JP4897635B2 - Liquid state detection sensor - Google Patents

Description

  The present invention relates to a liquid state detection sensor that detects the state of a liquid such as the concentration of a specific component contained in the liquid.

  A NOx selective reduction catalyst (SCR) may be used in an exhaust gas purification device that reduces nitrogen oxide (NOx) discharged from a diesel vehicle to a harmless gas, and a urea aqueous solution is used as the reducing agent. It is known that an aqueous urea solution having a urea concentration of 32.5 wt% may be used to efficiently perform this reduction reaction. However, an aqueous urea solution stored in a urea water tank mounted on an automobile is stored under severe environmental conditions, and the urea concentration may change due to changes over time. In addition, a different aqueous solution (for example, light oil) or water may be mistakenly mixed into the urea water tank. For this reason, a concentration sensor (liquid state detection sensor) for detecting the urea concentration is attached to the urea water tank so that the urea concentration of the urea aqueous solution can be managed, and the concentration detection is performed.

  By the way, it is known that the thermal conductivity of the urea aqueous solution varies depending on the concentration of urea contained in itself, and when the heating resistor having the property that the resistance value changes according to the temperature is heated for a certain time, The degree of change in the resistance value of the heating resistor varies depending on the urea concentration of the urea aqueous solution around the heating resistor. From this, the concentration of urea contained in the urea aqueous solution can be detected by grasping the degree of change in the resistance value of the heating resistor when the heating resistor is heated for a certain time (see, for example, Patent Document 1). .

In such a liquid state detection sensor, the resistance value change of the heating resistor is less affected by its own heat generation, so that the heating resistor is energized for a short period when measuring the urea concentration. The resistance value change of the heating resistor due to the energization for a short period is small, and in order to accurately measure the concentration, it is necessary to supply stable power to the heating resistor. Therefore, in general, for example, a constant current output circuit 500 as shown in FIG. 4 is configured to energize the heating resistor 507 to stabilize power supply. Specifically, the current detection resistor 502, the transistor 504, and the heating resistor 507 are connected in series to the power source 501, and are connected to the ground. A differential amplifier circuit 503 that detects a potential difference Vi between both ends of the current detection resistor 502 is connected to both ends of the current detection resistor 502, and an output of the differential amplifier circuit 503 together with a reference potential Vs supplied from a reference power source 506. This is input to the operational amplifier 505. The output of the operational amplifier 505 is input to the base of the transistor 504. Since the constant current output circuit 500 has such a configuration, even if the power supplied from the power source 501 becomes unstable and the current flowing through the current detection resistor 502 varies, the comparison result between the potential difference Vi and the reference potential Vs. By controlling the operational amplifier 505 based on the above, the magnitude of the current flowing between the collector and emitter of the transistor 504, that is, the current flowing through the heating resistor 507 is kept constant.
JP 2007-114181 A

  However, in the circuit configuration in which feedback control is performed to keep the current flowing through the heating resistor 507 constant using the differential amplifier circuit 503 and the operational amplifier 505 like the constant current output circuit 500, the number of electronic components increases. For this reason, the speed of the feedback control as described above tends to decrease. Since the detection of the urea concentration is performed by a short-term energization of the heating resistor, speeding up of the feedback control has been demanded so that the detection accuracy can be further improved.

  The present invention has been made in order to solve the above-described problems, and it is possible to increase the speed of feedback control for stabilizing the power supplied to the heating resistor and to further improve the detection accuracy of the liquid state. An object is to provide a liquid state detection sensor.

  In order to achieve the above object, the liquid state detection sensor according to the first aspect of the present invention is based on the heat dissipation characteristics of the heating resistor with respect to the liquid when the heating resistor is energized for a certain period of time. In the liquid state detection sensor for detecting the state of the liquid in the case, in order to energize the heating resistor, a power supply unit disposed on one end side of the heating resistor and a current flowing through the heating resistor are detected. In addition, the heating resistor is arranged on one end side of itself, the other end of itself is connected to the ground, and the heating resistor based on the magnitude of the current detected by the current detection unit Current adjusting means disposed between the heating resistor and the current detecting means, the power supply unit, the heating resistor, the current adjusting means, And the current detection means are connected in series in this order to form a closed loop circuit via the ground, and a reference voltage and a voltage at one end of the current detection means are input, and the comparison result of both voltages And a control means for controlling the driving state of the current adjusting means.

