Characterization of a sliding friction state in a steering system

Abstract

A method (100) for characterizing a sliding friction condition in an electrically assisted steering system (20) having a rack (22), a gear train (26) arranged at one end of a steering shaft (25) in meshing engagement with the rack (22), a steering motor (43), and a drive mechanism (129) electrically assisting a steering input to the steering system (20) via the steering motor (43), the method comprising: Connecting (S102) a torsion bar (54) and an inertia disc (52) to the steering shaft (25); Transmitting (S104) a periodic steering control signal from a controller (50) to a rotary actuator (42) while the steering motor (43) and the drive mechanism (129) assist the steering input, thereby causing rotation of the steering shaft (25) via the torsion bar (54) and the inertia disk (52) over a calibrated range of steering angles; Measuring (S106) a displacement of the rack (22) from a center position via a linear actuator (49, 149); Applying (S106) an axial force to the rack (22) that is proportional to the measured displacement in a direction that opposes the movement of the rack (22) while the periodic steering control signal is being transmitted; and Measuring (S108) a steering torque output value along the steering shaft (25) via a torque transducer (44) while the periodic steering control signal is being transmitted and the axial force is being applied; and Executing (S112) a control action with respect to the steering system (20) by the controller (50) when a difference between a maximum value of the measured steering torque and a minimum value of the measured steering torque, which occurs after the maximum value of the steering torque, at the start and at each reversal of the direction of the periodic steering control signal, exceeds a calibrated threshold value of approximately 0.3 Nm, which indicates the sliding friction condition.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the characterization of a sliding friction condition in a steering system. BACKGROUND

[0002] Vehicle rack and pinion steering systems incorporate an elongated flat gear or rack with teeth meshing with contact teeth of a rotating gear. The gear is rotatably mounted on a steering shaft. When a steering angle is transmitted to the steering axis by the rotation of a steering wheel, the gear train translates along the rack by engaging the rack teeth. This, in turn, moves the rack in a corresponding steering direction. Tie rods located at the distal ends of the rack are connected to the front road wheels of a vehicle via a corresponding steering arm. Therefore, a rack and pinion steering system effectively converts rotational motion relative to a steering axis into linear motion relative to the rack, while also providing an appropriate gear reduction.

[0003] The steering functionality provided by typical rack-and-pinion steering systems may be electrically assisted in some designs. For example, an electric steering motor may be controlled to transmit variable steering torque overlay via a drive mechanism on the rack itself, i.e., a rack-and-pinion electric power steering (EPS) system, or along the steering axis in a gear-type EPS or a column-type EPS system. For example, the publications DE 20 2011 050 806 U1, DE 10 2011 089 950 A1, CN 2 01 163 239 Y and Pfeffer, Harrer (ed.), Lenkungshandbuch, 2nd ed., Springer Vieweg, 2013, pp. 145-151, 242-244, disclose similar steering systems.

[0004] While EPS systems of all types generally perform well under most driving conditions, a transient phenomenon involving a sudden increase in required steering effort followed by an abrupt decrease in steering resistance may manifest itself under certain driving conditions. Such a condition, referred to herein as a sliding friction condition, can impact steering quality and overall driving feel.

[0005] It is therefore an object of the invention to improve the steering quality and driving feel.

[0006] This problem is solved by the features of the independent claims. Advantageous further developments are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of an exemplary test system for characterizing a sliding friction condition in a steering system as described herein. Fig. 2A and Fig. 2B are representative time diagrams of satisfactory and unsatisfactory performing steering systems, respectively, with measured steering output torque, rack displacement, and steering angle plotted on the vertical axis and time plotted on the horizontal axis. Fig. 3 is a flowchart describing an exemplary method for characterizing a steering sliding friction condition in a steering system using the test system shown in Fig. 1. DETAILED DESCRIPTION

[0007] Referring to the drawings, wherein like reference numerals refer to like components throughout the several views, an exemplary steering test system 10 is shown schematically in Fig. 1. The test system 10 includes a steering system 20 of the types described above, as well as steering control hardware 40 and a controller (C) 50 programmed to execute logic embodying a method 100 using the control hardware 40. Execution of the method 100 results in the characterization of a steering sliding friction condition in the steering system 20, as described below with reference to Fig. 2A-3 executed.

[0008] The steering system 20 may be embodied as an electrically assisted steering system of the type well known in the art, for example, a dual gear electric power steering (EPS) system as shown, or alternatively a gear EPS system or a column EPS system. As such, the steering system 20 of Fig. 1, an elongated rack 22 having a rack axis 17, a first end 33, and a second end 35. The steering system 20 also includes a gear housing 24 and a steering shaft 25 with the gear housing 24 disposed near the first end 33 of the rack 22. The steering system 20 may also include a steering assist motor 43 and a drive mechanism 129 within a drive housing 29.

