JP2011169205A - Pm detecting device - Google Patents

Abstract

A PM detection apparatus capable of accurately detecting the amount of PM deposited with a simple configuration.
A PM sensor 3 whose capacitance varies depending on the amount of accumulated PM, a variable resistor 4 and fixed resistors 5 to 7 are sequentially connected, and the PM sensor 3 is connected in parallel to the variable resistor 4 to be variable. A bridge circuit 9 in which a variable capacitor 8 is connected in parallel to a fixed resistor 5 adjacent to the resistor 4, a differential amplifier 14 connected to the measurement points c and d, and an output between the measurement points c and d. An absolute value signal generation circuit 15 for generating an absolute value signal of the voltage, and a variable resistor 4 is adjusted so that the absolute value signal becomes 0 by applying a DC voltage between the voltage application points a and b, and then the voltage A detection unit 16 that adjusts the variable capacitor 8 so that the absolute value signal becomes 0 by applying an AC voltage between the application points a and b, and detects the amount of deposited PM from the capacitance of the variable capacitor 8 at this time. With.
[Selection] Figure 1

Description

  The present invention relates to a DPF that collects PM in exhaust gas of an internal combustion engine and burns and removes the accumulated PM with high-temperature exhaust gas, and can accurately detect the amount of accumulated PM with a simple configuration. The present invention relates to a PM detection device.

  In vehicles equipped with an internal combustion engine such as a diesel engine, a diesel particulate filter (hereinafter referred to as DPF) is installed in the exhaust gas exhaust passage from the internal combustion engine to the atmosphere. That is, particulate matter (Particurate Matter; hereinafter referred to as PM) is collected. The DPF is a filter mainly made of ceramic and having a large number of honeycomb pores (or square pores). In the DPF, PM is collected by adhering to the surface of the honeycomb pores serving as exhaust gas passages.

  If an excessive amount of PM collected in the DPF accumulates, the exhaust pressure of the internal combustion engine increases and the characteristics of the internal combustion engine deteriorate. Therefore, in the internal combustion engine, if necessary, additional fuel injection control is performed to perform additional fuel injection after the main injection, thereby raising the exhaust gas temperature, thereby raising the DPF and burning and removing the PM accumulated in the DPF. To do. This operation is called DPF forced regeneration.

  If the amount of accumulated PM is large at the time of forced regeneration of the DPF, a large amount of PM is burned, the temperature rises excessively, and the DPF is melted. In order to avoid this, it is desirable to detect the amount of PM deposited and start the DPF forced regeneration based on the detected amount of deposition. However, conventionally, since the amount of deposition cannot be accurately detected, a relatively large safety factor is taken, and the DPF is forcibly regenerated before the detected amount of deposition becomes too large. As a result, DPF forced regeneration is executed at an interval shorter than necessary.

  However, if the DPF forced regeneration is executed at an interval shorter than necessary, extra fuel is consumed, resulting in a deterioration in fuel consumption. Therefore, it is desirable to accurately detect the amount of accumulated PM and perform DPF forced regeneration at the most appropriate time.

JP 2008-139294 A JP 2009-97410 A

  Previously, the present inventors obtained the knowledge that when a plurality of electrodes are installed in the DPF, the capacitance between the electrodes varies depending on the amount of PM deposited, and filed a PM sensor using this. However, since the capacitance of the PM sensor with electrodes installed in the DPF is very small, measurement with the conventional technology is difficult.

  In the prior art, a fixed capacitor or a fixed resistor is connected in series to a capacitance measurement target, and a voltage is generated between both ends of the capacitance measurement target by flowing a high-frequency alternating current. Is then input to the AD converter through a low-pass filter, and the capacitance is calculated from the voltage read by the AD converter.

  However, the capacitance of the PM sensor in which a plurality of electrodes are installed in the DPF is very small, for example, several pF to several hundred pF. When the frequency of the alternating current to be applied is several hundred KHz or less, the impedance is very high. Therefore, the detection circuit also needs to have a correspondingly high impedance. However, it is difficult to ensure a high impedance from the viewpoint of the distributed capacitance of the elements and wirings, and it is impossible to accurately measure the capacitance.

  On the other hand, if the frequency of the alternating current is increased in order to reduce the impedance, the frequency band used for communication is generated, causing problems of unnecessary radiation and the influence of standing waves generated in the wiring between the PM sensor and the detection circuit. Therefore, the characteristics of the PM sensor greatly change depending on the length of the wiring and the characteristic impedance. Furthermore, in order to calculate the capacitance from the read voltage, the AC current voltage must be accurate, but in order to obtain an AC power supply with an accurate amplitude, a complicated control circuit is required, which stabilizes the operation. It is disadvantageous in terms of sex and cost.

