Abstract:
An automatic transmission control apparatus controls a gear position by engaging and disengaging multiple frictional elements. A hydraulic pressure detector detects hydraulic pressure applied to the frictional elements. A failure determiner determines a failure, i.e., a dual-engagement of the frictional elements in accordance with detection signals of the hydraulic pressure detector. The hydraulic pressure detector detects first and second hydraulic pressure applied to the plurality of frictional elements. The second hydraulic pressure is greater than the first hydraulic pressure. The failure determiner determines a failure in accordance with a detection signals of the first and second hydraulic pressure detected by the hydraulic pressure detector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and incorporates herein by reference Japanese Patent Application No. 2004-105561 filed on March 31. 
   FIELD OF THE INVENTION 
   The present invention relates to an automatic transmission control apparatus that hydraulically controls a transmission mechanism of an automatic transmission. 
   BACKGROUND OF THE INVENTION 
   Conventionally, an automatic transmission is applied to an apparatus such as a vehicle. As disclosed in JP-A-2001-59570 (U.S. Pat. No. 6,357,289B1) and JP-A-2001-116134 (U.S. Pat. No. 6,375,591B1), a hydraulic control apparatus controls hydraulic pressure applied to frictional elements in an automatic transmission. Engagement and disengagement of each frictional element is controlled by the hydraulic control apparatus, so that a gear position is changed. Besides, hydraulic pressure, which is applied to each frictional element, is detected using a pressure switch, so that a failure is identified in accordance with a combination among detection signals of the pressure switches. 
   As shown in  FIG. 7 , one pressure switch detects hydraulic pressure applied to each frictional element in a hydraulic pressure control apparatus, in the same manners as those of the structures in U.S. Pat. No. 6,357,289B1 and U.S. Pat. No. 6,375,591B1. 
   Frictional elements include a reverse clutch (R/C)  1 , a low-reverse brake (LR/B)  2 , a low clutch (L/C)  3 , a 2-4 brake (2-4/B)  4 , and a high clutch (H/C)  5 . The frictional elements are engaged, and disengaged in accordance with hydraulic pressure. Control means  11  to  15  respectively switch hydraulic pressure applied to the frictional elements  1  to  5 . Each of the control means  11  to  15  is constructed of a solenoid valve and a spool valve, for example. The spool valve switches hydraulic passage in accordance with operation pressure of the solenoid valve. The manual valve  20  is connected with a select lever  30  via a wire or the like. The select lever  30  is operated by a driver to change a drive range (select lever position, gear position). When the drive range is changed, a manual valve  20  switches hydraulic pressure passages that are respectively connected to the control means  11  to  15 . In this situation, hydraulic pressure, which is applied to each control means  11  to  15 , is switched from one of line pressure and hydraulic pressure of a drain  22  to the other of the line pressure and the hydraulic pressure of the drain  22 , in accordance with the drive range. Each of pressure switches  40  outputs detection signal of hydraulic pressure applied to each frictional element. The detection signal of the pressure switch  40  is an on/off signal that is switched on a threshold that is set at detection main pressure. 
   For example, a gear position is changed from the second gear to the third gear in the D range (drive range) in accordance with a table of engagement shown in  FIG. 8 . As shown in  FIG. 9 , when hydraulic pressure  204 ,  205  applied to the H/C  5  increases, the pressure switch  40 , which detects the hydraulic pressure  204 ,  205  applied to the H/C  5 , is turned ON, so that a dual-engagement can be detected. In  FIG. 9 , hydraulic pressure  205  applied to the H/C  5  increases faster than normal hydraulic pressure  204  applied to the H/C  5 . Hydraulic pressure  200  is applied to L/C  3 , and hydraulic pressure  202  is applied to the 2-4/B  4  that is disengaged. 
   However, in the structures disclosed in U.S. Pat. No. 6,357,289B1 and U.S. Pat. No. 6,375,591B1, one pressure switch  40  is provided to each frictional element  1  to  5 , thereby, each pressure switch  40  can detect a condition of hydraulic pressure relative to one threshold. As shown in  FIG. 10 , when the 2-4/B  4  is not disengaged, hydraulic pressure  203  applied to the 2-4/B  4  is maintained high. In this case, when the threshold of the pressure switch  40  is set low, e.g., set at the first hydraulic pressure P 1 , a failure cannot be detected, or a dual-engagement may be detected after the dual-engagement arises. 
