Electrical Power Assisted Steering System

Abstract

A method of operating an electric power assisted steering system of the kind in which an electric motor applies an assistance torque to a part of the steering system as a function of an assistance torque signal, the assistance torque reducing the force required by the driver to turn a steering wheel, comprises during normal operation generating an assistance torque signal that is dependent on the torque applied by the driver, and during a fault operation in which the torque applied by the driver is not reliably known to the system generating an assistance torque signal that is derived from a first term that has a value that varies as a function of the angular position of the steering relative to a changeable datum, the angular position of the changeable datum being updated in the event that the direction of rotation of the steering changes.

Claims

1. A method of operating an electric power assisted steering system in which an electric motor applies an assistance torque to a part of the steering system as a function of an assistance torque signal, the assistance torque reducing a force required by a driver to turn a steering wheel, the method comprising during normal operation generating an assistance torque signal that is dependent on a torque applied by the driver, and during a fault operation in which the torque applied by the driver is not reliably known to the system generating an assistance torque signal that is derived from a first component or term that has a value that varies as a function of an angular position of the steering relative to a changeable datum, the angular position of the changeable datum being updated in the event that a direction of rotation of the steering changes.

2. The method according to claim 1 comprising resetting the changeable datum when the direction of rotation of the steering changes but only if one or more additional conditions are met.

3. The method according to claim 2 in which the additional condition is dependent of the value of the assistance torque signal at the time of the direction of rotation of the steering changes.

4. The method according to claim 1 comprising determining the changeable datum when a normal mode of operation is being used, ready to be employed in generating the assistance torque signal in the event that the assistance torque signal is lost or becomes unreliable.

5. The method according claim 1 comprising varying the first component at a high rate during an initial amount of movement away from the changeable datum, and thereafter varying at a lower rate or becoming saturated and not varying with increasing movement away from the changeable datum.

6. The method according to claim 1 further comprising modifying the value of the first component as a function of vehicle speed.

7. The method according to claim 1 that further comprises generating an assistance torque, during the fault operation, by the step of generating a second component or term that varies as a function of the position of the steering relative to a straight ahead position, the assistance torque being derived from a combination of the first component with the second component.

8. The method according to claim 7 comprising producing an assistance torque signal in the fault operation that includes a step of summing the first component and the second component.

9. The method according to claim 1 that further comprises generating an assistance torque signal during the fault operation by the step of producing a third component that varies as a function of steering angular velocity.

10. The method according to claim 1 further comprising a step of determining whether a reliable measure of torque is available, and switching from a normal mode of operation to a fault mode if it is not.

11. An electric power assisted steering system comprising a steering mechanism which operatively connects a steering wheel to road wheels of a vehicle, an electric motor operatively connected to the steering mechanism; a torque sensor adapted to produce a torque signal indicative of torque carried by a portion of the steering mechanism, and a signal processing means operable in a normal mode when an output signal of the torque sensor is reliably available and a fault mode when it is not, in which the signal processing means includes: a signal processing means which in the normal mode is adapted to generate an assistance torque signal that is dependent on the torque applied by the driver, and during a fault operation is adapted to generate an assistance torque signal that is independent of the driver applied torque, the signal processing means including a first sub-unit which in the fault mode generates a first term of the assistance torque signal that has a value that varies as a function of an angular position of the steering relative to a variable datum, the variable datum being reset when the direction of the rotation of steering reverses.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 is a schematic diagram of an electric power assisted steering system in accordance with the present invention working in a normal mode of operation;

[0046] FIG. 2 is a schematic diagram of the electric power assisted steering system of FIG. 1 working in a fault mode of operation

[0047] FIG. 3 is a block diagram illustrating the functional steps undertaken by the signal processor of the system of FIG. 1 in the normal mode;

[0048] FIG. 4 is a block diagram illustrating the functional steps undertaken by the signal processor of the system of FIG. 1 in the fault mode;

[0049] FIG. 5 is a graph showing the variation in the value of a first term generated in the process of producing the assistance torque signal as a function of the position of the steering relative to a changeable datum;

[0050] FIG. 6 is a graph showing the variation in the value of a second term generated in the process of producing the assistance torque as a function of the absolute column angle relative to the straight ahead position;

[0051] FIG. 7 is a graph showing the variation in the value of a gain term as a function of the column velocity Ncol; and

[0052] FIG. 8 is a graph showing the variation in a torque limit applied to the assistance torque signal value as a function of column angle relative to the straight ahead position.

