Tyre pressure detector

Abstract: A tire pressure detector of an automobile, comprises a magnet (38) having N and S poles and rotated in accordance with a pneumatic pressure in a tire, the N and S poles being rotatable in reverse, detecting means (5), arranged opposite to the magnet, for detecting magnetism and outputting a voltage signal changed to different magnetism level in accordance with rotation of the magnet, and determining means (40) for comparing the voltage signal with a positive or negative threshold value and determining whether the pneumatic pressure in the tire is within a suitable range, based on a polarity of the voltage signal represented by a comparison result.

Description: The present invention relates to a tire pressure detector adapted for detecting and monitoring pneumatic pressure in a tire attached to the body of an automobile or the like. In a conventional tire pressure detector attached to, for example, an automobile, a pressure sensor is mounted for each wheel and pneumatic pressure in the tire is directly detected by the sensor. A transmitter for transmitting an output of the pressure sensor is mounted on the side of a rotating tire, while the body has an antenna or the like for receiving a signal from the transmitter, so as to perform signal transmission therebetween. However, since the tire pressure detector of this type requires a transmitter and an antenna for transmitting and receiving signals, it has a complicated structure and the manufacturing cost is high. In addition, since an independent power source is required for a tire, it is difficult to detect exhaustion of a battery or to exchange the battery. To solve these problems, in recent years, a tire pressure detector of an improved type has been proposed, which has a sensor unit integrated with the tire, and a tire pressure detector mounted on the body. The sensor unit on the tire side has a magnet or the like which moves in accordance with the pneumatic pressure in the tire. The tire pressure detector has a detector unit for detecting movement of the magnet. FIG. 8 shows an example of the tire pressure detector of this type. FIG. 8 is a cross-sectional view showing the conventional tire pressure detector. A tire pressure detector 1 has a sensor unit 4, mounted on the outer peripheral surface of a rim portion 2a of a wheel 2, for detecting the pneumatic pressure in a tire 3 attached to the wheel 2. The tire pressure detector 1 also has a Hall element (detector) 5, fixed to the body side, for detecting a change in the sensor unit 4 and a monitoring circuit (not shown) for monitoring a detected value output from the Hall element 5. A communicating hole 6 is formed in the sensor unit 4. An inner portion 4a within the hole 6 communicates with an air chamber defined by the wheel 2 and the tire 3. The sensor unit 4 includes a rubber diaphragm 8 to which inner pressure of the tire is applied, a movable element 9 which can reciprocate along the axial direction of the sensor unit 4, a spring 10 for pushing the movable element 9 toward the air chamber 7, and a magnet 12 attached to the movable element 9 via an arm 11 projecting inward from the inner surface of the rim portion 2a of the wheel 2. In the tire pressure detector 1 having the above structure, the diaphragm 8 is deformed along the axis of the sensor unit 4 in accordance with the pneumatic pressure in the tire 3. Accordingly, the magnet 12 is moved together with the movable element 12 along the axis of the sensor unit 4. Thus, as the distance between the magnet 12 and the Hall element 5 is changed, the intensity of the magnetic field of the magnet 12, detected by the Hall effect of the Hall element 5, is changed. When the value detected by the Hall element 5 is out of a preset range, the monitoring circuit determines that the pneumatic pressure in the tire 3 is abnormal. In the conventional tire pressure detector 1 as described above, there is a problem that, if the distance between the magnet 12 and the Hall element 5 is not accurately set, a predetermined output cannot be obtained from the Hall element and the monitoring circuit may receive an erroneous result, e.g., an under pressure. Further, assuming that the temperature of an area around the tire 3 ranges from -40°C to 125°C, the output of the Hall element will considerably vary particularly at a relatively low temperature. Therefore, some temperature compensation is required, resulting in a complex circuit structure and a high manufacturing cost. Moreover, since the magnet 12 is exposed to the surrounding atmosphere, the magnetic force of the magnet may vary due to iron powder, in which case an accurate tire pressure