  According to a second aspect of the present invention, in addition to the configuration of the first aspect of the invention, the liquid state detection sensor may be configured such that the control means is configured to input the voltage at the one end of the current detection means to the control means. A control signal for controlling the driving or non-driving state is superimposed and input.

  According to a third aspect of the present invention, in addition to the configuration of the first or second aspect of the invention, the power supply unit includes a waveform of a voltage supplied from the power supply unit to the heating resistor. The noise removal means for removing the fluctuation noise generated in is provided.

  According to a fourth aspect of the present invention, in addition to the configuration of the third aspect of the invention, the noise removing means is a step-down linear regulator.

  In the liquid state detection sensor according to the first aspect of the present invention, the current detection means is disposed downstream (ground side) of the current adjustment means, and the potential difference between both ends of the current detection means can be detected based on the potential difference from the ground. Then, the current adjustment means can be controlled by the control means by comparing this potential difference with the reference voltage, so that a differential amplifier circuit for detecting the potential difference between both ends of the current detection means becomes unnecessary, and the electric circuit Since the configuration can be simplified, the speed of feedback control can be increased. Since the detection of the liquid state based on the heat dissipation characteristics of the heating resistor is performed through short-term energization of the heating resistor, the liquid state can be detected more accurately by speeding up the feedback control. Can do.

  The liquid state detection sensor according to the present invention detects the liquid state based on the heat dissipation characteristic of the heating resistor with respect to the liquid when the heating resistor is energized for a certain period of time. Examples of the object whose state is originally detected include concentration detection of a specific component contained in the liquid and liquid type detection.

  In the invention according to claim 2, the control signal for controlling the driving / non-driving of the control means is input not to the reference voltage input side but to the current detection means connected side. That is, when the control means is in a non-driven state, the output of the control means is maintained on the low potential side. Since the control of the current adjusting means by the control means is performed with a relatively low voltage, it takes time to return the output of the control means to a voltage that can be controlled by the current adjusting means when the control means is in a driving state. Is shorter than the case where the output of the control means is maintained on the high potential side and the control means is not driven. Therefore, the speed of the feedback control can be increased, and the liquid state can be detected more accurately.

  By the way, the waveform of the voltage supplied from the power supply unit to the heating resistor shows a so-called overshoot that temporarily exceeds the specified voltage until it stabilizes after the supply starts, or is temporarily lower than the specified voltage. In some cases, the camera may wobble due to an undershoot. Further, when the power supply unit is configured to include a booster circuit, the switching noise of the switching element that configures the booster circuit may be superimposed on the waveform of the voltage supplied to the heating resistor (this specification) In the book, fluctuations and switching noises that occur in such voltage waveforms are generically referred to as fluctuation noises). Therefore, in the invention according to claim 3, if the fluctuation noise generated in such a voltage waveform is removed by the noise removing means, stable power can be supplied to the heating resistor immediately after the voltage supply from the power supply unit is started. Thus, the liquid state can be detected more accurately.

  As such a noise removing means, if a step-down linear regulator is used as in the invention according to claim 4, there is no generation of fluctuation noise due to its own drive as compared with the case where a switching regulator is used. Stable power can be supplied to the resistor.

  Hereinafter, an embodiment of a liquid state detection sensor embodying the present invention will be described with reference to the drawings. First, the structure of a liquid state detection sensor 100 as an example will be described with reference to FIG. FIG. 1 is a longitudinal sectional view in which a part of the liquid state detection sensor 100 is cut away. In the liquid state detection sensor 100, the longitudinal direction of the level detection unit 70 (capacitor constituted by the outer cylinder electrode 10 and the internal electrode 20) is the axis O direction, and the side on which the liquid property detection unit 30 is provided is the front end side and attached. The side where the metal fitting 40 is provided is the rear end side.

