Abstract:
A control apparatus of power steering device having functions of monitoring intermediate troubles such as offset or drift out of signal troubles of torque sensor, and limiting the current for limiting the assist function is presented. In a control apparatus of power steering device for controlling a motor on the basis of a current control value calculated from a steering assist command value calculated by calculating means on the basis of a steering torque generated on a steering shaft, and a current value of a motor for applying a steering assist force to a steering mechanism, wherein the difference of a main torque signal and a sub torque signal of a torque sensor for detecting the steering torque is stored preliminarily, and the difference of the main torque signal and the sub torque signal during operation is compared with the stored value, and at least the current control value is limited when the difference based on the comparison is a condition larger than a first prescribed value and continues for a time longer than a first prescribed time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control apparatus of a power steering device for applying a steering assist force by a motor by a steering system of automobile or other vehicle, and more particularly to a control device of a power steering device having functions of monitoring troubles such as intermediate offset or drift among signal troubles of a torque sensor and limiting the assist function. 
     2. Description of the Related Art 
     A power steering apparatus for assisting a steering system of an automobile or other vehicle with an assist load by rotating force of a motor is designed to apply an assist load to the steering shaft or rack shaft by transmission mechanism such as gear or belt by way of reduction gears. In a conventional power steering device, in order to generate the assist torque (steering auxiliary torque) accurately, the motor current is controlled by feedback. The feedback control is intended to control the motor applied voltage so as to minimize the difference between the current command value and motor current detection value, and the motor applied voltage is generally controlled by the duty ratio of pulse width modulation (PWM) control. 
     A general configuration of a power steering device is shown in FIG. 1, in which a shaft  2  of a steering wheel  1  is coupled to a tie rod  6  of turning wheels by way of universal joints  4   a  and  4   b , and a rack-and-pinion mechanism  5 . The shaft  2  has a torque sensor  10  for detecting the steering torque of the steering wheel  1 , and a motor for assisting the steering force of the steering wheel  1  is coupled to the shaft  2  by way of a clutch  21  and reduction gears  3 . A control unit  30  for controlling the power steering device receives an electric power from a battery  14  through an ignition key  11  and a relay  13 , and the control unit  30  calculates the steering assist command value I of assist command on the basis of the steering torque T detected by the torque sensor  10  and the vehicle speed V detected by a vehicle speed sensor  12 , and controls the current to be supplied to the motor  20  on the basis of the calculated steering assist command value I. The clutch  21  is turned on or off by the control unit  30 , and it is turned on (coupled) in an ordinary running state. When the control unit  30  judges the power steering device to be abnormal, or when the power source (voltage Vb) of the battery  14  is turned off by the ignition key  11  and relay  13 , the clutch  21  is turned off (disconnected). 
     The control unit  30  is mainly composed of CPU, and a general function executed by a program in the CPU is shown in FIG.  2 . 
     Explaining the function and operation of the control unit  30 , the steering torque T detected and entered by the torque sensor  10  is compensated of phase in a phase compensator  31  in order to heighten the stability of the steering system, and the phase-compensated steering torque TA is inputted to a steering assist command value calculator  32 . The vehicle speed V detected by the vehicle speed sensor  12  is also inputted to the steering assist command value calculator  32 . The steering assist command value calculator  32  refers to a characteristic map (lookup table)  33  on the basis of the entered steering torque TA and vehicle speed V, and determines the steering assist command value I which is the control target value of the current to be supplied to the motor  20 . 
     The steering assist command value I is inputted to a subtractor  30 A, and is also inputted to a differential compensator  34  of feedforward system in order to enhance the response speed, and the deviation (I−i) determined in the subtractor  30 A is inputted to a proportion calculator  35 , and the proportion output is inputted to an adder  30 B and is also inputted to an integral calculator  36  in order to improve the characteristics of the feedback system. The outputs of the differential compensator  34  and integral calculator  36  are inputted to the adder  30 B, and the sum result in the adder  30 B, that is, the current command value E is inputted to a motor drive circuit  37  as a motor drive signal. The motor current value “i” of the motor  20  is detected in a motor current detecting circuit  38 , and the motor current detection value “i” is inputted to the subtractor  30 A and is fed back. 