DETAILED DESCRIPTION OF THE INVENTION

[0053] A typical electric power assisted steering system is illustrated in FIG. 1 of the accompanying drawings. The system comprises an electric motor 1 which acts upon a drive shaft 2 through an (optional) gearbox 3. The drive shaft 2 terminates with a worm gear that co-operates with a wheel provided on a portion of a steering column 5 or a shaft operatively connected to the steering column, for example acting upon the steering rack. Of course, this is not to be taken as limiting to the scope of protection we are seeking, and other power assisted steering systems are envisaged to which the invention has application. The steering column can be rotated by the driver applying a torque to the steering wheel 4.

[0054] The steering column 5 carries a torque sensor 6 that is adapted to measure the torque carried by the steering column 5 that is produced by the driver of the vehicle as the steering wheel (not shown) and hence steering column is turned against the resisting force provided by the vehicles road wheels (also not shown). The output signal T from the torque sensor 6 is fed to a first input of a signal processing unit 7. A shown the torque sensor is a dual channel device having two sets of sensing elements, providing for a degree of redundancy and enhanced safety, but a single channel device could be used.

[0055] A motor angular position sensor is provided which produces an output signal indicative of the angular position of the motor. Because the motor is directly connected to the steering column, this in turn enables a signal indicative of the absolute angular position of the steering column qcol relative to a fixed datum to be produced. The fixed datum is usually set to be zero at the straight ahead position of the vehicle and varies linearly as the steering moves away from the straight ahead position, positive value indicating a position in one direction (for example clockwise) from the straight-ahead and negative values indicating a position in the other direction. This sensor may produce an output signal expressed in degrees of rotation or any other convenient scale.

[0056] As an alternative, an angular position sensor may be connected directly to the steering column in other arrangements, or may form a part of the torque sensor.

[0057] Lastly, the system includes sensors for measuring the vehicle speed and vehicle lateral acceleration which produce respective output signals Vspd and Vacc. These could be omitted and the appropriate signals may be taken from these sensors where already provided on the vehicle for use in other systems, such as ABS or stability control systems.

[0058] The signal processing unit 7 acts upon these signals to produce, as its output, an assistance torque signal 9 that is passed to a motor controller (not shown). The motor controller converts the assistance torque signal 9, which is indicative of an amount of assistance torque demanded from the motor, into drive currents for the electric motor 1. The motor is then driven by these drive currents by applying PWM modulation to the switches of a motor drive bridge as is known in the art.

[0059] To produce this assistance torque signal 9 the processing unit 7 includes a number of sub-units, each of which performs a single processing step or a specific set of steps. These units work together to provide two distinct modes of operation: a normal mode in which the assistance torque is primarily based on the torque signal output from the torque sensor, and a fault mode of operation in which an assistance torque is generated without use of the torque signal. These are explained in more detail below with reference to FIGS. 3 and 4 in particular.

[0060] In each mode of operation, the value of the torque assistance signal 8 corresponds to the amount of assistance torque to be applied to the steering column by the electric motor 1. The value will vary from a minimum value corresponding to maximum output torque for the motor in one sense, through zero torque when the demand signal is zero, to a maximum motor torque of the opposite sense.

[0061] The motor controller 9 receives as its input the torque assistance signal and produces currents that are fed to the motor to reproduce the desired torque at the motor drive shaft 2. It is this assistance torque applied to the steering column shaft 5 that reduces the effort needed by the driver to turn the wheel.

Normal Mode of Operation

[0062] The function of the signal processing unit 7 in the normal mode is shown in FIG. 3. The torque demand signal 8 is made up of at least two components or terms. The first component is a torque component which depends on the amount of torque a driver is applying to the steering column through the wheel.

[0063] The second component or term is a damping component which is provided in order to improve the steering feel and/or enhance the safety of the system. Other torque components can be used in addition, for example to help to counter the effects of cross winds on the vehicle which can cause it to stray from an intended path.

[0064] The assistance torque signal is therefore derived as a function of the torque in the steering as measured by the torque sensor 6. The relationship between the measured torque and the assistance signal is essentially linear. However, other possible relationships may be used to map the torque to the assistance signal. In both cases, as torque increases the magnitude of the assistance signal increases. It will also be understood that the assistance torque signal may be dependent upon other parameters such as vehicle speed if required. In that case it is typical to reduce the value of the assistance torque signal at high speeds to enhance stability and increase it at very low speeds to ease parking manoeuvres.

Fault Mode of Operation

[0065] In the event that the torque sensor has a fault, or the signal from the torque sensor is otherwise deemed unreliable, the system will switch to operate in a second mode called a fault mode of operation. In this second mode the assistance torque is generated independent of the measured torque. Instead, an estimated amount of assistance torque is generated based on the angular position of the steering wheel.

