Abstract:
A power steering system including a sensor arranged or located in the steering system. The sensor detects an acoustic vibration or an acceleration signal in the steering system during a change in the steering angle and generates a sensor signal representing the vibration or the acceleration signal. A control unit receives the sensor signal and compares the sensor signal with a threshold value to detect a fault state; for example when the sensor signal is larger than the threshold value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to a power steering system; and, more specifically, to an apparatus for detecting friction in a steering gear of such a power steering system. 
         [0004]    2. Description of Related Art 
         [0005]    Rack-and-pinion power steering systems may utilize a steering gear having a ball screw. In addition, such systems may also use electric power assistance and are typically referred to as Rack Electric Power Assisted Steering (REPAS). Due to moisture and dirt entering the steering system, in particular the steering gear, rust may form in or on the components of the steering system. In particular on the ball raceways of the steering rack, on the balls of the recirculating ball gear and on the raceways of the recirculating ball gear, all of which increases the friction of the steering system. However, since a failure of the electrical assistance is to be expected only when the formation of rust has progressed a long way and the frictional forces have increased greatly, there is no initial adverse effect for the driver. That is since the electrical assistance compensates for the friction it may not be apparent until the assistance unit fails to overcome the frictional forces. For this reason, prompt warning of the driver is therefore desirable. 
         [0006]    Methods for detecting friction in a steering system are known; however, these methods are not capable of determining the friction within the steering gear separately from other influences such as the friction of the components connected to the steering gear; for example, the pivot bearings and shock absorbers. Therefore, such solutions require replacement of the entire steering system due to a finding of increased friction, even though the steering system itself does not exhibit any formation of rust or increased friction, that is the steering system or steering gear is undamaged. Accordingly, such methods are systems for detecting friction may lead to still functionally capable steering gears being undesirably replaced. 
         [0007]    The object of the invention is therefore to introduce a system that detects increased friction in a steering gear of a steering system. 
       SUMMARY OF THE INVENTION 
       [0008]    According to one embodiment or aspect of the invention an electrically assisted power steering system having a ball screw uses at least one sensor arranged in the steering system or in the vehicle. In this context, the sensor is designed in such a way that it detects an acoustic signal (air-borne noise) or an acceleration signal (vibration, structure-borne noise) of the steering system during a change in the steering angle and generates a sensor signal which represents the vibration/noise and transmits it to a control unit. The control unit is designed to compare the sensor signal with a threshold value and to detect a fault state if the sensor signal is larger than the threshold value. 
         [0009]    In one example, the sensor signal is examined for the presence of a fault situation; that is to say the friction in the system or steering gear has increased to an unacceptable amount. Any reduction in the measuring accuracy, possibly caused by indirectly measuring or determining the friction, is insignificant since rough quantification of the friction or of the state of the steering gear is sufficient for the desired purpose. Since an increase in the friction of the steering gear typically involves long-term changes it is conceivable for a fault state not to be detected until it has been found that the threshold value has been exceeded at least a certain number of times within a certain time period or with respect to a certain number of steering processes. The current steering angle, from which the change in the steering angle can be derived, is usually known in any case in the control unit or can be detected by a steering angle sensor. 
         [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating one example of a preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a schematic view of a power steering system according to the present invention in a vehicle. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0014]      FIG. 1  schematically illustrates an exemplary embodiment of a power steering system according to the present invention used with a vehicle  1  having wheels  2 , two of which can be oriented for steering by a steering gear  6 . In the present example, the steering gear  6  includes a ball screw, although the present invention may be used with other types of mechanical steering gears. In accordance with the exemplary embodiment, a driver of the motor vehicle  1  communicates a steering request via a steering wheel  3 . A difference between deflection of the steering wheel  3  and actual deflection of the steering gear  6  can be determined by a suitable deflection sensor such as a torsion rod  4  which connects the steering wheel  3  to a steering rack  10 . A control unit  7  controls an electric motor  5 , arranged on the steering rack  10  and functioning as an electric auxiliary motor, in such a way that a deviation between the deflection of the steering wheel  3  and the deflection of the steering gear  6  is compensated. The steering rack  10  which is driven by the electric motor  5  then moves the steering gear  6 , as a result of which the wheels  2  are oriented by the steering gear  6  as requested by the driver. As illustrated in  FIG. 1  a sensor  8  is provided which measures acoustic vibrations and/or acceleration signals in the steering gear  6  and transmits a sensor signal to the control unit  7 . 
