Control method for antilock braking systems

Abstract: An antilock braking system employing stress sensors installed in the wheel axle structure (36) of a motor vehicle. The stress sensors (29-32) measure a road surface frictional force (F) and a braking torque (T). A parameter calculating means (3) calculates a parameter M from these measured values. An ABS control means (8) terminates a braking pressure reduction process in accordance with the change in the parameter M . Thereby, the system can preclude the locking of the wheel (39) of the motor vehicle even when the brake is suddenly applied, can prevent that an "over control" of the braking pressure reduction occurs and makes it possible to minimize any braking loss.

Description: BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to a control method for operating an antilock braking system according to the preamble of claims 1 and 2. II. Prior Art If the wheels of a motor vehicle are locked upon application of the brake of a motor vehicle, rotation of the tire of the vehicle is braked and it is followed by tire slipping on the road surface. Such a condition results in a reduction of vehicle controllability. Antilock braking systems (ABS) are control systems capable of preventing the tire from locking during a braking process. Generally a conventional antilock braking system for motor vehicles employs a system such that the braking action is automatically controlled in accordance with the speed of the vehicle wheel. The vehicle wheel speed is measured with use of a wheel speed sensor. This wheel speed sensor has a structure measuring rotational velocity of the vehicle wheel by detecting the rotation speed of a geared hub attached on the axle of wheel with the use of a magnetic pick-up. Control procedure of a conventional antilock braking system is as follows: when the braking is suddenly applied, the locking of the wheel is detected by rapidly decreasing of the wheel speed. Then the control system reduces the braking pressure. Thereafter, when the wheel speed recovers by the braking pressure reduction, the braking is again applied. In such a control method, however, the effect of brake control is lost in fact during a period from the occurrence of the wheel locking to the resumption of the braking process. Consequently the controllability becomes unstable and it involves a longer stopping distance for the vehicle. In another conventional antilock braking systems, the control of the system is commenced by using an arbitrarily chosen control method and parameter settings. Thereafter the frequency of the tire slipping is counted during controlling, and the friction coefficient of the road surface is estimated on the basis of the counted slip frequency. When the slip frequently occurs. the control method is changed into a method suitable for a road surface having a lower friction coefficient, and when there is no slip, it is switched into a method suitable for a road surface having a higher friction coefficient. However, such a control method causes an increasing loss of the braking force during the tire slipping and results in a longer stopping distance for the vehicle. It is possible to control operations of antilock braking systems by using a measured value of a stress applied on a vehicle wheel. There are a photoelasticity method, a brittle coat method, a holography method, a strain gauge method, etc. as methods measuring stress about a structure. Generally, the strain gauge method is widely used, but it is difficult to detect an intended stress value because it receives several other forces than the stress to be measured. In particular, there is no method detecting side forces. The document EP-A-0 621 162 discloses a control method for operating an antilock braking system according to the preamble of claim 1 and claim 2, respectively. According to EP-A-0 621 162, an antilock brake system is disclosed which is mounted on a vehicle wherein, when the vehicle is braked in an emergency situation during running, an increase in the road surface friction force owing to an increase in the brake pressure is detected by a road surface friction force detecting device. The optimum control start point associated with an increase in the signal of the road surface friction force or the road surface friction coefficient provided by the road surface friction force detecting device or the road surface friction coefficient detecting device is decided by using a decrease in the wheel speed, using either the wheel speed or the derivative of the wheel speed. Depending on the circumstances, the brake pressure is then moved from the pressure increasing mode to the pressure retaining or decreasing mode. The document US-A-5 186 042 discloses a stress sensor and a stress measuring device which may be used for an antilock braking system. The document further discloses details concerning the construction of a stress detection sensor mounted to a wheel. SUMMARY OF THE INVENTION An object of the present invention is to provide a control method for operating an antilock braking system without using a measured value of speed of a vehicle wheel as control parameter. Another object of the present invention is to provide such a control method systems using as control parameter the values measured by stress sensors. A further object of the present invention is to prevent a braking