   Specifically, when hydraulic pressure applied to each frictional element  1  to  5  becomes greater than the first hydraulic pressure (low-threshold) P 1  in  FIGS. 9 ,  10 , the frictional element engages. When at least two of the frictional elements simultaneously engage, a dual-engagement arises. The gear is changed on the point A in  FIG. 10 , and hydraulic pressure applied to the 2-4/B  4  is supposed to decrease as shown by the hydraulic pressure  202 . When the threshold of the pressure switch  40  is set low, i.e., set at the first hydraulic pressure P 1 , disengagement of the 2-4/B  4  is detected on the point C in  FIG. 10  by the pressure switch  40 . In this case, if a failure arises, and the hydraulic pressure applied to the 2-4/B  4  is maintained high as shown by the hydraulic pressure  203 , the hydraulic pressure  203  may be determined to be in a failure condition on the point C or later. That is, when the pressure switch  40  does not detect the hydraulic pressure  203  to be less than the first hydraulic pressure P 1  on the point C or later, the hydraulic pressure  203  may be determined in the failure condition. Accordingly, the condition of the hydraulic pressure  203  may be determined at the point C or later after elapsing a long time from starting of the gear change on the point A. When the hydraulic pressure  203  applied to the 2-4/B  4  is maintained high, a dual-engagement arises on the point B in  FIG. 10 , on which the hydraulic pressure  204  applied to the H/C  5  increases over the first hydraulic pressure P 1  so that the H/C  5  engages. 
   On the contrary, as referred to  FIG. 9 , when the threshold of the pressure switch  40  is set high at second hydraulic pressure P 2 , and the hydraulic pressure  205  applied to the H/C  5  increases faster than a predetermined speed, a dual-engagement cannot be detected even after the dual-engagement arises. 
   Specifically, the threshold of the pressure switch is set high, e.g., set at the second hydraulic pressure P 2 , and the hydraulic pressure  205  applied to the H/C  5  may increase faster than the predetermined speed. In this case, the hydraulic pressure  205  applied to the H/C  5  increases over the first hydraulic pressure P 1  on the point B in  FIG. 9 , and the H/C  5  engages. Besides, the hydraulic pressure  202  applied to the 2-4/B  4  is still greater than the first hydraulic pressure P 1  on the point B in  FIG. 9 , and the 2-4/B  4  still engages. That is, a dual engagement arises on the point B in  FIG. 9 . As time elapses, the hydraulic pressure  205  increases after passing the point B, and the hydraulic pressure  205  becomes greater than the second hydraulic pressure P 2  on the point C in  FIG. 9 . At this point, increase of the hydraulic pressure  205  can be detected by the pressure switch  40 . That is, the dual-engagement cannot be detected in the zone between the point B and the point C, when the threshold of the pressure switch is set high, e.g., set at the second hydraulic pressure P 2 . 
   Accordingly, when the pressure switch has only one threshold on either the low-pressure side or the high-pressure side for each frictional element, not only a dual-engagement but also a failure of hydraulic pressure cannot be detected on one of the engagement-side and the disengagement-side. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing problems, it is an object of the present invention to produce an automatic transmission control apparatus that is capable of detecting a failure of hydraulic pressure, which is applied to frictional elements, on both sides of engagement and disengagement. 
   According to the present invention, an automatic transmission control apparatus controls a gear position by operating engagement and disengagement of multiple frictional elements. The automatic transmission control apparatus includes a hydraulic pressure detecting means and a failure determining means. The hydraulic pressure detecting means detects hydraulic pressure applied to at least one of the frictional elements. The failure determining means determines a failure in accordance with a detection signal of the hydraulic pressure detecting means. 
   The hydraulic pressure detecting means detects at least first hydraulic pressure and second hydraulic pressure. The second hydraulic pressure is greater than the first hydraulic pressure. The failure determining means determines a failure in accordance with a detection signal of the first hydraulic pressure that is detected by the hydraulic pressure detecting means. The failure determining means determines a failure in accordance with a detection signal of the second hydraulic pressure that is detected by the hydraulic pressure detecting means. 
   The hydraulic pressure detecting means detects hydraulic pressure that is applied to at least two of the frictional elements. The at least two of the frictional elements are probable of causing a dual-engagement. The failure determining means determines occurrence of the dual-engagement in accordance with the detection signal of the first hydraulic pressure that is detected by the hydraulic pressure detecting means. The failure determining means determines the dual-engagement in accordance with the detection signal of the second hydraulic pressure that is detected by the hydraulic pressure detecting means. 
