Sensor output device

Abstract

This output device for an oscillation circuit-type sensor improves measurement precision.　This output device for a sensor comprises a first identification unit that analyzes an output waveform of a first oscillation circuit section and identifies a characteristic of a sensor target, the first oscillation circuit section being provided with a first impedance and a second impedance of a different type than the first impedance, and the first impedance or the second impedance having a sensor function. The greater external environmental change sensitivity among the external environmental change sensitivity of the first impedance and the external environmental change sensitivity of the second impedance, and the external environmental change sensitivity of the characteristic identified by the first identification unit are roughly equivalent.

Description

[Technical Field]

[0001] The present invention relates to improvements in output devices for sensors. [Background technology]

[0002] Sensors convert changes in physical or chemical phenomena into changes in voltage, current, or other electrical quantities. Since such changes in the sensor are weak, a sensor output device is attached to the sensor to amplify or convert the change in the electrical quantity in the sensor so that it can be detected by a general-purpose detection device. For example, a Wheatstone bridge circuit is known as an output device for a sensor that amplifies changes in electrical resistance by being attached to a sensor (such as pressure-sensitive conductive rubber) that converts changes in pressure into changes in electrical resistance. Patent Document 1 discloses an output device that uses an oscillator circuit as an output device that responds to changes in electrical resistance (hereinafter sometimes simply referred to as "resistance"). In this sensor output device, the resistor (R) and capacitor (C) that make up the oscillator circuit act as a sensor, and output changes in the resistance as changes in frequency. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-197060 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0004] The oscillator circuit type sensor output device disclosed in Patent Document 1 suggests that highly accurate measurements are possible by using a capacitor with an accuracy of ±0.1 to ±0.2%. Here, the accuracy of the capacitor refers to the rate at which the capacitance changes in response to changes in the external environment, such as temperature. In other words, a capacitor with high accuracy has a small change in capacitance (sensitivity) in response to changes in the external environment. In this specification, this is sometimes referred to as having a small sensitivity to changes in the external environment. It is unclear what level of measurement accuracy can be achieved with this sensor output device. In particular, when the accuracy of a capacitor becomes almost as high as that of a resistor, for example, the accuracy of both becomes 10 -5 ~10 -6 / °C, it was unclear what the measurement accuracy of the sensor output device would be. [Means for solving the problem]

[0005] The inventors have conducted measurements using such an oscillator circuit type sensor output device and have noticed that the measurement accuracy (sensitivity to changes in the external environment) is equal to the accuracy (sensitivity to changes in the external environment) of the capacitor itself (the sensitivity to changes in the external environment is reduced). Here, "equal" includes an error within the range that can be understood by a person skilled in the art. As shown in Figure 1, the measurement accuracy of the oscillator circuit type sensor output device is ±1500×10 -6 / ℃. The accuracy of the capacitor in this case is ±60.0×10-6 / ℃, and the accuracy (rated) of the resistor is ±5×10 -6 / ℃. In FIG. 1, the vertical axis (left side) represents the rate of change of frequency, and the horizontal axis represents the passage of time.

[0006] As described above, by selecting resistors and capacitors with high accuracy, it is clear that oscillator circuit-type sensor output devices can also achieve high accuracy. In other words, a highly accurate output is possible with a simple circuit configuration. Such oscillator circuits also consume little power, which expands the range of uses for sensor output devices. A detailed explanation will be given with reference to Figure 1. The magnitude of the diurnal change in the output of the oscillator circuit type sensor output device that the authors conducted an experiment on was ±1500 × 10 for a change of approximately 8°C, as shown on the vertical axis of Figure 1. -6 (The figure shows the ratio of the frequency change, in units of 10 -6 The accuracy of the capacitor in this experiment was ±60×10 -6 / ℃, resistance accuracy (rated) is ±5×10 -6 / °C. In Figure 1, the horizontal axis indicates the passage of time. The data in Figure 1 was not obtained by directly measuring the frequency, but by counting the division period obtained by dividing the frequency using a 10 MHz clock. To achieve this level of measurement accuracy, changes in the waveform output by the sensor output device must be analyzed using a clock of 1 MHz or higher.