pressure reduction from being over controlled and to minimize a braking loss. The object underlying the present invention is solved by a control method for operating an antilock braking system comprising the features of claim 1 and claim 2, respectively. Further developments of the control method are specified in the subclaims. These and other advantages and objects of the present invention will be apparent upon reference to the following detailed description taken in connection with the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a graphical representation illustrating typical response of values measured by stress sensor means and parameters calculated from the measured values, when the brake of the vehicle is suddenly applied usable in a method according to the invention. Fig. 2 is a block diagram illustrating an embodiment of the method according to the present invention. Fig.3 is a graphical representation illustrating typical response of values measured by stress sensor means and parameters calculated from the measured values, when the brake of the vehicle is suddenly applied usable in a method according to the invention. Fig. 4 is a block diagram illustrating the embodiment of Fig. 3 usable in a method according to the invention. Fig. 5 is a plan view of an example of an uniaxial strain gauge usable in a method according to the invention. Fig. 6 is a schematic perspective view of a sensor segment. Fig. 7 is a schematic perspective view of a sensor segment. Fig. 8 is a schematic perspective view of a sensor segment. Fig. 9 is a block diagram of stress sensor means including the sensor segments as shown in Figs. 6 and 7. Fig. 10 is a block diagram of stress sensor means including the composite sensor segment as shown in Fig. 8. Fig. 11 is a schematic perspective view of an axle structure and a tire. Figs. 12, 13, 14, 15, 16 and 17 are schematic perspective view illustrating a variety of structures about the axle. Fig. 18 is a schematic perspective view illustrating a sensor segment in a sensor hole. Fig. 19 is a schematic perspective view illustrating another sensor segment in a sensor hole. Figs. 20 and 21 are schematic perspective views of examples of a plurality of sensor segments. Figs. 22, 23 and 24 are schematic perspective views illustrating sensor segments in the sensor hole. Fig. 25 is a schematic perspective view of a cubic sensor segment. Fig. 26 is a schematic perspective view illustrating the cubic sensor segment in the sensor hole. Fig. 27 is a schematic perspective view illustrating two cubic sensor segments in the sensor hole. Fig. 28 is a schematic perspective view of an example of sensor segment having uniaxial strain gauges and biaxial strain gauges. Fig. 29 is a schematic perspective view of an example of a sensor element having a plurality of sensor segments. Fig. 30 is a schematic perspective view illustrating the sensor segment shown in Fig. 28 in the sensor hole. Figs. 31, 32 and 33 are schematic perspective views of examples of sensor segments. Fig. 34 is a block diagram of stress sensor means. Figs. 35 and 36 are schematic perspective views of examples of the sensor segments of the cubic substrate. Fig. 37 is a schematic perspective view illustrating the cubic sensor segment in the sensor hole. Fig. 38 is a schematic perspective view of a test rack. Figs. 39, 40 and 41 are schematic perspective views of examples of the substrate made by an adhesive material. Fig. 42 is a schematic perspective view illustrating the sensor segment having the substrate made by the adhesive material in the sensor hole. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodimentes according to the present invention will be described hereinafter. Other embodiments will be apparent upon reference to the following detailed description. An embodiment of a control method for operating an antilock braking system with stress sensors according to the present invention will now be described. A stress sensor is a device which can produce output values used for control operation. The output values are the values proportional to a road surface frictional force F , and a braking torque T respectively. It is known that devices can be made with, for example, a strain gauge or an equivalent. Fig. 1 is a graphical illustration representing a typical response of the road surface frictional force F and the braking torque T detected by the stress sensor and the control parameters M calculated from the detected F and T whe the braking of the motor vehicle is suddenly applied. Fig. 1 shows that the road surface frictional force F and the braking torque T are continuously increased and the control parameter M given by an equation M = F - T is continuously decreased during a period (t 0 - t 1 ) in which the braking of the motor vehicle is being applied, after the sudden application of the brake at the time t 0 . As the parameter M has a negative value, a parameter Mw, which is the integral of the parameter M with respect to t , is also decreased with time passing by. For precluding the locking of the wheels of the motor vehicle, the desired control action for the movement process toward the braking pressure reduction mode is initiated at the time t 1 at which the parameter M has become higher than a predetermined threshold TH. From this time on, for some time, the parameter M keeps decreasing and then the degree of