   The dual-engagement arises when the at least two of the frictional elements simultaneously engage. 
   Each frictional element includes a return spring that generates resilient force biasing the frictional element in the direction in which the frictional element is released from an engagement condition. The frictional element engages when hydraulic pressure applied to the frictional element is greater than the resilient force of the return spring. The first hydraulic pressure is set in the vicinity of hydraulic pressure that is equivalent to the resilient force of the return spring. The second hydraulic pressure is set in the vicinity of hydraulic pressure that is equivalent to a minimum hydraulic pressure needed for engagement of the frictional element under a maximum load condition. 
   The first hydraulic pressure may be set at the hydraulic pressure that is equivalent to the resilient force of the return spring. The second hydraulic pressure may be set at the hydraulic pressure that is equivalent to the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. 
   The hydraulic pressure detecting means includes a first hydraulic pressure switch and a second hydraulic pressure switch. The first hydraulic pressure switch outputs an ON/OFF signal relative to the first hydraulic pressure as a threshold. The second hydraulic pressure switch outputs an ON/OFF signal relative to the second hydraulic pressure as a threshold. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is a diagram showing a hydraulic circuit of an automatic transmission control apparatus according to an embodiment of the present invention; 
       FIG. 2  is a diagram showing a circuit that includes pressure switches and resistances according to the embodiment; 
       FIG. 3  is a table showing a relationship between a combination of ON/OFF-conditions of the pressure switches and combined resistance of the resistances according to the embodiment; 
       FIG. 4A  is a cross-sectional side view showing the pressure switch in the OFF-condition, and  FIG. 4B  is a cross-sectional side view showing the pressure switch in the ON-condition according to the embodiment; 
       FIG. 5  is a time chart showing hydraulic pressure applied to frictional elements when a failure arises on the side of engagement according to the embodiment; 
       FIG. 6  is a time chart showing hydraulic pressure applied to the frictional elements when a failure arises on the side of disengagement according to the embodiment; 
       FIG. 7  is a diagram showing a hydraulic circuit of an automatic transmission control apparatus according to a prior art; 
       FIG. 8  is a table showing a relationship between engagement conditions of the frictional elements and drive modes according to the embodiment and a related art; 
       FIG. 9  is a time chart showing hydraulic pressure applied to frictional elements when a failure arises on the side of engagement according to the prior art; and 
       FIG. 10  is a time chart showing hydraulic pressure applied to the frictional elements when a failure arises on the side of disengagement according to the prior art. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   First Embodiment  
   As shown in  FIG. 1 , an automatic transmission control system includes hydraulic pressure switches  41 ,  42  that detect hydraulic pressure applied to frictional elements  1  to  5 . The pressure switches  41 ,  42  serve as hydraulic pressure detecting means. Each of the pressure switches  41 ,  42  respectively has a threshold, on which the pressure switch  41 ,  42  turns ON as hydraulic pressure detected by the pressure switch  41 ,  42  changes. The threshold of the pressure switch  41  is different from the threshold of the pressure switch  42 . The pressure switches  41 ,  42  have substantially equivalent structures excluding the threshold. 
   As shown in  FIG. 4A , terminals  100 ,  102  are not electrically continuous with each other, when hydraulic pressure applied to a diaphragm  104  is less than a detecting hydraulic pressure, i.e., the threshold. As shown in  FIG. 4B , the terminals  100 ,  102  become electrically continuous with each other, when hydraulic pressure applied to the diaphragm  104  is greater than the threshold. Therefore, the hydraulic pressure switch  41  turns OFF, when hydraulic pressure applied to the frictional elements  1  to  5  is less than the threshold. Besides, the hydraulic pressure switch  41  turns ON, when hydraulic pressure applied to the frictional elements  1  to  5  is greater than the threshold. 
   The first oil pressure (first hydraulic pressure) P 1 , which is a threshold set for the pressure switch (low-pressure switch)  41 , is set at hydraulic pressure that is equivalent to resilient force of each of return springs  1   a  to  5   a.  Each return spring  1   a  to  5   a  urges each frictional element  1  to  5  in the disengagement direction, in which the frictional element  1  to  5  is released from an engagement condition. The first hydraulic pressure P 1  may be set in the vicinity of the hydraulic pressure that is equivalent to the resilient force of the return spring  1   a  to  5   a.  On the contrary, the second oil pressure (second hydraulic pressure) P 2 , which is detected by the pressure switch (high-pressure switch)  42 , is set at hydraulic pressure that is equivalent to the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. The second hydraulic pressure P 2  may be set in the vicinity of the hydraulic pressure that is equivalent to the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. 
