Characterization of a static friction condition in an electrically assisted steering system

Abstract

A method for characterizing a sliding or stiction condition in an electrically assisted steering system includes transmitting a first steering control signal from a controller to a rotary actuator as a periodic steering conditioning signal, receiving a second steering control signal via a power steering motor throughout the test to electrically assist the steering, transmitting a third steering control signal to the rotary actuator to cause alternating rotation of the steering shaft to a desired angular rate at constant acceleration / deceleration, and measuring a steering torque output from the rotary actuator. A control action is taken when a difference in local maximum and minimum peak amplitudes of the steering torque output exceeds a calibrated threshold difference indicative of stiction.A system includes a rotary actuator, the steering system, a sleepwalker operable to measure a steering torque transmitted by the rotary actuator to the steering shaft, and the above-mentioned controller.

Description

TECHNICAL FIELDThe present disclosure relates to a method and system for identifying a stiction condition in an electrically assisted steering system.BACKGROUNDVehicle rack and pinion steering systems include an elongated flat gear or rack having teeth that mesh with contact teeth of a rotating gear. Such steering systems are known, for example, from the publications DE 10 2014 201 952 A1, DE 102011 002 012 A1, DE 10 2011 089 950 A1, DE 10 2016 208 241 A1, DE 198 01 982 A1, DE 102012 104 806 A1 and DE 10 2007 014 344 A1. The gear is attached to one end of a steering shaft. If a steering angle is transmitted to the steering shaft, for example, by rotation of a steering wheel, the gear is transmitted by engaging in the gear rod teeth, whereby the gear rod is again moved in a corresponding steering direction. Tie rods are disposed at the distal end portions of the rack. The tie rods are connected to the front road wheels of a vehicle by a corresponding steering arm. Therefore, rack and pinion steering gears effectively convert the rotational motion about a steering axis into an axial motion with respect to the rack, while also providing an appropriate level of gear reduction.The steering functionality provided by typical rack and pinion steering gears may be electrically assisted in some constructions. For example, an electric steering motor may communicate steering torque overlay along the steering axis, for example in an electric power steering (EPS) column or a gear rack EPS system, while in other constructions, such as a gear rack EPS system, the electric steering motor provides torque assist to a particular portion of the gear rack via a suitable drive mechanism. In an example of the latter construction, a rotor axis of a steering assist motor is disposed parallel to a longitudinal axis of the rack. A belt, chain, gear set or pulley system may be used to transmit torque from the off-axis steering assist motor to a ball screw device, and ultimately to the rack itself. Due to the close manufacturing tolerances, transient collisions may occur in the various transmission or torque-transmitting mechanisms of the steering system, such collisions possibly resulting in a sliding friction or "stiction" condition that may affect the overall steering precision and feel. The transient and largely unpredictable nature of such a static friction condition can complicate problem isolation, diagnosis, and correction.SUMMARYA method and system for characterizing a sliding friction or static friction condition in an electrically assisted steering system of the types described above are disclosed herein. According to the present approach, the steering system is in communication with a controller programmed to perform certain test steps of the present method. The method includes executing, in conjunction with active steering assist over the duration of the test, a controlled steering conditioning phase to induce the aforementioned stiction condition and generate a stiction metric for the steering system indicative of the stiction condition. The metric examines differences in the local maximum / minimum peak amplitudes of torque peaks occurring at any time during the tests, as indicated below, with threshold differences indicative of an unsatisfactory system.It is an object of the invention to improve the overall construction of the steering gear while simplifying the validation of the steering gear in relation to existing test systems and methods.This object is achieved by the features of the independent claims. Advantageous further developments are defined in the dependent claims.