     Herein, as the conventional torque sensor  10 , a type designed to output a main torque signal TM and a sub torque signal TS is used. This is for safety measure in case of failure of the torque sensor  10 , and whether trouble or not cannot be judged by one signal output alone. Hitherto, accordingly, monitoring the difference of the main torque signal TM and the sub torque signal TS, and when the difference more than a prescribed value continues for a prescribed time, it is judged to be abnormal, and the assist is cut off. 
     By using a characteristic map as shown in FIG. 3 having an abrupt rising characteristic, drift of torque sensor or offset voltage in a range not causing serious effect hitherto has come to have a large effect on steering when taking place abruptly while steering, and the assist amount may be excessive depending on the steering status, and an abnormal steering behavior despite the driver&#39;s will may occur, which may lead to a serious accident. 
     In the conventional trouble detection based on the difference between the main torque signal TM and sub torque signal TS, depending on the change in the drift or offset as shown in FIG. 4, such abnormal behavior may not be detected until the trouble is detected. Further, by setting a severe threshold for detecting trouble, if attempted to detect an intermediate trouble such as offset, considering the adjusting range of the torque sensor before shipping, a severe threshold may lead to wrong detection, and trouble cannot be detected correctly. 
     A small offset value generated gradually in a long course of time may cause a difference in right and left steering forces, but does not cause uncontrollable behavior of steering. In such a case, not leading to halt of assist, it is unpleasant for the driver, and steering error may occur, and therefore it is necessary to give some warning or limit the assist. 
     The present invention is devised in the light of such background, and it is hence an object of the invention to present a control apparatus of power steering device having functions of monitoring intermediate troubles such as offset or drift among signal troubles of torque sensor, inhibiting the assist in the event of a relatively large offset, and limiting the assist function when a trouble is detected in the case of a relatively small offset. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a control apparatus of power steering device for controlling a motor on the basis of a current control value calculated from a steering assist command value calculated by calculating means on the basis of a steering torque generated on a steering shaft, and a current value of a motor for applying a steering assist force to a steering mechanism, and this object is achieved by the configuration in which the difference of a main torque signal and a sub torque signal of a torque sensor for detecting the steering torque is stored preliminarily, and the difference of the main torque signal and the sub torque signal during operation is compared with the stored value, and at least the motor current output is stopped when the state of the difference by the comparison being different from a second prescribed value larger than a first prescribed value continues for a time shorter than a prescribed time. 
     The object of the present invention is effectively achieved by the configuration in which the steering assist command value is calculated on the basis of the output of a normal mode characteristic map, and when the state of the difference by the comparison being different from the first prescribed value continues for a prescribed time, it is changed over to a torque sensor offset abnormal mode characteristic map, or the current control value is limited according to the vehicle speed, or an intermediate characteristic map is used so as to change without any sense of strangeness when changing over from the normal mode characteristic map to the torque sensor offset abnormal mode characteristic map. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a structural diagram showing a schematic configuration of a power steering device; 
     FIG. 2 is a block structural diagram showing an example of control apparatus of a power steering device; 
     FIG. 3 is a diagram showing a characteristic example of the characteristic map; 
     FIG. 4 is a diagram for explaining the characteristic of the torque sensor; 
     FIG. 5 is a diagram for explaining the basic principle of the present invention; 
     FIG. 6 is a block diagram showing a configuration of a first embodiment of the present invention; 
     FIG. 7 is a block diagram showing an example of configuration of the torque offset abnormality detecting means; 
     FIG. 8 is a flowchart showing an example of operation of the first embodiment of the present invention; 
     FIG. 9 is a diagram for explaining the operation of the present invention; 
     FIG. 10 is a diagram for explaining the operation of the present invention; 
     FIG. 11 is a block diagram showing a configuration of a second embodiment of the present invention; 
     FIG. 12 is a diagram for explaining the operation of the present invention; 
     FIG. 13 is a diagram for explaining the operation of the present invention; 
     FIG. 14 is a block diagram showing a configuration of a third embodiment of the present invention; and 
     FIG. 15 is a flowchart showing an example of operation of the third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, the difference between a main torque signal TM and a sub torque signal TS in the initial state of shipment is stored in a nonvolatile memory, and the stored value is compared with the difference between the main torque signal TM and the sub torque signal TS during operation, and the assist is inhibited at a relatively large offset, and the assist function is limited when a trouble is detected at a relatively small offset. Therefore, the output can be limited in the event of a sudden increase in the assist torque, so that uncontrollable steering due to excessive assist can be avoided. 
