GB2454788A

GB2454788A - Active nibble control for electric steering

Info

Publication number: GB2454788A
Application number: GB0820691A
Authority: GB
Inventors: Darrel Recker; Joseph Mikhael Raad
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2007-11-14
Filing date: 2008-11-12
Publication date: 2009-05-20
Anticipated expiration: 2028-11-12
Also published as: GB0820691D0; GB2454788B

Abstract

A system and method for actively cancelling steering nibble in an electric power steering system 10 relying on the steps of detecting steering wheel nibble; calculating an active nibble cancelling torque based on the detected steering wheel 12 nibble signal; and using an electric motor in the electric power steering system as an actuator to apply nibble cancelling torque thereby eliminating nibble. An algorithm is provided for implementing the invention. Frequencies for cancellation are selected. Digital signal filtering is employed by a controller 50. A steering wheel vibration cancelling torque signal is added to an assist torque signal provided to the electric power steering system 10, thereby eliminating steering wheel vibration.

Description

ACTIVE STEERING NIBBLE CONTROL ALGORITHM

FOR ELECTRIC STEERNG SYSTEMS

Field of the invention

The invention relates to motor vehicle electric steering systems and more particularly to an algorithm for actively controlling steering nibble in an electric steering system.

Background of the invention

Steering nibble is the undesirable rotational vibration experienced by the driver at the steering wheel mainly during straight line driving. In some vehicles steering wheel nibble is the result of the chassis system responding to the tire and wheel force variations which eventually feed back in the form of slight rotations in the steering system. In many vehicles, steering nibble is caused by the presence of a front road wheel imbalance or front tire force variation. This steering wheel vibration occurs at the frequency of rotation of the front road wheels. The vibrations magnitude is maximized when these frequencies align with the steering/suspension resonant frequency, typically 12 to 20 Hz.

Brake judder is due to a chain of events, at the beginning of which there is unequal wear of the brake disks which leads to thickness variation of the brake disk. This disk thickness variation produces a harmonic modulation of the braking force during braking. The oscillations of the braking force in turn excite different modes of the wheel suspension, the vibrations being transmitted via a kinematic coupling to the steering system and in particular to the steering rod, if they lie in a specific critical frequency range, are in turn transmitted to the steering wheel and excite an oscillation of the steering gear, of the steering column and the steering wheel. For brake judder the vibration occurs at a frequency of once or twice times the rotational velocities of the front road wheels.

Steering wheel nibble and brake judder are customer concerns in many production automobiles. Original equipment manufacturers and their suppliers are investigating chassis modifications to address and reduce nibble and judder.

However, these modifications often have negative effects on other vehicle characteristics, and are typically expensive to implement.

There is a need to detect and actively control steering nibble and brake judder in an electric steering system without affecting steering feel, with the aim of reducing steering nibble related warranty costs.

Summary of the invention

In accordance with one aspect of the invention, there is provided a method for actively cancelling steering nibble in an electric power steering system which comprises the steps of detecting steering wheel nibble; calculating an active nibble cancelling torque based on the detected steering wheel nibble signal; and using an electric motor in the electric power steering system as an actuator to apply nibble cancelling torque thereby eliminating nibble.

In a preferred embodiment, the invention provides a method for actively cancelling steering nibble in an electric power steering system using a controller and an electric motor, which method comprises the steps of converting a selected wheel speed to a frequency; selecting a nibble order; determining nibble enable frequencies for cancellation; selecting a damping factor based on changes in vehicle velocity; calculating filter coefficients based on the selected damping factor, the selected nibble order and the wheel frequency; applying a gain scheduler to a steering column torque signal for the nibble enable frequencies; applying a digital filter to an output of the gain scheduler and using the filter coefficients to produce a nibble signal; calculating an active nibble cancelling torque signal from the nibble signal; and applying the active nibble cancelling torque signal to the electric motor to cancel steering wheel nibble vibration.