[0007] In Figure 1, the output change rate of the oscillator circuit type sensor output device changes significantly with the circadian rhythm, indicating that the output frequency changes due to the influence of temperature. Another object of the present invention is to further improve the accuracy of such an oscillator circuit type sensor output device. In the example of Figure 1, the inventors placed a reference resistor RA near the resistor RB, which acts as a sensor, and configured an oscillator circuit (second oscillator circuit section) using this reference resistor RA. This second oscillator circuit section is identical to the first oscillator circuit section except for the resistor RB (see Figures 2 and 4). The oscillator circuit (second oscillator circuit section) equipped with the reference resistor RA is sometimes referred to as a reference waveform output section. On the other hand, the oscillator circuit (first oscillator circuit section) equipped with the resistor RB, which acts as a sensor, is sometimes referred to as a measurement waveform output section. This reference waveform output section may be independent of the measurement waveform output section that includes the resistor RB as a sensor (see Figure 4), or may be shared with the measurement waveform output section except for the resistor portion (see Figure 2). The latter (Figure 2) is preferable from the viewpoint of locating the resistor RB and the reference resistor RA close to each other and reducing the number of parts.

[0008] In the example of Figure 1, the output from the measurement waveform output unit and the output from the reference waveform output unit were switched every 0.45 seconds, and the former was corrected with the latter. In other words, the latter output was subtracted from the former output. This process was then repeated. As a result, the magnitude of the diurnal change after correction on the chart was ±10 × 10 for a change of about 8°C. -6 The amplitude was ±1500 × 10, which is the amplitude before the differential processing. -6 is two orders of magnitude smaller than ±10×10 -6 This means that the measurement accuracy is improved. In the above, the aim is to improve the accuracy of correction by eliminating the time difference between the output from the measurement waveform output unit and the output from the reference waveform output unit. Depending on the required accuracy, a time difference between the two may be acceptable, but the time difference should be at least 10 seconds or less, and more preferably 1 second or less. To eliminate drift that occurs in electronic circuits, the smaller the time difference, the more effective it is.

[0009] It is preferable that the output from the measurement waveform output section and the reference waveform output section be a square wave, because it is easy to identify the frequency by using the rising and / or falling portions of the square wave. The waveform analysis unit 10 counts the number of clocks contained in one square wave of the output waveform of the measurement waveform output unit, thereby specifying the time of a half wavelength and thereby calculating the frequency. The output waveform from the reference waveform output section is processed in the same way. To clearly distinguish between the output waveform from the measurement waveform output section and the output waveform from the reference waveform output section, the amplitude of both and the ratio of the square wave to one wavelength can be changed. To specify the frequency, a waveform of 1 / 2 wavelength or more can be used, or a frequency divider circuit can be provided to lower the output frequency.

[0010] As described above, the output of the waveform analyzer 10 is digital data with a frequency, so that the output can be easily amplified and noise superimposed during transmission can be easily removed. When both the output wave from the measurement waveform output unit and the output wave from the reference waveform output unit are input to one waveform analysis unit 10, the output from the waveform analysis unit 10 is simply digital data representing the frequency, so it is not possible to determine which output wave it is from. Therefore, a signal that determines the first timing (e.g., time t1) at which the output waveform from the measurement waveform output unit is input to waveform analysis unit 10 is input from timing generation unit 21 of frequency correction unit (output correction unit) 20. Frequency subtraction unit 25 stores this first timing (time t1). Similarly, a signal that determines the second timing (e.g., time t2) at which the output waveform from the reference waveform output unit is input to waveform analysis unit 10 is input from timing generation unit 21. Frequency subtraction unit 25 stores this second timing (time t2).

[0011] This determines the position (time series arrangement) of the output waveform from the measurement waveform output unit and the position (time series arrangement) of the output waveform from the reference waveform output unit on a common time axis. The input signal from the timing generator and the output signal of the measurement waveform are preferably transmitted via an insulating device such as a photocoupler, as this makes the oscillator circuit less susceptible to external noise.

[0012] Frequency correction unit 20 corrects the frequency based on the output waveform from the measurement waveform output unit obtained at the first timing, using as a reference the frequency based on the output waveform from the reference waveform output unit obtained from the second timing. In the case of Figure 1, the correction method is as follows: frequency subtraction unit 25 subtracts the frequency based on the output waveform from the reference waveform output unit from the frequency based on the output waveform from the measurement waveform output unit. In the example of Fig. 1, the frequency obtained at the first timing is corrected by the frequency obtained at the immediately following second timing. Of course, the frequency obtained at the first timing may also be corrected by the frequency obtained at the immediately preceding second timing. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a chart showing an output waveform of a sensor output device according to the present invention. [Figure 2] FIG. 2 is a circuit diagram showing the configuration of a sensor output device according to an embodiment of the present invention. [Figure 3] FIG. 3 is a circuit diagram showing the configuration of a sensor output device according to another embodiment of the present invention. [Figure 4] FIG. 4 is a circuit diagram showing the configuration of a sensor output device according to another embodiment of the present invention. [Figure 5] FIG. 5 is a circuit diagram showing the configuration of a sensor output device according to another embodiment of the present invention. [Figure 6] FIG. 6 is a circuit diagram showing the configuration of a sensor output device according to another embodiment of the present invention. [Figure 7] FIG. 7 is a chart showing the output of the sensor output device of FIG. [Figure 8] FIG. 8 is a chart showing the difference in the output of the sensor output device of FIG.