     FIG. 5 shows a basic principle of the present invention, in which the stored difference between a main torque signal TM and a sub torque signal TS and the detected difference between the main torque signal TM and the sub torque signal TS are compared with a prescribed value 2 (&gt;prescribed value 1), and when the difference continues for a time shorter than a prescribed time 2 (&lt;prescribed time 1), the assist is inhibited. When the difference is smaller than the prescribed value 2 and larger than the prescribed value 1, and continues for the prescribed time 1 longer than the prescribed time 2, the assist is limited. Thus, a safer operation is realized by inhibiting or limiting the current command value. 
     In the current limiting method, the maximum current value of the current command value may be limited, or the maximum current value may be limited by multiplying the current command value by a specified gain. Further, when limiting the current command value, the limit value may be set slightly higher if the speed is low, or slightly lower if the speed is high. Further, in the event of abnormality, by changing over the normal mode characteristic map for assist to the torque sensor offset abnormal mode characteristic map, similar effects are obtained. Moreover, by using an intermediate characteristic map having an intermediate value when changing over from the normal mode characteristic map to the torque sensor offset abnormal mode characteristic map, a smooth change without sense of strangeness may be realized. 
     Referring now to the drawings, preferred embodiments of the present invention are described in detail below. 
     FIG. 6 is a block diagram of a first embodiment of the present invention corresponding to FIG. 2, which comprises a torque sensor offset abnormality detecting means  300 , a current command limit value generating means  310  and a subtractor  320 , and the output of the subtractor  320  is a steering assist command value I. The torque sensor  10  outputs a main torque signal TM and a sub torque signal TS having the characteristics as shown in FIG. 3, and the main torque signal TA is inputted to the phase compensator  31 , and is also inputted to the torque sensor offset abnormality detecting means  300 . The sub torque signal TS is inputted to the torque sensor offset abnormality detecting means  300 , and when an operation signal AB is outputted from the torque sensor offset abnormality detecting means  300 , the current command limit value generating means  310  feeds a limit current LT into the subtractor  320 . 
     FIG. 7 is a block diagram showing an example of internal structure of the torque sensor offset abnormality detecting means  300 , which comprises a difference calculator  301  for calculating the difference of the main torque signal TM and the sub torque signal TS, a changeover unit  302  for feeding the difference DS1 output from the difference calculator  301  into a memory (nonvolatile memory)  303  or a difference calculator  304 , a comparator  305  for comparing the difference DS2 outputted from the difference calculator  304  for comparing the stored value and present value and calculating the difference, with the prescribed value 1 and the prescribed value 2 (&gt;prescribed value 1) as thresholds, and a time measuring unit  306  for measuring the time of state signal NN outputted from the comparator  305  and issuing a specified operation signal AB before or after lapse of prescribed time 1 and prescribed time 2 (&gt;prescribed time 1). 
     In this configuration, the operation is explained by referring to the flowchart in FIG.  8 . 
     First, before shipping the products, that is, in the initial state, the difference between the main torque signal TM and the sub torque signal TS of the torque sensor  10  is calculated in the difference calculator  301 , and is stored in the nonvolatile memory  303  as offset data. That is, the changeover unit  302  is changed over to the memory  303  side, and the difference DS1 between the main torque signal TM and the sub torque signal TS is calculated in the difference calculator  301 , and the difference DS1 is stored in the memory  303  by way of the changeover unit  302  (stored value=MDS). Later, the changeover unit  302  is changed over to the difference calculator  304  side. 