  As described above, in the PM sensor in which the capacitance between the electrodes varies depending on the PM deposition amount, the capacitance cannot be accurately measured by the conventional technique, so the PM deposition amount is accurately detected. Is difficult.

  Here, the present inventors are examining capacitance detection of a PM sensor using a bridge circuit. Although described in detail in the embodiment, the capacitance of the variable capacitor is swept so that the capacitance of the PM sensor and the capacitance of the variable capacitor are balanced. When the capacitance of the PM sensor and the capacitance of the variable capacitor are balanced, the capacitance of the variable capacitor represents the capacitance of the PM sensor.

  However, in order to determine that the bridge circuit is balanced by the balance between the capacitance of the PM sensor and the capacitance of the variable capacitor, the capacitance of the variable capacitor is swept between the two measurement points on the bridge circuit. It is necessary to observe the voltage and identify the zero crossing point where the voltage goes from positive to negative or from negative to positive. On the other hand, an AD converter built in an electronic control unit (ECU) that is an in-vehicle computer can input only a positive signal. There is also a method using an analog comparator, but accurate measurement is difficult in terms of S / N. Therefore, a novel means / method is desired to identify the zero crossing point of the voltage between the two measurement points in the ECU. If the zero-cross point can be easily identified, it is possible to detect the capacitance of the PM sensor using a bridge circuit, and it is expected to simplify the PM detection device and improve the detection accuracy.

  The AD converter converts a voltage input in a predetermined voltage range into numerical data in a predetermined numerical range. The more the number of binary digits (bits) describing the data, the higher the voltage resolution, but on the other hand, the number of internal elements increases and the cost of the ECU increases. If the number of bits is reduced to reduce the cost of the ECU, the resolution of the voltage is lowered and the zero cross point becomes inaccurate. There is a need for a means and method that makes the zero-crossing point accurate even when the voltage resolution is low.

  Accordingly, an object of the present invention is to provide a PM detection apparatus that can solve the above-described problems and can accurately detect the amount of accumulated PM with a simple configuration.

  To achieve the above object, the present invention detects the amount of particulate matter (hereinafter referred to as PM) accumulated in a diesel particulate filter (hereinafter referred to as DPF) inserted in an exhaust gas exhaust passage from an internal combustion engine to the atmosphere. A PM sensor in which the capacitance between two electrodes arranged in the DPF varies according to the amount of PM deposited, an electrically controlled variable resistor, and three fixed resistors. A bridge circuit in which the PM sensor is connected in parallel to the variable resistor, and the electrically controlled variable capacitor is connected in parallel to one of the fixed resistors adjacent to the variable resistor; Of the four connection points of the bridge circuit, the connection point where the variable capacitor and the fixed resistor, the variable resistor and the PM sensor are connected, and the connection point located at the diagonal are the voltage. A DC power source and an AC power source for selectively applying a DC voltage and an AC voltage between the voltage application points, and the voltage application point among the four connection points of the bridge circuit. A differential amplifier having an input terminal connected to each of the two connection points, and an absolute value signal that represents the absolute value of the voltage between the measurement points based on the output of the differential amplifier Apply a DC voltage between the value signal generation circuit and the voltage application point to adjust the variable resistor so that the absolute value signal becomes 0, and then apply an AC voltage between the voltage application points. The variable capacitor is adjusted so that the absolute value signal becomes 0, and a detection unit that detects the amount of PM deposited from the capacitance of the variable capacitor at this time is provided.

  The detection unit sweeps the resistance value of the variable resistor in a predetermined range, and the absolute value is based on a resistance value change in which the absolute value signal approaches 0 and a resistance value change in which the absolute value signal moves away from 0. The resistance value at which the signal becomes 0 is extracted, the capacitance of the variable capacitor is swept within a predetermined range, and the capacitance change in which the absolute value signal approaches 0 and the absolute value signal moves away from 0 The capacitance at which the absolute value signal becomes 0 may be extracted based on the change.

  The variable capacitor may be connected in parallel with a compensation harness having a stray capacitance equal to the stray capacitance of the signal harness connecting the PM sensor and the variable resistor.

  In the variable capacitor, a plurality of fixed capacitors having different electrostatic capacities in proportion to powers of 2 are connected in parallel through switches, and are controlled to discrete composite capacities by a combination of opening and closing of the switches. May be.

  The present invention exhibits the following excellent effects.

  (1) The configuration is simple.

  (2) The amount of PM deposition can be accurately detected.

