JPH0522163B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a displacement measuring device that measures physical or mechanical displacement by converting it into a change in capacitance, and particularly relates to a measuring device that converts a detected amount into a digital amount and measures it using a digital processing device such as a microprocessor. It is something.

[Conventional technology]

Generally, when detecting capacitance, there is a drawback that errors occur in the detection results due to the influence of the dielectric constant or stray capacitance between the electrodes. Therefore, capacitance detection methods that are not affected by the above-mentioned effects have already been proposed. FIG. 1 is a principle diagram for explaining the principle of such a detection method. Figure A shows a movable electrode between two fixed electrodes EL _F.
A movable electrode _ELV is arranged, and the movable electrode _ELV moves to the left or right (see arrow R) in the figure in response to displacement of a physical quantity such as pressure or a mechanical quantity. In this case, when one of the capacitances CA ₁ and CA ₂ between the electrodes increases, the other decreases, that is, they change differentially. Here, the area of each electrode is S, the dielectric constant between the electrodes is ε, and the distance between the movable electrode EL _V and the fixed electrode EL _F is d. For example, as shown by the dotted line in Figure A, the movable electrode EL _V is Δd. The capacitances CA ₁ and CA ₂ when the capacitance is displaced by the same amount are calculated as CA ₁ =εA/(d−Δd) CA ₂ =εA/(d+Δd). Now, considering the sum and difference of these capacitances, CA ₁ + CA ₂ = εA・2d/(d ² − (Δd) ² ) CA _{1 −} CA ₂ = εA・2Δd/(d ² − (Δd) ² ), and therefore, by taking the ratio, (CA ₁ − CA ₂ )/(CA ₁ + CA ₂ )=Δd/d is obtained, and the displacement Δd is expressed as the capacitance value (CA ₁ − CA ₂ )/
It can be obtained by (CA ₁ + CA ₂ ). Similarly, in FIG. 1B, two fixed electrodes EL _F
On the other hand, if the movable electrode _ELV is arranged as shown in the figure and is displaced by Δd to the dotted line position in the figure due to displacement due to external pressure, etc., the following will occur. In this case, the capacity
CA ₁ is fixed and CA ₂ is variable, and its value can be expressed as CA ₁ −εA/d, CA ₂ =εA/(d+Δd) in the same way as above. Therefore, considering these differences, CA ₁ − CA ₂ = εA・Δd/d(d+Δd), and therefore, taking the ratio of CA ₁ − CA ₂ and CA ₂ , (CA ₁ − CA ₂ )/CA ₂ =Δd/d, and the displacement amount Δd can be detected as a change in capacitance value. As is clear from these equations, the amount of displacement is a function only of capacitance, so it is not affected by the dielectric constant or stray capacitance between the electrodes.
Therefore, it becomes possible to accurately detect the amount of physical displacement using the capacitance. By the way, conventional methods for detecting capacitor capacitance include applying high-frequency alternating current to the capacitor under test, and using the fact that the current flowing through the capacitor at that time is proportional to the frequency, power supply voltage, and capacitance to determine the capacitance. A method has been used in which the detected current is amplified using a differential amplifier or the like, and then calculated and converted into a displacement.

[Problem to be solved by the invention]

However, since such methods generally use analog circuits, they are inevitably subject to large fluctuations due to disturbances such as noise and temperature, which limits the detection accuracy. Furthermore, since impedance such as capacitance and resistance is affected by the power supply frequency or voltage, there is a drawback that measurement errors occur due to fluctuations in the power supply voltage or frequency. This invention was made in view of the above, and by digitizing the measuring device that converts the mechanical displacement amount into a capacitance value, it improves measurement accuracy and reduces power consumption. The purpose is to increase the

[Means to solve the problem]