     During the steering operation, the difference calculator  301  reads the present main torque signal TM (Step S 1 ), and the present sub torque signal TS is read in successively (Step S 2 ), and the difference DS1 between the main torque signal TM and the sub torque signal TS is calculated (Step S 3 ). The difference calculator  304  receives the difference DS1 through the changeover unit  302 , and reads out the stored value MDS from the memory  303  (Step S 4 ), and calculates the difference DS2 between the difference DS1 and the stored value MDS. Consequently, the comparator  305  judges if the difference DS2 from the difference calculator  304  is more than the prescribed value 1 (threshold) or not (Step S 5 ). That is, the stored value MDS is the offset component, and it is judged if abnormality has occurred or not due to increase of the difference DS1 becoming larger than the prescribed value 1. It is normal when the difference DS2 is smaller than the prescribed value 1, and counting of measuring time is cleared (Step S 6 ), and the process returns to the Step S 1 . 
     When the difference DS2 is more than the prescribed value 1, it is further judged if more than the prescribed value 2 or not (Step S 10 ), and when the difference DS2 is smaller than the prescribed value 2, it is judged if it continues for a prescribed time 1 or not (Step S 11 ). If the difference does not continue for the prescribed time 1, counting of measured time is increased, and the process returns to the Step S 1  (Step S 12 ), and when continued for the prescribed time 1, an operation signal AB is outputted, and a limit current LT is outputted from the current command limit value generating means  310 , and the assist is limited and the operation is terminated (Step S 13 ). 
     Further, at the Step S 10 , when the difference DS2 is more than the prescribed value 2, it is judged if the difference DS2 continues for a prescribed time 2 or not (Step S 14 ), if not continuing for the prescribed time 2, counting of measured time is increased, and the process returns to the Step S 1  (Step S 15 ), and when continued for the prescribed time 2, an operation signal AB is outputted, and a limit current LT is outputted from the current command limit value generating means  310 , and the assist is stopped and the operation is terminated (Step S 16 ). 
     FIG. 9 shows a mode of assist limiting, in which a characteristic A is a normal mode torque-current characteristic, and when an operation signal AB is outputted from the torque sensor offset abnormality detecting means  300 , the current command limit value generating means  310  outputs a limit current LT for limiting the motor current so that the steering assist command value I of the output of the subtractor  320  may be a characteristic B in FIG.  9 . Meanwhile, a vehicle speed signal V from the vehicle speed sensor  12  is inputted to the steering assist command value calculator  32 , and the current limit value may be increased as the vehicle speed V becomes lower as shown in FIG.  10 . 
     FIG. 11 shows a second embodiment of the present invention corresponding to FIG. 6, and in this embodiment, a normal mode characteristic map  33 A and a torque sensor offset abnormal mode characteristic map  33 B are provided as characteristic map, and a switch  330  is provided for changing over by an operation signal AB from the torque sensor offset abnormality detecting means  300 . The outputs of the normal mode characteristic map  33 A and torque sensor offset abnormal mode characteristic map  33 B are inputted to the steering assist command value calculator  32  by way of contacts “a” and “b” of the switch  330 . The characteristic of the normal mode characteristic map  33 A is a characteristic A indicated by broken line in FIG. 12, and the characteristic of the torque sensor offset abnormal mode characteristic map  33 B is a characteristic B indicated by solid line in FIG. 12, and when normal, the switch  330  is connected to the contact “a”, and the assist operation according to the characteristic A of the normal mode characteristic map  33 A is carried out. When an operation signal AB is outputted from the torque sensor offset abnormality detecting means  300 , the contact of the switch  330  is changed over from the contact “a” to “b”, and the assist operation according to the characteristic B of the torque sensor offset abnormal mode characteristic map  33 B is carried out, and the motor current is limited. 
     In the second embodiment, by changing over the normal mode characteristic map  33 A and the torque sensor offset abnormal mode characteristic map  33 B, the steering assist command value I is calculated, and since the difference is significant when changed over from the characteristic A to the characteristic B, the driver may feel a sense of strangeness in the steering operation. To avoid such problem, an intermediate characteristic map having an intermediate characteristic C as shown in FIG. 13 may be provided. That is, by changing over in gradual steps from a normal mode characteristic A to an intermediate characteristic C and a torque sensor offset abnormal mode characteristic B, sense of strangeness in operation due to change of characteristics may be prevented. 
