Vehicle travel direction estimating device

Abstract: A vehicle travel direction estimating device has a wheel speed calculator (6) for calculating the speed and acceleration Aw of the vehicle. Also, a gravity-type accelerometer (5) is provided for producing a gravitationally detected acceleration Am. When both Aw and Am are increased or decreased simultaneously, it is so determined that the vehicle is travelling forward, and when either one of Aw and Am shows increasing feature and the other one shows decreasing feature, or vice versa, it is so determined that the vehicle is travelling reverse. To detect this feature a calculator (9, 10) is provided to take differential values dAw and dAM of these accelerations Aw and Am. Using the differential values dAw and dAm, the vehicle travel direction is estimated.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device which calculates the estimated vehicle speed of a vehicle for use in wheel behavior control, including antilock brake control, and more particularly, to a device for estimating the direction of travel of the vehicle based on the output of the wheel speed and a gravity-type accelerometer. 2. Description of the Prior Art In antilock brake control devices and other wheel behavior control devices, estimating wheel slipping or spinning based on the calculated vehicle speed, and estimating the coefficient of friction between the tire and the road surface are important factors directly affecting the control performance of the device. The coefficient of friction between the tire and road is normally estimated based on the estimated value of vehicle acceleration during control, and the vehicle acceleration is estimated based on the estimated vehicle speed. Therefore, the performance of the wheel behavior control device is greatly dependent upon the precision of vehicle speed estimation. However, if the vehicle speed is estimated from the wheel speed alone, the estimating precision is reduced significantly when the tires slip or spin badly. Therefore, if the vehicle speed and vehicle acceleration are estimated based on both the wheel speed and the acceleration obtained from a reliable accelerometer, the estimating precision can be significantly increased. A typical type of accelerometer is the gravity-type accelerometer. As illustrated in Fig. 6, the gravity-type accelerometer detects acceleration by converting the displacement of a weight F caused by acceleration to an electrical signal using resistors, piezoelectric elements, differential transformers, end other devices. The gravity-type accelerometer measures an acceleration as a signed value, while the wheel speed speedometer usually measures a speed as an unsiged value, i.e. an absolute value. Therefore, the estimated vehicle acceleration calculated according to the wheel speed measured by such wheel speed speedometers will not have the same sign as the acceleration obtained from the gravity-type accelerometer when the vehicle is travelling in reverse direction. If the direction of vehicle travel is not identified by some means, it will not be possible to control wheel behavior when the vehicle travels in reverse direction. Although it is possible to use the sign (positive or negative) of the output of the gravity-type accelerometer, such sign has a poor accuracy, due to the effects of the installation precision, electrical drift and gain changes, and other relatively long-term variations. Furthermore, it is also affected by the slope of a road where the vehicle is driven on. In these circumstances the sign of the output cannot necessarily be used as the true sign of acceleration. Therefore, when using a gravity-type accelerometer, it is extremely important to solve these problems including the problem of the slope of the road. In the applicant's previously filed related application EP-A-0 492 626 corresponding to Japanese Patent application H2-406743, zero-point correction for the gravity-type accelerometer is introduced. By the use of the zero-point correction device as disclosed in the related application, it is possible to estimate the sign of the true acceleration with a certain degree of reliability using the sign of the value output by the gravity-type accelerometer after zero-point correction. In the above related application, however, the zero-point correction is carried out under the assumption that the direction of vehicle travel is correctly detected. If the zero-point correction is carried out based on a false assumption of the direction of vehicle travel, the correction will give an unexpected result much worse than without correction. Therefore, when a gravity-type accelerometer is used it is very important to correctly recognize the direction of vehicle travel. SUMMARY OF