  The liquid state detection sensor 100 of the present embodiment is a sensor for detecting the state of an aqueous urea solution used for reduction of nitrogen oxides (NOx) contained in exhaust gas of a diesel vehicle. Specifically, the level (liquid level) of the urea aqueous solution, the temperature, and the concentration of urea as a specific component contained in the solution are detected. As shown in FIG. 1, the liquid state detection sensor 100 includes a cylindrical outer cylinder electrode 10 and a cylindrical shape provided along the axis O direction of the outer cylinder electrode 10 inside the outer cylinder electrode 10. A level detection unit 70 including the internal electrode 20 is included. Moreover, the liquid property detection part 30 provided in the front end side of the internal electrode 20 and the attachment metal fitting 40 for attaching the liquid state detection sensor 100 to the urea water tank 98 (refer FIG. 2) are provided.

  The outer cylinder electrode 10 is made of a metal material and has a long and thin cylindrical shape extending in the axis O direction. A plurality of narrow slits 15 are intermittently opened along each bus bar on three bus bars that are equally spaced in the circumferential direction on the outer periphery of the outer cylindrical electrode 10. Further, at the distal end portion 11 of the outer cylinder electrode 10, an opening portion 16 for preventing a rubber bushing 80 interposed between the inner electrode 20 (described later) from coming off is formed on each bus bar where the slit 15 is formed. Each is provided. Furthermore, one air vent hole 19 is formed on a bus bar different from each bus bar where the slits 15 are formed at a position close to the base end portion 12 on the rear end side of the outer cylinder electrode 10.

  The tip 11 of the outer cylinder electrode 10 is positioned from the position of the opening 16 so as to surround the protector 130 that covers and protects the ceramic heater 110 in the radial direction of the liquid property detector 30 described later. Furthermore, it extends to the front end side in the axis O direction. And the most advanced part (lowermost part in a figure) of the outer cylinder electrode 10 is opened.

  Further, the base end portion 12 of the outer cylindrical electrode 10 is welded to an electrode support portion 41 provided at the distal end of a metal mounting bracket 40 while being engaged with the outer periphery thereof. The mounting bracket 40 functions as a pedestal for fixing the liquid state detection sensor 100 to the urea water tank 98, and a mounting hole (not shown) for inserting a mounting bolt is bowl-shaped on the rear end side of the electrode support portion 41. It is opened to the collar part 42 provided in the.

  On the opposite side of the electrode support portion 41 across the flange portion 42 of the mounting bracket 40, a housing portion 43 is formed that is surrounded by a wall surface standing upright from the flange portion 42 and has a concave shape. The housing 43 has a circuit for detecting the level, temperature, urea concentration, and the like of a urea aqueous solution, which will be described later, and an electric circuit with an external circuit (not shown) (an engine control unit (ECU) of an automobile in this embodiment). A circuit board 60 on which an input / output circuit or the like for performing a general connection is mounted is accommodated. Since the mounting bracket 40 is connected to the circuit board 60 so as to have the same potential as a wiring portion (not shown) forming the ground potential, the outer cylinder electrode 10 is grounded via the mounting bracket 40. Has been.

  The circuit board 60 is placed on a board placement portion (not shown) protruding from the four corners of the inner wall surface of the housing portion 43. The accommodating portion 43 is covered and protected by a cover 45, and the cover 45 is fixed to the flange portion 42. A connector 62 is fixed to the side surface of the cover 45, and a connection terminal (not shown) of the connector 62 and a pattern (an input / output circuit unit 290 described later) on the circuit board 60 are connected by the wiring cable 61. Yes. The circuit board 60 and the ECU are connected via the connector 62.

  A hole 46 penetrating into the accommodating portion 43 is opened in the electrode support portion 41 of the mounting bracket 40, and the base end portion 22 of the internal electrode 20 is inserted into the hole 46. The internal electrode 20 of the present embodiment is made of a long and thin cylindrical metal material extending in the direction of the axis O. On the outer peripheral surface of the internal electrode 20, an insulating film 23 made of a fluorine resin such as PTFE, PFA, ETFE, an epoxy resin, a polyimide resin, or the like is formed. Between the internal electrode 20 and the outer cylinder electrode 10, a level detection unit 70 is formed, in which a capacitor whose capacitance changes according to the level of the urea aqueous solution is formed.