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic illustration of a test system for characterizing a static friction condition of an exemplary electrically assisted steering system. FIG. 2A is a representative timing diagram of steering torque and steering angle illustrating the performance of a properly functioning electrically assisted steering gear, with signal amplitude plotted on the vertical axis and time plotted on the horizontal axis. FIG. 2B is an example timing diagram of steering torque and steering angle illustrating performance of a properly functioning electrically assisted steering gear with breakaway torque characteristics, on which signal amplitude is plotted on the vertical axis and time is plotted on the horizontal axis. FIG. 3 is an exemplary timing diagram of a steering conditioning input signal as part of the present method, with the steering angle signal amplitude plotted on the vertical axis and time plotted on the horizontal axis. FIG. 4 is a flow chart describing an example method for characterizing stiction characteristics of an electrically assisted steering system using the test system shown in FIG. 1.DETAILED DESCRIPTIONReferring now to the drawings, wherein like reference numerals refer to like components throughout the several views, an exemplary test system 10 is schematically illustrated in FIG. 1. The test system 10 includes an electrically assisted steering system 20, steering control hardware 40, and a controller (C) 50 programmed to execute logic including a method 100 using the control hardware 40. The steering gear 20 includes a rack 22 having a first end 33 and a second end 35 and a gear housing 24 including a gear box 26. Execution of method 100 that occurs while the steering assist is active results in characterization of a threshold free-wheeling torque condition, hereafter a sliding friction or "static friction" condition, in steering gear 20 as described in greater detail below with reference to FIGS. 2A-4.Because steering input in the form of an arranged steering angle is transmitted to a steering shaft 25 via a steering axis 13, typically from a steering wheel (not shown), but in the present controlled test environment via a rotary actuator 42, the gear train 26 disposed within the gear train housing 24 meshes with geared trains (not shown) along the rack 22. The toothed gear 26 thus translates along a longitudinal axis 17 of the toothed rack 22, as indicated by the double arrow A. This movement, in turn, moves the rack 22 in a corresponding direction to steer the front wheels of a vehicle (not shown) within which the electrically assisted steering gear 20 is to be deployed.Depending on the embodiment, a drive mechanism 129 and a drive housing 29 may be positioned near the second end 35 of the rack 22, as shown, which is typical of a rack and pinion electric power steering (EPS) system. A rotor axis 15 of the steering assist motor 43 may be aligned and operable to enable electrical assist of the steering maneuver, for example, by the drive mechanism 129, which in such an exemplary embodiment may be embodied as a suitable transmission, chains, belts, and / or other speed reduction mechanism. Although omitted from FIG. 1 for illustrative simplicity, the steering assist motor 43 and associated drive mechanism structure similar to the drive mechanism 129 and drive housing 29 may alternatively be positioned on or along the steering axis 13 to form a column or rack and pinion EPS system as mentioned above. A column EPS embodiment may also include a steering column and an intermediate shaft as is known in the art, wherein the steering assist motor 43 provides the steering assist torque to a separate drive mechanism mounted adjacent to the steering column. A toothed gear EPS system, in contrast, may place the steering assist motor 43 close to the gear housing 24 so that a steering assist torque directly acts on the rotatable toothed gear 26. Regardless of the embodiment, a bracket 27 and a base plate 28 may be used to secure the steering gear 20 in a test environment.An electric steering assist torque (arrow T A) is automatically provided to assist a steering maneuver by using torque overlay or additional steering torque established via the steering assist motor 43 at a level established in logic, typically an electronic control unit (not shown) of the steering assist motor 43. This occurs over the duration of the test described herein. The amount of torque overlay that assist motor 43 should generate may be determined, for example, based on an input torque signal from an internal torque sensor (not shown) disposed along steering axis 13 and a variety of other signals, such as vehicle speed, ignition state, etc., as is known in the art of EPS systems. Thus, electric power and steering control signals (arrow T 43) are provided to the steering assist motor 43 to allow the steering assist motor 43 to output the preprogrammed steering assist torque (arrow T A). As mentioned above, the steering assist remains over the test embodied by the method 100, and thus the electric power and steering control signals (arrow T 43) are provided to the steering assist motor 43 during conditioning and subsequent test execution.A transient sliding friction or static friction condition may occur when an electrically assisted system is used. It is herein recognized that a stiction condition may occur when certain types of linear actuators are