THE INVENTION The present invention has been developed with a view to substantially solving the above described disadvantages and has for its essential object to provide an improved vehicle travel direction estimating device. In order to achieve the aforementioned object, a vehicle travel direction estimating device according to the present invention comprises: wheel speed calculation means for measuring a rotational velocity of a wheel and for producing an absolute value thereof as a wheel speed of at least one wheel of a vehicle at each operation cycle; estimated vehicle acceleration calculation means for calculating an estimated vehicle acceleration Aw estimated from the wheel speed; gravity-type accelerometer for producing a gravitationally detected acceleration Am; first differentiating means for differentiating said gravitationally detected acceleration Am and for producing a first differential value dAm; second differentiating means for differentiating said estimated vehicle acceleration Aw and for producing a second differential value dAw; and detection means for detecting the direction of the vehicle travel based on said first and second differential values dAm and dAw. The differential dAw of the vehicle speed Aw estimated from the wheel speed and the differential dAm of the acceleration Am obtained from the gravity-type accelerometer are plotted on a map having the differential of the vehicle acceleration estimated based on the wheel speed on the horizontal axis and the differential of the output from the gravity-type accelerometer on the vertical axis, and the direction of vehicle travel is determined to be forward when the values approach the +45° line and reverse when the values approach the -45° line. A vehicle travel direction estimating device according to the present invention attempts to determine the direction of vehicle travel by a general evaluation of the behavior of the output of a gravity-type accelerometer and the behavior of the estimated vehicle acceleration based on the wheel speed. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given below and the accompanying diagrams wherein: Fig. 1 is a block diagram of an antilock brake control device equipped with a vehicle travel direction estimating device according to the present invention; Fig. 2 is a flow chart illustrating the operation of the vehicle travel direction estimating device shown in Fig. 1; Fig. 3 is a graph of the map of determination values used for determining the vehicle travel direction; Fig. 4 is an alternative example of the map in Fig. 3; Fig. 5 is another alternative example of the map in Fig. 3; Fig. 6 is a simplified diagram of a gravity-type accelerometer; and Fig. 7 is a view similar to Fig. 1, but showing a modification thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before proceeding to the detailed description of the device, the principle of operation for this device is described first. When the vertical axis represents the vehicle acceleration estimated from the wheel speed and also the output of the gravity-type accelerometer, and the horizontal axis represents time, the direction of vehicle travel is determined to be forward when the output of the accelerometer and the estimated vehicle acceleration show a change of vertically parallel movement to the horizontal axis, and is determined to be reverse when the change is vertically symmetrical to the horizontal axis. In other words, when both accelerations Aw and Am are increased or decreased simultaneously, it is so determined that the vehicle is travelling forward, and when either one of the accelerations Aw and Am shows increasing feature and the other one shows decreasing feature, or vice versa, it is so determined that the vehicle is travelling reverse. Of course, it is quite important to equalize the dynamic properties of the output of the accelerometer and the estimated vehicle acceleration, i.e. the mechanical and mathematical filtering delays of the two, because this is the basis of the comparison. To evaluate the above characteristics, i.e. parallel or symmetrical, a map having a horizontal axis representing the differential value dAw of the estimated vehicle acceleration Aw based on the wheel speed and a vertical axis representing the differential value dAm of the output Am of the gravity-type accelerometer is prepared. In the map, the direction of vehicle travel is determined to be forward when a plotted point is near the +45° line and reverse when the same is near the -45° line. Examples of such maps are shown in Figs. 3, 4, and 5. It can be concluded that evaluation is difficult when a plotted point is near either the vertical or horizontal axis. Therefore, the certainty of the forward or reverse direction of vehicle