  A pipe guide 55 and an inner case 50 for fixing the internal electrode 20 to the mounting bracket 40 are disposed at the proximal end portion 22 on the rear end side in the axis O direction of the internal electrode 20. The pipe guide 55 is an annular guide member joined near the end edge of the base end portion 22 of the internal electrode 20. The inner case 50 is a tubular resin member with a flange that positions and supports the internal electrode 20 so that the internal electrode 20 and the outer cylindrical electrode 10 are reliably insulated, and the tip side of the inner case 50 is the electrode support portion 41 of the mounting bracket 40. It is inserted in the hole 46. The inner case 50 is formed with a flange portion 51 that protrudes radially outward. When the inner case 50 is engaged with the electrode support portion 41, a hole in the electrode support portion 41 is formed from the housing portion 43 side. 46 is inserted. Then, the flange portion 51 abuts against the bottom surface in the housing portion 43, thereby preventing the inner case 50 from passing through the hole 46. In addition, the internal electrode 20 is inserted into the inner case 50 from the accommodating portion 43 side, but the pipe guide 55 is brought into contact with the flange portion 51 so that the inner electrode 20 is prevented from falling off the inner case 50.

  Further, an O-ring 53 and an O-ring 54 are provided on the outer periphery and the inner periphery of the inner case 50, respectively. The O-ring 53 seals a gap between the outer periphery of the inner case 50 and the hole 46 of the mounting bracket 40, and the O-ring 54 is formed between the inner periphery of the inner case 50 and the outer periphery of the base end portion 22 of the internal electrode 20. The gap between them is sealed. Thereby, when the liquid state detection sensor 100 is attached to the urea water tank 98 (see FIG. 2), the water tightness and the inside of the urea water tank 98 are prevented from communicating with each other through the housing portion 43. Airtightness is maintained. Note that a plate-like seal member (for example, rubber packing) (not shown) is attached to the surface on the tip side of the flange portion 42 of the mounting bracket 40, and when the liquid state detection sensor 100 is attached to the urea water tank 98, The watertightness and airtightness between the portion 42 and the urea water tank 98 are maintained.

  When the internal electrode 20 is assembled to the mounting bracket 40, the pipe guide 55 is pressed against the flange 51 of the inner case 50 by the two pressing plates 56 and 57. The insulating pressing plate 56 is fixed in the accommodating portion 43 with screws 58 in a state where the pressing plate 57 is sandwiched between the insulating guide plate 56 and the pipe guide 55. Thereby, the internal electrode 20 joined to the pipe guide 55 is fixed to the electrode support portion 41. A hole 59 is opened in the center of the holding plates 56 and 57, and two lead wires 90 (in FIG. 1) for electrically connecting the electrode lead wire 52 of the internal electrode 20 and a ceramic heater 110 described later. A two-core cable 91 containing only one of the lead wires 90 is inserted and electrically connected to the pattern on the circuit board 60. An electrode (not shown) on the ground side of the circuit board 60 is connected to the mounting bracket 40, and the outer cylinder electrode 10 welded to the mounting bracket 40 is electrically connected to the ground side.

  Next, the liquid property detection unit 30 provided at the tip portion 21 of the internal electrode 20 is a ceramic heater 110 as a liquid property detection element that detects the temperature of the urea aqueous solution and the concentration of urea contained in the present embodiment. Have The ceramic heater 110 is made of two plates made of an insulating ceramic, embedded in a heating resistor 114 (see FIG. 2) mainly composed of Pt or W, and fired. The ceramic heater 110 is supported by a holder 120 made of an insulating resin that is attached to the distal end portion 21 of the internal electrode 20. The holder 120 has a cylindrical shape with outer diameters of two steps, and the ceramic heater 110 in a state where the side where the heating resistor 114 is embedded is exposed at the tip end side having a small diameter is used as an adhesive. It fixes with the fixing members 125 and 126 which consist of. The rear end side, which is the large diameter side, is attached to the distal end portion 21 of the internal electrode 20, and a seal ring 140 is interposed between the outer peripheral surface of the internal electrode 20 and the inner peripheral surface of the holder 120. The water-tightness and air-tightness of the interior of the are secured. Further, the portion of the ceramic heater 110 exposed from the holder 120 is covered and protected by a protector 130.

