Sensor output device

Abstract

To improve the measurement accuracy of output devices for oscillator-type sensors. [Solution] The sensor output device of this invention comprises an oscillating circuit unit and a specific unit, the oscillating circuit unit having a first impedance having a sensor function and a second impedance not having a sensor function, and the specific unit measures the time of half a wavelength of the output from the oscillating circuit unit.

Description

[Technical Field]

[0001] This invention relates to an improvement on an output device for sensors. [Background technology]

[0002] Sensors convert changes in physical or chemical phenomena into changes in voltage, current, and other electrical quantities. Since the changes in such sensors are weak, the sensors are equipped with a sensor output device to amplify or convert the changes in the electrical quantity in the sensor so that they can be detected by a general-purpose detection device. For example, a Wheatstone bridge circuit is known to be used as a sensor output device that amplifies the change in electrical resistance, and is attached to a sensor (such as pressure-sensitive conductive rubber) that converts a change in pressure into a change 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 constitute the oscillator circuit are used as a sensor, and the change in its resistance is output as a change in frequency. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2008-197060 [Disclosure of the Invention] [Problems that the invention aims to solve]

[0004] Patent Document 1 discloses an oscillator-type sensor output device that suggests high-precision measurement is possible by using a capacitor with an accuracy of ±0.1 to 0.2%. Here, the accuracy of the capacitor refers to the percentage of change in capacitance 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 explained as having low sensitivity to changes in the external environment. It is unclear what level of measurement accuracy is achieved with this sensor output device. In particular, when the precision of the capacitor becomes as high as that of the resistor, for example, when the precision of both is 10 -5 ~10 -6 It was unclear what the measurement accuracy of the sensor output device would be when the temperature was / ℃. [Means for solving the problem]

[0005] The inventors have been performing measurements using such an oscillating circuit type sensor output device and have noticed that the measurement accuracy (sensitivity to external environmental changes) is equal to (or the sensitivity to external environmental changes is reduced to) that of the capacitor itself. Here, "equal" means that an error is included to the extent that a person skilled in the art can understand. As shown in Figure 1, the measurement accuracy of the oscillator circuit type sensor output device is ±1500 × 10 -6 It indicated / ℃. At this time, the accuracy of the capacitor was ±60.0 × 10⁻⁶ / ℃, and the accuracy (rated) of the resistor was ±5 × 10⁻⁶ -6 It was / ℃. In Figure 1, the vertical axis (left side) represents the rate of change of frequency, and the horizontal axis represents the passage of time.

[0006] Thus, by selecting resistors and capacitors with high precision, it is possible to achieve high precision even in oscillator-type sensor output devices. In other words, high-precision output is possible with a simple circuit configuration. Since such oscillator circuits consume little power, the applications of sensor output devices are broadened. A detailed explanation follows Figure 1. The magnitude of the diurnal variation in the output of the oscillator-type sensor output device that the authors experimented with is ±1500 × 10 for a change of approximately 8°C, as shown on the vertical axis of Figure 1. -6 (In the figure, it is shown converted to a ratio of frequency change, and the unit is 10) -6 The accuracy of the capacitor in the circuit used in this experiment was ±60 × 10⁻⁶. -6 / ℃, resistance accuracy (rated) is ±5 × 10 -6 The temperature was / ℃. In Figure 1, the horizontal axis represents the passage of time. Note that the data in Figure 1 was not obtained by directly measuring the frequency, but by counting the frequency division period obtained by dividing the frequency using a 10MHz clock. To achieve this level of measurement accuracy, the waveform changes output by the sensor output device must be analyzed using a clock of 1MHz or higher.

[0007] In Figure 1, the ratio of output change of the oscillator-type sensor output device shows a large diurnal variation, indicating that the output frequency changes with temperature. Another objective of this invention is to further improve the accuracy of such an oscillator-type sensor output device. In the example shown in Figure 1, the inventors placed a reference resistor RA near the sensor resistor RB and used this reference resistor RA to construct an oscillation circuit (second oscillation circuit section). This second oscillation circuit section is identical to the first oscillation circuit section except for the resistor RB (see Figures 2 and 4). The oscillation circuit equipped with the reference resistor RA (second oscillation circuit section) is sometimes called the reference waveform output section. On the other hand, the oscillation circuit equipped with the sensor resistor RB (first oscillation circuit section) is sometimes called the measurement waveform output section. This reference waveform output section may be independent of the measurement waveform output section, which includes the resistor RB that acts as a sensor (see Figure 4), or it may be shared with the measurement section except for the resistor (see Figure 2). The latter (Figure 2) is preferable from the standpoint of bringing resistor RB and the reference resistor RA close together and reducing the number of components.