travel evaluation can be expressed by the contour lines on the map. If the value Z of the contour line is used for determining the vehicle travel direction, the sign of Z represents the direction of vehicle travel and the absolute value of Z represents the degree of certainty of the determination. The value Z may be given by a numeric table on the map, or nay be calculated from a suitable function, Z= f (x,y) . If various parameters are defined such that: Aw is the vehicle acceleration estimated from the wheel speed; Am is the acceleration output by the gravity-type accelerometer; Aof and Aob are the zero-point correction amount applied to the gravity-type accelerometer in the forward direction and in the reverse direction, respectively; Acf and Acb are the acceleration output by the gravity-type accelerometer after correction in the forward direction and in the reverse direction, respectively; and kf and kb are the speed of zero-point adjustment in the forward direction and in the reverse direction, respectively, then the relationships Acf = Am - Aof (1) Aof = Aof + (Acf - Aw) * kf (2) can be defined in the forward direction, and Acb = -Am + Aob (3) Aob = Aob - (Aw - Acb) * kb (4) in the reverse direction. It is to be noted that in equations (2) and (4) 0 ≦ k ≦ 1. The unit of Ac, Am, Ao and Aw is the product of the gravity acceleration end the unit operating cycle time. Thus, the differential operations are expressed as the amount of chance during one operating cycle. A corrected acceleration Ac is determined using equations (1) and (2) when the result of the evaluation of vehicle travel direction is forward, and using equations (3) and (4) when the result of the evaluation of vehicle travel direction is reverse. It is to be noted that the equations (5) and (6) Acf = Am + Aof (5) Aof = Aof + (Aw - Acf) * kf (6) can be substituted for equations (1) and (2), and equations (7) and (8) Acb = -Am - Aob (7) Aob = Aob - (Aw - Acb) * kb (8) can be substituted for equations (3) and (4). Furthermore, sensitivity correction may also be applied in addition to the zero-point correction using equations Acf = (Am - Aof) * S Aof = Aof + (Acf - Aw) * kf * a S = S - (Acf - Aw) * kf * b for the forward direction, etc. If the value Z is expressed by a function, such a function may be expressed as follows. Z = f (dAw, dAm) (9) One example of the function f is f(x, y) = |x+y| - |x-y| (10). A map of the values obtained by this function is shown in Fig. 3. It should be further noted that maps as shown in Figs. 4 and 5 can also be used. Because Z is affected by noise and differences in the time constants of Aw and Am, it is preferable to use in place of the raw value Z the filtered value Zf, wherein the speed c of the filter is (0 < c < 1). The output Zf after filtering is defined by equation (11) as follows. Zf = Zf - (Z - Zf) * c (11) If c is chosen as a relatively small value, it will be more convenient to use a Zf value multiplied by 1/c as Zf = Zf * (1 - C) - Z (11') instead of (11). The extrimity of c-0 in (11') is the simple integration. When Zf>0, direction of travel is determined to be forward, and Zf<0 it is assumed to be reverse. When Zf > 0, Acf is used, and then Zf < 0, Acb is used. When Zf = 0, either direction may be applied, but it is natural to assume the forward direction of travel. Though the value Zf may be calculated at all operation cycles, the calculation may be stopped when the absolute value of Zf reaches a predetermined value and the value Zf at the time is stored in a memory until the vehicle speed becomes 0 (i.e., until the vehicle stops). Zf is reset to zero, when the vehicle starts moving again to resume the calculation of Zf from the cleared state. During the period in which calculation of Zf is stopped, it is preferable to stop calculation of Ao for the opposite direction and substitute the value Ao for the same direction, because the vehicle travel direction is assumed to remain in the same direction even if the behavior of Aw and Am temporarily suggests the opposite direction of travel. Specifically, calculation of Aob is stopped during the period in which Zf is fixed as Zf>0, and during which period it is assumed that Aob=Aof ; similarly, calculation of Aof is stopped during the period in which Zf is fixed as Zf<0, and during which period it is assumed that Aof=Aob . By taking this concept one step further, even if Zf is not fixed, it is also possible to normally calculate only one of the values Aof or Aob. Thus, when Zf ≧ 0, Ac = Am - Ao (1') Ao = Ao + (Ac - Aw) * k (2') are obtained and when Zf < 0, Ac = -Am + Ao = -(Am - Ao) (3') Ao = Ao + (Aw - Ac) * k (4') are obtained. Referring to Fig. 1, an antilock brake control device equipped with a vehicle travel direction estimating device