It is a circuit block diagram of PM detection apparatus which shows one Embodiment of this invention. It is a characteristic view of PM sensor used for PM detection device of the present invention. (A)-(d) is a schematic block diagram of PM sensor used for PM detection apparatus of this invention. It is a general | schematic internal circuit figure of the variable capacitor in PM detection apparatus of this invention. It is a general | schematic internal circuit figure of the variable capacitor in PM detection apparatus of this invention. It is an image figure which shows the harness wiring in PM detection apparatus of this invention. It is a schematic block diagram of the vehicle carrying the PM detection apparatus of this invention. It is a signal waveform diagram of each part of the circuit in the PM detection device of the present invention. It is a characteristic view of the voltage between measurement points (output voltage of a bridge circuit) with respect to the variable capacitor electrostatic capacity in the PM detection device of the present invention. It is a characteristic view of the signal after gain adjustment (output voltage of a gain adjustment circuit) with respect to the variable capacitor electrostatic capacitance which used the gain of the gain adjustment circuit in the PM detection apparatus of this invention as a parameter. It is a characteristic view of the output voltage of the differential amplification and low-pass circuit with respect to the variable capacitor electrostatic capacity using the gain of the gain adjustment circuit in the PM detection device of the present invention as a parameter. It is an image figure which shows data sampling of the absolute value signal at the time of using discrete variable capacitor | condenser electrostatic capacitance in PM detection apparatus of this invention, and specification of a zero crossing point.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, a PM detection device 1 according to the present invention is electrically controlled with a PM sensor 3 in which the capacitance between two electrodes arranged in a DPF 2 changes depending on the amount of PM deposited. Variable resistor 4 and three fixed resistors 5, 6, 7 are sequentially connected, PM sensor 3 is connected in parallel to variable resistor 4, and electrically controlled to fixed resistor 5 adjacent to variable resistor 4. Of the four connection points of the bridge circuit 9 and the variable capacitor 8, the fixed resistor 5, the variable resistor 4, and the PM sensor 3 among the four connection points of the bridge circuit 9. A point a and a connection point b located diagonally there are voltage application points a and b, and a DC power source 10 and an AC for selectively applying a DC voltage and an AC voltage between the voltage application points a and b. Four connections of power supply 11 and bridge circuit 9 Differential amplifier 14 having an input terminal connected to each of measurement points c and d, which are two connection points sandwiched between voltage application points a and b, through buffer amplifiers 12 and 13, respectively, and a differential amplifier The absolute value signal generation circuit 15 for generating an absolute value signal representing the absolute value of the voltage between the measurement points c and d based on the output of the output signal 14 and the absolute value signal by applying a DC voltage between the voltage application points a and b. The variable resistor 4 is adjusted so that becomes zero, and then the variable capacitor 8 is adjusted so that the absolute value signal becomes zero by applying an AC voltage between the voltage application points a and b. And a detector 16 that detects the amount of PM deposited from the capacitance of the capacitor 8.

  The DPF 2 is a conventionally known one and is made of a ceramic having a large number of honeycomb pores.

  As shown in FIG. 2, the PM sensor 3 has a characteristic that the capacitance increases in proportion to an increase in the amount of PM trapped in the DPF 2.

  The PM sensor 3a shown in FIG. 3A is provided with one cylindrical piece-like electrode 31 along one half of the outer periphery of the columnar DPF 2, and another one with a cylindrical piece along the opposite half. Two electrodes 32 are provided. As a result, when the two electrodes 31 and 32 face each other with the DPF 2 sandwiched from both sides and PM is collected in the DPF 2, the capacitance changes due to the influence of the PM existing between the electrodes 31 and 32.

  In the PM sensor 3b shown in FIG. 3B, one cylindrical electrode 33 is provided so as to cover the entire outer periphery of the columnar DPF 2, and another cylindrical electrode 34 is provided at the center of the DPF 2. Is. As a result, when the two electrodes 33 and 34 are arranged concentrically inside and outside of the DPF 2 and PM is collected in the DPF 2, the capacitance changes due to the influence of the PM existing between the electrodes 33 and 34.

  The PM sensor 3c shown in FIG. 3C is provided with one cylindrical electrode 35 so as to cover the entire outer periphery of the columnar DPF 2, and a plurality of lines are arranged in the center of the DPF 2 in a cylindrical shape. Another electrode 36 is provided.

  The PM sensor 3d shown in FIG. 3D is provided with two mesh-shaped electrodes 37 and 38 upstream and downstream of a cylindrical DPF 2, respectively.

  Returning to the description of FIG.

  The variable resistor 4 is electrically controlled by the detection unit 16 and changes its resistance value. For example, rotating a potentiometer with a stepping motor, rotating a rotary potentiometer with a DC motor and controlling the rotation angle with a rotation angle sensor, and switching a ladder circuit consisting of multiple resistors Thus, the resistance value changes stepwise between appropriate upper limit resistance values and lower limit resistance values steplessly or in appropriate steps.

  As the fixed resistors 5, 6 and 7, conventionally known resistors are used. The resistance values of the fixed resistors 5, 6 and 7 may be different from each other or may be equal to each other.

