Electric power steering device

Abstract

Electric power steering device for a vehicle, comprising: a steering mechanism operatively connecting a steering wheel to road wheels of the vehicle; an electric motor (1) operatively connected to the steering mechanism; a torque detecting device (6) configured to generate a first output signal indicative of the torque transmitted by a portion of the steering mechanism; a vehicle speed detecting device for generating a second output signal indicative of the speed of the vehicle; a signal processing device (7) adapted to receive the first and second output signals and to generate a torque target signal representing the torque to be applied to the steering mechanism by the electric motor (1); and a motor control stage configured to provide a drive current to the electric motor (1) in response to the torque target signal; wherein the device comprises a torque limiting device (150) configured to limit the torque and / or the maximum rate of increase of the torque after a restart of the device, characterized in that the limit value and / or the rate of increase are determined at least in part by information indicative of a state of the system before the restart and further comprising determining information before the restart indicating whether the shutdown was a normal or abnormal shutdown and applying different limit values and / or increase rates in the respective case.

Description

[0001] The invention relates to improvements in an electric power steering device of the type in which an electric motor is designed to apply an auxiliary torque to a steering component, for example the steering column, so that the effort required by the driver to steer the vehicle is reduced.

[0002] A generic electric power steering device and a method for controlling an electric power steering device with the features of the preamble of patent claim 1 and 8 respectively are known from DE 600 08 976 T2.

[0003] A simple electric power assisted steering (EPAS) system includes a torque sensor positioned to measure the torque level in a steering column. From this measurement, a controller calculates the value of a torque command signal, which includes an auxiliary torque component indicating the torque to be generated by an electric motor attached to the steering column. The motor applies auxiliary torque to the column in the same direction as the direction specified by the driver, thus reducing the effort required to turn the wheel.

[0004] When such a device is initially powered up, the controller may calculate that a high assist torque is required by the driver. This could occur if the device was unexpectedly turned off while the driver was turning at high speed and then restarted while still cornering. Herein, "power up" and "restart" and related terms should be read to include any condition where the EPAS device is initializing after a power failure, whether intentional or unintentional. In such a case, where the engine suddenly applies a high assist torque, the driver may become unsettled. Accordingly, it is known to control the rate at which torque can be applied after a power down using a torque limit that increases over time after restart.

[0005] WO 2005 092689 A1 proposes a modification whereby the speed of the vehicle after restarting is measured by a speed sensor associated with the vehicle. The rate at which the maximum torque that can be applied increases is varied depending on the speed. This can provide a different and faster return of torque for a stationary vehicle than for a fast-moving vehicle. This enables rapid re-establishment at times when the driver is least likely to be unsettled or where such unsettling is least dangerous, for example, when stationary.

[0006] The object underlying the invention is to improve an electric power steering system. This object is achieved by an electric power steering system according to claim 1 or by a method according to claim 8.

[0007] According to a first aspect, the present invention provides an electric power steering apparatus for a vehicle, comprising: a steering mechanism operatively connecting a steering wheel to road wheels of the vehicle; an electric motor operatively connected to the steering mechanism; a torque sensing device configured to generate a first output signal indicative of the torque transmitted by a portion of the steering mechanism; a vehicle speed detecting device for generating a second output signal indicative of the speed of the vehicle; a signal processing device configured to receive the first and second signals and to generate a torque command signal representing the torque to be applied by the motor to the steering mechanism; and a motor drive stage configured to provide a drive current to the motor in response to the torque command signal; wherein the device comprises a torque limiting device configured to limit the torque and / or the maximum rate of increase of the torque after a restart of the device, wherein the limit and / or the rate are determined at least in part by information indicative of the state of the system before the restart.

[0008] The information indicating the state of the system before the restart may include a state indicator (flag) indicating whether the most recent system shutdown prior to the restart was normal or abnormal. It may include a state indicator that is enabled or disabled to indicate one or the other of a normal shutdown or abnormal shutdown. In particular, it may include a measure of at least one of the following: engine speed, vehicle speed, and torque, all as obtained before the restart.

