Abstract:
The present invention discloses a self-calibration method for an electric power steering system, which can self-calibrate the sensors to a normalized state to prevent from signal distortion, whereby to maintain stable steering sense of the driver and promote robustness and performance of the EPS system. The self-calibration method includes a signal offset compensation tactic and a zero-point signal self-calibration tactic. The present invention determines whether to undertake self-calibration according to judgement tactics, including a sensor power supply judgement tactic, a sensor correctness judgement tactic, and a self-calibration triggering condition. The self-calibration method can increase the correctness of sensors, maintain the original steering-assisting function and promote robustness of the EPS system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a self-calibration method for an electric power steering system, particularly to an EPS self-calibration method normalizing signals of sensors. 
     2. Description of the Related Art 
     Before, a powerful and smooth-operation HPS (Hydraulic Power Steering) system was used as an auxiliary power source to lessen the driver&#39;s burden of operating the steering wheel. However, hydraulic fluid is likely to leak from hydraulic piping. Further, hydraulic fluid is likely to heat up and denature because of friction between hydraulic fluid and piping. Besides, hydraulic piping is usually very complicated. Thus, the EPS (Electric Power Steering) system has gradually replaced the conventional HPS system. 
     Compared with the HPS system using a hydraulic pump, an oil tank, hydraulic piping and a belt-pulley system that transmits engine power to drive the pump, the EPS system is implemented by electronic signals and thus has lower fabrication cost and maintenance cost. Further, the EPS system provides different amounts of power for different driving conditions. The EPS system does not output power unless the condition needs it. The EPS system makes the driver to steer the vehicle more easily and accelerates the response of the vehicle. Therefore, the EPS system will be a standard apparatus of vehicles soon or later. 
     The EPS system uses a torque sensor and a current sensor. The steering sense of the driver is greatly influenced by signals of sensors of the EPS system, such as the torque sensor and the current sensor. Therefore, the sensors of EPS systems should be calibrated to a normal state before delivery. Because of long-term use or displacement of installation positions, sensors will drift from their normal states and generates signals having slight errors. The errors are measured and compensated for to restore sensors to have the original setting or better setting. Environmental factors, such as temperature, or instable power supply, may cause erroneous auxiliary torque or discontinuous auxiliary force and affect the performance of the EPS system. In assembling an EPS system to a vehicle, sensor normalization takes time. In mass production, the total time and manpower spent in normalizing the sensors of all the vehicles is very considerable. Therefore, how to overcome sensor accuracy degradation caused by power supply variation and promote robustness of EPS systems is a problem the manufacturers desire to solve. 
     Accordingly, the present invention proposes a self-calibration method for an electric power steering system to overcome the abovementioned problems. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a self-calibration method for an electric power steering system, which can self-calibrate the sensors to a normalized state to prevent from signal distortion, whereby to maintain stable steering sense of the driver and promote robustness and performance of the EPS system. 
     Another objective of the present invention is to provide a self-calibration method for an electric power steering system, which uses a signal offset compensation tactic to increase accuracy of sensors, whereby to maintain performance of the EPS system and enhance robustness of the EPS system. 
     A further objective of the present invention is to provide a self-calibration method for an electric power steering system, which can overcome the problem that the conventional technology spends a great amount of time and labor to calibrate sensors of the EPS system. 
     To achieve the abovementioned objectives, the present invention proposes a self-calibration method for an electric power steering system, which comprises steps: detecting power supply values of at least one sensor module, and examining whether the power supply value meets a power supply condition: if the power supply value does not meet a power supply condition, interrupting operation of an electric power steering system; if the power supply value meets a power supply condition, examining whether the sensor module operates normally; if the sensor module operates abnormally, the electric power steering system switching auxiliary functions; if the sensor module operates normally, examining whether the sensor module meets a preset self-calibration triggering condition; if the sensor module does not meet a preset self-calibration triggering condition, and the EPS system uses the previous self-calibration value of the sensor module to undertake the function of the sensor module; if the sensor module meets a preset self-calibration triggering condition, setting a new self-calibration value, and the EPS system undertaking a normalization activity. 
