JP2004301823A - Drift cancellation device, and pressure measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a drift cancellation device suitable for a piezoelectric pressure sensor, and a pressure measuring instrument using the device. <P>SOLUTION: When measuring combustion pressure of an engine 10, a means 13 for generating a pulse at the timing when the engine 10 is brought into a prescribed condition (exhaust stroke) is provided in the first embodiment of the drift cancellation device, and a signal of the sensor 11 is sampled by the timing pulse to be subtracted from the original signal. A means for holding the minimum value of the pressure in each engine cycle is provided in the second embodiment, and the held minimum value is subtracted from the original signal. A drift value is subtracted from the output signal in real time with a simple constitution. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a drift cancellation device and a pressure measurement device using the device, and more particularly, to a drift cancellation device suitable for a piezoelectric pressure sensor using a piezoelectric element and a pressure measurement device using the device.

Conventionally, when measuring the pressure in an environment with a large temperature change, for example, when measuring the combustion pressure of a gasoline engine or a diesel engine, a charge generation type sensor using a single crystal piezoelectric element has been used as a pressure sensor. The output of the piezoelectric pressure sensor was converted into a voltage using a charge amplifier (charge detector) and output.
JP 05-172679 A

  However, the conventional piezoelectric pressure sensor has a problem that the output drifts. Drift is mainly due to the following two causes. The first cause is a fluctuation due to leakage of electric charges generated by the piezoelectric pressure sensor or a fluctuation due to leakage of electric charges from the periphery to a signal line of the sensor. The second cause is a fluctuation due to factors other than pressure due to thermal expansion of the sensor element or the sensor structure due to temperature change. Drift due to these factors is not negligible. Therefore, for example, even if the combustion pressure of the engine is measured, the pressure cannot be measured accurately in real time.

  SUMMARY OF THE INVENTION It is an object of the present invention to provide a drift canceling device suitable for a piezoelectric pressure sensor and a pressure measuring device using the device, which solves the above-mentioned problems of the prior art.

  In measuring the combustion pressure of the engine, in the present invention, the engine cycle is faster than the time change of the drift, the minimum pressure in the engine is almost equal to the atmospheric pressure, and further, the pressure measurement in the process other than the explosion process is not large. Focusing on the fact that the result is not so important, it is characterized in that the drift value, which is an unintended amount, is subtracted from the output signal.

  In the first embodiment of the drift canceling device of the present invention, a timing pulse generating means for generating a pulse at a timing when the device to be measured enters a predetermined state is provided. The main feature is to subtract from. Also, in the second, third and fourth embodiments of the drift canceling device of the present invention, means for detecting the minimum value of the pressure in each engine cycle is provided, and the detected minimum value is subtracted from the original signal. The most important feature.

  ADVANTAGE OF THE INVENTION In the drift cancellation apparatus of this invention, a drift value can be subtracted from an output signal in real time with a simple structure. Further, by providing a discharging means for the capacitor of the charge amplifier, the saturation of the measured value can be prevented.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 1 is a block diagram showing a configuration of a pressure measuring device using a first embodiment of the drift canceling device of the present invention. A charge generating pressure sensor 11 using a single crystal piezoelectric element is mounted on a cylinder 10 of the engine. The signal processing circuit 12 processes a signal output from the pressure sensor 11 and outputs an output signal (Vout) of a voltage proportional to the pressure in the cylinder 10 in real time.

  The timing pulse generation circuit 13 is provided, for example, by mounting a permanent magnet on the crankshaft or camshaft of the engine, and detecting the rotational position by a coil or a Hall effect element, so that the crankshaft or camshaft of the engine has a predetermined rotational phase position. For example, a pulse is detected by detecting that the engine has come to an exhaust process.

  FIG. 2 is a circuit diagram showing the configuration of the first embodiment of the drift canceling device (signal processing circuit 12) of the present invention. A well-known charge amplifier circuit A1 (charge-voltage conversion amplifier or a circuit that outputs a voltage such as an amplifier that handles those signals) converts a charge amount output from the sensor 11 into a voltage signal.