[0014] FIG. 2 shows the configuration of a sensor output device 1 according to an embodiment of the present invention. The sensor output device 1 comprises an oscillation circuit section 5, a waveform adjustment section 7, a waveform analysis section 10, a frequency correction section 20, and an identification section 30. The oscillator circuit unit 5 is a general-purpose oscillator circuit consisting of a resistor RB as a first impedance unit, a capacitor CA as a second impedance unit, and a comparator. The measurement resistor RB corresponds to part of a sensor made of pressure-sensitive conductive rubber, and its resistance changes depending on the pressure applied to the sensor. In this oscillator circuit section 5, a reference resistor RA is placed close to the resistor RB. The accuracy of the reference resistor RA with respect to temperature changes (sensitivity to external environment changes) is ±5×10 -6 / ℃, and its accuracy is higher than that of the resistor RB made of pressure-sensitive conductive rubber. The accuracy of the capacitor CA is ±60×10 -6 / ° C. The resistance RB for measurement and the reference resistance RA are switched by a switch SW1 serving as a switching unit.

[0015] The state of the oscillation circuit section 5 in which the resistor RB is selected is the measurement mode, and the state of the oscillation circuit section 5 in which the reference resistor RA is selected is the reference mode. In this example, an RC circuit is used as the oscillator circuit, but a person skilled in the art can easily imagine that an oscillator circuit can be constructed by combining inverters and NAND gates as shown in Figure 5, or by selecting at least two of the impedance elements consisting of a resistor, a capacitor, and a coil.

[0016] The waveform adjustment unit 7 changes the waveforms (square waves) of the measurement mode output and the reference mode output. In this example, the amplitude of each output waveform is changed by switching the switch SW2 in synchronization with the switch SW1. The waveform analysis unit 10 determines the frequency of the square wave. That is, it counts the number of clocks from the rising edge to the falling edge of the square wave. A clock of 1 MHz or higher is used. The time of the square wave (i.e., 1 / 2 wavelength) is determined from the number of clocks, so the frequency can be calculated. Of course, it is also possible to count the number of clocks contained in multiple consecutive square waves.

[0017] The frequency as digital data identified by the waveform analysis unit 10 is stored in a frequency storage unit 23 of a frequency correction unit 20 serving as an output correction unit. Since the output of the waveform analysis unit 10 is simply data related to frequency, it is not possible to determine from the data alone whether the data is derived from the measurement mode or the reference mode. Therefore, in this example, the switching timing (time t1) of the switch SW1 is controlled by a signal from the timing generation unit 21. The timing (time t1) at which the switching signal is output from the timing generation unit 21 is sent to the frequency storage unit 23 and stored in association with the data sent from the waveform analysis unit 10. As a result, the frequency-related data stored in the frequency storage unit 23 is linked to the time at which it was obtained.

[0018] The frequency subtraction unit 25 subtracts the frequency in the reference mode immediately after switching the switch SW1 (time t2) from the frequency in the measurement mode (time t1). Then, it subtracts the frequency in the reference mode immediately after switching the switch SW1 again (time t4) from the frequency in the measurement mode immediately after switching the switch SW1 (time t3). This process is repeated thereafter.

[0019] The identification unit 30 identifies a pressure change by comparing the change in frequency corrected by the frequency correction unit 20 with the calibration data obtained in advance. The identified pressure change, i.e., the change in the characteristics of the sensor target, is presented to the observer via a monitor (not shown). The circuit in Figure 2 has a small number of components and can be miniaturized. By using a strain gauge, which is widely used, as the resistor RB and integrating it with the miniaturized circuit of this invention, the strain gauge's lead wires can be shortened, enabling highly accurate strain measurements. A difficulty with measurements using strain gauges is that measurement results can be distorted by changes in the resistance of the lead wires due to changes in ambient temperature and by external noise superimposed on the lead wires. If the lead wire length can be reduced to a few centimeters or less, these difficulties can be overcome, enabling highly accurate measurements. Furthermore, if the object being measured is a conductor, the entire circuit integrated with the strain gauge, including the lead wires, can be covered with a conductor, eliminating the influence of external noise and enabling more accurate strain measurements.

