Patent Publication Number: US-2013233638-A1

Title: Electric power steering apparatus for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2012-0025090, filed on Mar. 12, 2012, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric power steering apparatus for a vehicle. More particularly, the present invention relates to an electric power steering apparatus for a vehicle which, by using two auxiliary steering units for assisting steering power when a steering wheel of the driver is manipulated, distributes an auxiliary steering power according to increases in a speed and a weight of the vehicle, reduces a load applied to any one of the auxiliary steering units, reduces operation noise, increases durability, lowers costs and allows for a sufficient auxiliary steering power at the same time by using a low output motor even when being applied to a truck or a bus requiring a relatively high steering power as compared with a car to improve driver convenience and improve the fuel ratio while increasing energy efficiency, and safely maintains a steering operation even if any one of the auxiliary steering units is operating normally while the vehicle is in motion. 
     2. Description of the Prior Art 
     In general, a power steering system used in a steering apparatus of a vehicle refers to an apparatus which assists a steering wheel manipulating power of the driver by using a power increasing unit when the driver manipulates the steering wheel of the vehicle so that the direction of the vehicle can be easily changed by a smaller force. 
     Such power steering systems are largely classified into electric power steering (EPS) systems and hydraulic power steering (HPS) systems. 
     Among them, the electric power steering systems are steering apparatuses for assisting a manipulating power of a steering wheel with a force of a motor, and are classified into a column type and a rack housing type according to locations of the auxiliary power mechanisms. 
       FIG. 1  is a diagram of a column type electric power steering apparatus according to the related art. 
     As illustrated in  FIG. 1 , the column type electric power steering apparatus includes a power steering mechanism  120  for providing a steering system from steering wheel  101  to two wheels  108  with auxiliary steering power. 
     One side of the steering shaft  102  is connected to the steering wheel  101  to be rotated together with the steering wheel  101  and an opposite side of the steering shaft  102  is connected to a pinion shaft  104  by means of a pair of universal joints  103 . 
     Further, the pinion shaft  104  is connected to a rack bar  109  through a rack-pinion mechanism  105  and opposite ends of the rack bar  109  are connected to the wheels  108  of the vehicle through a tie rod  106  and a knuckle arm  107 . In the rack-pinion mechanism  105 , a pinion gear  111  formed in the pinion shaft  104  and a rack gear  112  formed at one side of an outer peripheral surface of the rack bar  109  are engaged with each other so that if the driver manipulates the steering wheel  101 , a torque is generated in the steering shaft  102  and the wheels  108  are steered by the torque through the rack-pinion mechanism  105  and the tie rod  106 . 
     The power steering mechanism  120  includes a torque sensor  125  for detecting a torque applied to the steering wheel  101  by the driver and outputting an electrical signal proportional to the detected torque, an electronic control unit (ECU)  123  for generating a control signal based on an electrical signal transmitted from the torque sensor, a motor  130  for generating auxiliary power based on the signal transferred from the electronic control unit  123 , and a reducer  140  for transferring the auxiliary power generated by the motor to the steering shaft  102 . 
       FIG. 2  is a schematic diagram of a rack housing type electric power steering apparatus according to the related art. 
     As illustrated in  FIG. 2 , the rack housing type electric power steering apparatus includes a power steering mechanism  150  for providing a steering system extending from a steering wheel  101  to two wheels  108  with auxiliary steering power. 
     In the steering system, one side of a steering wheel  101  is connected to the steering shaft  102  to be rotated together with the steering wheel  101  and an opposite side of the steering wheel  101  is connected to a pinion shaft  104  by means of a pair of universal joints  103 . 
     Further, the pinion shaft  104  is connected to a rack bar  109  through a rack-pinion mechanism  105  and opposite ends of the rack bar  109  are connected to the wheels  108  of the vehicle through a tie rod  106  and a knuckle arm  107 . In the rack-pinion mechanism  105 , a pinion gear  111  formed in the pinion shaft  104  and a rack gear  112  formed at one side of an outer peripheral surface of the rack bar  109  are engaged with each other so that if the driver manipulates the steering wheel  101 , a torque is generated in the steering system  100  and the wheels  108  are steered by the torque through the rack-pinion mechanism  105  and the tie rod  106 . 