according to the present invention is shown. The antilock brake control device comprises wheel speed sensors 1 to 4 to detect the rotation of the four wheels, a gravity-type accelerometer 5 which produces an acceleration output Am, a wheel speed calculator 6 to calculate the wheel speed of each of the four wheels based on the signals from the wheel speed sensors, a wheel speed selector 7 to select the maximum, minimum, or average wheel speed of the four wheels depending upon the conditions, and an estimated vehicle acceleration calculation means 8 to calculate the estimated vehicle acceleration Aw based on the selected wheel speed. The antilock brake control device further comprises a first differentiation circuit 9 which outputs the differential dAm of the output Am from the gravity-type accelerometer 5, and a second differentiation circuit 10 which outputs the differential dAw of the estimated vehicle acceleration based on the wheel speed. An accelerometer zero-point correction amount adjustment speed setting means 11 is provided to receive the differential values dAm and dAw and produces an adjustment speed k (delay time) of the correction amount used for the zero point correction of the gravity-type accelerometer. A vehicle travel direction determination value output means 12 also receives the differential values dAm and dAw and reads from a table or a map such as shown in Fig. 3 or calculates the vehicle travel direction determination value Z using the differential values dAm and dAw. A filter device 13 receives the vehicle travel direction determination value Z and removes noise signal which may be produced during the differential operation by using equation (11). A forward correction calculation means 14 receives the estimated vehicle acceleration Aw estimated from the wheel speed, the acceleration output Am by the gravity-type accelerometer, and adjustment speed k, and calculates using equations (1) and (2) the acceleration Acf based on the corrected gravity-type accelerometer when the vehicle travel direction is forward. Similarly, a reverse correction calculation means 15 receives the parameters Aw, Am and kb, and calculates using equations (3) and (4) the acceleration Acb based on the corrected gravity-type accelerometer when the vehicle travel direction is reverse. A selection means 16 selects the output Acf from the forward correction calculation means 14 or output Acb from the reverse correction calculation means 15 according to the travel direction indicated by the determination value Zf output from the filter device 13, and outputs the corrected acceleration Ac which is the corrected version of the acceleration obtained from the gravity-type accelerometer. An estimated vehicle speed calculation means 17 is provided which calculates the estimated vehicle speed based on the wheel speed Vr from the wheel speed selector 7 and either one of the acceleration Ac based on the gravity-type accelerometer or the acceleration Aw obtained from the estimated vehicle acceleration calculation means 8. An antilock brake controller 18 detects wheel locking symptoms based on the wheel speed and estimated vehicle speed, and outputs an antilock brake control signal. Further provided are solenoid command output means 19 to 22 which output signals to the solenoids in the actuators 23 to 26 for controlling the brake hydraulic pressure based on the antilock brake control signals. The operation of the vehicle travel direction estimating device according to the present invention is described hereinbelow with reference to the flow chart shown in Fig. 2. The output Am from the accelerometer is read at Step #1, and the wheel speed is calculated in Step #2 based on the outputs from the wheel speed sensors 1 to 4. At Step #3, the representative wheel speed Vr is selected, and the estimated vehicle acceleration Aw is then calculated in Step #4. At Step #5 the differentials dAm and dAw of the acceleration Am output by the gravity-type accelerometer 5 and the estimated vehicle acceleration Aw based on the wheel speed are obtained. In Step #6, the adjustment speed k is output using the obtained differential values dAm and dAw. The adjustment speed k is a value between 0 and 1, and is obtained in a manner described in the applicant's previously filed related application (serial No. is not yet received) corresponding to Japanese Patent application H2-406743. In Step #7, the correction calculation is performed using equations (1) and (2) and the corrected acceleration Acf based on the gravity-type accelerometer is calculated when the vehicle travel direction is forward, and at Step #8 the correction calculation is performed using equations (3) and (4) and the corrected acceleration Acb based on the gravity-type