In order to achieve such an objective, the present invention
A displacement measuring device comprising two measuring capacitors C ₁ and C ₂ whose capacitances at least one of which changes in response to mechanical displacement has a charging/discharging circuit that charges each measuring capacitor and discharges it over a fixed period of time. , a capacitance-frequency conversion circuit that converts the time required for charging and discharging into a pulse signal with a frequency corresponding to the capacity; and a capacitance-frequency conversion circuit that counts the number of pulses from this capacitance-frequency conversion circuit until the counted value reaches a predetermined value. a first counter that outputs a count output when the clock pulse is reached; a clock signal source that generates clock pulses; and a clock signal source that starts counting clock pulses from the clock signal source when each capacitor starts charging or discharging; a second counter that stops counting the clock pulses upon output; and a second counter that receives the count output from the first counter, reads the count result of the second counter, and performs a predetermined calculation based on the count result. The present invention is characterized in that it is equipped with a digital arithmetic circuit that performs the following, and measures mechanical displacement based on the arithmetic results.

【Example】

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a block configuration diagram showing an embodiment of the present invention, FIG. 3 is a circuit configuration diagram showing the inside of the block in FIG. 2 in detail, and FIG. 4 is a time chart for explaining the detection operation of the present invention. FIG. 5 is a circuit configuration diagram showing another embodiment of the capacitance detection section. In FIG. 2, 1 is a capacitance detection section, 2 is a selection circuit for the detection section 1, 3 is a capacitance-frequency conversion circuit, 4 is a counter, 5 is a timer, 6 is a reference clock generation circuit, and 7 is a microprocessor (hereinafter referred to as ,μ−
Also called COM operation circuit. ), 8 is an optical transmission circuit, 9
1 is a power supply circuit using a battery, and 10 is a keyboard. As shown in FIG. 3, the detection section 1 is composed of capacitors C ₁ and C ₂ and the detection section selection circuit 2
are capacitors C ₁ , C ₂ and temperature measuring capacitor
C-MOS (complementary MOS) type analog switch SW2, SW that selects C _S and thermistor R _S
21, SW22, and the _capacitance -frequency conversion circuit ₃ includes analog switches SW1, SW11,
SW12 and the charging voltage of capacitor C ₁ or C ₂ are at a predetermined voltage level (threshold level)
is set when the resistance exceeds the specified time constant (resistance
R _f and a flip-flop Q1 (D type) which is reset after a certain period of time determined by capacitor C _f ). Note that when using a conventional general D-type flip-flop, a circuit (for example, a Schmitt circuit) for determining the threshold level is required at the front stage.
When using a -MOS type flip-flop, such a circuit is not required, and the switching voltage can be used as it is as the threshold voltage. Similarly, the timer 5 is composed of two-stage counters CT2 and CT3, and μ-
When the reset signal PO3 from the COM calculation circuit 7 is released, counting of the clock signal given from the reference clock generation circuit 6 is started, and the counter (CT1)
Counting is stopped by the count-up signal from 4. The μ-COM calculation circuit 7 is driven by a clock signal from the reference clock generation circuit 6 and performs various calculations and control operations. For example, send mode selection signals PO1 and PO2 to analog switch SW2 of detection section selection circuit 2 to select capacitor _C1 measurement mode, capacitor _C2 measurement mode, or temperature measurement mode (measurement using resistor R _S and capacitor C _S ). When not measuring, a reset signal PO3 is given to the counter 4 and the timer 5 to reset them, and when measuring, the reset signal PO3 is canceled to cause the counting operation to be performed.
The count-up signal from counter 4 is used as an interrupt signal.