     The power steering mechanism  150  includes a torque sensor  121  for detecting a torque applied to the steering wheel  101  by the driver and outputting an electrical signal proportional to the detected torque, an electronic control unit (ECU)  123  for generating a control signal based on an electrical signal transmitted from the torque sensor  121 , a motor  151  for generating auxiliary power based on the signal transferred from the electronic control unit  123 , and a belt transmission unit  153  for transmitting an auxiliary power generated by the motor  151  by means of a belt to the rack bar  109 . 
     Thus, the electronic power steering apparatus is configured such that a torque generated by rotation of the steering wheel  101  is transmitted to the rack bar  109  via the rack-pinion mechanism  105  and the auxiliary power generated by the motor  151  according to the generated torque is transmitted to the rack bar  109  via a ball screw  155  by the belt type transmission unit  153 . Thus, the torque generated in the steering system and the auxiliary power generated by the motor  151  are combined to axially move the rack bar  109 . 
     In the electric power steering apparatus according to the related art, a load applied to one power steering mechanism is increased regardless of speed and weight of the vehicle, causing severe operation noise and deteriorating durability. 
     Further, a truck or a bus requiring a relatively high steering power as compared with a car should have a high output motor, increasing manufacturing costs, lowering energy efficiency, and decreasing fuel ratio. 
     Further, if the power steering mechanism is not operating normally while the vehicle is in motion, the vehicle cannot be properly steered, and may cause a fatal accident. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an electric power steering apparatus for a vehicle which, by using two auxiliary steering units for assisting steering power when a steering wheel of the driver is manipulated, distributes an auxiliary steering power according to increases in speed and weight of the vehicle, reduces a load applied to any one of the auxiliary steering units, reduces operation noise, and increases durability 
     Another objective is to provide an electric power steering apparatus which lowers costs and allows a sufficient auxiliary steering power at the same time by using a low output motor even when being applied to a truck or a bus requiring a relatively high steering power as compared with a car to improve driver convenience and improves fuel ratio while increasing energy efficiency,. 
     Another objective is to provide an electric power steering apparatus which safely maintains a steering operation even if any one of the auxiliary steering units is not normally operated during a travel of the vehicle. 
     In order to accomplish this object, there is provided an electric power steering apparatus for a vehicle, including: a first auxiliary steering unit for assisting with steering power while rotating a steering shaft in a forward or reverse direction with a driving power of a first motor; a second auxiliary steering unit for assisting with steering power while sliding a rack bar in one or an opposite direction with a driving power of a second motor; a torque sensor for detecting a rotary torque of the steering shaft; a speed sensor for detecting a speed of the vehicle; and an electronic control unit for transmitting a target current value control signal to the first motor and the second motor respectively in response to input signals input from the torque sensor and the speed sensor, respectively. 
     As described above, according to the present invention, an electric power steering apparatus for a vehicle distributes an auxiliary steering power according to increases in speed and weight of the vehicle by using two auxiliary steering units for assisting steering power when a steering wheel of the driver is manipulated, reduces a load applied to any one of the auxiliary steering units, reduces operation noise, and increases durability 
     The electric power steering apparatus lowers costs and allows for a sufficient auxiliary steering power at the same time by using a low output motor even when being applied to a truck or a bus requiring a relatively high steering power as compared with a car to improve driver convenience and improves fuel ratio while increasing energy efficiency,. 