accelerometer is calculated when the vehicle travel direction is reverse. At Step #9, the determination value Z is output from a numeric table or equation (10) expressing the map shown in Fig. 3 to determine the direction of vehicle travel using the differentials dAm and dAw. Because there are cases in which the determination value Z is affected by the noise signals, filtering is applied to remove such noise signals. The filtered determination value Zf is then output at Step #10. At Step #11 it in determined whether the filtered determination value Zf is positive, zero, or negative. If positive or zero, the direction of vehicle travel is determined to be forward. In this case, the procedure thus advances to Step #12 to select the forward vehicle acceleraiion Acf. If Zf is negative, the direction of vehicle travel is determined to be reverse. In this case, the procedure advances is Step #13 to select the reverse vehicle acceleration Acb. The selected acceleration is set as the corrected acceleration Ac of the gravity-type accelerometer. Referring to Fig. 7, a modification of the vehicle travel direction estimating device is shown. According to the device shown in Fig. 1, the forward correction calculation means 14 carries out the calculation of equations (1) and (2) and, at the same time, the reverse correction calculation means 15 carries out the calculation of equations (3) and (4), regardless of forward or reverse direction of the vehicle travel. When the vehicle is travelling forward as detected by the vehicle travel direction determination value output means 12, the selection means 16 is so operated as to select output Acf from the calculation means 14, and when the vehicle is travelling reverse, the selection means 16 is so operated as to select output Acb from the calculation means 15. However, according to the device shown in Fig. 7, the selection means 16 is provided before the accelerometer zero-point correction amount adjustment speed setting means 11. The selection means 16 receives parameters Aw, Am, dAw and dAm. When the vehicle is travelling forward as detected by the vehicle travel direction determination value output means 12, the selection means 16 is so operated as to supply the parameters dAw and dAm to accelerometer zero-point correction amount adjustment speed setting means 11a and the parameters Aw and Am to forward correction calculation means 14, and when the vehicle is travelling reverse, the selection means 16 supply the parameters dAw and dAm to accelerometer zero-point correction amount adjustment speed setting means 11b and the parameters Aw and Am to reverse correction calculation means 15. By the arrangement of Fig. 7, equations (1) and (2) are calculated when it is detected that the vehicle is travelling forward, and equations (3) and (4) are calculated when it is detected that the vehicle is travelling reverse. As described hereinabove, since the correct direction of vehicle travel can be determined by using the differential of the output of the gravity-type accelerometer and the differential of the estimated vehicle acceleration based on the wheel speed, a vehicle travel direction estimating device according to the present invention can obtain the correct vehicle travel direction even if the zero point of the gravity-type accelerometer shifts. Thus, wheel behavior control of the vehicle can be correctly applied in both forward and reverse directions of vehicle travel. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention as claimed, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A vehicle travel direction estimating device comprising: wheel speed calculation means (6) for measuring a rotational velocity of a wheel and for producing an absolute value thereof as a wheel speed of at least one wheel of a vehicle at each operation cycle; estimated vehicle acceleration calculation means (8) for calculating an estimated vehicle acceleration Aw estimated from the wheel speed; gravity-type accelerometer (5) for producing a gravitationally detected acceleration Am; first differentiating means (9) for differentiating said gravitationally detected acceleration Am and for producing a first differential value dAm; second differentiating means (10) for differentiating said estimated vehicle acceleration Aw and for producing a second differential value dAw; and detection means (12) for detecting the direction of the vehicle travel based on said first and second differential values dAm and dAw. A vehicle travel direction estimating device according to Claim 1, wherein said detection means (12) detects the direction of travel to be forward when the signs of said first and second differential values are the same, and to be reverse when the signs of the differential values