Active disturbance rejection for electrical power steering system

Abstract

A method of controlling disturbances associated with electric power steering (EPS) systems maintains an original assist torque to feedback signal in the EPS, such as a column torque, and further minimizes the impact from the disturbance source to the feedback signal so that the disturbance is rejected while the original steering feel is maintained. The method further considers interaction of the rejection feature with other functions of the EPS. In one embodiment, relationships for isolating the disturbance are achieved by utilizing a combined feedback and feed-forward compensator.

Claims

1. A method of isolating a disturbance input (T.sub.disturbance(s)) in an electric power steering system, the system including a steering wheel for receiving a driver-applied torque for steering a vehicle, an electric motor coupled to supply a motor assist torque (T.sub.assist(s)) to the system to reduce driver steering input effort, and a vehicle component generating the disturbance input to the electric power steering system, the method including the steps of: (a) measuring a steering column torque, (T.sub.col(s)), and (b) isolating the disturbance input from the steering wheel by following the relationships: $\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{assist}}\left(s\right)}\approx p\left(s\right)$ such that the steering system dynamics from T.sub.assist to T.sub.col is controlled to maintain the original steering feel, where p(s) is the nominal system model from T.sub.assist to T.sub.col within a predetermined frequency range; and $\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{disturbance}}\left(s\right)}\approx 0$ such that T.sub.disturbance does not have an impact on T.sub.col, which isolates the disturbance from the driver, thus rejecting the disturbance.

2. The method of claim 1 wherein the relationships are achieved by utilizing a combined feedback and feed-forward compensator.

3. The method of claim 2 wherein the feedback compensator is defined by the equation C.sub.feedback(s)=H(s)P.sub.n.sup.−1 (s) and the feed-forward compensator is defined by the equation $C_{\mathrm{feedforward}}\left(s\right)=\frac{1}{1-H\left(s\right)},$ where H(s) is a filter and P.sub.n(s) is a model approximation.

4. The method of claim 3 wherein a choice of the filter H(s) is based upon characteristics of at least one of a disturbance frequency and a noise requirement, and wherein the filter may be one or more of a Low-pass filter, a High-pass filter, a Band-pass filter, a Selective filter, and a combination of one or more of these filter by either repeating the filter structure or designing multiple filters H(s) to span the specific disturbance frequency ranges.

5. The method of claim 4 wherein a design of the filter H(s) can be performed by one of an off-line and an on-line process by adjusting parameters of the filter to meet criteria for achieving a desired system performance.

6. The method of claim 5 wherein parameters of filter H(s) can be further designed by using one or more of a vehicle speed, a wheel speed, a column velocity and a column angle.

7. The method of claim 3 wherein a design of the model approximation P.sub.n(s) can be performed by one of an off-line and an on-line process by adjusting parameters of the filter to meet criteria for achieving a desired system performance.

8. The method of claim 7 wherein parameters of P.sub.n(s) can be further can be further designed by using one or more of a vehicle speed, a wheel speed, a column velocity and a column angle.

9. The method of claim 3 wherein filter H(s) is configured as a band pass filter such that a noise frequency is removed.

10. The method of claim 1 wherein step (b) can be selectively enable and disabled by the use of at least one of a hysteresis and a ramping function based on at least one of vehicle speed, a torque sensor value and a dialogistic algorithm to verify its effectiveness in order to avoid erroneous compensation.

11. The method of claim 9 wherein when step (b) is disabled, a control strategy is applied, the control strategy being one of (1) a switch to second disturbance compensation strategy that might still work under current vehicle conditions, (2) a hold the last control action before active disturbance rejection is totally disabled, and (3) a total disabling of the active disturbance rejection torque.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a schematic illustration of a prior art disturbance cancellation system.

(2) FIG. 1B is a schematic illustration of another prior art disturbance cancellation system.

(3) FIG. 2 is a schematic illustration of a disturbance cancellation system in accordance with the invention.

(4) FIG. 3 is a schematic illustration of an alternative embodiment of a disturbance cancellation system in accordance with the invention.

