CONTROL FOR ELECTRIC POWER STEERING

Abstract

A method of controlling an electric power assisted steering (EPS) system comprises the steps: During a first mode of operation of the EPS system driving a motor using two inverter bridges simultaneously, the nominal current that is applied to the motor at any time by each bridge being limited to a first value, and during a fault mode of operation driving the motor using only one of the two inverter bridges whereby the nominal current that is applied to the motor at any time in the fault mode is limited to a second value, the method varying the second value as a function of one or more operating parameters of the EPS system whereby the second value is set higher than the first value when it is deemed safe to do so.

Claims

1. Apparatus for detecting misalignment of a radar unit of a vehicle, the apparatus comprising: a rotational speed sensor arranged to measure the rotational speed of the radar unit about at least one axis and having an output for a signal indicative of the rotational speed a processor arranged coupled to the output of the rotational speed sensor; in which the processor is arranged to determine the misalignment at least in part based on the rotational speed measured by the rotational speed sensor.

2. The apparatus of claim 1, in which the least one axis consists of one generally vertical axis.

3. The apparatus of claim 1, comprising an accelerometer arranged to determine the acceleration of the radar unit along at least two axes, and having an output for a signal indicative of the acceleration coupled to the processor, with the processor being arranged to use the acceleration to determine the misalignment.

4. The apparatus of claim 3, in which the two axes are perpendicular to each other and to the at least one axis of the rotational speed sensor.

5. The apparatus of claim 1, in which the processor is arranged to determine the misalignment based upon the output of the rotational speed sensor by integrating the rotational speed.

6. The apparatus of claim 1, in which the apparatus is arranged so as to take measurements of the rotational speed regularly or continually over a period of time.

7. The apparatus of claim 6, in which the period of time comprises at least the period when an ignition of the vehicle is switched off.

8. The apparatus of claim 1, in which the rotational speed sensor is arranged to only measure the rotational speed if it exceeds a threshold.

9. The apparatus of claim 1, in which the rotational speed sensor comprises a gyroscope.

10. A vehicle having a radar unit and the apparatus of claim 1 attached thereto, in which the rotational speed sensor is attached to or integrated in the radar unit.

11. The vehicle of claim 10, provided with a further accelerometer coupled to the vehicle and able to determine the acceleration of the vehicle about two or three axes, with an output of the further accelerometer being coupled to the processor and the processor arranged to determine the misalignment based upon the acceleration of the vehicle.

12. A method of detecting misalignment of a radar unit of a vehicle, the method comprising measuring the rotational speed of the radar unit about at least one axis and having an output for a signal indicative of the rotational speed and determining the misalignment at least in part based on the measured rotational speed.

13. The method of claim 12, comprising determining, typically using an accelerometer, the acceleration of the radar unit along at least two axes and using the acceleration to determine the misalignment.

14. The method of claim 13, in which the two axes are perpendicular to each other and to the at least one axis about which the rotational speed is measured.

15. The method of claim 12, in which in which the least one axis consists of one generally vertical axis.

16. The method of claim 12, in which the method comprises determining the misalignment by integrating the rotational speed.

17. The method of claim 12, comprising taking measurements of the rotational speed regularly or continually over a period of time.

18. The method of claim 17, in which the period of time comprises at least the period when an ignition of the vehicle is switched off.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0046] Embodiments of the invention will now be described with reference to the Figures of the accompanying drawings in which:

[0047] FIG. 1 shows an embodiment of a dual-bridge EPS control system in accordance with the invention during a normal mode of operation;

[0048] FIG. 2 corresponds to FIG. 1 showing the EPS control system in a fault mode of operation;

[0049] FIG. 3 shows schematically the two lanes and the controller which sets the maximum nominal current available to the motor from each lane;

[0050] FIG. 4 is a flowchart of a control method in accordance with an embodiment of the invention; and

[0051] FIG. 5 shows a prior art method of operating a dual bridge EPS system where a single lane is used to provide an overrated current limit for a fixed period of time after a fault has occurred.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0052] FIGS. 1 and 2 represent a first embodiment of a dual-bridge EPS control system 100 in accordance with the present invention. Although a two lane system is shown, this is exemplary only and the invention may be applied to EPS systems have three or more lanes.

[0053] The EPS system comprises first 120 and second 120′ inverter bridges connected to and configured to drive first 130 and second 130′ motors, respectively. Each motor comprises a set of phase windings, in this example forming three phases connected in a star topology. The first bridge and motor form one lane, and the second bridge and motor form a second lane. The two lanes are identical in this example, of the construction of only a first lane will now be described.

[0054] A DC voltage is applied by a battery 110 between a supply rail and a ground rail and is connected to the 3-phase motor 130 via an inverter 120. The inverter 120 comprises three parallel arms (A, B, C), each of which has a pair of Silicon (Si) MOSFETs 122a, 122b, 124a, 124b, 126a, 126b connected in series between a supply rail and a ground rail. The motor phases A, B, C are connected to each other in a star configuration and branch off from between a respective pair of MOSFETs. As such, MOSFETs 122a, 122b are connected to a first phase A of the motor 130, MOSFETs 124a, 124b are connected to a second phase B of the motor 130, and MOSFETs 126a, 126b are connected to a third phase C of the motor 130. The pairs of MOSFETs associated with each phase (arms A, B, C) are connected in parallel to each other and to the battery 110. A power filter (not shown) may be provided between the battery 110 and the MOSFETs 122a, 122b, 124a, 124b, 126a, 126b.