decrease in the parameter Mw increases. From the time t 2 at which the effect of the braking pressure reduction appears, the road surface frictional force F and the braking torque T decrease, the parameter M begins to increase, and the degree of decrease in the parameter Mw is lessened significantly. Fig. 2 is a block diagram representing the function of the embodiment of the control method for operating an antilock braking system according to the present invention. A parameter M calculating means 3 calculates the parameter M from F and T detected by detecting means 1 and 2 (stress sensor means). A parameter Mw calculating means 5 calculates the parameter Mw by integrating the parameter M. A depressurization end detecting means 7 monitors the change in the slope of the parameter Mw, detecting the effect of the braking pressure reduction. An ABS control means 8 having an actuator terminates the control process for the braking pressure reduction when the effect has been detected. Such a processing prevents the braking pressure reduction from being over controlled and therefore makes it possible to minimize the braking loss. Amounts of change in parameters M and Mw vary in accordance with the variation in the vehicle velocity, the road surface friction coefficient, etc. Therefore it is necessary to modify the decision criteria for terminating the braking pressure reduction based on the change in the slope of the parameter Mw, and it complicates the judgment of the termination of the depressurization. Then, when a depressurization start detecting means 4 has detected the time t 1 at which the braking pressure reduction mode is initiated, the slope of the parameter Mw at the time t 1 is determined by a reference line calculating means 6. The reference line calculating means 6 produces a reference line "a" which is a straight line having a slope equal to the slope of the parameter Mw at the time t 1 . The depressurization end detecting means 7 detects a time t 3 at which the curve of the parameter Mw intersects the reference line "a". The ABS control means 8 having an actuator is terminated in the control process for the braking pressure reduction mode at the time t 3 . Preparation of the complicated decision criteria for terminating the braking pressure reduction is useless by using this control method. This simple method comparing the parameter Mw with the reference line "a" prevents the braking pressure reduction from being over controlled and makes it possible to minimize the braking loss. In the case where the reference line is equal to the straight line having the slope of the parameter Mw at the time t 1 , there is a case where a signal representing termination of the depressurization is output even when the braking pressure reduction has an insufficient condition. Hence, when the depressurization start detecting means 4 has detected the time t 1 at which the braking pressure reduction mode is initiated, the reference line calculating means 6 measures a first value of the parameter Mw at the time t 1 and a second value of the parameter Mw at a time t 4 preceding by a predetermined time from the time t 1 , and produces a reference line "b" passing the first and the second values of the parameter Mw. The depressurization end detecting means 7 detects a time t 5 at which the curve of the parameter Mw intersects the reference line "b". The ABS control means 8 having the actuator is terminated in the control process for the braking pressure reduction mode (at the time t 5 ). Such a method prevents the braking pressure reduction from being over controlled and makes it possible to minimize the braking loss. In the case where the braking pressure reduction control is performed in the vicinity of the peak of the road surface frictional force on the road having a very low road surface friction coefficient, if the braking pressure reduction is not sufficient, the road surface frictional force reaches the peak thereof again immediately after terminating the braking pressure reduction process. As a result, a wheel locking is caused and the vehicle controllability is lost. Then, the preceding time period Δt (Δt = t 4 - t 1 ) is modulated in accordance with the variation of the road surface friction coefficient. As the modulation of the period Δt changes the slope of the reference line "b", the time t 5 , at which the curve of the parameter Mw intersects the reference line "b", also changes, and makes it possible to adjust the time being terminated in the braking pressure reduction process. That is, the period for the braking pressure reduction process can be adjusted to the period suitable for the actual value of the road surface friction coefficient by using the method mentioned above. This control method performs the braking pressure reduction process during the period suitable for the road surface friction coefficient and prevents the braking pressure reduction from being over controlled and makes it possible to minimize the braking loss. A control method capable of using methods suitable for the value of the road surface friction coefficient in antilock braking systems with stress sensors will now be described. In this system, the stress sensor is a device which can produce output values used for control operation. The output value is the value proportional to a road surface frictional force F. It is known that the