(5) FIG. 4A is a graph of Column Torque, T.sub.col having a disturbance forcing function.

(6) FIG. 4B is a graph of Column Torque, T.sub.col of FIG. 4A after correction by active disturbance rejection.

(7) FIG. 4C is an overlay comparison of the states of Column Torque, T.sub.col shown in FIGS. 4A and 4B.

(8) FIG. 5 is a time dependent plot of the difference between compensated column torque and original column torque in accordance with the invention.

(9) FIG. 6A is a graph of Column Torque, T.sub.col, similar to FIG. 4A, having a different disturbance forcing function.

(10) FIG. 6B is a graph of Column Torque, T.sub.col of FIG. 6A after correction by active disturbance rejection.

(11) FIG. 6C is an overlay comparison of the states of Column Torque, T.sub.col shown in FIGS. 6A and 6B.

(12) FIG. 7 is a flow chart showing an embodiment of an EPS disturbance rejection system having an operational mode algorithm.

(13) FIG. 8 is a flow chart of an embodiment of an EPS having an active disturbance rejection system as an input to a lateral support module.

(14) FIG. 9 is a flow chart of an EPS system algorithm configured to monitor the effectiveness of active disturbance rejection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(15) Referring now to FIG. 2, there is illustrated an EPS disturbance rejection system having an alternative control structure and associated systematic design approach to reject T.sub.disturbance through a combined feedback compensator and feed-forward compensator design. The EPS system is configured to isolate a disturbance such that it does not affect the column torque, T.sub.col, rather than merely detecting the disturbance and then generating a compensating torque. The EPS system is configured as a control system that provides disturbance isolation by using a feed-forward compensator block. The feed-forward compensator provides an additional degree of freedom which allows the various transfer functions inputted to the column torque to be shaped or otherwise altered, for example, by modulation or attenuation. A feedback compensator allows the disturbance observed in the column torque to provide suitable feedback information. The feed-forward compensator is configured to shape the control loop such that there is a high sensitivity between the original assistance torque and the column torque and very low sensitivity between the disturbance force and the column torque.

(16) In one embodiment, the structure of the feedback and feed-forward compensator is configured to meet following criteria:

(17) $\begin{array}{cc}\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{assist}}\left(s\right)}\approx p\left(s\right)\mathrm{and}& \left(1\right)\\ \frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{disturbance}}\left(s\right)}\approx 0& \left(2\right)\end{array}$
In one aspect of this embodiment, there is provided:

(18) $\begin{array}{cc}{C}_{\mathrm{feedback}}\left(s\right)=H\left(s\right)P_n^{-1}\left(s\right)\mathrm{and}& \left(3\right)\\ C_{\mathrm{feedforward}}\left(s\right)=\frac{1}{1-H\left(s\right)}& \left(4\right)\end{array}$
By utilizing this aspect, the transfer function from T.sub.assist to T.sub.col may be described as:

(19) $\begin{array}{cc}\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{assist}}\left(s\right)}=\frac{p\left(s\right)p_n\left(s\right)}{H\left(s\right)\left[p\left(s\right)-p_n\left(s\right)\right]+p_n\left(s\right)}& \left(5\right)\end{array}$
The transfer function from T.sub.disturbance to T.sub.col is:

(20) $\begin{array}{cc}\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{disturbance}}\left(s\right)}=\frac{p_n\left(s\right)p\left(s\right)\left[1-H\left(s\right)\right]}{H\left(s\right)\left[p\left(s\right)-p_n\left(s\right)\right]+p_n\left(s\right)}& \left(6\right)\end{array}$