[0009] Depending on the EPS system embodiment, the drive mechanism 129 and the drive housing 29 may be positioned near the second end 35 of the rack 22, as shown, which is typical for a dual-gear rack EPS system. A rotor axis 15 of the steering assist motor 43 is aligned to enable electrical assistance of the steering action, for example, by the drive mechanism 129, which may be embodied as a suitable gear, chain, belt, and / or other speed reduction mechanism in such an embodiment. Although in Fig. Omitted from Figure 1 for illustrative simplicity, the steering assist motor 43 and drive mechanism structure similar to the drive mechanism 129 and drive housing 29 may alternatively be disposed on or along the steering axis 13 to form a column or gear 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, with the steering motor 43 providing the steering assist torque to a separate drive mechanism mounted adjacent to the steering column. A gear EPS system, in contrast, may place the steering assist motor 43 near the gear housing 24 so that assist torque acts directly on the rotatable gear transmission 26. Regardless of the embodiment, a bracket 27 and a base plate 28 may be used to secure the steering system 20 in a test environment.

[0010] The gear housing 24, located at the first end 33 of the rack 22, contains a rotatable gear train 26. When a steering input in the form of a commanded steering angle is transmitted to the steering shaft 25 via the 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 within the gear housing 24 engages meshing gears (not shown) of the rack 22. The gear train 26 thus translates along the rack axis 17, as indicated by the double arrow A.

[0011] This displacement, in turn, moves the rack 22 in a corresponding direction to steer the front wheels of a vehicle (not shown) in which the steering system 20 is employed.

[0012] It is recognized herein as a basis for practicing the present method 100 that a steering sliding friction condition may result in imperfections in the various mechanical interfaces of the steering system 20, such as meshing worm gears or other types of gears in a drive mechanism 129 of the drive housing 29 and the rack 22, when a vehicle equipped with the steering system 20 is in operation. The sliding friction condition is sporadic and transient, and is therefore difficult to replicate in a test environment. The method 100 thus takes steps to replicate real-world driving conditions in a controlled test environment for the purpose of isolating and correcting the steering sliding friction problem described above.

[0013] The control hardware 40, shown in Fig. 1, is therefore configured to solve this problem when used in conjunction with the controller 50 described below. The control hardware 40 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. It should be noted that the use of a torque transducer 44 coupled to the rotary actuator is merely exemplary. In this regard, the torque transducer 44 may comprise any suitable device for measuring and monitoring torque at the steering shaft 25 of the steering system 20, including, but not limited to, an internal torque sensor associated with the steering system 20.Since axis variation may be present in the components of the steering 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.

[0014] A compliant torsion bar 54 and an inertia plate 52, both arranged on the steering axle 13 as shown, are also used as part of the steering hardware 40. Together, the torsion bar 54 and inertia plate 52 are intended to simulate how the steering system 20 is arranged in a typical vehicle, i.e., with a steering column and intermediate shaft, clutches, and the like, ultimately connecting a steering wheel to the gear train 26. The materials of construction and the resilience / inertia provided by the torsion bar 54 and inertia plate 52 may vary with the design of the steering system 20 and are intended to simulate the stiffness and inertia characteristics of parts of the steering system above the level of the steering gear itself.In an exemplary configuration, the torsion bar 54 may provide a rotation rate of about 0.3 to 0.5 Nm / degree, and the inertia disk 52 has a moment of inertia of about 0.03 to 0.05 kg-m. 2 or 0.4 Nm / degree and 0.04 kg-m 2 in a different configuration.

[0015] Still referring to Fig. 1, a linear actuator 49 is disposed at the second end 35 of the rack 22 and connected to one of a pair of tie rods 75. A substantially identical linear actuator 149 may optionally be disposed at the first end 33, as shown. The controller 50 is placed in wired or wireless communication with each of the linear actuator 49 and / or 149, the rotary actuator 42, the torque transducer 44, and the rotary encoder 46, such that the controller 50 can provide a measured rack displacement (arrow D) 49 ) as a linear actuator position signal and axial force control signals (arrow F49 ) to and from the linear actuator 49 and / or 149.