In accordance with a second aspect of the invention, there is provided a system for actively cancelling steering wheel vibrations in an electric power steering system comprising a steering wheel in the electric power steering system; an electric motor for assisting steering of the steering wheel; a controller for controlling the electric motor and receiving a steering torque signal from the electric power steering system and providing an assist torque signal to the electric power steering system; a digital filter realized in the controller for detecting steering wheel vibration and calculating a steering wheel vibration torque cancelling signal; and means for adding the steering wheel vibration torque cancelling signal to the assist torque signal provided to the electric power steering system thereby eliminating steering wheel vibration.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an electrical power steering system; Figure 2 is an electrical power steering system; Figure 3 is a block diagram of torques represented in a typical electric power steering system; Figure 4 is a block diagram of an embodiment of an electric power steering system of the invention; Figure 5 is a block diagram of Front Wheel Frequency calculation; Figure 6 is a Bode plot diagram for the tuned resonator representing gain and phase as a function of front wheel frequencies and having a discrete damping factor, R=O.98 and a resonant frequency of 16 Hz; Figure 7 is a Bode plot diagram for the tuned resonator representing gain and phase as a function of front wheel frequencies and having a discrete damping factor, R=0.99 and a resonant frequency of 16 Hz; Figure 8 is a block diagram of the calculation of steering nibble cancelling torque of the invention; Figure 9 is a block diagram representing the determination of nibble order according to the invention; and Figure 10 is a block diagram of the invention applied to a particular nibble order.

Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve understanding of embodiments of the present invention.

Detailed description of the preferred embodiment(s) Referring to Fig. 1, an electrically assisted power steering system 10 includes a steering wheel 12 attached to a first end 14 of a steering shaft 16. A steering pinion gear 18, attached to a second end 20 of the steering shaft 16 opposite the first end 14, engages a steering rack gear 22 of a steering rack 24. Each end of the steering rack 24 includes a tie rod 26 attached to a steerable wheel and tire assembly 28 in a conventional manner. A steering torque sensor 30 is incorporated in the steering shaft 16 for detecting a steering torque applied by an operator to the steering shaft 16 by way of the steering wheel 12. A steering wheel angle sensor 40 senses a steering wheel angle. An electric motor 32 includes an output gear 34 mounted on an output shaft 36 for drivingly engaging an assist input gear 38 mounted on the steering shaft 16. Alternatively shown in Fig. 2, the electric motor 32' may have its output shaft 36' and an output gear 34' arranged to directly engage the steering rack 24'. A controller 50 receives signals representative of the torque of the steering shaft 16 between sensors 30 and 40.

In either Figs. 1 or 2, the electric motor may be a DC brush or brushless motor. It may utilize a three-phase alternating current induction motor. It should be noted that a variable reluctance motor may be substituted for the inductance motor without impacting the performance of the invention. Induction and variable reluctance motors are typically used in electrically assisted power steering systems because of their low friction and high torque-to-inertia ratio compared to larger electric motors.

Fig. 3 is a block diagram of a typical electric power steering system 60. In a typical electric power steering system 60, vehicle speed, V-, and steering column torque, T:.jin, signals are used, along with boost curves a 62 to determine the amount of assist torque, Ti--;--required to aid the driver in steering the vehicle. The assist torque, T1--1-, is carried out by the electric motor, (see Figs. 1 and 2).

The described embodiment utilizes the electric power steering system as an actuator to cancel steering nibble and brake judder actively by creating a digitally realized tuned resonator at the vehicle speed dependent nibble and/or brake judder frequencies. The output of the tuned resonator is fed back to the controller (see Figs. 1 or 2) as an additional assist force to the EPS rack to cancel the vibrations before they reach the steering wheel.

It is known that steering nibble is a vibration that occurs at 1X the rotational velocities of the front road wheels and brake judder occurs at lX or 2X the rotational velocities. The vibration is most prevalent when these frequencies align with the steering/suspension resonant frequency, typically 10 to 15 Hz. The preferred embodiment of the invention provides a very narrow rejection of frequencies using a software generated tuned resonator that dynamically adapts the frequency of the tuned resonator with front wheel speeds. If front wheel speeds are unavailable, the invention utilizes the vehicle speed. The precise tuning of the resonator provides the benefit of targeting the specific frequency to be rejected without generating a disturbance to other frequencies.

