Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
1998-09-29
2001-07-24
Louis-Jacques, Jacques H. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S043000, C180S421000, C180S422000, C180S443000, C180S446000
Reexamination Certificate
active
06266591
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a controller for an electric power assisted steering (EPAS) system for a vehicle and to an EPAS system.
In a known EPAS system comprising a controller controlling current to an electric motor which is mechanically connected to a component of the steering system, it is known to provide overall motor current limiting. For this purpose, the motor current is compared with a reference value and in the event that the motor current exceeds the reference value, power to the motor is cut off. The means which controls the power supply to the motor is supplied with a torque signal indicative of the torque applied to the vehicle steering wheel by the driver and as is well known the overall torque required to steer a vehicle generally diminishes with increasing vehicle speed. Whilst the current limiting arrangement provides a limit to the torque assistance provided by the motor and also provides protection for the motor and the drive circuits, the torque limit may not be appropriate for all driving conditions.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a controller for an electric power assisted steering system comprising a drive circuit and a current limiter for limiting current supplied by the drive circuit to an electric motor of the system as a function of vehicle speed.
It is thus possible to provide a controller for an electric power assisted steering system which limits the amount of current supplied to the electric motor and thereby limits the amount of torque developed by the motor.
It is also possible to ensure that the magnitude of the motor current does not exceed a predetermined value determined as a function of the vehicle speed.
Preferably, the maximum current magnitude supplied to the electric motor decreases monotonically as the vehicle speed increases.
Preferably, the current supplied to the electric motor is controlled by pulse width modulation.
A sensor may be arranged to measure the current supplied to the electric motor. The sensor may be arranged to measure the absolute value or the magnitude of the motor current. The sensor may be a sense resistor.
The electric motor may be of the three or more-phase star-connected brushless permanent magnet type. Alternatively, the electric motor may be of the delta-connected type, multi-phase bridge with switched reluctance motor type or the two-phase bridge type.
An over-current trip may be provided whereby the current supplied to the electric motor is monitored and a disable signal generated if the motor current magnitude exceeds a predetermined value.
Preferably, a latch is provided to maintain the disable signal until the latch is reset.
Preferably, a vehicle speed sensor is provided. The speed sensor may be a toothed wheel with a sensing element, for example a Hall-effect sensor or an optical sensor. Alternatively, the sensor may be a reed switch activated or deactivated by a rotating magnet. The speed sensor may alternatively be a variable reluctance type sensor.
Preferably, more than one speed sensor may be provided to improve the integrity of vehicle speed measurement.
Alternatively, the current limiter may be arranged as an electronic circuit for generating a first predetermined threshold signal and a further current limiter may be arranged as part of microcontroller software which generates a second threshold signal. Preferably, the first threshold signal is higher than the second threshold signal.
It is thus still possible to limit the amount of unwanted assistance torque provided by the system in the event of a fault in the microcontroller.
The current limiter may comprise a memory for storing current limit data as a function of vehicle speed, a comparator for comparing the current supplied by the drive circuit to the motor with the current limit, and a circuit for loading the current limit data into the memory and for periodically examining the stored data.
Preferably the data loading circuit loads the current limit data into the memory each time power is applied to the system.
Preferably the controller comprises a safety circuit for preventing writing to the memory during normal operation of the system.
Preferably the controller comprises a data processor for periodically calculating the current limit as a function of speed. Preferably the controller comprises a current measuring arrangement for measuring the current supplied by the drive circuit to the motor.
It is thus possible to provide an arrangement which is periodically programmed, for example at the start of every vehicle journey, so as to account for sensor variations and hardware drift. Such an arrangement is immune to correction of the current limit by subsequent malfunction during the journey. Preferably the current limit and sensor configuration are stored in the memory, such as random access memory, only when the supply of current from the drive circuit to the motor is interrupted, for instance by independent means.
According to a second aspect of the invention, there is provided an electric power assisted steering system including a controller in accordance with the first aspect of the invention.
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Horton Steven J
Wilson-Jones Russell
Andrus Sceales, Starke & Sawall
Broadhead Brian J
Louis-Jacques Jacques H.
TRW LucasVarity Electric Steering Limited
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