     Below, the embodiments are described in detail to make easily understood the objectives, technical contents, characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing the architecture of an EPS system according to one embodiment of the present invention; 
         FIG. 2  is a flowchart of a self-calibration method for an EPS system according to one embodiment of the present invention; 
         FIG. 3  is a flowchart of a self-calibration method for a current sensor according to one embodiment of the present invention; 
         FIG. 4  is a flowchart of the process to determine whether a current sensor meets a preset self-calibration condition according to one embodiment of the present invention; 
         FIG. 5  shows proportional calibration of current waveforms before according to one embodiment of the present invention: 
         FIG. 6  shows proportional calibration of current waveforms after according to one embodiment of the present invention; 
         FIG. 7  is a flowchart of a self-calibration method for a torque sensor according to one embodiment of the present invention; 
         FIG. 8  shows self-calibration of the torque sensor according to one embodiment of the present invention; 
         FIG. 9  shows normal operation of a torque sensor according to one embodiment of the present invention; 
         FIG. 10  is a flowchart of a process to determine whether a torque sensor meets a preset self-calibration triggering condition according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, the EPS system applies to a vehicle. The variation or abnormality of the power supply of any sensor of the EPS system may cause distortion of sensor signals or false actions of the EPS system. In order to avoid the abovementioned problems, the present invention addresses improving safety and robustness of the EPS system. Refer to  FIG. 1  and  FIG. 2 .  FIG. 1  is a block diagram schematically showing the architecture of an EPS system according to one embodiment of the present invention.  FIG. 2  is a flow chart of a self-calibration method for an EPS system according to one embodiment of the present invention. The EPS system of the present invention comprises a driving module  10 , at least one sensor module  12 , a filter circuit  14 , a voltage conversion circuit  16  and a microprocessor  18 . The driving module  10  is coupled to a vehicle steering system and outputs torque to assist the vehicle driver in rotating the steering wheel. The driving module  10  includes a motor driver, a motor and a decelerator. The motor driver is coupled to the motor. The decelerator connects the motor with the steering mechanism of the vehicle. The motor driver operates to assist the vehicle driver in rotating the steering wheel according to the control signals of the microprocessor  18 . A current sensor  22  of the sensor module  12  is connected with the driving module  10  and used to detect the operation status of the driving module  10 . A torque sensor  24  of the sensor module  12  is connected with the steering mechanism and used to detect the torque by which the vehicle driver rotates the steering wheel. The filter circuit  14  is connected with the sensor module  12  and used to filter out noise in the power supply signals for the sensor module  12 . The filter circuit  14  may be a low-pass filter circuit. The voltage conversion circuit  16  is connected with the filter circuit  14  and the microprocessor  18 . Firstly, in Step S 10 , use a voltage sensor  20  to detect the power supply signal of the sensor module  12 . The voltage sensor  20  is connected with the sensor module  12  and the filter circuit  14 . The voltage conversion circuit  16  (such as an analog-to-digital converter, ADC) converts the power supply signal into a power supply value. The microprocessor  18  examines whether the power supply value meets a power supply condition in order to learn whether the power supply of the sensor module  12  is normal before self-calibration. If the power supply value does not meet a power supply condition, the process proceeds to Step S 11  to interrupt the operation of the EPS system. If the power supply value meets a power supply condition, the process proceeds to Step S 12 , and the microprocessor  18  examines whether the power supply value is equal to the preset power supply value. The case that the power supply value is equal to the preset power supply value is the optimized condition. If the power supply value is not equal to the preset power supply value, the process proceeds to Step S 13  to compensate for the voltage offset of the sensor module  12 . If the power supply value is equal to the preset power supply value, or after Step S 13  has been executed, the process proceeds to Step S 14 , and the microprocessor  18  