  The known sample-and-hold circuit SH stores (updates) the voltage of the input terminal (IN) in the capacitor C1 based on the output pulse of the timing pulse generation circuit 13 input to the timing pulse input terminal (S / H), and The voltage of the capacitor C1 is output from an output terminal (OUT).

  The operational amplifier A2 and the resistors R1 to R4 constitute an operational amplifier. This circuit subtracts the output signal of the sample hold circuit SH from the output signal of the charge amplifier circuit A1, and outputs the result.

  FIG. 4 is a waveform diagram showing waveforms of main parts of the first embodiment of the drift cancellation device of the present invention. FIGS. 4 and 7 show, for example, the case where the drift value increases with time. However, the change in the drift value is exaggerated and the actual change in the drift value is smaller.

  The combustion pressure of the engine indicated by the output signal of the charge amplifier circuit A1 rises to several tens of atmospheres in the explosion process, but is approximately equal to the atmospheric pressure (1 atm) in the other processes. The waveform required for measuring the combustion pressure is the waveform data in the explosion process, and the shape of the waveform in the low air pressure portion is not so important.

  As described above, the timing pulse is generated by the timing pulse generation circuit 13 when the crankshaft or camshaft of the engine reaches a predetermined rotational phase position, for example, when the engine is in an exhaust process. The sample hold circuit SH samples, holds, and outputs the output signal of the charge amplifier circuit A1 based on the timing pulse.

  As a result, as the output signal (Vout), a signal in which the output signal is corrected to 0 for each exhaust cycle is output as shown in the figure. Similarly, when the drift value is decreasing, a signal in which the output signal is corrected to 0 for each exhaust cycle is output.

  FIG. 6 is a circuit diagram showing the configuration of the second embodiment of the drift canceling device of the present invention. FIG. 7 is a waveform chart showing waveforms of main parts of a second embodiment of the drift canceling device of the present invention. In this embodiment, means for holding the minimum value of the pressure in each engine cycle is provided, a method of subtracting the held minimum value from the original signal is employed, and further, the drift charge accumulated in the capacitor of the charge amplifier A1 is adopted. Discharge means is also provided.

6, the operational amplifier including the sample-and-hold circuit SH, the operational amplifier A2, and the resistors R1 to R4 is the same circuit as that of the first embodiment in FIG. An operational amplifier such as an amplifier operates with a power supply of ± 15 V, and the logic circuit operates with a single power supply of +15 V (high = 1 = + 15 V, low = 0 = 0 V).
The charge amplifier A1 includes a relay RL that is a means for discharging and resetting unnecessary drift charges accumulated in the capacitor C2. The driving will be described later.

  The buffer amplifier A3 is a buffer amplifier having a gain of almost 1, and includes a semi-fixed resistor VR and a detachable resistor Rs. The semi-fixed resistor VR is used for correcting the capacitance of the capacitor C2. The resistance Rs is a resistance for adjusting the sensor sensitivity prepared in pairs with the sensor 11, and the sensitivity of the sensor 11 is corrected by this.

  When the output of the second comparator CP2 is 0 (−15 V), the first comparator CP1 compares the output signal of the buffer amplifier A3 with the holding voltage of the sample and hold circuit SH, and outputs the output signal of the buffer amplifier A3. When the voltage is lower, the one-shot multivibrator circuit (hereinafter referred to as OM circuit) 1 is activated via the diode D4 of the OR circuit 1 which also performs level conversion, and the sample-and-hold circuit is activated via the OR gate circuit OR2. A sampling pulse of several microseconds is output to SH. The reason why the OM circuit 1 is provided is that the sampling pulse input to the sample-and-hold circuit SH needs to be longer than a predetermined width. The resistors R29 and R30 are resistors for level conversion. With this operation, the minimum value of the output signal of the buffer amplifier A3 is held in the sample hold circuit SH.

  Note that while the sampling pulse is applied to the sample hold circuit SH, the sample hold circuit SH is in a through state, and an input signal is applied to both of the input terminals of the first comparator CP1. The offset of the sample and hold circuit SH or the first comparator CP1 is adjusted so that the output of the first comparator CP1 becomes low.