cannot be detected. The Hall element 5 outputs a pulse signal as shown in FIG. 9, each time the tire 3 makes one rotation. In FIG. 9, a symbol T denotes a period determined by the rotation rate of the tire 3 and a symbol V denotes a peak value corresponding to the tire pressure. Thus, in the conventional tire pressure detector, it is necessary to compare the peak value V of the pulse signal with a preset reference voltage, in accordance with the rotation rate of the tire 3. If the comparison is performed at an inaccurate timing, the monitoring circuit will always receive an erroneous result of an under pressure. Furthermore, whenever the peak value V of the pulse signal is smaller than the preset reference voltage, the conventional tire pressure detector 1 determines that the pressure is insufficient. Therefore, even if a pulse signal is not obtained due to a malfunction of the detector, the detector may determine that the pressure is insufficient. Thus, the conventional detector has a drawback that a low pressure state is not distinguishable from a malfunction of the detector. In addition, the conventional tire pressure detector cannot detect tire pressure with high precision and reliability. The document EP 0 000 818 A1 which has been used to formulate the preamble part of claim 1, discloses a tire pressure detector of an automobile, comprising a magnet having N and S poles and rotated in accordance with a pneumatic pressure in a tire, the N and S poles being rotatable in reverse, and detecting means, arranged opposite to the magnet, to detect magnetism and output a voltage signal changed to different magnetism level in accordance with rotation of the magnet; i.e. corresponding to a rotation angle of the magnet. The angular position of the magnet is a measure of the tire pressure, and each position of the magnet will cause a different signal to be induced in a pick up coil as the magnet passes thereby during rotation of the vehicle wheel. An electronic logic circuit is connected to the pick up coil to interpret signals induced by the magnet. Finally, it is known from US 4,866,982 and from DE 44 25 398 A1 to use wave forms to decide whether a certain tire pressure or range of tire pressures is present. An object of the present invention is to provide a tire pressure detector which can detect whether the pneumatic pressure in a tire is within a suitable range with high precision and reliability, without an influence of a change in temperature or the like, and which can also detect a malfunction of the detector. Another object of the present invention is to provide a tire pressure detector, which can detect a pneumatic pressure in a tire without an influence of environmental conditions, such as a positional deviation due to low accuracy of attaching the sensor unit or the Hall element, so that the tire pressure can be detected accurately and reliably. These objects are solved by a tire pressure detector as claimed in claim 1. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic diagram for explaining a tire pressure detector of the present invention; FIG. 2 is a cross-sectional view showing the tire pressure detector; FIGS. 3A to 3C are diagrams showing the relationship between the direction of a magnet and the corresponding output voltage from a Hall element, when the tire pressure is within a suitable range; FIG. 4 is a block diagram of a monitoring circuit of the invention; FIGS. 5A to 5E are diagrams showing waveforms of outputs from a Hall element corresponding angles of a rotating magnet; FIG. 6 is a graph for explaining the operation of the present invention; FIG. 7 is a flowchart for explaining the operations of the present invention; FIG. 8 is a cross-sectional view showing a conventional tire pressure detector; and FIG. 9 is a diagram showing a waveform of an output of a Hall element of the conventional tire pressure detector. The present invention will be described with reference to FIG. 1. As shown in FIG. 1, an L-shaped pressure detector 31 is incorporated with a rim portion 2a of a wheel 2 to detect pneumatic pressure in a tire 3 attached to the wheel 2. Details of the structure of the pressure detector 31 will be described later with reference to FIG. 2. A Hall element 5 for detecting magnetism from the pressure detector 31 is arranged near one end of the pressure detector 31. A signal detected by the Hall element 5 is output to a monitoring circuit 40. A display unit 41 is connected to the monitoring circuit 40. The display unit 41 displays a pressure in the tire 3. Details of the pressure detector 31 will now be described with reference to FIG. 2. In FIG. 2, a through hole 6 is formed in a