  By the way, before the holder 120 is mounted, the core wires of the two lead wires 90 of the cable 91 are joined to the connector 119 of the ceramic heater 110 by caulking or soldering, respectively. Further, the insulating protection member 95 covers and protects the connector 119 and the lead wire 90 together with the joint portion. The two lead wires 90 are inserted through the cylindrical internal electrode 20 and connected to the circuit board 60.

  Next, the protector 130 is a metal protective member formed in a bottomed cylindrical shape. The opening side is fitted to the outer periphery of the small diameter portion of the holder 120. Further, a liquid circulation hole (not shown) is opened on the outer periphery of the protector 130, and the urea aqueous solution is exchanged inside and outside the protector 130. The liquid property detection unit 30 described above is attached to the distal end portion 21 of the internal electrode 20 via the holder 120 and is elastically supported in the outer cylindrical electrode 10 by the rubber bush 80. The rubber bush 80 has a cylindrical shape, and a protrusion 87 formed on the outer peripheral surface thereof is engaged with and fixed to the opening 16 of the outer cylinder electrode 10. A plurality of grooves (not shown) along the axis O direction are provided in each of the outer peripheral surface and the inner peripheral surface of the rubber bush 80. When the liquid state detection sensor 100 is attached to the urea water tank 98, liquid exchange and bubble removal between the urea aqueous solution flowing into the front end side of the rubber bush 80 and the urea aqueous solution flowing into the rear end side through this groove are performed. Is done.

  Next, the electrical configuration of the liquid state detection sensor 100 will be described with reference to FIG. FIG. 2 is a diagram illustrating an electrical configuration of the liquid state detection sensor 100. As shown in FIG. 2, the liquid state detection sensor 100 is attached to a urea water tank 98 as a liquid container, and includes a level detection unit 70 including a pair of electrodes (the outer cylinder electrode 10 and the internal electrode 20), and a heating resistance. The liquid property detection unit 30 including the ceramic heater 110 in which the body 114 is embedded is immersed in a urea aqueous solution as a state detection target liquid stored in the urea water tank 98.

  A microcomputer 220 having a known CPU, ROM, RAM, and the like is mounted on the circuit board 60 of the liquid state detection sensor 100. The microcomputer 220 includes a level detection circuit unit 250 that controls the level detection unit 70, a liquid property detection circuit unit 280 that controls the liquid property detection unit 30, and an input / output circuit unit 290 that communicates with the ECU. And are connected.

  The input / output circuit unit 290 controls the communication protocol in order to input and output signals between the liquid state detection sensor 100 and the ECU. Further, the level detection circuit unit 250 applied an AC voltage between the outer cylinder electrode 10 and the internal electrode 20 of the level detection unit 70 based on an instruction from the microcomputer 220, and flowed through a capacitor forming the level detection unit 70. The circuit unit converts the current into voltage and outputs the voltage signal to the microcomputer 220.

  Next, the liquid property detection circuit unit 280 applies a constant current to the ceramic heater 110 of the liquid property detection unit 30 based on an instruction from the microcomputer 220, and supplies the detection voltage generated at both ends of the heating resistor 114 to the microcomputer 220. It is the circuit part which outputs. The liquid property detection circuit unit 280 includes a differential amplifier circuit unit 230 and a constant current output circuit unit 300. The differential amplifier circuit section 230 performs A / D conversion on the difference between the potential Pin appearing at one end of the heating resistor 114 and the potential Pout appearing at the other end as a detection voltage, and outputs it to the microcomputer 220.

  The constant current output circuit unit 300 is a circuit unit for outputting a constant current that flows through the heating resistor 114. The constant current output circuit unit 300 includes a power supply unit 390, a noise removal unit 330, a switch unit 340, a MOS-FET 350, a current detection resistor 360, an operational amplifier 370, and a reference power supply 380.