  However, with this operation alone, for example, when the drift value increases, the held value of the sample-and-hold circuit SH is not updated, and the drift value is not subtracted. Therefore, a second comparator CP2 is provided so that the held value of the sample and hold circuit SH is updated every engine cycle even when the drift value increases.

  That is, when the hold state of the second comparator CP2 is released by the output of the sampling pulse, the output signal of the buffer amplifier A3 is supplied to the + terminal of the second comparator CP2, and Due to the action of R23, a voltage slightly higher than the held value of the sample and hold circuit SH, that is, a voltage higher by x in FIG. 7 is applied. Since the value of x does not affect the output waveform, an optimum value may be selected in consideration of noise and the like.

  In the state described above, when the output signal of the buffer amplifier A3 becomes higher at time t0, the output of the second comparator CP2 becomes high (+15 V), and positive feedback is applied by the diode D3 and the resistor R25. The high state is held.

  In the hold state, a voltage slightly higher than the hold value of the sample hold circuit SH, that is, a voltage higher by y in FIG. 7 is applied to the + terminal of the first comparator CP1 by the action of the diode D1 and the resistor R21. The value of y is, for example, a value larger than the maximum value of the temporal change amount of the drift expected in one engine cycle time. However, the larger this value is, the larger the error of the output signal becomes. Therefore, a small value is adopted within a range satisfying the above condition.

  If the output signal value of the buffer amplifier A3 decreases while the output of the second comparator CP2 is high, the output of the first comparator CP1 becomes high at time t1, and is output via the OM1 and OR2. A sampling pulse is output. The held value of the sample hold circuit SH is updated by the sampling pulse, the hold state of the second comparator CP2 is released by the action of the diode D2, and the OM circuit 2 is reset.

  The OM circuit 2 is a circuit that outputs a high signal when a sampling pulse is not generated for a predetermined time (for example, slightly longer than the longest time of the engine cycle), and the sampling value is low because the initial value of the sample and hold circuit SH is too low. When no pulse is generated, the OM circuit 1 is activated via the diode D5 to forcibly generate a sampling pulse. Further, it has a function of generating a charge reset signal of the charge amplifier A1 via the OR2, the OM circuit 3, the NOT circuit, and the RL to shift to a normal state. In a normal state, a reset is applied by a sampling pulse every engine cycle, so that the high state of the OM circuit 2 does not occur.

  When the held value of the sample hold circuit SH is updated at time t1, the held value of the sample hold circuit SH is updated continuously or intermittently until the output signal value of the buffer amplifier A3 reaches the minimum value at time t2. As a result, after time t2, the minimum value of the output signal value of the buffer amplifier A3 is held in the sample hold circuit SH. As a result of the above operation, the output waveform is as shown in the lower part of FIG.

  Next, the operation of discharging the drift charge accumulated in the capacitor C2 of the charge amplifier A1 will be described. The comparator CP3 outputs a high signal when the output signal of the buffer amplifier A3 exceeds the upper limit, and the comparator CP4 outputs a high signal when the output signal of the buffer amplifier A3 falls below the lower limit. The upper limit value and the lower limit value are respectively set outside the maximum value and the minimum value of the signal when there is no drift by a predetermined value.

  This high signal sets the FF through the OR circuit 3 which also includes diodes D6, D7 and R46 and also performs level conversion. Thereafter, when a sampling pulse is generated from OM1, the OM circuit 3 is activated via OR2, AND, and OR4, and generates a pulse having a length necessary for driving the relay RL, for example, 1 millisecond. This pulse drives the relay RL via the open-collector NOT circuit to short-circuit the capacitor C2 of the charge amplifier to discharge the accumulated charge. As the relay RL, for example, a reed relay operating at high speed can be adopted.

  With this operation, the output of the charge amplifier A1 becomes 0, and the sample hold value is updated to 0. Also, the FF is reset. Note that even when a reset pulse is generated from the OM2, the OM3 is started via the OR4 and the charge is reset.