  The variable capacitor 8 includes a rotary air variable condenser that is rotated by a stepping motor, a rotary air variable condenser that is rotated by a DC motor and a rotation angle sensor that controls the rotation angle, and a plurality of fixed capacitors having different electrostatic capacities. It is possible to obtain a plurality of composite capacitances with different values by changing the connection between the appropriate upper limit capacitance and the lower limit capacitance in a stepless manner or in an appropriate stepwise manner. The electric capacity changes.

  In the present embodiment, as shown in FIG. 4, the variable capacitor 8 includes a plurality of fixed capacitors 41 a to 41 h whose capacitances are different in proportion to powers of 2 and are connected in parallel via the switches 42 a to 42 h, respectively. Become. The switches 42a to 42h include relays and semiconductor switches.

The variable capacitor 8 is controlled to have a combined capacitance by a combination of opening and closing of the switches 42a to 42h. That is, when the capacitance of the fixed capacitor 41e is C ₀ , the capacitances of the fixed capacitors 41a to 41h are represented by C ₀ × 2 ⁿ (n = −3 to +4). When the switches 42a to 42g among the switches 42a to 42h are opened and only the switch 42h is closed, the capacitance of the variable capacitor 8 becomes 1 / 8C _{0 which} is the same as the fixed capacitor 41h, and the switches 42a to 42f are opened and the switches 42h and 42g are opened. When only is closed, the electrostatic capacity of the variable capacitor 8 becomes the electrostatic capacity (1/8 + 1/4) C _{0 in} parallel with the fixed capacitor 41h and the fixed capacitor 41g. In this way, the variable capacitor 8, from a minimum 1 / 8C ₀ up to (1/8 + 1/4 + 1/2 + 1 + 2 + 4 + 8 + 16) to C _0, is possible to change the capacitance stepwise ways 255 at 1 / 8C ₀ increments it can.

Further, as shown in FIG. 5, the plurality of fixed capacitors 41 a to 41 h are composed of a combination of unit capacitors 51 having a capacitance of C ₀ . For example, the fixed capacitor 41e includes only one unit capacitor 51. The fixed capacitor 41f is configured by connecting two unit capacitors 51 in series. The fixed capacitor 41g is configured by connecting four unit capacitors 51 in series. On the other hand, the fixed capacitor 41d is configured by connecting two unit capacitors 51 in parallel. The fixed capacitor 41c is configured by connecting four unit capacitors 51 in parallel. In this way, by using 45 unit capacitors 51 and using powers of 2 in series or in parallel, a plurality of fixed capacitors 41a to 41h having different capacitances in proportion to powers of 2 are formed. .

  Returning to the description of FIG.

  The variable capacitor 8 and the fixed resistor 5 of the bridge circuit 9 are preferably connected in parallel with a capacitance Cfc equivalent to the stray capacitance Cf associated with the PM sensor 3. Here, as shown in FIG. 6, the PM sensor 3 in the DPF 2 and the variable resistor 4 (not shown) of the bridge circuit 9 mounted on the circuit board 17 are connected by a signal harness 18. The stray capacitance Cf associated with the PM sensor 3 is provided by the signal harness 18. Therefore, if the stray capacitance Cf of the signal harness 18 is known, the capacitance Cfc can be realized by a capacitor as a single component. However, in this embodiment, the compensation harness 19 having the stray capacitance Cfc equal to the stray capacitance Cf of the signal harness 18, for example, the compensation using the same specification or the same product number as the signal harness 18 and the same length as the signal harness 18 is used. A harness 19 is connected to the variable capacitor 8 and the fixed resistor 5 in parallel. The compensation harness 19 is preferably wired along the signal harness 18 from the installation location of the circuit board 17 on which the bridge circuit 9 is mounted to the DPF 2 where the PM sensor 3 is installed.

  The bridge circuit 9 in FIG. 1 constitutes a so-called Wheatstone bridge having voltage application points a and b and measurement points c and d. The bridge circuit 9 is a resistance bridge made up of four resistors at the time of direct current, and from four alternating current impedances at the time of alternating current. It becomes an AC impedance bridge.

  As the DC power source 10, an in-vehicle battery power source or a secondary DC power source using a battery power source as a primary power source can be used. The DC power supply 10 can control whether or not to apply a DC voltage between the voltage application points a and b from the detection unit 16.

  The AC power supply 11 is for operating the PM sensor 3 and the variable capacitor 8 in the bridge circuit 9 as AC impedance, and is constituted by an oscillator, for example. The frequency is preferably a low frequency that does not cause the problem of unnecessary radiation, for example, several hundred KHz or less. The AC power supply 11 can control whether or not an AC voltage is applied between the voltage application points a and b from the detection unit 16.