  The power supply unit 390 includes a battery 310 that is a DC power supply and a booster circuit unit 320. Further, a noise removing unit 330 is connected between the power supply unit 390 and the heating resistor 114. The battery 310 is a power source that generates power for energizing the heating resistor 114, and has one end connected to the ground. The other end of the battery 310 is connected to one end of a booster circuit unit 320 configured by a known switching circuit. In the booster circuit unit 320, the voltage supplied from the battery 310 is boosted. One end of a noise removing unit 330 is connected to the other end of the booster circuit unit 320. The noise removing unit 330 is a step-down linear regulator (an example of a circuit configuration is shown in FIG. 2) having a known configuration. While the voltage once increased by the step-up circuit unit 320 is dropped, switching noise and battery voltage are reduced. By removing fluctuation noise generated in the voltage waveform such as voltage fluctuation caused by fluctuation, the voltage (and thus power) supplied to the heating resistor is stabilized. The noise removing unit 330 corresponds to the “noise removing unit” in the present invention.

  One end of the heating resistor 114 is connected to the other end of the noise removing unit 330. As described above, the heating resistor 114 is immersed in the aqueous urea solution and generates heat due to the current flowing through it. The other end of the heating resistor 114 is connected to the drain of an N-type MOS-FET 350 provided for adjusting the magnitude of the current flowing through the heating resistor 114. One end of a current detection resistor 360 provided for detecting the magnitude of the current flowing through the heating resistor 114 is connected to the source of the MOS-FET 350, and the other end is connected to the ground. As described above, the battery 310, the booster circuit unit 320, the noise removal unit 330, the heating resistor 114, the MOS-FET 350, and the current detection resistor 360 are connected in this order to form a closed loop circuit through the ground. The MOS-FET 350 corresponds to the “current adjusting unit” in the present invention, and the current detection resistor 360 corresponds to the “current detecting unit” in the present invention.

  Further, the output terminal of an operational amplifier 370 provided for controlling the magnitude of the current flowing between the drain and the source is connected to the gate of the MOS-FET 350. One end of a current detection resistor 360 is connected to the inverting input terminal (−input terminal) of the operational amplifier 370, and the non-inverting input terminal (+ input terminal) is connected to a reference power source 380 that generates a reference voltage. Has been. An output from the switch unit 340 controlled by the microcomputer 220 is also connected to the inverting input terminal of the operational amplifier 370, and a control signal (potential) for controlling the driving / non-driving of the MOS-FET 350 is a current. Superposed on the potential S appearing at one end from the detection resistor 360. The operational amplifier 370 corresponds to the “control unit” in the present invention.

  Here, the principle of detecting the state of the urea aqueous solution by the heating resistor 114 will be briefly described. Within a short period of time after the start of energization of the heating resistor 114, the heating resistor has not yet generated a large amount of heat, so the temperature of the heating resistor itself is almost the same as the temperature of the urea water liquid present around itself. Are the same. As the time elapses, the temperature of the heating resistor 114 itself increases continuously. Therefore, it is possible to measure the temperature of the urea aqueous solution by confirming in advance the correlation between the change in the resistance value of the heating resistor 114 and the temperature of the urea aqueous solution present in the surroundings in a short period from the start of energization. . In addition, it is known that a difference in thermal conductivity occurs between aqueous urea solutions depending on the concentration (the concentration of urea in the solution). That is, the rate of temperature rise of the heating resistor 114 varies depending on the thermal conductivity of the aqueous urea solution, that is, the concentration. From this, the urea concentration of the urea aqueous solution can be obtained based on the degree of change in the resistance value of the heating resistor 114 when the heating resistor 114 is heated for a certain time.