used, i.e., the ball screw mechanism mentioned above as part of the drive mechanism 129. Although omitted from the figures for illustrative simplicity, a ball screw mechanism as known in the art includes a threaded shaft defining a helical raceway with a collection of ball bearings. The method 100 proceeds from the standpoint of replication, isolation, and ultimately correction or prevention of the occurrence of the stiction condition by evaluating the breakaway torque characteristics that occur in the steering gear 20. Underlying the present approach is the realization made herein that the stiction condition, i.e., the predetermined threshold characteristics exhibiting breakaway torque, as described below, may be due to transient impingement of ball bearings within the drive mechanism 129. As will be described in more detail below with reference to FIGS. 2A-4, the method 100 therefore includes conditioning the steering gear 20 with a calibrated steering input so as to induce the static friction condition prior to measuring and evaluating all observed breakaway torque characteristics.With continued reference to FIG. 1, the control hardware 40 of FIG. 1 includes a rotary actuator 42, e.g., an electric motor, as well as a torque transducer 44 and a rotary encoder 46, each of which is coaxially aligned with the others on the steering axis 13 of the steering shaft 25. Because axis variation may be present in the control hardware 40, an extension shaft 45 may be coupled to the steering shaft 25 via a set of flexible couplings 48, as shown. The flexible couplings 48 serve to simplify the connection of the rotary actuator 42 to the steering shaft 25.One or more linear actuators 49 may optionally be disposed at opposite distal ends 33, 35 of the rack 22, as shown, and connected to rods 75. The controller 50 may be programmed to control the linear actuators 49 in a coordinated manner using a sensed force (arrows F D) from a pair of force transducers 41 so as to arrange a net zero force along the rack axis 17, i.e., the longitudinal axis of the rack 22, during the conditioning and measurement phases of the method 100. It will be appreciated that depending on the performance of the used servomechanism employed in one or more linear actuators 49, the actual force along the rack axis 17 may vary about the zero point, up to, for example, + / - 1200 N and 30 N, during the conditioning and measurement phases of the method 100.The controller 50 of FIG. 1 may be configured as a host machine, e.g., a digital computer or microcomputer, specifically programmed to perform the method steps of the method 100, an example of which is shown in FIG. 4. The memory (M) includes sufficiently concrete permanent memories such as magnetic or optical ROM (read-only memory), flash memory, etc., as well as RAM (random access memory), EEPROM (electrically erasable programmable read-only memory), and the like. An output signal (arrow 11) may be generated by the controller 50 and used to record diagnostic code or request execution of other control actions within the scope of the method 100.As part of the method 100, the controller 50 receives and detects a measured steering torque value (arrow T 44) from the torque transducer 44 and a measured steering angle (θ 46) from the rotary encoder 46, and outputs the commanded steering angle (arrow θ 42) to the rotary actuator 42. Electronic steering control signals (arrow T 43), which provide the electric steering assist indicated herein, are received by the steering assist motor 43, with such signals typically transmitted from and internally generated by an integrated electronic control unit (not shown) of the steering assist motor 43, as mentioned above, to determine and control the amount of torque overlap that the steering assist motor 43 should generate at a particular time. Thus, the electronic steering control signals (arrow T 43) ultimately allow the steering assist motor 43 to output a preprogrammed assist torque (arrow T A) as is also known in the art of EPS systems. Force control signals (arrow 111) are output from the controller 50 to the linear actuators 49 to arrange linear forces (arrows F) and thereby maintain a net zero force along the rack axis 17.Exemplary curves 60 and 62 are shown in FIGS. 2A and 2B, respectively, to illustrate the measured steering torque value (T 44). The amplitude (A) is plotted on the vertical axis, which is dependent on the signal, the torque or the steering angle. The time (t) is plotted on the horizontal axis. The upper and lower ranges of the arranged steering angle (θ 42), which are illustrated in FIGS. 2A and 2B, correspond to ±10°, and the measured steering torque value (T may correspond to 44) ±2 Nm, without being necessarily limited to these ranges.Steering torque disturbances may be detected in the measured torque value (T 44) by checking the local maximum / minimum (min / max) values of ± peak amplitudes and comparing the difference to a calibrated difference, for example 0.3 Nm, or about 0.2 to 0.4 Nm, using an exemplary range. The controller 50 of FIG. 1 is