  The buffer amplifier 12 receives the signal at the measurement point d with a much higher impedance than the resistance and capacitor of the bridge circuit 9 so as not to affect the state of the bridge circuit 9, and amplifies and transmits it to the subsequent stage. Similarly, the buffer amplifier 13 receives the signal at the measurement point c with high impedance and amplifies and transmits it to the subsequent stage.

  The differential amplifier 14 is an operational amplifier that amplifies and outputs a voltage difference between two input terminals.

  The absolute value signal generation circuit 15 includes a gain adjustment circuit 20 that amplifies the output of the differential amplifier 14 with an appropriate amplification degree by an operational amplifier and two gain commutators whose forward terminals are directed opposite to each other. Rectification circuit 21 that separates and outputs a positive voltage component and a negative voltage component from the output, and a smoothing that removes and smoothes the frequency component of AC power supply 11 from the separated signal of each component that is the output of rectification circuit 21 The circuit 22, the differential amplification / low-pass circuit 23 that subtracts and integrates the negative voltage from the positive voltage output from the smoothing circuit 22, and the maximum voltage value with respect to the output of the differential amplification / low-pass circuit 23 The clip circuit 24 regulates the maximum rated voltage of the digital circuit so as not to exceed it.

  The detection unit 16 is a programmable digital circuit that controls the DC power supply 10, the AC power supply 11, the variable resistor 4, and the variable capacitor 8, and includes an AD converter and is an output of the absolute value signal generation circuit 15. The voltage of the absolute value signal can be read as data. The detector 16 is preferably incorporated in an electronic control unit (ECU) that controls fuel injection and the like of the vehicle.

  The detector 16 applies a DC voltage between the voltage application points a and b, adjusts the variable resistor 4 so that the absolute value signal becomes 0, and then applies an AC voltage between the voltage application points a and b. Then, the variable capacitor 8 is adjusted so that the absolute value signal becomes 0, and the amount of accumulated PM is detected from the capacitance of the variable capacitor 8 at this time.

  By the way, when the resistance value of the variable resistor 4 and the capacitance of the variable capacitor are swept, the voltage between the two measurement points c and d on the bridge circuit 9 changes from positive to negative or from negative to positive. If the zero cross point at this time is specified, the balance of the bridge circuit 9 is obtained. However, in the present invention, the absolute value signal generation circuit 15 generates an absolute value signal representing the absolute value of the voltage between the measurement points c and d based on the output of the differential amplifier 14, and the detection unit 16 Then, the zero cross point is specified from the absolute value signal of the positive voltage. Therefore, in the detection unit 16, when the resistance value of the variable resistor 4 is swept within a predetermined range to obtain the balance of the resistance bridge, the resistance value change in which the absolute value signal approaches 0 and the absolute value signal move away from 0. Based on the change in resistance value, the resistance value at which the absolute value signal is 0 is extracted. Further, in the detection unit 16, when the capacitance of the variable capacitor 8 is swept within a predetermined range to obtain the balance of the AC impedance bridge, the capacitance change that the absolute value signal approaches 0 and the absolute value signal are 0. On the basis of the capacitance change away from the capacitance, the capacitance at which the absolute value signal becomes 0 is extracted.

  As shown in FIG. 7, the PM detection device 1 of the present invention detects the amount of PM accumulated in the DPF 2 inserted into the exhaust gas discharge passage 72 from the internal combustion engine 71 of the vehicle to the atmosphere. A PM sensor 3 is installed in the DPF 2. The bridge circuit 9, DC power supply 10 (in the case of a secondary DC power supply), AC power supply 11, differential amplifier 13, and absolute value signal generation circuit 15 are mounted on a circuit board 17. The circuit board 17 and the detection unit 16 can be installed in appropriate places such as a vehicle interior, an engine room, and a lower surface of the vehicle body. The circuit board 17 and the detection unit 16 may be integrated to form the same unit. The signal harness 18 and the compensation harness 19 are realized by, for example, a multicore cable in which a large number of four or more covered electric wires are accommodated in the same sheath.

  Hereinafter, the operation of the PM detection apparatus 1 of the present invention will be described.

  First, the basic principle of detecting the PM deposition amount will be described, and then the specific circuit operation will be described.

  In the PM sensor 3, the capacitance between the two electrodes changes substantially linearly as shown in FIG. 2 according to the amount of accumulated PM. Therefore, the amount of accumulated PM can be detected based on the capacitance of the PM sensor 3. This is because the PM, which is a conductor, is inserted between the electrodes, and the distance between the electrodes is apparently reduced and the capacitance is increased. Alternatively, the PM increases in the medium between the electrodes and the dielectric constant is increased. This is thought to be because the electric capacity increases.