  Based on this principle, in the liquid state detection sensor 100, the detection voltage V1 detected by the differential amplifier circuit unit 230 immediately after the energization of the heating resistor 114 is started (specifically, 10 ms after the energization starts). Based on the above, the temperature of the urea aqueous solution is measured by the microcomputer 220. Further, in the liquid state detection sensor 100, the detection voltage V2 detected by the differential amplifier circuit 230 and the heating resistor 114 after a short energization time of 700 ms has elapsed since the energization of the heating resistor 114 is started. The microcomputer 220 calculates a difference value from the detected voltage V1 detected immediately after the start of energization of the battery, and measures the urea concentration based on this difference value. In order to perform these measurements more accurately, it is important to pass a constant current through the heating resistor 114. That is, if the influence of the voltage fluctuation in the battery 310 or the switching noise in the booster circuit unit 320 is reduced and further a constant current is applied, the fluctuation of the resistance value of the heating resistor 114 itself due to an external factor is reduced, and an accurate measurement result Can be obtained.

  In the liquid property detection circuit unit 280, in the power supply unit 390, the power supplied from the battery 310 is once boosted in the boosting circuit unit 320, and then reduced in the noise removing unit 330, whereby the voltage due to switching noise and battery voltage fluctuations. It is possible to remove the fluctuation noise generated in the voltage waveform such as wobbling. The waveform of the voltage supplied from the battery 310 shows a so-called overshoot that temporarily exceeds the specified voltage until it stabilizes after the supply is started, or shows a so-called overshoot that temporarily falls below the specified voltage. There is a case. If the fluctuation noise generated in the voltage waveform is removed by the noise removing unit 330, that is, the step-down linear regulator, stable power can be supplied to the heating resistor 114 immediately after the start of power supply.

  The magnitude of the current flowing through the heating resistor 114 is detected as a potential S that appears at one end of the current detection resistor 360. This potential S is input to the operational amplifier 370 and compared with the potential T of the reference power source 380. The comparison result is input as an output to the gate of the MOS-FET 350, and the gate voltage is adjusted. If the potential S is higher than the potential T, the current flowing between the drain and source of the MOS-FET 350, that is, the current flowing through the heating resistor 114 is attenuated. If the potential S is lower than the potential T, the current flowing through the heating resistor 114 is amplified. By performing such feedback control, the liquid property detection circuit unit 280 can supply a constant current to the heating resistor 114.

  In the temperature measurement of the urea aqueous solution, it is necessary to detect the resistance value of the heating resistor 114 in a short time after the start of energization to the heating resistor 114 until the temperature change due to the heat generation of the heating resistor 114 itself occurs. There is. For this purpose, it is preferable that the current flowing through the heating resistor 114 is stabilized more quickly. In the present embodiment, a current detection resistor 360 that performs current detection is disposed downstream (ground side) from the MOS-FET 350 that performs current adjustment, and the potential S that appears at one end of the current detection resistor 360 is set to a potential difference from the ground. To be obtained as. The potential T of the reference power supply 380 to be compared with this potential S is also a potential difference from the ground. With this configuration, the circuit portion for obtaining the potential difference between both ends of the current detection resistor 360 is unnecessary, and the circuit configuration Simplification can be achieved and the reliability of the circuit can be improved. In addition, by simplifying the circuit configuration, the time until the control of the MOS-FET 350 by the operational amplifier 370 becomes effective can be further advanced, so that the speed of feedback control can be improved. Thereby, the detection accuracy of the temperature of urea aqueous solution and urea concentration can be improved.

  Further, even if the current flowing through the heating resistor 114 is fluctuated due to noise that cannot be completely removed by the noise removing unit 330 or noise due to external factors (for example, electromagnetic wave noise), the current detection resistor 360 is made to be more than the heating resistor 114. The noise can be detected by arranging on the ground side. In the feedback control performed to flow a constant current through the heating resistor 114, the influence of such noise is reduced, and the temperature and urea concentration detection accuracy of the urea aqueous solution can be increased.

  Further, since the control signal from the switch unit 340 that controls driving / non-driving of the operational amplifier 370 is input to the −input terminal side, the operational amplifier 370 is in a negative saturation state when not driven, that is, its output is 0V. It is in a state. It is known that it takes time for the operational amplifier 370 to return from the saturated state during driving, but in order to be able to output a relatively small voltage that is output as the gate voltage of the MOS-FET 350, the positive side It is faster to return from the negative saturation state than from the saturated state. As described above, when the control signal from the switch unit 340 is input to the negative input terminal of the operational amplifier 370, the time until the control of the MOS-FET 350 by the operational amplifier 370 becomes effective is further accelerated and the responsiveness is improved. can do.