device can be made with, for example, a strain gauge or an equivalent. Fig. 3 is a graphical illustration representing a typical response of the road surface frictional force F ("a", "b", "c") detected by the stress sensor, a first derivative F' of the road surface frictional force F , and a second derivative F" of the road surface frictional force F when the braking of the motor vehicle is suddenly applied. Curves "a", "b", and "c" representing the road surface frictional force F correspond to three maximum road surface friction coefficients respectively. The road surface having a higher maximum road surface friction coefficient shows the road surface frictional force having a larger peak value. Fig. 4 is a block diagram representing the function of this system of the antilock braking system which is usable in the invention. Immediately after applying the braking at the time t 10 , the road surface frictional force measured value F increases monotonically with elapsing time as shown in Fig. 3. Curves "a", "b" and "c" show the road surface frictional force measured values for the road surface having low, middle, and high maximum road surface friction coefficients respectively. The road surface frictional force F increases with increments of the maximum road surface friction coefficient. When a certain limit of frictional force obtained from the road surface, however, is exceeded, the measured value F has a peak value at a time t 12 and then begins to decrease. A timing detecting means 13 shown in Fig. 4 detects the time t 12 at which the road surface frictional force measured value F has reached the peak thereof. As peak values of the road surface frictional force measured value F vary in such a way that the values are about Pa, Pb, and Pc for roads having low, middle, and high maximum friction coefficients respectively, an estimating means 14 estimates the actual value of the maximum road surface friction coefficient from the magnitude of the measured peak value of the road surface frictional force. A control switching means 15 selects an optimum one among a plurality of control procedure or parameter settings programmed beforehand on the basis of analysis of the estimated maximum friction coefficient. Then, an ABS control means 16 having an actuator conducts the control of the antilock braking system in accordance with optimum control procedure or parameter settings selected by the control switching means 15. Because of a mechanical time delay, if the control process has been commenced after the road surface frictional force F has reached the peak value thereof, the road surface frictional force F causes to reduce when control effect brings into play actually and therefore the slip of the wheel has been increased. Then, an arithmetic means 12 takes the derivative F' of the road surface frictional force F. The timing detecting means 13 detects a time t 11 at which the derivative value F ' has reached the peak value thereof. As values of the road surface frictional force at the time t 11 vary in such a way that the values are Ya, Yb, and Yc for the curves "a", "b", and "c" respectively, the estimating means 14 estimates the actual maximum road surface friction coefficient from the relationship between these values of the frictional force and maximum road surface friction coefficient. The control switching means 15 selects an optimum one among a plurality of control procedure or parameter settings programmed beforehand on the basis of an analysis of the estimated maximum friction coefficient of the road surface. Then, the ABS control means 16 having the actuator conducts the control of the antilock braking system in accordance with optimum control procedure or parameter settings selected by the control switching means 15. As this method estimates the friction coefficient of the road surface before the road surface frictional force reaches the peak value thereof and commences the selected control operation, the slip of the wheel does not increase and therefore the optimum control of the antilock braking system can be carried out. The peak value of the derivative value F ' is difficult to find. For finding the peak of the derivative value F ', it is necessary to repeat the comparison of former data and latter data of the derivative value F '. However, this can be easily found by the following method. That is, the second derivative F " of the road surface frictional force F is calculated by the arithmetic means 12. The instant that the second derivative F " has switched from plus to minus is detected by the timing detecting means 13. This instant is the time t 11 at which the first derivative F' has reached the peak thereof. Just as in the case above mentioned, the estimating means 14 estimates the actual maximum road surface friction coefficient, the control switching means 15 selects an optimum control procedure or parameter settings, and then the ABS control means 16 conducts the optimum control of the antilock braking system. In this method, a memory for the past data of the derivative F' is not required, detection only of the instant that the second derivative F" is switched from plus to minus is required. Therefore, an estimation of the maximum friction coefficient of the road surface can easily be performed. The system described above employs the stress sensor means as F detecting means or T detecting