[0008] In the example of FIG. 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 by the latter. That is, the output of the latter was subtracted from the output of the former. And this was repeated. As a result, the magnitude of the corrected daily variation on the chart was ±10×10 -6 for the following amplitude. That is, the amplitude ±1500×10 -6 which is the value before the differential processing, became an amplitude ±10×10 -6 which is two or more digits smaller, indicating that the measurement accuracy has been improved. In the above, in order to improve the accuracy of correction, it is aimed to prevent a time difference from occurring between the output from the measurement waveform output unit and the output from the reference waveform output unit. Depending on the required accuracy, there may be a time difference between the two, but the time difference should be at least within 10 seconds, and more preferably within 1 second. To remove the drift generated in the electronic circuit, a smaller time difference is more effective.

[0009] The outputs from the measurement waveform output unit and the reference waveform output unit are preferably rectangular waves. This is because it is easy to identify the frequency by using the rising part and / or the falling part of the rectangular wave. The output waveform of the measurement waveform output unit counts the number of clocks included in one rectangular wave in the waveform analysis unit 10. Thereby, the time of 1 / 2 wavelength is specified, and thus the frequency is calculated. The output waveform from the reference waveform output unit is processed in the same way. In order to distinguish between the output waveform from the measurement waveform output unit and the output waveform from the reference waveform output unit, the amplitude of both and the ratio of the rectangular wave in one wavelength can be changed. To identify the frequency, a waveform of 1 / 2 wavelength or more can be used, or a frequency division circuit can be provided to lower the output frequency.

[0010] As described above, the output of the waveform analysis unit 10 becomes digital data of frequency. Therefore, the amplification of such an output can be easily performed, and the noise superimposed during transmission can be easily removed. When the output wave of the measurement waveform output unit and the output wave of the reference waveform output unit are both input to one waveform analysis unit 10, the output from the waveform analysis unit 10 is digital data simply representing the frequency, so it is impossible to determine which output wave it is from. Therefore, a signal for determining the first timing (for example, time t1) at which the output waveform from the measurement waveform output unit is input to the waveform analysis unit 10 is input from the timing generation unit 21 of the frequency correction unit (output correction unit) 20. The frequency subtraction unit 25 stores the first timing (time t1). Similarly, a signal for determining the second timing (for example, time t2) at which the output waveform from the reference waveform output unit is input to the waveform analysis unit 10 is input from the timing generation unit 21. The frequency subtraction unit 25 stores the second timing (time t2).

[0011] As a result, the positions (time series arrangements) of the output waveform from the measurement waveform output unit and the output waveform from the reference waveform output unit are determined on a common time axis. The input signal from the timing generation unit and the output signal of the measurement waveform are preferably in a form passing through an insulating device such as a photocoupler. In such a form, the oscillation circuit is less likely to be affected by external noise.

[0012] In the frequency correction unit 20, based on the frequency obtained from the output waveform from the reference waveform output unit at the second timing, the frequency based on the output waveform of the measurement waveform output unit obtained at the first timing is corrected. As a correction method, in the case of FIG. 1, the 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 subsequent second timing. Of course, the frequency obtained at the first timing may be corrected by the frequency obtained at the immediately preceding second timing.

Brief Description of the Drawings

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

[0014] The configuration of the sensor output device 1 according to the embodiment of the present invention is shown in Figure 2. This sensor output device 1 is composed of an oscillation circuit unit 5, a waveform adjustment unit 7, a waveform analysis unit 10, a frequency correction unit 20, and a specifying unit 30. The oscillation circuit unit 5 is a general-purpose oscillation circuit composed of a resistor RB as the first impedance unit, a capacitor (condenser) CA as the second impedance unit, and a comparator. The measurement resistor RB corresponds to a part of the sensor made of pressure-sensitive conductive rubber, and its resistance changes according to the pressure applied to the sensor. In this oscillation circuit unit 5, a reference resistor RA is arranged in proximity to this resistor RB. The accuracy (sensitivity to external environment changes) of this reference resistor RA with respect to temperature changes is ±5×10 -6 / °C, and its accuracy is higher than that of the resistor RB made of pressure-sensitive conductive rubber. That of the capacitor CA was ±60×10 -6 / °C. The measurement resistor RB and the reference resistor RA are switched by a switch SW1 as a switching unit.