[0055] The MOSFETS are also arranged in two groups with MOSFETS 122a, 124a, 126a on the “high” side of the inverter 120 and MOSFETS 122b, 124b, 126b on the “low” side of the inverter 120. The terms “high” and “low” are labels for ease of reference only. Each MOSFET 122a, 122b, 124a, 124b, 126a, 126b comprises a transistor channel and an intrinsic body diode connected in parallel with the transistor channel. The gate of each MOSFET 122a, 122b, 124a, 124b, 126a, 126b is connected to a gate driver to switch each MOSFET ON or OFF. Each gate driver received control signals from a control block (not shown).

[0056] In use, the controller applies voltage signals to the gate of each MOSFET to switch them ON and OFF rapidly in a predefined sequence, thus controlling the voltage applied to each phase of the motor and current flowing through the windings. This in turn controls the strength and orientation of the magnetic field produced by the windings, and hence the torque and speed of the motor. By using a sufficiently rapid pulse width modulation (PWM) switching pattern, a phase drive waveform can be applied that approximates the ideal sinusoidal waveform required to rotate the motor smoothly. This applies for both bridges 120, 120′ in normal operation, i.e. all MOSFETs of both bridges 120, 120′ are turned ON and OFF in a controlled manner during normal operation.

[0057] When the steering wheel 140 of a vehicle (not shown) is operated by a driver, a demand torque is detected by the system. The controller determines from this torque demand a current demand for each of the two motors 130, 130′, and appropriate PWM gate signals are applied to the switches to cause the motor to generate an assistance torque, dependent on the demand torque and vehicle speed, which acts on the steering rack to assist steering the vehicle.

[0058] The two motors 130, 130′ may be physically located in one housing or separately but, in either case, they act on the same steering rack and, in normal operation, each contribute approximately 50% of the steering power. This is easily achieved if the two lanes are substantially identical by providing the same PWM drive signals to the two inverter bridges from the controller. The motors 130, 130′ may conveniently be brushless 3-phase AC permanent magnet synchronous (PMSM) motors.

[0059] In normal operation, each motor applies the same torque. This means each inverter bridge applies the same current to the respective phase windings of the lane. Because only 50 percent nominal peak current is needed from each lane, the switches can be chosen to be of a relatively low rating where they can safely supply the 50 percent nominal peak current continuously if required, but not rated high enough to apply 100 percent of the nominal peak current from one lane continuously. If this was provided the switches would be expected to overheat leading to a failure. For ease of explanation this 50 percent limit will be considered hereinafter to be a first current limit value.

[0060] A fault can develop in either lane, often in the inverter bridge, that may render it unable to drive the motor. The most likely fault lies with the switching elements of the inverter bridges 120,120′. Other faults that can occur include a loss of connection between the bridge and the motor windings, or a short circuit or open circuit across one or more of the windings.

[0061] The system includes a fault detection unit which monitors the operation of each lane. When a fault is detected the fault detection unit instructs the controller to operate the EPS system in a fault mode of operation as follows.

[0062] In the fault mode, the faulty lane is switched off so that it does not provide any current that drives the motor. Instead of sharing the motor torque across the two lanes, all torque is provided by a single non-faulty lane. Where there are more than two lanes, the fault mode will share the torque across each of the remaining non-faulty lanes.

[0063] To ensure that the amount of steering assistance is not reduced, the nominal current limit is increased from the first current limit value to a second higher current limit value. The amount by which it is increased is set according to one or more operating parameters of the EPS system.

[0064] In the fault mode, the second current limit value is set at the 100 percent value for as long as possible, being reduced if the operating conditions indicate that it is not safe to do so due to overheating or a risk of overheating. In that case, the second value is de-rated to a reduced level. Notably, by monitoring the operational parameter continuously when in the fault mode the de-rated second value may be increased as soon as it is possible to do so without risk of overheating. It will be continuously varied to give the most assistance possible from the non-faulty lane so that the impact of loosing one lane is minimised.

[0065] A method of carrying out an embodiment of the invention is summarised in FIG. 4. In a first step S10, it is assumed that all lanes are correctly functioning. The nominal maximum current that can be supplied from each lane is set to 50 percent of the maximum current demand, with both lanes sharing the duties of providing torque to the EPS system.

[0066] The operational parameters of the motor are continuously monitored and if all is well the EPS system in step 11 continues to run in the normal mode of operation. If a fault in one lane is detected, that lane is turned off an the operation is switched to a fault mode in step 12. Initially in step 13 a value of the current limit is uprated to 100 percent for the remaining non-faulty lane so that the motor can apply the same level of assistance that it could in the normal mode. The operational parameters of the EPS system are monitored in step 14 and if it is deemed unsafe to run at the 100 percent uprating then in step 15 the second value is de-rated to a safe value less than 100 percent. It is known that 50 percent is safe so in a simple system it could jump down to 50 percent. However, it is preferable to de-rate as little as possible to give the maximum torque assistance to the driver. On the other hand, if the second value is below 100 percent and it is deemed safe to increase the second value, then in step 16 the second value can be increased up to 100 percent or as high as deemed safe at that instant.

[0067] The proposed method of the present invention has the inherent benefit of having the capability to reverse the de-rating. This means that if the boundary conditions change in a favourable way (e.g. the load conditions allow the temperature of power stage components to decrease), the available level of motor current can be increased back up again to 100 percent, and the later increased/decreased over time to give optimum steering assistance levels within the safe operating limits of the EPS system.

[0068] The method of the present invention may provide the following key benefits: [0069] No detailed characterization is needed to determine the allowable time for the overrating of the non-faulty lane(s) [0070] Derating to below 100 percent adapts to operating conditions [0071] Applicable to N lanes where N is any integer of 2 or higher

[0072] From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of, or in addition to, features already described herein.

[0073] Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

[0074] Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

[0075] For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and any reference signs in the claims shall not be construed as limiting the scope of the claims.