differ. A vehicle travel direction estimating device according to Claim 2, wherein said detection means (12) comprises means for obtaining a certainty value Z, an absolute value thereof representing the certainty of the determination and a sign thereof representing the vehicle travel direction, from a contour line on a map of which the vertical and horizontal axes represent said first and second differential values dAm and dAw, respectively. A vehicle travel direction estimating device according to Claim 3, wherein said detection means (12) comprises means for obtaining a certainty value Z, an absolute value thereof representing the certainty of the determination and a sign thereof representing the vehicle travel direction, at each operation cycle, and filtering means for producing a smoothed certainty value Zf by exponentially filtering the value Z. A vehicle travel direction estimating device according to Claim 1, further comprising: means (11) for producing an adjustment speed kf assuming a forward direction of travel in the producing an adjustment speed kb assuming a reverse direction of travel; means (14) for producing, assuming a forward direction of travel, corrected acceleration Acf based on said gravity-type accelerometer by correcting said gravitationally detected acceleration Am by a correction amount Aof; means (14) for producing, assuming a forward direction of travel, said correction amount Aof in the current operation cycle by adjusting the correction amount Aof obtained in the previous operation cycle, by a value which is the difference between said corrected acceleration Adf and said estimated vehicle acceleration Aw modified by said adjustment speed kf; means (15) for producing, assuming a reverse direction of travel, a corrected acceleration Acb based on said gravity-type accelerometer by correcting said gravitationally detected acceleration Am by correcting said gravitationally detected acceleration Am by a correction amount Aob; means (15) for producing, assuming a reverse direction of travel, said correction amount Aob in the current operation cycle by adjusting the correction amount Aob obtained in the previous operation cycle, by a value which is the difference between said corrected acceleration Acb and said estimated vehicle acceleration Aw modified by said adjustment speed kb; and means (16) for selecting one of corrected accelerations Acf and Acb according to the vehicle travel direction detected by said means (12). A vehicle travel direction estimating device according to Claim 1, further comprising: means (11) for producing an adjustment speed k assuming a direction of travel indicated by said vehicle travel direction detection means (12); means (14) for producing, assuming a forward direction of travel when a forward direction is indicated by said means (12), a corrected acceleration Ac based on said gravity-type accelerometer by correcting said gravitationally detected acceleration Am by a correction amount Ao; means (14) for producing, assuming a forward direction of travel when a forward direction is indicated by said means (12), said correction amount Ao in the current operation cycle by adjusting the correction amount Ao obtained in the previous operation cycle, by a value which is the difference between said corrected acceleration Ac and said estimated vehicle acceleration Aw modified by said adjustment speed k; means (15) for producing, assuming a reverse direction of travel when a reverse direction is indicated by said means (12), a corrected acceleration Ac based on said gravity-type accelerometer by correcting said gravitationally detected acceleration Am by a correction amount Ao; means (15) for producing, assuming a reverse direction of travel when a reverse direction is indicated by said means (12), sid correction amount Ao in the current operation cycle by adjusting the correction amount Ao obtained in the previous operation cycle, by a value which is the difference between said corrected acceleration Ac and said estimated vehicle acceleration Aw modified by said adjustment speed k. A vehicle travel direction estimating device according to Claim 1, wherein said detection means (12) further comprising: means for fixing said detected vehicle travel direction until the vehicle stops when the level of the certainty of detection reaches a predetermined level; and means for clearing said fixed direction when the vehicle restarts after a stops and for resuming the detection from the cleared state.

Inventors: NAKAURA TOHRU C O ITAMI WORKS, TAKATA KOJI C O ITAMI WORKS OF, NAKAURA, TOHRU, C/O ITAMI WORKS OF SUMITOMO, TAKATA, KOJI, C/O ITAMI WORKS OF SUMITOMO ELECTRIC
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: B60T8
IPC codes: G01P13, B60T8, B60W30