(21) Regarding equation (1) above, p(s) is the nominal system model from assistance torque to measurable state (column torque in this example) within an interested frequency range. Equation (1) states that the steering system dynamics from original assist torque to column torque is maintained. This maintains the original steering feel. Equation (2) states that the disturbance input does not have impact on column torque, this isolates the disturbance from the driver. With respect to equations (3) and (4), H(s) is a filter that allows the interested disturbance forcing function to pass through. This means that H(s)≈1 for certain frequency ranges that also contain frequency ranges of the disturbance. The type of filter characterized by H(s) can be very flexible depending on characteristics of disturbance to be rejected. Non-limiting examples of such filters include, but are not limited to, (1) Low-pass filters; (2) High-pass filters; (3) Band-pass filters; (4) Selective filters, or combinations of one or more of these filters. The filter combinations may be formed by either repeating the structure or designing multiple H(s) filters to span the specific disturbance frequency ranges. In one embodiment, the input parameters may be one or more vehicle states or combinations thereof, such as for example (1) Vehicle speed; (2) Wheel speed; (3) Column velocity; and (4) Column Angle. Referring to equations (5) and (6), p.sub.n(s) is a model approximation designed such that p(s)≈p.sub.n(s) within interested frequency range. Design of p.sub.n(s) can be realized by many techniques such as cascading the original plant with a lead-lag compensator to have unit gain and zero phase within interested frequency range. Alternatively, p.sub.n(s) can be simply obtained via an optimization method to match a proper, stable p.sub.n(s) to p(s) within the interested frequency range.

(22) Alternatively, the developed filter H(s) can be shared for both feedback and feed-forward loop configurations leading to a more simplified control architecture, shown in FIG. 3. Column torque information is sensed and computed using the model information block to obtain road disturbance information. The output of this computation is subtracted (or added depending on system polarity) from the total assist torque demand, which is a combination of original assist torque T.sub.assist and the filtered disturbance information from H(s). By doing this subtraction (or addition depending on system polarity) and passing original T.sub.assist information to filter H(s), effectively, steering feel issues are compensated for, due to the fact that filter H(s) will also filter out the frequency component in original assist torque T.sub.assist. This architecture may also be used with other existing methodologies where a developed ADR algorithm will affect steering feel.

(23) Furthermore, implementation of p.sub.n(s) can also depend on various vehicle conditions or driver interactions. For example, dynamics of the EPS might change depending on a particular operating mode, such as hands-on or hands-off. Ideally, compliance compensation takes such scenarios into considerations. Features that have hands-off detection capability can provide input to ADR module to indicate vehicle running conditions. For example, some embodiments of the EPS ADR system may include Lane Keeping Assist (LKA) and Lane Centering Assist (LCA) detection capabilities that provide additional assistive torque inputs to the EPS in response to particular sensor input scenarios. LKA provides a counter-steering torque to provide a driver assistive input that alerts a driver to adjust the vehicle trajectory to help guide a driver back to the center of a lane. LCA provides a torque input to the steering system in order to support maintaining the vehicle in the center of the lane. In addition to LKA/LCA, standard Hands-off detection modules may also provide operational input to the EPS. One embodiment of such a design is shown in FIG. 7, where hands on/off information may be provided by LKA/LCA module to indicate different compliance compensation required by the ADR system. Similarly, ADR may also provide necessary feedback to other features to indicate necessary consideration of such compensation as shown in FIG. 8.

(24) Assuming that p(s)≈p.sub.n(s) and H(s)≈1 are within the interested frequency range, then:

(25) $\begin{array}{cc}\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{assist}}\left(s\right)}\approx p\left(s\right)\mathrm{and}\frac{T_{\mathrm{col}}\left(s\right)}{T_{\mathrm{disturbance}}\left(s\right)}\approx 0& \left(7\right)\end{array}$

(26) This demonstrates that the proposed control structure can decouple a disturbance from the original assist torque such that the disturbance can be theoretically rejected 100% without modifying prior calibrations. Therefore, both disturbance rejection and minimal effect of steering feel are simultaneously achieved.

(27) The advantages of the EPS ADR system and method are that the filter, H(s) and plant approximation p.sub.n(s) can be tailored to a desired frequency or frequency range depending upon the disturbance characteristics. In addition, multiple disturbances sources i.e. brake pulsation, wheel imbalance, motor ripple, etc can be rejected by either repeating the structure or designing the H(s) and p.sub.n(s) to span the disturbance frequency range. Finally, the system flexibility enables simplification of plant model approximation and filter design if desired for real-time implementation.