sensor unit (the pressure detector) 31 of a tire pressure detector A. An inner space 31a of the sensor unit 31 communicates with an air chamber 7 defined by the wheel 2 and the tire (not shown). The sensor unit 31 includes a diaphragm 32 made of rubber or the like and a piston (movable element) 33, which is attached to a distal end portion of the diaphragm 32 and capable of reciprocating along the axis of the sensor unit 31. The piston 33 is formed of, for example, a lubricant resin. With the above structure, the diaphragm 32 is deformed in accordance with the pneumatic pressure in the tire (not shown) and accordingly, the piston 33 is reciprocated. A rotary shaft 34, extending along the axis of the sensor unit 31, is arranged in a distal end portion 31b of the sensor unit 31. One end 34a of the rotary shaft 34 is rounded so that it can be brought into contact with the piston 33 at a point. An edge portion 34b is formed on the end 34a of the rotary shaft 34 and rotatably supported in a recess portion 33a of the piston 33. With this structure, the rotary shaft 34 is capable of reciprocating together with the piston 33, while it is rotating. The other end 34c of the rotary shaft 34 is caulked so that the rotary shaft 34 may not be removed from a support 35 formed integral with the sensor unit 31. A threaded portion 36, to be engaged with the support 35, is formed on the side of the other end 34c with a lead of an amount of, for example, 6 mm. An annular magnet 38 is fixed by adhesive or the like to an edge portion 34d on a surface perpendicular to the axis of the rotary shaft 34. The upper portion of the magnet is the N pole and the lower portion thereof is the S pole. The rotary shaft 34 is capable of reciprocating in the axial direction with 3 mm stroke. The ends of a twisted coil spring (biasing member) 39 are fixed respectively to the edge portion 34d of the rotary shaft 34 and the support 35, so that the rotary shaft 34 is biased to be rotated in the axial direction can be urged. The twisted coil spring 39 has been compressed to a degree and twisted in the rotary direction of the rotary shaft 34 in advance, so as to rotate the rotary shaft 34 in a direction in which it is moved to a proximal end 31c of the sensor unit 31, when the pressure applied to the diaphragm 32 becomes a preset value. The sensor unit is prevented from rotating by means of a projection 31d and a recess portion 26 of a rim portion 2b, and fixed to a predetermined position. In the tire pressure detector A, when the pneumatic pressure in the tire (not shown) is within a preset suitable range (e.g., in the case of an automobile, 2.0 kg/cm 2 ), the Hall element 5 detects the S pole of the magnet 38, as shown in FIG. 2. When the pneumatic pressure in the tire (not shown) is reduced under the lower limit (to e.g., 1.7 kg/cm 2 ) due to a puncture or the like, the diaphragm 32 is biased by the twisted coil spring 39 and deformed toward the proximal end 31c. Accordingly, the piston 33 is moved backward to the side of the proximal end 31c. The rotary shaft 34, the end 34a of which is in contact with the piston 33, is rotated around the axis, so as to be moved backward 3 mm to the proximal end 31c by means of the threaded portion 36 engaged with the support 35. At this time, since the lead of the threaded portion 36 formed in the rotary shaft 34 is 6 mm, the rotary shaft 34 is rotated 180° around the axis. As a result, the polarities (the N pole and the S pole) of the magnet 38 attached to the rotary shaft 34 are inverted. The Hall element 5 detects the N pole of the magnet 38. The relationship between the rotation of the magnet 38 and the output voltage from the Hall element 5 will be described with reference to FIG. 3. As described before, the rotary shaft 34 is 180° rotated around the axis in accordance with the pneumatic pressure of the tire, thereby reversing the magnet 38 attached to the rotary shaft 34. FIG. 3 (a) shows the direction of the magnet 38 and the output voltage from the Hall element 5, in a case where the pneumatic pressure in the tire is 2.0 Kg/cm 2 . In this case, the S pole of the magnet 38 faces the Hall element 5 and the Hall element 5 outputs a lowest negative voltage. FIG. 3 (b) shows the direction of the magnet 38 and the output voltage from the Hall element 5, in a case where the pneumatic pressure in the tire is reduced to, for example, 1.85 Kg/cm 2 . In this case, the magnet 38 is rotated such that the boundary between the N pole and the S pole faces the Hall element 5. The Hall element 5 outputs positive and negative voltages of a relatively low level. FIG. 3 (c) shows the direction of the magnet 38 and the output voltage