programmed to condition the electric assist steering gear 20 as shown in FIG. 3 with active electric steering assist. FIG. 2A is representative of any acceptable torque response determined with the local peak amplitudes (points A 1, A 2) in each zone 65, and a difference (Δ=A 1- A 2) between each local max / min pair (A 1, A 2), which is compared to a calibrated value. Exceeding the calibrated value at any time during execution of the method may indicate an unsatisfactory result. By contrast, the curves 62 of FIG. 2B illustrate an exemplary unacceptable breakaway torque response in the steering gear 20, i.e., an unsatisfactory sliding friction / static friction condition. For example, FIG. 2B may represent a result of (A 1- A 2)=0.5 Nm, which is more than an exemplary calibrated value of 0.3 Nm, thus indicating an unsatisfactory result.FIG. 3 illustrates a representative periodic conditioning signal 70 having a calibrated frequency f CAL and ± steering angle (θ 42) plotted on the corresponding vertical axis and with time (t) on the horizontal axis. A purpose of transmitting the periodic conditioning signal 70 via the controller 50 of FIG. 1 is to encourage a transient and otherwise unpredictable stiction condition to manifest itself in a controlled test environment. Static friction due to ball insertion within the ball screw mechanism 29 of FIG. 1 may be difficult to repeat because the condition may take considerable time to develop and varies greatly between the steering systems depending on the drive surface and the level of vibration present. The conditioning method used within the scope of the present method 100 therefore intends to cause the inner ball bearings of the ball screw mechanism 129 to settle, a condition that, as recognized herein as part of the present approach, may result in a perceptible breakaway torque condition indicative of stiction. The disclosed conditioning method is not limited to the present test method 100, and other steering tests may benefit from such conditioning. In other words, upon completion of the conditioning portion of the method 100, any number of control actions and / or tests may be performed on a steering system.Sufficient steering conditioning may be provided to the steering gear 20 by transmitting the periodic conditioning signal 70 via the controller 50 of FIG. 1, and the resulting control of the rotary actuator 42 via an increasing conditioning phase t 1, a sustained conditioning phase t 2 in which a peak calibrated steering angle (θ 42) is maintained, and a subsequent conditioning phase t 3, as shown. The periodic conditioning signal 70 may be repeated a calibrated number of times. For example, in a particular embodiment that has proven effective during testing, a duration of 1 second (1 s) may be used for each of the periods t 1 and t 3 in FIG. 3, with a duration of 2 s used for the period t 2, with a corresponding steering angle of ±3° and calibrated frequency f CAL of about 8-12 Hz or about 10 Hz in another embodiment. Eight cycles of the periodic conditioning signal 70 may be used for a total of 32 seconds of steering conditioning in this particular embodiment. Conditioning for an insufficient amount of time or number of cycles may result in the problem not manifesting itself, while longer steering conditioning may not provide additional benefits.Referring to FIG. 4, an exemplary embodiment of the method 100 for identifying a state of breakaway torque in an electrically assisted steering gear 20 of FIG. 1 begins with step S 102, wherein the steering gear 20 is connected to the control hardware 40 and set in communication with the controller 50 described above. A steering control signal is received from the steering assist motor 43 of FIG. 1 This causes the steering assist motor 43 to electrically assist in the steering maneuver according to its own existing EPS control algorithms as mentioned above, which continues throughout the duration of the test.In steering assist active step S 104, the controller 50 transmits the periodic steering conditioning signal 70 of FIG. 3 to the rotary actuator 42 as another steering control signal for a calibrated amount of time, such as 32 seconds in the example above. As mentioned above, the periodic conditioning signal 70 has a calibrated steering frequency and a peak steering angle, for example, about 10 Hz and ± 3 degrees in the exemplary embodiment given above. The periodic conditioning signal 70 may be transmitted for a plurality of cycles, for example, the eight 4-second cycles over a 32 second duration shown in FIG. 3 At the same time, the controller 50 transmits the steering control signal (arrow θ 42) to the rotary actuator 42 as yet another steering control signal. This control action occurs after the calibrated duration of the conditioning signal 70 and while the EPS assist remains active.Receiving the steering control signal (arrow θ 42) in step S 104, the rotary actuator 42 causes the steering shaft 25 to rotate to a target steering angle rate with a constant acceleration or deceleration, for example, about ±10% degrees / second 2( ° / s 2). That is, the steering average acceleration is accurately controlled by the controller 50 of FIG. 1 to ensure smooth steering performance during the test. This, in turn, is to ensure that any torque disturbances detected as part of the method 100 actually occur due to a breakaway torque condition once the transmission engagement ceases, rather than due to any action or disturbance from the rotary actuator 42.As an example and with reference to FIGS. 2A and 2B as a reference, when starting from a reference / center or 0° steering angle, the steering shaft 25 may be accelerated at 10° / s 2 or another suitable constant acceleration value to a target angular rate, for example 10° / s. Thereafter, the steering shaft 25 is decelerated again to 0° / s, at -10° / s 2 in this case, in order to achieve a calibrated maximum steering angle, for example 10°. This alternating acceleration / deceleration pattern is repeated for the steering shaft 25 to return to the reference / center 0° steering angle. After a pause of about 1 s, the entire cycle is repeated in the opposite steering direction and continues for the duration of the test. The method 100 continues to step S 106 while the described alternating steering action has not yet been completed.Step S 106 includes measuring the torque output from the rotary actuator 42 while the steering shaft 25 rotates at the constant acceleration or deceleration at and from the target steering angle rate, respectively. Thus, torque transducers 44 may be coaxially connected along the steering rotational axis 13 of the steering shaft 25 such that rotation of the rotary actuator 42 imparts a steering output torque measured by the torque transducer 44. The torque transducer 44 in turn transmits the measured steering output torque (arrow T 44) to the controller 50. the method 100 proceeds to step S 108 once step S 106 is completed.Next, in step S 108, the controller 50 of FIG. 1 receives the measured steering torque (arrow T 44) and determines whether the difference (Δ) between the local max / min peak amplitudes of the measured steering torque (arrow T 44), at any location in the test exceeds a calibrated threshold indicative of the sliding friction condition. Such a threshold may be determined offline during testing, as the calibrated threshold may be expected to vary with the particular design of the steering gear 20, the EPS system, and / or the vehicle in which it is used. That is, some torque values may be perceptible to a driver of a vehicle as a steering column torque disturbance, while others are not fixed with the calibrated threshold amplitude in memory (M) of the controller 50 for the particular steering gear 20 being tested. The mentioned 0.3 Nm example is therefore only one possible threshold value. The method 100 proceeds to step S 110 when the difference (Δ) exceeds the calibrated threshold indicative of the sliding friction condition, as indicated by arrow Y. Otherwise, the method 100 proceeds to step S 112 as indicated by arrow N.Step S 110 may include executing a control action with respect to the steering gear 20 or the steering system 10 of FIG. 1. The controller 50 may generate an output signal (arrow 11 of FIG. 1 ) to record or output a first diagnostic code, i.e., a behavior indicative of the stiction condition. Additional control steps may include applying the first diagnostic code to validate a given construction of the steering gear 20 or steering system 10, for example, by rejecting a given construction during a validation process when the diagnostic code is recorded, or creating a construction specification, or possibly performing a maintenance action on the particular steering gear 20 being tested.Step S 112 is reached when the calculated difference (Δ) does not exceed the calibrated threshold difference, or in other words, when no perceptible stiction condition is detected. Step S 112 may include detecting, via the controller 50, a second diagnostic code, that is, via the output signal (arrow 11 of 1) when a threshold stuck friction condition is not detected. Because step S 112 indicates a fully functional steering gear 20 or system 10, additional control steps may include applying the second diagnostic code to validate a given construction of the steering gear 20 or system 10, for example, by assuming a given construction during a validation process or creating a construction specification.As used herein with reference to the disclosed values or ranges, the term "about" indicates that the disclosed numerical value allows some imprecision, for example reasonably close to or almost as high as, for example, plus ±10% of the stated values or ranges. If the imprecision provided by the term "about" is not otherwise understood by those of ordinary skill in the art with this ordinary meaning, then "about" as used herein indicates at least variations that result from ordinary measurement methods and the use of such parameters. Moreover, the disclosure of ranges includes the disclosure of all values of further divided ranges within the entire range.