(28) In an embodiment of a design method, the following steps may be used to develop the EPS ADR system. In a first step, a nominal transfer function for the EPS system is identified. Then, an interested frequency range, ω.sub.low≦ω≦ω.sub.high is determined based on different platforms and applications. In a brake pulsation compensation application, for example, co can be a function of wheel rotational frequency or position or velocity or combination of those. This may be calculated from, for example, wheel speed, vehicle speed, tone wheel pulses or received from a signal via CAN. Then, within the interested frequency range, a plant model P.sub.n(s), is designed such that P.sub.n.sup.−1(s)P(s)≈1. Design of P.sub.n(s) can be realized by many techniques such as, for example, cascading the original plant with a lead-lag compensator to have unit gain and zero phase within an interested frequency range. Alternatively, the plant model may be simply obtained via an optimization method to match a proper, stable P.sub.n(s) to P(s) within the interested frequency range. It is also possible to compute P.sub.n(s) on-line via measurement of other EPS states such as column position, column torque, column velocity, motor position, etc. As such, the method will compensate model changes due to wear or changes of vehicle conditions. Then, a filter H(s) is designed that matches the properties of the disturbance to be rejected, i.e. a low-pass filter, high-pass filter or band-pass filter or a highly selective filter. The interested frequency may be a function of wheel speed, vehicle operation conditions, etc. Alternatively, the interested frequency range may be designed off-line based on analyses of frequency property or statistics property.

(29) The features of the ADR system disclosed by the various embodiments described herein can be enabled or disabled as required to meet system needs. This can be achieved via a number of techniques, such as a hysteresis based approach or a ramping in/out. Furthermore, this feature can be monitored or diagnosed in order to avoid erroneous compensation by a number of techniques, i.e. comparing performance difference between with and without compensation. One embodiment of monitoring the effectiveness of active disturbance rejection is shown in FIG. 9. In this embodiment, the frequency of a target disturbance is monitored to ensure that disturbance rejection is effective. If disturbance rejection performance actually becomes worse, the ADR will be disabled. Criteria of effectiveness of the ADR system may be based, at least in part on, an observed increase in the disturbance pattern, such as by a cumulative sum control chart (CUSUM) or by generating an average value of disturbance torque via a moving average filter.

(30) In one embodiment, the EPS disturbance rejection method is applied to reject a periodic disturbance in EPS system without knowledge of the magnitude and phase of the disturbance. In another embodiment, the value of the disturbance frequency may be an initial approximation, rather than a known value. As an initial approximation, the disturbance frequency may be unknown and established as an initial guess or starting point based on previous system experience or overall vehicular system configuration parameters, i.e., vehicular tolerance or specification limits. The effectiveness of this method is demonstrated under several Examples, as listed below.

EXAMPLE 1

(31) The original torque assist, T.sub.assist, has a frequency components from 5-20 Hz. The disturbance, T.sub.disturbance, is a periodic signal with a frequency of about 12 Hz.

(32) The interested disturbance frequency range is 10-20 Hz, which is the typical range for smooth road shake. This information is used to define properties of the filter and model.

(33) FIGS. 4A-4C illustrate a column torque comparison under three different configurations, namely: (1) uncompensated column torque without disturbance; (2) compensated column torque with disturbance; and (3) uncompensated column torque with disturbance. It can be clearly seen that the disturbance is successfully isolated from the rest of the system when active disturbance rejection is enabled such that it has minimal impact on column torque. However, the original system without active disturbance rejection shows significant differences in the time-domain response. The difference of column torque between compensated column torque and original column is also shown in FIG. 5 to illustrate the same concept.

EXAMPLE 2

(34) In another embodiment of the invention, the EPS system and method may be used where disturbance frequency is different from that of T.sub.assist. In a first step, it is assumed that the frequency of T.sub.assist is about 5 Hz and the disturbance frequency is about 12 Hz. Referring now to FIGS. 6A-6C, it can be clearly shown that column torque with active disturbance rejection only contains one frequency component and is isolated from the disturbance.

(35) The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.