     When an intermediate characteristic map  33 C (contact “c” of the switch  33 ) is provided, a configuration of a third embodiment is as shown in FIG. 14, and its operation is explained by referring to a flowchart in FIG.  15 . The operation is same as in the second embodiment shown in FIG. 11 except that the torque sensor offset abnormality detecting means  300  outputs the operation signal AB in three stages. 
     First, before shipping products, the difference between the main torque signal TM and the sub torque signal TS of the torque sensor  10  is calculated and stored in the memory  303 , same as in the first and second embodiments. The contact of the switch  330  is connected to “a”, and the data of the normal mode characteristic map  33 A is put in the steering assist command value calculator  32 . During the steering operation, the difference calculator  301  reads the present main torque signal TM (Step S 20 ), and the present sub torque signal TS is read in successively (Step S 21 ), and the difference DS1 between the main torque signal TM and the sub torque signal TS is calculated (Step S 22 ). The difference calculator  304  receives the difference DS1 through the changeover unit  302 , and reads out the stored value MDS from the memory  303  (Step S 23 ), and calculates the difference DS2 between the difference DS1 and stored value MDS. Consequently, the comparator  305  judges if the difference DS2 from the difference calculator  304  is more than the prescribed value 1 (threshold) or not (Step S 24 ), and it is normal when the difference DS2 is smaller than the prescribed value 1, and counting of measuring time is cleared (Step S 25 ), and the process returns to the Step S 20 . 
     When the difference DS2 is more than the prescribed value 1, it is further judged if more than the prescribed value 2 or not (Step S 30 ), and when the difference DS2 is smaller than the prescribed value 2, it is judged if it continues for a prescribed time 1 or not (Step S 31 ). If the difference does not continue for the prescribed time 1, it is judged normal, and counting of measured time is increased, and the process returns to the Step S 20  (Step S 32 ), and when continued for the prescribed time 1, an operation signal AB is outputted from the time measuring unit  3067 , and the contact of the switch  330  is changed over from the contact “a” to “c”, and the data of the intermediate characteristic map  33 C is inputted to the steering assist command value calculator  32 , and the current command value is limited (Step S 33 ). 
     Further, judging if the current limiting continues for more than the prescribed value 2 or not (Step S 34 ), if not continued for a prescribed time 2, counting of measured time is increased, and the process returns to the Step S 20  (Step S 35 ), and when continued for the prescribed time 2, the contact of the switch  330  is changed from “c” to “b” by an operation signal AB, and the current command value is changed over to fail characteristic, and the operation is terminated (Step S 36 ). That is, the data of the torque sensor offset abnormal mode characteristic map  33 B is inputted to the steering assist command value calculator  32 . 
     On the other hand, when the difference DS2 is more than the prescribed value 2 at the Step S 30 , judging if continuing for the prescribed time 2 or not (Step S 40 ), and when not continuing for the prescribed time 2, counting of measured time is increased, and the process returns to the Step S 20  (Step S 41 ), and when continued for the prescribed time 2, an operation signal AB is outputted, and the assist is inhibited and the operation is terminated (Step S 42 ). 
     In the embodiment in FIG. 14, the normal mode characteristic map  33 A, the intermediate characteristic map  33 C and the torque sensor offset characteristic map  33 B individually have the assist inhibit region and the assist limit region. 
     In the embodiment, one intermediate characteristic map is used, but more intermediate characteristic maps may be used and changed over in multiple stages. The present invention can be applied not only in the column type and pinion type power steering device, but also in the rack assist type power steering device. 
     According to the present invention, intermediate troubles such as offset or drift are monitored out of signal troubles of torque sensor, and a trouble of torque sensor can be detected correctly, and if a trouble is detected, the current is limited, and the assist function is limited, so that a safe operation is realized. Moreover, the steering operation is free from any feel of strange sense because the current limiting is changed over depending on the vehicle speed or changed over in gradual steps.