means. Such stress sensor means has been disclosed, for example, in Japanese Patent Laid-Open Publication No.4-331336. The stress sensor means is installed in a hole formed on the wheel axle structure of the motor vehicle. An embodiment of the measurement device of a wheel operating force suitable for a motor vehicle, in particular, a passenger automobile will be described. Fig. 5 is a plane view of an example of an uniaxial strain gauge "e". A resistance element 23 acting as a sensitive part employs an Ni-Cr alloy film, a Si film, and the like. A base 27 is usually made of an insulation material such as polyimide resin, silicon oxide, and the like. Fig. 6 is a perspective view of a sensor segment "f" in which the strain gauges are mounted on a substrate 28. The substrate 28 is usually made of a metal or silicon or the equivalent. An "a" uniaxial strain gauge 29 is mounted on the obverse face of the substrate 28 and a "b" uniaxial strain gauge 30 is mounted on the reverse face of the substrate 28. Fig. 7 shows another sensor segment for measuring stress having a direction different from the sensor segment "f" shown in Fig. 6. This structure is the same as that of Fig. 6 except for the position of the sensor segment. A "c" uniaxial strain gauge 31 is mounted on the side face on the substrate 28 and a "d" uniaxial strain gauge 32 is mounted on the other side face on the substrate 28. An angle between the "a" strain gauge 29 and the "c" strain gauge 31 is 90'. Fig. 8 is a perspective view of a composite sensor segment having the "a", "b", "c" and "d" uniaxial strain gauges 29, 30, 31, and 32 shown in Figs. 6 and 7. Fig. 9 is a block diagram of stress sensor means including the sensor segments as shown in Figs. 6 and 7. In Fig. 9 each strain gauge constitutes one branch of a bridge circuit "g", the output of the bridge circuit "g" is amplified by a amplifier 33, and the amplified output is fed to a digital signal processor (DSP) 34. The output of the DSP 34, for example, can be connected to a personal computer for control use. Fig. 10 is a block diagram of stress sensor means including the composite sensor segment as shown in Fig. 8. In Fig. 10 there are two sets of the circuit shown in Fig. 9 and an output of each DSP 34 is input to a central processing unit (CPU) 35. Next, the axle structure of the vehicle will be described. Fig. 11 is a schematic perspective view of the axle structure and the tire. A horizontal direction ( F direction) with respect to the road surface, a vertical direction ( N direction) with respect to the road surface, and an axle center direction ( S direction) are defined as an axis X , an axis Z , and an axis Y respectively. A torsional moment (FB direction) about the axis Y is defined as FB. In the axle structure 36 a shock absorber 37 is installed. In the axle side a brake disk 38 is installed via a bearing. The tire 39 is installed in such a way as to cover the brake disk 38. A variety of structures about the axle are shown in Figs. 12 - 17. Numerals 40, 42 and 43 are the axle, a structure close to the axle 40, and a hole for the axle. A hole 41 for attaching the sensor as mentioned above is drilled into the structures as shown in Figs. 12 - 17. Fig. 18 is a schematic perspective view illustrating the sensor segment "f" in the sensor hole 41. When a force of F direction ( X axis) is applied to the axle, the substrate 28 of the sensor segment "f" receives a shearing strain. Two strain gauges, "a" strain gauge 29 and "b" strain gauge 30, expand and these resistances change by Δa and Δb. The DSP 34 determines a stress of F direction by the following equation: F = k F (Δa+Δb) When a force of N direction ( Z axis) is applied to the axle, the substrate 28 of the sensor segment "f" receives a contraction and an expansion stress and it bends. Two strain gauges, "a" strain gauge 29 and "b" strain gauge 30, contract and expand and these resistances change by - Δa and Δb. The DSP 34 determines a stress of N direction by following equation: N = k N (Δb-Δa) When a force of S direction ( Y axis) is applied to the axle, the substrate 28 of the sensor segment "f" receives a contraction stress. Two strain gauges, "a" strain gauge 29 and "b" strain gauge 30, contract and these resistances change by -Δa and Δb. The DSP 34 determines a stress of S direction by following equation: S = k S (Δa+Δb) When a force of FB direction is applied to the axle, the substrate 28 of the sensor segment "f" receives an expansion and a contraction stress and it is twisted. Two strain gauges, "a" strain gauge 29 and "b" strain gauge 30, expand and contract and these resistances change by a and - b. The DSP 34 determines a stress of FB direction by the following equation: FB = k T (Δa-Δb) Fig. 19 is a schematic perspective view illustrating another stress detecting device. Similarly, this embodiment can also determine the stress of each direction by calculating a change in resistance of a strain gauge. Another measurement device will be described. The sensor segment "f" shown in Fig. 8 is installed in the sensor hole 41 and the circuit shown in Fig. 10 is employed. When a force of F direction is applied to the axle, the substrate 28 of the sensor segment receives a shearing strain. The strain gauges, "a" gauge 29 and "b" gauge 30, expand and these resistances change by Δa and Δb. The strain gauges, "c" gauge 31 and "d" gauge 