Claims

A method (100) for characterizing a static friction condition using a rotary actuator (42) in an electrically assisted steering system (20), the steering system (20) comprising a rack (22), a pinion gear (26) disposed at an end of a steering shaft (25) in meshing engagement with the rack (22), a steering assist motor (43), and a drive mechanism (129) driven via the steering assist motor (43) to electrically assist a steering maneuver, the method (100) comprising: transmitting (102) a first steering control signal from a controller (50) to the rotary actuator (42) for a calibrated period of time, the first steering control signal being a periodic steering conditioning signal (70) having a calibrated steering frequency and a calibrated peak steering angle; receiving (102), via the steering assist motor (43), a second steering control signal during and after transmission of the first steering control signal, thereby causing the steering assist motor (43) to electrically assist the steering maneuver via the drive mechanism (129); transmitting (104), from the controller (50), a third steering control signal to the rotary actuator (42) after the calibrated duration and while still transmitting the second steering control signal, wherein the third steering control signal is an arranged steering angle that causes alternating rotation of the steering shaft at a constant acceleration and deceleration to and from a target steering angle rate, respectively, wherein the constant acceleration and deceleration is about ± 10 degrees / second 2 ; measuring (106) a steering torque output from the rotary actuator (42) while the steering shaft (25) is being alternately rotated; and executing (110), via the controller (50), a control action relating to the steering system (20) when a difference in local maximum and minimum peak amplitudes of the measured steering torque output exceeds a calibrated threshold difference indicative of the stiction condition.The method (100) of claim 1, wherein the calibrated frequency of the periodic signal is about 8-12 Hz and the peak steering angle is about ± 3 degrees.The method (100) of claim 1 or 2, wherein the measurement (106) of the steering torque output comprises using a torque sensor connected to the steering shaft (25) or to the rotary actuator (42) to directly measure the steering torque output.The method (100) of at least one of the preceding claims, wherein executing (110) a control action comprises rejecting the steering system (20) in a design validation process when a diagnostic code is detected.The method (100) of at least one of the preceding claims, wherein the threshold amplitude is in the range of about 0.2 Nm to 0.4 Nm.The method (100) of at least one of the preceding claims, wherein the steering system (20) comprises a linear actuator (49) connected to an end of the rack (22), the method (100) further comprising transmitting a control signal to the linear actuator (49) to arrange a net zero force along a longitudinal axis (17) of the rack (22).A system (10) comprising: a rotary actuator (42); an electrically assisted steering system (20) having a steering shaft (25), a rack (22) having a rack axis (17), a pinion gear (26) disposed at an end of the steering shaft (25) in meshing engagement with the rack (22), a steering assist motor (43) disposed off-axis with respect to the rack axis (17), and a drive mechanism (129) connected to the rack (22) and driven via the steering assist motor (43) to electrically assist a steering maneuver; a torque transducer (44) operable to measure a steering torque applied to the steering shaft (25) by the rotary actuator (42); A controller (50) programmed to identify a static friction condition in the steering gear (26), and wherein execution of the instructions of the controller (50) causes the controller (50) to: transmit (102) a first steering control signal from the controller (50) to the rotary actuator (42) for a calibrated period of time, wherein the first steering control signal is a periodic steering conditioning signal (70) having a calibrated steering frequency and a calibrated tip steering angle; transmit (104) a second steering control signal to the steering assist motor (43) simultaneously with the first steering control signal, thereby causing the steering assist motor (43) to electrically assist the steering maneuver; a third steering control signal to be transmitted from the controller (50) to the rotary actuator (42) after the calibrated duration and simultaneously with the second steering control signal, the third steering control signal being an arranged steering angle that causes alternating rotation of the steering shaft at a constant acceleration and deceleration to and from a desired steering angle rate, respectively, the constant acceleration and deceleration being about ± 10 degrees / second 2 ; determining a steering torque output to be measured by the rotary actuator (42) while the steering shaft (25) is being alternately rotated; and executing a control action with respect to the steering system (20) via the controller (50) when a difference in a local maximum and a local minimum peak amplitude of the measured steering torque output exceeds a calibrated threshold difference indicative of the stiction condition.The system (10) of claim 7, wherein the calibrated frequency of the periodic signal is about 10 Hz and the peak steering angle is about ± 3 degrees.The system (10) of claim 7 or 8, wherein the steering system (20) comprises a linear actuator (49) connected to an end of the rack (22), and wherein the controller (50) is operable to transmit a control signal to the linear actuator (49) to arrange a net zero force along the rack axis (17) of the rack (22).

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