varied depending on the surrounding environment, for example, a microphone having the inductance varied with the sound volume. Or it may be constituted to display the measured value by providing a display unit, or to control an equipment, based on the value measured by connecting to the equipment.

Claims: A humidity measuring apparatus comprising: a first element (2) having an impedance which varies in a non-linear curve in accordance with the value of ambient humidity; a second element (R1-R5) connected to said first element and having a variable impedance; means (7) for applying a potential difference across said first and second elements; a converter (5) for converting the voltage at the junction between the two elements into a digital value; setting means (6, SW1-SW5) for setting the impedance of said second element; and calculating means (6) for determining the ambient humidity from the output of said converter, including a detecting means (7) for detecting the ambient temperature of said first element (2) and wherein said calculating means (6) stores a plurality of parameters used for performing a plurality of linear approximations each corresponding to a section of said non-linear curve, and being adapted to select parameters in accordance with the impedance of said second element (2) as set by said setting means, said calculating means in operation performing a linear approximation on the output of said converter using the detected ambient temperature for obtaining a corrected value of the selected parameters. Apparatus according to claim 1, wherein the calculating means is adapted to make humidity measurements using the range of impedances available from said second element, and thereafter to select one of said impedances in accordance with the output of said temperature detecting means (7). Apparatus according to claim 2, wherein said setting means is adapted to set said second element to one of a plurality of fixed resistances, and said calculating means is adapted to calculate the value of the ambient condition if said digital value generated using the sensor element and said one fixed resistance lies within a predetermined range, and if the digital value does not lie within said predetermined range to cause said setting means to set said second element to a second value and repeat the process until a final value for the ambient condition can be determined by said calculating means. An apparatus according to any preceding claim, wherein said voltage source is applied intermittently.

Inventors: HATA SHIGEO, INTOH JUNICHI, SEKIYA TOSHIYUKI, SUZUKI KOJI, HATA, SHIGEO, INTOH, JUNICHI, SEKIYA, TOSHIYUKI, SUZUKI, KOJI, Hata, Shigeo, c/o Canon Kabushiki Kaisha, Intoh, Junichi, c/o Canon Kabushiki Kaisha, Sekiya, Toshiyuki, c/o Canon Kabushiki Kaisha, Suzuki, Koji, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: G01N27
IPC codes: G01R15, G01N27