from the Hall element 5, in a case where the pneumatic pressure in the tire is further reduced to, for example, 1.7 Kg/cm 2 (below the suitable range). In this case, the N pole of the magnet 38 faces the Hall element 5 and the Hall element 5 outputs a highest positive voltage. FIG. 4 is a block diagram of the monitoring circuit 40 shown in FIG. 1. A drive power source 42 supplies a predetermined voltage to a Hall element 5. The Hall element 5 outputs a voltage corresponding to the direction of a magnetic pole of a magnet 38, which is rotated in accordance with the pneumatic pressure in the tire as described above. Since the output voltage from the Hall element 5 is in the millivolt order, an amplifier 43 amplifies it to a predetermined level. The amplified voltage is output to a sample hold circuit 44 and an edge detector 50. The edge detector 50 detects a leading edge or trailing edge of an output signal from the amplifier 43, and outputs to a CPU 45 a signal indicating the detection. The signal output from the edge detector 50 is used as a trigger signal for executing a control program to be described later. The sample hold circuit 44 is activated by an activation signal AS supplied from the CPU 45 and holds the output voltage from the amplifier 43. An A/D converter 46 converts the output voltage held by the sample hold circuit 44 to a digital signal DS, and outputs the digital signal DS to the CPU 45 together with an A/D conversion end signal ES. The CPU 45 executes a control program (to be described later), calculates an approximate value of the pneumatic pressure in the tire (not shown) based on the digital value of the voltage output from the sample hold circuit, and determines a rough state of the pressure from the calculated value. The result of the determination is transmitted to a display unit 41, constituted by 7 segment LEDs or the like. The display unit 41 displays, for example, "suitable pressure", "intermediate pressure" or "under pressure". Details of the operation of the CPU 45 will be described later. Referring to FIG. 3, the relationship between the rotation angle of the magnet 38 and the output waveform of the Hall element 5. Actually, an output waveform is output from the Hall element, each time the tire makes one rotation. FIG. 3, however, shows only one waveform in order to make the explanation simple. Measurement values indicated below are those obtained under predetermined conditions. FIG. 5 (a) shows an output waveform of the Hall element 5 in a state where the tire pressure is lowered to about 1.7 Kg/cm 2 (beyond the range of the suitable pressure) and the rotation angle of the magnet 38 is 0°, that is, the N pole of the magnet 38 faces the Hall element 5. At this time, in the output waveform of the Hall element 5, the peak value on the positive side is maximum, whereas substantially no peak appears on the negative side. The peak values on the positive and negative sides were measured: 44.5 mV on the positive side and -1.5 mV on the negative side. Then, the air in the tire is pressurized. When the rotation angle of the magnet 38 is 45°, the output waveform of the Hall element 5 is as shown in FIG. 5 (b). In this state, the peak on the positive side is slightly lower than the maximum on the positive side, whereas a small peak appears on the negative side. The peak values on the positive and negative sides were respectively 39.3 mV and -4.4 mV. The air in the tire is further pressurized. When the rotation angle of the magnet 38 is 90°, that is, the boundary between the N and S poles faces the Hall element 5, the output waveform of the Hall element 5 is as shown in FIG. 5 (c). In this state, the peaks on the positive and negative sides are substantially the same. The peak values on the positive and negative sides were respectively, 27.1 mV and -14.1 mV. The air in the tire is further pressurized, and when the rotation angle of the magnet 38 is 135°, the output waveform of the Hall element 5 is as shown in FIG. 5 (d). In this state, the peak on the negative side is greater than that on the positive side. The peak values on the positive and negative sides were respectively, 6.9 mV and -37.8 mV. Further, when the tire pressure is increased to about 2.0 Kg/cm 2 (a suitable pressure), the rotation angle of the magnet 38 is 180°, that is, the S pole of the magnet 38 faces the Hall element 5. At this time, in the output waveform of the Hall element 5, as shown in FIG. 5 (e), substantially no peak appears on the positive side, whereas the peak value on the negative side is maximum. The peak values on the positive and negative sides were measured: 1.1 mV on the positive side and -46.9 mV on the negative side. Based on the aforementioned relationship between the rotation angle of the magnet 38 and the output waveform of the Hall element 5, the principle for detecting the tire pressure according to the present invention will be described below. In this embodiment, the ratio of the peak value (absolute value) of the waveform on the positive side to that on the negative side, at each rotation angle (0°, 45°, ... 