  In the present invention, when the AC impedance bridge is in an equilibrium state in the bridge circuit 9 of FIG. 1, the capacitance of the variable capacitor 8 and the capacitance of the PM sensor 3 are equal, and therefore, based on the capacitance of the variable capacitor 8. To detect the amount of PM deposited. However, prior to this, it is necessary to obtain the balance of the resistance bridge in the bridge circuit 9. This is because there are a plurality of combinations of resistance values and capacitances that give balance when attempting to balance only with an AC impedance bridge, and the capacitance is not fixed to one.

  The detection unit 16 controls the DC power supply 10 to apply a DC voltage between the voltage application points a and b. In this state, the variable resistor 4 is adjusted so that the bridge circuit 9 is balanced. Specifically, the detection unit 16 outputs the absolute value signal generation circuit 15 while controlling the variable resistor 4 so as to sweep the resistance value between the upper limit resistance value and the lower limit resistance value of the variable resistor 4. Read the absolute value signal. When the bridge circuit 9 is balanced, there is no voltage difference between the measurement points c and d, so that the absolute value signal is 0 or a minimum (hereinafter also referred to as a minimum including 0). The detection unit 16 fixes the variable resistor 4 to a resistance value at which the absolute value signal is minimized.

  Thereafter, the detection unit 16 stops applying the DC voltage, controls the AC power supply 11, and applies an AC voltage between the voltage application points a and b. In this state, the variable capacitor 8 is adjusted so that the bridge circuit 9 is balanced. Specifically, the detection unit 16 controls the variable capacitor 8 so as to sweep the capacitance between the upper limit capacitance and the lower limit capacitance of the variable capacitor 8, and the absolute value signal generation circuit 15 Read the output absolute value signal. When the bridge circuit 9 is balanced, there is no voltage difference between the measurement points c and d, so that the absolute value signal is minimal (including 0).

  When the bridge circuit 9 is balanced in direct current and alternating current in this way, the detection unit 16 detects the amount of PM deposited based on the capacitance of the variable capacitor 8. Since the capacitance of the variable capacitor 8 is a value given by the control unit 16, if the comparison table of this value and the amount of PM deposited is set in the detection unit 16 in advance, the detection unit 16 The amount of PM deposition can be read from the table and used as the detection result.

  The bridge circuit 9 is not limited to the balance at the direct current and the balance at the alternating current once, but preferably the balance at the direct current and the balance at the alternating current are alternately repeated a plurality of times.

  Next, a specific circuit operation in the PM detection device 1 of FIG. 1 will be described in the case where an AC voltage is applied between the voltage application points a and b to balance the AC impedance bridge. FIG. 8 shows point g (output of gain adjustment circuit 20), point h (+ side output of smoothing circuit 22), point k (−side output of smoothing circuit 22), point m (absolute value signal generation) in FIG. The voltage waveform at the output of the circuit 15 is shown.

  Now, it is assumed that the capacitance of the PM sensor 3 and the capacitance of the variable capacitor 8 both change stepwise from, for example, a state where the capacitance of the PM sensor 3 is balanced at 100 pF to 101 pF. When an alternating voltage is applied between the voltage application points a and b, a difference occurs in the alternating voltage between the measurement points c and d according to the difference in capacitance. When the voltage between the measurement points c and d amplified by the differential amplifier 14 is amplified by the gain adjustment circuit 20 with an appropriate amplification degree, the capacitance of the PM sensor 3 is at the point g as shown in FIG. An AC waveform appears at a timing corresponding to the timing of the step-like change. The peak-to-peak value of this AC waveform is proportional to the difference between the capacitance of the PM sensor 3 and the capacitance of the variable capacitor 8 (AC impedance bridge unbalance).

  When the positive voltage component and the negative voltage component at the point g are separated and smoothed in the rectifier circuit 21, the point h rapidly rises from the start of the AC waveform at the point g and stabilizes to a positive predetermined value. A direct current waveform appears, and at point k, a direct current waveform that suddenly falls from the start of the alternating current waveform at point g and stabilizes to a predetermined negative value appears.

  In the differential amplification and low-pass circuit 23, the difference between the point h and the point k is taken. That is, the sign of the negative predetermined value is inverted and added to the positive predetermined value. At the same time, when low-pass (low-pass; equivalent to integration) is performed, a DC waveform that rises slowly from the start point of the AC waveform at point g appears at point m. Data sampling as an absolute value signal may be performed when this waveform becomes stable after a predetermined time has elapsed. At the point m, the voltage of the signal from the differential amplification / low-pass circuit 23 is clipped to the power supply voltage Vcc of the digital circuit + the forward voltage drop Vf of the diode, so that the AD converter in the ECU constituting the detection unit 16 In addition, a negative voltage is not input, and a high positive voltage exceeding the maximum rated input voltage of the AD converter is not input.