  In FIG. 7, when the output signal of the buffer amplifier A3 exceeds the upper limit value at time t3, FF is set via OR3. Next, when CP1 becomes high at time t4, a sampling pulse is generated from OM1, and OM3 is activated via the AND circuit. Then, the relay RL operates to discharge the charge of the charge amplifier A1, the output of A1 becomes 0, and the sample hold value is updated to 0. The same applies to the operation when the value falls below the lower limit. The above operation prevents the output value of the charge amplifier from being saturated due to the drift.

  FIG. 3 is a block diagram showing the configuration of a third embodiment of the drift canceling device of the present invention. In this embodiment, an output signal of a charge amplifier A1 is converted into a digital signal by an A / D converter 20, and a drift cancellation process is performed in real time by a digital processing circuit 21 including, for example, a CPU, and a digital or analog waveform signal is output. Is output.

  The digital signal processing circuit 21 detects a minimum value (timing) based on the fact that the polarity of the difference from the immediately preceding value has changed from negative to positive from the noise removing unit 22 composed of a low-pass filter for preventing malfunction. The minimum value detecting means 23, the sample and hold means for storing the drift value (input signal value) at the timing when the minimum value is detected, and the adding means 25 for subtracting the drift value from the input signal are realized by a program. When the input signal value is equal to or more than the predetermined value, the minimum value detecting means 23 may not be operated. Further, a means for discharging the drift charge accumulated in the capacitor of the charge amplifier A1 as shown in FIG. 6 may be provided.

  FIG. 8 is a circuit diagram showing a configuration of a fourth embodiment of the drift canceling device of the present invention. FIG. 9 is a waveform diagram showing waveforms of main parts of a fourth embodiment of the drift canceling device of the present invention. The fourth embodiment is a modified example in which a part of the second embodiment shown in FIG. 6 is improved, and therefore, a configuration different from the second embodiment will be described.

  The fourth embodiment differs from the second embodiment in circuits related to the comparators CP1 and CP2. In the second embodiment, each of the comparators CP1 and CP2 generates a timing signal by comparing a signal based on the output signal of the sample-and-hold circuit with an input signal. In the fourth embodiment, The timing signal is generated by comparing a predetermined voltage with the output signal of the operational amplifier A2 without using the output signal of the sample and hold circuit.

  The two volumes VR2, VR3 and the buffer amplifiers BUF1, BUF2 generate a first predetermined voltage X and a second predetermined voltage Y lower than X. The second comparator CP2 compares the output voltage of the operational amplifier A2 with X, and when X is lower, RSFF is set. The analog switch SW using the MOSFET is turned on when the output Q of the RSFF becomes 1, and the voltage Y is supplied to the comparator CP1.

  The comparator CP1 compares the output voltage of the operational amplifier A2 with Y. When Y becomes higher, the output of the first comparator CP1 becomes high, and a sampling pulse is output via OM1 and OR2. The value held by the sample hold circuit SH is updated by this sampling pulse, and the RSFF is reset. As a result, SW is turned off, and the voltage of the + input terminal of the comparator CP1 becomes 0 by the resistor R50.

  When the sampling pulse is generated, the sample hold circuit is in the through state, the input signal is applied to both terminals of the operational amplifier A2, and the output signal becomes 0. Therefore, the input terminals of the comparator CP1 are both 0. In this case, the sample-and-hold circuit SH, the operational amplifier A2, or the first comparator CP1 is set so that the output of the first comparator CP1 becomes low. Adjust the offset of CP1. As a result, when a sampling pulse is being generated, CP1 is in a low state, and the sampling pulse is turned off at the fall of the pulse generated from OM1. Then, the sample and hold circuit enters a hold state.

However, if the voltage of the input signal further decreases, the output signal of the operational amplifier A2 becomes negative again, so that CP1 goes high again. This oscillation state is repeated as long as the voltage of the input signal continues to decrease, and ends when the input voltage starts increasing. Therefore, the lowest voltage is held in the sample and hold circuit.
In the fourth embodiment, with the above circuit configuration, the operation of the circuit can be easily adjusted by the two volumes VR2 and VR3 while satisfying the condition that Y is lower than X.