It receives it as an IRQ, reads the count output from the timer 5 via terminals PI0 to PI15, and performs predetermined arithmetic processing. The μ-COM arithmetic circuit 7 includes a keyboard 10 for instructing operations to adjust the zero point or span to avoid measurement errors, and a reference clock generation circuit 6 or a μ-COM arithmetic circuit to save power. A standby mode circuit 12 for intermittently operating 7, an optical transmission circuit 8 for transmitting and receiving information by light with a host computer on the control room side, and abnormality detection of a light emitting diode LED in the circuit 8. The circuit 11 etc. are connected. Since these circuits are not particularly relevant here, their details will be omitted. Note that 9 is a battery power supply circuit for supplying power to each required part. Below, we mainly explain the 3rd and 4th steps regarding the capacitance measurement operation.
This will be explained with reference to the figures. In the initial state, the mode selection signals PO1 and PO2 are not given from the μ-COM arithmetic circuit 7,
Counter (CT1) 4 is reset by reset signal PO3.
and timer 5 are in a reset state. Here, the measurement mode signal of capacitor _C1 as shown in Figure 4A is given, and the reset signal as shown in Figure 4B is applied.
When PO3 is released, capacitor C ₁ and switch
Since a path consisting of SW21, SW11, resistor R, and power supply _VDD is formed, capacitor _C1 is charged as shown in FIG. 4C. When this charging voltage exceeds the threshold voltage _VTH of flip-flop Q1 after _one hour t, flip-flop Q1 is set and an output is obtained from its output terminal Q. This output is applied to resistor R _f and capacitor C _f as well as to analog switch SW1. As a result, switch SW12 is opened and a charging circuit is formed by resistor R _f and capacitor C _f . At this time, the switch SW11 is switched to the position indicated by the dotted line, and the capacitor _C1 is discharged. When the charging voltage of the capacitor C _f reaches a predetermined value after a predetermined time t _c as shown in FIG. (t _c ) output pulse is obtained. Note that analog switch SW1 is also turned off by resetting flip-flop Q1, so switch SW
12 returns to the state shown in Figure 3, and the capacitor C _f
form a discharge circuit. Since the above time t ₁ is proportional to the size of capacitor C ₁ and resistor R, from the output of flip-flop Q1, capacitor C ₁
A pulse signal with a frequency proportional to the capacitance is obtained. This pulse signal is counted by the counter 4, and when it reaches a predetermined number, a pulse (count UP output) as shown in FIG. The timer 5 counts the clock pulses from the pulse generation circuit 6 when the reset signal PO3 is released, and the counting result is counted by the counter 4.
After receiving the UP signal, the μ-COM calculation circuit 7 outputs the terminal.
Read via PI0-PI15. Here, if the threshold voltage of the flip-flop Q1 is V _TH , it is expressed as V _TH = V _DD (1-e ^-t1/RC1 ), and therefore the charging time of the capacitor _C1 is _t1 ( (see FIG. 4D) is expressed as t ₁ =-RC ₁ log _e (1-V _TH /V _DD ). Further, the above-mentioned time t _C is similarly expressed as t _C =-R _f C _f log _e (1-V _TH /V _DD ). Note that since the values of R _f and C _f are known, the above t _C is selected to be a constant value. Therefore, by counting the clock pulses from the reference clock generation circuit 6 until the charging and discharging operation of the capacitor _C1 is counted n times, that is, by the output from the timer 5, the capacitor _{C1 is}
The charging/discharging time T ₁ can be determined by As _{is clear} from _FIG .
-1) times, it can be determined as T ₁ = nt ₁ + (n-1) t _C (1). In addition, in this way, n
The time measurement counter CT2 counts the times.
This is to improve the resolution of the CT 3, and the number n thereof is appropriately selected depending on the output frequency of the reference clock generating circuit 6, the resistance value of the resistor R, the capacitance value of the capacitor _C1 , etc. In one example, n=256 was selected. In this way, the charging and discharging time of capacitor C ₁
After determining T ₁ , the μ-COM calculation circuit 7 outputs the signal PO