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

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

[0017] The frequency identified by the waveform analysis unit 10 is stored as digital data in the frequency storage unit 23 of the frequency correction unit 20, which acts as an output correction unit. Since the output of the waveform analysis unit 10 is simply frequency data, it is not possible to determine from the data alone whether it originates from the measurement mode or the reference mode. Therefore, in this example, the switching timing (time t1) of 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 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 SW1 (time t2) from the frequency in the measurement mode (time t1). Then, it subtracts the frequency in the reference mode immediately after switching SW1 again (time t4) from the frequency in the measurement mode immediately after switching SW1 again (time t3). This process is repeated thereafter.

[0019] The identification unit 30 identifies the pressure change by comparing the frequency change corrected by the frequency correction unit 20 with pre-determined calibration data. 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 in many applications, as resistor RB and integrating it with the miniaturized circuit according to the present invention, the strain gauge lead wires can be shortened, enabling high-precision strain measurement. The difficulty in measurements using strain gauges is that the measurement results are disrupted by changes in the resistance value of the lead wires due to changes in ambient temperature and by the influence of external noise superimposed on the lead wires. If the length of the lead wires can be reduced to a few centimeters or less, the above difficulties can be eliminated, and high-precision measurements can be achieved. Furthermore, if the object to be 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 even higher-precision strain measurement.

[0020] In the example in Figure 2, the sensor function is handled by the resistor, but it is also possible to handle the sensor function by the capacitor (see Figure 3). In Figure 3, elements that perform the same function as in Figure 2 are given the same reference numerals, and their explanations are partially omitted. In the example shown in Figure 3, the reference capacitor CA and the measurement capacitor CB are switched by the switch SW101 in the oscillation circuit section 105.

[0021] Figure 4 shows the configuration of the sensor output device in another embodiment. In this sensor output device, the oscillation circuit section 205 for measurement mode (first oscillation circuit, measurement waveform output section) and the oscillation circuit section 305 for reference mode (second oscillation circuit, reference waveform output section) are separate components, and a waveform analysis unit 10 is attached to each of them. The timing of the outputs of the oscillation circuits 205 and 305 is controlled by timing signals input from the timing generation unit 21 to switches SW201 and SW301. This timing is then determined and used for frequency correction in the frequency correction unit.

[0022] In the example above, SW1 has two states to choose from: measurement mode and reference mode. However, it is preferable to also have a sleep mode. In this sleep mode, no power is supplied to either the reference resistor RA or the resistor RB. By providing such a sleep mode, power consumption can be reduced.

[0023] Figure 5 shows the configuration of a sensor output device in another embodiment. In this example, the oscillation circuit is configured with an inverter for digital circuits (e.g., Toshiba TC74HC04) and a NAND gate (e.g., Toshiba TC74HC00), and a three-state buffer is used for the switch SW3 that switches between measurement mode and reference mode. For example, Toshiba TC74HC126 or TC74HC125 can be used. This switch SW3 allows the user to select a reference mode, which activates the reference resistor RA, or a measurement mode, which activates the resistor RB, based on the signal from the timing generation unit 21. Furthermore, if an L-level signal from the timing generation circuit is input to the NAND gate, the gate output becomes L-level, oscillation stops, and the system enters a sleep mode, resulting in a current consumption of 20μA or less at DC5V.

[0024] For example, if a strain gauge with a resistance value of 120Ω is used as resistor RB, and the reference resistor RA is also set to 120Ω, then with a DC 5V power supply, a maximum current of 41.7mA will flow through the reference resistor RA and resistor RB. Therefore, by selecting the sleep mode, the circuit will not oscillate during unnecessary times, thus avoiding the flow of unnecessary current through the reference resistor RA and resistor RB.