Referring to Fig. 4, a block diagram of an embodiment of an electric power steering system 70 of the invention is shown. An active nibble control (ANC) algorithm 72 detects and identifies the magnitude of steering nibble and adds a nibble cancelling torque 74 (T-.), at the electric motor of the steering system. The ANC algorithm calculates front wheel frequencies, calculates tuned resonator coefficients and steering nibble signal, and calculates the steering nibble cancelling torque.

To calculate the front wheel frequencies the ANC algorithm uses one or both front wheel speed signals available from the vehicles central vehicle communication controller.

Fig. 5 is a block diagram of the calculation 80 of front wheel frequencies. One of the front wheel speed signals, such as the one having the most noise associated with steering nibble frequency, is selected 82. In the alternative, the average of both front wheel speeds is used. In the absence of wheel speed signals, the vehicle speed signal may be used. The speed signals are filtered 84 to reject high frequency noise and are compensated for any lags associated with filtering. The resultant filtered speed is used to calculate 86 front wheel frequencies, Wi,, by using a vehicle tire conversion factor from wheel speed to frequency.

The ANC algorithm implements a software generated tuned resonator filter applied to the column torque signal, T.:jiinr to produce a steering nibble signal SN(z).

SN(z) = Nz2 + Niz + N2. T(Z)--llflr, (1) Dz' + D1z + D-Where; N = (l-R); N1 = 0; N2 = (R-l); D = 1; D1 = -2Rcose; D2 = R2 R = Discrete decay factor; T-= Discrete sampling time in seconds; = Natural frequency in rad/sec.; e = Discrete time resonance frequency (Rad) = cT:.

The tuned resonator filter coefficients Ni and D are fixed. Coefficients N, N2, and D are a function of the discrete decay factor, R. The discrete decay factor, R, is increased very close to, without reaching, 1 to ensure maximum frequency rejection. The discrete decay factor, R, is lowered below 1 to speed up transient build up as W, varies. A good compromise for the discrete decay factor, or damping factor, R, is 0.985. In the event a compromise cannot be reached with satisfactory results, R adapts with vehicle acceleration. B. can be set close to 1 when the vehicle is being driven at near constant speeds, and lowered when the vehicle's speed is changing. The filter coefficient, D1, is the only term that needs to be dynamically adapted for varying front wheel frequency, Wr,. This ensures the selection of the only possible nibble frequency signal for a given front wheel frequency, Wi,, with strong rejection of neighbouring frequencies.

The calculated steering nibble signal identifies the instantaneous steering nibble torque magnitude, frequency, and phase experienced by a driver at the steering wheel. This signal has a 00 phase lag and gain of 1 at the front wheel frequency, Wi,, and is rapidly attenuated for frequencies away from W1.

Fig. 6 and Fig. 7 are Bode plot diagrams for the tuned resonator and represent gain and phase as a function of front wheel frequencies with a nibble frequency of 16 Hz. The discrete damping factor R = 0.98 is shown in Fig. 6 and the discrete damping factor R = 0.99 is shown in Fig. 7.

The ANC algorithm calculates 90 a steering nibble cancelling torque, T.-, as shown in the block diagram in Fig. 8. The steering column torque, T-iiinr,, is scaled by a gain scheduler 92 to select only frequencies within a predetermined steering nibble frequency range. This range is typically between 12 and 20 Hz. A tuned resonator filter 94 is applied to the modified column torque to construct a steering nibble signal, using Equation (1). The resultant signal is a sine wave whose amplitude, frequency, and phase completely match what the driver is experiencing as nibble at the steering wheel. The nibble signal is multiplied by a nibble control gain 96 to produce a steering nibble cancelling torque, T-.

The nibble cancelling torque, T-, is added to the assist torque, T--l-r, and is then carried out by the electric motor in the steering system.