   As shown in  FIG. 2 , the low-pressure switch  41  and the high-pressure switch  42 , which detect hydraulic pressure applied to each frictional element, construct a parallel circuit with resistances  51 ,  52 . The low-pressure switch  41  and the resistance  51  are connected in series, and the high-pressure switch  42  and the resistance  52  are connected in series in the parallel circuit. 
   As shown in  FIG. 3 , combined resistance of the parallel circuit changes in four degrees based on the ON/OFF conditions of the low-pressure switch  41  and the high-pressure switch  42 . Output voltage Vout of the parallel circuit varies in accordance with the combined resistance of the parallel circuit. The combined resistance of the parallel circuit changes based on the ON/OFF conditions of the low-pressure switch  41  and the high-pressure switch  42 . An engine control unit (ECU)  60  detects the ON/OFF conditions of the pressure switches  41 ,  42  in accordance with the ratio Vout/Vin between the output voltage Vout and the power-supply voltage Vin to determine a failure. The ECU  60  serves as a failure determining means. 
   Next, a dual-engagement, which arises when a gear position is changed from the second gear to the third gear in the D range, is described. 
   As shown in  FIGS. 5 ,  8 , the gear is changed on the point A in  FIG. 5 . Hydraulic pressure  205  applied to an H/C  5  increases faster than a predetermined speed, and the low-pressure switch  41 , which detects the hydraulic pressure  205  of the H/C  5  on the low-pressure side (first hydraulic pressure P 1 ), is turned ON, on the point B in  FIG. 5 . In this condition, the low-pressure switch  41 , which detects hydraulic pressure  202  applied to a 2-4/B  4  on the low-pressure side (first hydraulic pressure P 1 ), is still turned ON, on the point B in  FIG. 5 . Therefore, the ECU  60  determines the 2-4/B  4  and the H/C  5  to be causing a dual-engagement. When the hydraulic pressure  205  increases over the first hydraulic pressure P 1  on the point B in  FIG. 5 , and the high-pressure switch  42 , which detects the hydraulic pressure  202  on the second hydraulic pressure P 2 , is still turned ON on the point B in  FIG. 5 , the dual-engagement may be determined. 
   As shown in  FIGS. 6 ,  8 , when the gear position is changed from the second gear to the third gear in the D range, the 2-4/B  4  may not be released, i.e., hydraulic pressure  203  applied to the 2-4/B  4  may be maintained high. In this situation, the high-pressure switch  42 , which detects hydraulic pressure  203  of the 2-4/B  4  on the high-pressure side (second hydraulic pressure P 2 ), is determined to be still ON. Thereby, a dual engagement can be prevented by setting a predetermined time to a timer or the like, before hydraulic pressure  204  applied to the H/C  5  increases over the first hydraulic pressure P 1  and the low-pressure switch  41  is turned ON on the point C in  FIG. 10 . 
   Specifically, when the gear position is changed from the second gear to the third gear in the D range on the time point A in  FIG. 6 , the timer starts. The hydraulic pressure applied to the 2-4/B  4  is supposed to decrease as shown by the hydraulic pressure  202 , and is supposed to be lower than the second hydraulic pressure P 2  on the time point B in  FIG. 6 . However, when a predetermined time is elapsed after the timer starts, and the hydraulic pressure  203  is maintained higher than the second hydraulic pressure P 2  on the time point B, the ECU  60  determines the hydraulic pressure  203  of the 2-4/B  4  to be in a failure condition. If the 2-4/B  4  is maintained high, the hydraulic pressure  204  of the H/C  5  increases over the first hydraulic pressure P 1  on the time point C, on which the H/C  5  engages, and thus, the 2-4/B  4  and the H/C  5  causes a dual engagement. On the contrary, when the failure of the hydraulic pressure  203  of the 2-4/B  4  is detected on the point B, the dual engagement can be prevented on the point B. 
   In the above structure, hydraulic pressure applied to each frictional element is detected on both the high-pressure side and the low-pressure side using the two pressure switches  41 ,  42 . Therefore, failure of hydraulic pressure applied to each frictional element can be determined on both the engagement side and the disengagement side, while the gear position is being changed. Thus, various kinds of failure modes can be determined. 