     The electric power steering apparatus safely maintains a steering operation even if any one of the auxiliary steering units is not normally operated during a travel of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a column type electric power steering apparatus according to the related art; 
         FIG. 2  is a schematic diagram of a rack housing type electric power steering apparatus according to the related art; 
         FIG. 3  is a schematic diagram illustrating an electric power steering apparatus for a vehicle according to an embodiment of the present invention; 
         FIG. 4  is a perspective view illustrating the electric power steering apparatus for a vehicle according to the embodiment of the present invention; and 
         FIGS. 5 and 6  are sectional views illustrating a portion of the electric power steering apparatus for a vehicle according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same elements will be designated by the same reference numerals although they are shown in different drawings. Further, in the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. 
     In addition, terms, such as first, second, A, B, (a), (b) or the like may be used herein when describing components of the present invention. Each of these terminologies is not used to define an essence, order or sequence of a corresponding component but used merely to distinguish the corresponding component from other component(s). It should be noted that if it is described in the specification that one component is “connected”, “coupled” or “joined” to another component, a third component may be “connected”, “coupled”, and “joined” between the first and second components, although the first component may be directly connected, coupled or joined to the second component. 
     Hereinafter, an electric power steering apparatus for a vehicle according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 3  is a schematic diagram illustrating an electric power steering apparatus for a vehicle according to an embodiment of the present invention.  FIG. 4  is a perspective view illustrating the electric power steering apparatus for a vehicle according to the embodiment of the present invention.  FIGS. 5 and 6  are sectional views illustrating a portion of the electric power steering apparatus for a vehicle according to the embodiment of the present invention. 
     As illustrated in the drawings, the electric power steering apparatus for a vehicle according to the embodiment of the present invention includes: a first auxiliary steering unit  304  for assisting with the steering power while rotating a steering shaft  102  in a forward or reverse direction with a driving power of a first motor  303 ; a second auxiliary steering unit  312  for assisting steering power while sliding a rack bar  339  in one or an opposite direction with a driving power of a second motor  311 ; a torque sensor  301  for detecting a rotary torque of the steering shaft  102 ; a speed sensor  315  for detecting a speed of the vehicle; and an electronic control unit  309  for transmitting a target current value control signal to the first motor  303  and the second motor  311 , respectively, in response to input signals input from the torque sensor  301  and the speed sensor  315 , respectively. 
     That is, if the torque sensor  301  which detects a rotation direction of a steering shaft  102  rotated in conjunction with the steering wheel  101  when the driver manipulates the steering wheel  101  and a torque value thereof detects whether the driver intends to make a left turn or a right turn, and the torque by which the steering wheel  101  is manipulated, it transmits the data to the electronic control unit  309 . 
     Further, the speed sensor  315  provided in the vehicle detects whether the vehicle is stopped or is moving, and if moving, measures the travel speed and transmits the data to the electronic control unit  309 . 
     The electronic control unit  309  generates a target current value control signal and transmits the target current value control signal to the first auxiliary steering unit  304  and the second auxiliary steering unit  312  to assist with a manipulating power in a direction in which the driver manipulates the steering wheel  101  or an opposite direction in response to the input signals. 
     The first auxiliary steering unit  304  and the second auxiliary steering unit  312  rotate the steering shaft  102  and slide the rack bar  339 , respectively, in a direction the same as or opposite to the rotation direction of the steering wheel  101  of the driver in response to the control signal input from the electronic control unit  309  to assist with a steering power of the driver. 
     Here, the first auxiliary steering unit  304  includes: a first reducer  305  including a worm wheel  323  connected to the steering shaft  102  to be rotated, and a worm shaft  321  engaged with the worm wheel  323  to rotate the worm wheel  323  connected to the steering shaft  102 ; and a first motor  303  for driving the worm shaft  321  in a forward or reverse direction in response to a control signal input from the electronic control unit  309 . 
     The second auxiliary steering unit  312  includes: a ball nut  337  coupled to an outer periphery of the rack bar  339  via balls  341  to slide the rack bar  339  in one or an opposite direction while being rotated; a second reducer  313  including a nut pulley  333  coupled to an outer side of the ball nut  337  and to which a belt  335  is coupled, and a motor pulley  331  connected to the nut pulley  333  and the belt  335 ; and a second motor  311  coupled to the motor pulley  331  to drive the motor pulley  331  in a forward or reverse direction in response to a control signal input from the electronic control unit  309 . 