[0020] In the example of Figure 2, the sensor function is performed on the resistor side, but in some cases the sensor function is performed on the capacitor side (see Figure 3). In FIG. 3, elements having the same functions as those in FIG. 2 are given the same reference numerals, and their description will be partially omitted. In the example of FIG. 3, in the oscillation circuit section 105, a reference capacitor CA and a measurement capacitor CB are switched by a switch SW101.

[0021] The configuration of a sensor output device according to another embodiment is shown in FIG. In this sensor output device, an oscillation circuit section 205 (first oscillation circuit, measurement waveform output section) for the measurement mode and an oscillation circuit section 305 (second oscillation circuit, reference waveform output section) for the reference mode are separate entities, and a waveform analysis section 10 is attached to each of them. The output timing of the oscillation circuit sections 205 and 305 is controlled by a timing signal input to the switches SW201 and SW301 from the timing generation section 21. The time is identified and used for frequency correction in the frequency correction section.

[0022] In the above example, SW1 selects between two states: measurement mode and reference mode. However, it is preferable to also provide a sleep mode. In this sleep mode, power is not supplied to either the reference resistor RA or the resistor RB. Providing such a sleep mode can reduce power consumption.

[0023] Figure 5 shows the configuration of a sensor output device according to another embodiment. In this example, an oscillator circuit is configured with a digital circuit inverter (e.g., Toshiba's TC74HC04) and a NAND gate (e.g., Toshiba's TC74HC00), and a three-state buffer is used for switch SW3, which switches between measurement mode and reference mode. For example, Toshiba's TC74HC126 or TC74HC125 can be used. The switch SW3 allows selection between a reference mode in which the reference resistor RA is activated and a measurement mode in which the resistor RB is activated based on a signal from the timing generator 21. Furthermore, when an L-level signal from the timing generator is input to the NAND gate, the output of the gate goes L, stopping oscillation and entering sleep mode, reducing the current consumption at DC 5V to 20 μA or less.

[0024] For example, when a strain gauge with a resistance value of 120 Ω is used as resistor RB, the resistance value of reference resistor RA is also 120 Ω, and the power supply is DC 5 V, a maximum current of 41.7 mA flows through reference resistor RA and resistor RB. Therefore, by selecting sleep mode, the circuit will not oscillate when it is not needed, and unnecessary current will not flow through reference resistor RA and resistor RB.

[0025] Furthermore, according to the study by the present inventors, the measurement accuracy was improved by using a three-state buffer as the switch SW3. In the case of the analog switch SW1 used in Figures 2 to 4, the ON resistance is not necessarily a constant value, but if a three-state buffer is used as switch SW3, the ON resistance will be constant, which is thought to improve measurement accuracy.

[0026] FIG. 6 shows the configuration of a sensor output device according to another embodiment. In this example, a comparator with suppressed zero drift is used and the reference resistor RA is omitted to implement the reference mode. The change in resistance value calculated from the frequency change of this device was about ±80 ppm for a temperature change of ±5°C (see Figure 7). Figure 8 shows the difference in the resistance change. The standard deviation of the 50 data points at the beginning of Figure 8 is 1.0 × 10 -6 The short-term resistance change rate is ±1.0×10 -6 In this example, even without the reference resistor circuit, the amplitude of the diurnal frequency change was about 160 ppm, and highly accurate measurement results were obtained. This is due to the use of a comparator with suppressed zero drift. In this example, an STMicroelectronics comparator (TS3011) was used.

[0027] An oscillator circuit can also be constructed using a Schmitt trigger input inverter for digital circuits (for example, Toshiba's TC74HC14), but because the threshold for switching between high and low signals in an inverter is less certain than in a comparator, slight temperature changes can affect the inverter's threshold itself, potentially disrupting the measurement results.In contrast, a comparator with suppressed zero drift has a stable threshold and is thought to be less likely to disrupt the measurement results.

[0028] From a similar perspective, an operational amplifier or zero-drift amplifier can be used instead of the comparator. In the circuit of Figure 6, using a low-power comparator, operational amplifier, or zero-drift amplifier will achieve even greater power savings, and if the circuit has a sleep mode, it can oscillate only when necessary, achieving even greater power savings. In the circuit of Figure 6, the components required for selecting the reference resistance are omitted, which has the advantage that the circuit can be made smaller, is less susceptible to the effects of external noise, and requires less space when incorporating the circuit.