In another embodiment, the tuned resonator filter is independently applied to each front wheel frequency. Both filters are applied to the steering column torque signal, T:-lIlfl!,, and generates two steering nibble signals. The two results are combined, and used to calculate the nibble cancelling torque, Tj1. Separate filters will cancel separate wheel imbalances, i.e., imbalance on right and left wheels, the imbalances will be cancelled together. Another advantage to independent filters is that because they are independent, they can compensate for wheel speed differences.

It should be noted that steering nibble is typically a first order phenomenon. However, second order may be present with or without braking. First order nibble happens at 1X the frequency of wheel speed. Second order nibble happens at 2X the frequency, and can be present with or without braking. The preferred embodiment of the invention has independent gain schedulers that address the first and second order phenomenon.

A threshold defines the selection of the order. For example, over the predetermined threshold, the first order is selected and below the threshold, the second order is selected. In this way, the gain for each order can be independently addressed.

-10 -Brake judder is another Source of steering wheel vibrations during vehicle braking and is a first order and a second order phenomenon. Judder is caused by thickness variations or front brake disc warping. Judder vibration occurs at a frequency of once or twice the rotational velocities of the front road wheels. The tuned resonator filter is implemented to cancel brake judder by dynamically adapting to a factor of the frequency of the front wheel's frequency. The judder signal, a second order nibble, occurs at once the frequency of the wheel speed, or twice the frequency of the wheel speed, which happens at half the speed of the first order nibble, described above. In this embodiment a second tuned resonator is also applied, however, the second filter is applied to the second order. Therefore, one tuned resonator filter is applied to the first order nibble and another tuned resonator filter is applied to the second order nibble, also called brake judder.

To address both first and second order nibble, an active nibble control algorithm is presented in Figs. 9 and 10. Fig. 9 is a block diagram representing the determination of nibble order 100. A vehicle speed signal and wheel speed signals 102 are provided to the controller. A wheel speed 104 is selected.

The selected wheel speed is filtered 106 and converted to a frequency 108. The nibble order is selected 110 and provided as a nibble order signal 112.

Fig. 10 is a block diagram of an embodiment of the invention applied to a particular nibble order; first order, second order, or first and second order. The nibble order 112 is used to select enable frequencies 114 of first or second order phenomenon. A damping factor is selected 118 based on vehicle speed. For a constant vehicle speed, the damping factor, R, is close to one. For variable speed, the damping -11 -factor is less than one. The nibble order 112 is also used, along with wheel frequency and selected damping factor, to calculate filter coefficients 116. The nibble enable frequencies and steering column torque signal, T:jint,, are subject to the gain scheduler 92. The output of the gain scheduler 92 and the filter coefficients 116 are subject to application of the tuned resonator filter 94 and produce a nibble signal. The nibble signal is multiplied by a nibble control gain 96 to calculate the active nibble cancelling torque, Tir-.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments.

Various modifications and changes may be made, however, without departing from the scope of the present invention as set forth in the claims. The specification and figures are illustrative, rather than restrictive, and modifications are intended to be included within the scope of the present invention. Accordingly, the scope of the invention should be determined by the claims and their legal equivalents rather than by merely the examples described.

For example, the steps recited in any method or process claims may be executed in any order and are not limited to the specific order presented in the claims. Additionally, the components and/or elements recited in any apparatus claims may be assembled or otherwise operationally configured in a variety of permutations and are accordingly not limited to the specific configuration recited in the claims.

Benefits, other advantages and solutions to problems have been described above with regard to particular embodiments; however, any benefit, advantage, solution to problem or any element that may cause any particular benefit, advantage or -12 -solution to occur or to become more pronounced are not to be construed as critical, required or essential features or components of any or all the claims.

Claims

-13 - CLAIMS

1. A method for actively cancelling steering nibble in an electric power steering system comprising the steps of: detecting steering wheel nibble; calculating an active nibble cancelling torque based on the detected steering wheel nibble signal; and using an electric motor in the electric power steering system as an actuator to apply nibble cancelling torque thereby eliminating nibble.

2. A method as claimed in claim 1 wherein the step of detecting steering wheel nibble further comprises: creating a tuned resonator filter that is speed dependent wherein the speed is a vehicle speed, a front wheel speed, or both; determining a nibble order for the detected steering wheel nibble, the nibble order being a speed dependent nibble frequency; and selecting a damping factor for the detected steering wheel nibble.