[0016] That is, the controller 50 commands the application of a variable axial force along the rack axis 17 that is proportional to the actuator or rack displacement from a reference center rack position, i.e., a position in which the rack 22 is centered, and monitors the rate and magnitude of the applied axial force. The controller 50 also receives a measured steering output torque value (arrow T 44 ) around the steering axis 13 from the torque sensor 44 and a measured steering angle signal (θ 46 ) from the rotary encoder 46, and transmits the steering angle signal (arrow θ 42) to the rotary actuator 42 to control the rotary output of the rotary actuator 42. Thus, the controller 50 commands the rotary actuator 42 to apply a calibrated steering angle or torque to the steering system, while the linear actuator(s) applies a load to the rack 22. An integrated ECU (not shown) of the steering assist motor 43 then determines and controls the amount of torque overlay that the assist motor 43 should produce, for example, based on an input torque signal from an internal torque sensor (not shown) located along the steering axis 13 and a variety of other signals, such as vehicle speed, ignition state, etc., as is known in the art. Thus, electrical power and steering control signals (arrow T 43) to the steering assist motor 43 to enable the steering assist motor 43 to output a pre-programmed assist torque.

[0017] The control 50 of Fig. 1 may be configured as a host machine, e.g., a digital computer or microcomputer, specifically programmed to carry out the method steps of method 100, an example of which is shown in Fig. 3. For this purpose, the controller 50 is configured with sufficient hardware to perform the required steps, i.e., with sufficient memory (M), a processor (P), and other hardware, such as a high-speed clock, analog-to-digital and / or digital-to-analog circuitry, a timer, input / output circuitry and associated devices, signal programming and / or signal buffer circuitry. The memory (M) includes sufficient tangible permanent memory, 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.

[0018] Exemplary curves 60 and 62 are shown in Fig. 2A and 2B, respectively, to illustrate timing diagrams of exemplary acceptable and unacceptable test scenarios. Curve D 49represents the measured axial displacement of the rack 22 from the linear actuator 49, for example in millimeters (mm), while the curve θ 46 represents the steering angle transmitted to the steering axle 13. The curve T 44 represents the steering output torque measured by the torque transducer 44, with all hardware components shown in Fig. 1 and described above.

[0019] In Fig. 2A, that is, the exemplary acceptable behavior, note that the measured torque T 44 is relatively smooth. Without considering the changing amplitude of such a signal, the displayed power lacks significant torque peaks that indicate a sliding friction condition. The time diagram of the Fig. In contrast, Figure 2B shows an example of unacceptable sliding friction performance. Arrows 65 indicate torque spikes from the sliding friction condition that may occur when engaging mechanical components of the drive mechanism 129 or elsewhere in the steering system 20 are temporarily impinged before being quickly released.

[0020] The torque peaks 65 can be used to evaluate against a calibrated threshold to diagnose and correct the sliding friction problem in a particular steering system 20. For example, a control action may be taken with respect to the steering system 20, via the controller 50, when a difference between an absolute maximum value of the measured steering torque required to initiate rack displacement, indicated by points 68, and an absolute minimum value of the measured steering torque after breakaway torque (points 69), at the start of the test and all subsequent steering direction reversals shown in the periodic steering control signal, i.e., curve Θa2, exceeds a calibrated threshold indicative of the sliding friction condition.

[0021] With reference to Fig. 3 begins an embodiment of the method 100 for characterizing the sliding friction in the steering system 20 of Fig. 1 with step S102, in which the steering system 20 is connected to the control hardware 40 and placed in communication with the controller 50. In addition to connecting the controller 50 to the rotary actuator 42 and the torque transducer 44, step S102 includes connecting the linear actuator 49 and / or 149 of Fig. 1 with the rack 22 along the rack axis 17 and then placing the linear actuator 49 and / or 149 in communication with the controller 50. The method 100 proceeds to step S104 once the test system 10 is constructed.

[0022] Optionally, sufficient steering conditioning steps can be carried out by transmitting the periodic conditioning signal via the controller 50 from Fig. 1, and the resulting control of the rotary actuator 42 via a rising conditioning phase, a sustained conditioning phase in which a peak calibrated steering angle is maintained, and a falling conditioning phase. Such a periodic conditioning signal may be repeated a calibrated number of times. For example, in a particular embodiment, a duration of 1 second (1 s) may be used for each of the rising and falling phases, with a duration of 2 s used for the sustained conditioning phase, with a corresponding steering angle of ± 3° and a calibrated frequency F CALof approximately 8-12 Hz, or approximately 10 Hz in another embodiment. Eight cycles of the periodic conditioning signal can be used in this particular embodiment for a total of 32 seconds of steering conditioning. The actual values ​​will vary depending on the type of test being performed and the specific components of the system being tested. In general, conditioning for an insufficient amount of time or number of cycles may prevent the problem from manifesting itself, while longer steering conditioning may not yield any additional benefits.