         [0015]    The sensor  8  in the exemplary embodiment may include an acceleration sensor, an acoustic sensor, a torque sensor, a position sensor and/or a power consumption sensor. While different sensors  8  can be used, it is the friction, increased by the formation of rust or the penetration of dirt particles, which generates the noise and/or accelerations occurring during continuous steering. Accordingly, the sensor  8  is one that picks up acoustic vibrations and/or accelerations in the steering system. 
         [0016]    In this context, signals from sensors which are installed in the vehicle for other reasons can also be used beneficially for the purpose of use. In particular it is also conceivable to use a combination of the specified sensor types and to take into account their respective sensor signals jointly during the comparison of threshold values. In such a situation, the sensor signal, which ultimately represents the vibration of the steering gear  6 , is a combination of individual sensor signals. Data from sensors, along with data associated with the ambient conditions such as the temperature and air humidity, can also be taken into account. This can be done, for example, when the threshold value is set as a function of the measured ambient conditions. The intensity of the friction could thus be temperature-dependent, depending on the design of the steering system, for which reason it is advantageous to adapt the threshold value correspondingly. 
         [0017]    In an additional example, an acceleration sensor, already present in the vehicle, may also be is used. For example a sensor may include an acceleration sensor already present in an airbag triggering system. 
         [0018]    In addition, an acceleration sensor can also be used simultaneously to detect further faults in the steering gear such as, for example, jumping over the toothed belt, “normal” wear in the steering gear (for example wear of the sliding block which can lead to rattling of the toothing) or even in an extreme case fracturing of components (for example due to incorrect handling/repair) of the steering gear. 
         [0019]    The sensor  8  can be integrated into the control unit  7 . This has the advantage that the expenditure on additional cabling is eliminated. Alternatively or additionally, the sensor  8  can be arranged on the steering gear  6 , including the ball screw. Given that the most intensive generation of noise and/or the most intensive acceleration signal for fault situation detection is expected there. Accordingly the sensor  8  can generate sensor signals with increased measuring accuracy. 
         [0020]    The control unit  7  includes a predetermined threshold value. The threshold value is preferably an amplitude value or a frequency band. In this context, the control unit  7  is operative to compare a maximum modulation of the sensor signal with the threshold value. The modulation of the sensor signal provides reliable information about the intensity of the generation of noise in the steering gear. 
         [0021]    The control unit  7  can transmit a fault state to a human/machine interface for signaling to a driver. The transmission can occur here, for example, by means of a CAN (Controller Area Network) bus of the motor vehicle. 
         [0022]    The control unit  7  can be designed to deactivate the electric motor  5  of the power steering system after a fault state is detected. This can occur, in particular, after a restart of the vehicle  1  which takes place after the fault state is detected, since deactivating the electric motor  5  of the power steering system during travel can entail danger to the vehicle occupants and the surroundings if the control behavior of the vehicle is adversely affected by the loss of the power assistance. 
         [0023]    The power assisted steering system can have a filter unit  12  connected between the sensor  8  and the control unit  7  and designed to filter the sensor signal. Undesired signal components can be damped by means of the filtering, which increases the accuracy of detection of the fault state. As illustrated in the exemplary embodiment, the filter unit  12  may also be integrated into the control unit  7 . 
         [0024]    In particular, the filter unit can be designed to suppress frequency components of the sensor signal above an upper cutoff frequency. The upper cutoff frequency can be, for example, 10 kHz or less. In addition, it is also possible to provide a lower cutoff frequency below which frequency components of the sensor signal are suppressed. The lower cutoff frequency may be, for example, at least 100 Hz. 
         [0025]    The control unit  7  suitably evaluates the sensor signal after filtering by the filter unit  12  and, as already described, detects, under certain circumstances, a fault state when friction causes an unacceptable increase in the measured generation of noise in the steering gear  6 . The control unit  7  informs the driver of the motor vehicle  1  of the fault state by means of a human/machine interface  11 , for example by means of a warning lamp arranged on the dashboard. 
         [0026]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.