     The first auxiliary steering unit  304  is an electric auxiliary steering unit using a driving power of the first motor  303 , and the first reducer  305  adjusted by rotation of the steering shaft  102  is coupled to the steering shaft  102  so that the steering shaft  102  is rotated by the driving power of the first motor  303 . 
     The first reducer  305  rotates the steering shaft  102  to assist with a steering power of the driver with the driving power of the first motor  303 , by working in conjunction with the worm shaft  321  and the worm wheel  323  of the first reducer  305  to perform speed reduction according to gear ratio, and is done largely by the worm wheel  323  and the worm shaft  321 . The steering shaft  102  is connected to the center of the worm wheel  323  to be rotated in conjunction with the worm wheel  323  and the worm shaft  321  engaged with the worm wheel  323  is rotated by the driving power of the motor. 
     Then, the first motor  303  rotates the worm shaft  321  in a forward or reverse direction in response to a control signal input from the electronic control unit  309 , that is, a target current value to rotate the worm wheel  323 , thereby rotating the steering shaft  102  coupled to a rack-pinion mechanism  307 . 
     The second auxiliary steering unit  312  is an electric auxiliary steering unit using a driving power of the second motor  311 , and drives the motor pulley  331  and the nut pulley  333  connected to each other by the belt  335  with a driving power of the second motor  311  to rotate the ball nut  337 , thereby sliding the rack bar  339  in one or an opposite direction to assist with a steering power of the driver. 
     The second auxiliary steering unit  312  performs speed reduction according to a diameter ratio of the motor pulley  331  and the nut pulley  333  constituting the second reducer  313 , and if the motor pulley  331  coupled to the motor shaft  311 a, it is rotated in a forward or reverse direction of the second motor  311  in response to a control signal input from the electronic control unit  309 , the nut pulley  333  is rotated by the belt  335  to rotate the ball nut  337  coupled to the rack bar  339  via the balls  341 , thereby sliding the rack bar  339  to one or an opposite side to assist with a manipulating power of the steering wheel  101  of the driver through the rack-pinion mechanism  307 . 
     A screw groove  339   a  is formed on an outer peripheral surface of the rack bar  339  accommodated in the rack housing  306  and a screw groove  337   a  corresponding to the screw groove of the rack bar  339  is formed on an inner peripheral surface of the ball nut  337 , and a plurality of balls  341  are coupled to the screw grooves  339   a  and  337   a.    
     Thus, when the ball nut  337  is rotated, the rack bar  339  is slid in one or an opposite direction to operate a tie rod  106 , steering the wheels. 
     Here, the electronic control unit  309  transmits a target current value control signal to the first motor  303  and the second motor  311 , respectively in response to an input signal input from the torque sensor  301 , and when an input signal value input from the speed sensor  315  is larger than a set vehicle speed, transmission of the target current value control signal to the second motor  311  is stopped. 
     Thus, when the vehicle travels at a speed higher than a set vehicle speed, an operation of the second auxiliary steering unit  312  is stopped and the vehicle can be steered only with the auxiliary steering power of the first auxiliary steering unit  304 , making it possible to minimize power consumption. 
     Meanwhile, the power steering apparatus of the vehicle may include a weight mode switch  317  for selecting a high weight mode and a low weight mode according to a weight of the vehicle and transmitting the selected mode to the electronic control unit  309 . 
     In this case, the electronic control unit  309  transmits a target current value control signal to the first motor  303  and the second motor  311 , respectively, in response to input signals input from the torque sensor  301  and the speed sensor  315 . When the input signal input from the weight mode switch  317  corresponds to a high weight mode, a target current value control signal transmitted to the second motor  311  is increased, whereas when the input signal value input from the speed sensor  315  is higher than a set vehicle speed, transmission of a target current value control signal to the second motor  311  is stopped so that when the vehicle travels at a speed higher than a set vehicle speed, the vehicle can be steered only with an auxiliary steering power of the first auxiliary steering unit  304 , making it possible to minimize power consumption. 