180°) of the Hall element 5, is first calculated. Table 1 shows the value of the ratio of the positive peak value to the negative peak value, and the logarithm (log 10 ) thereof. As shown in Table 1, the ratio of the positive peak value to the negative peak value greatly varies. The difference between the maximum value of the ratio and the minimum value thereof is the greatest when the maximum value is 29.7 (the rotation angle is 0°) and the minimum value is -1.64 (the rotation angle is 180°). +/|- | log10(+/|-|) 0° 29.7 1.47 45° 8.9 0.95 90° 1.9 0.28 135° 0.2 -0.74 180° 0.02 -1.64 The relationship between the logarithm value log 10 x (x is the ratio of the positive peak value to the negative peak value) shown in Table 1 and the angle y of the magnet 38 is approximately expressed by, for example, the following equation (1), using the least square method or the like. y = 103 - 47.5(logx) - 8.4(logx) 2 - 4.9(logx) 3 The above equation (1) is graphed out as a curve, which is shown in FIG. 6. In the graph, the abscissa represents the logarithm value log 10 x, while the ordinate represents the angle y of the magnet 38. Using the above equation (1), the angle of the magnet 38 can be obtained from the ratio x of the positive peak value to the negative peak value. Assume that the relationship between the angle y of the magnet 38 and the pneumatic pressure z in the tire is given as a linear function and expressed by the following equation (2), where, when y=0 (° ), z=1.7 (kg/cm 2 ), and when y=180 (°), z=2.0 (kg/cm 2 ). z = (1/600)y + 1.7 The relationship between the ratio x of the positive peak value to the negative peak value and the pneumatic pressure z in the tire is given by the following equation (3) which is obtained from the above equations (1) and (2). z = 1.87 - 7.9 × 10 -2 (logx) - 1.4 × 10 -2 (logx) 2 -8.2 × 10 -3 (logx) 3 As described above, it is possible to calculate an approximate value of the tire pressure from the output of the Hall element 5. Based on the calculated value, the state of the tire pressure (suitable pressure, intermediate pressure or under pressure) can be accurately detected. An operation of the tire pressure detector according to this embodiment will be described with reference to FIG. 7. FIG. 7 is a flowchart showing a control program executed by the CPU 45. In the following explanation, it is assumed that the automobile is running. When power is supplied to the tire pressure detector, the CPU 45 executes a process of a step S1. In the step S1, it is determined whether a trigger signal is supplied from the edge detector 50. If a trigger signal is not supplied, the step S1 is executed again. If a trigger signal is supplied, a step S2 is executed, that is, a software timer is started. In a step S3, the CPU obtains maximum and minimum values of outputs of the Hall element 5, which are sequentially supplied from the A/D converter 46. In a step S4, the CPU determines whether a predetermined period of time, set to the software timer, has elapsed. If not, the flow returns to the step S3. In other words, the process of the step S3 is repeated, while the result of the step S4 is "No". The predetermined period of time set in the software timer is determined depending on a detectable speed of the automobile. For example, when the speed is 6 km/h or higher, the time required for the tire to make one rotation is about 1.2 second. Therefore, if a longer time, e.g., 1.5 second, is set to the software timer, at least one output of the Hall element can be detected within the set time. In this case, the peak values on the positive and negative sides of the output waveform of the Hall element 5 can be obtained, before the result of the step S4 becomes "Yes". When the result of the step S4 is "Yes", the flow advances to a step S5. In the step S5, the ratio of the positive peak value to the negative peak value, obtained in the step S1, is calculated. The calculated value is substituted for x in the above equation (3), thereby obtaining an approximate value of the tire pressure. In a step S6, the CPU determines whether the tire pressure is within a predetermined range. If it is considered that a normal detection is not performed, for example, if the tire pressure has a negative value, the CPU