Friction-based state of health indicator for an electric power steering system
A method for determining a state of health (SOH) value for an electric power steering (EPS) system in a vehicle includes estimating a first Self-Aligning Torque (SAT) value using a tire dynamics model, which includes modeled dynamics in the linear region of a lateral force acting on the vehicle tires. The method also includes estimating a second SAT value using an extended state observer and nominal parameters for the EPS system, and calculating a variance between the first and second SAT values. The controller monitors a progression of the calculated variance over a calibrated interval using the controller to thereby determine the SOH value, and automatically executes a control action using the SOH value. An EPS system for a vehicle includes a steering wheel, torque and angle sensors, a rack and pinion assembly, a steering motor, and the controller. A vehicle is also disclosed having the same controller.
1. A method for determining a state of health (SOH) value for an electronic power steering (EPS) system in a vehicle having tires and a controller, the method comprising:
estimating a first Self-Aligning Torque (SAT) value using a tire dynamics model, wherein the tire dynamics model includes modeled dynamics in the linear region of a lateral force acting on the tires;
estimating a second SAT value using an extended state observer and nominal parameters for the EPS system;
calculating a variance between the first SAT value and the second SAT value;
monitoring a progression of the calculated variance over a calibrated interval using the controller to thereby determine the SOH value, wherein the SOH value is a numeric value in a range of between 0 and 1 that describes the progression of the calculated variance; and
automatically executing a control action using the SOH value.
2. The method of claim 1 , wherein the vehicle includes a steering motor and a rack and pinion system, and wherein the nominal parameters describe the dynamics of the motor and the rack and pinion system.
3. The method of claim 1 , wherein automatically executing a control action includes at least one of: recording a diagnostic code and displaying an icon or message within the vehicle.
4. The method of claim 1 , including displaying the icon or message only when the SOH value is less than a calibrated threshold.
5. The method of claim 1 , further comprising dividing the range of between 0 and 1 of SOH values into a plurality of bands, and executing the control action in a different manner for each of the bands relative to the other bands.
6. An electric power steering (EPS) system for a vehicle, comprising:
a steering wheel configured to receive a set of driver steering inputs, including a steering torque and a steering angle;
a torque sensor configured to measure the steering torque;
an angle sensor configured to measure the steering angle;
a rack and pinion assembly;
a steering motor operable for passing a variable motor assist torque to the rack and pinion assembly at a torque level which depends in part on the steering angle and the steering torque; and
a controller having an extended state observer;
wherein the controller is configured to:
estimate a first Self-Aligning Torque (SAT) value using a tire dynamics model, wherein the tire dynamics model includes modeled dynamics in the linear region of a lateral force acting on the tires;
estimate a second SAT value using an extended state observer and nominal parameters for the EPS system;
calculate a variance between the first SAT value and the second SAT value;
monitor a progression of the calculated variance over a calibrated interval using the controller to thereby determine the SOH value, wherein the SOH value is a numeric value in range of between 0 and 1 that describes the progression of the calculated variance; and
automatically execute a control action using the SOH value.
7. The EPS system of claim 6 , wherein the controller is configured to automatically execute the control action via at least one of: recording a diagnostic code and displaying an icon or message within the vehicle.
8. The EPS system of claim 6 , wherein the controller is further configured to divide the range of between 0 and 1 into a plurality of bands, and to execute the control action in a different manner for each of the bands relative to the other bands.
9. A vehicle comprising:
a steering wheel;
tires which are steerable in response to driver inputs received by the steering wheel;
an electric power steering (EPS) system having a gear set and a steering motor, wherein the EPS system is configured to provide a motor assist torque to the gear set to thereby assist steering of the tires; and
a controller configured to execute a set of instructions from a tangible/non-transitory memory to thereby:
estimate a first Self-Aligning Torque (SAT) value using a tire dynamics model, wherein the tire dynamics model includes modeled dynamics in the linear region of a lateral force acting on the tires;
estimate a second SAT value using an extended state observer and nominal parameters for the EPS system;
calculate a variance between the first SAT value and the second SAT value;
monitor a progression of the calculated variance over a calibrated interval using the controller to thereby determine the SOH value, wherein the SOH value is a numeric value in a range of between 0 and 1 that describes the progression of the calculated variance; and
automatically execute a control action using the SOH value.
10. The vehicle of claim 9 , wherein the gear set is a rack and pinion assembly.
11. The vehicle of claim 9 , wherein the controller automatically executes the control action by at least one of: recording a diagnostic code and displaying an icon or message within the vehicle.
12. The vehicle of claim 9 , wherein the controller is further configured to divide the range of between 0 and 1 into a plurality of bands, and to execute the control action in a different manner for each of the bands relative to the other bands.