  In the circuit operation so far, when the AC voltage applied between the voltage application points a and b is used as a phase reference, the capacitance of the variable capacitor 8 is larger or smaller than the capacitance of the PM sensor 3. Then, the phase of the AC waveform between the measurement points c and d is reversed. When the voltage (peak-to-peak value) is defined as positive with respect to one phase (for example, when the capacitance of the variable capacitor 8 is smaller than the capacitance of the PM sensor 3), the other phase (PM sensor 3 Voltage (peak-to-peak value) is negative in the case where the capacitance of the variable capacitor 8 is larger than that of the variable capacitor 8.

  Therefore, as shown in FIG. 9, when the capacitance of the variable capacitor 8 is swept in the direction of increasing from the minimum value, the voltage (peak-to-peak value) between the measurement points c and d of the bridge circuit 9 is It gradually decreases from a large positive value, passes through the zero cross point, and reaches a large negative value.

  Next, since the output of the gain adjustment circuit 20 is obtained by amplifying the voltage between the measurement points c and d, theoretically, the peak-to-peak value of the output of the gain adjustment circuit 20 is the capacitance sweep range. It is proportional to the voltage (peak-to-peak value) between the measurement points c and d over the entire area. However, since the operational amplifier of the gain adjustment circuit 20 is actually saturated, as shown in FIG. 10, when the electrostatic capacitance of the variable capacitor 8 is swept in the direction of increasing from the minimum value, the gain adjustment circuit 20 The output (peak-to-peak value) shows a gentle slope when the gain is small, and the slope becomes steep when the gain is large, and reaches a peak at the saturation voltage.

  Next, the output of the differential amplification / low-pass circuit 23 shown in FIG. 11 is proportional to the absolute value of the output (peak-to-peak value) of the gain adjustment circuit 20 shown in FIG. Regardless of the magnitude of the gain given by the gain adjustment circuit 20, when the capacitance changes so that the output approaches 0, the output falls almost linearly to the right, and when the capacitance changes where the output moves away from 0, the output is It is almost linearly rising to the right. Therefore, it can be seen that the middle of the two changes (100 pF in this example) is the zero cross point. Therefore, with the absolute value signal that is the output of the absolute value signal generation circuit 15 clipped from the output of the differential amplification and low-pass circuit 23 as data, the capacitance change that approaches 0 and the electrostatic that the output moves away from 0 Based on the capacitance change, it is possible to extract the capacitance that becomes the zero cross point.

  Here, the output of the differential amplification / low-pass circuit 23 shows a gentle slope when the gain is small, and the slope becomes steep when the gain is large, and reaches a peak at the saturation voltage. In view of such characteristics, the gain of the gain adjustment circuit 20 also takes into account the voltage resolution and dynamic range of the AD converter of the detection unit 16 and the resolution when the capacitance of the variable capacitor 8 described later is discrete. The detection unit 16 is preferably set so that data used for specifying the zero-cross point can be suitably obtained. That is, when the output voltage is very small over a wide range of capacitance as in the case of gain = 1, data sampled with a low voltage resolution becomes 0 over a wide range of capacitance, and the zero cross point cannot be specified. On the other hand, if the gain is increased appropriately, the numerical value changes significantly with respect to the change in capacitance even for data sampled with a low voltage resolution, so that the zero cross point can be easily identified. On the other hand, when the change in the output voltage can be seen only in a narrow range of the electrostatic capacity as in the case of gain = 100, the number of data that can be sampled is small and it is difficult to specify the zero cross point.

  The gain of the gain adjustment circuit 20 may be determined in advance by obtaining an appropriate value through experiments or the like, or may be configured so that the gain of the gain adjustment circuit 20 can be controlled from the detection unit 16.

  In general, when trying to measure a value close to zero, the effect of noise is relatively large and the measurement accuracy decreases, but the effect of noise is eliminated by measuring many values from values far from zero to values close to zero. As a result, the measurement accuracy can be increased. Therefore, obtaining the zero cross point from the sweep result having an appropriate inclination contributes to increasing the measurement accuracy.

In the present invention, the variable capacitor 8 takes discrete values as shown in FIGS. 4 and 5. That is, the control for sweeping the electrostatic capacitance of the variable capacitor 8 is a control for changing the electrostatic capacitance to 256 pieces step by step (resolution) ΔC = 1 / 8C ₀ . For example, sweeping is performed in increments of 1 pF from 1 pF to 256 pF. At this time, the data input to the detection unit 16 is discrete data for each step ΔC as shown in FIG. Therefore, the detection unit 16 calculates a capacitance change in which the absolute value signal approximates to 0 and a capacitance change in which the absolute value signal moves away from 0 from these data. Here, each capacitance change is approximated by a straight line indicated by a one-dot chain line, but both capacitance changes may be approximated by a quadratic curve in a lump, and the pole may be obtained as a zero cross point.