     When an input signal input from the weight mode switch  317  corresponds to a low weight mode, the electronic control unit  309  stops transmission of a target current value control signal to the second motor  311  and increases a target current value control signal to the first motor  303 . When an input signal value input from the speed sensor  315  is higher than a set vehicle speed, transmission of a target current value control signal to the first motor  303  is decreased and a target current value control signal is stopped transmitting to the second motor  311  so that the vehicle can be steered only with an auxiliary steering power of the first auxiliary steering unit, thereby minimizing power consumption. 
     That is, operations of the first motor  303  and the second motor  311  are changed according to whether the weight of the vehicle is high or low and the speed of the vehicle is a high speed or a low speed, in which case the vehicle starts in the high weight mode, the first motor  303  and the second motor  311  are simultaneously driven and the driving power of the second motor  311  is increased, and if the increased vehicle speed exceeds a set vehicle speed value, an operation of the second motor  311  is stopped and only the first motor  303  is operated. 
     On the contrary, in the low weight mode, if the vehicle starts, only a driving power of the first motor  303  is increased, and if the increased speed exceeds the set vehicle speed, a target current value control signal to the first motor  303  is reduced to decrease a driving power of the first motor  303 . 
     Thus, when much freight is loaded on the vehicle or the number of passengers increases, the driver selects a high weight mode through a weight mode switch  317  to obtain more auxiliary steering power, and only the first motor  303  is operated to reduce unnecessary power consumption if the vehicle speed increases. 
     That is, when less auxiliary steering power is necessary as the vehicle speed is higher than a set value, only the first motor  303  is operated regardless of whether the vehicle is in the high weight mode or the low weight mode, and when a larger amount of auxiliary power steering power is necessary when the vehicle speed is lower than the set value, a high weight mode is selected as described above, the auxiliary power steering powers of both the first motor  303  and the second motor  311  can be used. 
     Thus, after the electronic control unit  309  determines whether the vehicle makes a left turn or a right turn from the signal of the torque sensor  301  and determines whether the vehicle speed is high or low from the signal of the speed sensor  315 , the first and second auxiliary steering units  304  and  312  distribute an amount of auxiliary steering power of the second auxiliary steering unit  312  and transmits a control signal when the vehicle speed is low and an operation of the second auxiliary power steering unit  312  is stopped when the vehicle speed is high. 
     Further, when the high weight mode is selected even at a low vehicle speed, the auxiliary steering power of the first auxiliary steering unit  304  increases above a normal state, increasing a target current value control signal transmitted to the first motor  303 . 
     That is because since the second auxiliary steering unit  312  coupled to the vehicle is located inside a dash board close to the driver seat, noise audible to the driver is relatively increased as compared with the first auxiliary steering unit  304  located below the engine room, noise transferred to the driver seat can be minimized and auxiliary power steering power can be efficiently used by further increasing an output of the first auxiliary steering unit  304 . 
     Further, a gear position sensor  319  for detecting a position of the transmission gear of the vehicle and transmitting the detected position to the electronic control unit  309  may be further provided. 
     In this case, when an input signal input from the gear position sensor  319  corresponds to a parking position or a rear gear position, the target current value control signal transmitted to the first motor  303  and the second motor  311  is increased, making it possible to use both the auxiliary steering powers of the first motor  303  and the second motor  311 . 
     Further, when the input signal input from the gear position sensor  319  corresponds to a parking position or a rear gear position and the input signal input from the weight mode switch  317  corresponds to a high weight mode according to selection of the weight mode switch  317 , the target current value control signal transmitted to the first motor  303  and the second motor  311  increases. 