examines whether the sensor module  12  operates normally, i.e. examines whether the sensor module  12  operates within a normal range. If the sensor module  12  operates abnormally, the process proceeds to Step S 16 , and the EPS system switches auxiliary functions, which will be described in detail thereinafter. If the sensor module  12  operates normally, the process proceeds to Step  18 , and the microprocessor  18  examines whether the sensor module  18 , the wheel speed, the vehicle speed, and the steering angle meet a preset self-calibration triggering condition. If the case does not meet the preset self-calibration triggering condition, the process proceeds to Step S 20 , and the microprocessor  18  adopts the previous self-calibration value of the sensor module  12  to compensate the value of the sensor module  12  to undertake detection. If the case meets the preset self-calibration triggering condition, the process proceeds to Step S 22 , and the microprocessor  18  undertake a normalization activity to update the calibration value of the sensor module  12 . 
     The sensor modules further comprises at least one current sensor  22  or at least one torque sensor  24 . In Step S 10 , the sensor module  12 , no matter whether it is the current sensor  22  or the torque sensor  24 , persistently uses the voltage sensor  20  to measure the power supply of the sensor module  12  and generate a plurality of power supply signals. The voltage conversion circuit  16  converts the plurality of power supply signals into a plurality of power supply values and records the power supply values in a memory. The power supply values are not only used to examine whether the power supply of the sensor module  12  is normal but also used to compensate for voltage offset in the succeeding steps. The present invention has different self-calibration methods to respectively calibrate and normalize the current sensor  22  and the torque sensor  24 . After Step S 12 , the current sensor  22  and the torque sensor  24  respectively have different voltage offset compensation tactics, different tactics to determine whether the sensor module operates normally, different tactics to determine whether a case meets the self-calibration triggering condition, and different ways to switch auxiliary functions. Below are described in detail the self-calibration methods for calibrating and normalizing the current sensor  22  and the torque sensor  24 . 
     Refer to  FIG. 3  a flowchart of a self-calibration method for a current sensor according to one embodiment of the present invention. Suppose that the sensor module  12  includes at least one current sensor  22 . In Step S 24 , the voltage sensor  20  detects power supply signals of the current sensor  22 ; the voltage conversion circuit  16  converts the power supply signals into power supply values; and the microprocessor  18  examines whether the power supply values meet a power supply condition. For example, the microprocessor  18  examines whether the power supply values are within 4.5-5.5V. The power supply is not regarded as at a normal state unless the power supply values are within 4.5-5.5V. If the power supply is not at a normal state, the process proceeds to Step S 26  to interrupt the operation of the ESP system. The power supply for the current sensor  22  varies in different conditions. Thus, a voltage offset compensation tactic is executed to make the current sensor  22  have the best performance corresponding to the current power supply value. If the power supply is at a normal state, the process proceeds to Step S 28 , the microprocessor  18  examines whether the power supply value of the current sensor  22  meets a preset power supply value, such as 5V. The case that the power supply value is equal to the preset power supply value is the optimized condition. If the power supply value is not at 5V, the process proceeds to Step S 30 . The voltage sensor  20  persistently detects the power supply of the current sensor  22  and generates a plurality of power supply signals. The voltage conversion circuit  16  converts the power supply signals into a plurality of power supply values and records them in a memory. Thus are defined the power supply values of the current sensor  22 . Then, take the half values of the power supply values. Next, the process proceeds to Step S 32  to convert the half values into mesial current values. Next, the process proceeds to step S 34  to set the mesial current values as the compensation for current offset in normalization undertaken by the EPS system. Thereby is finished the normalization activity to compensate for the current offset of the current sensor  22  in from Step S 30  to Step S 34 . If the power supply value is at 5V, or when Step S 34  is completed, the process proceeds to Step  36  to examine whether the current sensor  22  operates normally according to a normal operation range. In one embodiment, two current sensors  22  are used to detect the U-phase current and V-phase current of the motor. The microprocessor  18  examines whether the two current sensors  22  simultaneously meet the following two normal operation ranges:
 