  Needless to say, the present invention can be modified in various ways. For example, if the battery 310 can supply a voltage that is sufficiently higher than the voltage required for the heating resistor 114 to be supplied, the booster circuit unit 320 need not be provided. In addition, the switch unit 340 may be disposed at a position between the operational amplifier 370 and the MOS-FET 350 (indicated by a point U in FIG. 2), or a position between the noise removing unit 330 and the heating resistor 114 ( It may be arranged at a point V in FIG.

  Furthermore, the switch unit 340 may have a circuit configuration as shown in FIG. Specifically, a voltage dividing circuit in which two resistors 491 and 492 are connected in series is connected to the positive electrode side of the reference power supply 480, and one end of the voltage dividing circuit is grounded. Then, the voltage dividing point of the voltage dividing circuit is connected to the non-inverting input terminal of the operational amplifier 370. Further, the end of the switch unit 340 opposite to the end connected to the inverting input terminal of the operational amplifier 370 is connected between the reference power supply 480 and the resistor 491. Even in such a circuit configuration, the constant current output circuit unit 400 having the same function as that of the above embodiment is obtained.

  Further, the liquid state detection sensor 100 may be a sensor that does not have the level detection unit 70. In the liquid state detection sensor 100 of the above embodiment, the temperature detection of the urea aqueous solution and the urea concentration detection are performed, but the temperature detection of the urea aqueous solution may not be performed. Furthermore, instead of detecting the urea concentration, the liquid type may be detected.

  Further, the urea aqueous solution used as the liquid to be detected is merely an example, and other liquids may be used. The noise removing unit 330 may have a circuit configuration that simply removes noise using a capacitor. Further, the current adjusting unit is not limited to the MOS-FET 350, and may be configured by an NPN transistor or other switching element.

FIG. 3 is a longitudinal sectional view in which a part of the liquid state detection sensor 100 is cut away. 2 is a diagram illustrating an electrical configuration of a liquid state detection sensor 100. FIG. It is a figure which shows the circuit structure of the switch part 340 in the constant current output circuit part 400 as a modification. It is a figure which shows the constant current output circuit 500 of the conventional liquid state detection sensor.

Explanation of symbols

100 Liquid State Detection Sensor 114 Heating Resistor 330 Noise Remover (Noise Remover)
340 Switch unit 350 MOS-FET (current adjusting means)
360 Current detection resistor (current detection means)
370 operational amplifier (control means)
380, 480 Reference power supply 390 Power supply unit

Claims (4)

In the liquid state detection sensor for detecting the state of the liquid around the heating resistor based on the heat dissipation characteristics of the heating resistor with respect to the liquid when the heating resistor is energized for a certain time,
In order to energize the heating resistor, a power supply unit disposed on one end side of the heating resistor;
In order to detect the current flowing through the heating resistor, the heating resistor is disposed on one end side of the heating resistor, and the other end of the current detection unit is connected to the ground; and
A current adjustment disposed between the heating resistor and the current detection means for adjusting the magnitude of the current flowing through the heating resistor based on the magnitude of the current detected by the current detection means. Means and
The power supply unit, the heating resistor, the current adjustment unit, and the current detection unit are connected in series in this order to form a closed loop circuit via the ground,
The liquid further comprising: a control unit that receives a reference voltage and a voltage at one end of the current detection unit, and controls a driving state of the current adjustment unit according to a comparison result between the two voltages. Condition detection sensor.   2. The control means, wherein a control signal for controlling the control means to be driven or not driven is superimposed on the input of the voltage at the one end of the current detection means. The liquid state detection sensor described.   3. The liquid state detection according to claim 1, wherein the power supply unit includes a noise removing unit that removes fluctuation noise generated in a waveform of a voltage supplied from the power supply unit to the heating resistor. Sensor.   The liquid state detection sensor according to claim 3, wherein the noise removing unit is a step-down linear regulator.

Images