  Up to this point, the case where the AC voltage is applied between the voltage application points a and b to balance the AC impedance bridge has been described. However, the DC voltage is applied between the voltage application points a and b to balance the resistance bridge. The operation is simpler and can be understood if the horizontal axis in FIGS.

  As described above, according to the PM detection device 1 of the present invention, the capacitance of the PM sensor 3 can be known by searching for the capacitance of the variable capacitor 8 at which the bridge circuit 9 is balanced. The capacitance of the variable capacitor 8 is accurate because it is a value given by the detector 16 under control. Therefore, the detected PM accumulation amount is accurate.

  According to the PM detection device 1 of the present invention, the bridge circuit 9 can be balanced regardless of the amplitude value of the AC voltage applied by the AC power supply 11, and therefore the AC voltage amplitude value does not need to be accurate. . For this reason, the oscillator circuit which comprises AC power supply 11 can be made into a simple structure. As a result, the AC power supply 11 can be made highly reliable while being inexpensive.

  According to the PM detector 1 of the present invention, the output of the differential amplifier 14 based on the voltage between the measurement points c and d fluctuates positive and negative, whereas the absolute value signal generated by the absolute value signal generation circuit 15 is positive. Since the voltage is a voltage, only a positive voltage signal is input to the detection unit 16, and even when the detection unit 16 is configured by an ECU that is a digital circuit, the zero cross point of the voltage between the measurement points c and d is set. The identification process becomes easy.

1 PM detector 2 DPF (diesel particulate filter)
DESCRIPTION OF SYMBOLS 3 PM sensor 4 Variable resistor 5, 6, 7 Fixed resistor 8 Variable capacitor 9 Bridge circuit 10 DC power supply 11 AC power supply 12 Buffer amplifier 13 Buffer amplifier 14 Differential amplifier 15 Absolute value signal generation circuit 16 Detection part 17 Circuit board 18 Signal harness 19 Compensation harness 20 Gain adjustment circuit 21 Rectification circuit 22 Smoothing circuit 23 Differential amplification and low-pass circuit 24 Clip circuit 41a to 41h Fixed capacitor 42a to 42h Switch

Claims (4)

A PM detection device for detecting the amount of particulate matter (hereinafter referred to as PM) deposited in a diesel particulate filter (hereinafter referred to as DPF) inserted in an exhaust gas exhaust passage from an internal combustion engine to the atmosphere,
A PM sensor in which the capacitance between two electrodes arranged in the DPF varies with the amount of PM deposited;
An electrically controlled variable resistor and three fixed resistors are sequentially connected, the PM sensor is connected in parallel to the variable resistor, and one of the fixed resistors adjacent to the variable resistor is electrically connected A bridge circuit in which variable capacitors to be controlled are connected in parallel;
Of the four connection points of the bridge circuit, the connection point where the variable capacitor and the fixed resistor, the variable resistor and the PM sensor are connected, and the connection point located on the diagonal are the voltage application points. A DC power source and an AC power source for selectively applying a DC voltage and an AC voltage between the voltage application points;
A differential amplifier having an input terminal connected to each of measurement points, which are two connection points sandwiched between the voltage application points, among the four connection points of the bridge circuit;
An absolute value signal generation circuit that generates an absolute value signal representing the absolute value of the voltage between the measurement points based on the output of the differential amplifier;
The variable resistor is adjusted so that the absolute value signal becomes 0 by applying a DC voltage between the voltage application points, and then the AC voltage is applied between the voltage application points and the absolute value signal becomes 0. A PM detection apparatus comprising: a detection unit that adjusts the variable capacitor so that the amount of PM accumulated is detected from the capacitance of the variable capacitor at this time. The detection unit sweeps the resistance value of the variable resistor in a predetermined range, and the absolute value is based on a resistance value change in which the absolute value signal approaches 0 and a resistance value change in which the absolute value signal moves away from 0. Extract the resistance value for which the signal is 0,
The capacitance value of the variable capacitor is swept within a predetermined range, and the absolute value signal is 0 based on the capacitance change in which the absolute value signal approaches 0 and the capacitance change in which the absolute value signal moves away from 0. The PM detection device according to claim 1, wherein the capacitance is extracted.   3. The PM detection device according to claim 1, wherein a compensation harness having a stray capacitance equal to a stray capacitance of a signal harness connecting the PM sensor and the variable resistor is connected in parallel to the variable capacitor. .   In the variable capacitor, a plurality of fixed capacitors having different electrostatic capacities in proportion to powers of 2 are connected in parallel through switches, and are controlled to discrete composite capacities by a combination of opening and closing of the switches. The PM detection device according to claim 1, wherein

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