 V   phaseU : 0.5 V -4.5 V  
 
 V   phaseV : 0.5 V -4.5 V  
 
If the current sensor  22  operates abnormally, the process proceeds to Step S 38 , and the microprocessor  18  undertakes an EPS auxiliary function to switch the EPS system from a close-loop mode to an open-loop mode. Although the EPS system can keep on operating via the switching of the auxiliary functions in the case that the current sensor  22  operates abnormally, the performance of the EPS system is degraded more or less. If the current sensor  22  operates normally, the process proceeds to Step S 40 . As the operation current may vary with the usage condition, a judgement tactic is needed to determine whether the current sensor  22  meets a self-calibration triggering condition. Refer to  FIG. 4  a flowchart of the process to determine whether the current sensor  22  meets the condition of triggering self-calibration. In Step S 401 , examine whether the current in the DC side of the current sensor  22  is zero. In a DC-side current sensor, a resistor of a known specification is arranged in the side of the driving module  10 ; the voltage sensor  20  detects the voltage drop across the resistor; the current output by the battery to the driving module  10  is thus worked out according to the Ohm&#39;s law. When the driving module  10  does not operate, the DC side current approaches zero. If the DC side current is zero, the process proceeds to Step S 402  to record a plurality of current values of the current sensor  22 . For example, record a plurality of current values measured in a period of tome. Next, the process proceeds to Step S 403  to calculate the average of the current values and obtain a calibration value of the current waveform offset, and the calibration value of the current waveform offset is used as a new self-calibration value of the current sensor  22 . Thereby can be determined whether the current sensor  22  meets a self-calibration triggering condition. Next, the process proceeds to Step S 42 , and the microprocessor  18  of the EPS system undertakes normalization calibration. If the DC side current is non-zero, the process proceeds to Step S 404  to determine whether the steering wheel is rotated persistently. If the steering wheel is not rotated persistently, the process proceeds to Step S 44 , and the microprocessor  18  uses the previous calibration value to compensate the value of the current sensor  22 . If the steering wheel is rotated persistently, the process proceeds to Step S 405  to examine whether the torque applied to the steering wheel is a fixed value. If the torque applied to the steering wheel is not a fixed value, the process proceeds to Step S 44 . If the torque applied to the steering wheel is a fixed value, the process proceeds to Step S 406  to examine whether the EPS system operates in a close-loop mode. If the EPS system does not operate in a close-loop mode, the process proceeds to Step S 44 . Refer to  FIG. 5  for the proportional calibration of the current waveform before according to one embodiment of the present invention. The current sensor  22  has its own measurement errors. Further, the input current may have an offset. Thus, waveform inconsistency may occur in Phase U and Phase V and thus affect the performance of the EPS system. If the EPS system operates in a close-loop mode, the process proceeds to Step S 407  to record the periods of the current waveforms of Phase U and Phase V and find out the limit values of the current waveforms of Phase U and Phase V, wherein “●” in  FIG. 6  denotes the limit values (the peaks and troughs) of the current waveform of Phase U, and “◯” in  FIG. 6  denotes the limit values (the peaks and troughs) of the current waveform of Phase V. Next, the process proceeds to Step S 408  to calculate the limit values of the current waveforms of Phase U and Phase V and obtain a waveform proportional calibration value. The waveform proportional calibration value is used as a new self-calibration value of the current sensor  22 . Refer to  FIG. 6  for the proportional calibration of the current waveform after according to one embodiment of the present invention. Next, the process proceeds to Step S 42 , and the microprocessor performs normalization calibration and modifies the current waveforms of Phase U and Phase V to have an identical proportion.
 
     Refer to  FIG. 7  a flowchart of a self-calibration method for a torque sensor according to one embodiment of the present invention. Suppose that the sensor module  12  includes at least one torque sensor  24 . Refer to  FIG. 8  for self-calibration of the torque sensor according to one embodiment of the present invention, wherein the case of non-zero voltage corresponding to zero torque is calibrated to be the case of zero baseline corresponding to zero torque. In Step S 46 , the voltage sensor  20  detects power supply signals of the torque sensor  24 ; the voltage conversion circuit  16  converts the power supply signals into power supply values; and the microprocessor  18  examines whether the power supply values meet a power supply condition. For example, the microprocessor  18  examines whether the power supply values are within 4.5-5.5V. The power supply is not regarded as at a normal state unless the power supply values are within 4.5-5.5V. If the power supply is not at a normal state, the process proceeds to Step S 48  to interrupt the operation of the ESP system. The power supply for the torque sensor  24  may vary with the related electrical elements and degrade by long-term use, which results in slight signal errors. Thus, a voltage offset compensation tactic is executed to make the torque sensor  24  have the best performance corresponding to the current power supply value. If the power supply is at a normal state, the process proceeds to Step S 50 , and the microprocessor  18  examines whether the power supply value of the torque sensor  24  meets a preset power supply value, such as 5V. The case that the power supply value is equal to the preset power supply value is the optimized condition. If the power supply value is not at 5V, the process proceeds to Step S 52 . The voltage sensor  20  persistently detects the power supply of the torque sensor  24  and generates a plurality of power supply signals. The voltage conversion circuit  16  converts the power supply signals into a plurality of power supply values and records them in a memory. Thus are defined the power supply values of the torque sensor  24 . Then, take the half values of the power supply values. Next, the process proceeds to Step S 54  to work out a voltage offset value according to an initial power supply value, the mesial value of the current power supply, and a zero-torque voltage value at the initial power supply value, wherein the microprocessor  18  uses a voltage offset compensation algorithm to work out a new zero-torque voltage value. The voltage offset compensation equation is expressed by 
               new   ⁢           ⁢   zero   ⁢     -     ⁢   torque   ⁢           ⁢   voltage   ⁢           ⁢   value     =       1   2     ⁢     V       new   ⁢   _   ⁢   sensor     ⁢     _   ⁢   power         ⁢     {     1   +         V       initial   ⁢   _   ⁢   zeor     ⁢     -     ⁢   torque       -       1   2     ⁢     V       initial   ⁢   _   ⁢   sensor     ⁢     _   ⁢   power                 1   2     ⁢     V       initial   ⁢   _   ⁢   sensor     ⁢     _   ⁢   power               }             
wherein V new     —     sensor     —     power  denotes the current power supply value of the torque sensor  24 , V initial     —     zero-torque  the voltage value corresponding to zero torque value at the initial power supply value, V initial     —     sensor     —     power  the initial power supply value of the torque sensor  24 , and V initial     —     zero-torque  the preset value. Next, the process proceeds to Step S 56 , and the new zero-torque voltage value is set to be the voltage offset compensation value for normalizing the EPS system. Thus is completed the normalization calibration for the voltage offset compensation of the torque sensor  24  in from Step S 52  to Step S 56 .
 