  Although the embodiments of the present invention have been disclosed above, the present invention may have the following modifications. In the embodiment, the example in which the charge amplifier is used as the sensor signal input circuit is disclosed. However, a sensor signal input circuit using a capacitor is also conceivable. FIG. 5 is a circuit diagram showing a configuration of the second embodiment of the sensor signal input circuit. In this circuit, the output charge of the sensor 11 is converted into a voltage by a capacitor C4 and output through a buffer amplifier A4 having a high input impedance.

  In the embodiment, the example of the measuring device of the combustion pressure of the engine is disclosed. However, the drift canceling device and the pressure measuring device of the present invention may be any device as long as the measured object periodically becomes a known pressure state. The present invention can be applied to the measurement of the pressure of the measurement target.

It is a block diagram showing composition of a pressure measuring device using a 1st example of a drift cancellation device of the present invention. FIG. 1 is a circuit diagram showing a configuration of a first embodiment of a drift cancellation device of the present invention. FIG. 10 is a block diagram illustrating a configuration of a third embodiment of the drift cancellation device of the present invention. FIG. 3 is a waveform chart showing waveforms of main parts of the first embodiment of the drift cancel device of the present invention. FIG. 9 is a circuit diagram illustrating a configuration of a sensor signal input circuit according to a second embodiment. FIG. 6 is a circuit diagram showing a configuration of a second embodiment of the drift canceling device of the present invention. FIG. 6 is a waveform chart showing waveforms of main parts of a second embodiment of the drift cancel device of the present invention. FIG. 9 is a circuit diagram showing a configuration of a fourth embodiment of the drift cancel device of the present invention. FIG. 13 is a waveform chart showing waveforms of main parts of a fourth embodiment of the drift canceling device of the present invention.

Explanation of reference numerals

  Reference Signs List 10: cylinder, 11: piezoelectric pressure sensor, 12: signal processing circuit, 13: timing pulse generation circuit, 20: A / D converter, 21: digital processing circuit

Claims (4)

Timing pulse generating means for generating a pulse at a timing when the device under test enters a predetermined state,
A sample and hold unit that samples and holds an input signal corresponding to a pressure based on a signal output from the pressure sensor based on an output pulse of the timing pulse generation unit,
A drift canceling device comprising: a differential amplifying means for subtracting a voltage value output from the sample and hold means from the input signal. Sample and hold means for sampling and holding an input signal corresponding to a pressure based on a signal output from the pressure sensor,
First comparing means for comparing an output value of the sample and hold means with the input signal, and outputting a sampling signal to the sample and hold means when the input signal is lower;
The input signal is compared with a value higher than the output value of the sample and hold means by a first predetermined value. If the input signal is higher, the output is held in a high state. A second comparing means for increasing the output value of the sample and hold means input to the comparing means by a second predetermined value and releasing the hold state by the sampling signal;
And a differential amplifying unit for subtracting a voltage value output from the sample and hold unit from the input signal. Sample and hold means for sampling and holding an input signal corresponding to a pressure based on a signal output from the pressure sensor based on the sampling signal;
A differential amplifier for subtracting a voltage value output from the sample-and-hold unit from the input signal, a first predetermined voltage is compared with the input signal, and an output signal is generated when the input signal is higher. Second comparing means;
Flip-flop means set by an output signal of the second comparing means and reset by a sampling signal;
Switch means for outputting a second predetermined voltage lower than the first predetermined voltage based on an output of the flip-flop means;
First comparing means for comparing an output voltage from the switch means with the input signal and outputting a sampling signal when the input signal is lower;
A drift cancellation device comprising: Charge generation type pressure sensor means using a piezoelectric element,
Charge amplifier means for converting an output signal of the piezoelectric pressure sensor into a voltage signal corresponding to pressure,
A pressure measuring device comprising: the drift canceling device according to claim 2.

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