3. A method as claimed in claim 2 wherein the nibble order is a first order nibble frequency or a second order nibble frequency.

4. A method as claimed in claim 2 wherein the nibble order is a first order and a second order nibble frequency.

5. A method as claimed in claim 2 wherein the step of calculating an active nibble cancelling torque further comprises multiplying the detected steering wheel nibble by a nibble control gain.

-14 -

6. A method as claimed in any preceding claim wherein the step of detecting a steering wheel nibble is independently applied to each front wheel frequency.

7. A method as claimed in claim 6 further comprising the steps of: detecting two steering nibble signals, one for each front wheel frequency; and calculating the active nibble cancelling torque by combining both steering nibble signals.

8. A system for actively cancelling steering wheel vibrations in an electric power steering system comprising: a steering wheel in the electric power steering system; an electric motor for assisting steering of the steering wheel; a controller for controlling the electric motor and receiving a steering torque signal from the electric power steering system and providing an assist torque signal to the electric power steering system; a digital filter realized in the controller for detecting steering wheel vibration and calculating a steering wheel vibration torque cancelling signal; and means for adding the steering wheel vibration torque cancelling signal to the assist torque signal provided to the electric power steering system thereby eliminating steering wheel vibration.

9. A system as claimed in claim 8 wherein the digital filter is a tuned resonator filter that is speed dependent, the tuned resonator filter uses a damping factor, a nibble order and a wheel frequency to calculate the steering wheel vibration torque cancelling signal.

-15 -

10. A system as claimed in claim 8 wherein the steering wheel vibration is either a first order frequency that is one multiple of a road wheel frequency or a second order frequency that is two times the road wheel frequency.

11. A system as claimed in claim 8 wherein the steering wheel vibration has components that are a first order frequency that is one multiple of a road wheel frequency and a second order frequency that is two times the road wheel frequency.

12. A system as claimed in claim 8 wherein a steering wheel vibration torque cancelling signal further comprises: a steering nibble torque signal identified for a first front wheel frequency; a steering nibble torque signal identified for a second front wheel frequency; and the torque cancelling signal is calculated based on a combination of the steering nibble torque signals for the first and second front wheel frequencies.

13. A method for actively cancelling steering nibble in an electric power steering system using a controller and an electric motor, the method comprising the steps of: converting a selected wheel speed to a frequency; selecting a nibble order; determining nibble enable frequencies for cancellation; selecting a damping factor based on changes in vehicle velocity; calculating filter coefficients based on the selected damping factor, the selected nibble order and the wheel frequency; applying a gain scheduler to a steering column torque signal for the nibble enable frequencies; -16 -applying a digital filter to an output of the gain scheduler and using the filter coefficients to produce a nibble signal; calculating an active nibble cancelling torque signal from the nibble signal; and applying the active nibble cancelling torque signal to the electric motor to cancel steering wheel nibble vibration.

14. A method as claimed in claim 13 further comprising the steps of: detecting a first nibble signal for a first front wheel frequency; detecting a second nibble signal for a second front wheel frequency; and calculating an active nibble cancelling torque signal from a combination of the first and second nibble signals.

15. A method as claimed in claim 13 wherein the step of selecting a nibble order further comprises selecting a first order nibble frequency.

16. A method as claimed in claim 13 wherein the step of selecting a nibble order further comprises selecting a second order nibble frequency.

17. A method as claimed in claim 13 wherein the step of selecting a nibble order further comprises: selecting a first order nibble frequency; and selecting a second order nibble frequency.

18. A method as claimed in claim 13 wherein the step of applying a digital filter further comprises the step of dynamically adapting the digital filter as a tuned resonator filter to the front wheel frequencies.

-17 -

19. A method as claimed in claim 13 wherein the step of calculating an active nibble cancelling torque signal further comprises applying a 1800 out-of-phase cancelling torque in the electric motor of the electric power steering system.

20. A system for actively cancelling steering wheel vibrations in an electric power steering system constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.