[0023] In step S104, the controller 50 transmits the periodic steering control signal to control the steering angle (θ 42 the Fig. 1-2B) via the rotary actuator 42 of the Fig. 1. The periodic steering control signal may be a sine wave, for example, with an amplitude of 10 degrees and a frequency of 0.005 Hz, conveying a smooth steering start. For example, for a first quarter cycle and last quarter cycle of a sine wave used to provide the steering angle input signal, a smooth start and end may be achieved by doubling the frequency and halving the amplitude of such an input signal. The method 100 proceeds to step S106 once the steering shaft 25 rotates in accordance with the periodic steering control signal.

[0024] Step S106 proceeds in conjunction with step S104 and includes applying a calibrated axial force along the rack axis 17 via the linear actuator 49 and / or 149 of Fig. 1, as indicated by the double arrow F in Fig. 1. The value of the axial application force is proportional to the measured axial displacement of the rack 22 from the calibrated reference center rack position, for example, at 50 N / mm and in a direction resisting such displacement. That is, a "push / pull" action relative to the reference center position results in controlled operation of the linear actuator 49 and / or 149.

[0025] In one possible embodiment, the linear actuator 49 and / or 149 may include a linear variable differential transformer (LVDT), which, as known in the art, converts linear displacement into a corresponding electrical signal. An LVDT thus provides a rack displacement signal that can be used to calculate the axial application force, as described above, and allows the linear actuator 49 to act as a constant-rate spring. To improve performance, a special displacement sensor along axis 17 may be used to determine the displacement of the rack 22, which is then communicated to the controller 50 to determine the appropriate amount and direction of force to be applied (arrow F). The method 100 then proceeds to step S108.

[0026] In step S108, the torque sensor 44 measures the steering output torque (T 44) and communicates this measured value to the controller 50. The method 100 then proceeds to step S110.

[0027] Step S110 may include determining whether the test results from steps S102-S108 correspond to a pass / acceptable or fail / unacceptable steering system 20. In one example, the measured steering output torque (T 44 ) from step S108 to determine a difference between the absolute maximum value of the measured steering torque required to initiate rack displacement, ie points 68 of Fig. 2B, and a minimum value of the measured steering torque (point 69) that occurs after the "breakaway torque" of point 68, for startup and each subsequent steering direction reversal. This difference is compared to a calibrated threshold value indicative of the sliding friction condition. Generally, if the calculated torque difference, as described above, is, for example, 0.3 Nm or less, the sliding friction performance of the steering system 20 is considered acceptable. If the torque difference is above the calibrated limit, or 0.3 Nm in this embodiment, then the sliding friction performance of the steering system 20 is considered unacceptable. The method 100 proceeds to step S112 if, based on this comparison, the sample is unacceptable (-) and, in the alternative, to step S114 if the sample (+) is acceptable.

[0028] Step 112 may involve executing a control action with respect to the steering system 20 of the Fig. 1 if the preceding steps result in a decision that the steering system 20 has unacceptable performance. For example, the controller 50 may Fig. 1 record a first diagnostic code via an output signal, which is indicated by the arrow 14 in Fig. 1, if the steering output torque (T 44) exceeds the calibrated threshold mentioned above with reference to step S108, i.e., a torque difference indicating unacceptable sliding friction performance. Additional control steps may include applying the first diagnostic code to validate a given design of the manual steering gear 20, for example, by rejecting a given design during a design validation process when the diagnostic code is recorded or the creation of a design specification.

[0029] Step S114 is reached when the steering output torque (T 44) is at or below the calibrated threshold, or in other words, if no perceptible sliding friction condition is detected in steps S102 - S108. Step S114 may include recording a second diagnostic code (arrow 14) via the output signal from the controller 50 if the steering torque difference calculated based on the measured steering output torque (T 44 ) does not exceed the calibrated threshold mentioned above with reference to step S110. Since step S114 indicates a fully functional steering system 20, additional control steps may include applying the second diagnostic code to validate a given design of the steering system 20, for example, by accepting a given design during a validation process or the creation of a design specification.

[0030] As used herein with reference to the disclosed values ​​or ranges, the term "approximately" indicates that the disclosed numerical value allows for some inaccuracy, for example, reasonably close to or nearly as great as, for example, plus ± 10% of the stated values ​​or ranges. If the inaccuracy contemplated by the term "approximately" is not otherwise understood by those skilled in the art to have this ordinary meaning, then "approximately," as used herein, indicates at a minimum variations resulting from ordinary measurement techniques and the use of such parameters. Furthermore, the disclosure of ranges includes the disclosure of all values ​​of further subdivided ranges within the entire range.