     If the power supply value is at  5 V, or when Step S 56  is completed, the process proceeds to Step  58  to examine whether the torque sensor  24  operates normally according to a normal operation range. Refer to  FIG. 9  for normal operation of the torque sensor according to one embodiment of the present invention. In one embodiment, the torque sensor  24  has two symmetric torque measurement modules detecting the steering torque of the steering wheel. The microprocessor  18  examines whether the torque sensor  24  simultaneously meets the three following normal operation ranges:
 
 T   main :0.1-0.9 V   sensor power  
 
 T   sub : 0.1-0.9 V   sensor power  
 
 T   main   +T   sub   =V   sensor power  
 
wherein T main  denotes one torque sensor  24 , T sub  another torque sensor  24 ′, and 0.1-0.9V sensor power  represents the 10-90% input voltage of the power supply.
 
     If the torque sensor  24  operates abnormally, the process proceeds to Step S 60 , and the microprocessor  18  directly shuts off the EPS system, and power is no more supplied. If the torque sensor  24  operates normally, the process proceeds to Step S 62 . As the power supply may vary with the usage condition, a judgement tactic is needed to determine whether the torque sensor  24  meets a self-calibration triggering condition. Refer to  FIG. 10  a flowchart of the process to determine whether the torque sensor  24  meets the condition of triggering self-calibration. In Step S 621 , the microprocessor  18  examines whether the vehicle speed doesn&#39;t equal to zero. If the vehicle speed is zero, the process proceeds to Step S 64 , and the microprocessor  18  uses the previous calibration value to control the operation of the torque sensor  24 . If the vehicle speed is not zero, the process proceeds to Step S 622  to examine whether the wheels at two sides of the vehicle have an identical speed. If the wheels at two sides of the vehicle respectively have different speeds, the process proceeds to Step S 64 . If the wheels at two sides of the vehicle have an identical speed, the process proceeds to Step S 623  to examine whether the rotation angle of the steering wheel is a fixed value. If the rotation angle of the steering wheel is not a fixed value, the process proceeds to Step S 64 . If the rotation angle of the steering wheel is a fixed value, the process proceeds to Step S 624  to examine whether the steering torque of the torque sensor  24  is a fixed value. If the steering torque is not a fixed value, the process proceeds to Step S 64 . If the steering torque is a fixed value, the process proceeds to Step S 625  to obtain a new steering-torque voltage and use the new steering-torque voltage as a new self-calibration value of the torque sensor  24 . Next, the process proceeds to Step S 66 , and the microprocessor  18  undertakes normalization calibration to compensate for steering torque offset. 
     In conclusion, the present invention can self-calibrate the distorted signals of a current sensor or a torque sensor to a normalized state and thus promote the robustness and performance of an EPS system. The present invention further uses a signal offset compensation tactic to overcome signal distortion caused by variation or abnormality of power supply. Thereby is promoted the accuracy of the sensors and enhanced the performance and robustness of the EPS system. Therefore, the present invention can save the driver a lot of time and money originally spent in sending his vehicle to a maintenance factory for calibrating the sensors and normalizing the signals thereof. 
     The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit or characteristic of the present invention is to be also included within the scope of the present invention.