[0009] The provision of a controlled torque limit or torque ramp rate after power-down, according to the invention, enables the device to provide a different startup response after an abnormal power-down compared to that used after a normal power-down. This can offer significant safety advantages. Determining whether a power-down was normal or abnormal using information obtained before the power-down allows a strategy selection to be made quickly without having to wait for the speed sensors to be reinitialized. The applicant has now appreciated that such information may not always be available immediately after restart and that using prior information can fill this gap.For example, speed sensors are typically provided as part of an anti-lock braking system and may be too slow to produce a reliable output signal.

[0010] An abnormal shutdown may involve a shutdown while the vehicle is moving and torque is being applied to the steering. This is considered abnormal because it is not intended to occur and typically occurs if the battery power supply is disconnected or a fault occurs. This may be indicated by an engine speed or vehicle speed greater than a predefined threshold and perhaps a non-zero torque request. A normal shutdown would occur at the end of a trip with the vehicle stationary and the driver not applying any torque to the steering. This would be indicated by an engine speed of zero or near zero and perhaps little or no torque.

[0011] The device may include a memory storing information that can be used during use after a power-off and the next power-up to indicate whether the power-off was normal or abnormal. This may include a non-volatile memory that retains the information even in the event of a power failure.

[0012] By torque limit, it is meant that the device may include a comparison device configured to compare the magnitude of the torque represented by the torque command signal with the torque limit, causing the motor to apply a torque to the steering mechanism equal to the magnitude of the lower of the two compared values. The limit may be expressed as a fraction or as a percentage of the torque available for application to the steering mechanism by the motor.

[0013] The device may be configured to apply a zero torque limit for a specified period after a restart if the speed and / or torque indicated an abnormal shutdown prior to the restart. This ensures that the torque is applied gradually and allows time until a valid first and second signal are received from the speed sensor and torque sensor before assistance is applied. This may be a fraction of a second, a second, or perhaps more.

[0014] The torque limiting device can increase the limit from zero or near-zero to a higher limit or an unlimited value over a period of perhaps one second, five seconds, or a minute or more. The slower the increase, the less confusion is created, but the longer it will take to restore the full and normal assistance to be provided. The torque limiting device can be configured to cease operation once the torque limit reaches the actual torque target, since at that point the risk of driver confusion is eliminated because the steering now meets expectations.

[0015] During a restart procedure following an abnormal shutdown and while the threshold is applied, the device can ignore the first signal from the speed sensor and use a default measurement indicating the vehicle's speed before the restart. This ensures that incorrect information from a speed sensor that has not yet fully returned to operation is not used. The assumption that the speed will not change significantly during the relatively short duration of the restart thresholds is correct in most circumstances. The first signal can be used instead of the default speed if the restart follows a normal shutdown.

[0016] After the restart threshold period, the standard speed can be blended with the first signal until, after a period of time, the first signal is used exclusively. This can even be used before the end of the threshold period, so that the first signal is used alone once the threshold period ends. "Blending" means using a weighted average of the two signals (standard speed and first signal), where the weight applied to the standard is initially higher than that applied to the first signal until the end of the blend, where the weight applied to the standard is lower than that applied to the first signal.

[0017] The torque sensing device may comprise a torque sensor, such as an optical torque sensor, an induction-based sensor, or a magnetic-type sensor. This list is not intended to be exhaustive.

[0018] The vehicle speed detection device may comprise a pickup device that measures the wheel speed of the vehicle and processes this in conjunction with knowledge of the diameter of the tire mounted on the wheel. It may utilize input from an ABS sensor mounted on the vehicle.

[0019] The signal processing device may couple a dedicated processing circuit or chip or may comprise a standard processing resource.

[0020] According to a second aspect, the invention provides a method for controlling an electric power steering device for a vehicle having a steering mechanism, comprising the following steps: Measuring the torque in a part of the steering device; and Calculating a torque command signal indicating the torque to be applied to the steering mechanism by an electric motor; wherein, following a shutdown and the next subsequent restart, the method comprises limiting the torque that can be applied and / or the maximum rate of increase of the torque that can be applied by the electric motor, wherein the limit and / or the rate are determined at least in part by information indicative of a state of the system prior to the restart.

[0021] The method may include determining the information before the restart indicating whether the shutdown was a normal or abnormal shutdown and applying different thresholds and / or increment rates in each case.