32, change in resistance by Δc and Δd. The DSP 34 and CPU 35 determine a stress of F direction by the following equation: F = k F {(Δa+Δb)+(Δc+Δd)} When a force of N direction is applied to the axle, the substrate 28 of the sensor segment receives a contraction and an expansion stress and it bends. The strain gauges, "a" gauges, 29 and "b" gauge 30, contract and expand and these resistance change by -Δa and Δb. The strain gauges, "c" gauge 31 and "d" gauge 32, change in resistance by c and d. The DSP 34 and CPU 35 determine a stress of N direction by N = k N {(Δb-Δa)+(Δc+Δd)} When a force of S direction is applied to the axle, the substrate 28 receives contraction stress. The strain gauges, "a" gauge 29 and "b" gauge 30, contract and these resistances change by -Δa and -Δb. The strain gauges, "c" gauge 31 and "d" gauge 32, change in resistance by -Δc and-Δd. The DSP 34 and CPU 35 determine a stress of S direction by S = -k S {(Δa+Δb)+(Δc+Δd)} When a force of FB direction is applied to the axle, the substrate 28 receives an expansion and a contraction stress and it is twisted. The strain gauges, "a" gauge 29 and "b" gauge 30, expand and contract and these resistances change by Δa and -Δb. The strain gauges, "c" gauge 31 and "d" gauge 32, change in resistance by Δc and -Δd. The DSP 34 and CPU 35 determine a stress of FB direction by FB = k T {(Δa-Δb)+(Δc-Δd)} A further measurement device will be described . A plurality of units of the sensor segment "f" shown in Fig. 8 are installed in the sensor hole 41. When a force of F direction is applied to the axle, a plurality of substrates 28 receive a shearing strain. By using values representing the change of the strain gauge, the DSP 34 and CPU 35 determine a stress of F direction by Eq. 5 and the following equation: When a force of N direction is applied to the axle, a plurality of substrates 28 receive a contraction and an expansion stress and it bends. By using values representing the change of the strain gauge, the DSP 34 and CPU 35 determine a stress of N direction by Eq. 6 and the following equation: When a force of S direction is applied to the axle, a plurality of substrates 28 receive a contraction stress. By using values representing the change of the strain gauge, the DSP 34 and CPU 35 determine a stress of S direction by Eq. 7 and the following equation: When a force of FB direction is applied to the axle, a plurality of substrates 28 receive an expansion and a contraction stress and are twisted. The DSP 34 and CPU 35 determine a stress of FB direction by Figs. 20 and 21 show the examples of a plurality of sensor segments "f". Figs. 22 - 24 are schematic perspective views illustrating the sensor segments "f" in the sensor hole 41. A further measurement device will be described. A sensor segment shown in Fig. 25 is installed in the sensor hole 41. A circuit of this device is similar to one shown in Fig. 10. When a force of F direction is applied to the axle, a cube substrate 28 receives a shearing strain. By using values representing the change of the strain gauge mounted on each face of the cube substrate, the operation is performed about each sensor segment, and a stress of F direction is determined by F plural = f 1 +f 2 +f 3 + ··· When a force of N direction is applied to the axle, a cube substrate 28 receives a contraction and an expansion stress and it bends. By using values representing the change of the strain gauge mounted on each face of the cube substrate, the operation is performed about each sensor segment, and a stress of N direction is determined by N plural = n 1 +n 2 +n 3 +··· When a force of S direction is applied to the axle, a cube substrate 28 receives a contraction stress. By using values representing the change of the strain gauge mounted on each face of the cube substrate, the operation is performed about each sensor segment, and a stress of S direction is determined by S plural = s 1 +s 2 +s 3 + ··· When a force of FB direction is applied to the axle, a cube substrate 28 receives an expansion and a contraction stress and it is twisted. By using values representing the change of the strain gauge mounted on each face of the cube substrate, the operation is performed about each sensor segment, and a stress of FB direction is determined by FB plural = t 1 +t 2 +t 3 +··· Fig. 26 is a schematic perspective view illustrating the sensor segment "f" in the sensor hole 41. A further measurement device will be described. A plurality of units of the sensor segment shown in Fig. 25 are installed in the sensor hole 41. Fig. 27 is a schematic perspective view illustrating the two cubic sensor segments in the sensor hole 41. A circuit of this device is similar to one shown in Fig. 10. When a force of F direction is applied to the axle, the cube substrate 28 receives a shearing strain. By using values representing the change of the strain gauge mounted on each face of the cube substrate, a stress of F direction is determined by When a force of N direction is applied to the axle, a cubic substrate 28 receives a contraction and an expansion stress and it bends. By using values representing the change of the strain gauge mounted on each face of the cube substrate, a stress of N direction is determined by When a force of S direction is applied to the axle, the cubic substrate 28 receives a contraction stress. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of S direction is determined by When a force of FB direction is applied to the axle, the cubic substrate 28 receives an expansion and a contraction stress and it is twisted. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of S direction is determined by A further measurement device will be described. Fig. 28 shows an example in which a sensor segment employing uniaxial strain gauges and a sensor segment employing biaxial 90° strain gauges are installed on the substrate 28. Fig. 29 shows a sensor element "g" in which a plurality of sensor segments "f" are mounted. A wiring 46, a electrode 47, a cable 48, and a connector 49 are attached on the substrate 28. Fig. 30 is a schematic perspective view illustrating the sensor segment "f" shown in Fig. 28 in the sensor hole 41. A circuit of this device can employ, for example, a circuit shown in Fig. 34. When a force of F direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a shearing strain. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of F direction is determined by When a force of N direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a contraction and an expansion stress and it bends. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of N direction is determined by When a force of S direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a contraction stress. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of S direction is determined by When a force of FB direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives an expansion and a contraction stress and it is twisted. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of FB direction is determined by A further measurement device will be described. Figs. 31, 32 and 33 show examples of a sensor segment in which the strain gauge is mounted on the substrate. Figs. 35 and 36 show examples of sensor segment in which the strain gauge is mounted on the cubic substrate. Fig. 37 is a schematic perspective view illustrating the sensor segment "f" in the sensor hole 41. A circuit of this embodiment can employ, for example, the circuit shown in Fig. 34. When a force of F direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a shearing strain. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of F direction is determined by When a force of N direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a contraction and an expansion stress and it bends. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of N direction is determined by When a force of S direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives a contraction stress. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of S direction is determined by When a force of FB direction is applied to the axle, the cubic substrate 28 of the sensor segment "f" receives an expansion and a contraction stress and it is twisted. By using values representing the change of the strain gauge mounted on each face of the cubic substrate, a stress of FB direction is determined by An further measurement device will be described. By the calculations described above, stress of F direction which is equivalent to the horizontal direction with respect to the road surface, stress of N direction which is equivalent to the vertical direction with respect to the road surface, stress of S direction which is equivalent to the axle center direction, and stress of FB direction which is equivalent to the direction of the torsional moment about the axle have been determined. The axle structure with an installed sensor for detecting stress is fixed to a test rack as shown in Fig. 38. Then, a force having components of x " axis "f", y axis "g", z axis "h", and the torsional moment of y axis "g" are applied by an oil hydraulic cylinder 50. Table 1 shows outputs from the sensor segment. COMPONENT f COMPONENT n COMPONENT t COMPONENT s LOAD F f 1 n 1 t 1 s 1 LOAD N f 2 n 2 t 2 s 2 LOAD T f 3 n 3 t 3 s 3 LOAD S f 4 n 4 t 4 s 4 The axle structure is used within elastic deformation thereof. Therefore, the output of the sensor segment and the load have a proportional relationship. This relationship may be represented by the following equations: F orignal = α 1 x N orignal = α 2 x S orignal = α 3 x FB orignal = α 4 x The relationship about double, triple, and quadruple load may be represented by the following equations: F double = β 1 x F trio = γ 1 x F quart = δ 1 x By using Eqs. 29, 33, 34 and 35, the variation ΔF may be represented by the following equation: ΔF = (α 1 - δ 1 )x = ϕ 1  A linear expression for transforming from strain to force is produced by experiment so as to transform from x to . By using Eqs. 33 and 36, pure force F direction can be determined by the following equation: F pure = F-ϕ 1  Calculation for eliminating cross talk is performed by CPU or DSP. A further measurement device will be described. For high-accuracy measurement of an object stress, the position of the sensor is an important factor. Therefore, it is desirable that the sensor segment "f" is positioned at a neutral point which is unaffected by any other spurious force. The neutral point can be determined on the basis of the stress distribution of the axle structure, which is inspected by means of a finite element method, or the like. The size of the neutral point varies with respect to the shape of the axle structure. Therefore, it is desirable to use a small sensor. Another