1 or PO2 to set the switch SW21 to the detection mode of the capacitor _C2 , and measure the charging/discharging time _T2 of the capacitor _C2 . The operating mode in this case is exactly the same as above, and the time chart thereof is shown in the right half of FIG. Note that the charging/discharging time T ₂ is calculated as T ₂ =nt ₂ +(n-1)t _C () similarly to equation (1). The μ-COM calculation circuit 7 performs the following calculation based on the above equations () and (). T ₁ +T ₂ -2(n-1)t _C = -R (C ₁ + C ₂ ) log _e (1-V _TH /V _DD ) T _{1 -} T ₂ = -R (C ₁ - C ₂ ) log _e (1-V _TH /V _DD ) T ₁ -T ₂ /T ₁ +T ₂ -2(n-1)t _C =C ₁ -C ₂ /C ₁ +C ₂
...() As is clear from the explanation in the previous principle diagram, this equation is proportional to the displacement, so the μ-COM calculation circuit 7 can measure the displacement by performing the calculations described above. . In addition, in the above, the amount of physical displacement, for example, the differential pressure ΔP, was measured by differentially changing the capacitance of capacitors C ₁ and C ₂ , but as shown in Fig. 5, It is clear from the explanation of the principle diagram above that the present invention can be similarly applied to a case where C ₂ is fixed on the one hand and C ₁ is variable on the other hand. However, in this case, the pressure P is determined instead of the above-mentioned differential pressure ΔP, and the calculation formula thereof is expressed as follows in the same manner as above. P=C ₁ -C ₂ /C ₂ =T ₁ -T ₂ /T ₂ -(n-1)t _C ...() In addition, in the detection section 1, the stray capacitance C _ST is the capacitor under test C ₁ , C ₂ are included in parallel with each other, C ₁ →C ₁ +C _ST , C ₂ →C ₂ +C _ST , and the term C ₁ −C ₂ is not affected by the stray capacitance C _ST , but C ₁ +C Since 2C _ST will be added to the term ₂ , in order to correct this, (C ₁ − C ₂ )/
(C ₁ + C ₂ −2C _ST ), that is, the above ()
Instead of the equation, a constant value K is subtracted as shown in the following equation. T ₁ −T ₂ /T ₁ +T ₂ −2(n-1)t _C −K In the same way, for formula () to find the pressure P, (C ₁ −C ₂ )/(C ₂ −C _ST ) The calculation, ie, the calculation T ₁ -T ₂ /T ₂ -(n-1)t _C -K is performed. Next, a case in which temperature correction is performed will be explained. In this case, the signal from μ-COM calculation circuit 7 is
By selecting switch SW21 using PO1 and PO2, the temperature measurement mode is set. By this, capacitor C _S , switch SW21, SW1
1. A charging/discharging path via the SW 22 and the thermistor resistance temperature detector R _S is formed in the same way as in the measurement mode of the capacitors C ₁ and C ₂ . Note that although the resistor R is inserted in parallel with the thermistor R _S , the influence of the resistor R can be ignored because it is selected so that R _S <<R. Therefore, the charging time t ₃ by the capacitor C _S is t ₃ = _−RS C _S log _e (1 − V _TH /V _DD ), and the total charging and discharging time T ₃ is T ₃ = nt ₃ + It is given as (n-1)t _C. The temperature can be measured by storing the correspondence between the charge/discharge time _T3 and the temperature in advance in a storage section (not shown) of the μ-COM calculation circuit 7. In this case, it will be directly affected by fluctuations in the voltages V _DD and V _TH , but since it is an error in the correction amount, it will not be a big problem. Note that when converting the capacitance into a frequency signal, there may be an error due to fluctuations in the predetermined time t _C (=-R _f C _f log _e (1-V _TH /V _DD )), but the charging time t ₁ , t ₂ such that t ₁ ≫t _C and t ₂ ≫t _C , the above effects can be ignored. Also,
As is clear from the above formula () or (),
Since factors such as the resistance R and the voltages V _DD and V _TH are divided on the upper and lower sides, the influence of these fluctuations can be ignored. In addition, in the above, the amount of displacement was detected by specifically measuring the charging time of the charging and discharging operation of the capacitor, but it is also possible to detect the amount of displacement from the discharging time in the same manner as above. Needless to say. Furthermore, in the embodiment shown in FIG. 3 above,
Connect resistor R and flip-flop Q1 to capacitor
I decided to share it when detecting the capacitance of C ₁ and C ₂ , but
Of course, it is also possible to provide and detect these separately.