[0022] There now follows, by way of example, an embodiment of the present invention which is described with reference to the accompanying drawings, in which: Fig. 1 shows a schematic representation of an embodiment of an electric power steering system according to the invention; Fig. Figure 2 shows a schematic diagram of the circuitry within the processor of the embodiment of Fig. 1 functions performed; Fig. Figure 3 is an overview of the decision process implemented within the processor of the device of Fig. 1 is applied.

[0023] In Fig. An electric power steering device is shown in Figure 1 of the accompanying drawings. The system comprises an electric motor 1 acting on a drive shaft 2 through an optional gearbox 3. The drive shaft is terminated by a worm gear 4, which cooperates with a wheel provided on a portion of the steering column 5, a wheel or shaft provided on a portion of a steering column 5 being operatively connected to the steering column.

[0024] The steering column supports a torque sensing device, which in this example is a torque sensor 6, designed to measure the torque transmitted through the steering column, generated by the driver of the vehicle while turning the steering wheel against the resistive force provided by the vehicle's road wheels (not shown). The output signal T from the torque sensor 6 is fed to a first input of a signal processing device 7. This is typically an electrical circuit, in particular a dedicated integrated circuit such as an ASIC.

[0025] A vehicle speed detection device, in this example a wheel speed sensor, is also provided. This detects the speed of the road wheels, as is known in the art, by reading an ABS sensor associated with one or more of the wheels. Alternatively, it could be GPS-based or powered by the vehicle's speedometer. The sensor's output signal V is fed to a second input of the processing device.

[0026] Furthermore, an engine speed sensor 11 may also be provided, which measures the engine speed v. This may comprise a sensor that detects the current flowing through the vehicle's alternator—a current flow that indicates that the engine is rotating the alternator. The output signal of this sensor can be fed back into an input of the processing device 7.

[0027] The signal processing device 7 acts on the two input signals to generate a torque signal 8 as its output signal, which is passed to a motor control device 9. The motor control device and the associated control circuit convert the torque signal 8 into drive currents for the electric motor 1.

[0028] The value of torque signal 8 corresponds to the amount of auxiliary torque to be applied to the steering column. Normally, this torque corresponds to a torque setpoint signal and is determined by the torque specified by the driver. However, as will become apparent, this can be modified under certain circumstances, particularly after activation.

[0029] The processor is connected to a memory area 153 in which various values indicative of the system's status are stored. This is preferably a memory in which the values are retained even in the event of a power failure, so that the values written to the memory before a power-off can be accessed the next time the system is powered up. These values include an indicator of whether the engine is running, which is obtained from the vehicle speed sensor and / or torque sensor and / or engine speed sensor.

[0030] By shutdown, we mean an event in which the steering system is disconnected from power, either in a normal, planned manner or in an unplanned manner. This can occur, for example, if the battery power supply is temporarily interrupted (abnormal) or if the vehicle ignition is turned off by the driver at the end of a journey (normal).

[0031] The processor performs the function of controlling the motor to ensure that the correct level of assistance is applied. As described in Fig. As shown in Figure 2, the processor 7 includes a target calculation means 149 that calculates a torque target from the output signal of a torque sensor, as is well known in the art. This is then passed to a torque limiting means 150, which can limit the target torque to generate the torque signal sent to the motor. This limit is determined either by a speed allocator 152, which takes as its input a currently measured vehicle speed signal or a defined vehicle speed determined from information stored in memory 153 prior to a restart.

[0032] The output signal of the torque limiting device controls the increase in torque that can be applied to the engine after a restart, according to one or more of the relationships between speed and torque and time, as shown in Fig. 3 is shown.

[0033] Fig. Figure 3 shows the processing steps used to determine a relationship that the speed allocator applies to the vehicle speed to calculate a ramp rate.

[0034] As in Fig. As shown in Figure 3, two distinctly different restart strategies are applied by the torque limiting device depending on whether the restart follows a normal or abnormal shutdown. The strategies differ because it is understood that different strategies are to be used.

[0035] The left pane of the page lists some of the processing steps performed during operation of the EPAS device. Specifically, an engine speed measurement is taken and processed to control the setting of an "EPAS ASSIST" status indicator in memory, which indicates whether or not EPAS has been assisting. If the engine speed is above a predefined level, for example, 500 sec. -1, then EPAS is considered to be assisting. If it is below this, the engine is assumed to be off and EPAS is not assisting.