measurement device will be described. Embodiments mentioned above employ a sensor segment having a strain gauge, but it is not restricted to such a strain gauge. All other elements which are able to produce the variation by the strain can be used for the measurement device of the stress. Furthermore, the substrate itself can be made by using an adhesive material, functioning as an adhesive for the strain gauge in the hole. Figs. 39 to 41 are schematic perspective views illustrating examples of a substrate made by an adhesive material and Fig. 42 is a schematic perspective view illustrating a sensor segment having a substrate made by an adhesive material in the sensor hole 41. In the embodiments described above, the strain gauge is employed as the stress detecting element, but it can also employ a semiconductor pressure gauge, an optical fiber, etc. The sensing means for detecting the stresses acting on the vehicle wheel is attached into the small hole which is drilled at the stress measurement point of the axle of the vehicle or the structure located adjacent to the axle. The joint between the sensing means and the hole, for example, is achieved by the following methods: A tapered hole is drilled in the axle structure. The sensing means is embedded into a tapered pipe-like case. The tapered pipe-like case having the sensing means is tightly fitted into the tapered hole. It is also advisable to use simultaneously a clamping screw or adhesives. Another method is to use a silicide layer such as an alloy-silicide of gold and silicon. A hole is drilled in the axle structure. The sensing means is embedded into a pipe-like case. Gold is coated either on an inner wall of the hole or an outside of the pipe-like case. The other wall or side is coated by silicon. The pipe-like case is inserted into the hole. The alloy-silicide is grown by a thermal diffusion method, thereby the sensing means is tightly attached into the hole. The present invention may be embodied in other specific forms without departing from the scope or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive,

Claims: A control method for operating an antilock braking system comprising ABS control means (8), using a computer unit and at least one stress sensor device, each of which outputs measured signals proportional to a road surface frictional force F(t) and a braking torque T(t), characterized in that the computer unit calculates values of a parameter M(t) and a parameter Mw(t), which is the integral of the parameter M(t), wherein the parameter M(t) is defined by the following expression: M(t) = F(t) - T(t), every time when it retrieves values from a time to when a braking process is initiated, wherein the output signals are proportional to the road surface frictional force F(t) and the braking torque T(t) from the stress sensor device, and wherein the computer unit monitors the parameter M(t) and the parameter Mw(t) thus calculated, and in that the ABS control means (8) initiate a braking pressure reduction process at a time t1 when the parameter M(t) exceeds a predetermined threshold value S and terminate the braking pressure reduction at a time t3 when a curve representing the values of the parameter Mw(t) intersects a first reference fine (a), wherein the first reference line (a) is a straight line extending from the curve representing the parameter Mw(t) at the time t1 with a gradient equal to the gradient of the parameter Mw(t) at the time t1. A control method for operating an antilock braking system comprising ABS control means (8), using a computer unit and at least one stress sensor device, each of which outputs measured signals proportional to a road surface frictional force F(t) and a braking torque T(t), characterized in that the computer unit calculates values of a parameter M(t) and a parameter Mw(t), which is the integral of the parameter M(t), wherein the parameter M(t) is defined by the following expression: M(t) = F(t) - T(t), every time when it retrieves values from a time to when a braking process is initiated, wherein the output signals are proportional to the road surface frictional force F(t) and the braking torque T(t) from the stress sensor device, and wherein the computer unit monitors the parameter M(t) and the parameter Mw(t) thus calculated, and in that the ABS control means (8) initiate a braking pressure reduction process at a time t1 when the parameter M(t) exceeds a predetermined threshold value S and terminate the braking pressure reduction at a time t3 when a curve representing the values of the parameter Mw(t) intersects a second reference line (b), wherein the second reference line (b) is a straight line passing through a first value of the curve representing the parameter Mw(t) at a first time t1 and a second value at a second time t4 preceding the first time t1. The method according to claim 1 or 2, characterized in that the threshold value S is predetermined in advance depending on the road surface friction coefficient µ. The method according to claim 2 or 3, characterized in that the second value at the second time t4 of the parameter Mw(t) is predetermined in advance depending on the road friction coefficient µ.

Inventors: MIYAZAKI NAGAO, MIYAZAKI, NAGAO
Applicants: JAPAN ELECTRONICS IND LTD, JAPAN ELECTRONICS INDUSTRY, LTD.
Application year: 1996
Main classification: B60T8
IPC codes: B60T8, G01L5