[0066] FIG. 4 shows the functionality of the signal processing unit 7 in this mode and the control variables that are used by the signal processor to generate the assistance torque when operating in the second mode. As can be seen, the torque sensor signals are no longer used as an input in this mode. Instead, steering column angular position qcol, the vehicle speed vspd and vehicle lateral acceleration Vacc, and the steering column or motor angular velocity Ncol are used.

[0067] It can be seen in FIG. 4 that the signal processor produces an assistance torque signal 8 that is derived from three distinct components or terms, each of which is generated by a sub-unit of the signal processor. The three terms are as follows:

[0068] (a) a first term which varies as a function of the position Dqcol of the steering column relative to a changing datum,

[0069] (b) a second term which varies as a function of the position of the steering column relative to the fixed datum corresponding to the vehicle steering pointing straight ahead; and

[0070] (c) an optional third term which varies as a function of the velocity Ncol of the steering column.

[0071] These three terms are combined at sub-unit 12 within the signal processor to form the final assistance torque signal 8.

[0072] The first term is shown in FIG. 5 of the drawings. The graph shows on the X axis the position Dqcol of the steering relative to the changing datum and y axis is the value of the first term, expressed as a torque value in Nm. This component will be mapped on Dqcol and have axis going from 0 to high as the Dqcol will be reset on direction change. As can be seen this term is also modified as a function of vehicle speed, being scaled back at increasing vehicle speeds.

[0073] The key characteristic of the first term is that there is a rapid increase in the torque on initial movement away from the datum, followed by a reduced increase or saturation (no further increase) on further movement.

[0074] The changing datum from which Dqcol is measured is reset whenever the direction of rotation of the steering changes, a reversal of direction. As long at the steering is moving in one direction, or has come to rest, the datum will not be reset but will retain its last datum value. Thus, during use of the vehicle the position of the datum will be reset from time to time whenever the driver holds the steering at one fixed angle for a given time.

[0075] For example, the steering may be rotated away from the straight ahead position by the driver, and after rotating through one half a rotations in the clockwise direction is held stationary. The datum not be reset to 180 degrees from the straight ahead position as there is no reversal.

[0076] When stationary, the amount of assistance needed can be dropped off and so the value of the first component is reduced steadily over time towards zero.

[0077] If the driver starts to turn away further, in the same direction away from the straight ahead position, then the first component will again be applied in line with the mapped value.

[0078] If the steering is then moved in the return direction, back towards straight ahead, the datum is reset and the first component is calculated based on the graph using the new datum.

[0079] The second component or term is shown in FIG. 6. In this graph, the x axis indicates the steering position relative to the fixed straight ahead position, qcol, against on the y axis the value of the second term as a torque in newton metres. It can be seen that the second component varies in a piecewise linear manner, increasing as the steering moves away from the straight ahead position. The value is based on absolute steering angle and is symmetric across the straight ahead position, and so can be generated using a simple map that goes only from 0 to High. Multiple traces are shown as this component can also be scaled or limited with vehicle speed as with the first term.

[0080] The third component or term is shown in FIG. 7. This term is generally a damping term. The graph shows in the X axis the steering velocity Ncol varying from low to high speed, and on the Y axis the value of the third term expressed as a gain. Again this is also dependent on the vehicle speed in this example.

[0081] This third term damping can be used to provide an additional initial kick, kind of derivative term for the initial angle movement, as the vehicle moves away from the straight ahead position but then it later turns into damping control for larger angles. This configuration is used mostly at park, where the inertia to be overcome is higher, for the higher vehicle speeds it is damping all across the steering range.

[0082] The transfer functions for each of the three terms may be stored in a memory associated with the signal processing unit 7, as a look up table or map, or as mathematical equations, or as a mixture. Where a map is used, the signal processor may interpolate any required intermediate values. Where an equation is used, it is conveniently expressed as a set of piecewise linear equations.

[0083] The three terms are fed to a summation block as shown in FIG. 4 (question—is it right that the first two are added to the gain signal?). The output of the summation block is then, in this example, modified by a further torque limit term that applies a limit as a function of vehicle speed and column angle qcol as shown in FIG. 8

[0084] Whenever the signal processor deems that the datum is to be reset, the value of the assistance torque that is generated at the time will gradually be reduced to blend to the value that is determined according to the sum of the three components (Question—is this right?)

[0085] The output from the signal processor 7 in the fault mode may then be passed to the drive circuit to produce appropriate drive currents for the motor.

[0086] In use, a monitor is provided that determines if a reliable torque signal is available and to choose which mode to operate within. A warning may be issued to the driver when the system is operating in the fault mode.

[0087] In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. 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.