Claims

[1] A method (100) for characterizing a sliding friction condition in an electrically assisted steering system (20) having a rack (22), a gear train (26) arranged at one end of a steering shaft (25) in meshing engagement with the rack (22), a steering motor (43), and a drive mechanism (129) electrically assisting a steering input to the steering system (20) via the steering motor (43), the method comprising: Connecting (S102) a torsion bar (54) and an inertia disc (52) to the steering shaft (25); Transmitting (S104) a periodic steering control signal from a controller (50) to a rotary actuator (42) while the steering motor (43) and the drive mechanism (129) assist the steering input, thereby causing rotation of the steering shaft (25) via the torsion bar (54) and the inertia disk (52) over a calibrated range of steering angles; Measuring (S106) a displacement of the rack (22) from a center position via a linear actuator (49, 149); Applying (S106) an axial force to the rack (22) that is proportional to the measured displacement in a direction that opposes the movement of the rack (22) while the periodic steering control signal is being transmitted; and Measuring (S108) a steering torque output value along the steering shaft (25) via a torque transducer (44) while the periodic steering control signal is being transmitted and the axial force is being applied; and Executing (S112) a control action with respect to the steering system (20) by the controller (50) when a difference between a maximum value of the measured steering torque and a minimum value of the measured steering torque, which occurs after the maximum value of the steering torque, at the start and at each reversal of the direction of the periodic steering control signal, exceeds a calibrated threshold value of approximately 0.3 Nm, which indicates the sliding friction condition. [2] The method (100) of claim 1, wherein transmitting (S104) the periodic steering control signal includes transmitting a sinusoidal steering control signal. [3] The method (100) of claim 1, wherein the linear actuator (49, 149) includes a linear variable differential transformer (LVDT), the method (100) further comprising controlling (S106) the applied axial force via the LVDT to thereby enable the linear actuator (49, 149) to act as a constant rate spring. [4] The method (100) of claim 1, wherein performing (S112) a control action includes recording a diagnostic code in the memory (M) of the controller (50). [5] The method (100) of claim 4, wherein performing (S112) a control action includes rejecting the steering system (20) in a design validation process when the diagnostic code is recorded. [6] System 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 gear transmission (26) disposed at one end of the steering shaft in meshing engagement with the rack (22), a steering motor (43), and a drive mechanism (129) that electrically assists a steering input to the steering system (20) via the steering motor (43); a torque transducer (44) operable to measure a steering output torque along a steering axis of the steering shaft (25); a torsion bar (54) connected to the rotary actuator (42) via the torque transducer (44); an inertia disc (52) connected to the torsion bar (54) and the steering shaft (25); a linear actuator (49, 149) operable to provide a calibrated transverse application force along the rack axis (17); and a controller (50) programmed to characterize a sliding friction condition of the steering system (20), wherein execution of instructions by the controller (50) while the steering motor (43) and the drive mechanism (129) assist the steering input causes the controller (50) to: transmitting a periodic steering control signal to the rotary actuator (42) to thereby cause rotation of the steering shaft (25) via the torsion bar (54) and the inertia disk (52) over a calibrated range of steering angles; to measure a displacement of the rack (22) from a central position via the linear actuator (49, 149); applying an axial force to the rack (22) proportional to the measured displacement in a direction opposing the movement of the rack (22) while transmitting the periodic steering control signal; and measuring a steering torque output value along the steering shaft (25) via the torque transducer (44) while the periodic steering control signal is being transmitted and the axial force is being applied; and to execute a control action with respect to the steering system (20) when a difference between a maximum value of the measured steering torque and a minimum value of the measured steering torque, which occurs after the maximum value of the steering torque, at the start and at each reversal of the periodic steering control signal exceeds a calibrated threshold value of approximately 0.3 Nm, which is indicative of the sliding friction condition. [7] The system of claim 6, wherein the controller (50) is programmed to transmit the periodic steering control signal as a sinusoidal steering control signal. [8] The system of claim 6, wherein the linear actuator (49, 149) includes a linear variable differential transformer (LVDT) operable to control the applied axial force across the LVDT, thereby enabling the linear actuator (49, 149) to act as a constant rate spring. [9] The system of claim 6, wherein the control action includes recording a diagnostic code in the memory (M) of the controller (50).

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