[0029] The present invention is not limited to the above-described embodiments and examples. Various modifications within the scope of the claims and within the scope of those skilled in the art are also included in the present invention. Those skilled in the art will readily understand that the circuits shown in Figures 2 to 6 can all be integrated into an IC chip. The following items are disclosed below. (15) a first oscillation circuit unit including a first impedance and a second impedance of a type different from the first impedance, one of the first impedance and the second impedance having a sensor function; a first specifying unit that analyzes an output waveform of the first oscillation circuit unit and specifies a characteristic of a sensor target, In an output device for a sensor, the sensitivity of a larger one of the sensitivity of a first impedance to a change in the external environment and the sensitivity of a second impedance to a change in the external environment is equal to the sensitivity of a characteristic identified by the first identifying unit, The output device for a sensor, wherein a comparator, an operational amplifier, or a zero-drift amplifier in which zero drift is suppressed is used in the first oscillation circuit section. [Explanation of symbols]

[0030] 1. Sensor output device 5, 105, 205, 305 Oscillator circuit section 7 Waveform adjustment section

Claims

1. An output device for a sensor comprising a first oscillation circuit unit and a first identification unit, The first oscillation circuit unit includes: a first impedance having a sensor function, a 1-1 impedance of the same type as the first impedance having a sensor function but having a lower sensitivity to changes in the external environment than the first impedance having a sensor function, a first switching unit that switches between the first impedance having a sensor function and the 1-1 impedance, and a second impedance of a different type from the first impedance having a sensor function but not having a sensor function; a second impedance having a sensor function, a second-1 impedance of the same type as the second impedance having a sensor function but having a lower sensitivity to changes in the external environment than the second impedance having a sensor function, a second switching unit that switches between the second impedance having a sensor function and the second-1 impedance, and a first impedance of a different type from the second impedance having a sensor function but not having a sensor function; the first specifying unit analyzes the output waveform of the first oscillator circuit unit to specify a characteristic of a sensor target; a first impedance having the sensor function is selected by the first switching unit, or a second impedance having the sensor function is selected by the second switching unit, thereby switching to a measurement mode; The sensor output device is switched to the reference mode by the first switching unit selecting the 1-1 impedance or the second switching unit selecting the 2-2 impedance.

2. 2. The sensor output device according to claim 1, wherein the first specifying unit analyzes the output waveform of the first oscillator circuit unit using a clock of 1 MHz or higher.

3. 3. The sensor output device according to claim 1, wherein the first impedance having a sensor function, the first-1 impedance, and the first impedance are resistors, and the second impedance having a sensor function, the second-1 impedance, and the second impedance are capacitors.

4. 4. The sensor output device according to claim 1, wherein the first oscillation circuit section uses a comparator, an operational amplifier, or a zero-drift amplifier in which zero drift is suppressed.

5. 5. The sensor output device according to claim 1, further comprising a timing control section that suspends operation of said first oscillation circuit section.

6. 6. The sensor output device according to claim 1, further comprising an output correction unit that analyzes the output in the reference mode and corrects the output in the measurement mode.

7. 7. The sensor output device according to claim 1, further comprising a waveform adjustment unit that changes the patterns of the output waveforms in the measurement mode and the reference mode.

8. 8. The sensor output device according to claim 1, further comprising a timing generating unit that controls the first switching unit or the second switching unit to specify a time series arrangement of the output of the measurement mode and the output of the reference mode.

9. 9. The sensor output device according to claim 8, wherein the timing generating section outputs the measurement mode output and the reference mode output alternately and continuously.

10. An output device for a sensor comprising a first oscillation circuit unit, a second oscillation circuit unit, a first specifying unit, a second specifying unit, and an output correction unit, the first oscillation circuit unit includes a first impedance having a sensor function and a second impedance of a different type from the first impedance having the sensor function and not having the sensor function; the first specifying unit analyzes the output waveform of the first oscillator circuit unit to specify a characteristic of a sensor target; the second oscillation circuit unit includes a first-1 impedance and the second impedance, the first impedance being the same type as the first impedance having a sensor function and having a lower sensitivity to changes in an external environment than the first impedance having a sensor function; the second identification unit analyzes the output waveform of the second oscillation circuit unit to identify the characteristics of the sensor target; The output correction unit sets the output of the first oscillation circuit unit to a measurement mode and the output of the second oscillation circuit unit to a reference mode, and corrects the output of the measurement mode based on the output of the reference mode.

11. 11. The sensor output device according to claim 10, further comprising a timing generator that specifies a time series arrangement of the measurement mode output and the reference mode output.

12. 12. The sensor output device according to claim 11, wherein the timing generating section outputs the measurement mode output and the reference mode output alternately and continuously.

13. 2. The sensor output device according to claim 1, further comprising a timing generating unit that controls the first switching unit or the second switching unit to select the measurement mode, the reference mode, or the pause mode.

14. 2. The sensor output device according to claim 1, wherein the first switching section and the second switching section are made up of three-state buffers.

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