[0025] Furthermore, our investigations revealed that employing a three-state buffer as switch SW3 improved measurement accuracy. In the case of the analog switch SW1 used in Figures 2-4, the ON resistance is not necessarily constant, but if a three-state buffer is used as switch SW3, the ON resistance will be constant, which is expected to improve measurement accuracy.

[0026] Figure 6 shows the configuration of a sensor output device in another embodiment. In this example, a comparator with suppressed zero drift is used, and the reference resistor RA, which performs the reference mode, is omitted. The change in resistance value obtained from the frequency change of the device was approximately ±80 ppm for a temperature change of ±5°C (see Figure 7). Figure 8 shows the difference in resistance value change. The standard deviation of the first 50 data points in Figure 8 is 1.0 × 10⁻⁶. -6 It is weak, and the rate of resistance change over a short period of time is ±1.0 × 10⁻⁶. -6 It can be seen that the accuracy is approximately as shown. In this example, even without the circuit for the reference resistor, the amplitude of the diurnal variation in frequency was approximately 160 ppm, and highly accurate measurement results were obtained. This is due to the use of a comparator that suppresses zero drift. In this example, a comparator (TS3011) from ST Microelectronics was used.

[0027] While an oscillator circuit can be constructed using a Schmitt trigger input inverter for digital circuits (for example, Toshiba's TC74HC14), the high / low signal switching threshold of an inverter is less reliable than that of a comparator. Therefore, subtle temperature changes can affect the inverter's threshold itself, potentially disrupting the measurement results. In contrast, a comparator with suppressed zero drift is considered to have a stable threshold, making it less likely for measurement results to be disrupted.

[0028] From a similar perspective, operational amplifiers or zero-drift amplifiers can be used instead of comparators. In the circuit shown in Figure 6, using low-power comparators, operational amplifiers, or zero-drift amplifiers would further reduce power consumption, and if a sleep mode is included, oscillation can be initiated only when needed, achieving even greater power savings. In the circuit shown in Figure 6, the component required for selecting the reference resistor is omitted, resulting in a smaller circuit, reduced susceptibility to external noise, and the advantage of saving space during installation.

[0029] This invention is not limited in any way to the descriptions of embodiments and examples of the invention described above. Various modifications are also included in this invention, provided they do not depart from the scope of the claims and are easily conceivable by those skilled in the art. It will be easily understood by those skilled in the art that all the circuits shown in Figures 2 to 6 can be integrated into an IC chip.

[0030] This invention is not limited in any way to the descriptions of embodiments and examples of the invention described above. Various modifications are also included in this invention, provided they do not depart from the scope of the claims and are easily conceivable by those skilled in the art. It will be easily understood by those skilled in the art that all the circuits shown in Figures 2 to 6 can be integrated into an IC chip. The following matters are disclosed below. (15) A first oscillation circuit section having a first impedance and a second impedance of a different type from the first impedance, wherein one of the first impedance and the second impedance has a sensor function. A sensor output device comprising a first identification unit that analyzes the output waveform of the first oscillation circuit unit to identify the characteristics of the sensor target, In a sensor output device, the external environment change sensitivity of the larger of the first impedance and the second impedance is equal to the external environment change sensitivity of the characteristic identified by the first specific unit, A sensor output device in which a comparator, operational amplifier, or zero-drift amplifier with suppressed zero drift is used in the first oscillation circuit section. [Explanation of Symbols]

[0031] 1. Output device for sensors 5, 105, 205, 305 Oscillation Circuit Section 7 Waveform adjustment section

Claims

1. A sensor output device comprising an oscillating circuit section and a specific section, The aforementioned oscillating circuit section has a first impedance having a sensor function and a second impedance not having a sensor function. The specified unit is a sensor output device that measures the time of half a wavelength of the output from the oscillator circuit unit.

2. The sensor output device according to claim 1, wherein the specified unit measures the time of each half wavelength of the continuous output from the oscillating circuit.

3. The half wavelength of the output from the aforementioned oscillator circuit is a square wave. The sensor output device according to claim 1 or 2, wherein the specified unit counts the number of clock cycles from the rising edge to the falling edge of the rectangular wave.

4. The sensor output device according to claim 3, wherein the clock is 1 MHz or higher.

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