determine that the tire pressure detector has a malfunction. If the calculated value obtained in the step S5 is within the predetermined range, it is considered that the tire pressure detector does not have a malfunction. Then, it is determined in which one of the following states the tire pressure is classified: under pressure: about 1.7 kg/cm 2 intermediate pressure: about 1.85 kg/cm 2 suitable pressure: about 2.0 kg/cm 2 . The result of the determination is supplied to the display unit 41 in a step S7. The display unit 41 displays the range of the current tire pressure ("suitable pressure", "intermediate pressure" or "under pressure"). If the calculated value obtained in the step S5 is not within the predetermined range, since it is considered that the tire pressure detector has a malfunction, the result of the determination in the step S6 is "No". Then, the flow advances to a step S8, in which the CPU 45 transmits information representing a malfunction to the display unit 41. Accordingly, the display unit 41 displays a malfunction. As described above, according to the present invention, the magnet 38 is rotated in accordance. with the pneumatic pressure in the tire, so that the direction of the N or S pole of the magnet with respect to the Hall element 5 is changed. In accordance with the change of the direction of the magnet, the ratio of the positive peak value to the negative peak value of an output voltage supplied from the Hall element 5 is changed. The tire pressure is calculated on the basis of the value of the ratio. Therefore, even if the peak value of an output voltage from the Hall element 5 is changed by external factors, such as environmental conditions (e.g., a change in temperature) or positional deviation due to low accuracy of attaching the sensor unit or the Hall element, the value of the pneumatic pressure obtained by the calculation will be influenced little by the peak value itself, because it is calculated on the basis of the ratio of the positive peak value to the negative peak value of an output voltage from the Hall element 5, which is changed in accordance with the direction of the magnet 38. As a result, accuracy of detecting the tire pressure is increased. In addition, since a structure for temperature compensation is not required, the manufacturing cost is lowered. Although in the case of one wheel has been described above, if the automobile has a plurality of wheels, it is only necessary that the same tire pressure detector be provided for each tire. In the above embodiment, the magnet 38 is rotated 180° in the tire pressure ranging from 1.7 kg/cm 2 to 2.0 Kg/cm 2 . However, the present invention is not limited to this embodiment. To increase the range of the tire pressure which can be detected, it is possible to set the magnet 38 so as to be rotated 180° in a wider range of the tire pressure. Further, in the above embodiment, the range of the tire pressure (under pressure, intermediate pressure or suitable pressure) is displayed. However, the value of the tire pressure itself can be displayed in digital style. As has been described above, even if the peak value of an output voltage from the detecting means is changed by environmental conditions, such as a change in temperature, or positional deviation due to low accuracy of attaching the sensor unit or the Hall element, the influence on the detection result is suppressed, so that the accuracy of detecting the tire pressure is increased. In addition, since a structure for temperature compensation is not required, the manufacturing cost is lowered.

Claims: A tire pressure detector of an automobile, comprising: a magnet (38) having N and S poles and rotated in accordance with a pneumatic pressure in a tire; and detecting means (5), which is fixed to the body of the automobile and arranged opposite to the magnet, to detect magnetism and output a voltage signal at the N and S poles on a level corresponding to a rotation angle of the magnet; characterized in that a calculating means (40) is provided which calculates the pneumatic pressure in the tire based on a ratio of a positive peak value to a negative peak value of the voltage signal. The tire pressure detector according to claim 1, characterized by further comprising determining means (40) which determines in which of a plurality of pressure ranges the pneumatic pressure in the tire falls.

Inventors: FUKADA KAZUTOSHI, FURUICHI KENJI, GOTO MORITAKA, OHSHITA SAIICHIRO, YAMAMOTO MASAAKI, FUKADA, KAZUTOSHI, FURUICHI, KENJI, GOTO, MORITAKA, OHSHITA, SAIICHIRO, YAMAMOTO, MASAAKI
Applicants: FUJIKURA LTD, FUJIKURA LTD.
Application year: 1996
Main classification: B60C23
IPC codes: B60C23