【Effect of the invention】

As described above, according to the present invention, since the measurement circuit is configured digitally, it is possible to avoid the effects of noise, temperature, etc., and therefore it is possible to improve measurement accuracy. At the same time, power consumption can be saved. In particular, since the discharge of the capacitor is set to a known constant time and this time is subtracted from the measurement time as in the above equation (), highly accurate measurement that is not affected by the discharge time becomes possible. Furthermore, since the influence of temperature or stray capacitance can be corrected by calculation, it has the advantage that a compensation circuit for this purpose is not required. Furthermore, even when measuring pressure instead of differential pressure, this can be easily handled by simply changing the calculation formula, that is, by selecting an appropriate program, so the practical effect is extremely large. In particular, the above-mentioned dielectric constant ε, flip-flop threshold voltage V _TH , and power supply voltage V _DD vary depending on the temperature, but by performing calculations using the above formulas, etc., these terms can be removed from the calculation formula. This makes it possible to perform measurements unaffected by temperature.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram for explaining the principle of a method of detecting a physical displacement by converting it into a capacitance change, Fig. 2 is a block diagram showing an embodiment of this invention, and Fig. 3 is a block diagram of Fig. 2. FIG. 4 is a time chart for explaining the detection and conversion operations of the present invention, and FIG. 5 is a circuit diagram showing another embodiment of the capacitance detection section. Description of symbols 1...detection section, 2...detection section selection circuit,
3...Capacitance-frequency conversion circuit, 4...Counter, 5...
Timer, 6...Reference clock generation circuit, 7...μ-
COM operation circuit, 8... Optical transmission circuit, 9... Battery power supply circuit, 10... Keyboard, 11... LED abnormality detection circuit, 12... Standby mode circuit, C ₁ ,
C ₂ , C _S ... capacitor, Q1 ... flip-flop,
SW1, SW2...Analog switch, CT1~CT
3...Counter.

Claims (1)

[Scope of Claims] 1. In a displacement measuring device comprising two measuring capacitors C ₁ and C ₂ whose capacitance values differentially change according to mechanical displacement, each measuring capacitor C ₁ and C ₂ are charged to a predetermined voltage level, and the measurement capacitors C ₁ and C ₂
It has _a charging/discharging circuit that discharges the measurement _capacitors C ₁ , C ₂ within a predetermined time _{t c} , t _c according to A capacitance-frequency conversion circuit 3 that converts into a pulse signal with a frequency corresponding to the capacitance, and a capacitance-frequency conversion circuit 3 that counts the number of pulses from this capacitance-frequency conversion circuit 3, and outputs a count when the counted value reaches a predetermined value n. a first counter 4 that generates clock pulses; and a clock signal source 6 that generates clock pulses.
and a second counter 5 that starts counting clock pulses from the clock signal source when charging or discharging of each capacitor starts, and stops counting the clock pulses according to the count output from the first counter. and a digital calculation circuit 7 that receives the count output from the first counter, reads the count result of the second counter, and calculates the following formula (1) based on the count result. , a displacement measuring device that measures mechanical displacement from the calculation results. T ₁ −T ₂ /T ₁ +T ₂ −2(n−1)t _c …(1) T ₁ ; Counting result when measuring one capacitor T ₂ ; Counting result when measuring the other capacitor 2 Two measuring capacitors, one with a fixed capacitance value and the other with a capacitance value that changes according to mechanical displacement.
In a displacement measuring device having C ₁ , C ₂ , each measuring capacitor C ₁ , C ₂ is charged to a predetermined voltage level, and the measuring capacitor C ₁ , C ₂ is charged to a predetermined voltage level.
It has _a charging/discharging circuit that discharges the measurement _capacitors C ₁ , C ₂ within a predetermined time _{t c} , t _c according to A capacitance-frequency conversion circuit 3 that converts into a pulse signal with a frequency corresponding to the capacitance, and a capacitance-frequency conversion circuit 3 that counts the number of pulses from this capacitance-frequency conversion circuit 3, and outputs a count when the counted value reaches a predetermined value n. a first counter 4 that generates clock pulses; and a clock signal source 6 that generates clock pulses.
and a second counter 5 that starts counting clock pulses from the clock signal source when charging or discharging of each capacitor starts, and stops counting the clock pulses according to the count output from the first counter. and a digital calculation circuit 7 that receives the count output from the first counter, reads the count result of the second counter, and calculates the following formula (1) based on the count result. , a displacement measuring device that measures mechanical displacement from the calculation results. T ₁ −T ₂ /T ₂ −(n−1)t _c …(1) T ₁ ; Counting result when measuring one capacitor T ₂ ; Counting result when measuring the other capacitor