[0036] After a power off and restart, indicated by the dotted line, other processing steps are performed, as shown in the right part of the diagram of the Fig.3. Specifically, the "EPAS Assist" status indicator stored prior to shutdown / restart is consulted to see whether EPAS was providing assistance or not. A different ramp rate is used depending on the case. If EPAS was not assisting, a relatively fast ramp rate is used, taking approximately one second to reach the maximum torque limit. If EPAS was assisting, it is assumed that the shutdown was abnormal, and a slow ramp rate is used to perform a non-disturbing restart. This is represented in the figure as having a duration of X > 1. A suitable value for X might be 2 seconds.

[0037] It's also worth noting that the sensor-measured vehicle speed is used during a normal restart, while a default speed is used during a slow-increase restart. This default speed can be determined as the last measured speed before shutdown. Alternatively, it can be any predetermined speed, such as 10 mph, 20 mph, 30 mph, etc. This overcomes the problems of potential delays in obtaining an accurate speed measurement after an abnormal shutdown or restart.

[0038] The effect of the torque limiter's slew rate is to limit the rate at which the torque applied by the engine can increase, independent of the driver's command. Alternatively, it may be provided to limit the maximum torque that can be applied and the rate at which the maximum limit is increased over time. This may be done in conjunction with, or as an alternative to, limiting the slew rate. For further guidance on this subject, the skilled person is referred to the teachings of prior application WO 2005 092689 A1.

Claims

[1] Electric power steering device for a vehicle, comprising: a steering mechanism operatively connecting a steering wheel to road wheels of the vehicle; an electric motor (1) operatively connected to the steering mechanism; a torque detecting device (6) configured to generate a first output signal indicative of the torque transmitted by a portion of the steering mechanism; a vehicle speed detecting device for generating a second output signal indicative of the speed of the vehicle; a signal processing device (7) adapted to receive the first and second output signals and to generate a torque target signal representing the torque to be applied to the steering mechanism by the electric motor (1); and a motor control stage configured to provide a drive current to the electric motor (1) in response to the torque target signal; wherein the device comprises a torque limiting device (150) designed to limit the torque and / or the maximum rate of increase of the torque after a restart of the device, characterized by that the limit value and / or the increase rate are determined at least in part by information indicative of a state of the system before the restart and further comprising determining information before the restart indicating whether the shutdown was a normal or abnormal shutdown and applying different limit values and / or increase rates in the respective case. [2] An electric power steering apparatus according to claim 1, wherein the information indicating the state of the system before the restart includes a state indicator for indicating whether or not the most recent shutdown of the system before the restart was normal or abnormal. [3] An electric power steering apparatus according to claim 1 or 2, wherein the information indicative of the state of the system comprises at least one measure of the following: engine speed, vehicle speed and torque as obtained before the restart. [4] An electric power steering apparatus according to any preceding claim, comprising a comparison device arranged to compare the magnitude of the torque represented by the torque command signal with the torque limit value and to cause the electric motor (1) to apply a torque having a magnitude of the lower of the two compared values to the steering mechanism. [5] An electric power steering apparatus according to any preceding claim, adapted to apply a zero torque limit for a predetermined period of time after a restart if the speed and / or torque prior to the restart indicates an abnormal shutdown. [6] An electric power steering device according to any preceding claim, arranged to use, during a restart procedure from an abnormal shutdown and while applying a limit value, a standard measurement indicative of the speed of the vehicle before the restart. [7] An electric power steering apparatus according to claim 6, which is arranged to mix the standard speed with the first signal after a restart until after a period of time the first signal is used exclusively. [8] A method for controlling an electric power steering device for a vehicle having a steering mechanism, comprising the following steps: Measuring the torque in a part of the steering device; and Calculating a torque command signal indicating the torque to be applied to the steering mechanism by an electric motor (1); wherein after a shutdown and the next subsequent restart, the method comprises limiting the torque that can be applied and / or the maximum rate of increase of the torque that can be applied by the electric motor (1), wherein the limit value and / or the rate of increase are determined at least in part by information indicative of a state of the system before the restart, characterized by that further comprises determining information before the restart indicating whether the shutdown is a normal or abnormal shutdown, and applies different limits and / or increase rates in each case.

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