   In the above structure, a frictional element such as R/C  1  does not cause a dual-engagement. However, hydraulic pressure applied to the R/C  1  is detected using both the low-pressure switch  41  and the high-pressure switch  42 . In this case, the threshold of the high-pressure switch  42 , which is set on the high-pressure side, can be set higher than the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. Here, the frictional element, which is supposed to be engaged under the maximum load condition, may slip due to insufficient hydraulic pressure. However, the slipping condition of the frictional element can be detected by setting the threshold of the high-pressure switch  42 . 
   In the above structure, the first hydraulic pressure P 1  is set at the hydraulic pressure that is equivalent to the resilient force of the return spring. The second hydraulic pressure P 2  is set at hydraulic pressure that is equivalent to the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. When hydraulic pressure of a frictional element exceeds the first hydraulic pressure P 1 , i.e., the resilient force of the return spring, the frictional element is supposed in the engagement condition. When a drive mode is changed, and hydraulic pressure of one frictional element is maintained higher than the second hydraulic pressure P 2  for a predetermined period, failure of the hydraulic pressure of the frictional element can be detected, while hydraulic pressure of another of the frictional elements is less than the first hydraulic pressure P 1 . Therefore, failure of hydraulic pressure of each frictional element can be detected before frictional elements cause a dual-engagement. 
   In the above structure, the second hydraulic pressure P 2  may be set at hydraulic pressure that is higher than the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. In this case, the transmission is controlled under hydraulic pressure, which is lower than the second hydraulic pressure P 2 , in all kinds of drive modes. Therefore, a failure, in which hydraulic pressure does not properly decrease lower than a set value, i.e., the second hydraulic pressure P 2 , can be steadily detected. 
   In the above structure, the pressure switches  41 ,  42  are ON/OFF switches that are operated in accordance with hydraulic pressure. Therefore, the structure of the pressure switches  41 ,  42 , i.e., the hydraulic pressure detecting means is simple. 
   In the above structure, the high-pressure switch (second hydraulic pressure switch)  42  detects pressure of engagement of a frictional element, which is to be disengaged, while the gear position is being changed. Besides, the low-pressure switch (first hydraulic pressure switch)  41  detects pressure of engagement of another of the frictional elements, which is to be engaged, while the gear position is being changed. When both the high-pressure switch  42  of the frictional element, which is to be disengaged, and the low-pressure switch  41  of the frictional element, which is to be engaged, are turned ON, the ECU  60  (failure determining means) determines a dual-engagement to be caused. Thus, a dual-engagement can be steadily detected, while mistake of determination is restricted. 
   In the above structure, a combination of the ON/OFF conditions of the pressure switches  41 ,  42  are detected in accordance with the output voltage Vout of the parallel circuit. The output voltage Vout of the parallel circuit changes based on the combined resistance of the parallel circuit constructed of the pressure switches  41 ,  42  and the resistances  51 ,  52 . Thereby, the number of the signal wires, which is connected to the ECU  60  for detecting the combinations of ON/OFF conditions of the pressure switches  41 ,  42 , can be reduced to one. Thus, the number of the signal wires is reduced, and a wiring process of the signal wire can be simplified. Besides, ON/OFF conditions of both the low-pressure switch  41  and the high-pressure switch  42  can be detected using a circuit having a simple structure. 
   Other Embodiment  
   The hydraulic pressure detecting means may detect hydraulic pressure applied to each frictional element based on at least three kinds of thresholds of hydraulic pressure. The at least three kinds of thresholds of hydraulic pressure are different from each other. 
   The first hydraulic pressure P 1  is not limited to be set at the hydraulic pressure that is equivalent to the resilient force of the return spring. The second hydraulic pressure P 2  is not limited to be set at hydraulic pressure that is equivalent to the minimum hydraulic pressure needed for engagement of the frictional element under the maximum load condition. The first and second hydraulic pressure P 1 , P 2  may be set to be in another range, as appropriate. 
   As referred to  FIG. 6 , when the hydraulic pressure  204  reaches the first hydraulic pressure P 1 , and both the pressure switches  41 ,  42  are turned ON, the hydraulic pressure may be determined to be in a failure condition, instead of using the timer for restricting a dual-engagement in the above structure. 
   The two pressure switches  41 ,  42  may be provided to only frictional elements that may cause a dual-engagement. 
   The output signals of the pressure switches  41 ,  42  may be directly connected to the ECU  60  to detect the combinations of the ON/OFF conditions of the pressure switches  41 ,  42 , instead of using the parallel circuit in the above structure. 
   Various modifications and alternations may be diversely made to the above embodiments without departing from the spirit of the present invention.