Characterization of stick-slip condition in a steering system

1. A method for characterizing a stick-slip condition in an electrically-assisted steering system having a rack, a pinion gear disposed on an end of a steering shaft in meshed engagement with the rack, a steering motor, and a drive mechanism that electrically assists a steering input into the steering system via the steering motor, the method comprising:

connecting a torsion bar and an inertia wheel to the steering shaft;

transmitting a periodic steering control signal from a controller to a rotary actuator, while the steering motor and the drive mechanism assist the steering input, to thereby cause rotation of the steering shaft via the torsion bar and inertia wheel through a calibrated range of steering angles;

measuring a displacement of the rack from a center position via a linear actuator;

applying an axial force to the rack that is proportional to the measured displacement and in a direction resisting motion of the rack while transmitting the periodic steering control signal;

measuring a steering torque output value along the steering shaft via a torque transducer while transmitting the periodic steering control signal and applying the axial force; and

executing a control action with respect to the steering system, via the controller, when a difference between a maximum value of the measured steering torque and a minimum value of the measured steering torque occurring after the maximum value of the steering torque, at start-up and at each reversal of direction of the periodic steering control signal, exceeds a calibrated threshold value indicative of the stick-slip condition.

2. The method of claim 1 , wherein transmitting the periodic steering control signal includes transmitting a sinusoidal steering control signal.

3. The method of claim 1 , wherein the linear actuator includes a linear variable differential transformer (LVDT), the method further comprising controlling the applied axial force via the LVDT to thereby allow the linear actuator to act as a constant-rate spring.

4. The method of claim 1 , wherein executing a control action includes recording a diagnostic code in memory of the controller.

5. The method of claim 4 , wherein executing a control action includes rejecting the steering system in a design validation process when the diagnostic code is recorded.

6. The method of claim 1 , wherein the calibrated threshold value indicative of the stick-slip condition is 0.3 Nm.

7. A system comprising:

a rotary actuator;

an electrically-assisted steering system having a steering shaft, a rack having a rack axis, a pinion gear disposed on an end of the steering shaft in meshed engagement with the rack, a steering motor, and a drive mechanism that electrically assists a steering input into the steering system via the steering motor;

a torque transducer operable for measuring a steering output torque along a steering axis of the steering shaft;

a torsion bar connected to the rotary actuator via the torque transducer;

an inertia wheel connected to the torsion bar and to the steering shaft;

a linear actuator operable for providing a calibrated axial apply force along the rack axis; and

a controller programmed to characterize a stick-slip condition of the steering system, wherein execution of instructions by the controller while the steering motor and the drive mechanism assist the steering input causes the controller to:

transmit a periodic steering control signal from a controller to the rotary actuator to thereby cause rotation of the steering shaft, via the torsion bar and inertia wheel, through a calibrated range of steering angles;

measure a displacement of the rack from a center position via a the linear actuator;

apply an axial force to the rack that is proportional to the measured displacement and in a direction resisting motion of the rack while transmitting the periodic steering control signal;

measure a steering torque output value along the steering shaft via a torque transducer while transmitting the periodic steering control signal and applying the axial force; and

execute a control action with respect to the steering system, via the controller, when a difference between a maximum value of the measured steering torque and a minimum value of the measured steering torque occurring after the maximum value of the steering torque, for start-up and each reversal of the periodic steering control signal, exceeds a calibrated threshold value indicative of the stick-slip condition.

8. The system of claim 7 , wherein the controller is programmed to transmit the periodic steering control signal as a sinusoidal steering control signal.

9. The system of claim 7 , wherein the linear actuator includes a linear variable differential transformer (LVDT) operable for controlling the applied axial force via the LVDT to thereby allow the linear actuator to act as a constant-rate spring.

10. The system of claim 7 , wherein the control action includes recording a diagnostic code in memory of the controller.

11. The system of claim 10 , wherein the control action includes rejecting the steering system in a design validation process when the diagnostic code is recorded.

12. The system of claim 7 , wherein the calibrated threshold value indicative of the stick-slip condition is 0.3 Nm.