     Further, when the input signal input from the gear position sensor  319  corresponds to a parking position or a rear gear position and the input signal input from the weight mode switch  317  corresponds to a low weight mode, transmission of the target current value control signal to the second motor  311  is stopped and the target current value control signal to the first motor  303  is increased, making it possible to reduce power consumption and efficiently use auxiliary steering power. 
     Here, the vehicle speed value set to the electronic control unit  309  may vary according to the model of the vehicle, which vehicle speed value ranges from 20 km/h to 40 km/h, and the high weight mode and the low weight mode can be changed and set according to whether the vehicle is a car, a passenger vehicle, or a truck, according to the number of occupants, and according to the weight of loaded freight. 
     Further, the gear position sensor  319  may be a gear position sensor  319  conventionally used regardless of whether the transmission of the vehicle corresponds to an automatic transmission gear or a manual transmission gear or may be a separate gear position sensor  319  provided in a transmission gear. 
     Further, a first current sensor for measuring a current value of the first motor  303  and transmitting the current value to the electronic control unit  309  is provided in the first auxiliary steering unit  304 , and when a current value input from the first current sensor is different from a target current value transmitted to the first motor  303 , the electronic control unit  309  stops transmission of a target current value control signal to the first motor  303  and increases a target current value transmitted to the second motor  311 . 
     Further, a second current sensor for measuring a current value of the second motor  311  and transmitting the current value to the electronic control unit  309  is provided in the second auxiliary steering unit  312 . When a current value input from the second current sensor is different from a target current value transmitted to the second motor  311 , transmission of a target current value control signal to the second motor  311  is stopped and the target current value transmitted to the first motor  303  is increased. 
     Thus, if any one of the first auxiliary steering unit  304  and the second auxiliary steering unit  312  are not operating while the vehicle is in motion, auxiliary steering power of one operating normally is increased to continuously assist with a steering power of the driver. 
     As an example, when an error is generated in the first motor  303  of the first auxiliary steering unit  304 , the target current value transmitted to the first motor  303  becomes different from the current value input to the electronic control unit  309 . Thus, an operation of the first motor  303  is stopped by stopping transmission of a target current value control signal to the first motor  303  in the electronic control unit  309  and auxiliary steering power can be constantly maintained by increasing the target current value transmitted to the second motor  311 . 
     According to the present invention, an electric power steering apparatus for a vehicle distributes an auxiliary steering power according to increases in speed and weight of the vehicle by using two auxiliary steering units for assisting steering power when a steering wheel of the driver is manipulated, reduces a load applied to any one of the auxiliary steering units, reduces operation noise, and increases durability 
     The electric power steering apparatus lowers costs and allows for a sufficient auxiliary steering power at the same time by using a low output motor even when being applied to a truck or a bus requiring a relatively high steering power as compared with a car to improve driver convenience and improves fuel ratio while increasing energy efficiency,. 
     The electric power steering apparatus safely maintains a steering operation even if any one of the auxiliary steering units is not normally operated during a travel of the vehicle. 
     Even if it was described above that all of the components of an embodiment of the present invention are coupled as a single unit or coupled to be operated as a single unit, the present invention is not necessarily limited to such an embodiment. That is, among the components, one or more components may be selectively coupled to be operated as one or more units. 
     In addition, since terms, such as “including,” “comprising,” and “having” mean that one or more corresponding components may exist unless they are specifically described to the contrary, it shall be construed that one or more other components can be included. All of the terminologies containing one or more technical or scientific terminologies have the same meanings that persons skilled in the art understand ordinarily unless they are not defined otherwise. A term ordinarily used like that defined by a dictionary shall be construed that it has a meaning equal to that in the context of a related description, and shall not be construed in an ideal or excessively formal meaning unless it is clearly defined in the present specification. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Therefore, the embodiments disclosed in the present invention are intended to illustrate the scope of the technical idea of the present invention, and the scope of the present invention is not limited by the embodiment. The scope of the present invention shall be construed on the basis of the accompanying claims in such a manner that all of the technical ideas included within the scope equivalent to the claims belong to the present invention.